project iii me 360

8/12/2019 Project III Me 360

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Table of Contents:

Deliverable: Page #:

1. Safety Factors for Gears in Both Failure Modes 6

2. Coil and Wire Diameters of Spring 7

3. Observations 8

4. Key Parameters 3



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Key Parameters:

Symbol Definition Value

Fspring Force on spring 9.67 lbf

Wt Force due to torque 4.05 lbf

Kv Dynamic Factor 0.46Km Load Distribution Factor 1.00

Ka Application Factor 1.00

Ks Size Factor 1.00

KB Rim Thickness Factor 1.00

KI Idler Factor 1.00

J Bending Strength Geometry Factor 0.3

σb Bending Stress 6010.4 psi

dp Diameter of Pinion 0.4375 in

Dg Diameter of Gear 1.875 in

Cf Surface Finish Factor 1.00

Cp Elastic Coefficient 1966I Surface Geometry Factor 0.0938

σc Surface Stress 72850.4 psi

KL Life Factor 1.1

KT Temperature Factor 1.00

KR Reliability Factor 1.00

Sfb’ Bending Fatigue Strength5.7 ksi (gear)

29 ksi (pinion)

Sfb Corrected Bending Fatigue Strength6.27 ksi (gear)

31.9 ksi (pinion)

Nb gear Bending Factor of Safety for Gear 1.043

Nb pinion Bending Factor of Safety for Pinion 5.307CL Surface Life Factor 1.00

CH Hardness Ratio 1.00

Sfc’ Surface Fatigue Strength30 ksi (gear)

90 ksi (pinion)

Sfc Corrected Surface Fatigue Strength30 ksi (gear)

90 ksi (pinion)

Nc gear Surface Factor of Safety for Gear 0.412

Nc pinion Surface Factor of Safety for Pinion 1.235

Φshaft Major Diameter of Threaded Shaft 0.19 in

Δy Deflection of Spring 0.0156 in

K Spring Stiffness 619 lb/inNa Active Coils 2

G Modulus of Rigidity 11.5x106 psi

D Coil Diameter 0.255 in

d Wire Diameter 0.055 in



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Observations:

As seen from the safety factor calculations, the gear will fail during use due to surface loading.

With a factor of safety of 0.412, the pinion teeth will most likely grind into it causing a lot of debris to

collect in the system and eventually the gear will fail. This is likely due to the company wanting their

parts to fail after a certain amount of time forcing the customer to either purchase a new truck entirely

or order replacement parts in order to continue using the toy. To raise the factor of safety of the gear

one of two methods could be implemented. The first would be to increase the width of the gear, this

would divide the stress components by a larger number making them smaller, but this would require the

gear to be at least 22.5 inches wide. This is definitely too wide and would require heavy modification of

the design of the truck and this is to only achieve a factor of safety of 1. This method is not very

practical. The other method would be to choose a gear of a different material. If it was a steel gear, like

the pinion, then it would also have a bending factor of safety of 5.307 and a surface factor of safety of

1.235, much more reliable. If the surface factor of safety was still too small for comfort, the width could

also be increased slightly to lower the bending and surface stresses, resulting in higher factors of safety.

The coil and wire diameters of the spring will allow it to clear the threads on the shaft by 0.010” and

provide a force of 43 N on the clutch plates when the nut is twisted a half turn.

project iii me 360

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