EML 4501: Mechanical Systems Design



EML 4501: Mechanical Systems Design

Department of Mechanical and Aerospace Engineering

University of Florida

1st Design Report: Tire Pressure Gage

Prepared By: Sarah Fisk

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Abstract

This report involves evaluating the design of a linear tire pressure gage. Costs, time, materials, design features, tolerances and assembly order are among the things considered and discussed in this report.

TABLE OF CONTENTS

INTRODUCTION 3

OPERATION 3

PARTS AND DESIGN FEATURES 4

Tube 4

Plunger 6

Plunger Washer 7

End Cap 7

End Cap Washer 8

Spring 8

Head Piece 9

Rubber O-ring 10

Ruler 10

Pocket Clip 11

Orifice Plate 11

DESIGN COMPARISONS 13

ASSEMBLY ORDER AND TIME 14

MATERIALS AND PROCESSING 16

DIMENSIONS AND TOLERANCES 16

COST ANALYSIS 18

UNCERTAINTY ANALYSIS 18

APPENDIX 19

A: Part Drawings

B: Dimension Tolerance Calculations

C: Cost Analysis Calculations

D: Uncertainty Analysis Calculations

E: Resources

INTRODUCTION

The purpose of this project is for the student to gain experience in thinking about design considerations for mechanical assemblies. A s923 straight chuck pencil tire gage made by Milton Industries, Inc is examined in this project. Tire pressure gages have not changed much since the patent was submitted in 1956. It is a simple machine with simple principles so the majority of design work done on pressure gages today is probably aimed at making assembly easier and shorter. This purpose of this report is to detail the principles and methods behind reverse engineering higher end tire pressure gauge.

Interference fits are crucial in the function of this tire gage. Some of these interference fits include: the fit between the plunger and the inner side of the tube, the interference fits between the ribbed fins and the ruler, the interference between the rounded part of the ruler and the end cap, and the interference between the end cap and the rolled over edge of the tube.

OPERATION

This tire gage is for radial and conventional tires and measures pressures from 5 to 50 psi in one pound units.

When the tire pressure gage is used the rubber o-ring provides a seal between the head of the gage and the Schrader valve. The pin in the head of the gage releases the Schrader valve and allows air to flow through the small hole in the head of the gage. The air travels through a short chamber and then the orifice plate restricts the flow to provide the appropriate hydrostatic pressure. The orifice plate guides the air flow to the inside of the fins of the plunger, which creates a seal between the tube and the plunger so that air does not escape through the tube. The washer that is mounted on the nose of the plunger then presses against the spring as the nose of the plunger presses against the ruler. The spring provides the necessary counter balance to the tire pressure so that the ruler doesn’t simply go out as far as it can. The washer mounted flush with the end cap keeps that end of the spring in place. When the pressure is released, the interference fit of the end cap and the ruler keeps the ruler in place to give the user time to read the amount indicated, while the plunger and spring return back to their undisturbed position.

PARTS AND DESIGN FEATURES

A picture of the entire assembly is shown in Figure 1 and Figure 2 below. A drawing of this assembly is presented in Appendix A.1.

Figure 1: Tire Pressure Gage as Packaged

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Figure 2: The Pressure Gage with Ruler Extended

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Tube

The cylindrical shell forms the primary body of the gauge. It is shown in Figure 3 below. A drawing of the tube is presented in Appendix A.2

Figure 3: Pressure Gage Tube

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It must retain the pressure of the tire, and its inner diameter dictates the area and thus the force that the spring inside experiences. The cylindrical shell must also endure rough treatment inside a car or tool box, two likely places it will be found. One end of the tire gage is threaded, to screw on to the head of the gage. This end is shown in Figure 4 below.

Figure 4: Threaded End of Pressure Gage Tube

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The chrome plating serves to make the object more attractive to the customer. The chroming of the body also makes it durable against the elements, salted roads, and other rust-causing agents. The tube is rolled on the bottom edge to keep the end cap inside; this end is shown if Figure 5 below.

Figure 5: Rolled Edge of Pressure Gage Tube

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Plunger

The plunger catches the air pressure from the tire and pushes the spring a ruler to indicate the tire pressure. This plunger is shown in Figure 6 below. A drawing of the plunger is presented in Appendix A.3.

