External Gear Pump | Design , Advantages and Disadvantages

Gear pumps are less expensive but limited to pressures below 140 bar.It is noisy in operation than either vane or piston pumps. Gear pumps are invariably of fixed displacement type, which means that the amount of fluid displaced for each revolution of the drive shaft is theoretically constant.

External Gear Pumps

External gear pumps are the most popular hydraulic pumps in low-pressure ranges due to their long operating life, high efficiency and low cost. They are generally used in a simple machine. The most common form of external gear pump is shown in Figs.  and

It consist of a pump housing in which a pair of precisely machined meshing gears runs with minimal radial and axial clearance.One of the gears, called a driver,is driven by a prime mover. The driver drives another gear called a follower.As the teeth of the two gears separate, the fluid from the pump inlet gets trapped between the rotating gear cavities and pump housing.The trapped fluid is then carried around the periphery of the pump casing and delivered to outlet port. The teeth of precisely meshed gears provide almost a perfect seal between the pump inlet and the pump outlet.

When the outlet flow is resisted, pressure in the pump outlet chamber builds up rapidly and forces the gear diagonally outward against the pump inlet. When the system pressure increases, imbalance occurs. This imbalance increases mechanical friction and the bearing load of the two gears.Hence, the gear pumps are operated to the maximum pressure rating stated by the manufacturer.

It is important to note that the inlet is at the point of separation and the outlet at the point of mesh. These units are not reversible if the internal bleeds for the bearings are to be drilled to both the inlet and outlet sides.So that the manufacturer’s literature should be checked before attempting a reversed installation. If they are not drilled in this manner, the bearing may be permanently damaged as a result of inadequate lubrications.

Expression for the theoretical flow rate of an external gear pump

Let
Do =the outside diameter of gear teeth
Di= the inside diameter of gear teeth
L =the width of gear teeth
N=the speed of pump in RPM
VD=the displacement of pump in m/rev
M= module of gear
z=number of gear teeth
α= pressure angle

operation of an external gear pump
operation of an external gear pump

Advantages and disadvantages of gear pumps

The advantages are as follows:
1.They are self-priming.
2.They give constant delivery for a given speed.
3. They are compact and light in weight.
4. Volumetric efficiency is high.

The disadvantages are as follows:
1. The liquid to be pumped must be clean, otherwise it will damage pump.
2. Variable speed drives are required to change the delivery.
3. If they run dry, parts can be damaged because the fluid to be pumped is used as lubricant.

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