What is Hydrostatic Transmission?

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Hydrostatic Transmission

Transmission of power using interconnected devices from one point to another is called power transmission. Mechanical, electric, hydrodynamic, hydromechanical, and hydrostatic transmission are some categories of power transmission. This article included the topic hydrostatic transmission. But, a few details regarding other power transmission methods are also listed below.

Mechanical Transmission: These type of transmission system uses shafts, gears, torque converters, chains and belts for converting mechanical energy to kinetic energy. Transmission of power from an engine to the wheels of an automotive is an application.

Electric Transmission: In Electric transmission, an electric generator is used for converting mechanical energy to electric and with an electric motor this is converted back to mechanical energy. Electric transmission occurs in transformers.

Hydrodynamic Transmission: In hydrodynamic transmission, a hydrodynamic pump and hydrodynamic motor are connected together. The power generation is the result of the change in fluid velocity as it passes through the channel. An automatic car is one of the applications of hydrodynamic transmission.

Hydromechanical Transmission: A hydromechanical transmission uses a power split scheme for improving the efficiency of gearboxes and for providing flexibility. This transmission method converts the input energy to both mechanical and hydrostatic energy and is suitable for heavy-duty applications.

Now, we can move on to hydrostatic transmission. What is a hydrostatic transmission? Simply, it is a hydraulic system where the hydraulic pump or an accumulator will drive the motor using the fluid passing through flexible hoses. In hydrostatic transmission, gears are not required for converting rotating mechanical energy from one source to another. Because, when the displacement of pump and motor are fixed, then hydrostatic transmission itself will act as a gearbox. Hydrostatic transmission is suitable for applications that require variable output velocity or torque. Some of these applications include golf-course maintenance equipment, harvesters, tractors, trenchers, agricultural and large construction equipment. The advantages of the hydrostatic transmission system are:

  • With a constant input speed, hydrostatic transmission can deliver variable output speed and vice versa.
  • In a minimum time period, reverse direction of output rotation is possible.
  • Adjustment of speed, power and torque is possible with hydrostatic transmission.
  • Smooth and controlled acceleration.
  • Fast response.
  • Precise speed under variable load.
  • Hydrostatic transmission can be stalled without damage or overheating.
  • Ease of control.
  • Provide dynamic braking.
  • Hydrostatic transmission can transfer power from one prime mover to different locations.
  • Compact size.

The components required for a hydrostatic transmission system are transmission case(to hold components in place and for fluid transfer), a charge pump(for providing initial case oil pressure and to fill circuit with oil), the input shaft(to take power from the engine and to rotate the charge pump), axial piston pump(rotates the input shaft and to pump oil to the motor), hoses/passageway(to connect pump to the motor), pressure relief valves(provide alternative path for oil when pressure increases), motor(drives the output shaft), swash plate(changes the displacement of the piston pump), and check valve(used in closed loop circuit).

Also, Read: Hydraulic pumps Types- An Overview

How Does a Hydrostatic Transmission Work?

We know, every hydraulic system requires hydraulic fluids and it is stored in a reservoir. In a hydrostatic transmission system, when the engine runs, it will rotate the drive shaft and the input shaft connected to it. In a closed-circuit system, the movement of this input shaft will turn both the charge pump and piston pump. As a result, the charge pump will suck oil from the reservoir to the transmission case. The reciprocating motion of the piston, due to the change in the angle of a swash plate will cause the oil to pass through the hoses and towards the motor.

Also, Read: How Do Anti Lock Brakes Work?

Classification of Hydrostatic Transmission System

The hydrostatic transmission system can be classified according to the spatial arrangement, transmission ratio and circuit construction. Each of these classifications is mentioned below.

In-line, U-shaped, S-shaped and split are the 4 different hydraulic pump and motor configuration based on the spatial arrangement. The In-line configuration will contain a variable pump and a constant displacement motor connected directly in a line. U-shaped configuration is similar to In-line except, the motor is connected below the pump and both input shaft and motor shaft rotate in the same direction. Similar to U-shaped configuration, for S-shaped configuration the motor is below the pump/prime mover. But, the motor is placed behind the pump. In a split configuration, the motor and pump are separated using high-pressure hoses. This configuration has separate hoses for inflow and outflow of fluids.

The 4 classifications of hydrostatic transmission based on transmission ratio are the fixed-displacement pump and fixed-displacement motor, variable-displacement pump and fixed-displacement motor, fixed-displacement pump and variable-displacement motor, variable-displacement pump and variable-displacement motor.

Spatial flexibility is the only advantage of the fixed-displacement pump with fixed-displacement motor connection. This combination will have a constant transmission ratio. So, to obtain variable output speed, the speed of the prime mover needs to be changed. In a variable-displacement pump and fixed-displacement motor, the speed of the motor can be altered by changing the pump flow. The fixed-displacement pump and variable-displacement motor connection is commonly known as a constant power system. Because these connections will provide constant power and variable torque with variable output speed. The variable-displacement pump and variable-displacement motor is the most flexible configuration that produces variable output speed.

Closed-circuit and open-circuit transmission are the two classifications of hydrostatic transmission based on the circuit design. In open-circuit transmission, the fluid from the motor is directed towards the reservoir and the pump will again suck this fluid. But in closed-circuit transmission, the fluid from the motor will directly enter into the inlet of the pump and it requires a charge pump. A simple open circuit transmission system will contain elements like reservoir, suction strainer, pressure relief valves, pump, motor, conduit with connectors, return line filter and control unit. Other than that of the open circuit transmission system, a closed circuit transmission system requires a feed or charge pump unit and dual shock valve.


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