Group 25 - Snow Blower (Gasoline Powered)/Gate3

Revision as of 14:28, 16 November 2012

Purpose

The purpose of the following section of the project is to analyze the individual components that made up the product and to gather and understanding of how they operate and how they were made. The seven components that were analyzed in heavy detail were selected by their relative importance toward the snow blower as a whole. Things that were considered in order to hypothesize the manufacturing process were the shape, color, material, and particular indication marks. The following is what was concluded:

Project Management: Coordination Review

All members of the group are performing very well with each other. There is no need or concern for corrective action as everyone is pulling their own weight as far as showing up for meetings and fulfilling the tasks of which they are given. Everyone showed up on time to disassemble the snow blower and worked as a group so each person has a relatively good idea about how each of the components operates and interacts with other components. The only difficulty is that some members that have work outside would sometime hinder the ability of all members to gather at once so in response the group meetings would be scheduled at times of convenience for all. Sometimes only a some of the members were able to meet at any one given time.

Product Archaeology:Product Evaluation

Component Summary:

#	Component (# used)	Image	Function	Material	Part #	Manufacturing process
1	Gas Tank(x1)		Stores Fuel	Plastic	N/A	Injection Molding
2	Carburetor(x1)		Mixes fuel and air	Various metals Rubber Plastic	N/A	Die Casting
3	Valve stem(x2)		Allows air/fuel to enter cylinder	Steel	N/A	Die Casting
4	Spring retainer(x4)		Connects spring and valve stem	Steel	N/A	Stamping
5	Lifter(x2)		Lifts valve stem	Steel	N/A	Die Casting
6	Camshaft(x1)		Lifts the lifter	Steel	N/A	Forging&Machining
7	Piston(x1)		Change chemical energy to physical energy	Aluminum	N/A	Die Casting&Machining
8	Piston Rings(x3)		Seal off combustion chamber	Steel	N/A	Die Casting&Machining
9	Crankshaft(x1)		Transfers linear force to rotational torque	steel		Forging&Machining
10	Flywheel(x1)		Store rotational energy	Steel	N/A	Die Casting
11	Pull start reciever(x1)		Transfers force from pull start to crank shaft	Aluminum	N/A	Deep drawing
12	Spark Distributor(x1)		Create Electricity	Various metals Plastic Rubber	N/A	Stamping
13	Spark Plug(x1)		Create spark	Various metals Plastic	N/A	Injection Molding&Die cast
14	Cylinder Head(x1)		Closes combustion chamber/holds spark plug	Aluminum	N/A	Die cast
15	Muffler(x1)		Reduces engine noise	Aluminum	N/A	Stamping
16	Engine Body (x1)(crankshaft and connecting rod visible in picture)		Houses engine component	Aluminum		Die Casting&Machining
17	Pull start(x1)		Provides initial torque to start engine	Nylon Steel Plastic	N/A	Stamping&Die Casting
18	Electric Starter(x1)		provides initial torque to start engine	Various metals Plastic Rubber		Injection Molding&Die Casting
19	Pulley(xN)		Control tension in belt/drive belt	Steel	N/A	Die Casting
20	Wheels(x2)		Converts torque to linear motion	Rubber Steel	N/A	Molding&DieCasting
21	Drive shaft(xN)		Transfer torque	Steel	N/A	Die Casting
22	Belt(x2)		Transfers torque between two pulleys	Rubber	N/A	Die Casting&Molding
23	Retaining ring(x2)		Helps stabilize wheel	Steel	N/A	Die Casting
24	Retaining pin(x2)		Transfers torque from shaft to wheel	Steel	N/A	Stamping
25	Bushing(x2)		Supports drive shaft	Bronze	N/A	Die Casting&Machining
26	Roller bearing(x1)		Reduces friction in adjacent rotating parts	Steel	N/A	Stamping&Forging
27	Handle(x2)		Gives user directional control	Steel	N/A	Drawing&Stamping
28	Cable(x4)		Takes user input and moves corresponding components	Steel	N/A	Drawing
29	Key(x6)		Transfers torque between a shaft and a hole	Steel	N/A	Extrusion
30	Fuel line(x1)		Transports fuel	Rubber	N/A	Extrusion
31	Oil filler spout(x1)		Allows user to monitor oil level and add if needed	Plastic	N/A	Injection molding
32	Fuel shut off(x1)		Allows or restricts fuel flow in fuel line	Plastic	N/A	Injection molding
33	Axle(x3)		Transfer torque	Steel	N/A	Drawing
34	Worm gear(x1)		Transmits torque to a perpendicular shaft and increases torque	Steel	N/A	Thread rolling
35	Helical gear(x1)		Transmits torque from another gear to an axle	Steel	N/A	Gear hobbing
36	Spur gear(x8)		Transmits torque from another gear to a shaft	Steel	N/A	Die Casting
37	Gear box housing(x2)		Houses worm and helical gear	Aluminum	N/A	Die casting
38	Snow Blower housing(xN)		Provides a base for/protects the snow blowers components	Steel	N/A	Rolling
39	Impeller(x1)		Takes snow from the auger and ejects it through the chute	Steel	N/A	Rolling&Stamping
40	The Auger(x2)		Chops up and pulls snow back into the snow blower	Steel	N/A	Rolling&Stamping
41	Chute(x1)		Directs snow up and out of the snow blower	Steel	N/A	Rolling&Stamping
42	Driving wheel(x1)		Transmits torque from drive shaft plate to axle	Steel Rubber	N/A	Die casting
43	Knob(x2)		Covers levers on control panel for a better grip by the user	Plastic	N/A	Injection molding
44	Auger bushing(x2)		Supports auger axle	Steel	N/A	Die casting&Machining
45	Auger bushing plates(x4)		Fastens auger bushings to snow blower housing	Steel	N/A	Rolling&Stamping
46	Chain(x2)		Transmits torque between two spur gears	Steel	N/A	Rolling&Stamping
47	Bolt(x148)		Fastens two or more components	Steel	N/A	Rolling,Forging&Thread Rolling
48	Hex nut(x71)		Threads onto bolt to sandwich components in between	Steel	N/A	Extrusion,Sawing&Machining
49	Washer(x48)		Distribute the pressure of the bolt on the surface of the component being fastened	Steel	N/A	Stamping
50	Screw(x21)		Fastens two or more components	Steel	N/A	Rolling,Forging&Thread Rolling

