GearBox LockBack design

Look for a worm gear design. Depending on the configuration, it may not be backdriven.

Can you put an overrunning clutch on it?

Lead-screws can be designed to be non-backdriving, depending on how steep the lead is (in the same way a bolt going into a nut or threaded hole is non-backdring, turn the bolt, it goes into the threaded hole, put an axial force on the bolt, it will not move axially).

Cam-driven indexers are non-backdriving at the dwell position, but not through the whole range of motion, like worm drive gearing would be.

Generally, if the device is non-backdriving, the input member moves further than the output member, or the output member does not move at all. In the case of a lead screw, you have a small lead, i.e., it takes more rotations of the lead-screw per inch of axial displacement.

Typically, with gearing you are looking at a worm drive system, but the trad-off is that the input member i.e., the worm, must rotate faster or more times than the pinion gear that the worm is driving. A good book on lead screws or gearing, should show you how to calculate when a system will, and will not, backdrive. John

I think the most general rule of thumb is this: If the ramp angle is less than 15 degrees, the mating surfaces lock, and if greater than 15 degrees the mating surfaces loosen.

There is a smooth gradation of design, by the way, between a worm gear and two skew gears. It illustrates the 15 degree ramp business - skew gears will drive either way, worms wont (in general)

Brian Whatcott Altus OK

It's not quite as simple as that. If the only criterion for an irreversible mechanism was that it increased motion/rotation, the overdrive gear in your car would bring it to a screeching halt.

What makes a mechanism like a worm or screw irreversible involves friction. It's really little more than a simple inclined plane problem. Think of a block on a smooth ramp. If you push directly down on it from above, it might go down the ramp or it might not, depending on how steep the angle is and what the coefficient of friction is. The force pushing it down the ramp is the downward force multiplied by the sine of the ramp angle. If the friction force is greater, you're not going anywhere. In the case of a screw, you also have to contend with anything that might be preventing the nut from turning (in addition to the friction).

In a critical application I wouldn't rely on any mechanism to be naturally irreversible. Even though you might not be able to drive the mechanism backwards, if there is backward resistance it can lead to slipping over time, especially if there is vibration present. You should use a brake or sprag clutch if possible.

Don Kansas City

Hi Don & Brian,

John2005: Generally, the output member *decreases* motion, i.e., in the case of a worm gear, the input member (worm) may make several revolutions compared to the output member (pinion gear) which may only make 1 revolution per 10, 20, 40, etc. worm revolutions, depending on the drive ratio.

I did not mean to imply that this was always the case or a requirement, but that this is *generally* the case with a non-backdriving mechanism. You usually have this type of trade-off.

The analogy you and Brian used with a ramp, is a good way to describe it.

In the case of a cam, if the ramp has *any* incline, a roller follower will always backdrive the cam. At the cam dwell, there is zero incline, and the follower cannot backdrive the cam at the dwell position. However, the trade-off is that at the dwell, rotating the cam, does not move the follower for whatever duration the dwell lasts, but it will be non-backdriving for the duration of the dwell.

I was just saying that *in general* and depending on the mechanism, you have some type of trade-off with a non-backdriving device, where the input member usually moves for a greater distance or angular displacement, than the output member (at least for the non-backdriving portion of the gearing or device).

John

In worm gears, self-locking refers to gears that have less than 3 degrees of angle (I think it's in one of the AGMA specs) Same angle for self-actuating brakes.

The shallow angle worm gearing is used extensively where you use winding drums with overhauling loads (hoists, lifts, and certain cranes)

fwiw

Thank you for the comment. The 15 degree rule of thumb applies to taper pins particularly. Self holding tapers often have 3 degree tapers like Morse. Self-releasing tapers have tapers of 16 degrees or more. Milling machines etc.

Interestingly, Machinery's Handbook (at the 17th ed.) claims there is no safe lead angle at all for worm wheels back driven by a wheel. Vibration can inch the worm on a low-friction surface pair for any angle. Still, Mark's handbook admits worm and wheel hoists I see..

Brian Whatcott Altus OK