2002-04-14,  Rev. 2002-04-22. © 2002 Garrett D. Euler

Question:  How is bolt installation preload calculated?

Answer:  Bolt pretension, also called preload or prestress, comes from the installation torque T you apply when you install the bolt.  The inclined plane of the bolt thread helix converts torque to bolt pretension.  Bolt preload is computed as follows.

Pi = T/(K D)        (Eq. 1)

where Pi = bolt preload (called Fi in Shigley).
T = bolt installation torque.
K = torque coefficient.
D = bolt nominal shank diameter (i.e., bolt nominal size).

Torque coefficient K is a function of thread geometry, thread coefficient of friction mt, and collar coefficient of friction mc.  Look up K for your specific thread interface and collar (bolt head or nut annulus) interface materials, surface condition, and lubricant (if any).  ("Torque specs for screws," Shigley, and various other sources discuss various K value estimates.)  If you cannot find or obtain K from credible references or sources for your specific interfaces, then you would need to research to try to find the coefficients of friction for your specific interfaces, then calculate K yourself using one of the following two formulas listed below (Shigley, Mechanical Engineering Design, 5 ed., McGraw-Hill, 1989, p. 346, Eq. 8-19, and MIL-HDBK-60, 1990, Sect. 100.5.1, p. 26, Eq. 100.5.1, respectively), the latter being far simpler.

K = {[(0.5 dp)(tan l  +  mt sec b)/(1  –  mt tan l sec b)]   +   [0.625 mc D]}/D        (Eq. 2)

K = {[0.5 p/p]   +   [0.5 mt (D – 0.75 p sin a)/sin a]   +   [0.625 mc D]}/D        (Eq. 3)

where D = bolt nominal shank diameter.
a = thread profile angle = 60° (for M, MJ, UN, UNR, and UNJ thread profiles).
b = thread profile half angle = 60°/2 = 30°.
tan l = thread helix angle tan = p/(p dp).
dp = bolt pitch diameter.
mt = thread coefficient of friction.
mc = collar coefficient of friction.

D and p can be obtained from bolt tables such as Standard Metric and USA Bolt Shank Dimensions.

The three terms in Eq. 3 are axial load component (coefficient) of torque resistance due to (1) thread helix inclined plane normal force, (2) thread helix inclined plane tangential (thread friction) force, and (3) bolt head or nut washer face friction force, respectively.

However, whether you look up K in references or calculate it yourself, the engineer must understand that using theoretical equations and typical values for K and coefficients of friction merely gives a preload estimate.  Coefficient of friction data in published tables vary widely, are often tenuous, and are often not specific to your specific interface combinations and lubricants.  Such things as unacknowledged surface condition variations and ignored dirt in the internal thread can skew the results and produce a false indication of preload.

The engineer and technician must understand that published K values apply to perfectly clean interfaces and lubricants (if any).  If, for example, the threads of a steel, zinc-plated, K = 0.22, "dry" installation fastener were not clean, this might cause K to increase to a value of 0.32 or even higher.  One should also note that published K values are intended to be used when applying the torque to the nut.  The K values will change in relation to fastener length and assembly running torque if the torque is being read from the bolt head.

One should measure the nut or assembly "running" torque with an accurate, small-scale torque wrench.  ("Running" torque, also called prevailing torque, is defined as the torque when all threads are fully engaged, fastener is in motion, and washer face has not yet made contact.) The only torque that generates bolt preload is the torque you apply above running torque.

A few more things to be aware of are as follows.  Bolt proof strength Sp is the maximum tensile stress the bolt material can withstand without encountering permanent deformation.  Published bolt yield strengths are determined at room temperature.  Heat will lower the yield strength (and proof strength) of a fastener.  Especially in critical situations, you should never reuse a fastener unless you are certain the fastener has never been yielded.