Need help understanding a two-speed hydraulic motor circuit

[pode412]

I am currently working with a drilling/piling rig. I saw in multiple youtube clips that their rig can be controlled in the way that the auger, which is drove by a hydraulic motor, can be operated at a high rpm (low torque for getting rid of dirt above ground) and a low rpm (high torque for drilling). How do they achieve this while having a single fixed-displacement pump? How does a two-speed hydraulic motor works? Anything that would help me understanding this is appreciated.

Thank you.

 

[hydroman247]

By increasing or decreasing the displacement of the motor.

e.g. Say the pump is pumping 100L/min and the motor is 100cc, this = 1000RPM. If the motors displacement is changed to 200cc, the RPM will halve to 500RPM but the torque will double.

Some pumps have on/off type displacement change while others are infiniately variable to achieve a wide range of speeds and torques.

 

[Compositepro]

It is quite possible that there is a gearbox between the motor and auger. Reversing the motor can not only change rotation but also gear ratio.

 

[nebbiolo]

An hydraulic motor can function with a variable displacement exactly like a variable displacement pump.

The displacement is increased or decreased pressurizing a chamber where a piston is directly connected with the swash plate