Torque Limiting Clutches

[mmazzuc1]

Hey guys,

I have a DC motor that I will most likely chain drive a shaft with. The output shaft will need to be limited to 27-37 in-lb. The output shaft will rotate very slowly. (no specifics yet, but probably at least under 50RPM).

I need to find a cheap torque limiting clutch that in one direction will limit my torque to 27-37 in-lb, and in the opposite direction, not limit my torque at all. Also another option is having my torque limits set to different values for each rotational direction.

Is there a specific name for these types of clutches that allow for limitation in one direction, but no limitations or alternative limits in the opposite?

Size is not an issue at all for me, I just need something cheap. Limiter can be implemented on the sprocket, input shaft, or output shaft.

Does anyone have any suggestions to get me started in the right direction?

Thanks!

 

[hydtools]

Do you mean something like this?

http://www.bostongear.com/products/clutch/selecting-tl.html

Ted

 

[mmazzuc1]

Yeah, something similar to that. I would like it to be fully mechanical.

Functionally, @30in-lb I need the clutch to allow my output shaft to turn freely so that it doesn't burn my motor up after it has torqued to 30in-lb. Then motor will need to unscrew the lid off my valve. I calculated that the torque to unscrew my lid was about 5-7.5in-lb less then what is required to tighten it. So a bidirectional torque limiter should work, however its not necessary. I am only concerned to getting the lid on my valved tightened properly.

 

[mmazzuc1]

Also. I was wondering if a servo motor can be used for something like this? I am not sure how servos perform under loaded conditions though. I would think I could tighten the valve, set that position as my motors initial condition. Have it rotate my valve open, and then close it etc. Can the servo return the the starting position without skipping, vibrating, etc under a loaded condition (due to tightening)?

 

[hydtools]

With a torque limiting clutch, the input shaft does not tun freely when the torque limit is reached. It slips, limiting the torque delivered to the output, and the motor keeps turning. Over time the clutch will heat up as friction generates heat. See also a ratcheting clutch like that found in a cordless drill/driver.
In practice you may find that loosening the cap will require more initial torque than the tightening torque.

Ted

 

[mmazzuc1]

I see. Well in any case, I should be able to get away with a clutch that only slips in one direction? If I set my timing right on my system, the motor will only cause the clutch to slip for a small period of time, (1-2 seconds). My system will only open & close the valve one time every 2 weeks. Therefore, I might be able to get away with any wear issues on the clutch.

 

[MikeHalloran]

Given that you're using a DC motor, you can limit the torque applied just by limiting the current supplied to the motor. You may not want to bother.

You might want to check your torque calculations. I can't recall ever finding a screw that was easier to break loose than it was to tighten.

You also might want to build in a big fudge factor on the torque. Valve mechanisms and such tend to stick, badly, when left alone for periods like two weeks. Maybe you want to size the motor and drive so as to be just barely not quite strong enough to break the valve shaft. Then you don't need a clutch, and you can use a fuse as your ultimate torque limiter.






Mike Halloran
Pembroke Pines, FL, USA

 

[DAVIDSTECKER]

If you are fully opening and closing the valve, have you considered a ball valve and a linear actuator, maybe a simple threaded rod connected to your motor shaft and treaded pivot attached to the valve handle? Add a coupled of limit switches to the control circuit to stop the motor.
Best regards, David

 

[hydtools]

See this:

http://catalog.machinecomp.com/item...ch&by=prod&filter=0&categid=1001&prodid=1007#

Ted

 

[mikekilroy]

do you already have the motor? do you already have its drive? KISS principle is nice. I would not bother with a mechanical heat em up clutch when you probably need to buy a dc drive anyway from the sound of it. get help from a distributor/integrator for the right one and you can set the CW & CCW torque to the value you want, change it easily with a pot or computer program, and KISS.