Moment Acting on a Power Screw

[Gstyles]

Hi,
I have a power screw that closes a scissor like mechanism. The tapped portion which acts as the nut inflicts an axial force against the motion of the screw, however, the force originates below the axis of the screw, therby creating a moment at the center of the screw. I have been running the equations for calculating the required torque to move the load axially, but I am unsure how to account for this moment on both the nut and screw threads. It seems clear that this moment has an impact on the required torque but for the life of me I cant clearly see how to account for it. Maybe I've had too much coffee. Any help or guidance would be appreciated! Thanks!

 

[MintJulep]

I suppose if I wanted to figure it out on paper I would start with the chapter from something like Shigely, Spotts or Bickford that discusses where the torque goes when you tighten an ordinary nut and bolt. Your situation would seem to be simply a minor modification to that calculation, after you resolve the moment into a couple on opposing sides of the nut.

I don't have access to my copies at the moment, and am not sufficiently motivated to derrive the equations on my own.

It would seem far simpler to run a quick test, which would more accurately capture the specifics of your application.

 

[israelkk]

As a quick calculation which will be quite accurate I will resolve the moment into a couple of opposing forces on the nut/screw thread pitch diameter. Then multiply the some of the forces by the pitch radius and the coefficient of friction. This will give the added moment needed to overcome the moment due to the off axis force.