Bolt tensioners are mechanical fasteners that are used in place of standard hex nuts or bolts for easier and more precise tightening of large-diameter bolt and stud assemblies. They are sometimes called multi-jackbolt tensioners (MJTs). Many bolt tensioners apply torque via hydraulic or manual methods. Hydraulically-powered devices use fluid force. Manually-powered products utilize the turning of a wrench or other hand tool. Bolt tensioners that apply torque via other, non-hydraulic, non-manual methods are also available. For all types of bolt tensioner, torque rating is the maximum amount of rotational force that can be produced. Other important specifications include threaded diameter and thrust collar inner diameter. Threaded diameter may refer to the internal threaded diameter or the external threaded diameter, depending on the type of bolt tensioner. 

There are three basic types of bolt tensioners: nut replacement, bolt replacement, and thrust collar. Nut replacement or augmentation tensioners are designed to replace or assist an existing hex nut in a fastener assembly, to achieve a desired bolt tension. This is the most common type of bolt tensioner. With nut replacement devices, the key diameter is the external threaded diameter.  Bolt replacement tensioners are designed to replace an existing bolt or stud in an industrial fastener assembly. With these devices, the external diameter is most important. Thrust collars are bolt tensioners that consist of a ring with integral jackbolts (high-strength adjustable studs). They replace locknut assemblies and support bearings in large steel-mill roll mechanisms and similar applications, to facilitate servicing by maintenance personnel. 

Bolt tensioners are used in a variety of applications. For example, some products are designed for use with compressors. Others are used with foundation bolts, devices that help support buildings and other large structures. Bolt tensioners are also used with electricity-producing gas turbines, and to mechanically join turbines with electricity-producing generators. Specialized bolt tensioners are used to secure water-power related mechanical assemblies in hydroelectric facilities, as replacement fastening devices on ships and submersibles, and to secure reactor vessels in nuclear power plants. Additional applications for bolt tensioners include industrial presses and industrial pumps, steam turbines, wind turbines, and steel mill rollers.