ABSTRACT
The calculation of fastener stiffness, while based in part on experimental work, is theoretical. Since the stiffness is based on dimensional data, dimensional tolerances will have an impact. For example, a 1h-13UNC X 3 in. long hex head cap screw is assembled with a hex nut to give a grip length of 21/2 in. Standard tolerances for the overall length and the length of threads for this combination would produce a variance of 4% in the value of the stiffness. Another source of variation is in the elastic modulus. Many texts quote the elastic modulus of steel between 29 X 106 psi (200 GPa) and 30 X 106 psi (207 GPa). This is a 3% variation. Also, it has been found that fastener stiffness and hence deflection does not agree with experimental values when the fastener is "short." In regard to stiffness calculations, fasteners should be considered "short" if the length-to-diameter ratio (liD) is less than 5. However, these variations can easily be reduced if greater accuracy is required. It is a simple matter to produce load-deflection data for each lot of fasteners or even for each fastener. Only you can determine if the added accuracy will offset the added costs.
