The WOB-L principle has a unitary piston rod that ‘wobbles’ inside the cylinder as the crankshaft rotates. Unlike the more conventional articulated piston pump, the piston and piston rod are a single item, usually a single casting. The piston rod is mounted to an eccentric bearing assembly, which in turn is mounted to the motor shaft to convert rotary energy from the motor into linear motion of the piston within the cylinder.

The piston is sealed to the cylinder by a flanged disk cup that forms both a seal and mechanical guide for the piston. It runs without lubrication in contact with a low friction, surface coated cylinder of high heat conductivity. As the piston is driven up and down, air resistance on the upward stroke expands the disk’s seal on the piston against the cylinder wall to increase its efficiency, while compensating for the ‘wobble’ action.