Bolt Torque | What is it?

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Bolt Torque | What is it

Torque specifications frequent shop manuals. That's a given. Today, 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.

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."

Common Mistakes.
So what if you don't tighten a fastener sufficiently. That resolves the over-tightening scenario, but if a nut or bolt is not tightened enough, it will eventually work loose and drop off. On the other hand, getting too rigorous with a bolt in the tightening department will likely snap it off. Neither the "too loose" or the "too tight" situations work. That's why you should follow specific torque recommendations, and that's also why you should use an accurate, quality torque wrench.

Another big mistake is to test the accuracy of one torque wrench against another wrench. Mac Tools provides this example: "If a Mac wrench is used to tighten a bolt to 90-foot-pounds, and another torque wrench is used to loosen the bolt as an accuracy test for either wrench, then you will likely obtain different readings. This is a test that is frequently performed and it inevitably results in the assumption that one of the two torque wrenches is not calibrated correctly.

"What is not generally known or understood is that the break-away or break-loose torque is considerably less than the applied torque. This means that the torque required to loosen a bolt previously tightened to 90-foot-pounds would be considerably less than the 90-foot-pounds of applied torque. In view of the above, it is easy to see why one of the torque wrenches could be considered inaccurate. A torque wrench should be tested on a torque wrench-testing machine to determine its true accuracy."

As you can see, bolt torque isn't a complex topic, but it's pretty clear there's a bit more to torquing fasteners than you might first think. Before you break out the easy-outs and thread repair kit, follow the recommendations made by the Pros. It's something you won't regret.

Photo Guide:

bolt torqueIt's no secret that nuts, bolts and other fasteners come in all different types, shapes and sizes. The truth is, this isn't a coincidence. It's by design. Each fastener is designed to do a specific job in conjunction with a specific material. In essence, every type and grade of metal has a definite limit to which it can be safely
Bolt Torque | What is it?One of the most common types of torque wrench you'll find in motorsports or in professional shops is a "clicker" variety such as this. By definition, a torque wrench is a device that is used to measure or limit the amount of torque that is being applied at a given point (on a specific fastener).
Bolt Torque | What is itThis is a close up of the business end of a torque wrench manufactured by Mac Tools. Adjustment is straightforward: The lock ring is slid toward the handgrip and then the knurled handle is turned to the exact amount shown on the graduations. Setting the tool is like reading a micrometer: It's simple and extremely accurate.
Bolt Torque | What is itTorque wrenches are precision measuring instruments. Treat them accordingly. It's a wise practice to store the torque wrench in the container it is shipped or sold in. These Mac Tools torque wrenches are sold in protective plastic cases. And that's where they should take up residence until you need to use them.

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