[0001] This invention relates to pumps and is more particularly concerned with gerotor pumps,
sometimes known as N(N-1) type pumps.
[0002] Gerotor pumps are in common use as engine lubricating pumps, the inner rotor being
driven by a shaft geared to the crankshaft or being driven directly from the crankshaft.
They are positive displacement pumps in which discrete volumes of oil contained in
the pumping chambers pass between suction and pressure ports in the end plates. In
consequence the output flow fluctuates due to geometric effects and due to the compression
of the low pressure oil as it becomes exposed to the high pressure in the output port.
This compression effect becomes more marked if the oil in the sump becomes aerated
significantly due to the operation of the engine.
[0003] The effect of output flow fluctuation is to cause a pressure variation at a frequency
which is a multiple of the number of teeth on the driven rotor and the rotational
speed, and under certain circumstances this cyclic pressure variation can cause excitation
of the engine structure or associated systems sufficiently to emit undesirable noise.
[0004] The purpose of the present invention is to reduce the severity of volume fluctuation,
particularly where aerated oil is present. One benefit of this is to reduce undesirable
noise in sensitive installations.
[0005] According to this invention there is provided a gerotor pump comprising a housing,
an inner rotor having a lobed outer periphery and mounted for rotation in the housing,
an outer rotor encircling the inner rotor and having a lobed inner periphery, the
outer rotor being mounted in the housing for rotation about an axis which is eccentric
relative to the axis of rotation of the inner rotor, the lobes of the two rotors interengaging
to define a series of pumping chambers, and means defining radial surfaces in sealing
engagement with the two rotors at opposite axial sides thereof, at least one of which
surfaces has therein a circumferentially extending inlet port, a circumferentially
extending outlet port and sealing lands forming seals between the inlet and outlet
ports, the sealing land disposed at or adjacent the location at which the pumping
chambers have their maximum volume having in its face a groove extending circumferentially
from the outlet port towards but terminating short of the inlet port.
[0006] In preferred constructions according to the invention, the sealing land having said
groove therein has a circumferential length greater than the circumferential distance
between successive points of sealing contact between the two rotors at the location
of the said sealing land. The circumferential length of the said sealing land may,
for example, be 1½ to 2 times the circumferential distance between said successive
points of contact.
[0007] The invention will now be described in more detail with reference by way of example
to the accompanying diagrammatic drawings in which:
Figure 1 illustrates the general arrangement of a gerotor pump having a conventional
porting arrangement, and
Figure 2 shows the porting arrangement of a pump according to the invention.
[0008] Referring first to Figure 1 of the drawings, the pump comprises an inner rotor 10
mounted on a drive shaft 11 carried in a bearing in the pump housing 12, and an outer
rotor 13 rotatably mounted eccentrically with respect to the inner rotor in a cylindrical
recess 14 in the housing. The recess is closed off by an end plate (not shown) which
forms a seal with the axially outer end faces of the two rotors. The inner rotor has
lobes 15 on its outer periphery and the outer rotor has lobes 16 on its inner periphery,
the number of lobes on the outer rotor being greater by one than the number of lobes
on the inner rotor, so that a series of pumping chambers 17 are defined between the
two sets of lobes and have minimum and maximum volumes at opposite sides of the two
axes of rotation 18, 19 on extensions 20 of a line joining the two axes. The inner
end wall of the recess in the housing forms a seal with the adjacent ends of the two
rotors and is formed with a circumferentially extending inlet port 21 and a circumferentially
extending outlet port 22. The ports 21, 22 are shown in full lines in Figure 1 for
convenience, but are behind the rotors 10, 13. Where the lobes of the two rotors are
in deepest interengagement, the end wall of the recess has a first sealing land 23
separating the two ports, and where the lobes are in least deep interengagement a
second sealing land 24 separates the two ports. The rotor 10 is driven in the direction
of the arrow, so that as each pumping chamber 17 passes the first sealing land 23
its volume is a minimum but increases with continuing rotation, drawing working fluid
into the chamber from the inlet port 21. As each pumping chamber reaches the second
sealing land 24 its volume is a maximum and is sealed by that land, the circumferential
length of the land being substantially equal to the circumferential distance l between
the sealing points between the four lobes defining the chamber 17 at that point. As
the chamber moves past the land 24 and continues round, its volume reduces progressively
discharging its charge of oil to the outer port. In some constructions the sealing
land 24 is displaced a few degrees round in the direction of rotation of the rotors
to provide more time for the pumping chambers to fill, but the principle of operation
is unchanged.
