as large a load as a corresponding ideal machine using the same input force. Compound machines A compound machine is a machine formed from a set of simple machines connected in series with the output force of one providing the input force to the next. For example, a bench vise consists of a lever (the vise's handle) in series with a screw, and a simple gear train consists of a number of gears (wheels and axles) connected in series. The mechanical advantage of a compound machine is the ratio of the output force exerted by the last machine in the series divided by the input force applied to the first machine, that is Because the output force of each machine is the input of the next, , this mechanical advantage is also given by Thus, the mechanical advantage of the compound machine is equal to the product of the mechanical advantages of the series of simple machines that form it Similarly, the efficiency of a compound machine is also the product of the efficiencies of the series of simple machines that form it Self-locking machines In many simple machines, if the load force on the machine is high enough in relation to the input force , the machine will move backwards, with the load force doing work on the input force. So these machines can be used in either direction, with the driving force applied to either input point. For example, if the load force on a lever is high enough, the lever will move backwards, moving the input arm backwards against the input force. These are called reversible, non-locking or overhauling machines, and the backward motion is called overhauling. However, in some machines, if the frictional forces are high enough, no amount of load force can move it backwards, even if the input force is zero. This is called a self-locking, nonreversible, or non-overhauling machine. These machines can only be set in motion by a force at the input, and when the input force is removed will remain motionless, "locked" by friction at whatever position they were left. Self-locking occurs mainly in those machines with large areas of sliding contact between moving parts: the screw, inclined plane, and wedge: The most common example is a screw. In most screws, applying torque to the shaft can cause it to turn, moving the shaft linearly to do work against a load,