Page 454 - Mechatronics with Experiments

JWST499-Cetinkunt
JWST499-c07
440 MECHATRONICS Printer: Yet to Come October 9, 2014 8:41 254mm×178mm
Outlet port
Mechanical input shaft
Pressure
compensator
Displacement
volume
adjustment

Bias piston
Cam ring Control
Inlet port
piston
FIGURE 7.28: Vane pump: cross-section. Reproduced with permission from Eaton. Copyright
1999 Eaton. All Rights Reserved.

little bit as output pressure increases. Ideally, if there is no leakage, the flow rate would be
independent of the pressure.

(ii) Vane Pumps Vane pumps are known to operate more quietly than gear pumps
and used to have a cost benefit over piston pumps. More recent development has made
piston pumps both quieter and cheaper so they now totally dominate the market for mobile
hydraulics. A variable displacement vane pump is shown in (Figure 7.28). It works on the
principle that oil is taken in on the side with an increasing volume and the vanes connected
to the rotor then push the oil in to the part of the pump with decreasing volume, therefore
increasing the pressure of the oil before it reaches the outlet port. The so-called cam ring is
eccentrically mounted compared to the rotor of the pump, making the design unbalanced.
Changing the position of the cam ring in relation to the rotor changes the displacement
of the pump. The cam ring rests on a control piston on one side and the other side has a
bias piston. By changing the pressure in the control piston (Figure 7.28) the displacement
of the pump is changed. A control piston is used to adjust the pump displacement. The
maximum displacement is set by the displacement volume adjustment that limits the travel
of the bias piston.

(iii) Radial Piston Pumps A radial piston pump (Figure 7.29) is built on a rotating
shaft with the cylinder block rotating on the outside. As the piston follows the outer housing
of the pump on slippers, the offset from the central position creates the pumping motion.
The pistons maintain contact with the outer ring at all times. The input shaft rotation axis
is perpendicular to the plane of axes along which the pistons slide. The porting in the pivot
pin allows intake at low pressure into the cylinder (from the inner diameter side) as the
cylinder block passes by and pressurizes outflow as the cylinder block passes the outlet
port (through the inner diameter side). The number of pistons, diameter of the pistons, and
the length of their stroke determine pump displacement. The piston and cylinder sleeves
are manufactured and custom fitted to high tolerances for each pump. Radial piston pumps
are built so that more than one port discharges fluid at the same time and more than one
port is open to intake at the same time. This design attempts to reduce cyclic flow rate and
pressure pulsation in the output flow as a function of rotor angular position per revolution.