Imagine for a moment the bolts holding the wheels on your car were made of rubber. If you don't tighten them enough, when you turn a corner the bolts will stretch and your wheel will tilt to one side. If you tighten them up too much they will simply break. Of course your wheels aren't held on your car with rubber bolts, but did you know the metal bolts you do have function in much the same way?
Why Use a Torque Wrench?
A bolt sitting on your workbench is at rest regardless of whether it is made of metal or rubber. When you install the bolt in an application and begin to tighten it with a wrench it begins to stretch and load up a force along its length. Just like a rubber bolt, if you tighten a metal bolt too much it will break the bolt (or damage the application), but if it's not tight enough it will not have the proper preload to hold the application together. You could just tighten up the bolt as much as you can, but in most cases an engineer has calculated how much "give" should be between the bolted surfaces to assure minimal wear and maximum application life in whatever bumpy, cyclic environment the bolt is designed to be used. The answer to achieving the proper amount of preload is to use a torque wrench, and just as importantly to use it correctly.
Basics of a Torque Wrench
There are a number of torque wrench styles, but the most useful for the home mechanic is a "clicker" wrench. At first glance these may look a bit like a ratchet wrench, and they do have many similarities. The ratchet wrench usually has a ratcheting head with a switch for setting clockwise or counter-clockwise operation, and a handle for leverage. A torque wrench also has a pivot point just below the head, and the handle grip that is twisted to set the breaking force for the pivot. When the breaking force has been reached, the pivot gives an audible "click" indicating the proper torque has been achieved.
Using a Torque Wrench
Using a torque wrench correctly involves three primary concerns.
Use a smooth even pull to the stopping point. Jerking the wrench can cause the pivot point to break early. Not stopping when the wrench "clicks" will cause it to over torque.
When more than one bolt holds the surfaces together there is normally a sequence that should be used to bring the surfaces together in an even manner. Sometimes this is simply a "criss-cross", but sometimes the maintenance manual will have a numbering scheme – if so it should be followed. Just starting on one side and going in a circle around the part can cause the part to warp, crack, or otherwise cause damage.
In some cases the maintenance manual will require the threads be lubricated prior to tightening the bolt. Whether a lubricant is used or not has a considerable impact on how much torque is required to reach a given preload. Use lubricant if required, or not if so specified. If you don't follow this point you've wasted your time using a torque wrench.
Caring for Your Torque Wrench
Pardon the nanny notes, but unlike a sledge hammer or crow bar a torque wrench is a precision instrument and needs proper care. If it feels like a bolt is not getting tight enough by using your torque wrench, have the wrench tested at a calibration shop. If you need to remove a bolt, set the torque wrench aside and use a different tool. The clockwise/counterclockwise switch is for using the torque wrench on either clockwise or counterclockwise threaded bolts, not for removing a bolt or loosening it so you can re-torque. Don't turn past the "click". Store your wrench where it won't be bumped around.