Product Analysis

Product Complexity

• Shape Complexity Scale
  

Simple:A component that performs a single or inconsequential function. It has a basic design. It does not require any special material properties. It does not require any special manufacturing process. (i.e. die cast, injection molding, forging, etc...)

Semi-complex:A component with one or more functions that may have a moderately complex design (i.e. geometry, features, etc.). It may also have a moderately complex manufacture process (i.e. it can be made by CNC machines or additive process, but does not have to be).

Complex:A component that performs one or more critical functions. It will have sophisticated geometry and features. It requires a more complex manufacture process that may be unique to that specific component and it requires special materials.

• Interaction Complexity Scale
  

Simple:Consists of a single interaction to perform a single function (physical, electrical, etc.).

Semi-complex:Consists of one or more interactions between multiple components. May perform more than one function (physical, electrical, etc.).

Complex:Consists of one or more interactions between multiple components that requires specific component details for the interaction to work. The interaction may perform several functions.

Driving Wheel

The Driving Wheel was selected as one of the main components because of its importance to the driving unit and the fact that it accepts the most of the user signals within the driving unit.

• Component Function
  

Wheel that receives power from the fly wheel axle and can be moved side to side by the Transmission to adjust the speed. The Driving wheel is essentially the transmission of the snow blower as it uses the incoming rotational energy from the motor and adjusts the rpms and even the direction of the energy it passes out. The power comes from the motor and through a shaft with a metal plate on the end of it; what the driving wheel does is it is pressed against this plate perpendicularly and rolls along with the plate changing the direction of the power by 90 degrees. Where the interesting part comes from is that this Driving wheel can be altered as to the location of where it is being pressed against the plate, and as a result changing the rate of the power it puts out. If the user shifts into reverse the Driving wheel will translate to the other side of the plate and that will change the direction of the power by 180 degrees. For this part the inputs are signals from the user in the form translational kinetic energy to move it side to side and rotational kinetic energy from the motor. The outputs are rotational kinetic energy. It works in a snowy environment.

• Component Form

Since this part needs to transfer and alter rotational energy it needs to be a circular disk. It is made of what appears to be steel and the rather abstract shape indicates that is was cast in a die. On the circumference of the Wheel lies a hard rubber which is pressed against the adjacent plate. The reason for rubber is that it has a much larger coefficient of friction and ensures that the spinning plate will cause this wheel to spin. The Driving wheel does not have any special aesthetic work done to it as it lies inside of an enclosed unit. Its only purpose is to function, not to look appealing. This can be seen by the fact that it is just a cast piece of metal, no polishing or grinding was done to the metal or the hard rubber.