Figure 6: Plunger with Washer

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There are 6 wells on the plunger, most likely for grease. The grease is probably wiped on during assembly. The wells make this an easier process, most likely. The plunger is greased to allow it to slide along the casing more smoothly.

The diameter is slightly larger at the end of the fin, which provides an interference fit. This increase in diameter shouldn’t be so large as to provide too much frictional force.

The slot in the end of the plunger: If plungers are boxed together, this may keep them from getting lodged front-to-end in each other. It also provides a channel for the air to follow into the fin area.

The rounded head makes it easier to align the plunger with other elements (spring and washer) during the assembly process. If you don’t align it directly along the center axis the rounded head allows the plunger to slide into alignment.

The back is longer than the flap to keep the flap from contacting the orifice plate. The back rests on it instead. The flap is useful to increase air pressure and will make the seal better. The air will push the flap outwards.

The plunger provides a surface for the air pressure to act on one side and pushes against the ruler and spring on the other side. The plunger features an interference fit with the inner side of the tube.

This plunger would become less effective if the rubber hardened or became brittle, which would cause the plunger to jam or let air pass by it. The grease may also prevent rust formation.

Plunger Washer

The washer on the plunger helps in giving something for the spring to press against. If the spring pushed on the plunger itself, the rubber would give way and this might also cause undue wear on the plunger. This washer is shown in Figure 7 below. A drawing of the plunger washer is presented in Appendix A.4.

Figure 7: Plunger Washer

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End Cap

The main functions of the end cap are to provide a stop for the ruler so that it doesn’t slide out completely and to keep the ruler in place once the tire pressure is released. This end cap is shown in Figure 8 below. A drawing of the end cap is presented in Appendix A.5.

Figure 8: End Cap

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The red end cap has two flaps that provide an interference with the ruler. When the gage is removed from the Schrader valve, this interference keeps the ruler in place while the spring extends to its original length as the pressure is removed. It is large enough to not go trough the roller over edge of the tube, but small enough to slide freely through the tube. This is important for assembly. There is enough interference with the ruler to keep gravity from moving the ruler, but not enough interference to significantly effect the force balance. The ribs on the two flaps probably provide strength and keep the flaps from breaking when pushed on by the black washer.

The red end cap, as opposed to black or white, makes it easier to read because it provides more contrast with the ruler and the surroundings. It is dull so it won't reflect light, which makes it easier to see the reading against than the reflective tube.

End Cap Washer

The spring presses against the washer, not the end cap. The end cap on that end is not a complete circle, so if the spring pressed directly against that it might cause wear or breakage on the end cap. This washer is shown in Figure 9 below. A drawing of the end cap washer is presented in Appendix A.6.

Figure 9: End Cap Washer

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Spring

The spring used in this tire pressure gage is a pretty standard design. Its purpose is to provide the force balance for the tire pressure to push out the ruler the appropriate length. The spring in my gage had 33 coils. Two other classmates had 33 spring and 35 spring coils. This spring is shown in Figure 10 below. A drawing of this spring is presented in Appendix A.7.

Figure 10: Spring

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I went to calculators/comp_spring_k_pop.htm and entered the following information: n=31 (active coils, two less than total coils), G = 1.1x10^7 (middle of range for steels), D = .322 = diameter of coil. This resulted in k=1.076 lb/in.

My spring was not packaged in compression. If you shook the package you could hear it moving and the end cap didn’t bounce back if it was pushed in. Most others of these gages of my classmates were house in compression.

The ends are closed to prevent some tangling before and during assembly. The purpose of the spring is to provide the necessary force to counteract the force from the tire pressure. It’s what allows the magnitude of the tire pressure to be measured. The spring

The spring diameter will increase when compressed, so it needs to be a size such that this doesn’t provide too much interference with the tube.

Head Piece

The head of the tire pressure gage we were given is two pieces that are press fit, but I could not get them apart so for this report, they are modeled as one piece. A picture of the head piece is Figure 11, below. A drawing of the head piece is presented in Appendix A.8.

Figure 11: Head Piece

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The grip on the head made it much easier to screw together, especially when I had grease on my hands. This is useful for both assembly in the plant and regular use. One side of the head is threaded so it will screw onto the tube. This end is shown in Figure 12, below.

