Group 8 - Lawnmower (Gasoline Powered)
The following report documents the disassembly and reassembly process of a Toro series mower, made by Briggs and Stratton. This procedure was conducted in the Fall of 2012. Along the way, our group studied the product and developed an understanding of the engineering decisions that went into the creation of this product. Since the walk-behind, self-propelled mower is intended for home use, part geometry, component selection, and retail price were all determined through consideration of Toro’s desired customers; homeowners with small to medium-sized lawns. Analysis of the small 4-stroke gasoline engine used to power the product allowed the group to better understand the interactions and processes performed by this type of engine. In addition to the 4-stroke motor, we also reviewed the materials and manufacturing processes used for simpler components, such as the wheels and blade housing. Implementation of the GSEE (Global, Societal, Enviromental, and Economic) factors allowed us to put ourselves in the shoes of the designers that created the lawnmower. In the end, we found that the Toro series mower was well-engineered to meet the demands of its target customer base. Our proposed design revisions did not need to address any glaring problems with the product. Instead, they served to provide added convenience to the lawnmower.
Gate 1 - Project Planning
The purpose of this project is to reverse engineer a consumer product, which in our case is a Briggs and Stratton Toro-series lawnmower, in order to analyze it both as a whole and on a component level. We will look at such things as the manufacturing processes used to make the components, the considerations that went into designing he product, and some possible revisions to the product based on our work with it.
The plan to reverse engineer the lawnmower is to meet as a whole group at Evan’s house off campus. He has all of the necessary tools and equipment to disassemble the mower. We plan on going step-by-step taking apart each component of the lawnmower, taking photographs of the different components as we encounter them. We will work from the outside-in, starting with the handle, wheels, etc., and eventually moving into the engine block. From initial inspection, the tools that are required to disassemble each part of the lawn mower include, but will not be limited to:
- 3/8 and ½ drive socket set with multiple length extensions, many different metric sizes
- Multiple length, and size, Philips head and regular (flat head) screwdrivers
- Torx screwdrivers
- Needle nose pliers
- Rubber mallet
- Penetrating oil/lubricant (PB Blaster)
The estimated time that it will take to fully disassemble the lawnmower is 3-5 hours, depending on what complications we run in to. Some challenges we may encounter as we go are frozen and/or stripped bolts, and sheared bolts. We do not plan on removing any parts that are riveted together because we won’t be able to reassemble those parts correctly, so removing rivets will not be a challenge.
- Our group, as a whole, has at least a basic competency necessary to work with the mower, and specifically the engine; this includes skills with technical aspects such as disassembling the machine and identifying the functions of parts, as well as a grasp of the concept associated, i.e. how a gasoline engine works.
Project work is to be split up as evenly as possible between the five members of our group. Tasks that require specific skills or tools, such as solid modeling or component removal, will be assigned based on each member's strengths. Based on these strengths, we created titles for each member within the group:
- Project Manager , Kevin Bartosiewicz (email@example.com): Responsible for the successful planning, execution, oversight, and completion of a project. This includes coordinating information across the group, setting up team meetings, ensuring Gates are completed in a timely manner, and regularly assessing the collective productivity of the group. Strengths include consistent punctuality, effective written and oral communication, and strong organizational skills; hands-on skills and knowledge of combustion engines are limited.
- Technical Expert , Evan Farino (firstname.lastname@example.org): Responsible for overseeing of disassembly of lawnmower, as well as analysis of the different components and access to necessary tools. Strengths include several years' experience working with mechanical systems and good group collaboration; skill with computers (solid modeling, Wiki, etc.) are basic.
- Communication Liason , Anibal Martinez (email@example.com): Responsible for coordination of communication between our group and the professors, will take note of any question that the group has and address it to the professors and/or TA’s, and make sure that everyone in the group is aware of their tasks. Strengths are hands-on experience with combustion engines and good organizational skills; public speaking and time management are not strong points.
- Solid Modeler/Factotum , Jordan Pytlak (firstname.lastname@example.org): Responsible for computer-generated models of machine components, as well as helping any group member if they are having trouble and performing general work necessary for the project, but not covered specifically by any other person. Strengths include familiarity with Solidworks, good teamwork, and strong problem-solving; writing skills and combustion engine knowledge are limited.
- Group Collaborator/Reporter , Andrew Serwon (email@example.com): Responsible for editing and combining different pieces of each Gate from each member and posting them to the group Wiki page, as well as any additional work necessary. Strengths include strong writing skills and ability to learn quickly; time-management and hands-on skills are somewhat lacking.
Group meetings will be held weekly or bi-weekly (depending on availability) at 3:30 p.m. on Thursdays in the Silverman Library (Capen Hall). These meetings will include:
- discussion of upcoming projects (assigning tasks to members, creating group due dates, etc.)
- a check on the status of the current project
- working through any problems in the project that a member may be unable to overcome
- the addressing of member concerns (i.e. regarding other group members, upcoming due dates, or work loads)
Any meetings that require work with the lawnmower (disassembly, photography, part documentation...) will be held at Evan's house, and dates and times will be decided at the preceding Thursday meeting based on the availability of the group as a whole.
- Note: Any conflicts will be handled through group communication. Small conflicts will remain within the group, with any group members not involved in the conflict acting as mediators to resolve the issue. If this does not solve the problem, or the problem is too substantial for the group to handle, it will be discussed with a Teaching Assistant or professor of our MAE 277 class.
Based on pre-dissection observation, there are many bolts, all metric sizes, that are the majority of the “fasteners” holding each part together. As long as those come out, there should be no problem disassembling the lawnmower. There is nothing more difficult than just unscrewing pieces from other pieces.Development Profile : The Toro Company History). A majority of Toro corporate offices are located the United States. In the 1990s, the U.S economy was very strong under President Clinton’s time in office. In addition, the North American Free Trade Agreement was implemented in 1994. This resulted in the immediate reduction of export tariffs between the U.S and Mexico. In general, this was a prosperous time for manufacturers in America. As shown in the chart, Toro lawnmowers are sold in the United States, Europe, Australia, Asia, Canada, and Latin America. This lawn mower is intended to be accessible to consumers in both operation and price, many of whom own the product for residential use. Of course, the simple goal of a lawn mower is to effectively cut grass. Usage Profile :
Energy Profile : There is chemical, thermal, kinetic and electrical energy being used by the lawn mower. The engine has a container where gasoline is stored until it is ready to be used. The engine that our lawn mower has uses the four stroke cycle. This means that chemical energy from gasoline is the main source of energy for the engine. But in order for the engine to start up in the first place it needs input from the user.
- To start the mower, the user has to pull on a cord. This pulling allows the user to transfer kinetic energy through the cord to the flywheel, causing it to rotate. Through its connection to the crankshaft, the flywheel causes the crankshaft to move. This rotation causes the piston to move linearly within the cylinder; the crankshaft also causes the camshaft to rotate.
- The intake valve is opened in the cylinder by one of the cams on the camshaft. This allows a combustible gasoline-air mixture to enter into the cylinder. The piston is moving towards the bottom of the cylinder at this point due to the start cord being pulled (Intake Stroke).
- The rotation of the crankshaft causes the piston to move to the top of the cylinder, compressing the fuel and air mix (Compression Stroke).
- When the piston reaches the top of the cylinder, the spark plug fires providing energy in the form of electricity, which it draws from a source such as a battery or magnet. This spark will cause the fuel mixture to explode, converting its chemical potential energy into thermal energy. A controlled explosion will happen at this point which will push down on the piston head, transforming the thermal energy to kinetic energy so that the piston begins to move downward. This linear motion is the converted to rotation by the crankshaft (Power Stroke).
- The energy that the crankshaft received from the piston in the previous step is then transferred to the blades, which in turn spin to cut the grass, achieving the goal of the lawn mower.
- Finally, the remaining kinetic energy from the crankshaft's rotation causes the piston to move back towards the top of the cylinder, where the exhaust valve will open. The piston will force the combusted gases from the explosion out of the engine (Exhaust).
