In this issue we continue our discussion on hydrostatic transmission (HST), looking at different 
types of HST construction and the four different functional types.
The two types of construction HST are integral and non-integral:
- Integral – This construction combines the pump, motor and other hydraulic components into a single housing. The advantage of this is that it is cheaper and more compact, which may have axles, mounting surfaces and other components besides the HST.
- Non-integral – This is more common as power can be transmitted to one or more loads in areas that might otherwise be difficult to access. The pump is coupled to the prime mover (mostly the engine in forestry), the motor is coupled to the load, and they are simply connected by hose and tubing assemblies.
There must always be an optimum match between the engine and the load, allowing the engine to operate at an efficient speed while the HST adjusts to the operating conditions. The system should be balanced between efficiency and productivity. A highly efficient machine may have slow responsiveness (and poor productivity), while a very responsive machine could have poor efficiency due to energy that must always be available for the load.
The configuration of the HST will determine its performance:
- Fixed-displacement pump driving fixed-displacement motor – An inexpensive motor that has few applications because it is not very energy efficient. The pump has to be sized to drive the motor at a fixed speed under full load. If full speed is not required, fluid from the pump outlet passes over a relief valve, which wastes energy in the form of heat.
- Variable-displacement pump driving a fixed-displacement motor – This creates constant torque at any speed because the torque only depends on fluid pressure and motor-displacement. If the pump displacement is increased or decreased, it will increase or decrease the motor speed, with torque staying constant. Therefore, power increases with pump displacement.
- Fixed-displacement pump driving a variable-displacement motor – This achieves constant power by varying the motor displacement to maintain the product of speed and constant torque. If the motor displacement decreases, the motor speed will increase but the motor torque will decrease, resulting in constant power.
- Variable-displacement pump driving a variable-displacement motor – This versatile configuration can provide infinite ratios of torque and speed to power.
In the next issue, we will finish up our talk by discussing open and closed circuit HST systems. Source: www.hydraulicspneumatics.com