[0001] The present invention relates to an axial piston pump of the type generally used
in hydraulic systems.
[0002] A conventional axial piston pump comprises a rotating cylinder block which is supported
on a drive shaft which is, in turn, supported on bearings, to enable the shaft and
cylinder block to rotate together. The block contains a plurality of pistons. Each
piston is fitted, by means of a universal joint, with a slipper pad. The slipper pads
contact and react against a load surface of a swashplate which surrounds the drive
shaft. The load surface is inclined at an angle to the axis of rotation of the drive
shaft.
[0003] The swashplate is held stationary in relation to the rotating cylinder block. Therefore,
the action of the slipper pads against the angled load face of the swashplate causes
a reciprocating action in the pistons.
[0004] The reciprocation of the pistons causes oil to be drawn into the cylinder via an
inlet port located in the pump housing, a kidney-shaped inlet port situated in a valve
plate located between the cover and an opposed, adjacent valve face of the cylinder
block. The valve face of the cylinder block and the opposed face of the valve plate
lie in a plane perpendicular to the rotational axis of the drive shaft.
[0005] The oil is discharged via slots in the valve face of the cylinder block and on through
a kidney-shaped outlet port in the valve plate. This discharged oil is subsequently
directed through loading pistons housed within the cover and finally on through an
outlet port provided in the cover.
[0006] The pump design must ensure that clearance between valve plate and cylinder block
face is controlled in order to minimise leakage without incurring excessive frictional
losses.
[0007] This controlled clearance can be achieved in two ways. The first method has the valve
plate rigidly mounted to the pump casing with the cylinder block connected to the
drive shaft by a loose fitting spline. The main pumping pistons load the cylinder
block hydrostatically against the valve plate. The clearance in the spline allows
the cylinder block to articulate, thus accommodating manufacturing tolerances and
deflections arising from the loads generated within the pump. This articulation facilitates
the alignment of the cylinder block valve face and the valve plate. Such a conventional
pump is shown in accompanying figures 7a and 7b.
[0008] The second configuration has the cylinder block rigidly mounted to the drive shaft
and the valve plate is floating in the axial direction. The valve plate is loaded
against the cylinder block by a series of loading pistons. The second arrangement
affords the advantage that the low inertia valve plate can follow the cylinder block
runnout with minimum vibration. The higher inertia of the floating cylinder block
version leads to high amplitude vibrations and consequently the valve plate clearance
in adversely affected.
[0009] The subsequent technical description within this specification deals with the floating
valve plate pump, although it is to be understood that the invention is not limited
for use with only such pumps.
[0010] In a floating valve plate pump, the loading pistons are used to load the valve plate
onto the cylinder block in the axial direction of the shaft and are designed to prevent
separation of the porting interface valve face, thereby minimising a loss of pressurized
fluid into the pump casing.
[0011] The displacement of the pump can be varied from zero to a maximum by altering the
angle of the swashplate using, for example, control pistons situated within the pump
housing. The control system, for controlling the angle of the swashplate, requires
that the friction between a rear, curved side of the swashplate and a complementary,
but inversely, curved swashplate cradle (which seats the swashplate) be kept to a
minimum. This can be achieved by means of a hydrostatic bearing system which is supplied
with lubricating oil at a pump outlet pressure. Alternatively, a roller bearing can
be used, but this feature has the disadvantages of high cost, increased noise level
and limited life.
[0012] Although such pumps have been widely used, they do have short-comings related to
efficiency, reliability and cost which the invention addresses.
[0013] The valve plate provides a mechanism for transferring fluid to and from the cylinder
block. It is important to maintain the design clearance between the valve plate and
cylinder block in order to optimize leakage and frictional losses. If loading is excessive,
it results in seizure between the static valve plate and the rotating cylinder block.
[0014] To date, the valve plate has been loaded against the cylinder block by four (for
example) circular loading pistons. Each of the discrete pistons imparts a localised
load onto the valve plate. This results in distortion of the valve plate, leading
to undesirable variation in the clearance between the valve plate and the cylinder
block and in the extreme, metal to metal contact can occur. In areas of high clearance
leakage increases and the pump's efficiency is reduced. Areas of low clearance increase
the risk of seizure and the pump's reliability is adversely affected at extreme operating
conditions.
[0015] The conventional way of providing the hydrostatic low friction bearing between the
rear of the swashplate and the swashplate cradle requires a feed of high pressure
oil from the pump outlet port. This has conventionally been achieved by means of a
series of interconnecting drillings from the outlet port at one end of the pump, via
the pump casing, to the swashplate bearing which is at the opposite end of the pump
to the outlet port. The drilling provided in the pump casing is complicated and relatively
expensive to manufacture.
