JUNE-JULY 2018 Nov-Dec 2017 - Copy - Page 91

BOOK IN SERIES Yet, these pumps have some drawbacks too, i.e. a design focused on sustaining around 150 – 280 bar, their unsuitability for high flow applications, rather loud noise and a rather poor overall efficiency. Nonetheless, the strong demand for these hydraulic generators prompts manufacturers to carry out research and improve their products by employing special materials, accurate heat treatments, minimum coupling tolerance between wheels and surface precise finish in order to reduce the drawbacks mentioned above. External gear pumps (Figure 4.5) are essentially made up of two twin gearwheels geared with each other that are held in a totally smooth stator housing so as to prevent leakages between moving and fixed parts; the 8-shaped bearings, held in the stator along with gears, counterbalance side hydraulic thrusts by means of dedicated seals. Bearings or bushings are positioned in the two coversopposite the stator where gear spindles revolve; the transmission shaft between the prime mover and the pilot wheel is in the hole of the front cover provided with a static seal (see Figure 4.9 and Figure 4.10). Like in most rotary and reciprocating pumps, the inlet has a larger bore than the outlet and both of them are usually positioned one opposite the otheron the stator housing. When the motor is started (Figure 4.6), lead wheel A, firmly connected to it, makes the driven gear B move into the opposite direction. The rotation of the teeth that come out of the mesh entails a vacuum whose volume is equal to the space between the two teeth; in other words, as shown in the Figure, the tooth x that was previously in the space between teeth y and z rotates clockwise, thus clearing a space (previously inexistent, hence a vacuum) equal to this volume. During this very short phase (rotational speed can range from few hundreds revolutions per minute to 3000 rpm), the fluid pushed by the atmospheric pressure on the free surface of the tank fills the space between the teeth w and k of gear A and it is driven clockwise to the outlet. In the following phase, the vacuum generated by the tooth y allows teeth z and r of gear B to drive anticlock wise the oil to the right part of the stator. Obviously, these phases alternate so as to conclude the revolution. Figure 4.5 When the actuating circuit is operated, delivery pressure should be the same throughout the whole upper chamber; actually, the inevitable leakages cause the pressure of the whole active part (oil transferring chambers) to be proportionally distributedand to decrease from the outlet to the first space after suction. The pressurised fluid inside the pump subjects gearwheels to considerable radial loads (Figure 4.7). 91 Another problem is the accurate positioning of gearplane faces vis-à-vis cover surface plates. If they do not form a perfect right angle with tolerances even lessthan 2-3 thousandths of millimetres, this would promote substantial leakages, early wear and the ensuing seizing. Figure 4.5 www.ghmediabusiness.com That is why it is important to use components with more suitable tolerances and methods to limit axial and radial Figure 4.7