Gate 3 Group 15 2011

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Contents

Introduction

At this point in the project we have completely dissected our weed wacker and have begun the analysis of our product on the component and subsystem level. Our repot includes an analysis of our team with specific detail on the group conflicts, a component summary, a product analysis, a solid model assembly, an engineering analysis and multiple redesigns for our product on the component or subsystem level.


DSC03505.JPG

Image 3.1 - Crankshaft, Piston, and Housing

Cause for Corrective Action

At this point in the project our group is working very well together. We never seem to face any problems working together: whether it is working in the lab, doing the write-ups, or working on the assignments in class. We have all become friends making it much easier to work together because we know each other’s strengths and weaknesses making it simple to decide what each person should do.

The biggest dilemma we encountered in putting Gate 3 together was with the three dimensional diagrams of the components. We initially believed the modeling could have been in a two dimensional AutoCad program, however upon rereading the gate and visiting Professor Olewnik in office hours we discovered that we needed to do three dimensional modeling. Our CAD expert had not used any three dimensional drawing programs in a few years. Because of this development he had to relearn Inventor in order for us to complete this portion of the gate. Being proactive in this section of Gate 3 proved to be vital because it enabled our CADs-man to become competent in Inventor and ultimately make our drawings.

A common theme in trying to coordinate our group meetings is that it is very difficult to find a time to meet for extended periods in the lab. We were able to combat this by meeting whenever the majority of the group could. However, this required us to be even more active in informing our group members of what was going on so that everyone was always up to date. Luckily the night that we budgeted to get most of our lab work done everyone was able to attend, so there was not a lot of wasted time in relaying information from one group member to another.

While we were doing work in the lab there was only one challenge that we encountered. For part of Gate 3 we were required to determine how each component of our weed wacker was made. There were various parts such as the spring shaft that we were unable to determine the manufacturing process. To make sure this was resolved one of our group members went to Instructor Cormier’s office hours with a list of the parts we unsure of their production, and Instructor Cormier was able to help us complete this section of the Gate.

As of now the only major unresolved challenge that remains for our group is when we will complete the lab portion of Gate 4. The further we get into the semester the more demanding our classes will become. Taking this into consideration one of our goals is to have Gate 4 finished well before it is due. This way we will not have to worry about having to put the Gate together while others things are going on. This could be problem because there is a limited amount of time between now and the Thanksgiving Break, and over the break we cannot work on it as we have in the past because obviously the lab will not be open. As a group we have decided that one of the members will take the weed wacker home, and we will go over to their house to put our product back together. Doing this ensures that we will have ample time to complete the write up portion of the Gate without worrying about anything else. Achieving this goal will not be easy, we will be forced to be very proactive and constantly communicate with one another. Working hard and efficiently will be vital to our success, and will be the overall factor in determining when we are able to finish the last two gates of the course project.

Component Summary

Table 3.1 - Brief summary of all of the components.

Part Quantity Function Part Number Material Manufacturing Process Photo
Large Housing Gasket 1 Seals connection between housing and plastic case None Black rubber Die cut Large Gasket-15.jpg
Small Housing Gasket 1 Seals connection between housing and engine block None Black rubber Die cut Smaller Gasket-15.jpg
Carburetor Gasket 1 Seals connection between choke and carburetor None Black rubber Die cut Small Gasket-15.jpg
Ground Wire 1 Connects Magneto to a ground 3 Copper, Plastic insulation Drawn and coated in plastic insulation 123-15.jpg
Crankshaft housing 1 Houses crankshaft 530-028679 Aluminum Die cast 1243-15.jpg
Cylinder/block 1 Contains and directs combustion 28681 Aluminum Die cast, then milled 12e43-15.jpg
Piston 1 Converts combustion to translational mechanical energy HK1010 Steel Die cast and then milled 1d2e43-15.jpg
Flywheel 1 Spins magnets to power magnetos 12407-68 Aluminum, ceramic magnets Die cast 1d2e4d3-15.jpg
Magneto 1 Generates a current when magnets pass by RF1837 Copper wires, plastic casing and insulation, steel internals Wires drawn, steel forged, plastic molded 1d2e4dd3-15.jpg
Carburetor 1 Controls gas and air flow into cylinder A199 Brass, Steel Die cast and forged parts, milled center hole 1d2e4dfd3-15.jpg
Crankshaft 1 Turned by piston to create rotational motion C Steel Cold rolled, then milled and threaded, forged counterweights 1d2de4dfd3-15.jpg
Bearing 2 Smooth motion between crankshaft and housing Nope Hardened Steel, plastic casing Die casted and polished balls, cut extruded pipes, injection molded plastic casing and spacer 1d2des4dfd3-15.jpg
Clutch 1 Transmit rotational motion from crankshaft to the spring shaft M Steel, spring steel The clutch is cast then milled,