[0009] Referring now to Figure 2, the porting arrangement in a pump according to the invention
is shown. In this arrangement, the length of the second sealing land 24 is increased,
reducing the circumferential length of the outlet port 22 but leaving the inlet port
21 unchanged. The length of the land 24 in this construction is typically up to 2
times the circumferential length l of the pumping chambers as they pass that land.
A groove 26 extends generally circumferentially along the land from the outlet port
22 towards but terminates well short of the inlet port 21. The groove has its greatest
cross-section at its end opening to the outlet port and the cross-section reduces
progressively to zero at a position spaced from the opposite end of the land by a
distance l equal to the distance between the seals of the adjacent pumping chambers.
[0010] In operation of the pump described with relation to Figure 2, used as a lubricating
pump for an internal combustion engine, the oil which is contained in a pumping chamber
is abruptly exposed to the outlet port oil pressure as the pumping chamber moves beyond
the downstream end of the sealing land in the arrangement of Figure 1 and this results
in a degree of unwanted noise. If the oil has not become aerated the lengthening of
land 24 in itself would result in compression of the oil in the pumping chamber owing
to the fact that the volume of the pumping chamber has begun to reduce during its
past movement the sealing land 24. The effect can be severe if the oil in the pumping
chambers is not aerated. On the other hand, if the oil in the pumping chambers is
aerated, the compression may be insufficient to raise the pressure in the chamber
to the value existing in the outlet port. The presence of the groove 26 operates to
permit more gradual equalization of the pressures in the outlet port and the pumping
chambers. When the oil is not aerated oil can flow from the pumping chamber along
the groove to the outlet port reducing the pressure in the pumping chamber gradually.
When the oil is aerated, oil can flow along the groove from the outlet port to the
pumping chamber to raise the pressure in the pumping chamber. The progressive nature
of this transfer of oil prevents sudden volume changes and hence sudden pressure changes
as the pumping chambers come into communication with the outlet port, and thus reduces
noise and vibrations which may occur in some installations.
[0011] The maximum length of the groove is such as to prevent significant direct leakage
past land 24 from the outlet to the inlet port, and for this reason the groove will
commence not normally nearer to the inlet port than a distance equal to l.
1. A gerotor pump comprising a housing, an inner rotor having a lobed outer periphery
and mounted for rotation in the housing, an outer rotor encircling the inner rotor
and having a lobed inner periphery, the outer rotor being mounted in the housing for
rotation about an axis which is eccentric relative to the axis of rotation of the
inner rotor, the lobes of the two rotors interengaging to define a series of pumping
chambers, and means defining radial surfaces in sealing engagement with the two rotors
at opposite axial sides thereof, at least one of which surfaces has therein a circumferentially
extending inlet port, a circumferentially extending outlet port and sealing lands
forming seals between the inlet and outlet ports, the sealing land disposed at or
adjacent the location at which the pumping chambers have their maximum volume having
in its face a groove extending circumferentially from the outlet port towards but
terminating short of the inlet port.
2. A pump as claimed in claim 1, wherein the sealing land having said groove therein
has a circumferential length greater than the circumferential distance l between successive
points of sealing contact between the two rotors at the location of the said sealing
land.
3. A pump as claimed in claim 2, wherein the circumferential length of the said sealing
land is between 1 and 2 times the circumferential distance between said successive
points of contact.
4. A pump as claimed in claim 2 or claim 3, wherein the groove has a cross-sectional
area which increases progressively in a direction towards the outlet port and commences
substantially at said circumferential distance l from the upstream end of said sealing
land.
