Abstract: An improved spring assembly for driving the throwing arm of a ball throwing machine. The assembly is mountable on a central transverse drive shaft of the machine. It includes a primary spring with an outwardly extending loop for loading against an abutment as the shaft is rotated in a first direction about its axis and for unloading in the first direction upon disengagement of the abutment. A secondary spring assembly is mountable to the drive shaft and includes a looped end engaging the outwardly extending arm of the primary spring assembly at a point approximately midway along its length. The secondary spring loads against the loop of the primary spring in response to rotation of the drive shaft, and unloads to drive the first loop forcibly in the first direction upon disengagement of the abutment by the first loop.