The spring is drawn and then hot wound

1d2des4ddfd3-15.jpg
Pull Start 1 Turn flywheel to start engine 29191 Plastic, nylon Injection molded parts, drawn and woven rope 1d2deds4ddfd3-15.jpg
Clutch Shroud 1 Grabbed by clutch to turn spring shaft None Plastic, steel Injection molded plastic, extruded and formed pipe 1d2deds4dddfd3-15.jpg
Washer 1 Reduces friction in clutch shroud None Steel Forged 1d2deds4ddddfd3-15.jpg
Head Spring 1 Created tension in head None Steel The spring is drawn and then hot wound 1d2deds4dddcdfd3-15.jpg
Trigger Mechanism 1 Controls throttle None Plastic, steel spring Injection molded plastic. The spring is drawn and then hot wound 1d2gfcdfd3-15.jpg
Support Handle 1 Ergonomic way to hold the weed wacker None Plastic Injection molded plastic 1d2gfcdfgd3-15.jpg
Air Filter and Casing 1 Filter air entering the engine 89 Plastic casing, foam filter, steel springs Injection molded plastic, drawn and tempered spring, foam is machined 1d2gfcdfgrd3-15.jpg
Throttle Cable 1 Connect trigger to throttle 5300274549 Steel, plastic Drawn and braided wire, injection molded casing 1d2gfcdfgrgfd3-15.jpg
Key Piece 1 Stabilizes Crankshaft None Steel Forged 1d2gfcdfgrg6fd3-15.jpg
Muffler 1 Quiets engine None Steel, Spring Steel Die casting and drilled. The spring is drawn and then hot wound 1d2gfcdf5grg6f65u5d3-15.jpg
Choke Plate 1 Aids in engine starting, richens mixture None Steel Forged 1d2gfcdf5grg6fud3-15.jpg
Kill Lever 1 Kills engine None Steel, Plastic Forged steel, molded plastic 1d2gfcdf5grg6fu5d3-15.jpg
Spark Plug 1 Ignites mixture AutoLite258 Steel, Plastic Rolled steel, die cast ceramic 1d2gfl5d3-15.jpg
Spring shaft 1 Transmits motion from crankshaft to head None Spring steel The spring is drawn and then hot wound 1d2gfl5dt3-15.jpg
Shaft Housing 1 Protects the spring shaft None Steel Extruded, cut and formed pipe 1d2gfl5drrt3-15.jpg
Main Casing 2 Protects engine, ergonomics 027517-8,027476 Plastic Injection molded 1d2gfl5uydrrt3-15.jpg
Head Guard 1 Protects user from debris 530094699 Plastic Injection molded 1d2gfl5uytrdrrt3-15.jpg
Elbow Spacer 1 Spaces magneto and cylinder None Aluminum Die cast 1d2gfmrrt3-15.jpg
Head Assembly 6 total pieces Holds and feeds trimmer line 94522,94523,94521,94488 Plastic Injection molded 1d2gfmrrrt3-15.jpg
Head Spacer 1 Spaces head from main shaft None Steel Forged 1d2gfqmrrrt3-15.jpg
Screws 29 (in varying sizes) Attach Parts None Steel Forged and rolled 1d2gfqmrerrrt3-15.jpg

Product Analysis

Component Function and Component Form

Part Main Casing

  • Function: The engine casing provides a base for engine parts to be mounted in. It also covers the engine and internal parts protecting them from any impact. In addition the casing protects the user from the extremely hot parts of the engine.
  • Does the Component help Perform Multiple Functions?: The main casing performs multiple functions; protecting the user form the machine and protecting the engine from impact.
  • Associated Flows: No flows are associated with the main casing; the product could function without this component. The only purpose it to provide protection for the user and the product.
  • Functional Environment: The casing has standard pressure applied to it, but a high operating temperature due to its proximity to the engine.
  • General Shape: The shape of the casing is somewhat irregular, but it could most be defined as a rectangle.
  • Notable Properties: The casing does not have any symmetry; it is similar to a rectangle.
  • 1/2/3 Dimensional: The main casing is a 3 dimensional component.
  • Dimensions: Approximately 1 foot high, 6 inches wide, and 6 inches deep.
  • Relation of Shape to Function: The casing is in a shape that allows it to completely surround the engine; keeping it protected.
  • Weight: Approximately 12 ounces.
  • Impact of Manufacturing Decisions on Geometry: Injection molding is the only way to make the main casing. This process allows the manufacturer to create the intricate geometry the part requires. To create this component in any other process would have been much more expensive.
  • Specific Material Property Necessary to Function: To perform its function the plastic needs to be strong (to protect the engine) and a good thermal insulator (to protect the user). The material that was used also had to be light so that it did not add too much weight to the product.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Societal - The casing protects the user from the moving parts and heat of the engine.
    • Societal - Plastic is light weight making the weed wacker easier to carry and maneuver.
    • Economical – The casing was made through injection molding which is relatively cheap driving down the price of the tool.
  • Aesthetic Properties: The main casing is made out of a smooth, glossy plastic. It has an irregular shape because it needs to cover up the engine.
  • Aesthetic Purpose: The aesthetic purpose of the main casing is to neatly hide the components that make up the engine. It provides an attractive profile to the weed wacker.
  • Color and Why: The main casing of our weed wacker is grey and black. This provides for a nice color scheme for product. There are also various Craftsman logos on the casing that served as advertisement for the product
  • Surface Finish: The surface finish on the main casing is smooth and glossy.
  • Surface Finish for Function of Aesthetics: The weed wacker was given a smooth glossy, surface finish for aesthetic reasons. It provides the tool with an exterior that is pleasant to look at and touch.

