Gate 1 Group 15 2011
Gate One: Project Planning and Proposal
Before starting on a major project it is important to understand what you are getting yourself into; this particularly important in a group setting. It is very important to know the strengths and weaknesses of each individual so that it is possible to maximize the group’s effectiveness. In this gate of the project we will go over the approach we will take in disassembling and reassembling our product, we will go over what tools we will need to do this, and address our strengths and weaknesses as individuals and a group as a whole.
Figure 1.1 - The 1991 Craftsman 26cc Weed Wacker
Table 1.4 Projected Plan for Disassembly
|Part Description||Fastener||Tool Required||Time (minutes)|
|Muffler Cover||2 Self-Tapping Screws||Phillips Driver||3|
|Air Filter||2 Machine Screws||Allen Wrench||1|
|Carburetor (from motor)||2 Machine Screws||Phillips Driver||2.5|
|Carburetor (from throttle)||2 Bolts and Nuts||Crescent Wrench||2|
|Spark Plug Ground||Friction||Hand||.5|
|Engine Cover||8 Machine Screws||Allen Wrench||3|
|Pull Start Coil||4 Machine Screws||Allen Wrench||2|
|Shaft Joint Cover||2 Machine Screws||Allen Wrench||1|
|Hose Clamp||1 Machine Screw||Flat Head Driver||1|
|Trigger and Housing||1 Machine Screw||Allen Wrench||1|
|Forward Hand Grip||1 Screw with Knob||Hand||1|
|Head Guard||2 Self Tapping Screws||Flat Head Driver||2|
|Head||1 Machine Screw||Phillips Driver||.5|
|Strap Loop||1 Bolt and Nut||Crescent Wrench||1|
|Total Disassembly Time||21 minutes|
(All times listed are with the assumption that all the parts are going to come off easily, however we know that this is not the case and the dissection will indeed take longer than the time we have listed.)
- Several metal components are rusted and may strip easily, delaying our product dissection
- There have been a few home repairs on this product that may force us to deviate from the disassembly plan that would be used on a new product. Most notably, the rear hand grip and trigger line are held together with a hose clamp.
- The product was not in a running state before the project began (gas line was cut and the primer was ripped out), so it is highly unlikely that it will run after reassembly unless replacement parts are installed.
- Due to the restrictions imposed on us for Gate One, we have as yet been unable to inspect the actual engine to determine its condition. A rusted and/or deteriorated engine will hinder dissection
Table 1.1 - A short profile of the team members.
|Brent Haseley|| -Experience with tools and machines
-Worked 4 years in warehouse maintenance
| -Not CAD expert
-Very busy with overwhelming schedule and workload for other classes
|Charles Kalbfell|| -Quick learner
-Proficient writer and editor
| -Not familiar with 3-D modeling
-Poor presentation skills
|Frances Kalbfell|| -Good writer
-Willing to work hard
| -Tendency to wait until the last minute
-Limited experience with AutoCAD
|James Quirk|| -Proficient in CAD and 3-D modeling
-I have trouble focusing on one task
|James Ziccarelli|| -Performs confident work with hands when dealing with a product
- Has great interpersonal and communication with others
| - Not totally efficient with AutoCAD or Pro-Engineer
- Busy between school and a part-time job
Table 1.3 - a general timeline of projected project deadlines.
|Task||Subtasks||Stating day||Due Date|
|Gate 1||September 9, 2011||October 10, 2011|
|Project Management Decisions|| Work Proposal
|September 12, 2011||September 28, 2011|
|Project Archeology|| Preparation and Initial
|September 30, 2011||October 10, 2011|
|Gate 2||October 10, 2011||October 31, 2011|
|Cause for Corrective Action||October 10, 2011||October 20, 2011|
|Product Archaeology||Physical Dissection||October 10, 2011||October 17, 2011|
|Documentation of Dissection|| Notes in lab
Written up documentation
| October 10, 2011
October 10, 2011
| October 17, 2011|
October 19, 2011
|Gate 3||October 26, 2011||November 16, 2011|
|Cause for Corrective Action||October 26, 2011||October 30, 2011|
| Component Summary
Solid Model Assembly
| October 26, 2011
November 1, 2011
October 26, 2011
October 26, 2011
| November 3, 2011|
November 12, 2011
November 8, 2011
November 10, 2011
|Component Assessment Questions||November 1, 2011||November 12, 2011|
|Gate 4||November 14, 2011||December 5, 2011|
Critical Project Review
|Cause for Corrective Action||November 14, 2011||November 18, 2011|
|Product Reassembly||November 14, 2011||November 18, 2011|
|Design Revisions||November 14, 2011||November 28. 2011|
|November 14, 2011||November 29, 2011|
|Gate 5||December 2, 2011||December 16, 2011|
| Finalization of Deliverables
| December 2, 2011
December 2, 2011
| December 8, 2011|
December 10, 2011
| Technical Report
| December 6, 2011
December 2, 2011
| December 12, 2011|
December 14, 2011
- For all gates our group has made plans to meet at least one time each week out of class and lab to ensure that we get the gates done ahead of time.