Figure 12: Head Piece, Threaded End

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The head has an opening just large enough to fit around a standard Schrader gage. Unlike most other designs on the market today, there is no small protrusion to release the pressure from a Schrader valve.

If the width of the center post is too large, there is no room for the hole in the head and if it is too small it doesn’t sit well on the Schrader valve. The end of the head piece with the post is shown below in Figure 13.

Figure 13: Head Piece, Post End

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Rubber O-ring

The rubber O-ring is found press fit between the two pieces of the head. It provides a good seal for the Schrader valve. A drawing of the O-ring is presented in Appendix A.9.

Ruler

The ruler allows the user to read the pressure reading. The ruler comes out of the tube a distance that is dependant on the tire pressure. This ruler is shown in Figure 14, below. A drawing of the ruler is presented in Appendix A.10.

Figure 14: Ruler

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This ruler, pushed on by the rubber plunger, is held in place by the end cap. The pressures are marked in psi and N/mm^2 gradations, color-coded for each type on each side for quick reference.

The ruler has a rounded end that is bigger than the rest of the ruler to stop the ruler from falling out of the end of the tube. It interferes with the end cap. I tried to pull the ruler through in that direction and it would not go.

Pocket Clip

The pocket clip allows the user to clip the tire pressure gage to a piece of their clothing. This is convenient for the user, especially if they are checking the pressure of numerous tires. This pocket clip is shown in Figures 15, below. A drawing of the pocket clip is presented in Appendix A.11.

Figure 15: Pocket Clip

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It requires interference and stiffness so that it fits snugly over the tube.

The pocket clip is easy to slide over the material of a pocket and didn’t break or warp too much when pulled and put in torsion.

Orifice Plate

The orifice plate is housed between the head of the tube and the threaded end of the tube. The orifice plate provides hydrostatic pressure by choking the flow. The orifice plate is shown below in Figure 16, below. A drawing of the orifice plate is presented in Appendix A.12.

Figure 16: Orifice Plate

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The hole size in the head is trivial because it doesn’t mate to another hole and the orifice plate provides the air path. The orifice plate is shaped to direct air through its hole. This means the hole doesn’t have to be aligned with the hole in the head and the size of the channel connecting the two doesn’t need to be considered heavily. The orifice plate is shown sitting in the head in Figure 17.

Figure 17: Orifice Plate in Head

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The threads between the head and the tube don’t have to be air tight due to the orifice plate.

DESIGN COMPARISONS

I compared the tire pressure gage given in class with a $1 round-head design pressure gage. My observations and comparisons are listed below.

The straight head sealed better than the round gage and was easier to have a seal form. The O-ring in the head was made of some soft plastic or rubber and was easily pulled out.

The head is press fit to the body, but is relatively easy to take off.

The plunger was made of hard plastic, which I don’t think would seal as well. The plunger was also made of two pieces. There were no oil or grease reserves on this plunger. It seemed that the plunger was oiled, rather than greased.

There were two rings on the ruler – one metal with a square hole and the other a tight-fit rubber. The end cap didn’t provide any interference fit with the ruler, but did have an interference fit with the tube. The rubber washer seemed to keep the ruler from moving when the airflow stopped. This might be a cheaper way to provide the necessary interference fit than the flaps on the end cap.

The metal of the whole assembly was thinner and less dense than the one given in class.

The spring, which came out noticeably bent, was housed in what seemed like more compression than the gage given in class.

ASSEMBLY ORDER AND TIME

The assembly order and assembly times are shown in Figure 1 below. The total time for assembly is 65.34 seconds. All times shown in the chart are in seconds

