Hydraulic axial piston unit with multiple valve plates

An axial piston hydraulic unit has a valve plate assembly positioned between a head and a rotatable cylinder barrel. The assembly includes a pair of valve plates stacked together to function as a single valve plate. The valve plate assembly defines a restricted flow path for providing initial communication between an approaching piston bore of the rotatable barrel and an intake port and a discharge port extending through the pair of plates. A first portion of the flow path is defined in the valve plate adjoining the head and a second portion of the flow path is defined in the valve plate adjacent the barrel. The stacked valve plates are hydraulically held together by a pair of pressure balancing devices.

TECHNICAL FIELD 
This invention relates generally to hydraulic axial piston units and more 
particularly to one having a device for gradually increasing fluid 
communication between the piston bores in a rotatable barrel and intake 
and discharge passages in the housing. 
BACKGROUND ART 
Axial piston hydraulic pumps and motors commonly have a hardened valve 
plate stationarily positioned between a portion of the housing and a 
rotatable barrel. The valve plate has a plurality of kidney shaped ports 
extending therethrough for fluid communication between the piston bores of 
the barrel and intake and discharge passages in the head. The valve plates 
also typically have metering slots or grooves at the leading edge of the 
kidney ports for gradually increasing fluid communication between the 
piston bores and the respective intake and discharge passages in a manner 
which decreases hydraulic shock to alleviate noise and cavitation. 
The valve plate of one such pump is made relatively thin so that the slots 
can be stamped to expedite the manufacturing process of the valve plates. 
A portion of the head forms the bottom of the slot. One of the 
disadvantages found with such thin valve plates is that the thickness of 
the valve plate is one of the factors determining the size of the orifice 
area of the metering slot. This limits the flexibility of controlling the 
orifice area as the piston bores open into the intake or discharge 
passages. It has also been found that high stresses are induced in the 
valve plate if the proper relationship of the plate thickness and slot 
size is not used. 
Thus, it would be desirable to utilize the advantages of the thin valve 
plate design within a hydraulic axial piston unit while permitting a wider 
variety of metering slot geometries to be used for optimizing the 
operating characteristics of the axial piston unit. 
The present invention is directed to overcoming one or more of the problems 
as set forth above. 
DISCLOSURE OF THE INVENTION 
In one aspect of the present invention, an axial piston hydraulic unit 
includes a head having intake and discharge passages therein, a rotatable 
barrel having a plurality of equally spaced circumferentially arranged 
piston bores opening toward the head, and a valve plate assembly 
positioned between the head and the barrel and secured against rotation 
relative to the head. The valve plate assembly has at least two valve 
plates stacked together to function as a single valve plate and has 
axially aligned arcuate shaped intake ports and axially aligned arcuate 
shaped discharge ports extending therethrough to selectively sequentially 
serially communicate the piston bores with the intake and discharge 
passages as the barrel rotates. The valve plate assembly defines a 
restricted flow path between the head and the end of the barrel for 
establishing initial communication between an approaching piston bore and 
one of the discharge or intake ports wherein a first portion of the flow 
path is defined in the plate adjacent the head and a second portion of the 
flow path is defined in the plate adjacent the barrel.