- The cycle is restarted from step 2, and continues until either the user lets go of the throttle on the mower's handle (causing the flywheel, and thus the piston, crankshaft, camshaft, intake and exhaust valves, and blades, to stop moving) or all of the gasoline stored in the tank is used.
Complexity Profile : The lawnmower is a combination of many different parts that work together. There are many small components such as bolts and screws that keep the lawnmower together as well as springs. The major components consist of:
- Self-propelled system: A plastic handle is pushed down by the user while the safety lever is against the handle, which allows a transmission to engage on the axle connecting the rear wheels. The transmission runs off of a belt that is connected to the main shaft of the engine under the deck. It uses gears to turn the horizontal circular motion of the shaft to vertical motion to move the wheels forward along the ground. As the user pushes the handle farther, the faster the mower moves.
- Engine: The engine is the most complex part of the lawnmower. It is started by a pull start. This means that the user pulls a string that is connected to the flywheel on top of the engine. As it spins from the outward pull, the internals of the motor are in synchronized motion. As long as the pull start is pulled hard enough, the spark plug will spark and ignite the fuel and air mixture, then the engine runs on its own without the user constantly pulling on the string. The internals are extremely complicated, and have to run in sync with each other perfectly or the engine will encounter major failure.
- Other, less complex systems: Included are the adjustable height, the blade and the chute. The adjustable height works by adjusting the angles of the four wheels. The user pulls a tab out and lifts or lowers the deck and then pushes the tab into the desired height level “notch”. The blade is bolted to the bottom of the engine’s shaft and turns with the revolutions of the motor. There are two chutes on this lawnmower. One on the side opens when the user pulls it up. There is a spring holding it down against the side of the deck until the user lifts it. There is a hole in the deck behind that plastic piece. When it is lifted, grass can exit the hole. The second shoot is on the back of the deck. This one works the same way as the one on the side, and the user can attach a bag behind it to catch the grass, unlike the side chute.
The adjustable height and chutes are not very important in the overall working condition of the mower. On the other hand, the self-propelled system with the safety lever, and the engine, have to work together in order for the lawnmower to cut the grass and move forward at the same time. Also, the blades have to be attached to cut grass. Once the motor is started, the blades are spinning under the deck. Then, the user holds down the safety lever as they push down on the plastic handle, which gives the mower forward motion while cutting the grass. This is a necessity for the lawnmower to function as it was designed to.Material Profile : Materials that are clearly visible include steel, cast iron, plastic and rubber. The handle is made of steel tubing; the individual components of the motor and transmission are made of steel plate. The throttle and safety cable is made of steel cable. The motor and deck are made from cast iron due to the complex contours and shapes. There are various hardness’ of plastic throughout the lawnmower. The wheels are made of a very hard, thick plastic for strength. The chute covers, gas tank, and propelling handle are made of a similar plastic, and the engine cover is made of a thinner, flexible plastic. The gas line and belt is made of rubber. What cannot be seen from the outside observance of the mower are gaskets in the engine and transmission, which should be made of rubber. Other internal parts should be made of steel as well.
User Interaction Profile : The user starts the mower by priming the engine, and then pulls the pull-rope while keeping the lever pressed down against the handle. Once started, just stand behind and push it wherever the grass needs to be trimmed. The user must at least know rudimentary engine parts like the throttle and choke, these can also be looked up in the owners manual if not familiar with gasoline engines at all. The mower is very easy to use, however it does take a little bit of brute strength to start. It is self-propelled so you don’t have to push it the whole time and tire yourself out, just walk behind it and point where you want to go. Some maintenance is required, that comes with all gasoline engines. Regular oil fills, keep the filter clean, make sure spark plugs are working, and keep it topped off with gas. Not too difficult for the everyday person. On the mower, the blades must be kept sharp in order for the machine to perform its job efficiently; this can be done at home, but it is recommended that it be done by a professional in order to ensure the sharpening is done correctly and to minimize the risk of the user getting injured.
Product Alternative Profile : There are very few alternatives to this product:
|Gasoline/Electric Manual-Push Lawnmowers||Lawnmowers, either powered by combustion or electricity, that have no form of self-propulsion and must be pushed by the user.||These models are generally less expensive than self-propelled mowers.||User-propelled models create require the user to propel the machine forward as well as to turn it, causing more effort to be needed.|
|Completely Manual Lawnmower ("Reel Mower")||These models have no engines, and are propelled and powered solely by manual work by the user.||Much less expensive than any gasoline- or electric-power models; gives off no emissions.||User must put in large amounts of work to propel the machine, and mowing time is generally longer than with a powered model.|
|Ride-On Lawnmowers/Tractors||Riding mowers are ideal for large plots of land, such as parks, fields, and farms. They are generally gas-powered.||Allow grass to be cut much quicker and easier, as the user needn't put more effort in than pushing a pedal and steering; models with seats and only a spot to stand can be found; some models include cup holders.||Generally much more expensive than push models, and the size of residential grassy areas generally don't rationalize the extra money necessary to purchase a riding model rather than a push model.|
|Neglect||Leaving the grass to grow without cutting it.||User doesn't have to put in any work; no gasoline/electricity necessary.||Extremely long grass is unsightly; may warrant rodents and other pests to live among the grass; owner could face fines from local government.|
|Livestock||Animals such as goats, cows, rabbits, and others that eat grass could be set on their own to maintain the lawn.||Animals require no gas or electricity; potentially sustainable due to reproduction; feces can be used as fertilizer.||Require a constant source of food, may not be viable if grass growth is erratic; owner may face fines from local government.|
Overall, the model we have chosen is the best choice for residential use; it is within the middle range of pricing, requires less fuel and maintenance than some alternatives, and can perform its task with a small amount of work from the user, all while performing its intended job consistently. Self-propelled mowers similar to the one we chose generally average around $400; user-propelled combustion/electric mowers generally average about $250; completely manual models average around $150; riding mowers generally cost between $1,000 and $2,000; uncut grass fines are usually about $150.
Gate 2 - Product Dissection
The purpose of this Gate in the project is to being the dissection of our lawnmower. Here, we will document the process of dissecting the product, and then analyzing the arrangement of the subsystems throughout the mower.
Preliminary Project Review
The goals that were initially set forth in the Work Proposal have been met with general success. While the plan that had been outlined was not entirely descript, it left room for our group to assess our needs and goals as we went along. Ideally, it would have been best to create an all-encompassing, detailed dissection plan/proposal from the very start, but this is the first group project of such a magnitude that most members have dealt with. As such, group members may have felt hesitant to take the lead since a strong team dynamic and level of comfort had yet to be established in the early stages of the project. At the time of writing, we have collectively developed a better sense of what is expected from by the professors, as well as what we can expect from each other. In the following two sections, we will look at each facet of our original work and project proposals to discuss what pertinent information may have been absent and how, or if, this affected our group’s performance.
Work Proposal Review:
- As outlined, the lawnmower was dissected at our Technical Expert’s residence. The proposal did not set forth an exact date or time, but these details were easily agreed upon through consideration of each member’s availability. Luckily, all members were available at the same time, on fairly short notice (2-3 days ahead of time). To avoid the possibility of a schedule conflict in the future, our timetables will be discussed at the early stage of each project gate.
- A dissection plan was not explicitly provided in Gate 1. This is likely the result of two circumstances. Firstly, the majority of the group has not done a complete mower (and engine) teardown in the past. Secondly, our Technical Expert was trusted to have intimate knowledge of this process. At the expense our Gate 1 score, we did not take the initiative to research the specifics or elaborate on our knowledge. This could have turned out to be very problematic, but our Technical Expert did, in fact, competently lead and help the other group members through the dissection process. In the remaining gates, it will be necessary to have a more structured approach in order to avoid a possible roadblock if our group collectively lacks a particular skill or skillset.
- While we did not allude to the specific tasks of each member for the dissection process, our roles were established as we went along, rather naturally. For example, the Project Manager and Group Collaborator had the least experience in breaking down an engine, so they were charged with documenting a photographing the steps taken during dissection. In addition, it was a good chance to broaden their knowledge of 4 stroke engines as the other members explained the inner-workings of our project as we progressed. This builds confidence in technical knowledge for future gates. The Technical Expert was in charge of leading the teardown. Since the remaining two members also were comfortable in a hands-on setting, they jumped in when it was possible to work on two or more components at a time.