Part: Piston

  • Function: Translate the energy created by the combustion of the gasoline into translational mechanical energy.
  • Does the Component help Perform Multiple Functions?: No, this component’s only function is to convert the energy created in combustion into translational kinetic energy.
  • Associated Flows: This component undergoes translational motion and transfers it to the crankshaft.
  • Functional Environment: High heat and high pressure due to its proximity to the engine. (It is physically inside the combustion chamber, also the friction from the translational motion creates more heat.)
  • General Shape: A cylindrical shape with an arm extending through the bottom to translate kinetic energy into translational rotational energy.
  • Notable Properties: The rings on the piston create an air tight seal to help create higher compression and a more efficient engine. Contains axial symmetry.
  • 1/2/3 Dimensional: 3 Dimensional
  • Dimensions: Approximately 1 inch in diameter, 1.5 inches tall
  • Relation of Shape to Function: Needs to be milled to specific dimensions to create a tight seal in the cylinder to create the highest possible compression without too much energy being lost.
  • Weight: 9 ounces
  • Material: Steel
  • Impact of Manufacturing Decisions on Geometry: The piston had to be made out of steel because of the environment it performs in. The component would be cast then milled in order to cut down on cost and then insure the tolerances of the part.
  • Specific Material Property Necessary o Function: Strength, rigidity, durability, and ability to perform well in high heat environment.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic- the material chosen for the component was steel; which is a relatively cheap metal.
    • Societal- the need for the piston to create a good seal with the cylinder affects the efficiency of the engine which in turn is seen by the consumer.
    • Environmental – a more efficient engine results in fewer pollutants in the air.
  • Aesthetic Properties: Very smooth and shiny.
  • Aesthetic Purpose: No aesthetic purpose visually because it’s located inside the engine.
  • Color and Why: Silver, this is the color of raw manufactured steel.
  • Surface Finish: Very smooth
  • Surface Finish for Function of Aesthetics: Function, this part must have very high tolerances to function correctly. The smooth finish allows the piston to move easily, with less friction, inside the cylinder.

Part: Engine Block

  • Function: To create a safe and controlled environment for the combustion to take place.
  • Does the Component help Perform Multiple Functions?: Yes, the component is used to hold many of the engine components together to help create a secure and efficient working engine.
  • Associated Flows: Translational energy of the piston. The engine block also receives mass from the carburetor in the form of air and gasoline.
  • Functional Environment: High heat and high pressure due to the fact that the combustion of the gasoline is happening inside the block and many moving parts are located inside of the engine block.
  • General Shape: Contains a cylindrical hole in the middle of the block, while the outside of the block contains many horizontal rows of metal spaced evenly apart that help dissipate heat more effectively while also reducing the engine blocks weight.
  • Notable Properties: The cylindrical hole inside the block needs to be milled to specific dimensions to have the piston fit precisely inside the engine block.
  • 1/2/3 Dimensional: 3 Dimensional
  • Dimensions: Approximately a 5x5 inch base while consistently getting smaller towards the top to about a 3.5x3.5 inch area.
  • Relation of Shape to Function: The shape of the cylindrical hole is designed to create minimal friction between itself and the piston. The outer casing is designed to dissipate heat and reduce weight.
  • Weight: Approximately 4 pounds
  • Material: The engine block is made out of steel.
  • Impact of Manufacturing Decisions on Geometry: The complex geometry of the engine block would cause the need for the block to be die casted. Then the cylindrical hole located on the bottom of the block would need to be milled out to a specific diameter.
  • Specific Material Property Necessary o Function: Strength, rigidity, durability, and ability to perform well in high heat environment.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic- the material chosen for the component was steel; which is a relatively cheap metal.
    • Economic- the need for the cylinder to create a good seal with the piston affects the efficiency of the engine which in turn is seen by the consumer.
    • Environmental – A more efficient engine results in fewer pollutants in the air.
  • Aesthetic Properties: The outside of the engine block looks like a heat sink. With various fins creating maximum surface area for this part.
  • Aesthetic Purpose: The cylinder block looks like a heat sink so that it can dissipate as much heat as possible.
  • Color and Why: Silver, this is the color of raw manufactured steel.
  • Surface Finish: Not overly smooth besides for where the cylinder is milled out inside the engine block.
  • Surface Finish for Function of Aesthetics: Function, this part must have very high tolerances to function correctly. The smooth finish allows the piston to move easily, with less friction, inside the cylinder. The outside of the engine block contains no purpose and therefore doesn’t need a surface finish for function of aesthetics.

Part: Spring Shaft

  • Function: Transmit rotational motion from the crankshaft to the head.
  • Does the Component help Perform Multiple Functions?: No, the single function of this component is to transmit rotational energy from the crankshaft to the rotating head of the weed wacker.
  • Associated Flows: Rotational Motion.
  • Functional Environment: Elevated heat due to friction, standard pressure, and high torque. The component is also in a very greasy environment because grease is used to reduce friction between the spring shaft and shaft casing.
  • General Shape: Long thin cylinder.
  • Notable Properties: The component must be long enough to connect the clutch shroud to the head of the weed wacker. It must also be eery flexible with excellent geometric memory (this is why this component is made out of a spring). The material used must also be strong enough to withstand the constant abuses it faces while in use.
  • 1/2/3 Dimensional: 3 Dimensional
  • Dimensions: Approximately 4 feet long 1 inch wide and 1 inch high
  • Relation of Shape to Function: Must be small enough to fit inside main shaft housing and long enough to connect the clutch shroud and the head of the weed wacker. In order to perform its function it was not necessary for the component to be in the shape of a cylinder, for example it could have performed it function just as well if it was a long thin rectangle.
  • Weight: Approximately 1 pound.
  • Material: Spring Steel
  • Impact of Manufacturing Decisions on Geometry: The shape of the spring shaft was partially determined by the manufacturing process that was used to make it. The manufacturer was able to make the spring shaft by drawing and a circular profile can easily be achieved through drawing.
  • Specific Material Property Necessary o Function: Elasticity of spring steel and strong enough to withstand the transfer of rotational motion from the crankshaft and the head of the weed wacker.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Societal - using a spring for the shaft allows for bent shaft, to minimize back strain for the user as opposed to that of a straight shaft.
    • Economical – typically bent shaft weed wackers cost less money than straight shaft weed wackers do saving the consumer money.
  • Aesthetic Properties: A collection of intertwined thin, long cylinders compose the spring shaft. It is approximately 4 feet long and runs the length of the main shaft.
  • Aesthetic Purpose: None, it is not visible when the product is assembled.
  • Color and Why: The spring shaft of our weed wacker is black because it has gone through extensive metal treatments and there has also been a considerable amount of grease staining.
  • Surface Finish: The spring shaft has a relatively smooth surface finish.
  • Surface Finish for Function of Aesthetics: The spring shaft is smooth for functional reasons. Since the spring shaft is smooth it can freely rotate inside the main shaft with minimal friction.

