Estimation of Yield Clamping Force and Ultimate Clamping Force of Threads

(Proopsal of formula in consideration of Polar sectional coefficient and Shear Strength)

Abstract

Conventionally,The yield tightening force and the ultimate tightening force are basic indices for plastic region tightening, which are calculated using Mises stress and the assumptions concerning the equivalent diameter and the shear stress distribution. The recent study concerning the breaking torque of bolt ,however,shows that Mises stress is valid in the case where shear strength/tensile strength ration is nearly equal to1/√3 . This suggests that for the property classes other than 12.9, the equation should be amended taking into account the strength ration of the property class (e.g., 0.65 for PC 8.8). To confirm the correctness of this concept, three kind of tests, i.e., tensile, tortional and tightening tests are performed for test pieces machined from bolts of pc8.8 and 12.9. The results show that the equations should be amended considering the strength ration of the bolt material for pc 8.8 does not fit the experimental results.The reason is thought be the work hargening of the test piece under purely torsional loading. Therefore , the equation should include this effect.