[0016] The present invention sets out to provide an axial piston pump in which the leakage
gap between the cylinder block and the valve plate is minimised but without causing
seizure or excessive frictional losses.
[0017] Furthermore, the invention sets out to provide an arrangement in which distortion
of the valve plate is obviated or mitigated, thereby avoiding local variations in
the thickness of the oil film between the valve plate and the cylinder block.
[0018] Additionally, the invention sets out to provide an arrangement in which oil can be
supplied to a hydrostatic bearing provided between the swashplate and the swashplate
cradle, but without requiring a complicated drilling of the pump casing.
[0019] According to a first aspect of the invention there is provided an axial piston pump
comprising a drive shaft, a cylinder block rotatable with the drive shaft, a plurality
of pistons provided within the cylinder block, a swashplate situated at one axial
end of the cylinder block for causing reciprocation of the pistons when the said cylinder
block is rotated, and a valve plate situated at a second axial end of the cylinder
block; wherein eitherone of the cylinder block orvalve plate is urged against the
other to form a hydrostatic seal between the cylinder block and a face of the said
valve plate; characterised in that a spiral groove bearing is provided between the
valve plate and the cylinder block.
[0020] In a preferred embodiment the cylinder block is fixed relative to the axial direction
of the drive shaft and the valve plate is urged against the cylinder block.
[0021] Alternatively, the valve plate can be fixed in the axial direction of the drive shaft
and the cylinder block urged against it.
[0022] Preferably, the spiral groove bearing will be provided in the valve plate. It can
be formed by a plurality of spirally orientated grooves. Alternatively, the grooves
can be straight. If preferred, the spiral groove bearing can be provided in the cylinder
block.
[0023] The grooves of the spiral groove bearing can be very shallow.
[0024] According to a second aspect of the invention there is provided an axial piston pump
comprising a drive shaft, a cylinder block rotatable with the drive shaft, a plurality
of first pistons provided within the cylinder block, a swashplate situated at one
axial end of the cylinder block for causing reciprocation of the pistons when the
said cylinder block is rotated, a valve plate situated at a second axial end of the
cylinder block and held stationary relative to the direction of rotation of the cylinder
block and urged against the said second end of the cylinder block to form a hydrostatic
seal between the cylinder block and a face of the said valve plate, wherein the valve
plate is urged against the cylinder block by means of a second piston. Characterised
in that the second piston has an arcuate load face.
[0025] Preferably, the piston will be kidney-shaped.
[0026] According to a third aspect of the invention there is provided an axial piston pump
comprising a drive shaft, a cylinder block rotatable with the drive shaft, a plurality
of pistons provided within the cylinder block, a swashplate situated at one axial
end of the cylinder block for causing reciprocation of the pistons when the cylinder
block is rotated; the said swashplate being provided with a curved back, the curved
back being seated within a curved recess in a swashplate cradle the said swashplate
being capable of swivelling within the said recess; characterised in that a hydrostatic
bearing is formed between the said curved back of the swash plate and the curved recess
of the swashplate cradle, and high pressure oil is supplied to the said hydrostatic
bearing via a passage provided in at least one of the said pistons and via a hole
provided in the body of the said swashplate.
[0027] Preferably, a pair of hydrostatic bearings are provided between the swashplate back
and the swashplate cradle. One of these may be fed directly by means of the said drilling
in the swashplate and the other is fed via the first bearing and via a drilling provided
in the body of the swashplate cradle, which links the two bearings together. A control
orifice can be provided in the feed drilling in the swash plate to modulate the pressure
at the hydrostatic bearings and minimise the leakage. Each of the pistons can comprise
a hole for allowing oil to be fed to the swashplate.
[0028] In a preferred embodiment, the load surface of the swashplate is provided with a
wear plate, upon which slippers, provided at the respective ends of the pistons, move.
The slippers each comprise a drilling to allow oil to escape from their respective
piston and the wearplate comprises a drilling which communicates with the drilling
in the swashplate.