Part: Crankshaft

  • Function: Convert translation motion of the piston to rotational motion that is necessary to spin the head.
  • Does the Component help Perform Multiple Functions?: Yes, the converted rotational motion is used to both turn the head as well as rotate the flywheel, enabling the attached magnets to power the magnetos to create a spark in the cylinder.
  • Associated Flows: Translational and rotational motion.
  • Functional Environment: Elevated temperature due to proximity to combustion in cylinder as well as friction during rotation.
  • General Shape: Rod, with counter weight and offset pin at the end that attaches to the cylinder arm. The other end tapers slightly, and has a slot cut for a Woodruff Key.
  • Notable Properties: Some parts of the shaft have several axis of symmetry, as most of the shaft is simply a thin, uniform cylinder. One end is very irregular, due to how the counterweight must be formed to balance the moment caused by the connection to the moving piston.
  • 1/2/3 Dimensional: Three dimensional, as the basic geometry of the part is a long and narrow cylinder.
  • Dimensions: 6.25 inches long, with a radius ranging from .125 inches at the narrow end to .25 inches at the counterweight.
  • Relation of Shape to Function: The crankshaft rotates inside of the housing, so a cylindrical shape is necessary for this to occur at high speeds. The shape of the counterweight is such that its center of mass is positioned to negate the moment caused by the piston connection.
  • Weight: .5 pounds. Made out of steel, a dense material, so it is slightly heavier that other parts of similar size that are constructed out of aluminum.
  • Material: Steel, because high strength is necessary as the part is under a lot of torque during operation of the product. The added weight of steel over aluminum is a necessary trade off.
  • Impact of Manufacturing Decisions on Geometry: The part was mostly manufactured on a lathe, which is responsible for its cylindrical geometry. This is intentional because it is necessary for the operation of the product.
  • Specific Material Property Necessary to Function: The high strength of steel enables the shaft to withstand the high stress placed upon it during the operation of an internal combustion engine.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic - there are materials that are stronger than steel, but steel is much cheaper than the alternatives.
    • Societal - The placement of the counterweight reduces vibration in the engine, resulting in a much more comfortable experience for the user.
  • Aesthetic Properties: Fine finished steel, so very shiny. The counterweights are matte.
  • Aesthetic Purpose: There is no direct purpose for the surface aesthetics. However, the aesthetics of the shiny shaft are a result of the fine finish necessary to reduce friction in the engine.
  • Color and Why: Silver, because this part is not visible when the product is assembled, so it is left raw. Also, most coloring methods are not suited for the friction applied to the shaft.
  • Surface Finish: Fine steel finish, result of lathe formation.
  • Surface Finish for Function of Aesthetics: The fine finish is for function, as it is necessary to allow the shaft to rotate freely without unnecessary friction.

Part: Crankshaft Housing

  • Function: This part holds the crankshaft in place so that is can rotate freely. It also is necessary to connect the cyllinder to the rest of the product.
  • Does the Component help Perform Multiple Functions?: Yes. Its first and most important function is to hold and support the crankshaft, but it also provides a base to which several other parts of the are mounted.
  • Associated Flows: Rotational motion is transferred through the housing by the crankshaft. The housing itself, however, does not move in reation to the rest of the product during operation.
  • Functional Environment: Due to its close proximity to the cylinder, the housing is subject to heat radiated by the combustion. It is not directly affected by friction, as a bearing separates it from the crankshaft, but bearings do heat up during use, and this heat eventually reaches the housing.
  • General Shape: Very irregular, vaguely box shaped. There are several points at which other parts attach, resulting in a very irregular shape.
  • Notable Properties: Lightweight, due to its aluminum construction. Matte finish.
  • 1/2/3 Dimensional: Three dimensional, as it is a hollow part with moving parts inside.
  • Dimensions: 3.5 inches at the widest point, 3 inches tall.
  • Relation of Shape to Function: Several other parts attach to the housing, so it must be shaped in a way that allows those parts to be secured tightly.
  • Weight: .25 pounds, due to aluminum construction.
  • Material: Aluminum, as reduced weight is desired in this application, while high strength is not.
  • Impact of Manufacturing Decisions on Geometry: The product has a rough matte finish as a result of the die casting manufacturing process. This is acceptable because this part does not come in direct contact with other moving parts.
  • Specific Material Property Necessary to Function: Aluminum is stronger than most polymers, while remaining lighter than steel. this tradeoff is desired in a part that is relatively large but not under an excessive amount of stress.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Societal - Aluminum construction reduces weight, which creates a more comfortable experience for the user.
    • Economic - Aluminum is lightweight while remaining cheap. Complex polymers, some of which may be suitable for such applications, are much more expensive than aluminum.
  • Aesthetic Properties: Matte grey finish, caused by die casting process. Not painted or unnaturally colored in any way.
  • Aesthetic Purpose: There is no aesthetic purpose, as this part is not visible when the product is assembled. Therefore, expense is spared by not finishing to a higher grade or painting.
  • Color and Why: Dull grey color, as this is the color of raw aluminum. This is left raw because the part is not visible when the product is constructed, so there is no reason to paint it.
  • Surface Finish: Matte, rough, as a result of die casting process.
  • Surface Finish for Function of Aesthetics: Neither, this part does not need to be precise to function correctly, so it is left raw to cut manufacturing costs.

