Speed control mechanism

Claim 1

The combination of a speed control mechanism and hydraulic motor comprising: an hydraulic motor; a valve body having a bore therein; a main spool valve having a central land and two outer lands thereon mounted in said bore; a pressure fluid supply port communicating with said bore and normally closed by the central land on said main spool valve; control ports communicating with said bore between said central and said outer lands and connected to said hydraulic motor; return ports communicating with said bore and closed by said outer lands when said central land closes said pressure fluid supply port; throttling slots in each outer land; first actuator means for displacing said main spool valve from its position blocking said pressure fluid supply port and thereby allowing the pressure fluid supply port to communicate with a first control port to deliver fluid under pressure to said hydraulic motor, and allowing a second control port to communicate with a second return port through a throttling slot in one of said outer lands, said throttling slot and said second return port defining an orifice through which fluid returning from said hydraulic motor must pass; cam means for rotating the main spool relative to said bore to adjust the size of the orifice defined by said throttling slot and said second return port when said main spool valve is displaced; and pressure-responsive, metering valve means connected to each of said return ports and responsive to the pressure of fluid returning from said hydraulic motor and fluid passing through said orifice to control the maximum volumetric rate of flow through said return ports from said hydraulic motor.