[0029] Embodiments of the invention will now be described by way of example and with reference
to the accompanying drawings in which: -
Figure 1 is a section through a pump in accordance with the present invention;
Figure 2 is a section through Y-Y of the pump shown in Figure 1 and schematically
showing a feed path for oil in accordance with the third aspect of the invention;
Figure 3 is an exploded perspective view showing a valve plate in accordance with
a first aspect of the invention and in combination with four conventional loading
pistons;
Figure 4 is a valve plate in accordance with the first aspect of the invention in
combination with a loading piston according to a second aspect of the invention;
Figure 5 is a perspective view of a swash plate in accordance with a third aspect
of the present invention;
Figure 6 is a perspective view of a cylinder block in accordance with the invention;
and
Figures 7a and 7b are side and top views, respectively, of a conventional floating
cylinder block type of axial piston pump.
[0030] The general construction of the pump according to the present invention is the same
as that of the conventional pump described above.
[0031] As can be seen from Figures 1 and 2, the pump comprises a drive shaft 2 which is
fitted with a cylinder block 4. The cylinder block 4 is keyed to the said drive shaft
2 by means of a portion 7 of the drive shaft 2 which is generally square in cross-section
and fits within a similarly profiled recess in the cylinder block 4. The cylinder
block 4 is fixed to the drive shaft 2 in such a manner that rotation of the drive
shaft 2 causes the cylinder block4 to rotate. The drive shaft is supported by bearings
3 and 5 to facilitate rotation. The cylinder block 4 is fitted with nine pistons 6a
- 6i.
[0032] Each piston is reciprocally movable in a direction parallel to the axis of rotation
of the cylinder block assembly. A ball 8a - 8e is provided at the end of each piston
and is received within a socket in a respective slipper pad 10a - 10e.
[0033] A swashplate 12 is provided within a swashplate cradle 14. The swashplate 12 has
a curved back 16, which is part-circular in profile. The swashplate cradle 14 is provided
with a swash plate seating surface 18 which is curved to the same degree as the rear
of the swashplate 16. This allows the swashplate to swivel within the swashplate cradle
14.
[0034] During use, the swashplate 12 will be positioned within the swashplate cradle 14
with its load face 20 inclined such that a normal to the load face 20 is at an angle
with the rotational axis of the drive shaft 2.
[0035] The angle of inclination of the swashplate 12 can be adjusted by means of a pair
of control pistons (not completely shown) which move an arm 24 which is received within
the swashplate 12. The angle of inclination of the swashplate is adjusted by means
of the control pistons 22 which move the arm, thereby moving the swashplate. The direction
of movement of the pistons 22 is into and out of the page as seen in Figure 1.
[0036] The rotation of the cylinder block 4 causes the pistons 6a - 6i to reciprocate as
the piston slippers 10a - 10i act against the load face of the swashplate 12.
[0037] Avalve plate 26 is provided at the other end of the cylinder block 4.
[0038] The valve plate 26 is loaded against the valve face of the cylinder block by four
loading pistons 30a-30d. These pistons 30a-30d serve to prevent separation of the
valve plate from the cylinder block valve face, thus minimising the loss of pressurized
fluid into the pump casing. Each piston is provided with a seal 31a-31d about its
perimeter.
[0039] The valve plate is provided with a kidney-shaped inlet port 28 and two kidney-shaped
outlet ports 29a and 29b.
[0040] A kidney-shaped recess is provided on the face 52 of the valve plate 26 which addresses
the cylinder block. This recess communicates with the outlet ports 29a and 29b.
[0041] During use, reciprocation of the pistons causes oil to be drawn into the cylindervia
the kidney-shaped inlet port 28 and the kidney-shaped slots 27a-27i in the valve face
of the cylinder block. The oil is discharged via the kidney slots 27a-27i, the valve
plate outlet ports 29a-29b, the loading pistons 30a-30d and finally an outlet port
25 in the cover.
[0042] In accordance with a first aspect of this invention, a spiral groove bearing 50 is
provided in a peripheral region of the cylinder block facing face 52 of the valve
plate 26.
[0043] The spiral groove bearing 50 is formed from a plurality of grooves 54, which are
formed so as to spiral inwardly from the periphery of the cylinder block facing face
52 of the valve plate towards the centre of this face 52.
[0044] Because the valve plate is held stationary relative to the rotating cylinder block
during use , oil is driven into the grooves of the spiral groove bearing 50 and creates
a hydrodynamic pressure. This provides a self-compensating effect and significantly
reduces the risk of seizure. This arrangement enables the valve plate 26 to be loaded
more heavily, thereby reducing leakage.
[0045] In accordance with the second aspect of the invention the four conventional valve
plate loading pistons 30a - 30d are replaced with a single kidney-shaped piston 60.
This can be seen in Figure 4.
