• Positive displacement
pump (Gear Pump): a
specific amount of fluid
passes through the pump
for each rotation
• Centrifugal pump (Vane
Pump): no specific
amount of fluid flow per
rotation; flow depends on
speed of blades
Positive displacement vs Centrifugal pump
• Displacement relates to how the output of the pump reacts to
– Non-positive-displacement pumps produce flow variations
due to internal slippage.
– Positive-displacement pumps produce a
constant output per cycle
• A non-positive-displacement pump has large
– Allows fluid slippage in the pump
– Results in varying flow output as system load varies
Key Feature of a V-Displacement
• Fixed Displacement Pump
–A fixed volume of fluids
will be provided in each
_The volume of supplying
fluids may be change
based on demands
If the pump is the heart of a hydraulic system then
the valve is the brain.
Valves are used to perform a large variety of
governing and controlling functions.
• pressure control valves
• flow control valves
• check valves (non-return valves)
• directional control valves
b) c) d) e) Conical
g) Spool valve
Aspects of classification
The velocity of hydraulic cylinders can be influenced by the flow rate.
This can be done by an adjustable pump or by a flow rate control valve.
The flow rate is changed by changing an orifice surface area or by changing the
pressure difference on a resistance or dividing the flow rate.
Advantages of flow valves
good dynamic behaviour
the relatively high energy losses
Restrictor valves (throttle valves)
Flow control valves
Flow dividing valves
The function of hydraulic motors is the
opposite of hydraulic pumps:
It draws in oil and displaces it, converting mechanical force
into fluid force.
Oil under pressure is forced in and spilled out, converting
fluid force into mechanical force.