hydraulic pumps performance and characteristics

Hydraulic Pumps

>>Performance and Characteristics

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

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)

Volumetric efficiencyThis indicates the amount of leakage, which takes place within the pump and involves considerations such as manufacturing tolerances and flexing of the pump casing.

Actual Pump Output, Q QA = (VD np ηV) /1000 where:Q: L/minVD : cm3/rev

ηV: Volumetric efficiency (decimal)

OR… QA = (VD np ηV) /231 where:Q: GPMVD: in3/rev

ηV: same as above (no units)

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 =

Mechanical efficiency

Mechanical efficiency can also be computed in terms of torque, and called torque efficency:

overall efficiency

The ratio of power output to power input to the pumpOr the Product of both volumetric and mechanical efficiencies is known as the overall efficiency

Torque to Drive a Pump

TA = (ΔP VD)/(2π ηm)

where:TA : Newton meters torque required

ΔP : pressure rise across the pump in MPaVD : Pump displacement in cm3/rev

ηm : Pump mechanical (torque) efficiency – a decimal

OR…

Torque to Drive a PumpEnglish Units

TA = (ΔP VD)/(2π ηm)

where:TA : is torque required

ΔP : pressure rise across the pump in PSIVD : Pump displacement in inches3/rev

ηm : Pump torque efficiency – a decimal

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

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

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

Typical Performance curves for pumps

Other Factors affecting pump performance

• Presence of foreign particlescause damage to the internal surfaces of a pump.

• Foams and bubblesGenerate noise and causes cavitation

• Overheating of oilpoor 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

Comparative analysis of pumps

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

Catalogue example

Double pumps

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

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

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…

Pumps Selection

• Flow rate requirement• Operating speed• Pressure rating• Performance• Reliability• Maintenance• Cost and• Noise.

Pump Symbols Any fixed displacement

pump Variable displacement

pump Variable displacement –

Pressure Compensated