Clutches and brakes transmit power to control starts and stops in mechanical power transmission systems. Hydraulic brakes and clutches are driven by pressurized fluid, pneumatic brakes and clutches are driven by compressed air or gas, and electromagnetic brakes and clutches are activated by an electrical signal. Electromagnetic clutches effectively transmit torque in conveyor drives, while pneumatic clutches and brakes come in constricting, expanding, and caliper disc brake forms. Cam clutches are used to transmit torque in one direction. They do this via wedging elements or cams that lock the clutch's inner and outer races. Cam clutches are used in overrunning, indexing, and backstopping applications.