Pumps - Introduction.ppt

PUMPS

Introduction• Heart of hydraulic system, converts mechanical energy into

hydraulic energy.• The mechanical energy is delivered to the pump via a prime

mover such as an electric motor.• Due to mechanical action, the pump creates a partial vacuum

at its inlet.• This permits atmospheric pressure to force the fluid through

the inlet line and into the pump.• The pump then pushes the fluid into the hydraulic system.

Pumping theory

Classifications of Pumps

• Hydrodynamic or non positive displacement pumps– Used for low-pressure, high-volume flow

applications• Hydrostatic or positive displacement pumps– Used for fluid power applications

Classifications of Pumps• Hydrodynamic or non positive

displacement pumps– Most common types are centrifugal

(impeller) and axial (propeller) pumps

– Provide smooth continuous flow– The output flow is reduced as the

resistance in the system circuit increased.

– Priming is required.– No positive internal seal against

leakage

Classifications of Pumps

• Hydrostatic or Positive displacement pumps– Ejects a fixed quantity of fluid per revolution of

the pump shaft.– Pump output flow is constant and not depend on

the system pressure.– A pressure relief valve is used to protect the pump

against overpressure.

Classifications of Pumps

Gear Pumps

• External gear Pump

Gear Pumps

• External gear Pump

Gear Pumps

Gear Pumps• External gear Pump– There are actually two cylinder volumes where oil could fill inside

the pump if there were no gear teeth.– However one half of these two volumes is taken up by the gear

teeth of both gears.– Volumetric displacement and Theoretical flow rate

• Do – outside diameter of gear teeth (in, m).

• Di – inside diameter of gear teeth (in, m).• L – width of gear teeth (in, m).• VD – displacement volume of pump (in3/rev, m3/rev)• N – rpm of pump• QT – theoretical pump flow-rate

Gear Pumps

• External gear Pump– Volumetric displacement and Theoretical flow rate

QT= VD × N

Gear Pumps• External gear Pump– Volumetric displacement and Theoretical flow rate

whereb – width of gearm – module of gearz – number of teeth in driver or driven gearα – pressure angle of the gearN – rpm of the pump

Gear Pumps• Efficiencies of Pump– Volumetric efficiency (ηv)

– Mechanical efficiency (ηm)

– Overall efficiency (ηo)

=

Exercises• A gear pump with the following specification runs at 1400

rpm. Module = 3mm/toothGear width = 15mmNumber of teeth = 12Pressure angle = 200

a. Calculate the theoretical discharge [Ans : 0.0163 m3/min ]b. Calculate the hydraulic power produced by the pump when working against a pressure of 100 bar [Ans : 2.716 KW]

Exercises

• Find the actual delivery of the gear pump with the following specification:Outside diameter of the gear = 80mmInside diameter of the gear = 60mmGear width = 20mmSpeed of the pump = 1600 rpmVolumetric efficiency = 88%

[Ans : QT = 0. 0703 m3/min, QA = 0.0618 m3/min]

Exercises

• A gear pump has a 3-inch outside diameter, a 2-inch inside diameter and 1-inch width. If the actual pump flow at 1800 rpm and rated pressure is 28gpm, what is the volumetric efficiency? [Ans: 91.3%]

• Note:– 1 m3 = 1000 litres– 1 litre = 10-3 m3

– 1 gallon (gal) = 231 in3

– 1gal = 3.785 litre– 1 inch = 0.0254 m

Gear Pumps

• External gear Pump– Volumetric efficiency• There must be a small clearance (about 0.001inch)

between the teeth tip and pump housing• Some of the oil at the discharge port can leak directly

back toward the suction port. This internal leakage is called pump slippage.• The higher the discharge pressure, the lower the

volumetric efficiency because the internal leakage increases with pressure.

Gear Pumps

• External gear Pump– Volumetric efficiency

Gear Pumps

• External gear Pump– This pump uses spur gear (teeth are parallel to the axis of the gear),

which are noisy at relatively high speeds. – To reduce noise and provide smoother operation, helical gears (teeth

inclined at a small angle to the axis of the gear) are sometimes used.– However, these helical gear pumps are limited to low pressure

applications (below 200 psi) because they develop excessive end thrust.

– Herringbone gear pumps eliminate this thrust action and thus can be used to develop much higher pressures (up to 3000psi) and provide greater flow rates with much less pulsating action.

Gear Pumps

SPUR HELICAL HERRINGBONE

Gear Pumps

• Internal gear Pump– This design consists of an internal gear, a regular spur gear,

a crescent – shaped seal, and an external housing.– As power applied to an external gear, the motion of gears

draws fluid from the reservoir and forces it around both the sides of crescent seal, which acts as a seal between the suction and discharge ports.

– When the teeth mesh on the side opposite to the crescent seal, the fluid is forced to enter the discharge port of the pump.

Gear Pumps

• Internal gear Pump

Gear Pumps• Internal gear Pump

Gear Pumps

• Lobe Pump– This pump operates in a fashion similar to the external gear

pump.– But unlike the external gear pump, both lobes are driven

externally so that they do not actually contact each other.– They are quieter than other types of gear pumps.– Due to smaller number of mating elements, the lobe pump

output will have a somewhat greater amount of pulsation, although its volumetric displacement is generally greater than other types of gear pumps.

Gear Pumps

• Lobe Pump

Gear Pumps

• Lobe Pump

Gear Pumps

• Gerotor Pump– The name Gerotor is derived from “Generated Rotor”.– The inner rotor has “N” teeth and the outer rotor has “N+1”

teeth.– The inner gear rotor is power driven and draws the outer gear

rotor around as they mesh together.– The volumetric displacement is determined by the space

formed by the extra tooth in the outer rotor.– Gerotor pumps are generally designed using a trochoidal

inner rotor and an outer rotor formed by a circle with intersecting circular arcs.

Gear PumpsGerotor Pump

Gear Pumps

• Gerotor Pump

Gear Pumps

• Screw Pump– An axial flow positive displacement unit.– Three precision ground screws, meshing within a close-fitting housing,

deliver nonpulsating flow quietly and efficiently.– The two symmetrically opposed idler rotors acts as rotating seals,

confining the fluid in a succession of closures or stages.– The idler rotors are in rolling contact with the central power rotor and

are free to float in their respective housing bores on a hydrodynamic oil film.

– There are no radial bending loads.– Axial hydraulic forces on the rotor set are balanced, eliminating any

need for thrust bearings.