Slip clutch torque loss

[lucky-guesser]

People Smarter Than Me,

I’m currently working on a slip clutch mechanism that is spun by a drill, but I am having some issues with maintaining a constant torque. The slip mechanism is working well, holding the working loads that it needs to and then breaking free when it should, but the goal is that when the gears bind up you break them free with a wrench and then go back to business as usual.

The stack up is a shoulder that is part of the drive shaft, clutch pad, gear that spins over the drive shaft, another clutch pad, thrust bearing, and a nut threaded on the driveshaft. The clamping from the nut holds the clutch pad-gear sandwich against the shoulder on the opposite end. The thrust bearing is to allow the gear to spin freely once max tq is reached without it spinning off the nut as well (so it doesn’t have to be manually reset).
The issue I am having is that the clutch is losing its holding force after breaking free. I am getting consistent results by setting the clamping force by measuring the torque applied to the nut that clamps the whole system together. But after the clutch breaks free and spins maybe 10 extra revs before stopping, the nut is little more than hand tight vs the 50 lb-in it was set at.

With the thrust bearing the nut is not spinning lose, I think what’s happening is the clutch is wearing ever so slightly enough that I am losing compression and thus my holding friction. There is noticeable heat buildup in the system just from running, even without slipping, but you can still hold the parts in your hand so I don’t think there’s enough of a temp change for that to be the issue.

My clutch pads are TN-9005, the gear is powdered metal and everything else is mild steel. If you have ideas for a more wear resistant clutch material, a way to help keep constant pressure on the clutches after wearing, or if you think I’m going in totally the wrong direction any help would be appreciated.

P.S. This whole assembly is only a few inches long and if successful would be made in the thousands each year so size and cost are both concerns to keep in mind. Thanks.

 

[TugboatEng]

You don't have any springs?

 

[lucky-guesser]

You don't have any springs?

From what I am seeing, to get the force needed it looks like I would need a light-medium die spring (550lbs approx). Most are larger than I'd like so I could cut them in half but they are still somewhat cost prohibitive. That said, the places I've looked are also known for having a considerable price mark up. I will keep that in mind but I have some some looking on that already.

 

[3DDave]

"Torque" of a screw is mainly a reflection of the tension in the thread. That tension is a function of elongation. If there is a fixed location for a nut and the intervening material wears, then the screw is no longer elongated and there is no tension. No tension = no torque measured.

The elongation you are probably depending on is conveniently measured in millionths of an inch. Any loss of material in the clutch is more than that, removing the preload.

As TugboatEng suggests, this is usually managed by using springs to keep the force more constant as the friction material wears.

 

[TugboatEng]

Belleville springs are cheap and can be adjusted to your exact needs by different stacking arrangements. I would start there.

 

[Compositepro]

Clutch wear will certainly be an issue, but I would not at all assume that nut movement is not an issue if it is not securely locked in place. There is no difference between micro wear of the friction material or micro movement of the nut in this loss of compression on the clutch.

 

[TugboatEng]

Both cases have the same solution.

 

[Compositepro]

Most adjustment screws or nuts should have something to lock them in place other than random friction. This nut is subject to vibration, spring load, and starting and stopping torque due to inertia.

 

[MintJulep]

Another spring option:

https://www.smalley.com/wave-springs