Many purpose built forestry machines make use of hydrostatic transmission (HST). But what is hydrostatic transmission and why would we use it?
HST accepts rotary power from a primary source, such as the internal combustion engines in our forestry equipment, and transmits this power to a load. The power source can really be anything that supplies rotary power. What is important is that the power source and the power destination can have completely different operating characteristics. This would not be possible with the conventional mechanical transmission which, for example, we have in most cars. The HST must regulate speed, torque, power and in some cases, direction of turn. It is even possible for a HST to drive a load from full speed in one direction to full speed in the opposite direction, with infinite variations of speed between these two points, with the primary power source operating at constant speed.
How does it work? A pump connected to the primary power source (e.g. internal combustion engine in a skidder), generates oil flow to drive a hydraulic motor which drives the final application. If the displacement of the pump and motor are fixed, then the HST is simply acting as a gearbox to move power from one place to another (engine to the load). However, if variable-displacement pumps and/or motors are used, then the speed, torque or power can be regulated.
HST has many advantages over conventional power transmission systems, some are listed below:
- It transmits high power in a compact size.
- It operates efficiently over a wide range of torque to speed ratios.
- It can maintain controlled speed, even in reverse, regardless of the load.
- It can transmit power from one location to several other remote locations, even if the position and orientation of the locations changes.
- It does not creep at zero speed.
- It will give a faster response than similarly rated mechanical transmissions.
- It is able to provide dynamic braking.
In the next issue, we will talk about the different types of HST construction and the four different functional types of HST’s. Source: www.hydraulicspneumatics.com