- Each group member plans on going to the lab at least one time each week.
- The group also meets briefly at the end of every lecture to discuss progress of the project.
Table 1.2 - A brief description of the roles assigned to each group member.
|Brent Haseley||Documenter: I will be responsible for documenting and|
photographing every step of the product dissection.
Wiki Designer: I will be the person doing most of the
|Charles Kalbfell||Project Manager: I will be tasked with setting meeting|
dates, internal group deadlines, resolving conflicts,
and keeping the project on track. I will also read over everybody’s
|Frances Kalbfell||Chief Editor: I will be in charge of thoroughly going over|
everyone’s parts to make sure that they are technically
and grammatically correct. I will also have to put all of the parts
|James Quirk||Communication Liaison: I am in charge of communication between|
my group members and the instructors. When the group
needs a question answered, I am the person who expresses this via email
|James Ziccarelli||Main Technical Expert: I am in charge of the actual product dissection.|
I will also be responsible for developing a thorough knowledge of the
physical product and understanding how it will be reassembled.
- Majority rule, based on voting
- Project Manager has deciding vote in the event of a tie
- Team members agree to have individual work completed before deadlines to prevent any complications
- Everyone is comfortable correcting and being corrected by each other, recognize the importance of constructive criticism
Initial Product Assessment
The weed wacker was invented in the 1970s by George Ballas , and it has been a staple of the lawn care industry ever since. The design of weed wackers has continued to evolve and improve, resulting in the machines we know today. The product we are working on is a 26cc Craftsman weed wacker that was manufactured in 1991. The Craftsman weed wacker was intended to be sold in areas of the world were people have lawns and where they would have sufficient discretionary income to allow the purchase of luxury items such as a tool that performs such specific (and non-essential) jobs as a weed wacker. The typical areas where these criteria are met are in middle and upper class regions of the United States and Europe, though they have steadily migrated to areas in South America, Asia, Australia, and Africa as well. There are now people on all populated continents that tend to ornamental lawns.
One of the more pressing economical and global concerns at the time of the widespread adoption of the weed wacker was the price of gasoline. In early 1990 the average U.S. price for a gallon of regular gasoline was about $1.07 and over the course of the next year it went to as high as $1.42 (which represents about a 33% increase). Therefore fuel economy would have been a major consideration for the designers of any product in 1991 - especially an item such as a weed wacker that is not a necessity. So it is safe to assume that this particular weed wacker was designed to be as fuel efficient as possible while not sacrificing too much power. This ties in very nicely with one of the big global concerns of the day, keeping the environment clean . During the early 1990’s people began to become increasingly worried about what pollutants were doing to the environment. This might have caused the designers of our weed wacker to make sure our product was as fuel efficient as possible and wasn’t exhausting excessive dangerous chemicals into the environment. The overall goal of designers of any tool is to make it efficient (and there are many different criteria for “efficient”).
Currently Craftsman ships products to over 90 countries around the world . However, our weed wacker was almost certainly not sold in all of these countries. Craftsman has diversified its products and suppliers since the 1990’s so it is very likely our weed wacker was only available in America and well-off areas of Europe. Craftsman would have only been interested in selling their product where there was sufficient demand to ensure profitability, so they most likely did not try to establish markets in less affluent areas, or in areas where the climate meant there was little lawn maintenance (whether because of deserts or harsh winter conditions). Their marketing strategy would have been geared towards people in regions where their product could have made them an adequate profit even after establishing a support infrastructure. It is also worth noting that the weed wacker has a relatively small impact on society since its only function is to make a lawn look neater.
