The simple spur gear pump uses two meshing gears closely fitted into a housing. One of the gears is driven by the engine accessory drive shaft, and this gear drives the other. As the gears rotate into a direction , the space between the teeth on the inlet side becomes larger. Fluid is sucked in at this stage, trapped between the teeth and the housing and carried around to the discharge side of the pump.
As the fluid comes to discharge the teeth of the two gears come into mesh, decreasing the volume, and force the fluid out the pump discharge.
A small amount of fluid is allowed to leak past the gears and around the shafts for lubrication and cooling(known as Case Lubrication of EDP). This fluid drains into the hollow shafts of the gears and is picked up by the low pressure at the inlet side of the pump. A weak relief valve holds the oil in the hollow shafts until it builds up to a pressure of about 3-6 psi. This ‘Case’ pressure is maintained so that if the shaft or seal becomes scored, fluid will be forced out, rather than air being drawn into, the pump. Air would otherwise displace the fluid needed for lubrication and the pump would be damaged due to less lubrication and overheating.
As the pump output pressure increases, there is a tendency for the case to distort and allow increased leakage. To prevent this, some pumps have high-pressure oil from the discharge side of the pump fed through a check valve into a cavity behind the bushing flanges. The bushings are forced tight against the sides of the gears, decreasing the side clearance and minimising leakage, also compensating for bushing wear.
