A car jack uses hydraulic or mechanical force to lift vehicles. Jacks using hydraulic force depend on pressure. A pump plunger takes oil through two cylinders. When pushed back, the plunger opens the suction valve and forces the oil into the pump chamber. When pushed forward, the oil relocates to the cylinder chamber and the suction valve closes in order to build enough pressure to lift the vehicle in the air.
Mechanical jacks, often called scissor jacks, use levers and screws to lift a vehicle. They are inserted under the vehicle, and an Allen wrench is used to turn the screw in the center of the jack clockwise. As the screw turns, the jack elongates, and the vehicle raises. Turning the screw in the opposite motion brings the vehicle back down to the ground.
There are two types of hydraulic jacks. Bottle jacks have a vertical shaft that holds the bearing pad that lifts the vehicle. These are common in the automobile industry, as they can lift 1 1/2 to 5 tons of weight, but are also used in the medical fields to raise and lower stretchers and patient lifts. Floor jacks have a horizontal shaft that enables a vehicle to be lifted further in the air than a bottle jack. At around 200 pounds, these jacks are considerably larger, but they are also able to lift 4 four to 10 tons of weight.
Mechanical jacks vary, as they can use ratchets, screws or reels to force the lift. Each brand is labeled with its weight capacity, which can range from 5 to 150 tons.