The intended impact for the consumer is relatively straightforward for this product. Weed wackers are intended to simplify lawn maintenance, allowing for a more esthetic presentation with reduced effort (more satisfaction while enjoying more free time). These machines are very effective in attaining these goals, as evidence by their broad market penetration. Though this is a relatively mature product, there is still continuing development. Current environmental concern are pushing manufacturers to look to alternatives to 2 cycle engines; the most promising are 4 cycle wackers (primarily Honda), or rechargeable electric weed wackers (Worx and multiple others).
There are some weed wackers that can do many different jobs because of available attachments, but the 1991 26cc Craftsman weed wacker we are dissecting this semester fills a very specific role. This machine is intended to be used to trim vegetation in areas around the world where people maintain decorative lawns. You would never try to cut an entire lawn with a weed wacker because it would be very tedious, and it probably would not look good anyway because it would result in a very uneven cut. Trying to cut anything bigger than small to medium weeds would similarly be out of the question. If you tried to cut anything too thick it would merely result in overheating the engine and broken string, and if you tried to cut something like a vine it would most assuredly get tangled in the head of the weed wacker and either the engine or transmission (or both) would seize. The ideal use of a weed wacker like this is to cut the grass and other small plants on the perimeter of a small lawn, or to trim around trees or other objects.
Almost every landscaping company in America uses weed wackers to finish off their trim work, but they almost certainly do not use a 26cc Craftsman weed wacker from the early 1990’s. There may be other considerations (for instance, some companies prefer to use slightly older weed wackers with adjustable carburetors because they allow you to change the power to fuel economy ratio with the turn of a screw whereas this feature is illegal on newer models), but product evolution means the more modern a weed wacker is the better you would expect it to perform. Regardless, this particular weed wacker is not durable enough to handle the rigors of commercial use – it was clearly designed to a lower price point, yet it is still tough enough to be acceptable to a residential homeowner. In that light, this is a very efficient design. It is a low maintenance device that is very good at doing what it was designed to do, and it is durable enough to provide a very long service life for a once-a-week user intent on tidying his/her lawn.
There are a multitude of different types of energy utilized in our 26cc Craftsman weed wacker, the most prominent of which are inside the machine itself. The energy of the system all comes from the chemical energy that is provided by combusting the gasoline that is put into the machine. The actual ignition of the gas is accomplished by the sparkplug. There is a flywheel in the powerhead that serves many functions – it is what the recoil starter engages to start the engine, its mass smooths out the power pulses generated by the engine, it incorporates an integral fan that provides engine cooling, and it has magnets on it that generate electricity as they spin by an ignition coil. From here this electricity is converted by an electronic ignition module, and then it is fed out to the sparkplug. The plug then forces the electricity to jump across a gap to get to the electrode, providing the spark that initiates the combustion process. The energy in the gasoline is then released resulting in a controlled explosion. This explosion then causes the piston to move down, which causes the crank shaft to move, and then its rotational inertia forces it back up, which results in another combustion cycle. The crank shaft then imparts rotational kinetic energy to the head of the weed wacker, which does the actual cutting.
There is also user input energy that goes into making our weed wacker a functional tool. Our weed wacker does not feature automatic/electric start, so to initiate operation the user must apply some amount of force to the rip-cord. This provides the energy necessary to begin the self-propagating operation as outlined above. Our weed wacker does not have any type of propulsion, so the user must carry the machine while using it. Even though it may seem negligible there is still also energy involved with the user squeezing the trigger on the handle of the weed wacker, which eventually results in the release of gasoline into the combustion chamber.
All of the information above can also be found in the chart provided below.
