The main characteristics of a hydraulic torque wrench which set it apart from other powered wrenches of similar function are that it must generate torque using only hydraulic means it must be self ratcheting, and it must include an accurate method of determining the amount of torque applied.
In 1985, the Bolt Council : Research Council on Structural Connections, aka RCSC, specification permitted the use of the calibrated wrench method of installation, but with a clearer statement of the requirements of the method and its limitations.
In the calibrated wrench method the wrench is calibrated or adjusted to shut off when the desired torque is reached. In practice, several bolts of the lot to be installed are tightened in a calibrating device that directly reads the tension in the bolt. The wrench is adjusted to shut off at bolt tensions that are a minimum of 5% greater than the required preload.
Some manufacturers utilize a holding pawl design to keep the wrench locked in position prior to each power stroke, others use varying designs, which as in all industries have debatable faults or claimed advantages.
Hydraulic torque wrenches typically offer accuracy of ±3% and have a high degree of repeatability making them well suited to applications where large bolts are involved and a high degree of accuracy is required.
A hydraulic torque wrench is significantly quieter, lighter weight and more accurate than pneumatic impact wrenches capable of similar torque output, making it an appealing alternative for many users to the very loud and cumbersome impact wrenches or torque multipliers which were formerly the only viable option for working with very large nuts and bolts until the hydraulic torque wrench was introduced.
Bolt tensioners are an alternative to hydraulic torque wrenches, but, not as commonly used.