Gearbox Maximum Torque

[MarkoRamius1086]

Team 48 was going to use a 256:1 Banebot planetary. I was looking around in the North Carolina pits, when I happened into their area. I noticed them installing a hanger for the vertical bar, but I didnt see any motors or transmissions. When my team and I was working on a hanger we found a really nice 40:1 wormgear... but it was too pricy. Well, back to 48. Upon further inspection I saw they had a Fisher-Price and a parallel silver gearbox. I had never run into Banebots transmissions before this point, probably due to their reputation. There was a small orange sticker on that thing and I was fairly shocked about the reduction... until that moment I didnt think it was possible to cram such a small reduction into a small transmission.

[A_Reed]

Originally Posted by MarkoRamius1086

Team 48 was going to use a 256:1 Banebot planetary. I was looking around in the North Carolina pits, when I happened into their area. I noticed them installing a hanger for the vertical bar, but I didnt see any motors or transmissions. When my team and I was working on a hanger we found a really nice 40:1 wormgear... but it was too pricy. Well, back to 48. Upon further inspection I saw they had a Fisher-Price and a parallel silver gearbox. I had never run into Banebots transmissions before this point, probably due to their reputation. There was a small orange sticker on that thing and I was fairly shocked about the reduction... until that moment I didnt think it was possible to cram such a small reduction into a small transmission.

It was only 4 stages of 4:1.

But, Sadly we had to remove the whole system when we found out our nice clean 'ground down' welds snapped under the forces required pull the robot up. Note for next year don't grind welds to make them look 'pretty'.

And yes you can figure out the maximum torque of any gearbox you just need to apply the principles of machine design and properties of materials to determine the physical properties of the material the gears and shafts are made from and then the actual max stress that can be absorbed by the gears and shafts and apply a factor of safety to insure the parts never break.

[Joe Johnson]

Failure modes matter. Is it a shaft bending, teeth shearing off, etc.

It is hard to know from your description what happened. The gears teeth may have been a secondary failure.

But... ...once you know how it fails you can estimate at what value it will break.

It is probably outside the scope of a ChiefDelphi.com thread to cover the HOWs of these estimates. But perhaps we can give it a shot once we know more about what failure we are trying to predict.

Joe J.

[Peter Johnson]

Originally Posted by Joe Johnson

It is probably outside the scope of a ChiefDelphi.com thread to cover the HOWs of these estimates. But perhaps we can give it a shot once we know more about what failure we are trying to predict.

But it's fun (and a good educational experience) to dig into the HOWs!

http://www.bostongear.com/pdf/gear_theory.pdf is a wonderful reference for gear theory. See page 6 of the PDF for the Lewis formula (Barth revision) for safe static stress on gear teeth, for example. Given the face width, diametral pitch, pitch diameter, pitch line velocity (or RPM), and material, the formulas and tables on that page will give you the max torque a gear can take (at least on the teeth).

[Joe Johnson]

Originally Posted by Peter Johnson

But it's fun (and a good educational experience) to dig into the HOWs!

http://www.bostongear.com/pdf/gear_theory.pdf is a wonderful reference for gear theory. See page 6 of the PDF for the Lewis formula (Barth revision) for safe static stress on gear teeth, for example. Given the face width, diametral pitch, pitch diameter, pitch line velocity (or RPM), and material, the formulas and tables on that page will give you the max torque a gear can take (at least on the teeth).

Thanks for the reference.

I didn't mean to imply that we can't get into the HOWs a bit but I just wanted to be clear that ChiefDelphi.com is no substitute for an in depth engineering class.

And... ...Don't get me started on the Lewis formula! It is a good starting point but it is clearly too conservative for many many applications (including every FIRST robot I have ever encountered).

Bill Beatty and I agree (I can't find the thread but trust me, he does) on this point -- a simple beam bending stress calculation is a more appropriate estimate for FIRST gear teeth.

Joe J.

[Dick Linn]

It may not be relevant, but the Banesbot max recommended torque is 85 ft-lb for all the larger P80 Series Gearboxes

[Kevin Sevcik]

Norm,

Two important questions that might help us determine failure mode:

1. What was the diameter of the spindle you were winding onto?
2. Did you support the opposite end of the shaft with a bearing block, or was this a 150 lb cantilevered load on your gearbox?

Second question is the more important, as 150 lbs of side load on one of these gear boxes is probably going to translate through the output bearings and put a heck of a load on the final planetary stage.

Also, if only you'd have mentioned this while you were at Lone Star. I'd have loved to take a look at the thing in person. If only to determine whether I should continue to assume these gearboxes are really a vast improvement over the '07 ones my team had welded back together at GLR.

[EricVanWyk]

Thanks for all the insight, and apologies for latency in this response. I wanted to be sure I knew a little more before responding, but I haven't been able to pull more information or pictures up on/of our specific failure. The busted-box is in "a bag somewhere over there". Which, "there" they mean, I do not know.

The spindle was mounted directly on the gearbox output, but it was supported on the other side by a pillow block: A single plate held the gear box (4 mounting holes) and the pillow block (2 mounting holes).


I'd like to convert this thread into an "Idiots Guide to Torque" or a "Mentor's Guide to Teaching Torque", if possible. My vision of it is the following:
* A spread sheet similar to Dick Lynn's information for the N most commonly used gearboxes in FRC ((85 ft lbs for a P80)).
* A simple beam bending stress calculator, per Joe Johnson's suggestion, with perhaps 3 example gearboxes already entered. This will probably benefit from a picture of a gear with the important dimensions labeled.
* An explanation of when the simple beam calculator is appropriate, and pointers towards how to do it more accurately. Maybe this is where we mention the life cycle calculations?
* An explanation of how "how's" matter. For example, pictures of busted gears and how they failed. "Gears look like this? Too much side load!" "Gears look like that? Need more lube!" etc.

Thanks again, all, and I'll try to take pictures after ATL.

EDIT/PS:
When I teach a new topic at FRC, I usually give a 2-10 minute "whats up" to everyone, and then point 1 or 2 students in the right direction to dig deeper. If this resource could be architected similarly, that would be fantastic.

[IKE]

Originally Posted by Joe Johnson

Bill Beatty and I agree (I can't find the thread but trust me, he does) on this point -- a simple beam bending stress calculation is a more appropriate estimate for FIRST gear teeth.

Joe J.

As Joe is eluding to, the cycle count and service life are quite low for FRC robots.
Last year 217 saw around 100 matches. At 2:15 minutes per match, it is less than 4 hours of runtime. Even at the 4500 rpm (cim speed) this is only 10^6 cycles. This is relatively low in terms of most design lifes, but within the fatigue regime of design. Really though the initial stage is seeing the lower torques. With a 50:1 or more, you are now down to 2x10^4 cycles. This is down within the infant death for most powertrain systems.
Thus I tend to agree with Dr. Joe that design so that the teeth do not yield, and you should be good to go.

Hmmm. maybe a simple set of equations would be good for this and helpful to the community.

IKE

[AustinSchuh]

Originally Posted by Peter Johnson

http://www.bostongear.com/pdf/gear_theory.pdf is a wonderful reference for gear theory. See page 6 of the PDF for the Lewis formula (Barth revision) for safe static stress on gear teeth, for example. Given the face width, diametral pitch, pitch diameter, pitch line velocity (or RPM), and material, the formulas and tables on that page will give you the max torque a gear can take (at least on the teeth).

Here's another calculator that I've been using lately. It sounds like it's an online version of what's referenced in that document.

http://www.rushgears.com/Tech_Tools/horsepower.php