Table 1: Assembly Order and Times

|Action |Time |Type |

|Head: |19.4 | |

|Pick up O-ring |1.43 |Handling |

|Pick up larger head piece |1.13 |Handling |

|Place the O-ring in the larger head piece |3 |Insertion |

|Move larger head piece to press fit machine |1.5 |Insertion |

|Pick up smaller head piece |1.13 |Handling |

|Move smaller head piece to press fit machine |1.5 |Insertion |

|Press fit the head pieces |2.5 |Insertion |

|Take press fit head from machine |1.13 |Handling |

|Pick up orifice plate |2.45 |Handling |

|Place the orifice plate in the threaded end of the head |2.5 |Insertion |

|Store head with pin side down |1.13 |Handling |

|Body: |45.94 | |

|Pick up tube |1.13 |Handling |

|Place the tube in a holder |1.5 |Insertion |

|Pick up black end cap washer |2.45 |Handling |

|Pick up the ruler |1.13 |Handling |

|Place the black washer on the ruler |5.5 |Insertion |

|Pick up the end cap |1.13 |Handling |

|Place the end cap on the ruler |5.5 |Insertion |

|Pick up spring |1.13 |Handling |

|Slide the spring on the ruler |5.5 |Insertion |

|Drop this sub-assembly into tube |2 |Insertion |

|Pick up gold plunger washer |2.45 |Handling |

|Pick up plunger |1.13 |Handling |

|Place gold washer on plunger |2 |Insertion |

|Drop plunger in the tube |1.5 |Insertion |

|Pick up pocket clip |1.13 |Handling |

|Slide pocket clip over the end of the tube |2.5 |Insertion |

|Take the body out of the holder |1.13 |Handling |

|Pick up head sub-assembly |1.13 |Handling |

|Screw head on |6 |Insertion |

|Total Time (in seconds) |65.34 | |

If you put the ruler/cap/washer assembly in the tube and then try to put the spring in the tube, you run into several problems. The washer gets caught at the entrance of the tube and sometimes ended up on top of the tube. The spring also got caught on the ruler and required some jiggling to get it inside.

The assembler can’t screw on the head while the tube is in the holder because the orifice plate sometimes falls out when the head is turned upside down.

The assembler could put the orifice plate on the tube and then screw the head on, but it is harder to align. If the plate is placed in the head first, the walls aid in alignment. The misalignment sometimes interfered with screwing the head down.

After testing different assembly orders and techniques, I found it was easier to slide the pocket clip over the end of the ruler than to snap it on. This also removes the pinching hazard. The break in the clip is still needed, however, to make sure there is enough interference to keep the tube from sliding out of the clip.

The bent head design is more advantageous in one situation: The straight head won’t work if there’s not enough clearance – like on motorcycles. In this case, the bent head design is better.

MATERIALS AND PROCESSING

The pocket clip was made by sheet metal folding and most likely made of brass.

The tube, orifice plate and head are made of some kind of brass, but are finished with chrome. The finishes can be scratched off with a file. Chrome plating is extremely cheap and easy to do, but makes the product much more appealing to the customer. You can cover up the parts you don’t want plated with polymers and just put the whole part in. The head appears to be machined, most likely on a CNC lathe. The brass is easy to lathe, but hard to die cast. The engraving was most likely done by a rolling process and the cross hatching could be done on the lathe or in a rolling process.

The metal spring used in the gauge is likely formed from drawn wire, a process that yields uniform diameters and roundness. It is made of a cheap steal.

If the orifice plate is made wavy the plate will become flat when screwed down. This is probably why it’s made of a soft material. It is most likely stamped and then the hole is drilled through the center.

The tube is most likely made by cold extrusion and then the edge will be rolled over with a specialized tool.

The plunger is most likely made of polyurethane because it sinks and burns with a yellow flame. It seems to be die cast.

The end cap piece burns slowly with a small yellow flame and sinks in water. It is probably made out of ABS. Failure of the flaps holding the ruler in place after some time (creep, brittleness) may cause the ruler to slide in and out inconveniently.

The ruler sinks. It doesn’t light when a flame is applied to it, and produces dripping and a burnt hair smell. It is made of nylon. It is most likely die cast.

The O-ring in the head produced a self-extinguishing fire that burned out after producing white smoke. This might be made of PVC. The o-ring is probably a thermoset. The ruler and end cap are made of thermo plastics – nylon, ABS, and polyethelyne.

With injection molding, dies can last many cycles and produce many hundreds of thousands of parts before wearing out. Parts can be made quickly with easily-transported thermoplastic pellets, dyed quickly, and generally be easily and cheaply produced and manipulated.

DIMENSIONS AND TOLERANCES

Table 2 below shows all the functional dimensions and tolerances. All closure equations and accompanying calculations for these values are shown in Appendix B.

