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

It's easy to figure out the counterweight/constant spring if we have only one single load like tubes used in previous year. Now we have to deal with loads that varies from 8 lb to 50 lb. How can we precisely move the elevator up and down and not fried the motor when holding a position. A linear electrical actuator(worm screw) with no back drive will be nice but we don't have the machine capability.

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

Instead of trying to counterbalance, think instead of applying a friction brake of some sort. Try thinking about how you brake a bicycle tire, and see if you can do something like that with your spool that lifts the elevator. Pneumatic would be a great way to generate that braking force without burning out a motor!

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

window motors, snow blower motors, and van door motors (these have the most torque) have worm drive. Use those.

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

CIMs or mini-CIMs with a RAW box from andymark could work well

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

These are 30 watts or less, meaning they are very slow.

Given lack of machining, I agree with the bike brake idea. Otherwise I would say use a steel punch into a hole in a 1st stage gear in the gearbox, pneumatically actuated. 1538 did this with great success in 2013. Requires some precision machining to line up the punch with the hole though.

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

In the past we've successfully used both bicycle disk brakes and ratchet/ pawl systems to hold loads at position without running the motors.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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.

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

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

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

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.

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

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.

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

We opted for the KOP drive system, but we're equipping it with holonomic wheels, so we are considering "recycling" the 4" rubber tires as brakes. We'd build a clamp that fits around the wheel, probably with a brake lining of carpet, and open and close it pneumatically. Not sure how often we'd have to change the carpet - my guess is after a about 12-24 hours of practice, and between tournaments.

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

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."