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Unread 01-29-2007, 02:06 PM
Joe Johnson's Avatar Unsung FIRST Hero
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Banebots 36mm gearbox: Recommendations for use

As readers of these fora know, the 56mm Banebots transmission has a failure mode that involves the double D joint between the last carrier stage and the output shaft.

The 36mm gearbox shares this same failure mode.

I have done some testing and I have the following recommendation with regard to the use of these gearboxes:
My analysis is below. I think the numbers are somewhat conservative but I don’t think they are way out of line with what teams will see.

This will be my advice to teams in summary:

36mm w/FP: If you plan to stall the motor, use ratio less than 37:1
36mm w/BB: If you plan to stall the motor, use ratio less than 55:1

Yes, I realise that I am recommending that the 64:1 gearbox in the kit is actually under designed. I am calling the as I see them. I plan on doing a test soon on this gearbox to see if I am whacked or not. My crystal ball is not perfect, but I am going with the best data I have right now.


Finally, I do not include dynamic effects in these calculations. If you are expecting the mechanism to have significant impact loads, I recommend even lower ratios.

As always, your mileage may vary.

Joe J.

Rockwell C to Tensile Yield:
RC 22 – 115 Ksi
RC 23 – 117 Ksi
RC 24 – 119 Ksi
RC 25 – 123 Ksi
RC 26 – 125 Ksi

************************************************** *********************************
36mm
Carrier hardness:
RC 21.2, 23.8, 23.3
Gear Brass:
RA 31.4, 38.5, 35.7 << suspect due to small surface to test
Shaft:
Did not test

Given the analysis of the D on the 56mm gearbox, due to scaling, the 36mm D should take 16% of 350in-lbs failure for the 56mm gearbox. But the yield of the carrier on the 36mm gearbox is harder (117Ksi rather than 64Ksi) so it should 183% stronger due to better material. The net effect should be that the joint should fail at 29% of the value as the 56mm gearbox or (100in-lbs).

If that is true, then I predict that the 36mm gearbox with a FP motor (with 12V stall = .42N-m = 3.7in-lbs) will fail if the effective ratio (the ratio including losses due to efficiency) is 27:1 or higher.

In order to get 27:1 it will take 3 stages. Again using 90% per stage, I predict that the FP motor on the 36mm gearbox with a ratio of 37:1 or greater will fail.

Similarly for the BB motor in the kit (12V stall = .28N-m = 2.5in-lbs) will fail if the effective ratio (the ratio including losses due to efficiency) is higher than 40:1.

In order to get 40:1 it will take 3 stages. Again using 90% per, predict that the BB motor on the 36mm gearbox with a ratio of 55:1 or greater will fail.

Again, note that these are not one time failure predictions but a failure that will fail upon repeated cycling back and forth.

Now to the actual torque to failure test: I used a 64:1 gearbox. The input torque required to fail the gearbox with the output shaft locked was .4N-m (3.5in-lbs). The peak (after failure, the torque grows as the shaft plows through the carrier) was 0.7N-m

This is scarily close to the stall torque of the FP motor and not too far away from the the BB motor. Also, this does not include any dynamic loading of the gearbox.

But, based on the 3.5 n-lbs failure, I get that the D joint fails at 3.5in-lbs * (4 * .85)^3 = 140in-lbs. Note while this is higher than the 100in-lbs predicted above, that number was not 1 time failure load, but a load that if cycled caused failure, so I am not too worried at this difference.
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Unread 02-01-2007, 05:16 PM
Joe Johnson's Avatar Unsung FIRST Hero
Joe Johnson Joe Johnson is offline
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Re: Banebots 36mm gearbox: Recommendations for use

UPDATE:
I have finally had a spare minute from the 56mm DD issue to get some time to actually evaluate my prediction with respect to the 36mm gearbox.

As you know from above, I recommended that teams that use the Banebot motor, keep the ratio below 55:1, much lower with impact loads.

I am in the middle of my test but I have enough to give a prelimary update.

Used the 64:1 kit transmission for theses test.

Preliminarily, used 6V and ran 50 +stall -stall cycles with the roto "locked" Actually I could not get a good lock on the output shaft so it rotated 5 deg or so.

The backlash in the joint opened up to about 10deg after these 50 cycle.

I repeated the test with 12V, again another 50 +stall -stall cycles (careful to stop when the motor gets hot and blow air on it for a bit).

The backlash in the joint opened up to about 15 deg after these 50 cycle.

I allowed the gearbox output shaft to now rotate about 20 deg or so and immediately stop -- a pretty rough test, because it is enough freeplay to let the motor reach full speed.

No noticeable increase in backlash.

I will continue the test tomorrow. But, I believe that the joint may be better than I had projected. There are lots of possible reasons. One may be that the materials are workharding with each impact.

This is encouraging results because I suppose even if I DO eventually get the gearbox to fail, it is not a 1 hit, boom and your done type failure but something that takes time. This will allow teams to do R&R after each match if needed.

There is a great quote from Raul-the-magnificent that I think about a lot, I don't have it exactly but the basic idea is this: "In FIRST, you can either have comfortable safety margins or you can have a competitive robot, you can't have both."

True dat.

Joe J.
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Unread 02-02-2007, 05:10 PM
Joe Johnson's Avatar Unsung FIRST Hero
Joe Johnson Joe Johnson is offline
Engineer at iRobot
AKA: Dr. Joe
FRC #0246 (Overclocked)
Team Role: Engineer
 
Join Date: May 2001
Rookie Year: 1996
Location: Boston, MA
Posts: 2,282
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Re: Banebots 36mm gearbox: Recommendations for use

Another Update:

I am impressed with the 64:1 36mm gearbox included in the kit.

I eventually got the gearbox to fail but it was not easy.

Data:
At 350 HARD impact cycles* The backlash in the motor has opened up to 30.5 degrees (I have a way to measure this now before I was eyeballing it) but the gearbox seems fine other than that.

At 500 HARD impact cycles, the gearbox had 35.5 degrees of backlash. As I was re-assembling the test rig, I rotated the output shaft without power (back and forth a bit -- nothing hard, just positioning things to continue the test), we the gearbox locked up. I was eventually able to get the gearbox freed up by wiggling back and forth more, but when I powered the gearbox would run in either direction with no load, but would lock up in one direction if under medium load conditions.

Taking the gearbox apart, the D had opened up asymetrically (my stops must not have been as hard in one direction as the other). It is hard to tell exactly what the exact cause of the bind is because when the gearbox was apart everything moved freely. I believe that the asymetrical wear in the hole was causing the carrier to bind in one direction.

Conclusions:
The gearbox is considerably more robust than I expected. I believe that the test was pretty aggressive and the gearbox took 500 hard impacts to fail.

I believe that the Banebots 64:1 36mm gearbox is a pretty robust solution for many types of FIRST applications.

Bottom Line:
With some care not to repeatedly run the motor into hard stops at full speed**, I believe that this gearbox will serve teams very well.

Joe J.

*1 cycle = + 12 V to motor, free rotation of output shaft for approx 20 deg, Impact stop, -12 V to motor, free rotation of output shaft for appox -20 deg., impact stop.

**I am still a little concerned about non-obvious repeated impact cases. For example using this gearbox for a feedback loop for a large, heavy robot arm with a very rigid connection between the arm and the gearbox (say, chain). The potential problem is that the arm may act as an impact load every time the feedback loop changes the sign of the correction (which is often).
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