SELECTION OF HYDRAULIC MOTORS

HYDROSTATIC DRIVES Power transmission

systems

Consists of a positive

displacement pump driving a positive displacement motor.

They are compact,

powerful, readily controlled.

FEATURES OF HYDROSTATIC DRIVES

Remain stalled and undamaged under full power load at low power loss. Hold a preset speed accurately against driving or braking loads.

Operate efficiently over a wide range of torque or speed ratios.

Provide faster response than any other type of transmission.

Transmit high power with low inertia.

TYPES OF HYDROSTATIC DRIVES Open- system

A open system is one where the hydraulic fluid is returned into a large unpressurised tank at the end of a cycle through the system

Closed- system

A closed system is where the hydraulic fluid stays in one closed pressurized loop without returning to a main tank after each cycle.

PRINCIPLE CIRCUIT DIAGRAM FOR OPEN LOOP AND CLOSED LOOP SYSTEM

REPLENISHING PUMP In closed loop system the pump and the

motor have small amount of internal leakage.

Results in fluid loss This must be replenished to prevent

cavitation. Separate pump called replenishing pump

must be added.

HYDROSTATIC DRIVE ARRANGEMENTS Variable displacement pump, Fixed

displacement motor

Fixed displacement pump, Variable

displacement motor

Variable displacement pump, Variable

displacement motor

HYDROSTATIC DRIVE CHARACTERISTICS

VARIABLE DISPLACEMENT PUMP, FIXED DISPLACEMENT MOTOR

The output power can be varied by adjusting the pump displacement which changes the motor speed.

This drive configuration is known as CONSTANT TORQUE TRANSMISSION.

FIXED DISPLACEMENT PUMP, VARIABLE DISPLACEMENT MOTOR

This drive configuration is known as CONSTANT

POWER TRANSMISSION.

This type of characteristic is useful in drives of

coiling machines, tension control devices, winding and unwinding machines etc.

VARIABLE DISPLACEMENT PUMP, VARIABLE DISPLACEMENT MOTOR This type is generally used for applications

requiring a combination of torque and power characteristics.

The characteristic is a combination of the previous

two, as both pump and motor displacement can be varied. The speed is adjusted by either adjusting pump or

motor or both.

BRAKING OF HYDROSTATIC DRIVES Hydrostatic drives can be slowed down and stopped more smoothly and rapidly than most drives. This is because the fluid motors are smaller and have considerably less inertia than the equivalent electric motors and mechanical drives. Two braking systems are ideal for hydrostatic drives: 1.regenerative 2.dynamic.

BRAKING SYSTEMS With REGENERATIVE BRAKING , the inertia of the

driven machinery begins to drive the fluid motor as a pump, which in turn drives the pump as a motor. The kinetic energy is quickly absorbed stopping the fluid motor and its connected equipment.

In DYNAMIC BRAKING, flow from the pump to the

fluid motor is cut off by a valve. Load inertia drives the fluid motor as a pump and the output is forced through a relief valve, creating a cushioned braking effect.

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