Hello,
We have an arm that weighs about 6 pounds that is mounted on two posts. It rotates on a rod that is about 4" from the end of the robot. That arm has a gear attached to it and a chain leading to a window motor. However once the arm gets past center it can’t control the arm, and thus the arm comes “crashing,” in a sense, down. The motor has a worm gear, yet it can’t stop the motor momentum. Is there some motor that we can get that has a stronger worm gear? I will get the motor name and number once we get to school later today. It is a motor from a few years ago, with an 11mm shaft that turns into a screw end, if that helps any veterans.
well i would suggest that you change the motor…or make a lighter arm. 
if that helps of course.
hmmm
would counter weighing the arm work?
We got a “gas spring” from McMaster-Carr to hold most of the weight of our arm.
they can only be shipped by ground (no air shipping) so if you want to try it you’ll need to order it soon.
Also, the spring we got has a 6.3" stroke, and is rated 60 lbs. It has that much force when it’s in the extended position, and has 75 lbs force when compressed all the way.
With a worm gear, the arm should never backdrive like that. If anything, the motor would just stall out when trying to lift. It sounds like something (chain, set screw, worm gear itself?) is not fully engaged and/or is experience so much torque that it slips. What is the sprocket ratio between the motor output and the arm itself?
Could you use 2 motors?
The motor he’s talking about is the (dearly departed) van door motor, I’m assuming from the fact that it has a screw thread on the end. It’s no longer legal anyways, so you’ll have to change it for a different motor, but for informational purposes it’s a worm-gear transmission and it is, in fact, backdriveable. I could go into all the technical details, but suffice to say, worm-gear transmissions can still be designed to be backdriveable. It simply depends on the lead angle of the worm-gear. If the lead angle is less than the friction angle, the worm-gear won’t backdrive… usually. This usually works out to a lead angle of around 8 degrees.
At any rate, to answer your actual question, I don’t think there’s a non-backdriveable motor in the kit that will help you. The only motors of this type in the kit are the window motors. They have a stall torque of around 9 lb-ft. That means the center of mass of your 6 lb arm will have to be less than 1.5 feet for your unmodified arm to be able to lift itself from a horizontal position. However, as someone here noted, counter balancing your arm may work, as would gearing down the window motor.
In summary, your current motor sounds illegal. Your options are to switch to a window lift motor and either gear it down enough to move your arm, or counterbalance your arm enough that you can directly drive it.
thanks
we switched the motor out with the new window motors and it works much better