A hydrostatic transmission provides improved maneuverability, but at a cost. The efficiency of a hydrostatic transmission is always lower than a discrete-gear transmission. A discrete-gear transmission will typically have an efficiency of 95% or greater, meaning that 95% of the input energy is delivered to the load (wheels). A hydrostatic transmission has an efficiency of around 80%. Some well designed units will have an efficiency slightly above 85%, but none can approach the efficiency of a discrete-gear transmission. A designer always poses the question: Does the gain in vehicle productivity offset the loss in efficiency and resultant higher fuel cost?
In addition to increased maneuverability, a hydrostatic drive vehicle offers several other advantages:
1. It operates over a wide range of torque/speed ratios. Once a gear ratio is selected with a direct-drive transmission, the only speed variation available is that achieved by controlling engine speed. Once the engine speed reaches a maximum, the transmission must be shifted to a lower ratio to increase vehicle speed. With a hydrostatic transmission, vehicle speed is continuously variable from a slow creep up to a maximum.
2. It can transmit high power with low inertia. When a large mass is rotated at a given speed, it takes an interval of time to change this speed. A hydrostatic transmission adds little inertia to the total rotating mass associated with vehicle operation; consequently, a hydrostatic transmission vehicle tends to change speed more quickly (have less inertia) than a direct-drive or automatic shift transmission vehicle.
3a. It provides dynamic braking. A hydrostatic drive vehicle can be stopped by destroking the pump. Imagine that you are traveling forward and you suddenly move the swashplate control to the neutral position. What will happen? A pressure spike will develop, and fluid will flow across the relief valve. The vehicle’s mechanical energy will be converted to heat energy, and the vehicle quickly slows (probably sliding the wheels).
3b. It remains stalled and undamaged under full load. Vehicle hydrostatic transmissions are almost always designed for wheel slip to occur before a relief valve is actuated. The relief valve’s role is to clip off peaks and attenuate shocks, as described in part 3a. If the vehicle loses traction and bogs down, the pressure increases until the relief valve opens. Stalling the vehicle in this manner does not damage the transmission. Holding it in a stalled condition causes the fluid temperature to rise, and this is undesirable. [Most HST pumps today are available with a pressure limiter function that provides the “stall and undamaged” feature with little heat generation. The pressure limiter destrokes the pump by shifting the swashplate much like a pressure compensator.]
4. There is no interruption of power to wheels during shifting. Anyone who has watched the driver of a direct-drive vehicle with discretegear transmission shift gears while climbing a hill can appreciate the advantage of continuous power flow over a speed range.
