This document discusses hydraulic pumps, including:
- Pumps are not continuous flow devices and have discrete chambers that collect and discharge flow through valve plates. The design of these components affects pressure variation.
- Actual pump flow is determined by displacement, speed, efficiency terms accounting for volumetric (leakage) efficiency and mechanical (friction loss) efficiency.
- Volumetric efficiency depends on manufacturing tolerances while mechanical efficiency depends on bearing friction and fluid turbulence.
- Formulas are provided to calculate theoretical flow, actual flow accounting for efficiencies, torque required to drive the pump, and power delivered versus power input accounting for overall efficiency.
- Factors like fluid properties, speed, foreign particles,
2. General Issues
Pumps are not strictly continuous
flow devices. Discrete chambers are
involved.
Flow is collected for discharge
through valve plates
Design of the valve plate and the
pump mechanism affects pressure
pulses and variation (ripple) of torque
and pressure
3. General Issues
Our theoretical displacements can be used
to determine theoretical pump flow
Qth =Displacement (cc/rev) * Speed (rpm)
Actual flow is a linear function of pump
displacement, speed, a units constant, and
an efficiency term
Two kinds of inefficiencies to account for
losses:
Volumetric efficiency (slip)
Mechanical efficiency (Friction losses)
4. Volumetric efficiency
This indicates the amount of leakage, which takes place
within the pump and involves considerations such as
manufacturing tolerances and flexing of the pump
casing.
6. Mechanical efficiency
This indicates the amount of energy lost by
friction in bearing and other moving parts and Energy losses
due to fluid turbulence.
mech eff =
8. overall efficiency
The ratio of power output to power input to the pump
Or the Product of both volumetric and mechanical
efficiencies is known as the overall efficiency
9. Torque to Drive a Pump
TA = (ΔP VD)/(2π ηm)
where:
TA : Newton meters torque required
ΔP : pressure rise across the pump in MPa
VD : Pump displacement in cm3
/rev
ηm: Pump mechanical (torque) efficiency – a decimal
OR…
10. Torque to Drive a Pump
English Units
TA = (ΔP VD)/(2π ηm)
where:
TA : is torque required
ΔP : pressure rise across the pump in PSI
VD : Pump displacement in inches3
/rev
ηm: Pump torque efficiency – a decimal
11. Power to Drive the Pump
The hydraulic (theoretical) power
delivered by the pump is
QActualΔP/600 or QactualΔP/1714
for SI English units
(note this is actual pump flow, not theoretical)
Shaft power to drive the pump is
given by Psp = Phydr / ηo where:
η o = ηv ηm which is total pump efficiency
12. What Determines ηv & ηm ?
ηv is a function of clearance spaces, system
pressure, viscosity and pump speed
Leakage flow at a given pressure is relatively
fixed regardless of pump speed
It is also affected by fluid viscosity as lower
viscosity fluid will result in higher leakage and
lower volumetric efficiency
13. What about Torque (mechanical)
Efficiency?
Torque efficiency is a function of
speed and fluid viscosity
Higher pump speeds will result in
lower efficiency as viscous friction is
speed dependent
Lower viscosity fluid can reduce
viscous losses but acts negatively on
volumetric efficiency
15. Other Factors affecting pump
performance
• Presence of foreign particles
cause damage to the internal surfaces of a pump.
• Foams and bubbles
Generate noise and causes cavitation
• Overheating of oil
poor lubricant and increases the internal leakage,
reducing pump capacity
• Wrong selection of oil.
select the oil in accordance with the ambient
temperature and follow the instructions of pump
manufacturer
17. Cavitation
Pump cavitation can occur due to entrained air
bubbles in the hydraulic fluid or vaporization of
the hydraulic fluid
To control cavitation keep the suction pressure
above saturation pressure of fluid by:
Keeping suction line velocities below 4 ft/sec
(~1m/s) and pump inlet lines as short as possible
Minimize inlet line fittings; mount pump close to
reservoir; use low-pressure drop filters on inlet, and
use proper oil
20. Sizing Pumps
Component sizing begins with the LOAD
Load and actuator will determine
Flow requirement for this circuit
Pressure range required by the circuit
(We’ll do this with cylinders and motors… soon)
Total and simultaneous flow requirements
Select for the maximum load pressure
Add pressure drops that will occur in valves,
lines and fittings
21. Pump Sizing
With pump outlet pressure and flow known
we will consider speed.
Industrial apps will use synchonous speed of
electric motors. Generally 1750 rpm, or possibly
1100. ($ decides)
Small diesel apps such as skid loaders can
operate directly from engine crankshaft and will
have engine speed. (2000-3000 rpm).
Larger diesel apps – pump splitter with gear
reductions possible to optimize speed
22. Pump Sizing
Determine appropriate speed for your app
Use the equation for pump flow, solved for
displacement
VD = 1000Q/p (np ηV)
What shall we use for ηV ??
This is a function of speed, pressure, and
fluid viscosity
Look for vendor data or curves and adjust…