A New Method of Screw Strength Calculation

By considering several factors usually neglected, a more realistic picture of screw strength is obtained. These are: thread geometry, combined tensile and shear stresses, and uncertainty in the exact value of the friction coefficient. This uncertainty increases the tightening torque necessary to obtain a required tensile load, and reduces the effective screw strength considerably. Taking all these factors into account, the calculated screw strength is much smaller, but more realistic, so that smaller safety factors can be employed, yet more reliable designs obtained. The equations derived point the way towards two methods for increasing screw strength. These are: use of preload-indicating washers; and turning the screw back by a certain amount after the initial tightening. The first method alone can double the usable screw strength.