Bearings for gear pumps

The bore of a bearing bush for a gear pump has a first recess located in a zone of load concentration in the bore, the first recess being supplied with a fluid under pressure. A second recess in the bore is located wholly outside the zone in which concentrated loads occur and acts as a reservoir for fluid, to prevent cavitation within the bore as a result of low pressures caused by radical movements of a shaft within the bush. A passage connects the second recess with a zone of lower pressure within the pump.

This invention relates to bearing bushes for use in external gear pumps, 
that is pumps in which liquid is displaced by the action of two externally 
meshed toothed gears. 
As described in U.S. Pat. No. 3,528,756 which is incorporated herewith by 
reference the gears of a gear pump are supported in a housing by means of 
bearing bushes having bores through which the axles of the gears pass, and 
also having bearing surfaces which abut the end faces of the gears to 
provide a seal therewith. It is known that, in use, the high pressure at 
the pump outlet results in the axles of both the driving and driven gears 
being urged towards the sides of the bores which are adjacent the pump 
inlet, and provides areas of load concentration within the bushes. The 
precise area of load concentration may vary with changes in operating 
conditions of the pump, but remains within a zone, hereinafter referred to 
as the critical zone, which lies generally at the first quadrant, relative 
to the direction of gear rotation, from a plane which includes the axes of 
the meshing gears. This critical zone can readily be observed in a bush 
which has been dismantled from a normally operated pump. It has been 
proposed to provide a recess in the bore within the critical zone and to 
supply this recess with liquid from the pump outlet. Thereby providing a 
hydrostatic bearing to support the concentrated load and to assist in 
dissipation of heat. 
It is also known from the foregoing reference to provide a passage which 
communicates with the bore of the bush and which opens on to end faces of 
the bush, this passage allowing fluid to escape from the bore to a lower 
pressure zone within the pump housing. As shown in the above reference 
this passage may comprise a groove which extends along the bore of the 
bush and opens on to its end faces. 
It has been found that as a result of wear on the gears, particularly the 
appearance of flats on the gear teeth, the gear axle moves relative to the 
bearing bore, resulting in transient zones of increased clearance between 
the axle and the bore. Low pressure in these transient zones results in 
cavitation which causes severe damage to the bearing bush. 
It is an object of the invention to provide a bearing bush in which damage 
from the foregoing causes is substantially reduced. 
According to the invention there is provided a bearing bush for a gear 
pump, said bush having within its bore a first recess which lies wholly 
within a critical zone of the bore, as hereinbefore defined, a first 
passage for supplying liquid under pressure to said first recess, a second 
recess in said bore, and a second passage through which fluid can flow 
from said bore to a zone of reduced pressure, said second recess and said 
second passage lying wholly outside said critical zone.

A bush 10 for a driven gear is shown in FIGS. 1 and 2. A generally similar 
bush 10A for a meshing driving gear abuts the gear 10 in a known manner. 
The bush 10 is generally cylindrical but is provided with a flat 11 which, 
in use, abuts a corresponding flat on the bush 10A. The line B--B 
indicates a plane which includes the axes of the two bushes 10, 10A for 
meshing gears. The bush 10 is provided with cut-outs 12, 13 which 
respectively communicate with the pump inlet and outlet. Pressure in the 
pump outlet and reaction between the meshing gear teeth results in a force 
on a shaft of a gear within the bush 10. The line of action of this force 
is indicated at 14 and is inclined at 60.degree. to the plane B--B. As a 
result of changing operating conditions of the pump the direction of the 
line of action may vary over a critical zone indicated at 15. This zone 15 
lies generally in the area of the first quadrant, in the direction of 
rotation R of a gear within the pump, measured from the plane B-- B. For 
the generality of pumps the zone 15 may be considered as lying between 
20.degree. and 100.degree. from the line B--B in the direction of rotation 
R. In any specific pump the zone 15 has a circumferential extent of 
approximately 20.degree., and in the described example extends between 
50.degree. and 70.degree. from the plane B--B, for the bush 10 of the 
driven gear, and between 40.degree. and 60.degree. from that plane in the 
bush 10A of the driving gear. 
Within the bore 16 of the bush is a recess 17 whose length is less than the 
axial dimension of the bush and which is centred on the mean line of 
action 14. The recess 17 communicates through a first passage 18 with the 
cut-out 13 so that, in use, high pressure liquid from the pump outlet is 
supplied to the recess 17 and provides a hydrostatic bearing adjacent the 
area of concentrated load. The recess 17 overlaps the critical zone 15, 
and in the specific example extends between 55.degree. and 80.degree. from 
the plane B--B. In the example the axial length of the recess 17 is 
approximately 70% of that of the bore 16, to provide an adequate 
restriction to flow between the recess 17 and the end faces of the bush 
10. The recess 17 has a depth of 1.5 mm. In alternative embodiments the 
recess 17 may have a circumferential extent of between 20.degree. and 
40.degree.. 
A further recess 20 in the bore 16 has an axial extent equal to that of the 
recess 17 but lies wholly outside the critical zone 15. Fluid entering the 
recess 20 provides a reservoir from which liquid can flow to any transient 
low pressure zone occurring within the bore 16 as a result of relative 
movement of the gear axle. Cavitation within the transient low pressure 
zones is thereby prevented and it has been found that erosion of the bore 
16 is virtually eradicated, even following severe tests in which a pump 
having bushes according to the invention was operated with deliberately 
mutilated gear teeth. 
An end 21 of the recess 20 is spaced by not less than 15.degree. from the 
closest part of the zone 15. Alternatively, if an adjacent end 22 of the 
recess 17 lies outside the critical zone 15, the end 21 of the recess 20 
willbe spaced by not less than 15.degree. from the end 22 of the recess 
17. The circumferential extent of the recess 20 is in the present example 
160.degree., but may be between 130.degree. and 190.degree.. The axial 
extent of the recess is not greater than 70% of the bearing surface of the 
bore 16, so that if the load of the gear axle is transiently shifted to a 
direction generally opposite to the line of action 14, the zones of the 
bore 16 at the ends of the recess 20 will adequately support that load. 
The depth of the recess 20 is 1.5 mm. 
A second passage is constituted by a V-groove 23 which is located 
approximately 30.degree. from the plane B--B in a direction opposite to 
the direction of rotation R. The groove 23 opens on to the end faces of 
the bush 10. In the present embodiment the groove 23 is interrupted by the 
recess 20, but in other embodiments the groove 23 and recess 20 may be 
separate. In the present embodiment the groove 23 has two functions, one 
of which is to supply fluid to the recess 20 from a zone within a 
counterbore 24 (FIG. 1) in the bush 10 adjacent the end face of the gear 
therein. Fluid flowing from the counterbore 24 to a zone 27 of reduced 
pressure within the pump housing 25. This flow prevents excessive increase 
in the pressure in the zone 24 and also serves to maintain the recess 20 
full of fluid.