In steam engine technology, part of the valve gear used by some designs to give motion to the valve. It may best be described as an auxiliary crank. It is essential to ensure that the valve events occur in the correct sequence in relation to the movement of the piston inside the cylinder. The valve spindle is therefore driven off the motion of the piston by connecting it through links and levers to the connecting rod, crank or driving axle, depending on the design of the valve gear. Where inside cylinders are used, it is usual to derive the valve motion off eccentrics fitted to the driving axle. The eccentric consists of a circular disc, called a sheave, fitted to the axle so that its centre is offset from that of the axle. An eccentric rod is attached to the sheave by means of a strap which allows the sheave to rotate within the strap. When the axle rotates, the eccentric produces a fore and aft motion at the leading end of the eccentric rod. This, in conjunction with the expansion link and the setting of the reverser, is used to give motion to the valve spindle.