Gearbox Maximum Torque
 

Disclaimer - I am an electrical guy, please excuse my ignorance.

Some of the students prototyped a lifter mechanism that featured and subsequently destroyed a P60 132:1 gearbox. The final planetary stage was almost entirely bald.

It is easy to gut-feeling see why it broke in retrospect, but I'd like to be able to teach them how to predict this ahead of time. Trouble is, I couldn't find any specification on this gearbox that would give me a good place to start.

What is the name of this 'max torque' spec that I am looking for, and where do I find it? Is there a collection/spreadsheet of this info for common FRC gearboxes?

Thanks!


PS: If it helps you understand the number I want: I'd measure this by stalling one side of the gearbox and cranking on it with a torque-meter until it broke. Does this number change with speed?

PPS: What other info am I missing?

Re: Gearbox Maximum Torque
 

Here is Dr Joe's recommendations on the 42mm Banebots gear boxes, which where the predecessor to the P60 model. I don't know about any for the P60.

Are you using a FP motor or on of the RS-555 motors?
Keep in mind the FP can generate significantly more torque than the RS series motors the P60 was designed for. It does work with the FP but you have to be careful how you use it because there is a risk of destroying the gearbox (which unfortunately you now have first hand experience with). I am also electrical/programing so that is the extent of my knowledge :o

Re: Gearbox Maximum Torque
 

Many people would argue you could have predicted it's failure when you saw the "Banebots" sticker on it. The brand has become somewhat notorious for poor design, workmanship, and material quality. Although, I cannot comment on that specific model, so I'll leave it at that. Perhaps things have improved since those days.

To better answer your question, yes, the failure of a gearbox can absolutely be predicted. The problem can be approached a few different ways:
- All factors and loads known/given, lifetime to be found
- All factors and lifetime given, maximum load to be found
- Lifetime and load given, design/sizing of the gears to be found

After a machine design course spending a few weeks on gear train design, it is not a terribly difficult process to analyze a gear train, but it does take a little while. There's probably about a dozen pages in my notebook that run through all the calculations.

Also, in gear train design, there are a lot of "fudge factors." Usually they are referred to as "K factors." There are about 8 or so of them that deal with temperature, gear quality, mounting configuration, lubricants, etc and can greatly affect the design and calculation of the gear train. Many times, these K factors cannot be calculated and must be estimated. So, while the failure (or success) of a gearbox can be calculated, its lifespan is only estimated to some reasonable degree of certainty. Typically in engineering design, when we talk about lifespan, we're talking greater than 10^4 cycles. For your application, you're just concerned with if it will work for your robot.

Honestly, I've never done the above calculations for a FIRST Robot. There just isn't time to fully analyze everything. Over time, you just learn what works, and develop a rule of thumb, and build off past designs. If you're concerned, you throw it into FEA to give you a rough idea if you're screwed or way in the clear.

To determine the safe loading for that gearbox, the easiest way would be a destructive test, like you mentioned. Lock one side, put a torque meter (digital torque wrench perhaps, rod and weights?) on the other side, and have at it until something breaks. If you wanted to, the maximum safe load could be calculated, but you'd need all the gear pitches, tooth counts, material, face width, hardness, etc.

There is not any listing I know of that indicates the maximum allowable torque for common gearboxes used in FRC.

Re: Gearbox Maximum Torque
 

Look on the vendor's spec sheet for the gearbox. If there is no spec for it, caveat emptor.....

64:1 is a steep ratio for a gearbox, 132:1 is asking for trouble if the thing is heavily loaded (as a lifter would be)

Re: Gearbox Maximum Torque
 

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.

Re: Gearbox Maximum Torque
 

It was only 4 stages of 4:1.:rolleyes:

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.

Re: Gearbox Maximum Torque
 

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.

Re: Gearbox Maximum Torque
 

Heh. The quality of the P60 is significantly greater than the commonly failing gearboxes from 07-08. (Oh jeez...the horror of replacing those as a freshman...:p )

Re: Gearbox Maximum Torque
 

Cou you post a picture of the damaged parts of the gearbox?

We played with a banebots P80 256:1 Gearbox for our Hanging Mechanism that was later distroyed. Apparently the motor was driven the wrong way and stalled against a stop for the arm for too long and I believe this stressed the carrier plates inside of the gearbox, but I'm not entirely sure. I'll have to crack it open next time I'm in the shop and see.

Re: Gearbox Maximum Torque
 

I am the teacher/lead at 2468, the Team Eric is referring to in this post.

We have used the Banebots for three years now in a variety of applications and have had great success as well as appreciate the service we have received from them.

No information is provided by Banebots as to whether it would be able to handle the stress we placed it under.

Another mentor (physics teacher) ran through all the calculations with a group of students to decide on the FP and the Banebots P60 132:1 mentioned in the post.

The apparatus was mounted in the middle of the chassis. It was positioned within about 6 inches of the rear chassis "C" channel. We had a carbon fiber hook on the bar connected by spectacord to a spindle attached to the output shaft of the P60. The motor was mounted on a 1/4" aluminum plate that was attached to 80/20 rails.

The last output gear was stripped of the teeth in a lift. It was probably the 15 or 16 lift we had made with the system. It would lift the robot above the tunnel in less than 6 seconds.

Maybe this will help with the discussion.

Thanks,
Norman

Re: Gearbox Maximum Torque
 

We are often lulled into the assumption that a gearbox can handle whatever job we are applying it to for the desired speed, but with ratios this high, they can certainly self-destruct if too much motor is used. It is important not to use a motor that can yield more torque at the output than the gearbox can handle. Planetaries by their nature have very small teeth for their ratios, which is why they are so compact, but this limits their output torque compared to more robust designs. For high-ratio, low torque applications were compact size is a primary consideration, they are great. But if you are planning to push a planetary hard with a torquey motor, beware.

Re: Gearbox Maximum Torque
 

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).

Re: Gearbox Maximum Torque
 

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.

Re: Gearbox Maximum Torque
 

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

Re: Gearbox Maximum Torque
 

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