someone who takes the place of another (as when things get dangerous or difficult)
a toothed wheel that engages another toothed mechanism in order to change the speed or direction of transmitted motion
a mechanism for transmitting motion for some specific purpose (as the steering gear of a vehicle)
wheelwork consisting of a connected set of rotating gears by which force is transmitted or motion or torque is changed
equipment consisting of miscellaneous articles needed for a particular operation or sport etc.