Management Proposal Review:
- From the start of the project, it was understood that the workload for each assignment was to be split as evenly as possible by each group member. In general, this has been accomplished. When delegating tasks needed to complete each gate, a group discussion is held to determine which member has the skills to handle the job, and the time required to complete said job is considered. When possible, or necessary, tasks are completed collectively. For example, the dissection process was completed as a group, as described above. After this collaborative effort, the written portion of Gate 2 was discussed and divided between members based on their title.
- Project work is split up as evenly as possible between the five members of our group. Specific tasks are assigned based on each member's area of expertise. Upon dissection on the lawnmower, it was determined that each member had the knowledge required to complete any area of the written assignment. As such, these tasks were delegated based on the time required and personal interest. The only exception was found in the “How To” portion of this gate. We found our Technical Expert to have the most intimate knowledge of the mower breakdown. In addition, completion of the final draft and wiki upload was to be performed by our Group Collaborator. Out of fairness, and in order to leave time for error checking, each written task must be submitted roughly three days prior to the due date.
- The group meetings have been held often and successfully. In our initial proposal, we did not outline a meeting schedule, but we had agreed to make ourselves available every Tuesday and Thursday at 3:30pm, as needed. Via email and our interactions preceding every MAE277 lecture, we assess what has been accomplished, and decide whether or not a deeper discussion needs to take place. Generally, we end up meeting at least once a week, sometimes twice a week. The Project Manager mediates these sessions, and we have gravitated to a regular itinerary. We normally begin by reviewing what we are expected to determine, both individually and as a group, and then move on to finding solutions to any roadblocks. For example, if a member is unsure of the best way to present a collection of data and information, a friendly argument regarding which type of graph would most easily aid the reader ensues. Or, if a task requires the research of additional information, this data is obtained as soon as possible during the meeting.
- Thus far, we have not had any member conflicts. As it was described earlier, the workload is divided as fairly and evenly as possible through member discussion. Tasks are determined collectively, not dictated by a sole member. In the future, if a member falls behind in their assignment, we will collectively step in and help finish the workload. However, this is a situation that we have yet to encounter.
Listed below is the step-by-step process we took in disassembling the lawnmower. The steps are numbered in the order in which each part was removed. Listed with each part is whether or not the part is intended to be disassembled, the tools used to disassemble it, and the difficulty of its removal (on a scale from 0 to 10, where 0 is no work necessary, and 10 is extremely difficult to remove. The scale is based on the amount of energy spent removing a part, and how accessible each bolt or screw holding the part on was; also, the factor of needing more than 2 hands is considered in the rating.)
|Component Number||Component Name||Is the Component Intended to be Removed?||Tools Used||Difficulty||Image|
|1||Handle (Safety Lever and Self-Propulsion Engagement)||Intended to be removed due to the use of a nut that requires only a hand. The cables are not intended to be removed.|| Hand: Unscrew two "nuts."
Needle-nose pliers: Pull off cable mount from bracket next to carburetor and the other from bracket on rear axle; pull each side of the handle outward off of the bolts.
|2||Wheels (Front and Rear)||Not intended to be removed because the bolts are hard to access between the wheel and mower deck.||1/2-inch Wrench and 1/2-inch Regular Socket: Each wheel is connected to mower deck with two bolts.||7|
|3||Blades||Intended for removal because they need to be changed or sharpened in order to cut grass well.||5/8-inch Regular Socket: Hold blade with heavy gloves and turn bolt.||2|
|4||Rear Chute||Not intended to be removed because you must take other parts off to access the screws holding it onto the deck.||3/8-inch Socket: Remove the 4 bolts and pull off.||3|
|5||Rubber Flap (Behind Chute and Plastic Shield)||Not intended to be removed, but can be easily if the user does not like it.||Phillips Head Screwdriver: Unscrew 1 screw on each side and pull off.||2|
|6||Transmission (Gear Box)||Not intended for removal because it is sealed with an RTV-like substance and the gears have to be lined up correctly.|| T20 Torx: Remove the 8 torx screws
Putty Knife: Wedge between the two piece case and pry the case off, breaking the seal.
|7||Gears||Not intended for removal because they are in a sealed case.||Hand: Pull wheels outward and the gears will separate and come off.||1|
|8||Belt||Not intended for removal because the user cannot remove it without removing the gear box in order to take it off of the pulley on the worm gear.||Hand: Pry belt off of pulley on the engine output shaft and the pulley underneath gear box.||3|
|9||Engine||Not intended for removal due to the fact that multiple parts are connected to the motor that have to be taken off before the motor is removed.||1/2-inch Regular Socket: Unscrew 3 bolts holding motor on deck.||4|
|10||Engine Cover||Intended for removal due to the fact that it is only held on by easily accessible Phillips head screws and can be taken off for normal inspection of engine parts.||Phillips Head Screwdriver: Unscrew 3 screws and pull both pieces off||2|
|11||Gas Tank||Not intended for removal because other parts have to be removed before the user can access the screws holding it on.|| 8mm Socket
Pliers: Unscrew bolts surrounding the pull start; pinch hose clamp on bottom of tank and slide down the rubber hose
|12||Air Cleaner and Cover||Intended for removal since it is only held on by 1 accessible screw and cleaning or changing of air cleaner is recommended for proper maintenance.||Regular Screwdriver: Unscrew 1 screw and use hand to unscrew knob in center of cover, pull off, air cleaner will fall out, or pull out.||2|
|13||"Shell" with Pull-Start||Not intended to be taken off because it is bolted on in various places.||3/8-inch Socket with Extension: Remove all bolts on all sides of the shell, lift off engine.||4|
|14||Spark Plug||Intended for removal because it may need to be replaced after a lot of use to maintain engine health.||20mm Deep Socket: Unscrew and lift out of hole.||1||See image for part 13, the "Shell."|
|15||Muffler Cage Protector||Not intended for removal because there is no need to remove it.||8mm Regular Socket: Remove the screws on both ends of cage and pull off.||3|
|16||Carburetor||Not intended for removal because bolts are hard to access and is a delicate, necessary, part to the functionality of the engine.||3/8-inch Regular Socket with Extension: Remove bolts holding carburetor onto engine block and pull off throttle return spring.||5|
|17||Head||Note: The head and all parts following (muffler excluded) are not intended for removal because they are extremely important for proper functionality of the engine and are difficult to access.||1/2-inch Regular Socket: Remove the 8 bolts while engine is in vice or held by another person, pull off.||4|
|18||Lower Engine Case Cover||See Note above.|| 3/8-inch Regular Socket: Remove bolts holding case onto engine block.
Rubber Mallet: Turn engine downward and tap around the edges of the case until it frees from gasket and engine block, slide off output shaft. Challenge: The case would not separate from gasket and block, had to use force with mallet to break it free.
|19||Flywheel||See Note above.||15/16-inch Regular Socket: Unscrew Bolt while holding output shaft in place, pull off input shaft.||3|
|20||Muffler||Not intended for removal because you must take off cage and small cover before the user can access the 2 bolts holding it onto the engine.|| 8mm Regular Socket: Remove the 4 bolts holding on small cover to access 3/8 bolts underneath.
3/8-inch Socket: Remove 2 bolts holding muffle to engine, pull off.
|21||Governor||See Note above.||Gravity: Governor will fall off if it is face-down. If not, lift off with hand.||0|
|22||Camshaft||See Note above||Hands: Pull out.||3|
|23||Piston Rod and Piston||See Note above||8mm Socket: Unscrew 2 bolts and remove bottom piece of piston rod. Turn crankshaft so that the piston rod and piston can slide out next to crankshaft.||5|
|24||Crankshaft (Input and Output Shaft)||See Note above.||Hand: Pull out.||2|
|25||Breather Assembly||See Note Above||8mm Socket: Unscrew 2 bolts and lift off.||3|
|26||Valves and Valve Springs||See Note Above||Hand: Lift out valves from top of engine block and pull out springs through space that was covered by the breather assembly.||2||See picture for part 25, the Breather Assembly.|
The subsystems involved:
- Pull cord
- Blade Housing
- Air Filter
These subsystems are connected as follows:
- Carburetor to Engine: The purpose of the carburetor is to deliver a fuel and air mixture to the cylinder, so the carburetor and engine need to be connected together. Since the carburetor is gravity fed it needs to be located somewhere below the fuel tank. The carburetor is connected to engine by two bolts.