• Manufacturing Methods

The rather abstract design of the wheel and the fact it is made of metal indicate that it was most likely cast in a die. The seam that would normally be left behind cannot be seen as the hard rubber is covering where it would be. The hard rubber circumference was most likely melted and allowed to cool on the surface that it currently resides, or in other words it was cast. Rubber was used because the component needs a high coefficient of friction; the metal was used because the part needed to be strong and abrasive. Economically steel and rubber and the casting process are relatively cheap and as such this part could be made easily for not that high of a cost.

• Component Complexity

The Driving wheel itself is not very complex, looking at it from an engineering perspective or as someone who designs objects with a modeling program can see that it is just a wheel. Even the method of manufacturing, its geometry and material are all very common and easy to come by. Its function though is actually rather complex. The fact that the wheel can translate along its axle yet because of the axle shape the wheel must always spin with it. So it has the ability to spin with the axle and at the same time with the user input it can translate along it.

Wheels

The tire was chosen as one of the important components of the snow blower beacause it is required for the snow blower to be moved. It is also needed for the snow blower to keep its traction to the floor in the wet conditions.

• Component Function
  

The main function of the tires is to allow the snow blower to be moved easily from one location to another. Another function that the tires help to perform is to provide traction for the snow blower in the wet and slippery environment that it is meant to be used in. The grove pattern that is seen all around the wheel surface allows it to make direct contact with the ground as well as increasing the coefficient of friction which will allow more traction. The tires are connected by an axle that converts the rotational energy from the driving gear system to a linear motion along the ground.

• Component Form

The tires are in a cylindrical shape with perfect symmetry when cut in half. It is primarily three dimensional. The cylindrical shape of the tires couples the function that its suppose to perform very well because this shape allows the user to push the snow blower from point A to point B with ease. If it was any other shape other than a cylinder, a square or hexagon for example, it will be very difficult to move. The diameter of both tires is also identical which allows it to easily move in a linear direction. If the dimensions were different, the snow blower will tilt and shift to the side with the smaller dimensions.

The tires are made from solid rubber and steel. The frictional properties of rubber are needed for this component to function. This is because rubber generally has a high static friction on concrete which will allow it to have good traction and also prevent it from slipping. The ridged pattern in the rubber wheel will increase the static friction allowing it to have even more traction. If another material was used, wood, plastic or metal for example, it will have less of a frictional force which will allow the snow blower to slip under the wet and icy conditions. The rigidness of the steel rim is also needed for it to function. This is important because the tires are on the floor with all the rocks and gravel that might be kicked up while it is in motion and if it is not hard and rigid like steel it will break easily.

Global factor: Rubber and steel are widely accessible worldwide.
Social factor: Since the tire is made of solid rubber with steel rims, the user will not have to deal with popped tires. The solid rubber tires also allow for low maintenance.
Economic factor: The cost of rubber and steel are relatively cheap so mass production the tire will be cheap. The tires do not serve any aesthetic purpose. The tires are black because these rubber tires contain carbon black. This is used in manufacturing the tires because it improves wear resistance and wet traction. The surface of the tires is rough. This is for functional reasons because the roughness of the tires will cause it to have more traction in the snow.

• Manufacturing Methods

http://www.youtube.com/watch?v=hG_d7H5XuYs
The tire has two components that have to be manufactured separately, the inner steel rim and the outer solid rubber. The inner steel rim is manufactured by forging which is evident due to its smooth surface. The material or the shape of the rim did not impact this decision because it could have been made by many other methods like die casting, investment casting, etc. But by choosing to forge the rim instead allows it to be a lot harder and sturdier. This will allow it to last a lot longer in the harsh weather conditions that the snow blower is meant for. As seen in the video link above, three processes were used to make the solid tire: extrusion, rolling, and die casting. I think that the material choice did impact this decision because I believe that this would be the simplest way to make a solid rubber tire. Another reason why I think that this was the way that the rubber was manufactured is because you would need the rubber to fill in the molds to form the groves that gives it more traction.

• Component Complexity

The function of the the tire is not very complex as its purpose is to take the rotational power given to it and apply it to the ground in the form of translational power. The manufacturing process of the solid rubber tire however is actually quite in depth because it requires several processes.

