A jack, or hydraulic car jack, is a device that uses force to lift heavy loads. The primary mechanism by which force is applied varies, depending on the specific type of jack, but is typically a screw thread or a hydraulic cylinder. Jacks may be categorized based on the type of mechanism used to generate the lifting force, typically mechanical power, hydraulic power, or pneumatic power.

Mechanical jacks, such as the commonly used car jacks, lift heavy equipment and are rated based on the lifting capacity, which is typically expressed in terms of the number of tons that the jack can handle. Hydraulic jacks tend to have higher lifting capacities than mechanical jacks owing to the amount of force that can be generated by the hydraulic cylinders which product the lifting action. Common forms of hydraulic jacks include bottle jacks and floor jacks.

Hydraulic jacks function based on a concept in fluid mechanics known as Pascal’s Principal. Essentially, if two cylinders (a large and a small one) are connected by an incompressible fluid, and a given amount of pressure is applied to one cylinder, that same pressure is imparted to the second cylinder through the fluid connecting them. However, because pressure is equal to force per unit area, the cylinder that has a larger area will experience a force multiplication effect. Even though the pressure on both cylinders is the same, the force which is produced by the larger cylinder will be higher, proportionally higher based on the area of the cylinder.

Hydraulic jacks depend on this basic principle to lift heavy loads: they use pump plungers to move oil through two cylinders. The plunger is first drawn back, which opens the suction valve ball within and draws oil into the pump chamber. As the plunger is pushed forward, the oil moves through an external discharge check valve into the cylinder chamber, and the suction valve closes, which results in pressure building within the cylinder.

Bottle jacks became popular in the early 1900s when the automobile industry began to take off. Also called hand jacks, bottle jacks provided an easy way for an individual to lift a vehicle for roadside inspection or service. The design of the hydraulic bottle jack resembles a milk bottle, thus earning bottle jacks their name—today, they range in size and lifting capability from one hundred pounds to several tons. How does a bottle jack work? Bottle jacks feature a vertical shaft, which supports a platform (called a bearing pad) that directly bears the weight of the object as it is lifted.

Although they are most commonly used in the automobile industry (1.5 to 5-ton jacks are the range of capacities typically used to lift cars), bottle jacks have other uses as well. In the medical industry, they can be used in hydraulic stretchers and patient lifts. In industrial applications, they can be found as pipe benders used in plumbing, in cable slicers for electrical projects, and as material lifts within warehouses. Their ability to lift heavy loads plays a big role in enabling the repair of large agricultural machinery and in many construction operations. Bottle jacks can be secured within a frame, mounted on a beam, or simply used as free-standing to allow for repositioning as needed.

What is a floor jack? Unlike bottle jack shafts which operate vertically, the shaft in a floor jack is horizontal—the shaft pushes on a crank that connects to a lifting pad, which is then lifted vertically. Floor jacks typically provide a greater range of vertical lift than bottle jacks and are available in two sizes. The original jack is about four feet long, a foot wide, and weighs around 200 pounds—they can lift 4-10 tons. A more compact model was later made, which is about three feet in length and can lift 11/2 tons.  Although “mini jacks” are also produced, they are not a recognized standard type of floor jack. Typically, one of the first two sizes should be used.

The hydraulic cylinders used in jacks can experience failures requiring servicing or cylinder replacement to restore the jack to a safe operating condition. Some common failure mechanisms for hydraulic cylinders include:

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Hydraulic jacks can lift heavy loads but require maintenance to preclude the development of problems with the hydraulic components that could endanger operators, such as problems with the cylinders that assist in generating the lifting force.

To learn more about other types of lifting mechanisms, consult our guide on lifts, or visit the Thomas Supplier Discovery Platform to locate related suppliers for lifts, hydraulic jacks, or hydraulic cylinder repair services.

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