Helical spline teeth machined on the shaft engage matching splines on the inside diameter of the piston. The outside diameter of the piston carries a second set of splines, of opposite hand, which engage matching splines in the housing. As hydraulic pressure is applied, the piston is displaced axially within the housing – similar to the operation of a hydraulic cylinder – while simultaneously, the splines cause the shaft to rotate. When the control valve is closed, oil is trapped inside the housing, preventing piston movement and locking the shaft firmly in position.

Starting Position	End Position
The piston is completely bottomed out. Bars indicate starting positions of piston and shaft. Arrows indicate directions they will rotate. The housing with integral ring gear remains stationary. For the sake of clarity, shaft flange with bearings are not shown.	As fluid pressure is applied, the piston is displaced axially while the helical gearing causes the simultaneous rotation of the piston and shaft. Helac's unique double helix design compounds rotation: shaft rotation is about twice that of the piston. Applying pressure to the alternate port will return the piston and shaft to their original starting positions.