Hydrostatic Transmission System

 Hydrostatic transmissions do not make use of the hydrodynamic forces of the fluid flow.

HYDROSTATIC TRANSMISSION SYSTEM

1. What is Hydrostatic Transmission?

• Hydrostatic transmission is special case of energy transmission system where the mechanical energy of the input drive shaft is converted into pressure energy in the nearly incompressible working fluid and then reconverted into mechanical energy at the output shaft.

• Hydrostatic transmissions do not make use of the hydrodynamic forces of the fluid flow. There is no solid coupling of the input and output. The transmission input drive is a central hydraulic pump and final drive unit(s) is/are a hydraulic motor, or hydraulic. Both components can be placed physically far apart on the machine, being connected only by flexible hoses.

• Hydrostatic transmission consists of a drive wherein the hydraulic energy input element is a pump and the output element is a hydraulic motor.

• Usually hydrostatic transmission pumps and motors are designed and matched to optimise energy transmission.

2. Difference between Hydrostatic Transmission and Hydraulic Energy Transmission Systems

The primary difference between hydrostatic transmission system and a hydraulic system equipped with hydraulic pumps and motors is that an hydrostatic transmission is a whole unit in which pump and motor are specifically matched to work together. Also, hydrostatic transmission controls are designed to provide the specific functions to enable the transmission to perform specific tasks.

3. Advantages of Hydrostatic Transmission

Hydrostatic transmission offer many important advantages over other forms of power transmission. Depending on its configuration, anhydrostatic transmission system:

(i) transmits high power in a compact size;

(ii) exhibits low inertia;

(iii) operates efficiently over a wide range of torque-to-speed ratios;

(iv) maintains controlled speed (even in reverse) regardless of load, within design limits;

(v) maintains a preset speed accurately against driving or braking loads;

(vi) can transmit power from a single prime mover to multiple locations, even if position and orientation of the locations changes;

(vii) can remain stalled and undamaged under full load at low power loss;

(viii) does not creep at zero speed;

(ix) provides faster response than mechanical or electromechanical transmissions of comparable rating; and

(x) can provide dynamic braking.

4. Basic Hydrostatic Transmission System Circuits

• The two basic hydrostatic transmission system circuits are open circuit and closed circuit. The terms 'open circuit' and 'closed circuit' describe how the hydraulic lines in the conducting circuit are connected.

• In an open circuit, the flow path of the fluid is not continuous and is interrupted by the reservoir.

• In closed circuit, flow path remains uninterrupted.

5. Open-Circuit Hydrostatic Transmission System

A simple open-circuit hydrostatic transmission system is presented in Fig.8.23. The major elements of the open-circuit are indicated in Fig.8.23.

6. Closed Circuit Hydrostatic Transmission System

• A typical closed circuit hydrostatic transmission system is depicted in Fig.8.24. 

• Elements: A closed circuit hydrostatic transmission system essentially consists of all the elements of open circuit. In addition, the following two elements are included: (i) Feed or charge pump to replace the leakage oil; and (ii) Dual shock valve to protect the system from the damage in case of pressure over ride and over running condition.

• To improve the performance and to fulfill other desired tasks, many other appropriate components such as accumulator, cooling system, special purpose valves, etc are included in the circuit.