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Power Screws and Force
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Posted by: mike22

09/24/2004, 07:10:06

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I have a device that I need to estimate a failure torque for. It is a 3-pronged wheeler puller, with an extra base, to clamp it all together. Basically I have a power screw on it, which is tightened with a torque wrench. The screw is then in compression. Is the compression in the screw given by T = 0.2DF, or is it the much more complicated power screw equation I have seen in books (is generally given as T as a function of the Force, the lead, the pitch, etc...). Or are the two equvalent, with the 0.2 using standard values?? I seem to remember the T = 0.2DF having something to do with pre-tensioned screws, but the /calculators/torque_calc.htm page suggests different.


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Re: Power Screws and Force
Re: Power Screws and Force -- mike22 Post Reply Top of thread Forum
Posted by: zekeman

09/24/2004, 13:43:09

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I'm not crazy about using "empirical" formulas for this.
I would use the basic approximate formula:
T=F*{(tan(alpha)+mu)/(1-mu*tan(alpha))}*.5*D where:
alpha=the helix angle of the screw=invtan( p/(pi*D)
mu= friction coefficient
D=pitch dia of screw
p= screw pitch

For example if the screw/nut is lubricated and steel/steel, the approximate low end friction coeff=0.11 and if the screw is an acme 1/2 inch-10 ,the term in parentheses above is:
The torque equation is then:
which is considerably lower than the the formula you cite and would give a less conservative result.
It should be noted that if there is bearing friction during the torqueing process it would increase the T somewhat.

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