How a Hydraulic Jack Multiplies Force with Oil
A hydraulic jack lets a small push lift a heavy load by using oil in a sealed system. Here is Pascal's principle, the pump and check valves, and the trade-off in distance.
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A hydraulic jack lets one person raise a load far heavier than they could lift, by pushing a small handle that moves oil through a sealed system. The trick is that pressure in a trapped liquid acts equally everywhere, so a small piston can push on a big piston with a much larger force. The price is distance: the big piston moves much less than the small one. This article walks through the principle, the parts of a typical jack and the safety points that sources mention.
Pascal's principle
The French mathematician Blaise Pascal established the law in 1653 and published it in 1663, according to Wikipedia's article on Pascal's law. It says that a change in pressure at any point in a confined, incompressible fluid is passed on throughout the fluid, so the same change occurs everywhere. Wikipedia quotes the older wording that the change is transmitted "equally and undiminished" in all directions.
Pressure is force divided by the area it acts on. If the same pressure acts on a small piston and a large one, the force on each is the pressure multiplied by its area. So the ratio of the output force to the input force equals the ratio of the two areas.
A worked example
NASA's Glenn Research Center explains this with simple numbers. A force of 1 pound pressing on a piston of 1 square inch creates pressure in the fluid. That same pressure acts on a second piston of 10 square inches and lifts a 10-pound weight. The ideal force gain, called the mechanical advantage, equals the area ratio, here 10 to 1.
Wikipedia gives a similar example: if the output piston has 50 times the area of the input piston, a 1 newton push becomes a 50 newton force at the output.
Where the energy goes
This does not create energy from nothing. NASA points out that the volume of fluid pushed out of the small cylinder equals the volume that enters the large one. In its example the small piston has to travel 10 inches to raise the big piston by 1 inch. Wikipedia gives a larger-ratio case: pushing the small piston down 100 cm raises the large one only 2 cm. In both, input force times input distance equals output force times output distance, the same bargain a lever makes.
Inside a typical jack
A real jack must lift in many small strokes, because the pump chamber is small. Wikipedia's article on hydraulic jacks describes how the pump manages this using oil and two one-way valves, called check valves.
- When the pump plunger pulls back, it draws oil from the reservoir through a suction check valve into the pump chamber.
- When the plunger pushes forward, the suction valve closes and a discharge check valve opens, sending the oil into the main cylinder.
- Pressure builds in the cylinder and raises the load.
- The closed valves keep the oil from flowing back between strokes.
Wikipedia notes that the oil is incompressible and self-lubricating, which suits the job.
Floor jacks and bottle jacks
Two common shapes are described. A floor jack uses a horizontal piston that pushes the short end of a bellcrank, a pivoting lever, while the long arm moves the lifting pad up. Its low profile lets it fit under vehicles, and Wikipedia says it should be used only on a hard, smooth, level floor. A bottle jack has a vertical ram that rises out of a cylindrical body. Its range is limited, so it suits short lifts, and blocks can be used to gain height.
Safety notes from the sources
Wikipedia says that a jack not designed for a particular vehicle needs extra care with ground conditions, the jacking point and stability when fully extended. It also says hydraulic jacks are typically used for shop work, rather than as an emergency jack carried in a vehicle. For the correct way to lift a specific vehicle, follow the manufacturer's manual.
The limits of the principle
Pascal's law, as Wikipedia states it, applies to confined, incompressible fluids at rest, and it is usually applied to static systems. The area ratio is also an ideal figure: NASA calls it the ideal mechanical advantage, so a real jack, with friction and other losses, delivers somewhat less.
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