Portable prestressing jacks are used at precast plants for single-strand tensioning before concrete is poured. These jacks are self-contained units, equipped with motor, pump, ram and controls. They have maximum working capacities ranging between 45,000 and 52,000 pounds. In general, two workers are required to operate this type of jack. One worker lines up the ram with the strand to be tensioned and secures the strand in the chuck, while the other worker operates the jack's controls.
An electric motor powers a hydraulic pump to supply pressure to the jack. A 10-hp, three-phase electric motor is commonly used and offers high efficiency. A number of different controls and gauges are necessary on a pretensioning jack. One valve controls the direction the ram moves. Other controls allow selection of initial or final tensioning or easeoff to release the pressure in the cylinder slowly. One gauge is used to measure tension during initial tensioning, and another during final tensioning.