Gearing a Rotating Arm

[philso]

Originally Posted by Chris is me

Arms can be tricky!

I would avoid driving a long lever arm with a live 1/2" hex axle, ...

If you must motorize a long lever arm, it's probably best to do the final reduction for the arm using chain, with the sprocket bolted to the arm to transmit torque. 973 RAMP has some good videos on how to design a simple durable arm power transmission system. The chain can deal with some of the shock loading the arm will face a bit better than a final gear stage could, as there is some flex in the chain and the sprockets are engaged at multiple teeth versus 1 or 2 teeth for a geared system.

A team here in Houston twisted some 1/2" hex so that it looked like part of a wrought iron railing last year by transmitting the torque from the sprocket to the arm through the 1/2" hex.

Originally Posted by JamesCH95

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

The RS550 and RS775's rely on fan cooling. If the stall current is high enough, the windings will start smoking pretty quickly, possibly causing permanent damage to the motor.

[MrForbes]

We made a wooden cam last year to raise an arm, I think the rate and load would be similar to what you're trying to do. The students come up with some neat ideas!

[Sperkowsky]

We plan on running a small arm on a cim geared by a versa planetary 100:1. The arm should be done this week and will definitely be tested next week. We may tone the gearing down and if it seems like a situation of durability we may substitute our P80 in. But, It looks like it will be fine. One thing I will see is do not directly drive the axle with the motor. Use chains, belts, or gears. We plan on using gears.

[Lil' Lavery]

I've learned the hard way that it's better to be overly cautious with your ratios when designing an arm, and especially so when you need precision control. A more aggressive gearing may get you to the position you need to be faster, but it's also going to apply more torque to the motor which means it's going to draw more current. As a result, you're going to get an arm that "spring to life." Fine tune movements will be difficult, since by the time you apply enough power to get the motor moving with an aggressive gearing, it's going to be moving fast. Further still, the more you gear back, the less you have to stall your motor in order for your arm to hold position (if you have to stall it at all).

[Collin Stiers]

careful with the versa planetaries. if you look at the published load specifications it says that a CIM on a 100:1 is well outside of the load limits for those gear boxes. Also your speed seems high. My team is doing a similar thing to you however our arm mass and length is larger. We plan on using two mini CIM's we will have a total of a 150:1 gear ratio. 50 of that will come from a 3 stage gem planetary then another 3:1 reduction by chain and sprockets. also be carefull with what you use as the axle. we are using a 1 inch aluminum pipe.