Muffler

-The muffler was chosen due to the consideration of its impact upon the user in terms of sound.

• Component Function
  

The general purpose of the muffler/exhaust pipe is to channel the residual heat and fumes from the engine block out and away from the snow blower and dampen the noise level. At the end of the pipe is a small box with holes in it to try and reduce the sound. The function process involved with it is heat and exhaust and sound enter the muffler pipe from the engine and they all leave through the holes of the muffler box. The muffler reduces the sound to quite an extent even though when in use the snow blower is still loud. This particular component can be used in almost any type of environment.

• Component Form
  

The exhaust pipe is short and straight with a small, rectangular box on the end of it; very 3 dimensional. The pipe itself is ideal for channeling material in a certain direction, but the box on the end however is necessary because of the noise dampening material that is held inside. Because this does not need to be very strong or corrosion resistant it was made out of a lightweight metal and sits at about a ½ lb. The metal feels like it could be Aluminum, which fulfills its requirements of lightweight yet relatively tough so that the muffler wouldn’t break off easily if ever bumped into. Aluminum is also very cheap so it could be manufactured and purchased without much concern. Because it lies on the outside and is visible the part was given a smooth surface, not polished but smoothed out. Also since this piece gets very hot it was not painted and so it has the light silver color of aluminum which is relatively appealing.

• Manufacturing Process
  

The box on the end of the short pipe is made of two pieces that were die casted and are designed with small clamp fits around the outside so that they can snap together and hold in place. Die casting is hypothesized by the abstract geometry of the pieces and because it is a relatively cheap and common way to work with aluminum. There are also seams around the outside of the pieces and small circles on the inside from the presser which indicate casting.

• Component Complexity
  

The muffler is not very complex for it is simply a metal box on the end of a small pipe. It does not interact with any other parts for it is not a moving part.

Worm Gear

• Component Function
  

The worm gear serves to translate the flow of rotational energy in the auger shaft 90 degrees to the standard gear in the auger differential and consequentially the auger axle. It is necessary for the direction of rotational energy flow to change so that the snow in the front of the snow blower can be thrown backwards into the impeller housing and impeller blades. The second function of the worm gear, while not as prominent but just as crucial, is to gear down the rpm’s of the engine crankshaft and auger shaft to a more manageable speed for the auger axle and blades. The rpm speed of the engine is far too fast for the auger blades to handle. In order for the snow blower to be socially acceptable (i.e. safe) and not a machine capable of ripping a man’s arm off, the auger axle needs to be slowed down. The worm gear operates in a very hot and moist environment within the auger differential because the interacting gears are spinning at high speeds causing lots of friction and the differential is filled with heat absorbing lubricant.

• Component Form

The worm gear is a 3-dimensional, axis-symmetrical shaft with deep spiraling channels. The component is the shape of a rod so that it can effectively channel rotational energy. It also has deep spiraling channels cut into it’s axle so that the linear motion of the thread in relation to the axle can drive the teeth of the standard gear, which happen to act perpendicular to the worm gear axle. The worm gear is made from steel because it needs to be strong under high temperatures and high stress. Steel is a socially favorable choice of material because it is durable and ensures the safety of the product in the eyes of the consumer. The worm gear does not have an aesthetic purpose because it is hidden away inside the auger differential which is not intended to be opened often. The worm gear has a smooth surface finish so that friction is limited and rotational work lost due heat of friction is limited.

• Manufacturing Methods

The worm gear was made by a method known as Infeed Worm Rolling (http://www.youtube.com/watch?v=P89Y4Ht7SCs ). Infeed Worm Rolling is really the only method in which a quality worm gear can be made quickly and efficiently. The method, which involves two large shaping wheels pressing against a steel shaft and cutting out spiral channels, only takes about 18 seconds and produces a worm gear with virtually no imperfections. Very little waste metal is created in the process as well, making it environmentally preferable. The steel worm gear really couldn’t be produced as well with any other method. Infeed Worm Rolling, while only capable of manufacturing worm gears, can produce worm gears in a cheap, quick and quality manner; making it the most economically practical method. It is also socially practical because it is quick and simple, and thus will not hold up production of the snow blower.

• Component Complexity

The worm gear is complex in terms of it's form because the method of manufacture (Infeed Worm Rolling) is unique to that type of gear (although variations of rolling can be used to create the thread on screws and bolts). The worm gear is semi-complex in terms of interactions because it performs two very important functions; that is translating torque at a ninety degree angle and gearing down the rpm's of the engine.