Energy Profile Table
Table 1.5 - Shows paths of various energy forms through the system.
|Types of Energy Used in the System||How it is Used in the system||How Energy is imported|
|Internal Combustion||The most prominent form of energy in a weed wacker comes from the gasoline you put into the machine. The weed wacker’s engine produces power by internal combustion that turns the shaft. From there a transmission modifies it so it spins the head (and string) parallel to the ground.||Chemical potential energy is added in the form of gasoline. The gasoline is drawn into the combustion chamber where it is ignited.|
|User input energy||Human energy is also need so that the weed wacker can run. Someone has to pull the rip-cord so the weed wacker will start. A person has to pull the trigger so that the engine will get gas. Since it cannot move on its own someone needs to carry the weed wacker around.||The user does work on the system.|
|Rotational Kinetic Energy||Energy is converted into kinetic energy as the motor spins the shaft and head (this also creates momentum – you can see this because when you turn off the weed wacker the head continues to spin).||The combustion of the gasoline results in the piston going up and down, which moves the crankshaft that turns the drive shaft.|
|Heat||Substantial thermal energy is created by all the above mentioned processes (to the point where many weed wackers actually stop working due to overheating). All the explosions and friction create immense amounts of heat.||Heat energy is brought into the system through friction and chemical combustion.|
|Electrical Energy||Two magnets on a flywheel (called a magneto) spin around and pass a coil. As they spin past they generate an electrical current that then passes though the ignition system and into the sparkplug. The plug then creates a spark at the electrode gap.||Electrical energy is generated in the system when spinning magnets pass a coil.|
|Chemical Energy in Fuel||Chemical energy is stored in the bonds of the gasoline. When they are broken through combustion the controlled explosion’s energy becomes kinetic energy, which powers the system.||Chemical energy is imported to the system when gasoline is poured into the weed wacker.|
|Potential energy stored in a Spring||Potential energy is stored in the various springs of a weed wacker. There are springs in the shafts of many weed wackers, the trigger system, the head, and in the pull cord’s recoil mechanism.||The user introduces potential energy when he compresses (or stretches) a spring.|
|Kinetic Energy||Energy is converted into kinetic energy inside the engine itself. The piston of the weed wacker moves up and down due to the explosion caused by the gasoline, causing the crank shaft to move.||Kinetic energy enters the system made when the piston moves due to the controlled explosion of the gasoline. Kinetic energy is also stored in the rotating head, shaft, and string.|
In order to simplify our analysis of our weed wacker we broke it down into three separate sub-sections; the head, the shaft, and the power plant. (The breakdown of these sections can be seen in Figure 1.2) Since we have yet to disassemble the weed wacker we made some educated guesses regarding the approximate number of components we believe the weed wacker contains.
Figure 1.2 - The product simplified into three sections.
Starting from the ground up, the first section is the head of the weed wacker. This is where the cutting line is dispensed and rapidly rotated in order to actually cut the vegetation. After studying this section we concluded that it contains around fifteen components and six mechanical fasteners, most of which are flat-head screws.
The second section of the weed wacker is the least complex mechanically. It contains the hollow aluminum tube that serves as the backbone (or frame) for the tool. Inside this tube is the driveshaft that transmits engine power to the cutting head. This tube is also where the handle and throttle trigger are located. This section appears to be made up of about 20 or so different components. Many of the parts, such as the trigger, are actually made of multiple pieces that work in unison to provide their overall function. Holding this section together are about 10 fasteners, most of which are either flat-head screws or Allen screws. The largest metallic piece of hardware on most weed wackers appears to be made of brass – those fasteners join the operator’s handle to the shaft. Since this piece is designed to be adjustable for multiple operators, a non-corrosive material is necessary – user adjustments are typically done in the field and seized hardware there is much more difficult to overcome than it would be in a shop. Also, the fastener for the trigger on the throttle is made of plastic. This is because the throttle has to move (a fixed throttle would obviously be unacceptable), and yet it is subject to constant vibration. The elasticity of the plastic absorbs or dampens the vibrations, resulting in a fastener that is moveable but that does not back out over time. While this subassembly is the most straightforward, its importance cannot be overstated; since it provides the user interface with the machine (the handle) and the engine control apparatus (the throttle) ergonomics must be a key element in its design.
Finally we come to the third section of the weed wacker, which is far and away the most complex part of the tool. This section contains the internal combustion 2-cycle engine that is the power supply for the product (a simple diagram of a 2-sroke engine can be seen in Figure 2). The engine contains the highest parts count, which we are estimating to be approximately 90 different parts and at least 30 fasteners . These fasteners will most likely include but are not limited to flat-head and Phillips-screws, Allen screws, solid metal keys and/or metric sized bolts and nuts. The operating conditions within the engine mandate the materials these fasteners and parts are constructed of. For instance, the temperature in and near the cylinder is extremely high, and air cooling is not the most efficient means of dissipating by-product heat. However, cost, complexity (especially maintenance), and weight considerations preclude the use of a water cooling system.