- Air Filter to Carburetor: The air filter get rid of any debris in the air going into the intake before that air is sent to the carburetor. This is to ensure that no damage is caused to the insides of the engine by foreign material and to help make the combustion process more efficient. The air filter is connected to the engine by one screw.
- Pull Cord to Engine: The Pull cord is connected to the flywheel. When the user pulls on the cord, it transfers kinetic energy to the flywheel, which in turn causes the piston and crankshaft to start moving. There is a magnetic strip that is part of the flywheel. This strip induces an electrical current into two copper wires stationed next to the flywheel. This electrical energy is transferred to the spark plug. This creates the spark that ignites the fuel and air mixture inside of the piston cylinder. The pull cord is wrapped around a spool. This spool engages the flywheel when it is pulled. This transfers the energy from the user to the engine, which allows the engine to start its four stroke cycle.
- Engine to Blades: The lawnmower engine is connected to the blades via a drive shaft. The drive shaft transfers the kinetic energy created from the combustion of the fuel and air mixture to the blades. This allows the blades to cut the grass and fulfill the purpose of the lawnmower. The blades are connected to the engine by a single bolt.
- Handle to Blade Housing: When the user pushes on the handle, it transfers kinetic energy to the wheels, which cause the whole lawnmower structure to move. The handle is connected to the rest of the lawnmower by two bolts. One on each side of the handle.
- Engine to Blade Housing: The engine is connected to the blade housing with bolts. The blades are located just below the engine. This is also the area where the drive shaft connects to the blades. The engine is connected to the blade housing with 3 bolts.
- Wheels to Blade Housing: The wheels are connected to blade housing with some bolts. The wheels transform the kinetic energy provided by the user into rotational energy, which allows the lawnmower to move. Each of the four wheels are connected to the blade housing with two pairs of bolts and nuts.
- Muffler to Engine: The muffler is connected to the engine with three bolts. The purpose of the muffler is to dampen the noise created by the operation of the engine. The muffler is connected to the engine by two bolts.
How the connections are implemented: All of the subsystems are implemented automatically when the engine is started except the wheels and handle. After the pull cord is pulled and the engine starts, the blades start to spin due to the force applied by the piston onto the driveshaft. The piston is powered by the combustion of the air that that comes through the air filter and mixes with gasoline in the carburetor. After combustion the exhaust gasses are forced through the muffler. This all happens without any human interference. However, in order to get the mower to move, the user must push on the handle and transfer that energy to the wheels.
- GSEE influences: The designers wanted the mower to be as easy as possible to operate (societal) and able to be operated in all climates (global) so they would not have to build different variants, which cost more money (economical). They wanted the average homeowner to be able to operate it without problems (societal) or much maintenance (economical). The product is designed to be mass-produced on an assembly line to minimize cost and maximize production (economic) so all of the subsystems are built separately and then put together at the end. The engine is not built buy the lawnmower company TORO. They contract out the engine production to Briggs & Stratton, which is a very good engine builder and has all the tools, machines, and know-how to produce engines for the TORO company. TORO researched the cost difference of designing and building their own engines and saw that is was cheaper to contract out instead of building their own plant (economical/ environmental). As with any gasoline engine, you will have exhaust fumes that pollute the environment but the engine tries to maximize output energy without wasting any gasoline or losing it to friction (environmental).
- The mower is built to maximize performance so all of the connections are physical so the energy is transferred between subsystems is instantaneous. The physical connections also help to keep the cost down.
- Reasons for subsystem placement: The subsystems are placed so that the whole mower will be balanced and safe. The engine is place directly in the middle of the blade housing, the wheels are evenly spaced and are holding up the blade housing which is holding up everything else. The air filter is mounted to the carburetor so is can mix clean air with the fuel. The muffler is not facing at the user so fumes won’t be blowing towards him and the handle is at a certain distance away so that with an average step, the users feet will be a safe distance away from the blades. The pull rope extends up the handle and is held there by an eyelet so the user can start the mower from the same place he will operate it from.
- Placement restrictions: You cannot put the blades next to the handle, which is not practical or safe. The muffler and air filter should be separated so the engine isn’t taking in exhaust fumes. The pull rope shouldn’t be near the blades to ensure safety upon starting.
Gate 3 - Product Analysis
This Gate of the project will consist of analysis of the product on a component and subsystem level. With the product dissected, we will examine the manufacturing processes used to make each part of the product, as well as the factors the went into the decisions to use each process, and we will suggest some revisions for the product based on our experiences with it over the course of the semester.
In Gate 2 of this project, there was one unresolved challenge facing the group:
- Clearly-Outlined Roles: In the past we had not decided upon each member’s task in the early stages of each assignment. This resulted in members having undefined tasks until the due date was realized to be quickly approaching.
- Solution: In preparation of this gate, the group met shortly after Gate 2 was completed and created a detailed plan of which members were responsible for each portion of the assignment. This allowed members to gain extra benefit from lecture material that was relevant to their task in this gate. In addition, this was helpful to the member responsible for the solid-modeling portion of Gate 3. Since it had been a fair amount of time since any of us have done any modeling (if ever), this allowed for an ample period to get reacquainted with the process.
While working on this gate, we also encountered a new temporary setback:
- For a short time, we did not schedule any group meetings or discussions. However, this is purely due to the fact that each member had at least 4 exams scheduled in a period of 3 days. The weight of exam grades on one’s GPA required a lot of extra study effort and little work on this project during that period of time.
- Future Solution: Look ahead to upcoming exams and start working on project assignments before the heavy studying sessions begin.
Evidence for certain manufacturing processes are riser marks (injection molding, die casting, investment casting), parting lines (injection molding, die casting), positive draft angles (injection molding, die casting, forging), axially symmetrical features (turning), elongated forms with elongated features (extrusion, drawing),axially asymmetric or non-molded holes (drilling), intricate detail (machining, investment casting), specific surface finishes (machining, grinding), and thin, sheet-like metal (rolling).