Electrical Start

The electric starter is not a necessary part for the snow blower to function normally and this is exactly why it was picked for further analysis. The electric starter was put on our snow blower as an extra, easier mode of starting the engine. Using the electric starter as opposed to the pull start gives the user the option for a less strenuous and more intuitive method for starting the snow blowers engine. It widens the range of users that will be able to operate the snow blower because you no longer need the moderate strength it takes to use the pull start.

• Component Function
  

The function of the electric starter is to provide an alternate way to start the snow blowers engine. This component does not help provide multiple functions. The electric starter requires an input of electrical energy via the plug on the top of the engine and user input to depress the engine start button. The output is waste heat due to electrical resistance and rotational, mechanical energy that spins the flywheel. The environment this component will be operating in can potentially exceed 200°F due to the heat of the engine.

• Component Form

This component is cylindrical in shape and has a wire running from it to a rectangular shaped receiver. The high rotational speeds this component will be achieving means it must be axis-symmetric. This is the primary reason for its shape. This component is composed of plastic, copper, aluminum, and rubber. When this component was manufactured the process used were impacted by the selection of the materials. Because there are so many parts in the electric start I will be focusing on the plastic body of it. The plastic body of the electric starter was chosen for its high electrical insulation properties. The type of plastic chosen was based on the high heats (relative to the melting temperature of most polymers) that this component has to withstand. The insulating plastic cover was influenced by the safety standards of societies it is used in. The cover also protects the insides of the electric starter from the outside environment. The esthetic properties of this component are pretty limited to the colors chosen for its plastic and rubber parts. This component does not have an esthetic purpose. The body of the component is black with some exposed aluminum. The plug portion of the component is off white with a black button. The color of the plug portion was chosen so it would stand out against the red engine covers and the black button on top stands out against the plug. The outside surface of the component is mostly smooth except for the exposed aluminum which is slightly rough to the touch. The surface finishes are esthetic and do not contribute to the functionality of the component.

• Manufacturing Methods

To plastic body of the engine starter was manufactured using injection molding. There are small raised seams where the two halves of the mold came together. Injection molding was chosen because the body of the engine starter is plastic and the shape is not very complex. The shape did not impact the manufacturing method very much because this part could have easily been made in a lathe because of its symmetry. Injection molding was chosen over other manufacturing processes because for large production it is far less expensive. Injection molding does require and industrial electricity supply so it must be produced in a developed society with industrial electricity available.

• Component Complexity

Electrical Motor had three shapes for each part

• Motorbox:Motor box had simple shape of rectangular box.
• lead:lead was 16 inch wire rapped with rubber.
• plug:Plug was rectangular box with plug and another rectangular box shaped grap.

Cam Shaft

• Component Function
  

• Component Form

• Manufacturing Methods

• Component Complexity

Auger Blades

• Component Function
  

The auger blades sole function is to physically import snow from the external environment and throw it into the path of the impeller. The auger blades play host to energy flow in the form of translating rotational energy from the auger axle into linear energy in the flying snow. The auger blades also conduct mass transfer in the form of importing snow from the external environment and exporting it to the impeller. The auger blades function partially outside the auger housing and partially inside. This allows them to serve as mediator of snow between the outside environment and the internal impeller.

• Component Form

The auger blades are a pair of 3-D spiraling blades. The blades spiraling shape coupled with it’s rotational motion acts to feed the snow into the back of the impeller housing. The auger blade is made of steel so as to ensure that it can maintain integrity and performance in low temperatures. Steel is a widely available material and is cheaper than other potential materials that meet the material requirements. The auger blades are painted white so as to be aesthetically pleasing because they are visible to the user. They are painted with a more durable paint than the typical household grade so the paint does not come off under high velocities and low temperatures. There is no functional purpose to the auger blade paint and finish; as the act of feeding snow does not require it.

• Manufacturing Methods

The auger blades were fabricated by investment casting. They are metal and thus could not be produced by injection molding. Additive processes are not typically compatible with metal materials and are not economical in the case of the auger blade. The auger blades have no parting lines and have a geometry that would not be able to be recreated by rolling, forging, extruding, or die cast methods. The geometry is also too variable to make milling, or any other subtractive process an option.

