lay shaft

 A secondary shaft, as in a sliding change gear for an automobile; a cam shaft operated by a two-to-one gear in an internal-combustion engine. It is generally a shaft moving more or less independently of the other parts of a machine, as, in some marine engines, a shaft, driven by a small auxiliary engine, for independently operating the valves of the main engine to insure uniform motion.  [Webster 1913 Suppl.]