Clutch Gears

[Brickthus]

Some people have expressed an interest in knowing more about how to use the while 24-tooth clutch gear, so here goes:

Clutches are often used to drive limited travel devices from a motor. They absorb excess motor rotation once the device has reached its end-stop.

The balance is between the application turning too slowly and the clutch slipping too much.

Clutches are often used with PF medium motor, 71427/43362 gearmotor or monkey motor.

They're used less often with the PF XL motor or the 52192 RC car motor because those motors have more torque.

- The 8421 crane is an exception, using a 52192 motor with 8:24, 8:clutch, worm:8, 8:rack for the jib extension and 8:24, 12:20, 8:clutch for the cable spool. This shows the extra thought that goes into a set, about the torque of the rack and cable spool; the rack has a heavy load so it has gearing after the clutch whereas the cable has less torque and uses the clutch in the last stage.

Generally, have the clutch gear do about 50-100rpm.

- A PF medium motor or 71427/43362 gearmotor does about 300rpm at no load, so if your application does 30-100rpm, it's good for the first stage to be 8:24 onto the clutch gear.

- Slower applications (3-33rpm) would put the clutch as the second of three 8:24 stages from a 300rpm motor. This is a bit like the crane jib.

- Faster applications are unlikely to gain anything by using a clutch gear. Either too much torque is required in order to achieve the speed (in which case the clutch gear would slip too much) or a fast application would not be gearing down from the motor. If you want the wheels to turn at 300rpm, put a medium motor on each wheel!

Static short-travel applications (e.g. point motor for 9V points, driven by PF medium motor):

- use 8:24 clutch gear followed by 8:24 and 8:rack

- may vary second 8:24. For a faster rack, replace with 12:20 or 16:16. May use a lever instead of a rack (e.g. PF automatic machine)

- for a slower rack, keep the clutch gear 8:24 stage at 3x rack speed, adding more gearing-down between the clutch and the motor, rather than between the clutch and the rack.

It is usually better to drive onto the teeth of the clutch gear and take the output from the middle.

The exception might be if you take the output off the teeth onto a larger gear (usually 40 teeth, occasionally 36, or 28 for an old differential).

Do you have any other clutch applications?

Mark

[vmln8r]

Thank you Mark, that was very informative.

I have used a clutch in a pneumatic walking beam engine, as part of the flywheel to reduce wear on the gears which span very fast as it was geared up (I should probably mention that the engine didn't work).

I also found the clutch placement a little odd in the 'Barcode Truck' (8479) after reading your explanation. The grabber arm has gearing after the clutch whilst for the linear actuator the clutch is towards the end. It might be why my truck fails to lift a wheel without my help. Of course it could be the glued flex cables I am using...

[CP5670]

Good explanation. I find that it often takes a little trial and error to determine where exactly the clutch should go in a geartrain. As you said, you have to get a balance between transmitting enough torque for the mechanism to work while still making sure it slips if something stalls.

I also found the clutch placement a little odd in the 'Barcode Truck' (8479) after reading your explanation. The grabber arm has gearing after the clutch whilst for the linear actuator the clutch is towards the end. It might be why my truck fails to lift a wheel without my help. Of course it could be the glued flex cables I am using...

8479 is a special case because it's meant to use the original light gray clutch gears from 1997, which behave a little differently from the modern kind. See my post here.

[Jetro]

8479 is a special case because it's meant to use the original light gray clutch gears from 1997, which behave a little differently from the modern kind. See my post here.

Thank you!! I had never noticed the difference, but this has solved a problem I had with a tricky clutch gear situation.

[Erik Leppen]

It is usually better to drive onto the teeth of the clutch gear and take the output from the middle.

This one strikes me as odd. Why would that make any difference? Could you explain that?

[Brickthus]

This one strikes me as odd. Why would that make any difference? Could you explain that?

Driving the teeth requires less torque from the motor. Most applications are gearing down rather than up.

The gear system should increase torque as it reduces speed.

Driving the teeth there is more torque on the clutch itself than on the teeth because the teeth have larger diameter.

Driving from the middle there is less torque on the clutch than on the axle because the axle has small diameter. This defeats the object of increasing torque.

Mark

[Carsten Svendsen]

Sorry for bumping, but does this mean that if I put a clutch gear on my buggy motor at 1000rpm at a 1:1 ratio (meaning directly) it wouldn't work as intended? 

To clarify, does the clutch gear need to be going at a slow speed in order to function correctly?

[mocbuild101]

1 hour ago, Carsten Svendsen said:

To clarify, does the clutch gear need to be going at a slow speed in order to function correctly?

No, speed doesn't affect it - it's the torque (difference) that makes it slip.

So if it was connected to a buggy motor, and resistance (torque) was applied to the output, it would slow the motor down until there was enough torque to make the clutch slip.