Figure 1.3 - a simplified 2-stroke engine.
In the first two sections the parts are made of plastic, aluminum, or steel (the fasteners). All the plastic components are molded and then assembled and fastened together. The aluminum was chosen because of its high strength to weight ratio, and it is cut, bent, and finished to form the interchangeable parts that give the machine the form we are familiar with. The vibration, heat, high forces, and the requisite tolerances of internal combustion engines mean the engine is the most complex part in terms of engineering. There is a very high parts count inside the engine, and the majority of them are made of steel or aluminum, but there are multiple types of steel. Then there is also a plastic housing that encompasses the whole engine. This plastic has to be light, but it also has to offer protection for the machine and the operator. The parts inside the engine are machined to very tight tolerances, and then they are carefully assembled in a precise sequence to create a working engine. The casing that protects the engine is also clearly designed and molded to fit this specific engine type.
In isolation none of the sub-assemblies seem fairly complex (the possible exception being the engine). The reality is that they are all the product of considerable engineering, and they come together to create a durable, inexpensive machine that is very efficient at carrying out its intended function. For example, the spring loaded spool and a fixed blade on the cutting head provide a simple way to keep the string at an optimal length, and the spool is easy to take out to reload as string is exhausted. In fact, the directions are conveyed in just a few icons molded into the spool itself. This simplifies operation and maintenance – and if these conditions are not met there would be user frustration, deterring people from using or even purchasing the product. This rotating head is connected to either a simple transmission system or a spring drive shaft – either method ensures that the power is available in the correct orientation (parallel to the ground) for optimal cutting. Both methods are effective; the spring is cheaper to manufacture, but it is less durable. A plastic guard is affixed to this tube immediately above the head to protect the consumer and bystanders from flying debris during use. Further up the shaft is the handle, which gives the user stability and control of the weed wacker, and then the throttle trigger, which provides engine control. The trigger consists of a lever which is finger activated, pulling a cable that runs directly to the carburetor. When the trigger is pulled, the carburetor allows increased fuel flow, resulting in more power. Possibly the most important function of this tube is easy to overlook – it protects the drive shaft, resulting in a maintenance free and effective means of transmitting engine power to the cutting head. Finally we come to the engine. As mentioned, the engine is comprised of many small parts and fasteners which all have a specific role in the generation of power for this portable outdoor tool. The unique demands of the operating environment mean many parts are made from expensive metals, but there is no way around that – the combustion power pulses and high rotational speeds call for materials stronger than aluminum and mild steel, and so the their extra cost has to be allowed.
A weed wacker represents considerable engineering. It has to be durable and affordable, and it needs to be simple enough that a homeowner can perform routine maintenance and the occasional small repair. At first glance it might sound simple – have a small motor spin a piece of plastic string to cut unwanted vegetation, but that does not tell the whole story. We have a very small motor with a tremendous power to weight ration that is attached to a long shaft that is then enclosed in a protective tube. The power is then oriented correctly, and it is supplied to a cutting head that incorporates a regulating methodology to ensure the string length remains within a very narrow range. The total machine is controlled by a choke lever, a throttle trigger, and the occasional bump on the ground. There are three distinct sub-assemblies, each of which is made up of myriad pieces, each of which is designed for a specific role. Then they all complement each other and seamlessly interact to give us a product that is safe, durable, easy to use, and affordable.
A weed wacker is made out of metal, plastic and rubber. The majority of the visible parts are either plastic or aluminum, and most of the fasteners (such as the screws, nuts and bolts) are made of steel. Rubber is utilized in the fuel system, handles, and in sealing items such as gas cap washers and o-rings. There are traces of paper in the filters and gaskets, but that comprises only a miniscule percentage of overall material content. The majority of the parts that are not visible while the product is still assembled are made out of common metals such as aluminum and stamped steel, but there are some engine components that are made out of more exotic metals. These parts are made out of metal because they are subject to high stress loads (such as the engine internals, drive shafts, and gears), and these plastics or other less durable materials could simply not stand up to the stress loads that result from routine operations. Both the pull-cord (rope) and the spark plug (ceramics and metal) were left off the charts because of they did not fit into a well-defined category.