• Component Complexity

Solid Modeled Assembly

Engineering Analysis

In choosing the dimensions of the worm gear, an engineer might use the analysis process to determine an efficient worm gear design that can translate enough torque from the auger drive shaft to the auger axle and blades so that it can chew through heavier snow without sacrificing too many rpm's. Design implications include the worm gear thread lead, the worm gear radius, the radius of the helical gear and the kinetic coefficient of friction between the worm gear and helical gear. A worm gear that has too large a radius will have greater speed (rpm's) but may not have enough torque to make it through thick snow. A worm gear with a smaller radius will convert plenty of foot pounds but may not spin very fast.

purpose

Determine the effect of the worm gear radius on the torque conversion ration.

diagram

Assumptions

• The worm gear thread lead (l) is equal to 0.375"
• The normal pressure angle (α_n) is equal to 20 degrees
• The kinetic coefficient of friction between the worm gear and helical gear (μ) is 0.57. (Steel on Steel)
• The input torque of the auger drive shaft is 50 ft*lb. (M_1)
• The radius of the helical gear (r_2) is 1.5"
• The output torque of the worm wheel (M_2)
• The diameter of the worm gear (d_1)
• The diameter of the worm wheel (d_2)
• The worm lead angle (Y)

Governing Equations

M2 = (M1 * d2/d1)*[cos(an – utanY) / (cos(an) * tan(Y+u))]
Rg = (# teeth on wheel) / (# teeth on worm)

Design Revision

1.
Snow blowers are known to be very loud and quite irritating to both the user and others in the proximity while they are in use and the major reason for that is the muffler. A possible fix for this would be to have the snow blower built with a better (more sound suppressant) muffler. Briggs and Stratton have several models in the categories of Lo-Tone and Super Lo-Tone that range in prices that are relatively more expensive than the stock muffler. While this will not be beneficial towards the Economic standpoint of the revision, but it will be much better in terms of the societal factor. This is guaranteed since significant suppressant of the extreme noise will not only increase customer satisfaction, but it will also impress those within the proximity and may incline them to purchase this improved snow blower as well. This addition would not cause any extra strain on the product and therefor would not demand an increase of serviceability. While this improvement would see good sales in America for various reasons, overseas money is more tight and people may not find it necessary to pay extra for a little sound so Globally this addition may only see true success in America.

2.
One possible design revision for the snow blower is the replacement of the two solid rubber tires with two air filled tires. This implementation would not only reduce the overall weight of the product but would also reduce the cost as there would be less rubber needed. The only issue with this is that the air filled tires can go flat or may puncture as where the solid would not. Even though the customer may have to fill air or replace the tire every once in a while the fact is that the overall price would be less and maneuverability would increase due to less weight. On a Global factor the altitude of the location of use may require a stronger tire seal and more or less air since the atmospheric pressure changes for different locations. Societally speaking the reduction of weight and increase of maneuverability would increase customer satisfaction, while the requirement of more frequent serviceability may harm satisfaction. Economically change would be in favor since with less material there would be less cost and with less weight the product could be shipped at a lower cost per unit. Since it is a small adjustment a change in the assembly process would be minimal to none. The only features changing on the entire product would be subtraction material from the two tires.

3.
Another possible design alteration or the Eager 1 Snow Blower would be the replacement of the current plastic gas tank with that of a metal one. The current plastic tank is susceptible to melting due to extreme heat which with its current location on top of the motor that gets extremely hot, could cause a possible hazard if it were to melt. There are many metals available in the current market that are just as light weight as plastic per unit mass and have a much higher melting point so to eliminate the issue at hand. One possible material would be aluminum, which is extremely light weight, fire resistant and yet it is tough enough to be just as durable as the plastic. Aluminum is quite cheap in America however other parts of the world it may be more expensive including tax so different parts of the world might find this revision unfavorable. Aluminum currently sits at a refined price of $0.89 per pound while plastic is harder to determine as there are many different forms of plastic, but the average of most is about $1.23. This means that an aluminum gas tank would be less expensive and would make the product cheaper. Environmentally aluminum can be recycled at any time as where thermosetting plastics cannot be recycled, which is what the current gas tank is thought to be made of.

References

http://www.youtube.com/watch?v=P89Y4Ht7SCs
http://www.briggsandstratton.com/support/frequently-asked-questions/Is%20there%20a%20silencer%20available%20for%20my%20engine/
http://www.infomine.com/investment/metal-prices/tin/
http://www.roymech.co.uk/Useful_Tables/Drive/Worm_Gears.html
http://www.technologystudent.com/gears1/worm1.htm
http://www.roymech.co.uk/images9/gear_worm.gif