A torque wrench is a tool that measures the tension applied to the bolt being tightened. Bolts can only take so much pressure, and torque wrenches are designed so that you can tighten bolts to the optimal tension level. Too tight, the nut or bolt may break or be damaged; not tight enough, whatever piece of machinery or item you are working on may be weakly structured or unsafe for use. You can set a torque wrench to a specific torque setting. The signal upon reaching the set torque varies depending on the type of wrench.
The simplest and most basic type of torque wrench is a beam wrench. It has an indicator with a scale or dial to indicate the applied torque force. You have to constantly check the indicator as you work to know how much force you are applying and when you've reached the set torque level. The most accurate way to reach the optimal torque is to tighten the bolts gradually. When there is more than one bolt to tighten, it's better to partially tighten all the bolts and then go back over each one until you reach the designated torque. This prevents an imbalance between the different bolts, which can cause faulty assembly and malfunctioning. Apply smooth, constant force and avoid using your full body weight to give the bolt a final tightening, which can throw the entire assembly off balance.
Electronic torque wrenches can be set to the required torque level, and they give a digital readout of the torque level on an LCD or LED screen. Some torque wrenches give an audio indication at reaching torque level, and another more sophisticated kind clicks at the preset level, with a mechanism that indicates when to stop tightening. Torque wrenches must be calibrated at regularly to make sure the readings are accurate.