# which motor for arm?

i need some help…i didn’t know in which motor use for the arm ??
we try 2 keyang motors but when the arm goes up it fall down

Two Keyang motors should be able to hold up an arm since they have worm gearing. How much would you say your arm weighs and can it be lightened?

was the worm gearing included in the kit box…?
if yes how does it look? and how am i supposed to use it?

The Worm gear is built into the Keyang motors and make them hard but not impossible to back drive. We gear down even more (with gears or chain and sprockets) to slow the motor and add torque.

It would be helpful to know the length of the arm you are working with and the mass of the arm. It would also help to know the mass of the end effector, or whatever you have at the end of the arm.

Then calculate:

Arm torque=(mass of arm * .5 * length of arm) + (mass of end effector * length of arm)+ (mass of ringer * length of arm)

For a safe operating margin you probably want to roughly double this value…

The Keyang motors output about 10 Nm of torque (7.5 foot pounds) at stall. Two of them working together should deliver 20Nm of torque. If you gear that down about 5:1, you will have 100Nm of torque to drive your arm… that is about 75 foot pounds. The arm will take (very) roughly five seconds to complete one full 360 degree rotation… if you only need 90 degrees of rotation, that will only take one second… which is a bit fast. You may want to gear the motors down closer to 8:1 or 10:1.

Since the motors have worm drive built in to them (notice how the window motors are different from the planetary geared globe and banebots gear motors) they should resist a fair degree of backdrive, and be able to hold you arm in position quite nicely.

Do, however, make sure that you leave some safety margin in there… as your robot drives and bumps around the floor the sudden impact forces on the motors could be quite high and could end up damaging the gearhead on the motors. Use pulleys, sprockets or some other mechanism between your motor and your arm to manage the torque.

Hope that helps,

Jason

I would definitely use the FP motors with the white gearboxes. We’ve used them in the past on every one of our robots.