2004 by W.H.Dornfeld. All rights reserved.Power Screws:Power ScrewsLooking at a square thread screw, we unwind one turn:αLead2πrmThis shows an inclined ramp with anglemrLeadπα2tan1−=1WrmrcLeadµThread frictionCollar frictionMean thread radiusMean collar radiusLoad on nutµc 2004 by W.H.Dornfeld. All rights reserved.Power Screws:Power Screw TorqueThe torque required to raise the load W isand to lower the load, we flip two signs:+−+=ccmraiserrWT µαµαµtan1tan++−=ccmlowerrrWT µαµαµtan1tan2WrmrcLeadµµc 2004 by W.H.Dornfeld. All rights reserved.Power Screws:If the thread form is not square but has an angle 2φ, replace the thread friction µ with the effective frictionPower Screw Thread Angleφµµcos=eThe effect:• Square: 2φ = 0, φ = 0, 1/cosφ = 1.0• Acme: 2φ = 29°, φ = 14.5°, 1/cosφ = 1.033• Unified: 2φ = 60°, φ = 30°, 1/cosφ = 1.15The thread angle effectively increases surface friction between 3 and 15%3 2004 by W.H.Dornfeld. All rights reserved.Power Screws:Power Screws - OverhaulingIf the collar friction is small (e.g., it may have a ball thrust bearing), too small a thread friction may let the weight screw down on its own.This can happen when(the numerator goes negative).This is the same case for a weight sliding down a ramp when the incline angle α exceeds tan-1µ.mrLeadπαµ2tan =<αLead2π rmµ4++−=ccmlowerrrWT
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