Finding torque for a HAB climb


#1

So I know by now everyone has seen the Lego four bar prototype, but i was doing some math and the amount of torque needed to drive a 20in four bar with a 130 lb load just gets too much. What gearboxes would you guys use to get a large reduction like 256:1 for two cims??


#2

I would start with a large sprocket or pulley reduction of 2:1 then versa planetaries. If that is still too high for their load ratings, the AM sports can handle higher load. Consider 775 pros instead of cims. This is going to be used only once or twice a match with the motors running full I assume.


#3

I have a related question. If I want to use a drum to wind cable on an arm similar to the 'Snow Problem mechanism, does the drum count as driven or driven when modelling the system as an arm?

I.e. is it a reducer or a multiplier. Idk why I can’t get my head around this.


#4

Chain as your last stage is just a good idea, if the word torque came up in talking about the mechanism.


#5

I’m agreeing with @Tom_Line on the overall approach. I’d also be weight-conscious on the rest of the robot, which can reduce your motor needs.

You could also consider using pneumatics to assist the start, where the motors see the highest load since the robot is the furthest out on the arm. How much and where is an exercise I’m leaving to people smarter than me.


#6

I under that chain is better than belts in this situation, but is chain also better than gears for high torque applications like this?


#7

Thank you very much I will look further into this


#8

Chain vs gears depends on the size of either one


#9

Ok, how about a 1:2 ratio? Say 18:36 gears vs 18:36 sprockets + chain.


#10

You might be able to run a PTO off your drive with some extra reduction to slow it down.


#11

Yes, it would be wise to use chain for the final reduction to the arm. Probably #35 chain if you need to use a sprocket small enough to fit under the drivetrain.

I would also recommend giving this some thought.


#12

Why not have the system set up over-center for the majority of the match, then with a ton of surgical tubing assist the beginning of the lift. Your motor just has to overcome the over-center tension, then it can be significantly easier from there.


#13

Yeah I know #35 chain, I was just wondering the relative strength of gears vs chain for high torque situations.


#14

I always default to chain because our machining capabilities are limited, and tensioning chain is easier than getting low tolerance c to c distances right on gears.


#15

For a high stress application like this, parts can deflect enough for the teeth of a set of gears to disengage and skip (assuming you are using 20dp gears, like most teams tend to use). Chain can be more tolerant of these kinds of deflections and still works under heavy load without skipping teeth.


#16

Ok, that makes sense.


#17

Looking for ~200 ft-lb of torque in an FRC robot, I wouldn’t try to apply that as a rotation of an axle, but as a force well away from the axle. Maybe a lead screw or a rack/pinion setup around the middle or front of the robot?


#18

In this application, it really does not. Chain is definitely better than gears here for at least the final stage.

The reasoning is pretty simple - this is a long, high torque arm. This is susceptible to high shock loads and long sustained loads.

A sprocket and chain will dampen shock to a small extent, and will spread the load across all teeth the chain is currently engaged with. A gear mesh will only engage 1 to 2 teeth at a time.

I’m sure with a comically small sprocket and chain, versus a comically large gear, you can contrive some scenario where the gear would work better as the last stage in a very high load arm reduction. But really, just don’t do it. Some of us have been burned before…