Whole Product Analysis

Our group conducted an entire product analysis and it can be seen if you visit the link Whole Product Analysis

Manufacturing Methods

Part: Main Casing

  • Manufacturing Method: Injection molding
  • Evidence: The casing was made utilizing injection molding which is evident through part lines and tapered edges.
  • Did Material Choice Impact Manufacturing: Yes, because it is much cheaper and easier to mold plastic than do anything else with it.
  • Did Shape Impact Manufacturing: Yes, because you needed a certain geometry that is easy (as well as chap) to make with injection molding.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Societal- The casing helps dress up the machine. It also protects the user from the heat of the engine.
    • Societal Using plastic makes the weed wacker lighter than using a material such as metal.
    • Economic- Plastic is cheaper to manufacture and buy than metal.

Part: Piston

  • Manufacturing Method: Die cast and milled
  • Evidence: The inside of the piston contains a complex geometry and a reset mark, while the outside is milled and is evident through the visible lines around the outside of the piston and the fact it has a very smooth finish.
  • Did Material Choice Impact Manufacturing: Yes, because steel can be molded in a die cast but still be strong enough once it’s retained its shape.
  • Did Shape Impact Manufacturing: Yes, the inner geometry is much too detailed to be milled out or machined out and the most economical and logical way is for it to be die cast, while the outside can be resized and smoothed by milling.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic – The piston was die cast to save the manufacturer money and cut down on waste.
    • Societal – The piston needed to be milled smooth and to a specific diameter to create a better engine efficiency which is seen by the consumer.
    • Environmental – A more efficient engine results in fewer pollutants in the air.

Part: Engine Block

  • Manufacturing Method: Die cast and milled
  • Evidence: The outside of the engine block contains reset marks, while the inside cylinder has a smooth finish and lines up and down it to show that is was milled.
  • Did Material Choice Impact Manufacturing: Yes, because steel can be molded in a die cast but still be strong enough once it’s retained its shape.
  • Did Shape Impact Manufacturing: Yes, because the engine block contains many detailed geometries which could possibly be machined out to achieve the shape but this process would be too costly and too lengthy. The fact that this piece can be made in a cast makes this product faster and cheaper to make
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic - The ability to cast this part makes it cheaper to produce meaning lower costs to the manufacturer and the consumer.
    • Societal – The piston needed to be milled smooth and to a specific diameter to create a better engine efficiency which is seen by the consumer.
    • Environmental – A more efficient engine results in fewer pollutants in the air.

Part: Spring Shaft

  • Manufacturing Method: The spring shaft was made by drawing and tempering steel. Springs are made by hot winding a heated wire around a mandrel, removing it immediately, and cooling it in oil. The spring must then be heat treated to make it strong and able to handle stress and return to its original shape. In the case of this component, multiple springs are then wound together for increased strength.
  • Evidence: When we cut the spring shaft Instructor Cormier gave us in half we discovered that it was made out of a collection of intertwined springs.
  • Did Material Choice Impact Manufacturing: Yes, the process described above is the best way to make a spring out of steel.
  • Did Shape Impact Manufacturing: Yes, to manipulate steel to a wire you must draw it. Solid steel that is milled into this shape would not be able to maintain the flexibility needed. Because of this the only way to make the spring shaft is the above process.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic- Drawing steel is much cheaper than trying to make the same part any other way.
    • Environmental-When creating a spring in this fashion there is less waste then in any other process.

Part: Crankshaft

  • Manufacturing Method: Lathed steel rod, die cast counterweight, milled Woodruff Key slot.
  • Evidence: The rod is very smooth and cylindrically symmetrical. The counterweight has a rough, matte finish and slight flashing marks. The Woodruff Key slot is cut very precisely, indicating that it was precision milled. Also, since the rest of the shaft was lathed, it would be impossible to die cast the slot.
  • Did Material Choice Impact Manufacturing: Yes, many factories are very well equipped to work with steel, so the precision geometry would not be excessively difficult to produce.
  • Did Shape Impact Manufacturing: Yes, the precision that is necessary for the shaft to rotate smoothly would be impossible to achieve through die casting, so lathing would be necessary. The Woodruff Key slot also had to be precise to prevent the key from falling out due to a loose fit. The counterweight, however, makes no contact with other surfaces, so the rough finish left by die casting was acceptable.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic - Lathing is much cheaper than five axis milling, which could also have been used to manufacture the part.
    • Economic - The counterweight is die cast instead of milled, as the better surface finish provided by milling is not necessary for function.
    • Societal - The fine finish provided by lathing reduces engine vibration, resulting in a more comfortable user experience.

Part: Crankshaft Housing

  • Manufacturing Method: This part was die cast.
  • Evidence: There are slight flashing marks along the edges of the part, and there are also small venting marks visible on some surfaces.
  • Did Material Choice Impact Manufacturing: Slightly. Polymers can be injection molded, which is very similar to die casting, but not completely identical. Aluminum must be die cast.
  • Did Shape Impact Manufacturing: Yes, the part does not have intricate surfaces or angles that are impossible to create with die casting. This is acceptable, because the part is able to function correctly without intricate geometry.
  • Influence of Global, Societal, Economic, and Environmental Factors:
    • Economic - Die casting is much cheaper to do on a large scale than milling, which reduced the production cost of the product.

