back-driveable vs. not

[Stephen P]

Does "non-back drivable" mean that a motor can't be turned when off? Or can it not go in reverse? Or not be pushed backwards while going forwards? And are the window motors non back-driveable, meaning when suspending the robot via winch they won't release when turned off?

[Pyro16]

I’m pretty sure that it means that it can’t be moved if its off. If it’s the window motor it does goes in both directions and I don’t thing will turn because it’s a worm drive.
Why don’t you test it to make sure?

[KenWittlief]

dont try to find out by testing it - someone on here should know one way or the other

if it is a high ratio worm gear and you try to force it, you can strip the teeth off the gear - then how will it be able to eat steaks or apples?!

[Matt D]

non backdriveable means that you cannot physically move it (in either direction). Only the motor can move it. One way to do this is with a worm drive (although worm gears with pitch larger than ~10% can be backdriven).

Here is more info on the window motors (found by searching CD):

The anti-backdriving capability of those motors is awesome, so we try to use it to our advantage. Modifying the motor is just not allowed within the rules.

I know you don't want to argue the point, but I just can't help myself. The seal drag and other resistive forces are nothing compared to the weight of the window. Even though the window is going down, gravity is still acting on the motor (just helping). The motor's backdriving resisistance (or the linkages' backdriving ability) allows the window to stay up when the motor is off. The second the motor starts to move, the backdriving ability of the worm is greatly reduced (sliding friction vs. static friction) and gravity actually pulls the window down and the motor just controls the fall.
http://www.chiefdelphi.com/forums/showpost.php?p=217050&postcount=8

[Stephen P]

non backdriveable means that you cannot physically move it (in either direction). Only the motor can move it. One way to do this is with a worm drive (although worm gears with pitch larger than ~10% can be backdriven).

Here is more info on the window motors (found by searching CD):


http://www.chiefdelphi.com/forums/showpost.php?p=217050&postcount=8

This anti-back drive device may be adequate for suspending a 5 pound window, but what about a 130 pound robot?

[KenWittlief]

its not only designed to hold the car window open, but to prevent a thief from pushing it down and to keep the windows from sliding down when the car hits a pothole.

[Salik Syed]

its not only designed to hold the car window open, but to prevent a thief from pushing it down and to keep the windows from sliding down when the car hits a pothole.

i'm not sure about the window but i can guarentee (is that how u spell it?) that van doors can be back driven w/ a 130 lb robot!

[Chris Hibner]

i'm not sure about the window but i can guarentee (is that how u spell it?) that van doors can be back driven w/ a 130 lb robot!

The Van Door Motor is designed to be back driven (so you can open the doors of the van manually in case the motor fails).

The window motor is designed NOT to be back driven (so people can't break into the vehicle).

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

[Paul Copioli]

We have used the window motors for 3 of the past 4 years and in every case they did not back drive.

1. 2000 used as our winch motor and hung after the power was shut off

2. 2001 used as our conveyor extender motor and held 200 lbs of pre-load without back driving

3. 2003 used as one our strap motor and one as our bin lift motor. Neither were backdriven. The bin lift motor held me up (180 lbs) when it was shut off.

This year we will be using the window motors for an application that requires them to hold position while turned off.

-Paul

[Max Lobovsky]

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."

[Paul Copioli]

I will try to put this to bed so everyone is comfortable about the window motor and its ability to resist back driving. Two pieces of info:

1. Torque at breaker limit for window motor (torque when breaker starts to trip) ~ 5 N-m

2. Speed at which this occurs ~ 50 RPM

The max torque the window can put out is MUCH lower (at least 10 times) than the backdriving torque required to destroy the worm or break free. From experience, I can tell you that the worm breaks before the motor breaks free.

In theory, you could put a large enough diameter spool so that the 130 lbs of robot weight would break the motor, but then you would not be able to even lift yourself up. If you design the spool and motor combination to actually lift the robot up, then you are nowhere near back driving the motor.

-Paul

[blindguyinanorg]

we used the window motors last year for our fold out pushing arms that weighed 5 lbs+ each. they were place and the bottoms of the arms. we had to reaplace them several times because the load stripped the gearbox.

[NoRemorse]

In theory, you could put a large enough diameter spool so that the 130 lbs of robot weight would break the motor, but then you would not be able to even lift yourself up. If you design the spool and motor combination to actually lift the robot up, then you are nowhere near back driving the motor.

This is brilliant. Pretty simple actually, but i didn't look at it that way. Thanks!

[ShadowKnight]

aye, we also are using the window motors...a combination of 2 actually because we have the weight and we can...less strain on each motor and all...and after running some of the claculations, we can lift approximately 600 pounds with the motors in combination with gearing down the motors. So yeah...that eliminates the need what so ever to build a devise that prevents the motors from back driving and we really have no worries about breaking the worm gear, especially after some of the testimonials afore mentioned.

[Aignam]

If you simply need to keep a platform raised, or something to the effect of that, you might consider keeping a motor running just enough so the torque holds it in place, but does not change the position of said platform, etc.

[Dick Linn]

Just be mindful and use the correct diameter cable (if you are using cable). There is a definite working strength with cable, and it depends quite a lot on how you fasten it and the radius of the attachment used for the cable or whatever diameter pulleys are used. Leave a reasonable safety margin.

A 3/16 inch cable could be used with plenty of safety margin, or you could snap a cable of 1/8 inch or much larger if you don't engineer it correctly. If you have a cable running around a 1/4 inch bolt or something small, do you realize what stress that causes on the cable? Anything less than a couple of inches diameter should be carefully considered, even if you are oly lifting the robot a few times in the course of competition. Do be careful!