One of the most critical factors when it comes to reliability of a threaded-fastener joint is preload—the force the tightened fastener exerts on an assembly. Preload is a function of many variables, including fastener material and finish, head style, and lubrication. So, in the end, there is no simple, totally reliable way to compute the precise preload needed for every application. However, the following equations serve as useful guides in relating preload to three of the more important factors in threaded-joint design: total bolt load, tensile stress, and tightening torque.
via Calculating the Proper Preload for Threaded Fasteners | Fasteners content from Machine Design.