Table 2: Functional Dimensions and Tolerances

|Dimension |Nominal |Tolerance |Description |

|Rp lgst |0.17 |0.0013 |largest plunger radius |

|Rt inside |0.164 |0.0012 |inside radius of tube |

|interference |0.006 |0.0025 |between plunger lip and tube |

|Rpwo |0.16 |0.0005 |outside radius of plunger washer |

|clearance |0.004 |0.0017 |between plunger washer and tube |

|Rpwi |0.1185 |0.0005 |inside radius of plunger washer |

|Rpnose |0.12 |0.00095 |radius of nose of plunger |

|interference |0.0015 |0.00145 |between washer and plunger |

|Relgst |0.1615 |0.00125 |largest radius of end cap |

|clearance |0.0025 |0.00245 |between end cap and tube |

|Resm |0.142 |0.0011 |small radius of end cap |

|Rt roll |0.1515 |0.00105 |radius of rolled over tube edge |

|int lg |0.01 |0.0023 |so end cap doesn't fall out |

|cl sm |0.0095 |0.00215 |so end cap sticks out |

|L ruler |0.0816 |0.0007 |half width of ruler |

|L ec hole |0.083 |0.0007 |half width of hole in end cap |

|w matl |0.0266 |0.00011 |width of material of end cap at corners |

|clearance |0.0014 |0.0014 |between ruler and hole in end cap |

|R ec flap |0.07 |0.0006 |From center to center of end cap flap |

|interference |0.0116 |0.0013 |interference between flap and ruler |

|R I ec w |0.1285 |0.001 |inside radius of end cap washer |

|R o ec w |0.161 |0.001 |outside radius of end cap washer |

|clearance |0.003 |0.0022 |between tube and outside of washer |

|clearance |0.0131 |0.00199 |between ruler and inside of washer |

|R orifice |0.177 |0.001 |radius of orifice plate |

|interference |0.013 |0.0022 |so plate doesn't slide down tube |

|R in head |0.1795 |0.0014 |inside radius of head - threaded part |

|clearance |0.0025 |0.0024 |between head and orifice plate |

|Ltube |5 |0.005 |length of tube |

|Lbul to w |0.23 |0.009 |length of bullet from back to washer |

|Lec to w |0.5 |0.009 |length of end cap from top to lip |

|t ec w |0.017 |0.001 |thickness of end cap washer |

|t p w |0.025 |0.001 |thickness of plunger washer |

|L spring |4.148 |0.1 |length of spring |

|clearance |0.02 |0.015 |clearance between spring and tube |

|R spr |0.144 |0.016 |outside radius of spring coil |

|H post |0.212 |0.02 |height of post |

|interference |0.1 |0.05 |amount Schrader valve depressed |

|R hi |0.156 |0.0015 |inside radius of head |

COST ANALYSIS

The unit costs for each piece, assembly and materials are shown below in Table 3 below. The calculations, equations and resources for these values are shown in Appendix C. The calculations were done for manufacturing 1,000,000 units. The material, labor, overhead, shipping and packaging costs were considered in this analysis. However, some of these values seemed to be negligible and affected the unit cost very little.

Table 3: Unit Costs

|Assembly |$0.72 |

|Standard Parts |$0.10 |

| |Spring |$0.06 |

| |O-ring |$0.02 |

| |Plunger Washer |$0.01 |

| |End Cap Washer |$0.01 |

|Manufactured Parts |$1.2744 |

| |Head |$0.3269 |

| |Tube |$0.376 |

| |Orifice Plate |$0.0515 |

| |End Cap |$0.10 |

| |Plunger |$0.10 |

| |Ruler |$0.20 |

| |Pocket Clip |$0.12 |

|Total Cost per Unit |$2.0944 |

The most obvious thing that can be done to reduce the price is move the manufacturing and assembly to another country where worker rates are considerably lower. It might also be worth it to look into making some of the manufactured non-standard parts easier to machine.

UNCERTAINTY ANALYSIS

The uncertainty will get higher as the pressure reading increases. The uncertainty in the pressure measurement reading was found to be 1.6 psi. These calculations are presented in Appendix D.

APPENDIX

A: Part Drawings

B: Dimension Tolerance Calculations

C: Cost Analysis Calculations

D: Uncertainty Analysis Calculations

E: Resources

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