Re: High torque that can be purchased.
 

We run two 775's through AM planetaries (making them the equivalent of a cim). From that point, we run them into a toughbox mini (12.75:1), then through a 36:14 reduction to an intermediate shaft, then through a 45:15 reduction on our final shaft.

Total reduction: 100:1.

It's actually still not nearly as high as I would like - we'll probably correct that before states because we occasionally get some wobble in the arm and we run the motors around 20% power.

Re: High torque that can be purchased.
 

The RS775 has enough stall torque to put 150 ft-lbs of torque on the output of a 256:1 transmission. This does not count shock loading.

Banebots has warned that the 256:1 has a MAXIMUM operating load of 35 ft-lbs.

Re: High torque that can be purchased.
 

And my experience has been that surviving repeated use even at less than this rated max load is certainly not a guaranteed thing. These gearboxes simply fatigue out, even at lower loads.

Re: High torque that can be purchased.
 

I agree with Jessek (team 1885) that a quick simple fix, which can INSTANTLY be verified, is gas struts. You can purchase them as replacement parts for the rear hatches on SUV's or run to your local camper dealer and pick them up for $20 or so. Once installed, you only need to have the arm "balance" itself, parallel to the floor. The advantage over surgical tubing is that when compressed, the struts exert a greater force than when fully extended, which is when you need them least; yet when fully extended they still exert considerable force. We used paired window motors and a 5:1 sprocket with the struts to lift a ridiculously heavy arm with ease. In fact, the motors were used to PULL the arm down!

Re: High torque that can be purchased.
 

McMaster-Carr has a good assortment of gas springs and mounting hardware. Can't be shipped by air, though.

Since there seems to be a problem with the BB transmissions due to shock loading when the drive direction changes, you might want to consider adding some slip to the drive system. We used a V belt to get about 4:1 reduction for our arm. It's driven by a single window motor, counterbalanced by a latex tube spring. And we can move the arm without the motor turning, the belt will slip when needed. But it's a light arm....if your arm is heavy, you may need more than this.

Re: High torque that can be purchased.
 

You're hitting those loads for very short durations during normal operation.

Re: High torque that can be purchased.
 

Any time you change direct at what is perceived "instantly" you are putting a tremendous load on the gearbox for a very short instant (similar to how impulses and momentum yield huge forces in collisions). If you exceed the working load by say 5x 3 or 4 or 5 times a match in different directions, you are slamming the components that are breaking back at force at above their load limits, for a short moment, causing wear and tear on the parts.

Think about a car, generally the transmission should not have issues with the torque output of the engine, but if you are going 30 or even 15mph down the road and slam it into reverse, something is going to break or get damaged. Or if you are familiar with manual cars, letting the clutch out too fast can make the entire vehicle lurch, you are accelerating your entire drivetrain (transmission back) to match your engine's speed almost instantly, though in most cars you will burn out the clutch.

Re: High torque that can be purchased.
 

Our team ran into problems with the amount of torque it took to drive our arm, and went through several designs before getting one that lasted. The current one we have now has survived one regional, and even was able to provide enough torque to right our robot as it was falling over. The current transmission uses two rs-775 motors, each going into a 64:1 BB p60's. 64:1 is the highest ratio that we thought safe to use, since each rs-775 puts out about .5 ft/lbs at stall torque, over 64:1 and the motors stalling could strip the gearbox. After the p60, there is a 2:1 stage of spur gear reductions. They are 20 pitch, 20 degree pressure angle, with a .5" face width, to handle the torque. The smaller face width gears from andymark could be iffy, as there is already 128 ft/lbs on them at stall. Then, we have two stages of sprocket reductions, 2.33:1, and a 4.5:1 which directly drives the arm. The first stage of sprocket reductions uses andymark #35 chain sprockets, a 12 tooth, and a 28 tooth mounted on a special hub with a 3/4 bore and key to handle the torque. The last stage is a 12 tooth #35 chain steel sprocket with a 3/4" bore from mcmaster, and a 54 tooth sprocket also from mcmaster. It has worked great so far, no problems at all. If you don't a way to accurately locate holes, creating the proper meshing for the spur gears could be tricky. We also tried an entirely spur gearbox, 312:1 with the same 4.5:1 final sprocket reduction, only to find that the teeth stripped out during Thursday practice. We then built the current one in the remaining half a day, much thanks to Team 701, and the Nasa Machine Shop, and were able to make the first round of Friday matches. Here is the new arm transmission, putting out max torque, (~1300 ft/lbs) and living to tell the tale.

http://www.youtube.com/user/LelandRo...uVXu3$@#$@#$@#(2:03)

Re: High torque that can be purchased.
 

Just from watching them in competition, there are many arm designs out there that jerk around a lot more than that. I cringe when I think of what that is doing to those tiny tiny little gears in the BB transmission.

Re: High torque that can be purchased.
 

Yep, unfortunately I haven't watched a competition yet, but even low loads like that could probably trash the gearbox.

Now that I think about it, that arm with all the weight on the end is going to bounce every time your driving changes direction or you collide with another robot and will probably smack the gears around in the banebot transmission a bit.

Re: High torque that can be purchased.
 

FP motor + 20:1 BB transmission + 4-start lead screw + fancy-schmancy gusset + surgical tubing = no problems for our "high torque" arm, because it isn't high torque at all!

Without the lead screw connected, the arm can be moved with gentle pressure from one finger, even when fully extended and holding a tube. After looking at the beautiful, smooth operation of 2056's arm this year, we're looking at the possibility of upgrading to a gas shock intead of (or in addition to) the tubing.

Re: High torque that can be purchased.
 

Can you explain this? My high school physics knowledge tells me that surgical tubing is elastic (and near "perfectly elastic"), and the force increases with distance (Hooke's law).

Re: High torque that can be purchased.
 

You are correct that the force does increase with distance. The advantage is that the surgi tube fatigues much quicker than the gas strut leaks so the strut will be more consistent over time. Of course you can replace the surgi tube on a regular basis.

Re: High torque that can be purchased.
 

This is indeed what we did (replaced the tubing on a regular basis).

Note that surgical tubing follows Hooke's-ish law. Rubber bands and other such things are not quite springs, and the exact characterization varies, but it generally is not exactly F = -kx.

Re: High torque that can be purchased.
 

You are correct, I was generalizing, as far as I know there is nothing that is perfectly elastic in the true sense.

The other thing is that surgi tube is not perfectly uniform, there is certainly some variation from batch to batch as well as within the individual batch.