Calculate hold down force for rotating shaft

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Discussion Overview

The discussion revolves around calculating the hold down force required to prevent a rotating steel shaft from moving under a specified torque. The context includes considerations of friction, material properties, and the mechanics of clamping.

Discussion Character

  • Technical explanation, Mathematical reasoning, Debate/contested

Main Points Raised

  • One participant presents a scenario involving a .850" diameter steel shaft subjected to a torque of 125 ft-lbs and seeks to determine the necessary down force to prevent rotation.
  • Another participant suggests that the required force depends on the speed at which the shaft needs to be stopped.
  • A clarification is made that the participant needs to know the clamping force required before applying the torque to ensure the shaft does not move.
  • It is proposed that the coefficient of static friction between the shaft and the clamp block should be considered, with the normal force from the clamp generating a static frictional force sufficient to counteract the applied torque.
  • Participants are reminded to consider the strength of all components involved to avoid failure.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the calculation of the necessary clamping force and the factors influencing it, indicating that multiple views and approaches are present in the discussion.

Contextual Notes

There are unresolved aspects regarding the exact coefficient of friction to use and the implications of the materials involved, as well as the specific conditions under which the calculations should be made.

deserttech
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I have a .850" diameter steel shaft that will have a torque of 125 ft-lbs put on it. I need to calculate how much down force I need to stop the shaft from rotating. I suppose we can assume that it will be steel on steel.

Going with a coefficient of friction of about .35 I tried to calculate with the typical CoF problem of a box and an incline, but I don't know how steep to make the incline.

How can I figure out how much force it will take to stop a .850" diameter shaft from rotating under a torque of 125 ft-lbs?
Thanks
 
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deserttech said:
How can I figure out how much force it will take to stop a .850" diameter shaft from rotating under a torque of 125 ft-lbs?
Depends on how quickly you want to stop it.
 
A.T. said:
Depends on how quickly you want to stop it.
Maybe, I didn't describe it fully. I need to know how much force it will take to clamp the shaft before I apply the torque so that it doesn't move. I'm not sure how to calculate that.
 
deserttech said:
Maybe, I didn't describe it fully. I need to know how much force it will take to clamp the shaft before I apply the torque so that it doesn't move. I'm not sure how to calculate that.

You can get a pretty good start by looking up the coefficient of static friction between the material of the shaft and the material of the clamp block. The force that you apply to the clamp block will be the normal force generating a static frictional force; this frictional force acting on a radius of .425" has to generate enough torque to resist the 125 lb-ft that you'll be applying.

You'll want to consider the strength of everything involved to make sure nothing will break.
 
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