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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