High Torque Arm design questions

[apples000]

Okay, I have a few more questions.
I think we're going to be using some gearbox with a single chain reduction.
We are going to drive the arm pivot because we want the arm to be able to rotate 180 degrees.

This transmission can backdrive, so I'm looking for ways to prevent it.

I have three ideas:

Worm gearbox (worm reduction + chain):
I need a single start worm. The goal is (at stall) 180 ft-lbs at the joint, or 43 ft-lbs at the drive sprocket. In in-lbs, that's 518 in-lbs on the worm gear.

At half speed, I have 7.2 rpm at the arm, 30 rpm at the worm gear, and 900 rpm at the worm.

Both of these meet operating specs for 12 pitch ground/hardened worms running with 30 tooth bronze worm gears.

The disadvantage is that it is expensive, and large shock loads could damage it.

Locking gearbox:
I'd use a dog clutch somewhere. My concern is backlash if it's early in the reduction, and force to engage/disengage it if it's near the end.

Also, I could go with something like the robonaut's claw locking thing where they have preset positions it locks in. I'm not sure if I like limiting myself to only a handful of presets though.

Any ideas/suggestions?

[EricH]

Originally Posted by apples000

Okay, I have a few more questions.
I think we're going to be using some gearbox with a single chain reduction.
We are going to drive the arm pivot because we want the arm to be able to rotate 180 degrees.

This transmission can backdrive, so I'm looking for ways to prevent it.

I'll point you to one of the robots from 2005, namely 330's. Large sprocket driving a single pivot, dead-axle (sprocket directly attached to the base of the arm). To get to the sprocket from the FP motors (x2), the power first went through 1 FP gearbox/motor to a common shaft, to a small sprocket, to a medium sprocket on an intermediate shaft via chain, to a small sprocket on the same intermediate shaft, to the big sprocket. No backdriving without a bit of force (>8 lb@5'), and it could lift us under power). Also no lock...

There are a number of methods for reducing force on the chain; try running 2 chain runs in parallel, which will cut the amount of force each chain has to take in half. Also note the counterweight/counterforce suggestions. The counterweights will help stop backdriving.

For purposes of locking an arm, a small pneumatic cylinder fired into holes will do the trick. Or a small servo into a slot. Something like that.

[DampRobot]

Worm gears all the way!

Worm gears are great. They're nonbackdrivable, and lend themselves to both manual and closed loop control. Basically, where you put them they stay, which means that you can have a much simpler and less finicky control loop. I think we just had a P loop on our worm powered claw tilt this year, and had it not been for the precise angle requirements, open loop control would have worked just fine.

12 DP seems like a reasonable pitch based on my experience, but you could probably get away with as high as 20 DP.

Realistically, whenever I've seen large shock loads running through a worm gearbox, either the spacers around the worm or the thrust bearings on the worm are what get damaged. When the worm gear teeth are 3/16" of hardened steel, and your washers on your thrust bearing are 3/64" of unevenly supported stainless, it's not hard to see what will fail first. Of course, you can bump up the bearings to angular contact bearings, and make all your spacers steel, but at that point, you're likely going to see other things break before the worm gear. We ran a worm gear setup as our claw tilt this year, and shock loads (like stopping the claw suddenly) caused the screw holes holding the sprocket to the claw and the hex on the worm gearbox sprocket to round out before the worm and worm gear were damaged.

If you do go worm, you're going to want to throw a lot of power at it. I'd guess (without having run any numbers) that you'd need at the very least a CIM to power whatever you're describing. We ran a 2 CIM, 1 lead worm + 2 spur reductions gearbox this year, and observed efficiencies in the 6-7% range. That gearbox was the result of a lot of institutional knowledge too, I doubt our earlier versions were even that efficient. Worm gears are great, but they do dump a lot of power.

We've been doing worm gearboxes for about four years now. Let me know if you'd like to see some of our gearbox CAD. I'd be happy to share designs and experience.

[Kevin Leonard]

Personally, if I have to lift anything in FRC, I'd prefer a 4-bar lift any day.
#mytwocents
Plus they can be pneumatic, and 20 likes pneumatics.

[Jared]

Worm gears are pretty good for this type of application.

We used a two start worm on our climber with decent success. We used a 2 start 16 pitch worm and a 20 tooth worm gear. We must have climbed our practice tower hundreds of times with the setup (2 CIMs direct driving the worm) and we had no issues.

Worm gears aren't indestructible though. We broke two, one bronze, one steel, once we increased our climber to 3 CIMs and doubled the speed.

I think you can do better on efficiency than the previous poster has mentioned.
Our climber, which was two belts, the worm reduction described above, two #25 chains, and a 5 start lead screw, and we saw over 50% efficiency (measured by the amount of time it took the robot to lift itself).

Making sure the worm/worm gear are in the exact right spot is very important. We added .002" extra to the center to center like we do with spur gears. Also, make sure you put plenty of grease on the gears.

[IKE]

To the OP,

Chain's do not "handle torque", they handle tension as your rating called out. Chains are excellent for doing what you are looking to do.

With a big enough sprocket, you could make thousands of foot pounds of torque with 25 pitch chain. It just might be a a very big sprocket. As Archimedes said, with a large enough lever, I could move the world.

As far as pivots go, FRC 33 used large bushing for the 2005 and 2007 robot. The joints were really nice to work with and gave smooth operation, though the arm construction could be difficult.

With high torque though can come high forces, and you do need to be mindful of the loads. If you would like help doing calculations and layouts, let me know and we can figure these out as a an open case study.

We would start with calculating the amount of load and how far/fast we wanted to move it, and thus would dictate a minimum motor power. Based off of that, we can pick a motor, and then figure what sort of methods we could use to get the right torque.

[the.miler]

If you're interested in counterbalancing your arm, this year we figured out a way of using surgical tubing to perfectly balance our collector arms throughout their range of motion, which could possibly be upscaled to deal with heavier arms.

I'm hoping that the student who worked on that system will chime in here (I can only tell you that it worked like a charm and integrated into our design quietly, inside a frame tube where nobody ever noticed it without us pointing it out). The value of counterbalancing the arm was that it allowed us to use a much smaller pneumatic cylinder than we otherwise would have, reducing air usage while giving us very quick actuation in both directions. Similar benefits would have been in store if we'd used a motor to actuate the arms.

The links show a mechanism (and components) for counterbalancing a lift gate that works on the same principles. The first link also has an entertaining video that shows it in action:

http://www.northerntool.com/shop/too...BTOWbnB85IBZSh

http://www.etrailer.com/Accessories-...Fc1afgod9HgA3A