What’s the Difference Between Bearing, Shear, and Tear-Out Stress?

It is important to calculate the proper area. Depending on the design, a pin or bolted joint might share a load over multiple surfaces In the second image, the load will be distributed over double the area.

In a rigid assembly, the preload is higher than that of the service load—thus the service load will, or should, have little effect on the fastener tension forces. This reduces fatigue failure even when a dynamic of fluctuating load is applied. This only applies if the designer considered the difference of the tensile strength and endurance limit of the fastener. A cyclic stress on a joint will be designed to the endurance limit, not the tensile strength of the fastener. For example, if a fastener has a minimum tensile strength of about 150 Kips, but the endurance limit is 15 Kips, a joint under a cyclic load will not be able to exceed 15 Kips or it can fail prematurely due to fatigue.

According to a technical memorandum for determining bolt load published by NASA, bolts should typically generate a clamping force that, if designed properly, will carry less than 20% of the external loads on the bolts. The majority of the work is performed by the compressive energy induced on flanges while the bolts are tightened.

Many variables affect preload, including tensile strength, finish, head style, and lubrication. Lubricants will reduce the torque required to produce a clamping force. Coatings may reduce or increase required torque. Multiplying the standard dry torque by as much as 0.45 to 1.70 can compensate for lubrications and coatings. For example, Molybdenum grease, film, and paste can reduce the torque needed to produce a proper clamping force by 30%, 40%, or 45%, respectively. Unfortunately, there is no simple, totally reliable way to compute the precise preload needed for every application (for more information, check out "Calculating the Proper Preload for Threaded Fasteners"). Factors such as the bolt head, thread type, washer, and fastener plates also help increase surface area to prevent creep that will cause the preload to be reduced and potentially cause slippage or introduce another mode of failure. There are standards that can also help guide you in preloading your fasteners. However, this is not a quick fix, as each standard focuses on a different application or technology. ASTM, ANIS, ASME, IOS, and others may overlap while others might have gaps between them.