Page 94 - demo

BOOK IN SERIES

The type we are now going to describe should not be mistaken the other by a sheet with a clearance hole. Leading wheels
with the double staggered gearpumps. We can see in Figure are axial and solidly connected to a single transmission shaft
4.6 (Single suction and delivery phases) that the fluid flowing and obviously the casing has a single outlet. The double pair
out of the upper outlet cannot be delivered uniformly. As of staggered gears reduces pulsations by about 70 % during
a matter of fact, the oil held in each meatus is alternately delivery.
driven by the right and left teeth, thus causing a pulsating
delivery along with slightly variable pressures, noise and If noise is too loud and yet we want to use this very type of
vibrations. Luckily, the cycle is not remarkably pulsating, so pump, we can resort to external helical gear pumps: they are
it does not have the trend shown in the left part of Figure quite noiseless but more expensive and they can sustain a
4.15: as previously explained, the expulsion of the last drops lower nominal pressure than standard pumps because of the
from a specific tooth coincide with the beginning of the first substantial loadsbetween the bushings and the teeth (Figure
phase of a tooth of the other wheel. Actually, the curve trend 4.17).
in standard pumps depends on the number of gear teeth: the A famous international company has recently launched a
more teeth a gear has, the less it pulsates; yet, a high number special gear pump that partially meets the need for variable
of teeth cause different types of technical problems. flow. A single casing holds two separated external gear
pumps that share delivery and suction hoses; the parts are
Pulsating flow
in gear pumps mutually axial and connected to the single transmission
shaft. If the hydraulic circuit requires a limited flow, the flow

Maximum ow variation Standard ow variation is generated by a single pump, while the other pump is on
(it does not occur in gear pumps) in standard gear pumps
Q Q standby due to its dedicated valve. When more flow (hence
more power) is needed, a Load Sensing signal switches the
t t valve of the second pump thus increasing the flow. However,
Figure 4.15 this device cannot be classified as a variable flow pump
because flow distribution, albeit automatically, is affected by
the displacement of the two pumping parts: if both of them,
at a certain rpm, pump 10 l/min, system flows can amount to
as little as 10 or 20 l/min.

Figure 4.16
In double staggered gearpumps (Figure 4.16), the teeth of
the second pair of gears are offset by a half tooth spacing
94 (a tooth of the second pair is arrayed in the middle of the Figure 4.17
space between two teeth of the first one) and divided from

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| Global MDA Journal | NOV-DEC 2017