Page 137 - Fluid Power Engineering

Hydraulic Pumps 111

The tip clearance leakage is affected by the tip clearance, the number
of teeth, and the pump exit pressure. An increase in the number of
teeth increases both the local losses and resistance to internal leakage.
The excessive wear of the pump casing increases the tip clearance and
consequently the tip clearance leakage.
The side clearance leakage takes place through the clearance
between the gears’ sides and the side plates. In the case of pumps
operating at low pressure levels, this leakage is not so high; therefore,
the side plates are fixed. Then, the wear on the side plates increases
this leakage. However, the pumps operating at high pressures pres-
ent higher leakage through this path. Therefore, they should include
an arrangement for the hydrostatic compensation of the side clear-
ance. The side plates are pushed towards the gears under the action
of a pressure force (see Fig. 4.22). The pump exit pressure is commu-
nicated to act on a part of the side plate’s area (6). This area is well
calculated to generate the force necessary to produce the required
tightness without too much increase in the friction torque. In this
way, the side clearance is automatically adjusted according to the sys-
tem pressure. At low-pressure levels, the leakage is reduced, and a
smaller tightening force acts on the wear plates. In addition, the wear
of the side plate has no significant effect on the side clearance leakage
since it is constantly pressed against the gear side.

The Pulsation of Flow in Gear Pumps
The flow at the pump exit is pulsating due to the variable rate of
delivery from the pump chambers. The following relation gives the
frequency of pulsation:

f = 2 z n (4.31)
where f = Flow pulsation frequency, Hz
n = Pump speed, rev/s
z = Number of teeth per gear
For the gear pump, the pulsation coefficient is calculated by the following
expression:
2
σ = π cos 2 γ × 100% (4.32)
4(z + 1)
A gear pump with ten teeth per gear and a pressure angle
2
2
γ = 25° has a flow pulsation coefficient σ = π cos 25/{4(10 + 1)} =
0.184 = 18.4%.
Oil Trapping and Squeezing in Gear Pumps
During the normal operation of the pump, as the tooth comes to the
meshing point, a volume of oil becomes trapped in the space between
two successive teeth. The oil trapping takes place where the gears