JUNE-JULY 2018 Nov-Dec 2017 - Copy - Page 92

BOOK IN SERIES thrusts. Old and expensive types of gear pumps (made up of perfectly smooth lateral faces and a stator with very precise shim adjustments) were replaced by radial and axial compensation pumps, also known as self-balancedpumps, which consist of a casing and balancing bushings made of aluminium alloy, treated steel gearwheels and front seals made of reinforced nitrile mixture or viton. Axial and radial compensations are made possible by placing two balancing or compensation bearings opposite the plane faces of the gearwheels and axially floating between the covers and the gears themselves. Figure 4.10 Figure 4.8 Inaxial andradial balance, the pressurised fluid, pushed through tiny and accurately measured openings between the outlet and the bushings, exerts a thrust on their two back parts (cover sides). This keeps them solidly connected to the wheel but it allows an adequate lubrication thanks to an accurate design of the thrust force. As a result, gears perfectly mesh with bushings while the lubricating film prevents the faces of the parts from wearing out; the spindles solidly connected to the gearwheels do not need more bearings and the single bearing, if used, is positioned over the end of the transmission shaft (Figure 4.10). Radial balance design must also take into account the fact that teeth in mesh cannot fully expel oil. As a result, tiny fluid drops are ‘squeezed’ between the engaging wheels, thus entailing (depending on the incompressibility of the liquid) local overpressures that act in a radial manner vis-à- vis gear shafts. In addition, during gear disengagement, the volume between the teeth suddenly increases even before the contact with the sucked fluid. Consequently, the central part of bushings must have some interstices in order to discharge this fluid; these are the only points where delivery and suction areas come into contact and the overpressure-prone fluid discharges in micro-areas subjected to early vacuum (Figure 4.11). 92 Figure 4.9 Figure 4.11 www.ghmediabusiness.com