book in series
Bevel gear pumps, which are widely employed for commercial
vehicles, usually have displacements between 40 and 80
cm3/rev, working pressures of 350 bar, peak pressures of 400
bar and a maximum speed of 2000 ÷ 2500 rpm.
former inlet becomes the outlet.
Axial piston pumps - Variable displacement
Figure 4.66
Displacement variation in axial piston pumps results from the
adjustment of the inclination of the axis between the plate
and the cylinder block. The variation must be possible in every
moment of the operational phases and with displacements
ranging from zero (pistons forming a right angle with the
plate = maximum pressure, minimum flow) to the maximum
angle allowed. In addition, it must always be possible (closed
circuits) to reverse the flow (the inlet becomes the outlet and
vice versa), always under the same conditions of variability
without reverting the revolution direction of the prime mover.
Although every type of axial piston pump can undergo the
displacement variation, it is usually performed in in-line or
bent axis rotary cylinder block piston pumps. Figure 4.67 and
Figure 4.68 briefly show the shifting device of the plate or the
c ylinder block; controllers are dealt with more in detail in the
next chapter .
Figure 4.67
Assuming 25° is the maximum angle to which pump maximum
displacement cmax corresponds, during the variation of the
angle , the displacement of the pump cx can be determined
by means of the following formula:
cx = cmax • 25sinsin
Given a bent axis pump whose maximum displacement cmax
at 25° is 150 cm3/rev, nominal pressure is 180 bar, volumetric
efficiency v = 0.9, overall efficiency g = 0.85 and rotational
speed of 1750 rpm, if the inclination is changed to 12°, its
displacement c12° is:
The design of variable displacement bent axis pumps differs
from fixed displacement pumps only in the larger space of
the mobile cylinder block.
In Figure 4.67 flow variation is performed manually through a
hand wheel/worm screw. When the angle is maximum, flow
too is maximum: the less the angle between the cylinder
block and the axis of the transmission shaft is, the less the
flow rate is; flow is zero as long as the two axis are in line.
Flow direction can be reversed by reversing the angle: the
JuNe-july 2018 | Global MDA Journal | 67