Page 148 - Fluid Power Engineering

122 Cha pte r F o u r

much greater than the vane retraction force since the pressure
in the leading chamber acts on a small portion of the vane tip
area. The pumps having this design suffer from severe wear at
these zones. Here comes the main advantage of the intra-vane
structure. At this position, the blade root is not pressurized
since it is connected with the trailing nonpressurized chamber.
Only the intra vane is pressurized. The net forces at the vane
can be kept small at all times; thus, the wear during normal
operation is extremely reduced.
• The vane separates two chambers of different pressures, with
high pressure facing the vane trailing side (see Fig. 4.34d). At
this position, most of the blade tip area is subjected to the
pressure in the trailing chamber. Therefore, both the vane
root and intra vane are pressurized to create the required
tightening force.

4.7 Variable Displacement Pumps

4.7.1 General
Variable displacement pumps are much more expensive and of a
more complicated design compared to fixed displacement pumps.
However, the designers use the variable displacement pumps for two
reasons: economy and control.
Economic Reasons
The hydraulic systems designers try to minimize the hydraulic power
generation (PQ) whenever it is not needed. Several solutions satisfy
this requirement, for example, by using convenient valves and an
accumulator assembly. The use of variable displacement pumps with
pressure compensators is among the most widely used solutions. The
flow characteristics of this class are illustrated in Fig. 4.38.
Another important issue is that hydraulic systems operate within
a wide range of pressures and flow rates. Therefore, these systems
need prime movers of very high power. Thus, the use of displacement
pumps with constant power controllers offers a reasonable solution to
this problem. This text presents examples for these controllers.

Control Reasons
A wide range of controllers of displacement pumps are available,
which are intended to control the flow rates and direction of flow to
the hydraulic cylinders and motors, aiming to control the magnitude
and direction of their velocity. Among these controllers are:
• Hydraulic proportional controllers, for open-loop control of
the pump displacement