back-driveable vs. not

[Max Lobovsky]

Originally Posted by Chris Hibner

I would be willing to bet that the window motor cannot be back driven due to the weight of a 130 lb robot.

You cannot make a statement as simple as this. There is a torque at which something will give. In this case, the worm gear will probably be the first thing to break. The torque backdriving the motor depends on not just the weight of the robot, but also the radius of the winch. i.e. if the robot weighs 130lb and is directly hanging from a winch of radius of 0.1 feet, you will have 130*0.1 = 13ft*lbs of torque acting on the motor. If someone has some data on these motors, that would be great.

[Dick Linn]

There are other older threads on the door motor/winches. I suggest searching the forums and you'll get some good info. We put a winch spool on the van door motor assembly (maybe 1 1/2 diameter in the center and 3" for the end plates. You can turn it relatively easily by hand, so we are putting a positive hold mechanism (like a cleat or an ascender for rock climbing) onto the cable to keep it from unreeling once we lift.

Worm drives can definitely be back-driven if the lead angle is too great. There is some info about this at bostongear.com at http://www.bostongear.com/products/enclosed/faqs.html (and possibly in the "gearology" tutorial, too) and a mention about back drive efficiency (Eb) somewhere at http://www.roton.com/worms.jsp. I saw this from a random search, so don't take it as hard fact: "A rule of thumb for the worm drive is they wilt not back drive above a 20:1 ratio. If vibrations occur, however, the ratio will need to be higher. Self-locking in the gear pass depends on the coefficient of friction between the worm and the worm wheel, which depends on the choice of materials and the lead angle of the worm, which is a function of the desired ratio."