Entire Product Manufacturing

Our group conducted an entire manufacturing analysis on our product and it can be seen if you visit the link Complete Product Manufacturing Analysis

Component Complexity

Whenever you are evaluating parts of a machine it is very important to know how complex the underling manufacturing processes are. However, this could be very difficult for the average user to ascertain. That is why we created a manufacturing complexity scale for every component in our 26 cc Craftsman weed wacker. The component complexity scale is a rating of the difficulty it takes to manufacture a given part. For the sake of simplicity we arbitrarily selected a scale of 1 to 4, as explained below.

In our scale a 1 was awarded to any part that was easily produced through a single manufacturing process. This kind of piece would include components that were made by forging, die cutting, die casting, or injection molding. A 2 was awarded to any piece that required two simple manufacturing processes, such as forging twice or forging and injection molding. On our scale any component that required a difficult process warranted a rating of 3; this included things like milling. Finally, any component that had to be made through one difficult process in conjuncture with other processes was classified as a 4. An example of this would be any piece that had to be die cast and milled.

Both the component functions and geometry also played huge roles in the rating each component received. A complex manufacturing process is needed to make complex geometry, so our scale inherently rates the parts with more intricate geometry as harder to manufacture than those without it. Similarly the more important functions are typically done by pieces that were either very complex or needed tighter tolerance, and those are the pieces that received scores of 3 or 4 on our scale.

Interaction Complexity Scale

Our interaction complexity scale utilizes a 1-3 rubric, with 1 being parts that interact with few other components and 3 being parts that work in concert with many other parts to create the desired outcome.

  • A 1 on our scale represents a component that has no significant interaction with any other parts. An example would be the plastic injection molding casing that covers the engine. This casing has no significant function in the generation of power, but it protects the engine and serves an aesthetic role.
  • A 2 on our scale is a component that has minor involvement with other parts of the weed wacker. It may be something that is only connected to a few other parts, but it still does is relatively important to the overall function of the weed wacker. An example of this would be a bearing. A bearing is a simple part that does not interact with many other parts – it is typically used to hold another component in place. Its primary functions are to reduce friction and maintain the alignment of other critical pieces. This means that while it is only connected to one other component, it is intrinsically necessary for the production of power.
  • A 3 on our scale is a component that is used in combination with many other parts, or one that is vital in a primary goal of the tool (such as creating power). An example of this type of part would be our piston. The piston is a central part of an engine – it creates the required compression and the subsequent power for the machine. The piston is connected to the crankshaft which helps move the piston through the cylinder. This is a simplified explanation for how important a piston is and how a piston is involved with many other parts of the engine; the goal here is merely to explain our scale.

Component Manufacturing Complexity Scale:

  • 1-Simple one process manufacturing
  • 2-Two simple process manufacturing
  • 3-One difficult process
  • 4-Combination of one difficult process and one simple

Interaction Complexity:

  • 1-No interaction between parts besides being connected
  • 2-Minor component involved with few moving parts to produce an outcome
  • 3-Major component involved with many moving parts to produce an outcome
Part: Main Casing
  • Complexity: The main casing has intricate detail on its inside and outside.
  • Complexity Scale: 1
  • Complexity of Interaction: It protects the engine from possible impacts and the consumer from the engine.
  • Interaction Scale: 1
Part: Piston:
  • Complexity: Multiple steps are needed to create the piston and to achieve its finish for an efficient part.
  • Complexity Scale: 4
  • Complexity of Interaction:  : The piston plays a major part in the creation of energy in the engine. This is done through the conversion of rotational motion to move the piston into translational motion to create the compression needed to move the crankshaft.
  • Interaction Scale: 3
Part: Engine Block
  • Complexity: There are multiple steps needed to create the engine block to be able to achieve its finish inside the cylinder to make it efficient and also the creation of the outside to dissipate heat.
  • Complexity Scale: 4
  • Complexity of Interaction: The Engine block is used to help contain the compression created from the piston pushing the gases into a confined area. The block needs to be able to withstand high pressures and high temperatures while moving and translating this compressed energy to help the crankshaft rotate.
  • Interaction Scale: 3
Part: Spring Shaft
  • Complexity: Multiple steps and layers needed to create the flexibility and strength required when manufacturing the spring shaft.
  • Complexity Scale: 4
  • Complexity of Interaction: Translates rotational energy from the crankshaft in the engine down to the head of the weed wacker.
  • Interaction Scale: 3
Part: Crankshaft
  • Complexity: The crankshaft is a simple design, however it takes multiple manufacturing processes to create the part.
  • Complexity Scale: 4
  • Complexity of Interaction: The crankshaft is a major component of the engine. It converts the translational energy of the piston and turns it into rotational energy.
  • Interaction Scale: 3
Part: Crankshaft Housing
  • Complexity: Complex design, its main purpose are to dissipate heat and protect crankshaft however it is made using only one manufacturing process.
  • Complexity Scale: 1
  • Complexity of Interaction: Protect the inner workings of the engine and help reduce heat.
  • Interaction Scale: 1
Entire Product Complexity

Our group analyzed the complexity of every component of the weed wacker and it can be seen by visiting Entire Product Complexity

Solid Modeling Assembly

Image 3.2 - Full Assembly

Image 3.2 - Full Assembly

Image 3.3 - Cylinder

Image 3.3 - Cylinder

Image 3.4 - Piston

Image 3.4 - Piston

Image 3.5 - Crankshaft

Image 3.5 - Crankshaft

For our solid model assembly we chose to do the most important features of our small gasoline engine. We believed these piece to be the cylinder, the crankshaft, and the piston. These three components were chosen because they account for the epicenter of energy transformation. The piston is housed in the cylinder, and the crankshaft is attached to the piston. The energy making process starts when the piston moves upwards in the cylinder using mechanical energy. The linear mechanical energy of the piston compresses the gas and air mixture, which is ignited by a timed spark created by the spark plug. The combination of the air/gas mixture and the spark results in combustion. This controlled combustion drives the piston downward through the cylinder which causes the crankshaft to turn. Some of the resultant energy forces the piston back up, and this cycle is repeated as long as the engine continues to run. This assembly is of paramount importance from an energy standpoint. Throughout this process, chemical energy is converted into mechanical energy, and ultimately into rotational energy.

