Page 453 - Mechatronics with Experiments

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JWST499-c07
JWST499-Cetinkunt
ELECTROHYDRAULIC MOTION CONTROL SYSTEMS 439
Pumps

Negative
Positive
displacement
(centrifugal) displacement

Gear Vane Piston

Radial Axial

FIGURE 7.26: Hydraulic pump categories: Positive displacement pump types: gear, vane,
piston (radial and axial).

side pushes the gear towards the low-pressure side. The relation between the driven and
driving shaft causes imbalance that will have to be taken up by the bearings. Gear pumps
are always of fixed displacement type. Despite these drawbacks gear pumps are popular
because of their simplicity, low cost, and robustness. The unmeshing of gears around the
inlet port creates a low pressure and results in the suction action. Similarly, the squeezing
action of the gears on the fluid between the teeth and pump housing as well as when the
meshing between teeth begins around the outlet port results in the increased pressure at the
outlet port. The flow rate is determined by the input shaft speed of the pump and its fixed
displacement. The pressure is determined by the load as long as it is not too high to the
point of stalling the input shaft or breaking the pump housing. However, as the pressure
increases, the leakage increases. Hence, the flow rate versus pressure curve drops down a

Outlet port

Driven gear
(by input shaft)
Volume on this
side decreases as teeth Volume in this side
mesh and pressure increases as teeth unmesh
increases and pressure decreases
Idler gear
Pump casing

Inlet port
FIGURE 7.27: Gear pump cross-section views: two-dimensional cross-sectional view.
Reproduced with permission from Eaton. Copyright 1999 Eaton. All Rights Reserved.