|Handle||Allows the user to push and steer the lawnmower.||Plastic, Steel||Plastic: Injection molding; Steel: Forming, Drilling|
|Wheels||Convert the kinetic energy input by the user into rotational energy, which allows the lawnmower to move.||Plastic||Injection Molding, Drilling|
|Blades||Cut grass by spinning, using energy from driveshaft.||Steel||Drilling, Grinding, Die Casting|
|Transmission Housing||Holds the gears in the rear axle together.||Plastic||Injection Molding, Drilling|
|Engine Cover||Prevents debris from entering the engine.||Plastic||Injection Molding|
|Gas Tank||Serves as a reservoir for fuel until it is needed by the engine.||Plastic||Injection Molding|
|Air Filter Cover||Prevents extra debris from clogging the air filter.||Plastic||Injection Molding|
|Spark Plug||Converts electrical energy into thermal energy in order to ignite fuel and air mix in the combustion chamber.||Steel, Ceramic||Steel: Drilling, Die Casting; Ceramic: Molding, Drilling, Machining|
|Muffler||Dampens the noise created by the engine.||Steel||Rolling, Pressing, Die Casting, Drilling|
|Carburetor||Mixes fuel with air; sends fuel-air mixture to combustion chamber.||Aluminum||Die Casting, Drilling, Milling|
|Engine Head||Has a hole where the spark plug is inserted; holds various fins for the purpose of cooling the engine.||Iron||Drilling, Machining|
|Flywheel||Stores kinetic energy, then uses that energy when the engine is not producing power; it also contains a magnet which provides electrical energy to the spark plug.||Iron||Die Casting, Drilling, Milling|
|Governor||Limits the speed of the camshaft in the engine.||Plastic, Iron||Iron: Drilling, Machining; Plastic: Injection Molding|
|Camshaft||Holds piston rod and various cams that push on the push rods, which open the valves in the combustion chamber.||Steel||Drilling, Milling, Grinding|
|Piston Rod||Connected to the crankshaft and rocker arms; when the piston rod pushes the elongated part of a cam, it forces the valves in the combustion chamber to open.||Aluminum||Die Casting|
|Piston Head||Uses energy from ignited fuel-air mixture to push piston rod, which turns the crankshaft and camshaft.||Aluminum||Drilling, Machining, Die Casting||(See picture for Piston Rod)|
|Valves||The intake valve allows the fuel and air mixture to enter the combustion chamber; the exhaust valve allows the gases formed during the combustion process to exit the chamber.||Steel||Forging, Grinding|
|Crankshaft||Transforms the linear motion of the piston into a rotational motion.||Steel||Milling, Die Casting, Drilling, Grinding|
|Component||Component Function||Component Form||Manufacturing Methods||Component Complexity|
|Blade||In general, the rotating blade of a lawnmower is arguably the most important component of the product. Naturally, the task of a lawnmower is to cut grass and it is the blade that performs this function. The spinning of the blade is the result of its connection to the driveshaft via a metal adapter and 3 bolts. The rotational kinetic energy of the driveshaft is translated to the blade, as the two are connected. The length of the blade is positioned horizontally and follows the rotation of the shaft around its central vertical axis. In order to contact the grass, the blade is exposed to its operating environment on the underside of the blade housing.||The blade is rectangular in shape, with 2 teeth along two opposite corners. There is also a slight curvature to the blade that extends from one of these opposing ends to the other. The blade’s thickness is small in magnitude compared to the lengths of its side, and as such it may be considered as a 2 dimensional object (it is about 21 inches long, 2-1/2 inches wide, and 1/8 inch thick, and weighs about 1 pound). This small thickness is necessary to the function of cutting grass. A thicker, duller blade would not be suitable for cutting. For example, imagine cutting steak with a thin, sharp butcher’s knife versus using the shaft of a hockey stick to perform the same task. In addition, a notably thicker and heavier blade would require more rotational force from the output shaft to get it spinning. This component is made of steel. The strength of steel is required for proper cutting. Sharpened edges are possible with this material, as is relatively high structural rigidity. Comparatively, a plastic blade of the same dimensions would be flimsy and likely to break when cutting thick and/or wet grass. The blade is not seen by the user and therefore the aesthetic purpose is negligible. The dimensions and look of the steel blade are purely a result of its functional requirements. When the blade is new, the surface finish is smooth, which aids in cutting force.|| Blanks of steel are punched or stamped to quickly produce the shape of the blade. The lack of riser marks and parting lines verify the use of this manipulative method. As stated, this method is quick and costs little, and is effective for high volume steel pieces. Generally, rapid heating and cooling is used in this type of manufacturing process to strengthen and harden the steel. In addition, a grinder is used to sharpen the blade and smooth out any imperfections. While our used blade may appear slightly dull and rough, a new blade would be smooth and sharp at the edges, which are two types of surfaces achievable through grinding. A variety of GSEE factors are related to the manufacturer’s decision to form the blade of steel:
||The blade in "not very" complex. The shaping procedure is a quick step via stamping. The blade is essentially a long rectangle that requires some grinding to achieve the sharp edges. Its connection to the crankshaft is fixed, and thus it is simply activated when the motor is running.|
|Blade Housing||This component has a variety of functions necessary to ensure a proper user experience. Firstly, the blade housing serves as the mounting location for the wheels, engine, and handle bar. Secondly, a view of the underside of the housing reveals how the grass clippings are channeled away from the blade by way of part geometry. In addition, this component prevents the user from coming into contact with the spinning blade, which is located on the underside of the housing. Lastly, the housing is exposed to the environment and clearly visible, so its color and finish serve an aesthetic purpose.||In general, the form of the blade housing resembles a shallow cylindrical shell of a 23-inch diameter, with box-like edges at the front and rear-facing sides, which give it a length of 34 inches from front to rear. Overall, it weighs about 19 pounds. The basic profile is axis-symmetric, however there are various cut-outs and drilled holes that are necessary to mount the aforementioned components. The 5-inch height of the housing is directly related to the blade that is (naturally) found within the blade housing. It needs to be high enough to clear the blade and leave ample space for the grass clippings to exit the housing. Also, its length and width take 2 main functions into consideration; first, the track of the wheels must be wide enough to provide stability for the lawnmower, but must not be excessively cumbersome and thereby limit maneuverability; and second, the housing must provide a structurally solid base for the engine and its related weight. Steel was chosen as the material to provide strength and durability for the structure. Aesthetically speaking, the housing features a smooth, red finish. The quality of the finish likely results from trying to create an appealing product. In terms of color, the mower is simply one of the 3 common lawnmower colors: green, red, and black. The contrasting red color against the green grass may be useful so that user can clearly judge the mowers boundaries and stay clear of objects in its path.||The housing is formed from relatively thin steel. One can assume the steel was first rolled to the desired thickness, and then stamped to create the necessary shape. The lack of parting lines and riser marks support these assumptions. Other metals could have been used and the process would likely have been the same. Stamping is a very efficient method for high-volume components. The choice to create the deck from one piece of steel provides structural support in 2 ways. This can be seen as an effort to ensure safety, which is related to societal concerns for manufacturing. Economically, as stated, the stamping process is very cost effective for a mass produced metal object. The steel is also finished to protect against the elements, which addresses global manufacturing concerns.||The deck/blade housing is considered to be a “not very” complex component. While it is a central connection for a number of other components, it does not move or rotate, and the manufacturing process is very simple.|