User Interaction Profile
Beginning with the removal of the product from its original box (and assuming it is already fully assembled) the first order of business is to load the trimmer head with string. The easiest way to accomplish this is to remove the spool from the head by depressing it into the head and then turning the spool ¼ turn counterclockwise. The spool will then “pop out” and separate from the head. Next cut approximately 20 feet of string and thread it half way into the provided hooks inside the spool – this serves as an anchor for the string. There will then be 2 ten foot lengths of string that need to be wound around the spool. Leave about 6 inches of each end free, and pull these pieces down into the slots in the spool (these act as temporary restraining clamps). Finally, lace the ends through the grommets in the head and replace the spool onto the head. A sharp pull on both ends will seat the spool and lock it into position for operation (see video for visual aid).
The next procedure calls for filling the fuel tank. It is important to note that this is a two stroke engine and therefore requires a pre-mixture of gasoline and oil in order to operate properly. This has the potential to be overlooked by a careless user who might instead fill it with pure gasoline. This would result in major damage being done to the engine, causing it to seize and rendering it forever unusable. It is also relevant to remember that the corrosive nature of ethanol precludes the use of fuel containing more than about 5% alcohol (precluding the use on many premium grades of gasoline since ethanol is a common octane booster).
Before the device is actually operated, the user should obtain eye and hearing protection and should tailor the position of the handle and throttle trigger to their own liking – this results in much less user fatigue. While there is a small shield attached to the shaft near the head, it is not sufficient to stop all projectiles that may be thrown by the spinning trimmer line. A shirt, long pants - preferably denim - and proper footwear should also be worn to protect the user. While not absolutely necessary, many professionals wear a full face shield to prevent facial lacerations.
With these precautions the user can then use the product by holding it by the two designated handles and operating the trigger with his upper hand. The engine may now be started. The first step is to slide the electric kill switch to the “ON” position. Then to make starting a cold engine easier the choke should be engaged. The user then has to squeeze the primer bulb 3 times to start the flow of fuel into the carburetor (once the machine is operating vacuum will pull the fuel into the carb. Then pull the recoil starting cord a few times until the engine begins to sound as though it is close to running. At that point the choke should then be decreased to half, and the user should pull on the starting cord until the engine starts. It should be allowed to warm up for approximately thirty seconds, and then the choke may be shut off entirely. The head of the device should then be moved into contact with undesirable plant growth. When the trimmer line becomes too short, the head should be lightly bounced on the ground to feed out more line.
The product interfaces are very intuitive and simple. There are only two points of contact (three, if the optional support strap is attached), and each serves a specific purpose. (There are actually other, infrequently used interfaces like the pull handle on the starter, the primer bulb, the choke pull, and the “ON/OFF” switch, but we will focus on the primary operating controls.) The bottom hand grasps the handle, and it carries most of the product’s weight and directs where the head will cut. The operator’s top hand is the one closest to the engine and it controls the engine’s speed and acts as a counterbalance. A trigger is ideal, as it is extremely intuitive and it allows the user to maintain a natural grip on the machine. Also, it is interesting to note that as the engine is revved to higher speeds that cause more vibration in the shaft the trigger is moved closer to the shaft, allowing for a more confident grip when it is most needed.
The user’s lower hand is used to control the movement of the trimmer head. It also serves as a fulcrum to balance the weight of the engine against the weight of the head. This is ideal because it allows the upper hand to focus entirely on controlling the machine without the distraction of having to support it as well.
Ease of Use
Thanks to intuitive product interfaces, the weed wacker is very simple to operate; it has a very shallow learning curve. The most challenging task is simply carrying the full weight of the machine and not letting the head drop too low (“scalping” vegetation and doing damage to the lawn).
The trigger mechanism provides very little resistance when squeezed, so it will not fatigue the user in any significant way. Padded handles also contribute to the ergonomics of the product, as they eliminate a significant amount of the vibrations caused by the running engine. The muffler is also relatively large, resulting in a material reduction of sound pollution. These same engines are frequently used to power large scale radio controlled model airplanes.
