Hydraulic rotary actuators use a pressurized, incompressible fluid to rotate mechanical components. They are faster and more powerful than pneumatic actuators because the high pressures used in hydraulic systems produce greater torque. Hydraulic rotary actuators use two types of rotational elements: circular shafts and tables. Typically, circular shafts include a keyway while tables include a bolt pattern for mounting other components. Single-shaft devices provide an output on only one side of the actuator. Double-shaft devices provide outputs on both sides of the actuator. To convert linear motion into shaft rotation, helical spline teeth on the shaft engage matching splines on the inside diameter of a piston. As hydraulic pressure is applied, the piston is displaced axially within the housing and the splines cause the shaft to rotate. When a control valve is closed, hydraulic fluid is trapped inside the housing and the shaft is locked in place. Single-piston devices drive a rack that rotates the pinion. Double-piston and four-piston devices drive racks on both sides of the pinion. Single-chamber and double-chamber rotary vanes that are actuated by pressurized air are also available.