As a group, we chose to utilize Autodesk Inventor Professional 2012 program to complete our solid model assembly. We considered three main factors: cost, previous use, and presentation. Autodesk student licenses are free to download and easy to use, especially in comparison with other 3D models such as ProEngineer Wildfire 5.0 (student edition), which costs upwards of $150 dollars. The second factor was our previous experience with this program. James Quirk has used this program before, and he found it very easy to re-acclimate himself to Autodesk Inventor. The final factor we dealt with revolved around the overall professionalism of the presentation. Autodesk Inventor is a neat and constructive device for presenting a solid model assembly – we felt it was the optimal way to show how the cylinder, the crankshaft, and the piston interact and work together.


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Engineering Analysis

When designing a weed wacker one of the most important things to consider is how long the weed wacker can run on a single tank of gasoline. In the following analysis we will discuss how many linear feet of grass we can trim on one full tank of gasoline.


Problem Statement:

Determine how many linear feet of grass can be cut with our 1991 26 cc Craftsman weed wacker on a single tank of gasoline.

Diagram:

Stupiddiagram.png Image 3.6 - The path traveled by the operator of the weed wacker.

Assumptions:

  • Assume that the drag on the head is negligible.
  • Assume that the weed whacker is acting on full throttle (head spins).
  • Assume that gasoline contains 1.3 x 108 J/gal.
  • Assume that gas/oil ratio is 50:1 so the presence of oil is negligible.
  • Efficiency of the weed wacker is 30%.
  • Fuel tank holds 0.1328125 gallons.
  • Assume that the strings are six inches long and the mass of the string is negligible.
  • There are no power losses in the weed wacker besides those in the engine.
  • Assume that the person walks in a straight line at a speed of 1mph (walking speed with a weed wacker).
  • Assume engine has 1.2 hp (from product manual #1).
  • The weed wacker will trim as fast as the user can walk.


Governing Equations:


Governingequationsg152001.png

Calculations:


Governingequationsg1h52.png

Solution Check:

Our units throughout the calculations make sense. We took Joules per gallon and multiplied by the number of gallons in our gas tank to discover how many joules of potential energy our weed wacker’s fuel tank contained. We then took that number and factored in the efficiency of our engine to find the total energy our engine produced. Using this in conjunction with the given power output of the weed wacker we are able to determine how many seconds it can run before using all its gasoline. The total time our weed wacker can run multiplied by the average walking speed of a person operating a weed wacker will tell us how many linear feet of grass can be cut on one tank of gasoline. The answer we found was in the units that we wanted (feet), and the number seems reasonable. You can run a weed wacker for a very long time without needing to fill it up.


Discussion:

Our estimate of 8465 linear ft of grass cut by our Craftsman weed wacker is an ideal case. It shows the upper bound of the amount of grass that could be cut on a single tank of gas. However, this number is probably not achievable in a real world setting because of the assumptions we made before solving the problem. A change in most of the assumptions will cause the amount of feet that could be cut by our product to decrease. If we were to factor in power loss due to slippage in the clutch or the drag force created by the blade we would see a substantial decline in the footage we were able to cut. Similarly, if a skilled operator could walk faster than 1 mile per hour we would see a material gain in the operating range. Then there are other factors – such as changing the assumption about the quality of gasoline – that could have either a positive or negative effect on the outcome.

Design Revisions

The product we are dissecting this semester is a 1991 26 cc Craftsman weed wacker. It was designed to be light-weight and easy to use, and its primary target is residential homeowners who perform their own lawn maintenance. While this product is well designed and engineered, it is not perfect; there are a multitude of design revisions that would make our weed wacker a more effective and economical lawn care tool.

Bent Shaft to Straight Shaft

The most prominent change that should be considered in a redesign of this product would be changing from the currently employed bent shaft to a straight driveshaft.

There is nothing inherently wrong with a bent shaft weed wacker, but it is not optimal for long term use. Bent shafts eliminate a transmission device, so they are less costly, but the resulting weed wackers do not last as long as their straight shaft counterparts. Moreover, they are less efficient in transferring engine torque because power is lost as the shaft experiences unwanted motion (in the form of flex) while it is rotating. A straight shaft is superior because it results in a more substantial connection between the clutch and the head of the weed wacker; since a straight shaft is a solid rod essentially all the energy that goes into the shaft is delivered to the head of the weed wacker.

While a straight shaft will cost more initially, the difference will be recouped in the long run. Apart from saving money on gasoline and oil (because it is a more efficient method of transferring rotation) a straight shaft machine will last much longer, and it is this increased lifespan that produces the real savings. Another by-product of switching to a straight shaft is the increased flexibility the tool would provide – the robustness of straight shaft design means many additional attachments are viable, and this enhanced capability should be factored into a cost/benefit analysis. On the downside, a straight shaft may be a bit harder to engineer ergonomically (it is far easier to adjust the degree of bend than it is to find an optimally oriented transmission), but the reality that a straight shaft is more economical and environmentally friendly outweighs that factor. A better tool that lasts longer and releases less pollution into the environment is a better design, and given the relatively modest cost one that the manufacturer of our product probably should have selected.