Regular maintenance is required when dealing with weed wackers in order to keep it in optimal operating condition. The most obvious task the user must complete is refilling the fuel tank. In order to keep the product lightweight, the fuel tank is rather small and must be replenished frequently. This small fuel capacity also allows for relatively frequent user breaks, reducing monotony (at least to a degree). Trimmer line must also be refilled regularly, as it breaks off during use.
The product also requires regular cleaning and periodic lubrication. Wet grass clippings, saps, and mud form an adhesive semi-solid that will detract from the performance of the product. Long vegetation can also wrap around the cutting head, greatly increasing friction and reducing cooling, both of which can be disastrous for the longevity of the machine. These materials can also build up in the head, preventing it from feeding line properly. Once dried, it can be easily scraped out with a flat screw driver or putty knife, and many operators spray the head with a silicon lubricant or Pam cooking spray (though oil based lubricants can damage grass, especially freshly cut edging). The controls like the throttle cable and choke pull are long lasting, but they should be periodically lubricated to ensure a long service life. The carburetor has a propensity to gum up if gasoline is allowed to sit in it for a long time. This jellied gas plugs up the tiny holes in a carburetor and makes it so that the machine cannot take in sufficient fuel, rendering it inoperable. This means the fuel should be run through regularly, and many operators use a fuel additive like Stabil if they anticipate reduced usage. In any event, operating efficiency is enhanced if the carb is serviced regularly.
The air and fuel filters on a weed wacker are also very important because of the environment these machines are operated in. Weed wackers are used in places that are constantly dusty with considerable amounts of debris flying around. These filters’ job is to catch as much of that as they possibly can. They frequently get dirty and need regular cleaning and replacement, but they are what keep harmful particulates out of the engine. On the exhaust side, there is a small screen located between the muffler and the exhaust port that prevents sparking or “popping” when concentrated fuel-rich exhaust gasses come into contact with the atmosphere. Unfortunately, these screens can become clogged by combustion by-products. It can be a bit harder to service then the input filters (requiring the removal of the muffler) but once it is clogged it effectively strangles the engine, resulting in very poor performance (or even rendering the tool inoperable).
All of the regular maintenance tasks are simple enough for an uninitiated user to complete with simple hand tools. More advanced maintenance, such as internal engine problems or transmission issues usually require a more seasoned background to be appropriately corrected.
Table 1.7 Maintenance Schedule
|Cleaning out grass clippings||Before each use|
|Fill gas tank||Every 30 minutes runtime|
|Add line||Varies with usage|
|Clean carburetor||2 times a year|
|Clean air filter||Monthly|
|Replace in-tank fuel filter||Annually|
|Clean exhaust screen||Annually|
Project Alternative Profile
The product we will be doing our project on over the course of the semester is a 1991 Craftsman 26cc Weed Whacker. We know this is quite an old model, and we will be comparing it to more contemporary models such as the 2011 Husqvarna 28cc Full Crank Gas Trimmer and the 2011 Craftsman 27 cc* 2-Cycle Full Crank Brushcutter Combo Weedwacker® Gas Trimmer.
