The hydraulic jack is one of the more important and interesting tools used to repair vehicles. For lifting tasks that are beyond the limits of a screw jack, it is the only practical alternative. There are many different types of hydraulic jack, from bottle jacks to car hoists, but they all operate using the same basic principle. A small piston operated by a hand lever or motor pumps fluid from the reservoir into the main cylinder. Most jacks use a special blend of oil for the hydraulic fluid but some use air, although these are more commonly known as pneumatic jacks. Jacks are not only used to lift vehicles but also to support tires, engines, transmissions and other heavy objects.
The typical hydraulic jack has a large main cylinder and a smaller pumping cylinder. The main cylinder contains a large piston that does the lifting, and around this cylinder is the fluid reservoir. Inside the jack casing, there are three pipes that connect the cylinders and the reservoir together. When the jack is used for lifting, fluid is pulled through one pipe to the pumping cylinder during the upstroke, and then pushed into the main cylinder through another pipe during the downstroke. A check valve prevents the fluid from traveling back to the pumping cylinder. When a release valve is turned, fluid flows out of the main cylinder and back to the reservoir through the third pipe.
The hydraulic jack works like a set of gears or pulleys. Work done by the pumping piston over a long distance moves an object over a smaller distance. This is possible because of Pascal's principle, which states that a pressure change at one point in an enclosed fluid results in a pressure change across the entire fluid. This principle only applies for incompressible fluids, such as oil and water which do not change volume as their pressure increases. Jacks that use compressible fluids, such as air and other gases, lose some of the input energy in changing the volume of the fluid.
There are several types of hydraulic jack used in garages and workshops. The bottle jack is a compact unit that is especially useful for lifting in confined spaces, but it takes a long time to extend. The floor jack is preferred for lifting the end of a car because it can be rolled under and extended in one quick action. They are often seen being used by pit crews during car races, and there are even floor jacks especially designed for this task. The toe jack resembles an upside-down bottle jack with a hook mounted on the side, and lifts an object by pushing itself off the ground. Scissor lifts use a hydraulic jack to extend a frame and lift a vehicle a short distance off the ground. They are popular with small garages because they are much cheaper than floor hoists, and they can be moved around easily.
Hydraulic jacks are also used on a range of supports for car components. The work stand uses a jack and support frame to hold heavy parts in place, such as the transmission and differential. The engine crane has a lever arm attached to a jack and support frame with wheels, allowing the engine to be lifted out of the bay and taken away. The tire lift has a jack and cradle for lifting a tire, and also has a support frame with wheels.
Hydraulic jacks are also used for many tasks that do not involve lifting. Log splitters are bottle jacks that have a wedge attached to the end of the main piston. The log is held in place by a support frame and the wedge is driven into it by the jack. There are also hydraulic presses have a jack mounted to a support frame that are used for changing gears and bearings, as well as straightening bent metal rods.