|Camshaft||The camshaft directly serves one purpose, which is to open and close the intake and exhaust valves with the proper timing. Indirectly, the camshaft helps make sure the engine runs smoothly by controlling both the entrance of the fuel-air mixture into the combustion chamber, and the expulsion of the combusted mixture from the chamber. This, in turn, allows the piston to move properly, which turns the crankshaft; that rotation is transferred to the drive shaft, which spins the blade. There is also a gear on the crankshaft that meshes with the gear on the camshaft, so the camshaft’s rotation comes from the rotation of the crankshaft, which is caused by the translational movement of the piston. In order to take part in this process, the camshaft is, logically, located within the engine block.||The camshaft is mostly axially-symmetric, aside from the two cams. It is a primarily three-dimensional object, as the shaft needs to be a certain length and the cams need specific heights and angular offsets from each other. Additionally, the gear on the shaft needs to be large enough to mesh with that of the crankshaft. The shaft has a total length of about 3-3/4 inches and a diameter of 1/2 inches (both dimensions include the 1/16-inch bevel around the flat part at each end). The gear has a diameter of 3 inches, with 48 teeth placed around its circumference, and a thickness of 1/4 inches. The gear is centered at 5/8 inches from one end of the shaft. From the opposite end, the centers of the two cams are placed at 1-1/16 inches and 2-5/16 inches. The cams have a thickness of 3/8 inches, a total height (from pointed tip to rounded bottom) of 7/8 inches, and an offset of about 95° from each other. All together, the camshaft assembly weighs about 2 pounds. The shaft must be round in order for it to rotate within the engine, with a round gear so that its rotation can be driven by the adjacent gear on the crankshaft. Additionally, the cams must be raindrop-shaped in order to push the valves open for the split-second timings for which they are needed. The shaft itself is made from steel, while the gear and cams are made from plastic. We had expected the shaft to be made of some particularly strong metal, but the plastic parts came as a surprise, considering the large amount of heat a wear the parts must endure in order to function within the engine. However, if the plastic is strong enough to withstand those conditions, it is a better choice by virtue of plastic parts being cheaper to manufacture than steel, although the plastic parts would undoubtedly wear out much quicker than the steel with continued use. For these reasons, the decision to use plastic was probably most influenced by economic consideration, while the steel was probably driven by societal and possibly environmental factors, because it is unlikely to break down within the mower’s lifespan, and would therefore create less waste material as well as less frustration by the user who would have to take the machine to a specialist to find out that the shaft broke. The camshaft does not need to be aesthetically pleasing as it is housed within the engine and rarely, if ever, seen by the user, and it is clear that aesthetics were not considered when the part was designed. It does have a very smooth finish, but that was influenced by the fact that it needs to rotate easily.||The steel shaft has no injection marks, which rules out casting and molding; it could have been turned, but that would require using a tool sharp enough to cut the steel down from its original shape, which doesn’t seem feasible when taking into account the time constraints and consumer demand (societal factor) of the product, as well as the relatively low level of accuracy necessary in the part (as compared to a camshaft necessary in a racecar engine, for example); such reasoning rules out investment casting as well, as the molds are only used once in that process (economic factor), and this part would need to be mass-produced. Due to the fact that it is metal, injection molding was not possible (as this process is used for plastics). Also, the fact that the shaft is round, not flat, removes the possibility of it having been rolled. This leads us to believe it was most likely extruded, drawn, or possibly forged, and then machined in order to accept the pins that attach the cams and gear to the shaft. The cams and gear were all very clearly injection molded, as revealed by the riser marks and parting lines left on them by their respective molds. Injection molding is one of the cheapest and most efficient manufacturing processes for creating mass-produced plastic parts, because the molds can be used several times, and they can generally produce many of the plastic pieces at a time.||The camshaft overall is a “very complex” component. The processes used to manufacture its sub-components are all fairly simple (no more than putting molten plastic into a mold or pushing softened steel through a circular hole), but when assembled, the cams need to have an exact angular displacement between their peaks. In the engine assembly, the gear must mesh with that of the crankshaft at a specific point (marked by a line on each gear) so that the valves open at precisely the correct moment. This makes its complexity rating increase, although not significantly.|
|Engine Block||The engine block integrates the cylinder walls, intake and exhaust ports, crankcase, and other internals associated with these cavities. As a result of the large amount of heat during the 4-stroke process, namely combustion, the block must be able to withstand very high temperatures. Combustion provides mechanical energy which drives the transmission and propels the mower. In addition, the structure must be extremely rigid and inflexible. The rotation and movement of the crankshaft and pistons require precise and repeatable paths, and as such the dimensions and surfaces inside the block are necessary to be of high tolerances. The block resides below the engine cover and is mostly out of view, obscured by other connections such as the muffler.||The block resembles a 13 inch x 13 inch x 10 inch rectangular prism, but with the aforementioned cavities and recesses cut out of it. The shape is purely created to house all the necessary integrated components and it offers no aesthetic value. The block is made of cast-iron. This material provides strength and resistance to high temperatures; two critical qualities that provide durability. Another metal, such as aluminum, could be used, but it is more expensive to manufacture. Aluminum blocks are desirable for their low weight in addition to strength and heat resistance. The exterior surfaces of the block are rough. They are not intended to offer an visual appeal. Inside, however, surfaces like the cylinder walls must be as smooth and exact as possible, so as not to provide unnecessary friction. While these finishes are often appealing to enthusiasts, the precision is purely driven by engine functionality. Without its internal components, the block weighs roughly 17 lb.||A mold is created created for the block, and liquid metal is poured into said mold. The die-casting process is supplemented by machining processes such as milling and drilling of internal cavities. Grinding is performed to create exacting dimensions and smooth surfaces free of any defects. This is necessary for internal walls and exterior points of connection to other subsystems. These precision machining processes are required for proper engine operation, and is its implementation is not dependent upon the manufacturer’s choice of metals. Additionally, lack of axis symmetry and detailed inner-geometry forbids other processes such as turning or extrusion. Economically, the use of cast-iron is cheaper than using the lightweight aluminum. From a safety standpoint, by consequence of societal manufacturing factors, the use of a strong material like a metal is necessary to support all of the interactions and processes that occur inside the block. The metal also withstands climates across the globe. Whether the mower is used on a hot summer day, or is stored for a long period during winter months, the block is durable enough to withstand various climates throughout the product’s life-cycle. Since the block will likely last past the lawnmower’s life cycle, there is no need for most users to worry about replacement, and the block can simply be recycled at a scrap yard.||The engine block is a very complex component. It features a multi-process manufacturing method, required high tolerances, interacts with numerous components, and it must last for the life of the mower.|
|Muffler||The muffler essentially performs two functions: expelling the exhaust from the engine and quieting the noise coming out of the exhaust port. It has a tube connected to the exhaust vent of the engine, and is attached to the side of the engine so that it is exposed to the outside, which allows the exhaust to be expelled.||It is almost the shape of a rectangle with one corner removed, with a larger length of 7-3/4 inches, a smaller length of 3-3/4 inches, a larger height of 3-3/4 inches, a smaller height of 2 inches, and a uniform thickness of 1-3/4 inches. In one side is an intake hole of 1-inch diameter, and in the other is a collection of 33 holes with ½-inch diameters collected within the area of a 1-1/2-inch diameter circle, through which the exhaust leaves the system. Over this exhaust port, there is a horseshoe-shaped hood with a width of 2-3/8 inches at the circular end and 2 inches at the opposite end, a length of 4-1/8 inches, and a ½-inch, outward-turned lip along the hood’s curved edge, and a height of 5/8 inches. The fact that it is basically a three-dimensional box shape I what allows it to do its job: any sound from the engine travelling through the connection tube is dispersed into the large open space of the muffler, while a relatively miniscule amount exits through the small exhaust port holes, along with the actual gas exhaust. From there, the hood is positioned to prevent the user from directly inhaling the exhaust. We believe the muffler and hood to be made from steel, a material that needed to be able to withstand the heat given off by the engine and the temperature of the exhaust, and yet be light enough that it doesn’t drastically affect the overall weight of the mower (the muffler and hood together weigh about 1.5 lb). Considering the facts that each of the potential manufacturing processes are fairly easy and inexpensive to perform and that steel is easily attainable, they make sense for parts that must be mass-produced. Aesthetically, the muffler is nothing special. It was most likely treated with an anti-rust coat because it sits on the outside of the mower, exposed to the weather, but it was not painted, presumably because the paint would also need to stand up to the large amounts of heat from the engine and exhaust, and most commercially available paints are unable to withstand that heat.|| Considering that these parts are made from a pliable metal, we came up with two possible scenarios for its manufacturing:
Either way, it is clear from the positive draft angles of the parts that they were made from some sort of mold. When considered, creating these parts from molding makes more sense than the next best alternative, which would be to start with solid blocks of steel and cut away sections in order to make the appropriate cavities and holes, which would prove to be more costly and time consuming than necessary, especially with the thickness of the steel used in the parts.
