Fluid Power - (ME353)- Lec7

8/17/2019 Fluid Power - (ME353)- Lec7

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Fluid Power Systems (ME353)

Fall 2012

Lecture 7


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Motors in seriesSame flow rate for each motor.


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Motors in parallelDifferent flow rate for each motor


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Motors in parallel with flow control

Different flow rate for each motor with flow control valvefor each one.


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Braking circuits for Hydraulic Motors

Braking circuits are used to slow hydraulic motors to a stop

– Inertia of a heavy rotating load can continue to turn the motor shaft

– Braking occurs when fluid discharged from the motor outlet port is

forced to pass through an adjustable pressure control valve before

returning to the reservoir


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Open-Loop and Closed-Loop HydraulicMotor Systems

An open-loop hydraulic motor system uses a layout typical of a basichydraulic system

– Pump moves fluid from a reservoir, through a directional control valve,

to the motor

– Fluid is then returned from the motor to the reservoir through the same

control valve

Closed-loop hydraulic motor systems continuously circulate fluid between

the pump and the motor without returning it to a system reservoir

These systems use a replenishment circuit to replace fluid lost through

leakage


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Open circuit design


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Hydrostatic Drives

Hydrostatic drive systems consist of the basic components typically found in

other hydraulic motor circuits

When compared to conventional transmissions, hydrostatic drives:

– Have a high power output – to – size ratio

– May be stalled under full load with no internal damage

– Accurately maintain speed under varying load conditions

– Provide an almost infinite number of input/output speed ratios


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Hydrostatic drives provide effective transmission of power and allow easy

adjustment and control of:

– Output shaft speed

– Torque

– Horsepower

– Direction of rotation

Hydrostatic drives may be open or closed circuits – Open circuit has the layout of a basic hydraulic motor circuit

– Closed circuit has the outlet of the pump directly connected to the inlet

of the motor and the outlet of the motor directly connected to the inlet of

the pump


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Open circuit design


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Closed circuit design


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Four combinations of pump/motor arrangements can be used

– Fixed-displacement pump and motor

(Maximum horsepower, torque, and output shaft speed are fixed)

– Fixed-displacement pump and variable-displacement motor

(Maximum horsepower is fixed, Torque and speed are variable)

– Variable-displacement pump and fixed-displacement motor

( Horsepower and output shaft speed are variable, Torque output is fixed) – Variable-displacement pump and motor

(Horsepower, torque, output shaft speed are variable)

Hydrostatic drives are typically considered hydrostatic

transmissions when both the pump and motor have variabledisplacement

This combination allows manual or automatic control of torque,

speed, and power output

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Fluid Power - (ME353)- Lec7