An animation of the hydraulic rotary actuator shown in the video posted by AvE. This is not a hydraulic motor as the output shaft only rotates 360 degrees in total, but the output torque is substantially higher.
Original video: https://www.youtube.com/watch?v=yZ04iC3J6Mc
The outer housing (yellow) is fixed. Hydraulic pressure drives the piston (green) up and down along the axis of the housing. The output shaft (red) is free to rotate but constrained axially.
The piston is engaged with the housing via a left-handed thread. This causes the piston to rotate at it travels up and down.
The piston is also engaged to the output shaft via a right-handed thread. As the piston moves down, the output shaft is forced to rotate. The rotation of the piston and output shaft are in the same direction, causing the total output rotation to be the sum of:
(piston displacement * piston-housing thread lead) + (piston displacement * piston-shaft lead)
In this model, the piston-housing and piston-shaft leads are the same, though this is not a physical requirement. The result is the rotation of the shaft is double the rotation of the piston. Different leads on the engaging threads can result in more or less output rotation at the expense of torque and internal friction.
Post time: Jun-12-2017