Acme Thread Power Screw driving and driven 2

[bryansonnier]

I'm looking at making a design using an acme thread power screw. the basis of the design is a rotary actuator will slowly turn a shaft at 600 in-lb of torque plus also being turned will be an ACME thread power screw which will be compressing some belleville spring washers. the shaft will have a ratcheting system so that when the actuator finishes its turn the load on the belleville washers will be held by the ratchet. The actuator will then get a signal to turn back the other way triggering the ratchet to release allowing the bellevilles to push the power screw back the other direction also turning the shaft. the load on the shaft when driven by the springs will be 600 in-lbs.

My question is can an acme thread be used in both a driving and driven function? Would a lead angle of 45degrees be needed or is there some other optimum angle if that functionality is possible. would a square thread be better for this? The shaft rotation would be only 90 degrees so i think i would use multiple thread starts to distribute the load. The main unknown is would a 45degree angle do what i'm looking for or would a slightly lesser angel work?

 

[MintJulep]

It's pretty hard to follow your description. Even a simple sketch would be so helpful in communicating your design.

If you hope to use standard ACME threads, then you don't get to choose lead angle independently. There are standard combinations of diameter and pitch.

You can play with materials of nut and screw, coatings and lubrication to get self-locking or back-driving ability.

There are other screw forms that are probably "better" for what I imagine is your application.

Having said that, your design (as I imagine it) seems pretty complex. Turn a shaft, then turn it back, right?

You have an actuator that apparently can be driven in both directions. Why not just drive the shaft in both directions and avoid the power screw and springs altogether?

If you can't drive the shaft in both directions, why not just wind up a torsion spring in one direction and let the spring directly turn it back the other?

Instead of constraining the possible solutions by explaining how you think the problem should be solved, why don't you tell us what you need to accomplish.

 

[GregLocock]

From what I can remember the tan of the pitch angle must be greater than the coefficient of friction of the interface, if you want to be able to back drive a thread.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Jboggs]

Why not use a standard acme screw commercial jack? They have already worked through all the design questions, and their sales people can help you select the right model. Do you need some brand name references?

 

[bryansonnier]

Yes the actuator can drive in both directions, but its slow. I need to snap in one direction, hence the spring. I was going to set up a switch with a cam on the shaft to tell the actuator when to stop. There will be an external signal for telling the actuator when to start in either direction.

Judging by the replies, it looks like the terminology i'm looking for is the ability to back-drive.

Thank you for your help. A quick look, and i think thats what i need.

 

[bryansonnier]

Jboggs,

I have a very defined rotation that i need to maintain (90deg) i may be able to find a shelf item but i'm thinking it will need to be custom. If you have some helpful references though i'm all ears. thank you.

 

[hydtools]

https://www.roton.com/screw-university/formula-calculators/

Ted