13.4.1 Hydraulic Jacks

Hydraulic jacks use oil as the lifting medium, the body of the jack acting as a reservoir for the oil. When the jack is operated, oil is passed under pressure through a system of non-return valves to the base of the ram chamber. As more oil is delivered with each stroke of the operating lever, and as the oil cannot be compressed, it forces the ram out of the chamber providing a lifting motion. Lowering is achieved by releasing a valve which allows the oil to return to the reservoir. The operation of the release valve may be by depressing a lever, turning a screw or opening the valve directly. The ram is then returned by gravity, although in some cases this may be spring assisted.

The controlled use of hydraulics, where known pressures are applied over known areas, enables jacks to be used for load weighing purposes. Calibrated gauges may be fitted to some models to enable them to be used for this purpose.

Hydraulic jack bodies are commonly manufactured from aluminium, steel or cast iron. The material used affects the design, size, self-weight and capacity of the jack.