What torque do I need for a particular fastener?
Torque guides can be found on the Fastenal Engineering & Design Support webpage within the Other Resources category. If your particular fastener is not represented in these torque guides, contact the Fastenal Engineering & Design Support team.
How do coatings or platings affect the torque-tension relationship?
A coating or plating changes the surface of the fastener. In general, most coatings increase the lubricity of the fastener over the clean and dry condition. This reduces the amount of friction or surface roughness and means that when the fastener is tightened, more energy goes into stretching the bolt (preload) than is lost to friction between the mating threads or the bearing surface of the bolt or nut. A lower torque will now achieve the same preload in the fastener before the coating was applied. If coatings or platings are employed, the installation torque should use a K-factor appropriate for the internal and external thread conditions.
What is the K-factor and how do I use it?
The K-factor is derived from the short-form torque equation. When calculating how much torque is required to achieve a certain preload in a bolt, many different factors can have a significant effect. Geometric variables, friction between the threads, and friction between the bearing surfaces and clamp members all play a roll in how much of the input torque is transferred to tensile force in the bolt. It is rare that any of these variables are known for a given application. A simplified formula for evaluating the preload-torque relationship emerged that reduced all the unknown or difficult to determine variables into one value, K. So the equation now appears as T = KDF where D is the nominal diameter of the fastener, F is the desired preload for the bolt, and K is the K-factor. Typically, this value must be estimated or determined empirically. You use the K-factor when you must calculate an installation torque for an assembly.
Is it better to apply torque to the bolt head or the nut?
In general, there is no difference between applying torque to the bolt or nut. There are, however, situations where it would matter such as:
- If the through-hole produces interference - then it would be better to apply torque to the nut side of the joint.
- If the clamp member material is different on one side of the joint - here friction plays a role and you would want to apply torque to the side that has the lower frictional coefficient.
- If the through-holes have different diameters - this could change the way the bearing face contacts your clamp members (might be better to apply torque from opposite side).
- If the bolt is very long, it might be better to apply torque to the nut to avoid torsional wind-up effects.
In summary, if you have the same material in contact with the bolt head and nut, the same sized through-holes (no interference) through the clamp members, and the same size/type of bolt head and nut (heavy hex pattern for both, for example), then it makes no difference which side of the joint you apply the torque to.