Table 1.7 Weed Wacker Comparison
|Weed Whacker Model||Description||Advantages||Disadvantages|
|1991 Craftsman 26cc Weedwacker -2 cycle engine|| -Throttle trigger
-48’’ shaft length
-16’’ cutting path
| -Controlled fuel settings:
-More fuel less oxygen for more power
-Less fuel more oxygen for better fuel economy
| -More harmful emissions|
|2011 Husqvarna 28cc Full Crank Gas Trimmer|| -2 cycle engine
-T25 Bumper Head
-54’’ shaft length
| - Bumper Head with a two hole string feed
- lightweight, 9.7 pounds
- Noise reduction technology
| -Two hole string feed can be hard to maintain|
-Can’t select fuel settings on the carburetor
- No available attachments
- Braided wire cable drive
|2011 Craftsman 27cc* 2-Cycle Full Crank Brushcutter Combo Weedwacker Gas Trimmer|| -2 cycle engine
-Gas mix powered
-57’’ shaft length
-14’’ cutting path
| -Multiple attachments including : an edger, a cultivator, a blower, a brushcutter, a pruner, a hedge trimmer, and a pole saw
- Available electric start
-Dual Line Hassle Free™ Cutting Head
- Shaft drive system
| -Poor distribution of weight|
-Can’t select fuel settings on the carburetor
- A more narrower cutting path
1991 Craftsman 26 cc Weed Wacker
All weed wackers perform the same basic function - they trim the parts of a lawn that a lawnmower cannot cut. However, this does not mean that all weed wackers are the same. The 1991 Craftsman 26cc Weedwacker differs in many ways from the models sold today. Even though it was designed over two decades ago this Craftsman model has one clear advantage over current models in that it has multiple fuel settings. Beyond actually tuning the carburetor (something current clean air standards prohibits) the user of the 1991 model can control the air fuel ratio using a switch on the side of the weed whacker. If a user needs more power all they have to do is select the higher setting. If conditions warrant, they can also opt to select a lower power setting to maximize fuel economy. This is very important because of the rising fuel costs, and it also lets a user reduce “wear and tear” on the engine. At its highest setting this particular model can operate at up to 7500 RPM, yet it idles at a relatively low 2800-3200 RPM (10).This range allows for a lot of flexibility and that is one of the metrics for efficiency.
A distinct disadvantage of such an old model is the noise output. In response to complaints about the noise of 2 cycle power plants engineers have been specifying larger mufflers to quiet the wackers without adversely affecting their power output. This was apparently not a major priority when our unit was developed, so we expect our model would be much louder than a comparable weed wacker sold today. Another issue with our model would be how it transfers power to the head. Since it is a curved shaft weed wacker our model eschews a conventional transmission and instead utilizes a braided wire cable drive. While there is nothing inherently wrong with this system it is much less robust than a shaft drive unit. Older weed whackers also tend to create more emissions than their newer counterparts. Until 1995 the emissions of 2 stroke weed wacker engines were essentially unregulated. Older engines may have been more powerful, but they could release up to 25-30% of their unburned oil and gas into the air (10).
2011 Husqvarna 28cc Full Crank Gas Trimmer
The 2011 Husqvarna 28cc Full Crank Gas Trimmer is a newer and more well-known model of weed whacker. This model utilizes a HT25 bumper head with a two-hole string feed, which allows a user to store more line in the head of the machine . As a result, there is less downtime because there is less need to stop the machine to re-string the head. This weed whacker happens to be a very light unit at 9.7 pounds making it a good choice for the casual home owner. The Husqvarna also has a curved shaft which allows to user to easily control the weed wacker.
Even though it is a new model the Husqvarna trimmer has some distinct disadvantages when compared to the other weed wackers we looked at. New regulations have made it illegal to sell any weed wacker with an adjustable carburetor. This makes it impossible for the user to adjust fuel settings, which could be seen as a huge inconvenience. This sealed carburetor paradoxically results in more maintenance costs – instead of retuning a carburetor to account for any variation in performance, a new carburetor has to be purchased and installed. For a new trimmer it is surprising that the Husqvarna does not have the ability to link with any attachments - the only thing you are able to do with this unit is trim a lawn.
2011 Craftsman 27 cc 2-Cycle Full Crank Brushcutter Combo Weedwacker® Gas Trimmer
The Craftsman 27 cc* 2-Cycle Full Crank Brushcutter Combo Weedwacker® Gas Trimmer is another new model on the weed whacker market. This trimmer has two very prevalent advantages over the other model we analyzed in that it has the ability to use attachments and it uses a shaft drive. There are currently 6 attachments that this model can hook up with to perform various maintenance chores (see Figure 6 to view the attachments). The other major advantage of this machine is that the head is turned buy a solid shaft drive as oppose to a braided wire cable. This means the unit will be very durable and could survive commercial use, whereas the other trimmers are limited to individual use.
Figure 1.7 -A display of all the different attachments that the 2011 Craftsman weed-wacker can be used with. 
The largest criticism of the modern Craftsman is that it is heavier than the other two models, a disadvantage exacerbated by poor weight distribution. This makes it awkward to handle, and can lead to increased operator fatigue. This is not the result of poor design, but rather of a different design objective – heavier parts are necessary for a unit designed to stand up to heavier workloads and more challenging attachments. Versatility and durability have a price, and that is why different machines are appropriate for different users.
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