Hydraulic elevators are elevators which are powered by a piston that travels inside a cylinder. An electric motor pumps hydraulic oil into the cylinder to move the piston. The piston smoothly lifts the elevator cab. Electrical valves control the release of the oil for a gentle descent.
Hydraulic elevators are used extensively in buildings up to five or six stories high. Sometimes, but rarely, up to 8 stories high. These elevators, which can operate at speeds up to 61 meters (200 ft) per minute, do not use the large overhead hoisting machinery the way geared and gearless traction systems do.
