gearing design calculation question

Technical Motors
1

487 stripped a cluster gear in the Atwood gearbox.
Its not clear why, but we’d like some advice on the design.
We used the old motors and the plastic housing from the Atwood jack. We took the 80 tooth steel gear and welded it onto a new output shaft. This gives a free speed of 450 RPM.
This new 5/8" output shaft goes straight to the wheel.
We may or may not have adequate alignment and spacing of the shafts and we are worried because its too late to change it.
Basic Transmission design question:
Using the Atwood ‘copper’ cluster gear, there is 11 nm torque coming in to the cluster gear’s 16 tooth output. (2.2 x 44/9)
The 80 tooth spur gear is mated to the 16 tooth on the cluster gear and that increases the final output torque to 55 nm.
The steel spur gear has a face width of approx .375".
Question: Based in using the ‘coppery’ cluster gear (rated at 90,000 psi) and the stock spur gear 32 dp, hardened steel, 80 teeth, 3/8" face width, are we overloading this step in the gearing?
If we could get 1/2" face width, would that put us over the top?
We must replace the stripped shaft/gear assembly, but we’d like to make the new one more reliable.

Thanks, Charlie Affel, Team 487

2

Are you locking out the slip clutch inside that gearbox?

This is a very bad idea, imho.

I have not done the math, but I am pretty sure you are pushing on a rope on this one.

The gearbox you are discussing has a slip clutch inside of it for a reason: The loads are huge if you allow the full stall torque of the motor ripple through the gearbox.

If you have not locked out the clutches, I am surprised that you are able to damage the gearbox at all.

Basically, you can do the math yourself if you want to.

Figure out the load on the tooth (assume one tooth carries the load).

Put the load at the tip (for safety factor purposes).

Approximate the tooth as a rectangular beam.

The max. stress is then

Max Stress = Mc/I

M = Force on Tooth X Tooth Height

c = (Thickness of Tooth at Base) / 2

I = (1/3) (Face Width) (Thickness of Tooth at Base)^3

Do all your calculation in lbs and inches and you will get a number for stress in psi:

Good steel, unhardened has a yeild stress of 80,000 - 90,000 psi
Aluminum 30,000 psi or so (it can be better, but again, be safe).
Hardened steel can be 100,000 - 120,000 psi or so (harder if really need it, but things get pretty brittle up in those regions).

Good luck.

Joe J.

3

Thanks for the prompt reply and the formula.
We did omit the clutches from version 1.
Drawing board is still open.
Its just a prototype now. We plan the final machine to be complete by the Maryland State Fair on Labor Day.

Charlie Affel

4

I’ve been interested in using the Atwood jack gearing, but couldn’t find any info about it. Where did you find it?

5

The details are on Chief Delphi elsewhere (some from 2002).
Summary:
Older motor has 9 teeth, odd Diametral Pitch (36) only matched by the ‘coppery cluster gear’ in the jack.
This gear has 16 and 44 teeth.
The next gear in the drivetrain has 80 teeth.
Advice per our experience & Joe’s advice above: Use the intermediate shaft that came with the plastic gearbox. Keeping the clutch as well. Lengthen this this 3/8" shaft and extend it through the case and use it as the output.
This results in a 225 RPM free speed.
Discard the old final output shaft and the last stage of reduction.
Prior to this, we made a new shaft (for the 80 tooth gear) and cover and had failures due either to misalignment or overloading (as Joe predicted). Of our two gearboxes that we made this way, one has been fine through a lot of use and the second failed twice.
CA