Four Factors Summary for straight shaft to bent shaft

  • Economic- More efficient energy transmission in a straight shaft than in a spring shaft.
  • Economic- A solid, straight shaft lasts longer than a spring shaft meaning the user has to replace the tool less often.
  • Environmental- When using a solid shaft there is less of an energy loss therefore the user has to use less gas to produce the same power, producing less harmful emissions.
  • Societal- A solid shaft weed wacker can use various attachments that a bent shaft weed wacker cannot making it more appealing consumers.


Second Piston Ring

When we discussed how we could redesign our product to maximize its performance we decided one of the first things we would look at would be the piston. Redesigning this component would only marginally increase manufacturing costs, but it could result in a drastic increase in the performance of our engine. Our piston currently has only one piston ring, but in our proposed redesign we would like to add an additional ring. By doing so we would increase the compression inside the motor, and as a result we would create a more efficient engine.

Adding a second piston ring would greatly enhance two key attributes of our weed wacker. There would be a substantial increase in horsepower, and at the same time the fuel efficiency of our engine would also increase - saving the consumer substantial money over the life of the product. These gains would be large enough to be noticed by a user, yet they would not be accompanied by a material increase in manufacturing cost; a groove for the second ring could easily be added during the same process that carves the groove for the first ring. This redesign would also positively affect the environment; by increasing fuel economy our enhanced machine would produce fewer emissions.

Summary of Four Factors for the second piston ring

  • Economic and Environmental- increased compression would lead to increased fuel economy which would save the user money and would expel less harmful emission into the environment.
  • Societal- The increase in power may make this product more attractive to potential buyers.


Grease Fittings

Transmissions generate a lot of heat and so they require copious amounts of grease to survive for a long service life. However, many people do not grease their weed wackers frequently enough because it is a time consuming and messy procedure. While typical users do not perform a rigorous analysis, their de facto decision is to replace worn out parts rather than perform routine maintenance on their machine. This leads us to believe that a very beneficial design change would be to include a grease fitting on the head of our weed wacker. A simple zerk type grease fitting would greatly simplify lubrication at a nominal cost. Grease fittings are one-way valves that provide a passageway for grease from a grease gun to the inside of a mechanical device. This allows grease to be forced into something like a transmission or bearing, and then when the parts warm up the grease becomes more fluid, allowing it to seep into where it is needed. Even if you ignore that the resulting machine is much more robust, the simplification of routine maintenance would greatly increase durability, making the inclusion of a grease fitting a prudent addition. The grease fittings would not be a costly add-on; you can buy them for under 2 dollars per fitting [3].

Summary of the Four Factor for Grease Fittings

  • Societal- They make it easier to maintain the weed wacker.
  • Economic- They help extend the life of the weed wacker because people will grease their weed wackers more often.


Automatic Oil Injection

One of the biggest detriments of any 2 cycle engine is that they release large amounts of pollutants into the environment. A unique feature of two cycle engines is that they do not have complex lubrication systems, instead of a reservoir, pump, and filter they require you to mix oil into the fuel. After this oil performs its lubrication duties it is combusted with the fuel. In practice, many people tend to add too much oil to the mixture (either intentionally to minimize engine wear or through carelessness), and that extra oil is also burned and emitted into the environment. This is why weed wackers, (and two-stroke engines as a whole) come under scrutiny as “dirty” machines. For every person who adds excess oil to weed wackers there are counters who don’t enough (or any). This is also a major dilemma for manufacturers because not adding enough oil to a weed wacker can drastically effect the lifetime of the tool.

Taking this into consideration, we believe a good design change for our weed wacker would be to add a subsystem that automatically adds the correct amount of oil to the intake charged – the need to pre-mix gas and oil is eliminated, instead the operator only has to periodically replenish an oil tank. By incorporating this type of system to our weed wacker our engine would use substantially less oil than it currently does while maintaining performance. This would also prevent catastrophic damage resulting from users not adding sufficient oil to their weed wacker.

Implementing this system would be beneficial for the environment as well as the consumer in a variety of ways. Clearly the most prominent of these benefits is that a weed wacker with an Autolub system (a name brand automatic oil injection system for motorcycles) would burn much less oil and therefore produce less harmful emissions. Although it would add to the initial cost of the product another major advantage would be that it would be more economical because users would not need to spend as much money on oil each year; instead of having to mix oil in with their gas on every fill up they would probably only have to fill their weed wacker with oil once every season. There is also the non-financial advantage of simplifying maintenance and operations – a tool that is easier to use is always attractive to consumers.

Summary of the Four Factors for Automatic Oil Injection

  • Societal- It makes it easier to use the weed wacker because you no longer need to add oil to the gasoline.
  • Economic- It only adds oil when you need it, and it does not burn as much oil. Which means that you need to add oil much less frequently.
  • Environmental- It makes it so that you do not burn oil as often. The engine runs much cleaner, and produces less harmful emissions.

Sources

[1]
"Craftsman Model 358.797121 Grass Trimmer Operator’s Manual | Weed Eater World Manuals." Weed Eater World. Web. 09 Oct. 2011. <http://www.weedeaterworld.com/Manuals/craftsman-manuals/craftsman-model-358-797121-grass-trimmer-operators-manual/


[2]
“Autolub INC” 11/16/2011 <http://www.autolub.com/products.html>


[3]
"Marshall's Industrial Hardware" 2011 <http://www.marshallshardware.com/products/productList.aspx?uid=2-583>