Gearing a Rotating Arm

[JamesCH95]

Originally Posted by asid61

JVN calculator is my go-to for rotary stuff. Just plug in your ratios and it spits out speed.
For your application, I get a speed of 0.27 seconds to go 90* if I used 2 miniCIMs on 100:1 versaplanetaries (no extra reduction).

1) JVN Design Calc is very good for this sort of thing

2) Read and understand your chosen transmission's applications manual there are a number of motor/gearing combinations that quickly exceed VexPro's recommendations (or bane-bots, or whatever). For example: running a mini-cim with a 2x10:1 vesaplanetary is counter-indicated, but running a mini-cim with a 9:1+4:1+3:1 gives you 108:1 reduction and is approved. I recall that when BB planetaries were first used widely in FRC there were a number of complaints about the transmissions failing, and it was because they simply weren't rated to deal with an RS775 geared down 256:1 (for example).
[moving on from addressing the above post]

3) Arms can usually stall to hold position, but you must make sure this current is acceptable (see JVN calc for this). I've designed arms that stalled RS775s to hold position with no issues, use your judgement here

4) If you counter-balance, and I think most arms should be counter-balanced, I suggest working out the spring geometry to counteract gravity. I.e. you want the most counter-balancing torque when the arm is horizontal, and the least amount (or zero) when it is vertical (meaning its CG is over the pivot point).

5) Some amount of damping on your counter-balance is fantastic for smooth control. We really like using air cylinders to counter-balance because they provide a constant restoring force, damping, and mechanical hard-stops all in one.

6) Back to actual arm gearing... my general practice is to pick the power I want the arms to have, which drives my motor decision. I gear as much as possible with a planetary transmission, staying inside the MFGs load guides, then I do I final sprocket reduction to the arm itself. Sprockets let me keep my drive motors down low in the robot, and generally help with arm packaging (many small details that I won't go into here). Chain is also a great place to couple on an encoder for arm control.

Examples of 95 robots' arm gearing:

2011: 2xRS775 + 64:1 bane bots + 14:72 sprocket reduction (IIRC), motors were mounted under the 'deck' of the robot, shoulder joint was near the height limit. Motors stalled to hold position. Some minor pneumatic counter-balance

2015: 2xCIM + 50:1 versa planetary + 16:64 sprocket reduction, pneumatic counter-balance kept arm neutral without game pieces, motors stalled to hold position. This robot needed over-balance protection in code since it could pick the arm+game piece up fast enough to flip itself over.