JUNE-JULY 2018 Nov-Dec 2017 - Copy - Page 93

BOOK IN SERIES Some systems need the direction reverse of the prime mover but their wheels cannot be inverted; under these circumstances gear pumps known as reversible pumps are employed because their inlets can turn into the outlets and vice versa as they have the same bore. Standard gear pumps (Figure 4.14) usually have the following characteristics: l Working pressure up to 250 bar l Peak pressure up to 280 bar l Rotational speed 500 ÷ 3000 rpm l Absolute suction pressure 0.75 ÷ 0.85 bar Figure 4.12 Bushing openings (Figure 4.12) play another important role as they allow leakages to pass from the delivery area to the suction area where the leaked fluid mixes with the fluid from the tank. The 3-shaped seal sets the balance area and separates the suction area from the delivery area. It is supported by an anti- extrusion ring, with the same shape as the seal, so as to avoid the extrusion of the seal parts where it is not supported due to play. l Fluid viscosity 15 ÷ 150 cSt l Overall efficiency 0.75 ÷ 0.85 l Contamination sensitivity l Loud noise at high pressures Some versions have two or three pairs of gears, which allow them to have only one inlet but to exploit more outlets, thus avoiding the coaxial system. The pump leading gear is generally set to revolve clockwise; anticlockwise revolution occurs when the back cover is disassembled and wheels are inverted (i.e. the leading wheel is replaced by the driven wheel and vice versa). This operation must be carried out carefully in order to avoid forcing and the position of the balancing bushing vis-à-vis the housing must be marked so as not to reassemble it wrongly; its positioning vis-à-vis the inlet and the outlet is essential as well (Figure 4.13). 93 Figure 4.13 www.ghmediabusiness.com Figure 4.14