When bolts are fastened to a device or structure, they require a certain amount of preload torque tension in order to properly support the final load. Preload tension is the amount of tension applied to the bolt by a device like a torque wrench or spanner that tightens the bolt into place before the load it will eventually support is added (thus, preload). It is important to apply the correct amount of preload tension when fastening a bolt in order to meet the requirements of the specific bolt material and grade, the load it will carry, and many other aspects of the load. If proper preload conditions aren’t met, a bolt may fail or a unit may become damaged because of the inadequate fastening.
In order to properly fasten bolts, it is important to follow standards for torque tension. The SAE International has outlined a series of standards both for bolt sizes and torque tension on these bolts in relation to the substrate material.
SAE Bolt Standards (a sample)
These grades are available at http://www.unified-eng.com/scitech/bolt/boltmarks.html. The SAE
Fastener Standards Manual can be purchased directly from SAE at this website:
http://books.sae.org/book-hs-4000/2009. It features more gradings for bolts as well as other fasteners.
In order to make torque tensioning standards effective, a little physics is involved. Two different equations are used to determine the force needed.
For a reusable connection: Fi = 0.75AtSp
For a permanent connection: Fi = 0.9AtSp In these equations, Fi is the “recommended preload force.” At is the tensile area of the bolt and Sp is the proof strength of the bolt. This equation can be difficult to solve due to the difficulty measuring Fi, but there is another equation that can help figure it out.
T = K Fi d In this equation, d is the nominal diameter of the bolt and K is a known variable, the correction factor that depends on the material, size, surface friction and threading of the bolt. Most bolts have a 0.15 to 0.3 K. The following chart shows various K values for different materials:
unlubricated mid-size steel bolt
non-plated black finish
Based on this information, we can determine different torque tension needs for different grades of bolts. For instance, a grade 8 bolt used on three different materials:
Example SAE Torque Tensioning Standards
Material and K value
Nominal size and Threads per inch
Stress Area in squared inches
Torque in inches per pound