Hydrostatic drives are widely recognized as an excellent means of power transmission when variable output speed is required. Typically outperforming mechanical and electrical variable-speed drives and gear-type transmissions, they offer fast response, maintain precise speed under varying loads, and allow infinitely variable speed control from zero to maximum.

Unlike gear transmissions, hydrostatics have a continuous power curve without peaks and valleys, and they can increase available torque without shifting gears. But despite the superior performance of hydrostatics, a major drawback has been higher cost compared to their mechanical counterparts.

Manufacturers, however, continue to boost performance levels, produce smaller and lighter packages, and offer advanced electronic controls. These factors now make hydrostatics an economical choice for many applications.

A basic hydrostatic transmission is an entire hydraulic system. It contains pump, motor, and all required controls in one simple package. Such a system provides all the noted advantages of a conventional hydraulic system -- such as stepless adjustment of speed, torque, and power; plus smooth and controllable acceleration; ability to be stalled without damage; and easy controllability -- with the convenience of single-package procurement installation.

Early hydrostatic transmissions were intended primarily for low-cost applications such as farm equipment and garden tractors. But improved designs -- particularly in controls -- have made these transmissions suitable for a broad range of applications.

As a result, light-duty units (less than 20 hp) are being used on equipment such as lawn tractors, golf-course maintenance equipment, and small machine tools; medium-duty units (25 to 50 hp) on skid-steer loaders, trenchers, harvesters, and other such vehicles; and heavy-duty transmissions (approximately 60 hp and higher) on agricultural and large construction equipment.

Part of the reason for the increasing attractiveness of hydrostatic transmissions is improved design of pumps and motors -- particularly higher flow and pressure ratings in a more compact package. For example, several years ago, most pumps could be expected to deliver about 0.125-gpm flow per pound of pump. Currently available pumps deliver about 0.5 gpm/lb, a 400% increase. Similarly, older motors provided about 0.5 hp/lb of motor; new motors provide about 2.5 hp/lb.

Performance: Hydrostatic transmissions are commonly available with at least three standards of output performance:

  • Variable-power, variable-torque transmissions are based on a variable-displacement pump supplying a variable-displacement motor. They can provide a combination of constant torque and constant power. These units are adjustable, flexible, and expensive.
  • Constant-torque, variable-power transmissions are based on a variable-displacement pump supplying fluid to a fixed-displacement motor under constant load. Speed is controlled by varying pump delivery. This is considered the best general-purpose drive, with wide speed ranges, up to 42:1, and simple controls.
  • Constant-power, variable-torque transmissions are based on a variable-displacement pump with a power limiter, driving a fixed-displacement motor. The forte of this unit is efficiency, but speed range is usually limited to 4:1.

Drive configurations: Hydrostatic transmissions usually take one of two general configurations, split or close coupled. A split transmission consists of a power unit with the hydraulic pump, heat exchanger, filters, valves, and controls mounted on a reservoir. The hydraulic motor is remotely mounted and connected to the power unit through hose or tubing. Split transmissions are typically used in heavy-duty applications because they offer wide flexibility in configuring a system for the most efficient use of space or best weight distribution.

Integrated, or close-coupled, transmissions have a hydraulic pump and motor that share a common valving surface. This arrangement provides an extremely short oil-flow path, eliminating high-pressure oil leaks either to the reservoir or to the environment. A cast casing or housing provides a self-contained oil reservoir, structural support for the rotating elements, and heat dissipation. They are usually bolted directly to a mechanical differential axle to form a hydrostatic transaxle. Close-coupled transmissions are typically found in light-duty applications, where tight space constraints require compact units, while high-volume production mandates easy assembly.

Transmission sizing: Hydrostatic transmission size normally is based on the corner horsepower of the work function. Corner horsepower is the product of the maximum force and maximum speed required by the function, even though these two conditions rarely occur simultaneously. Corner horsepower for vehicle propulsion is

Hc = ( Ft * V ) / 3,600n

where Hc = corner horsepower, kW; Ft = maximum vehicle tractive force, N; V = maximum vehicle speed, km/h; and n = final drive efficiency, %.

Transmission corner horsepower, Ht, is the product of maximum output torque (generally at a specified maximum pressure) and maximum output speed:

Ht = ( Tt * N ) / 9,550

:where T = theoretical torque at maximum system pressure, N-m; t = torque efficiency, %; and N = maximum transmission speed, rpm.

Initial transmission selection is made by comparing the results of these calculations. Selection is refined by considering the effects of duty cycle, final-drive ratio, rolling radius, primer-mover speed, and design life.

Electronic controls: Control capabilities for hydrostatic transmissions have advanced from simple remote electrical actuators to packages that offer complete optimization of machine performance. For example, electronics on paving equipment not only controls the transmission, including speed and rate of acceleration and deceleration, but also steering, paving height, rate of material flow, road crown, slope on curves, and so on.

While not currently economical for every application, proportional controls offer a reasonable payback in most traction drives and propel systems through fuel savings and increased productivity. Acceptance will quicken when the added benefits in addition to primary control are recognized. One such feature is performance monitoring, another is system diagnostics -- relating when servicing is needed, when failure is imminent, or where a failure has occurred. Such features are relatively easy to add into software because many of the variables needed are already measured for control.