JUNE-JULY 2018 JAN-FEB 2018 | Page 65

BOOK IN SERIES Unlike the pressure in the fluid film between the bushes and the rotor/stator unit, the pressure in these basins offsets the pump axial loads. Figure 4.34 In everyday language, standard pumps with single inlet and outlet are referred to as ‘round’ and radially balanced pumps as ‘square’ (Figure 4.35) due to their shape. Figure 4.37 Vanes must be balanced in order to ensure high performances. An effective system consists in putting double vanes with opposed sharp edge in the slots (Figure 4.36). The tightness between vanes and the stator virtually doubles and, in order to reduce the ensuing friction, opposite cavities are manufactured on the sides so that the pressurised fluid flows through them and, only in the delivery area, it offsets stator pressure. Figure 4.38 Figure 4.35 The pressure at the base of the vane in intravane systems (Figure 4.37) depends on its positioning: the base of the vane is alternately subjected to suction and delivery pressure during the revolution. The slot between the small insert and the vane is constantly subjected to delivery pressure. The fluid is routed to the base of the vane through small double clearances between its seatand the external part of the rotor (Figure 4.38 – A e B –). The constantly pressurised fluid flows to the cavity between the insert and the vane via a clearance between the slot and the plane face of the rotor (Figure 4.38 – C –), which is routed by the wear plate. The insert, driven by the pressurised fluid to the ring, does not move in the lower part of the slot. Figure 4.36 In single vane pumps, the thrust fluid at the base of each vane is always at delivery pressure. This ensures an optimum tightness, but it also results in a strong friction on the stator wall in suction areas, which makes it wear out quickly. This may 2018 Global MDA Journal 65