|The muffler is a “not very complex” component. It is made from a fairly cheap, easy to obtain material and manufactured using some of the most basic and simply performed manufacturing processes. As its only interaction is being bolted to the engine and receiving exhaust from the combustion chamber, it is not complex in that respect as well.|
|Piston||The piston uses energy from the combustion of the air-fuel mixture to rotate the crankshaft, which in turn rotates the camshaft and drive shaft for the blade and wheels. It is also used to push the exhausted mixture out of the combustion chamber through the exhaust valve. This component would be useless if it could not perform under the heat and pressure inside the engine’s combustion chamber, and it is such an integral part of the mower that the whole system would be rendered pointless if the piston did not work.||The piston consists of an axially symmetric, round piston head and a long, flat piston rod that are pinned together. The head is a component of all three dimensions, as it must have a diameter large enough to receive the blast from the combustion and force all of the exhaust out, and it must also be tall enough that it has a place to connect to the rod. The rod, however, can be thought of in only two dimensions: it needs to be long enough to push the head from bottom dead center position to top dead center position in the combustion chamber, as well as wide enough to fit around the crankshaft it is attached to. Although, when designing the rod, it is necessary for it to be thick enough to withstand the large amounts of energy released from the combustions without breaking. The head has a diameter of 2-5/8 inches. Its bottom is curved, like a bridge support, and has a height at the edges of 1-5/8 inches, and a height at the center of 1-1/4 inches (all dimensions include the 1/16-inch bevel around the top edge). When looking at the curved part, there is a hole of ½-inch diameter centered ½ inch from the top of the curve (3/4 inch from the top of the piston head), and half-way between both sides. Rotated 90°, the head takes on a T-shape, with a base width of 2-1/8 inches and a top-cross thickness of ½ inch. Around the head are three metal rings with 2-3/4 inch diameters. These rings can be compressed to fit inside grooves in the head, and they fill the space in between the piston head and the combustion chamber walls in order to ensure that a minimal amount of combustion energy gets past the piston and that a maximal amount of exhaust is pushed out of the chamber. The piston rod is about4-7/8 inches long, with a width of 1-1/2 inches at the bottom and 1 inch at the top. On the bottom, there is a piece which can be unscrewed in order to attach and detach the rod from the crankshaft. This piece completes the 1-inch hole in the bottom portion of the rod which the crankshaft goes through. In the top of the rod there is a ½-inch hole which accommodates the pin which connects the rod with the head. As a whole, the piston has a weight of about 3 pounds. The holes through which the head is connected to the rod and the rod is connected to the crankshaft must be round so that they are able to successfully convert the translational motion into rotational motion, but other than that there are no functional factors that dictate the component’s shape. We believe the piston rod and head to be made of aluminum, a material that had to be strong enough to withstand the heat and impact of the combustions, but not so strong that it would drive the cost of the machine up too much. The piston was clearly not designed aesthetically. It has a mainly smooth surface finish on the surfaces which are rubbing against other surfaces, but there is some rougher surface texture on the more detailed areas of the component which suggest machining of those areas.||The piston rod seems to have been die cast, as determined from the riser marks and parting lines on the part. Again, it would be wasteful to investment cast or completely machine this part due to its relatively low level of complexity and need to be mass-produced, and it would be impossible to use turning due to the strength of the material and the non-axially-symmetric shape of the part. We believe the piston head to have been die cast as well, with machining and drilling later on. Parting lines on the head suggest its being die cast, but the detailing on the drilled-into sides, as well as the grooves around the circumference, seem too intricate for a cast, which is why we think it was machined. We also think it would have been easier to simply drill a hole through the part after it was finished than to integrate the hole into the cast; this would also allow for smaller tolerances on the size of the pin placed in the hole.||The piston is a “very complex” component, as its interactions with other components, and even those between its own two sub-components, are very complex and have very small tolerances; this is despite the relatively simple methods with which the component was most likely manufactured.|
|Wheels||The 4 wheels of the lawnmower, located at the corners of the deck, allow for horizontal movement. They are exposed to the operating environment and placed in such away that they directly contact the ground. Without the wheels, the user would be scraping across his/her lawn. Motivation is provided in one of two ways. Like any other object with freely moving wheels, the operator can simply push the mower in a desired direction. Since this lawnmower is a self-propelled unit, the primary driving force of the rear wheels is the power generated from the motor. As the engine spins, it turns the gearbox, which in turn translates this rotational kinetic energy to the wheels. The front wheels are not powered, and simply provide a rolling surface. However, while the rear wheels are fixed in path, the front wheels allow for a full turning radius. In short, the rear wheels help the lawnmower move forward, and the front wheels are used to maneuver about a desired path.||Like most wheels, the the mower's are axis-symmetric. All 4 wheels have the same dimensions (8 inch diameter, 2 inches wide, approximately 1 pound in weight). The symmetry and uniformity is necessary to provide an even rolling surface. There is also a tread pattern across the width of each tire to provide traction. Each is made of hard plastic. Naturally, larger tires like the those found on an automobile or made of rubber. However, the lawnmower wheels will primarily be used at comparatively slow speeds over fairly flat surfaces. As such, the wheels are not required to offer any elasticity or give. For this reason, a simple plastic wheel is suitable and common for this application. The decision to use plastic is much cheaper for the manufacturer, and the user need not worry about flat tires or replacing worn wheels. The wheels do not contribute a large aesthetic influence on the design of the mower. The plastic tires are black in color, mimicking that of rubber, with a white wheel. They are utilitarian in look, and attention to styling is not given the same priority as that of the wheels on a car or truck.||It would appear that the wheels and tires are manufactured via injection molding. There are three details that contribute to this determination. First, the pieces are plastic, so methods used for metals (die-casting, forging, etc.) are not realistic. Additionally, each component has visible parting lines and riser marks, both properties of an investment cast product. Hence, the material choice had great effect and the manufacturing technique employed, and the object’s shape was simply a result of its functional requirements.||While the wheel is an extremely necessary functioning component of any lawnmower, it is not very complex. It connections are fixed, the outward-facing surfaces of the tire are not required to be particularly smooth, and the material choice is that of a simple hard plastic.|
Note: Component complexities are "not very," "moderately," or "very" complex. These ratings take into account the difficulty of the manufacturing process used, the connections of each analyzed component to other components, and the type of movement of the component under normal operation of the mower.
Solid Modeled Assembly
One of the most important parts of a lawn mower, are the blades. Without them, you wouldn’t have a lawnmower. In the beginning, some person probably sat down and figured out how to attach a sharpened blade to the output shaft of an engine to create a motorized lawnmower. It needed to be sharp enough to slice the grass as it spins, and sturdy enough to hold up to the spinning and stress put on from cutting large amounts of grass or brush. Once the design was created, it was made and attached to the output shaft. Testing would be physical, since there was no such thing as simulations or really even a household computer at the time of its creation. Things to watch for while cutting the grass is if the blade spins evenly-that it is centered on the shaft, and if it can cut all of the grass that the lawnmower passes over with accuracy(no flexing in the blade or missed patches of grass). For an engineer to do a proper analysis of the blade on a single shaft lawnmower, all they would really need is to know the center of mass for the blades, the sharpness needed to cut the grass, and the speed at which the blades must spin in order to cut grass well.
- The center of mass can be found by using statics-since each side has to be symmetric to cut grass evenly, the center of mass is just the midpoint from each end of the blade and the midpoint of the width of the blade.
- The sharpness would be found simply by doing field testing, since there is no real formula for seeing how sharp something needs to be to slice through various types of grass.
- The speed, or RPM’s, of the blades can be measured if the engineer knows the RPM’s of the motor. Since the blade spins directly from the motor’s output shaft, the revolutions per minute of the blade will be the same as the revolutions per minute of the motor.
- This change impacts global, societal, economic, and environmental needs. Since it is stronger and more durable, the blade can be used to cut wide varieties of grass and small brush all around the world. Since it is more effective at cutting grass, lawns will look more presentable within the community and the environment will have a better appearance. Also, despite a higher initial cost, the maintenance of the blade is greatly reduced by using stainless steel or similar hard steels. The fact that a user will not have to try and get the blade off of the shaft and sharpen it saves them money and gives them a sense of relief.
- As with the blade, the steel cable is more expensive than the nylon rope, but will never need replacing. This means that it can be used all over the world, in any conditions and put under whatever stress necessary to get the engine started, without breaking like the nylon rope. The maintenance cost for this would be nothing; therefore the initial extra cost will make up for not needing to be replaced.
- The metal instead of plastic will be harder to make and cost more than the plastic, but should never break like the plastic under most conditions. This durability factor will appeal more to a customer, and will make up for the difference in cost. Also, the lawnmower can be used in a location where replacement parts are not readily available, and the user doesn’t have to be worried about it failing.
Gate 4 - Product Explanation
The purpose of this Gate is to reassemble to mower and draw conclusions from the data gathered in the past Gates, as well as to look at the design of the product as a whole, rather than on a component level, and propose revisions for the overall product design.
Critical Project Review
At this point of the semester-long project, we have been able to complete the assignments with few noteworthy issues. Also, we have yet to encounter any group conflicts.
- Resolutions: In the previous coordination review, the group encountered one roadblock. Due to a short period that featured numerous exams for each group member, we decided to forego our regularly scheduled weekly meetings. However, this was a non-issue for Gate 4, as we had the luxury of a week-long break from all classes. We met before this break and were able to use this time off to move ahead on Gate 4.