[0046] The piston 60 comprises six outlet apertures 62a - 62f. These are aligned with six
similarly sized and shaped outlet apertures 64a - 64f provided in a single kidney-shaped
outlet port 66 of the valve plate 26. During operation of the pump, oil can escape
by means of the outlet apertures 64a - 64f and subsequently on out through the outlet
apertures 62a - 62f in the piston 60. The kidney-shaped inlet port of the valve plate
26 is identical to that of the conventional valve plate 26.
[0047] The kidney shaped piston 60 is fitted with a seal 61 about its perimeter; this corresponds
to the seals 31a-31d of the prior art loading pistons.
[0048] Preferably, the kidney-shaped piston 60 will be used in conjunction with a spiral
groove bearing 50, but it will operate successfully without the presence of such a
spiral groove bearing.
[0049] By adopting a single piston 60, the valve plate loading becomes evenly distributed,
resulting in less distortion of the valve plate. As a result of this reduction in
distortion of the valve plate, local distortions in the oil film thickness caused
by using discrete pistons can be avoided, providing less leakage minimum friction
and higher reliability. Because the valve plate distributes the loading more evenly,
the valve plate is less susceptible to deformation and the thickness of the valve
plate can be reduced. This means that costs can be saved.
[0050] If desired, the valve plate 26 plus loading piston 60 could be manufactured as a
single integral component.
[0051] I n order to facilitate adjustment of the swash plate 12 within the swashplate cradle
14, a pair of hydrostatic bearings 70 and 72 are provided in the curved surface 16
of the swashplate 12. This can best be seen from Figure 5. These bearings are fed
with oil at pump outlet pressure.
[0052] The high pressure oil is supplied to the hydrostatic bearings 70 and 72 from the
outlet port via openings provided in the nine pumping pistons 6a - 6i, through holes
in the respective slipper pads 10a - 10i and via a feed hole 80 which extends through
the wearplate 13 and the swashplate 12. The feed hole 80 directly feeds hydrostatic
bearing 72. Hydrostatic bearing 70 is fed by means of a drilling 82 (shown in Figure
1 and schematically in Figure 5) in the swashplate cradle 14. Drilling 82 connects
the two bearings 70 and 72. An orifice 83 is fitted in the drilling 80 to control
the pressure at the bearings and limitthe leakage rate.
[0053] The position of feed hole 80 relative to the outlet port will determine the pressure
of the oil supplied to the bearings. For maximum pressure at the hydrostatic bearings
70, 72, the feed hole and outlet port would need to be aligned.
[0054] Each time a slipper pad passes across the feed hole 80 in the wearplate 13, a pulse
of high pressure oil is fed to the hydrostatic bearings. This can be seen schematically
in Figure 6.
[0055] This arrangement reduces the complexity of machining components to provide the oil
supply and thereby reduces the cost of the pump. It also provides a self-cleaning
action for the control orifice 83, reduces fiction between swashplate 12 and cradle
14 and minimises leakage.
[0056] The third aspect can be combined in a single pump along with one or more of the first
and second aspect of the invention. Alternatively, the third aspect of the invention
can be employed separately.
[0057] The first and third aspect of the invention can be used in a floating cylinder block
pump described earlier. All three aspects of the invention could be incorporated in
the hydraulic motor variant of the pump. When applying the arcuate loading piston
principle to motors it wi be necessary to provide two loading pistons, one adjacent
to the supply port (equivalent to the pump's high pressure outlet port) and the second
adjacent to the return port (equivalent to the pump's low pressure inlet port). The
fitting of two pistons permits rotation in either the clockwise or anit-clockwise
direction.
[0058] Upon making reference to the foregoing description, which is given by way of illustrative
example only, many further modifications and adaptations wi II suggest themselves
to those versed in the art. The scope of the present invention is not intended to
be limited to exclude such modifications and adaptations.
1. An axial piston pump comprising a drive shaft (2), a cylinder block (4) rotatable
with the drive shaft (2), a plurality of pistons (6a-6i) provided within the cylinder
block (4), a swashplate (12) situated at one axial end of the cylinder block (4) for
causing reciprocation of the pistons (6a-6i) when the said cylinder block (4) is rotated,
and a valve plate (26) situated at a second axial end of the cylinder block (4); wherein
either one of the cylinder block (4) or valve plate (26) is urged against the other
to form a hydrostatic seal between the cylinder block (4) and a face (52) of the said
valve plate (26); characterised in that a spiral groove bearing (50) is provided between
the valve plate (26) and the cylinder block (4).
