# Mechanism needed

• Automotive
Hello,

for my current physics-project i am searching a mechanism:
When there is to much friction on a rotating axle, it should reverse the direction and go on, till the mechanism starts all over again. I would love to solve this with pure mechanical parts, like springs gears etc...

If you have any ideas on how to realize this mechanism, or even an existing mechanism, please share your thoughts with me. Im happy about every idea.

Thanks, Chris

p.s.: and sorry for my english, im from Austria ;)

berkeman
Mentor
Welcome to the PF.
When there is to much friction on a rotating axle, it should reverse the direction and go on, till the mechanism starts all over again. I would love to solve this with pure mechanical parts, like springs gears etc...
Can you describe the behavior a bit more? What is applying frictional torque to the axle? What is driving the axle against that frictional torque? Will reversing the direction of axle rotation zero out the frictional torque, or will it remain just as strong after the reversal?

Tom.G
Gold Member
I don't know how practical, but this concept comes to mind.
• concentric shafts, one the drive the other driven
• a torque-limiting clutch, as used in some battery operated screwdrivers
• planetary gear system coupling the two shafts
• when the torque-limiting clutch slips it drives a pawl that shifts (or locks?) the planetary gear train
Still lots of details to work out!

Cheers,
Tom

"....drives a pawl that shifts (or locks?) the planetary gear train"

thus reversing the rotation of the driven shaft. (Is this what you mean to say?)

Chris184
Welcome to the PF.

Can you describe the behavior a bit more? What is applying frictional torque to the axle? What is driving the axle against that frictional torque? Will reversing the direction of axle rotation zero out the frictional torque, or will it remain just as strong after the reversal?

Yes sure. There is a torqueslip built into a linear actuator. The linearactuator shall be driven in and out by this mechanism. When reversing the direction the friction will remain as strong as before the reversal.

I don't know how practical, but this concept comes to mind.
• concentric shafts, one the drive the other driven
• a torque-limiting clutch, as used in some battery operated screwdrivers
• planetary gear system coupling the two shafts
• when the torque-limiting clutch slips it drives a pawl that shifts (or locks?) the planetary gear train
Still lots of details to work out!

Cheers,
Tom

Hey Tom, thanks for your idea. It was helpful. Even tho i did not completely understand the pawl. The problem which comes in my mind is that the pawl has to be unlocked again, since i want the mechanism to be repeatet more than twice. Do you understand my concerns, or did i misunderstand you?

Last edited:
Tom.G
Gold Member
"....drives a pawl that shifts (or locks?) the planetary gear train"

thus reversing the rotation of the driven shaft. (Is this what you mean to say?
Yes.

Do you understand my concerns, or did i misunderstand you?
Probably a bit of both!

When reversing the direction the friction will remain as strong as after before? the reversal.
I think that shoots down my (shadow of an) approach. But I do wonder how the driven element doesn't immediately switch back to the first direction.

Baluncore
2021 Award
Two gears that act forward or one of which reverses. Used on the lead screw on a lathe when thread cutting.
Take a look at a thread cutting lathe to see how Left/Right thread is selected.
You could over-centre spring load the gear mount plate so that when it is overloaded it toggles or flips to the other mode and reflects direction.