- Growth: Additionally, in our most recent meetings, we have been able to tackle group discussions and proposals very quickly and effectively. This is a result of a collective recognition of each member’s role and knowledge of the product/project at hand. For example, when delegating tasks for this particular gate, each member had already looked ahead at the assignment and considered what section(s) would best fit his skill set. As a result, this topic required little discussion in our meeting and all members were able to start on their tasks immediately. This extra amount of time has allowed us all to turn in our work the Group Collaborator ahead of schedule, as well.
The lawnmower was most likely originally assembled by hand due to the many pieces and materials combined into one machine. The assembly is very similar to the disassembly, although you have to pay attention to the correct order of things, which makes the assembly harder than the disassembly.
Each step was rated on a scale of difficulty based on the time it took to assemble each part, the amount of tools needed, and the precision needed to fit each part correctly.
|Valves and Valve Springs|| Hand: Push springs in through breather space and push valves upward into 2 holes underneath
|Breather Assembly||8mm Socket: Tighten both bolts on each end, do not torque a lot.||3|
|Crankshaft, Input Shaft, and Output Shaft||Hand: Push input shaft into top hole of engine block.||2|
|Piston and Piston Rod|| Hand and Regular Screwdriver: Push piston into cylinder while squeezing the 2 piston rings so the piston will fit into cylinder.
||8 (Pushing the rings in while forcing piston into cylinder requires much patience and accuracy.)|
|Camshaft||Hand: Push end of camshaft into bored hole in engine block.||2|
|Governor||Hand: Push onto end of camshaft and line up with metal lever entering engine block.||4 (Alignment with metal lever is difficult.)|
|Lower Engine Case Cover||3/8-inch Socket: Tighten all bolts, in a criss-cross pattern, to necessary torque specification.||4|
|Head||1/2-inch Socket: Tighten all bolts, in a criss-cross pattern, to necessary torque specification.||4|
|Spark Plug||20mm Deep Socket: Screw spark plug into hole firmly.||3|
|Carburetor|| 3/8-inch Socket with Extension: Tighten both bolts holding carburetor onto block.
|Air Cleaner/Intake System|| 8mm Socket: Tighten both bolts holding plastic case onto carburetor.
|Muffler|| Hand: Line up gaskets between muffler and engine.
||5 (Lining up gaskets is difficult.)|
|Muffler Cage||8mm Socket: Tighten 3 bolts holding muffler cage onto engine.||3|
|Flywheel||15/16-inch Socket: Tighten nut holding flywheel onto input shaft to correct torque specification.||4 (Have to hold output shaft still while tightening.)|
|"Shell" with Pull Start||3/8-inch Socket: Tighten all bolts holding shell onto engine.||3|
|Oil Filler Neck||Hand: Push bottom into hole near the bottom of the engine block.||3|
|Gas Tank|| 8mm Socket: Tighten 3 bolts holding gas tank onto pull start.
|Rear Chute||3/8-inch Socket: Tighten 4 bolts underneath deck and 1 bolt on rear side of deck.||3|
|Rear Axle, Gear Box, and Rear Wheels|| 1/2-inch Socket and ½-inch Wrench: Tighten both bolts on each wheel to deck.
||7 (Difficult to align gears correctly)|
|Front Wheels||1/2-inch Socket and 1/2-inch Wrench: Tighten both bolts on each wheel to frame.||4|
|Rubber Flap and Plastic Shield|| 3/8-inch Socket: Tighten screws on each side of the deck to hold plastic shield on, tighten 2 screws on top of deck to secure pulley shield.
|Engine|| 1/2-inch Socket: Align and tighten 3 bolts holding engine onto mower deck.
|Blade||5/8-inch Socket: Hold blade with thick glove and tighten bolt.||3|
|Engine Cover|| Hand: Pull pull-start cord through hole.
|Handle||Hand: Align handle in grooves and tighten “nuts”. Push plastic knobs on the end of black cords into designated holes on mower deck and engine, attach end of steel cable to gearbox and engine safety. Lastly, push pull cord into ring on right side of handle.||4|
Figure B shows a simple representation of a crank-slider mechanism. As the crank rotates clockwise from the 9 o’clock position to the 3 o’clock position, the connecting rod, and thereby the piston, move to the right. Then, as the crank continues to rotate from the 3 o’clock position back the 9 o’clock position, the connecting rod and piston traverse back to the left. The rightmost position of the piston is referred to as Top Dead Center, "TDC," (minimum volume in combustion chamber), and conversely, the leftmost position corresponds to Bottom Dead Center, "BDC," (achieving maximum volume in combustion chamber). In a 4-stroke engine, it will take 2 complete rotations of the crankshaft to complete a cycle (Table 1); consider the cycle when beginning at TDC (Note: Rotation/Movement of the Crank and Slider is in relation to Figure B):
|Stroke/Process Number||Description of Process||Rotation of Crankshaft (Crank), Clockwise||Movement of Piston (Slider)|
|1||Intake air-fuel mixture into the cylinder.||3 o'clock to 9 o'clock||TDC to BDC|
|2||Compress mixture. Spark plug fires just before TDC and initiates combustion.||9 o'clock to 3 o'clock||BDC to TDC|
|3||The hot gases drive the piston to BDC. The torque transferred to the crankshaft via the connecting rod can be used to power the mower’s cutting blade.||3 o'clock to 9 o'clock||TDC to BDC|
|4||Expel exhaust gases as piston is driven towards TDC.||9 o'clock to 3 o'clock||BDC to TDC|
- Length of the Crank: R = (1/2)(x_BDC – x_TDC), where x_TDC is the piston extension at top dead center, and x_BDC is the piston extension at bottom dead center.
- Piston’s Displacement from TDC: x = R – R cos(θ) + L cos(φ), where L is the length of the connecting rod, θ is the crank angle, and φ is the angle between the connecting rod and the "ground".
- Electric Starter: An electric starter would be more convenient to use than the current method of starting the lawnmower engine, the pull cord. The pull cord requires the user to prime the engine and then pull the cord in order to start up the engine. With the electric starter all the user has to do is push a button or flip a switch and the lawn mower will be ready for use. The added ease of use would probably be popular with the intended market for this product: suburban homeowners (societal factor). But there are some disadvantages to having an electric starter. It would be more expensive than the traditional pull cord system. The electric starter is also more complex than the pull cord system. This would require the user to have a certain amount of knowledge about electrical systems if they wanted to perform maintenance on it (societal). Overall it is easier to maintain and repair a pull cord system than an electrical one. Furthermore, since the engine would be dependent on the electric starter to work, the whole lawnmower would be unusable if the batteries on the electric starter ran out of energy. This would require the user to either recharge the battery, which might take some time depending on the battery, or replace the battery entirely. In addition, it is more expensive to make an electric starter than a pull-cord starter system, and the added cost would be passed on to the customer (economic).
- Front Swivel Wheels: Replacing the static front wheels, with wheels that can freely spin would increase maneuverability, safety, and ease of use. The mower would easier to maneuver because instead of having to lift the front wheels off the ground every time you wanted to make a slight course adjustment. For the same reason, the mower is now safer (societal). When the user lifts the front wheels off the ground, he or she exposes the cutting blade and that is cause for concern. Small animals, children or limbs could be injured if the operator is not being careful. Not having to lift the front up makes the mower easier to use as well. Now you don’t have to be strong enough to lift the mowers wheels up and can just let the drive system do all the work as you just guide where it needs to go (societal). There would be a minimal price increase in the mower or you could install them yourself. Do it yourself Front swivel wheel kits sell for around $50 (economical).
- Easy Storage: To make the mower easily stored and transported the blade housing deck could fold in half, use a more compact engine. The handle would be completely retractable and foldable and wheels that could fold into the blade deck. The overall integrity of the mower would not be affected very much. By using one-way hinges, clips and pins the mower would be just as strong as a one-piece design. The power may decrease a little just because the use of a smaller engine. People would be able to store them in much smaller spaces (global). With the cost of shipping so high and being able to condense the mower into a much smaller space, the company could fit almost double the amount mowers per shipment. The overall cost may increase a little because of the extra labor and parts involved (economical).
Gate 5 - Delivery
This Gate is the final part of our semester-long project, and as such consists of the finalization of the project, including an in-class oral presentation of our project, editing of the previous Gates' deliverables, and a technical report to summarize the whole analysis process.