BOOK IN SERIES
The type we are now going to describe should not be mistaken with the double staggered gearpumps. We can see in Figure 4.6( Single suction and delivery phases) that the fluid flowing out of the upper outlet cannot be delivered uniformly. As a matter of fact, the oil held in each meatus is alternately driven by the right and left teeth, thus causing a pulsating delivery along with slightly variable pressures, noise and vibrations. Luckily, the cycle is not remarkably pulsating, so it does not have the trend shown in the left part of Figure 4.15: as previously explained, the expulsion of the last drops from a specific tooth coincide with the beginning of the first phase of a tooth of the other wheel. Actually, the curve trend in standard pumps depends on the number of gear teeth: the more teeth a gear has, the less it pulsates; yet, a high number of teeth cause different types of technical problems.
Q
Maximum �ow variation( it does not occur in gear pumps)
Pulsating flow in gear pumps
t
Q
Figure 4.15
Standard �ow variation in standard gear pumps t
the other by a sheet with a clearance hole. Leading wheels are axial and solidly connected to a single transmission shaft and obviously the casing has a single outlet. The double pair of staggered gears reduces pulsations by about 70 % during delivery.
If noise is too loud and yet we want to use this very type of pump, we can resort to external helical gear pumps: they are quite noiseless but more expensive and they can sustain a lower nominal pressure than standard pumps because of the substantial loadsbetween the bushings and the teeth( Figure 4.17).
A famous international company has recently launched a special gear pump that partially meets the need for variable flow. A single casing holds two separated external gear pumps that share delivery and suction hoses; the parts are mutually axial and connected to the single transmission shaft. If the hydraulic circuit requires a limited flow, the flow is generated by a single pump, while the other pump is on standby due to its dedicated valve. When more flow( hence more power) is needed, a Load Sensing signal switches the valve of the second pump thus increasing the flow. However, 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.
94
| Global MDA Journal | NOV-DEC 2017
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( a tooth of the second pair is arrayed in the middle of the space between two teeth of the first one) and divided from
Figure 4.17
www. ghmediabusiness. com