A torque wrench is a wrench designed to let you tighten nuts or bolts without over-tightening them, which is prevented in a variety of ways.

 

Lets go back to the lug nuts. If the lug nuts on your car need to be tightened to 75 foot pounds you need to make sure you have a way of knowing when you've reached 75 foot pounds. Are you just going to use a regular wrench and guess?

 

That's why I said above that "hopefully" you used a torque wrench. A standard torque wrench will have a gauge on it that shows you how much force you are applying. As you tighten the nuts the needle will point to the amount of force being used which will allow you to apply exactly the right amount.

 

Most wrenches of this type are fairly long to allow sufficient leverage to generate enough force. If you're tightening your lug nuts with a 16" wrench you're going to have a much easier time than if you try it with a 10" wrench, for example.

 

While some types of these wrenches just have a gauge to tell you how much force you're applying, others actually have mechanisms that prevent you from applying too much force. For example, some wrenches have a clutch that disengages when the correct amount of force is applied so that you cannot tighten the bolt any further.

 

Now, what if you're working with bolts that are much larger than the lug nuts that hold your wheels on? What if you need to tighten something to 10,000 feet pounds instead of 100? In these cases there is another type of wrench, a hydraulic torque wrench, that uses hydraulic power to apply the necessary force to the bolts. Hydraulic torque wrenches have been around since the mid-60's but recent improvements in their design have made them smaller, lighter, and and more easily able to fit into tight spaces. This kind is considered better than pneumatic wrenches because they are quieter, lighter, and more accurate.