2. An axial piston pump according to Claim 1, wherein the cylinder block (4) is fixed
relative to the axial direction of the drive shaft (2) and the valve plate (26) is
urged aginst the cylinder block (4).
3. An axial piston pump according to Claim 1, wherein the valve plate (26) is fixed
in the axial direction of the drive shaft (2) and the cylinder block (4) is urged
against the valve plate (26).
4. An axial piston pump according to any preceding Claim, wherein grooves (54) of
the spiral groove bearing (50) are provided in the valve plate (26).
5. An axial piston pump according to Claim 1, 2 or 3, wherein grooves of the spiral
groove bearing (50) are provided in the cylinder block (4).
6. An axial piston pump according to any preceding Claim, wherein the spiral groove
bearing (50) comprises a plurality of spirally configured grooves.
7. An axial piston pump according to any one of Claims 1 to 5, wherein the spiral
groove bearing is formed from a plurality of straight grooves.
8. An axial piston pump according to any preceding
Claim, wherein the grooves (54) forming the spiral groove bearing (50) are very shallow.
9. An axial piston pump comprising a drive shaft (2), a cylinder block (4) rotatable
with the drive shaft (2), a plurality of first pistons (6a-6i) provided within the
cylinder block (4), a swashplate (12) situated at one axial end of the cylinder block
(4) for causing reciprocation of the pistons (6a-6i) when the said cylinder block
(4) is rotated, a valve plate (26) situated at a second axial end of the cylinder
block (4) and held stationary relative to the direction of rotation of the cylinder
block (4) and urged against the said second end of the cylinder block (4) to form
a hydrostatic seal between the cylinder block (4) and a face (52) of the said valve
plate (26), wherein the valve plate (26) is urged against the cylinder block (4) by
means of a second piston (60) characterised in that the second piston (60) has an
arcuate load face.
10. An axial piston pump according to Claim 9, wherein the second piston (60) has
a kidney-shaped load face.
11. An axial piston pump according to Claim 9 or 10, wherein the second piston (60)
comprises a plurality of outlet apertures (62a-62f); each aperture (62a-62f) being
aligned with a respective similarly configured outlet aperture (64a-64f) in the valve
plate (26).
12. An axial piston pump according to any one of Claims 9 to 11, wherein the valve
plate (26) and second piston (60) are integrally formed.
13. An axial piston pump comprising a drive shaft (2), a cylinder block (4) rotatable
with the drive shaft (2), a plurality of pistons (6a-6i) provided within the cylinder
block (4), a swashplate (12) situated at one axial end of the cylinder block (4) for
causing reciprocation of the pistons (6a-6i) when the cylinder block (4) is rotated;
the said swashplate (12) being provided with a curved back, the curved back being
seated within a curved recess in a swashplate cradle (14), the said swashplate (12)
being capable of swivelling within the said recess (14); characterised in that a hydrostatic
bearing (70,72) is formed between the said curved back of the swashplate (12) and
the curved recess of the swashplate cradle (14), and high pressure oil is supplied
to the said hydrostatic bearing (70,72) via a passage provided in at least one of
the said pistons and via a hole (80) provided in the body of the said swash plate
(12).
14. An axial piston pump according to Claim 13, wherein a pair of hydrostatic bearings
(70, 72) are provided between the swashplate back and the swash plate cradle.
15. An axial piston pump according to Claim 14, wherein a first of the hydrostatic
bearings (72) is fed directly by means of the said hole (80) provided in the swashplate
(12), and the other (70) is fed via the first hydrostatic bearing (72) and via a passage
(82) provided in the body of the swashplate cradle (14), which passage (82) links
the two bearings (70, 72).
16. An axial piston pump according to any one of Claims 13 to 15, further comprising
a control orifice (83) provided in the said hole (80) in the swashplate (12) for modulating
the pressure at the hydrostatic bearing or bearings (70,72).
17. An axial piston pump according to any one of Claims 13 to 16, wherein each of
the said pistons (6a-6i) comprises a hole for allowing oil to be fed to the swashplate
(12).
18. An axial piston pump according to Claim 17, wherein the load surface of the swashplate
is provided with a wear plate (13) upon which slippers (10a-10i), provided at the
respective ends of the pistons (6a-6i) move; each slipper (10a-10i) comprising a passage
to allow oil to escape from its respective piston (6a-6i), and the wear plate (13)
comprising a passage which communicates with the hole (80) in the swashplate (12).
19. An axial piston pump substantially as herein described with reference to figures
1, 2 and 3, 4, 5 or 6 of the accompanying drawings.