Wherever a means of power transmission and variable speed are required, we typically think of using mechanical and electrical variable-speed drives and gear-type transmissions. However, there exists another equally excellent means of transmitting power, and that is through hydrostatic drives. While offering a fast response, hydraulic drives can maintain precise speed under varying loads all the while allowing infinitely variable speed control from zero to maximum.
Gear transmission systems usually have a discontinuous power curve with peaks and valleys. For increasing available torque, you need to shift gears. Hydraulic drives can overcome both these shortcomings. However, despite their superior performance, hydrostatics have a major drawback – higher cost compared to their mechanical counterparts.
However, manufacturers are driving down the economics of using hydrostatic drives. They are producing smaller and lighter packages, while boosting performance levels and offering advanced electronic controls. Many applications now prefer to use hydrostatics to other types of drives.
Hydrostatic drives have several advantages, the most significant being – a basic hydrostatic transmission is an entire hydraulic system. The simple package contains all the required controls, the motor and the pump included. The single unit provides all the advantages of a conventional hydraulic system – ability to be installed without damage; easy controllability; an entirely stepless adjustment of speed, torque and power; with smooth and controllable acceleration. All this comes with the simple convenience of a single-package procurement and installation.
Earlier, hydrostatic transmissions were limited to low-cost applications such as garden tractors and farm equipment. However, with improved designs, especially in control systems, hydrostatic transmissions are now suitable for a wide variety of applications.
This has resulted in the use of light-duty units of less than 20HP being used on equipment such as small machine tools, maintenance equipment for golf courses and lawn tractors. Medium-duty units of 25-50HP are used on vehicles such as harvesters, trenchers and steer loaders. Agricultural and large constructional equipment mostly use the heavy -duty transmission equipment rated for 60HP and above.
The increasing attractiveness of hydrostatic transmission is partly due to the improved design of motors and pumps that result in higher flow and pressure ratings in more compact packages. For example, where earlier pumps could deliver only 0.125-gpm flow for every pound of pump, current pumps can deliver more than 0.5gpm/lb., representing a four-fold increase. Similarly, where older motors could provide only about 0.5HP/lb., newer motors offer 2.5HP/lb. with ease.
Today, you can have hydrostatic transmissions with at least three standards of output performance – Variable-power/Variable-torque, Variable-power/Constant-torque and Constant-power/Variable-torque. Additionally, you can select hydrostatic drive configurations such as close-coupled or split-coupled. The transmission size is specified by corner horsepower of the work function. You obtain corner horsepower by multiplying the maximum force required with the maximum speed requirement, although you may never require these two conditions simultaneously.
Earlier control capabilities of hydrostatic transmissions were limited to simple remote electrical actuators. Today, they have advanced to packages offering complete optimization of the machine performance.
Fuel savings and increased productivity make hydrostatic proportional controls economical to use in most traction drives and propel systems, although they may not be economical for every application.