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Bolt Torque
By Wayne Scraba/
Printable version
Degree of DifficultyEasy
Estimated Time15 minutes
15 minutes
These days, when consulting your shop manual, you’ll sometimes find stretch measurements for certain fasteners along with torque angle figures but generally speaking, “bolt torque” is still king. In many cases, the use of a torque wrench is the only way possible to measure the tension that is brought to bear on a fastener.
Bolt Torque
What is Torque?
Let’s rewind for a second. What is torque? Simple. Torque is the twist or the resistance to rotation. When used in reference to a fastener, torque is the resistance to turning of the bolt or nut. Torque is based upon the fundamental law of the lever: Force x distance = the torque or twist around a point. Torque is most commonly measured in foot-pounds or inch-pounds. In practice, if one pound of force is applied one foot from the center of the fastener, the resulting torque developed would be referred to as one foot-pound of torque. If the resistance measurement is in inches, the resulting torque would be referred to as twelve inch-pounds of torque.
Simple enough. Yet there’s more: Applying the exact amount of torque to a nut or bolt induces the correct amount of tension or elongation in the bolt that is necessary to hold the parts together. By applying the correct amount of specified torque to a bolt, the danger of distortion to the final part or adjoining parts is eliminated.
Stretch and Spring
It is also very important to understand that for a fastener to properly function, it must be “stretched.” ARP (the racing fastener company) notes that the ability of a fastener material to rebound (like a spring) is what really provides the clamping force. Different materials tend to react differently to these conditions. As a result, different fasteners are designed for different tasks.
What if you over-tighten a bolt? ARP points out that if a fastener is over-torqued, it stretches too much. Because of this, the yield will have been exceeded, and for all intents and purposes, the fastener is ruined.
Heat and Tension
Another big factor in the torque picture is heat, primarily in aluminum. Both Mac Tools and ARP note that the thermal expansion rate of aluminum is greater than steel. Because of this, it is possible to stretch a fastener beyond yield as the aluminum expands when heated. The solution? Produce a more flexible bolt.
There’s a definite limit on how much a specific type and grade of metal can be stressed or stretched safely. In essence, that is the elastic limit of the material. Mac Tools states: “After the correct fastener and material have been chosen, the design engineer establishes the exact amount of torque to be applied. This torque specification will induce a stress or elongation of the bolt of approximately 60-70 percent of its ultimate elastic limit.”
Another factor is tension. What is it? According to Mac Tools, tension is straight pull and is measured in pounds. Torque wrenches are at times referred to as “tension wrenches.” This is incorrect. Mac states: “Wrenches that are designed to measure or limit the amount of torque applied to a nut or bolt are definitely torque wrenches.”
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