Elevator counterweight / constant spring and ability to lift different number of tote

[cmrnpizzo14]

Theoretically couldn't you just design for the heaviest weight that you expect to hold? It wouldn't be ideal but if you think about it, 40 lbs is actually quite a small range so it should work.

[Sh1ine]

Originally Posted by cmrnpizzo14

Theoretically couldn't you just design for the heaviest weight that you expect to hold? It wouldn't be ideal but if you think about it, 40 lbs is actually quite a small range so it should work.

Even if you split the difference (0 lbs with no load and 50 lbs full load) and put a 25 lb CF spring your motor would always be working, but at 1/2 the force and 25 lbs should be able to be held by a cim (or bag motor) without too much issue.

[nixiebunny]

The fusee pulley is one solution. It automagically changes its "gear ratio" as the distance traveled increases, so you can use a gearmotor and optimize its lifting speed for the small stack but still have plenty of torque for the tall stack.

[pmangels17]

Why not use a worm drive? We used an Andymark Wormbox in 2011 for our elevator with decent success (we had other racking problems irrelevant of the gearbox) and the RAW Box, as already suggested, could work. We actually used a custom worm drive last year to winch back a really heavily loaded catapult arm, but if you don't have resources to custom build, use a COTS worm gearbox. You can mate with a CIM and get a ton of power out of a reliable box.

[lamk]

Originally Posted by pmangels17

Why not use a worm drive? We used an Andymark Wormbox in 2011 for our elevator with decent success (we had other racking problems irrelevant of the gearbox) and the RAW Box, as already suggested, could work. We actually used a custom worm drive last year to winch back a really heavily loaded catapult arm, but if you don't have resources to custom build, use a COTS worm gearbox. You can mate with a CIM and get a ton of power out of a reliable box.

We actually use the said gear box for our aerial assist bot to arm a catapult but we have to use a one way bearing and clutch to arm and fire it. For an elevator we want to be able to move up and down at will. However if we only aim for 3 totes and use 2 sims we may be able to get away without a braking system.

[pmangels17]

If you use a worm you don't have to use a one way bearing, they can be driven both ways without a braking system. They are also a good way to have a compact gear reduction that slows the CIM motors down slow enough to be usable in an elevator.

[Cyberphil]

Originally Posted by nixiebunny

The fusee pulley is one solution. It automagically changes its "gear ratio" as the distance traveled increases, so you can use a gearmotor and optimize its lifting speed for the small stack but still have plenty of torque for the tall stack.

I have never heard of one of these. Although I am not working with a team this year, I may use this on my own practice design. Seems like it will do the trick.

[Jared]

Worm drives CAN backdrive- especially multi start worms. As they wear in, they can backdrive a vibrating and shaking robot.

[pmangels17]

While yes that is certainly true, depending on the number of starts (or threads) on the worm, the torque required to backdrive is often much higher than will ever be seen in an FRC application, though vibration could foreseeably unwind a worm drive at a very slow rate.

[Nathan Streeter]

Originally Posted by Jared

Worm drives CAN backdrive- especially multi start worms. As they wear in, they can backdrive a vibrating and shaking robot.

This is certainly true.

As a note, if anyone is looking at the AM WormBox, we used it last year as the initial (and largest) reduction for the arm on our 2014 robot, Kaizen. The WormBox has a multi-start worm gear and was very much back-drivable by the 'reaction forces' of our robot accelerating and stopping. We counter-balanced this arm reasonably well using surgical tubing to enable us to gear the arm faster... with this counter-balance the arm wouldn't backdrive the WormBox when sitting statically (at least when not holding a ball), but could be backdriven if you pushed the arm to a desired position.

The WormBox worked pretty well for us (enabling us to have compact, lightweight, low-backlash gearing), but we did preemptively insert one or two thrust bearings axially on the worm-shaft, which I would recommend to anyone that uses it, particularly if they expect impact loading on the mechanism. We only got away with cantilevering the output gear for a while before we wore down the 7075 Aluminum gear that we mated to it and the press fit of a bearing on the output shaft of the WormBox. This was in issue with our implementation though, not the WormBox.

[JamesCH95]

DeWalt, and other drill transmissions, have anti-back-drive pins in them that can work quite nicely for this purpose. As a bonus: they work in BOTH directions AND there's a white paper on how to interface them with FRC motors called "Nothing But DeWalts."