Any wrench that uses anything other but human effort can be classified as a power wrench. The power to operate the wrench can come from an electric power source, a pneumatic power source or hydraulics. These powered wrenches work in a similar fashion, the power source is just different. It is just not power wrenches that are commonly used in industry, power screwdrivers and nut runners are very common tools.
There are a number of different wrench designs; open end wrenches can be slipped on a nut from the side, socket wrenches are put over the nut from the top. As well as the gripping portion of the wrench they can be straight handled when they are strictly hand tools but when powered they are normally a socket design. A hydraulic wrench uses sockets of different sizes; they fit to the drive mechanism of the wrench which normally is a square of a given size. Small socket wrenches have a half inch square drive; a hydraulic wrench can have a one inch or even a one and a half inch drive.
Hydraulic wrenches are usually found in heavy construction and heavy industry where often the dimension across the flats on a nut can reach six inches or more. The common powered wrenches which are used around the home are usually electrically powered and in light industry, the favorite power source is pneumatics or air under pressure.
One of the more common wrenches to be powered is an impact wrench. Of all the powered wrenches, perhaps an impact wrench is the most powerful. Impact wrenches store power in the form of a hammer which is integral with the operating mechanism. When the trigger of the wrench is pulled, there are small hammers in the wrench which create an impact which in turn gets translated into a rotary motion. As the nut or bolt is subjected to small impacts, it slowly comes loose or tightened to a preset torque.
All power wrenches require a source of power. In the case of a hydraulic wrench it runs from a power pack which sends hydraulic oil under pressure into the piston or motor used to perform the function. In some instances a powered impact wrench requires special heat treated sockets due to the stress.