BEST MODE FOR CARRYING OUT THE INVENTION 
An axial piston hydraulic unit 10 includes a multi-piece housing 11 having 
a head 12 suitably attached to a casing 13. The hydraulic unit 10 may be 
either a pump or a motor and has a rotatable barrel 14 disposed within the 
housing and secured to a shaft 16 suitably mounted for rotation about a 
longitudinal axis. The barrel is resiliently biased toward the head 12 by 
a spring 17 disposed between a ring 18 secured to the shaft and a ring 21 
secured to the barrel. The barrel has a plurality of equally spaced 
circumferentially arranged piston bores 22 each of which has a 
conventional kidney slot 23 opening toward the head. A plurality of 
pistons 24 are individually reciprocatably positioned within the piston 
bores 22. A slipper 26 is conventionally swivably connected to an end of 
each piston. A swashplate 27 is positioned within the housing and has a 
planar cam surface 28 engaged by the slippers to reciprocate the pistons 
within the piston bores in a conventional manner. 
A valve plate assembly 31 is positioned between the barrel and a planar 
surface 32 of the head 12 and is secured against rotation relative to the 
head by a pair of pins 33. The valve plate assembly includes a pair of 
valve plates 34,36 stacked together to function as a single valve plate. 
The valve pirates have axially aligned arcuate shaped intake ports 37 
continuously communicating with an intake passage 39 in the head and a 
plurality of arcuate shaped discharge ports 41-43 continuously 
communicating with a discharge passage 47 to selectively sequentially 
serially communicate the piston bores with the intake and discharge 
passages as the barrel rotates. Assuming that the barrel rotates clockwise 
relative to the valve plate assembly as viewed in FIG. 2, the intake ports 
37 have a leading edge 48. The discharge port 41 would be the leading 
discharge port and have a leading edge 49. 
The valve plates 34,36 define a restricted flow path 50 between the head 
and the end of the barrel for establishing initial communication between 
an approaching piston bore and the intake port 37. Similarly, the valve 
plate assembly 31 defines a restricted flow path 51 for establishing 
initial communication between an approaching piston bore and the discharge 
port 41. In this embodiment, the restricted flow paths are substantially 
identical and only the flow path 50 will be described in detail with 
common reference numerals applied to the flow path 51. A first portion of 
the flow path is defined by a first slot 52 in the plate 34 adjoining the 
head 12. A second portion of the flow path is defined by a second slot 53 
in the plate 36 adjoining the barrel 14. The first slot opens into the 
intake port 37 and extends a predetermined distance from the leading edge 
48 thereof. The second slot 53 also opens into the intake port and extends 
from the leading edge a distance greater than the predetermined distance 
of the first opening. Another portion of the flow path is defined by an 
elongate aperture 54 extending through the plate 34 and being 
circumferentially spaced from the first slot 52. A portion of the elongate 
aperture 54 opens into the second slot 53 to define a fixed size orifice 
56. Another portion of the flow path is defined by an aperture 57 in the 
plate 36 and continuously communicating with the elongate aperture 54. 
An alternate embodiment of the flow path 50 is disclosed in FIGS. 6 and 7. 
It is noted that the same reference numerals of the first embodiment are 
used to designate similarly constructed counterpart elements of this 
embodiment. In this embodiment, however, the flow path includes only first 
and second slots 52,53 both of which open into the intake port 37. 
Moreover, in this embodiment the predetermined distance that the opening 
52 extends from the leading edge 48 is somewhat greater than the 
predetermined distance in the first embodiment. 
The stacked valve plates 34,36 are hydraulically held together by inner and 
outer concentric pressure balancing devices 58 and 59 positioned radially 
inwardly and outwardly of the intake and discharge ports. The pressure 
balancing devices are substantially identical in design and function and 
only the outer pressure balancing device will be described in detail. The 
outer pressure balancing device includes an annular drain groove 61 in the 
head 12 facing the valve plate 34, an annular drain groove 62 in the 
barrel 14 facing the plate 36, an annular array of circumferentially 
spaced arcuate pressure balancing slots 63 extending through the plate 34 
and continuously communicating with the drain groove 61, and an annular 
array of circumferentially spaced pressure balancing slots 64 extending 
through the valve plate 36 and continuously communicating with the groove 
62 and the slots 63. Preferably the slots 63 are offset relative to the 
slots 64 so that each pressure balancing slot in the plate 34 continuously 
communicates with a pair of the pressure balancing slots in the plate 36. 
The valve plates can be relatively thin for simplifying the manufacturing 
process. For example, the thickness in one embodiment is about 1.6 mm such 
that the openings through the valve plates can be easily made by stamping. 
Since stamping becomes somewhat difficult when the width of the openings 
is less than the thickness of the plate, the use of thin valve plates 
permits extremely narrow openings to be stamped therein thereby increasing 
the ability to optimize the slot configuration for improved operating 
characteristics. While only two plates are shown in this embodiment, three 
or more thinner valve plates may alternatively be stacked together to 
function as a single valve plate. 
Industrial Applicability 
The operation of the present embodiment will be described as if the barrel 
14 rotates clockwise relative to the valve plate assembly 31 as viewed in 
FIG. 2 so that the kidney slots 23 of the piston bores 22 in the barrel 
sequentially communicate with the intake port 37 and the discharge ports 
41-43. As the leading edge of each kidney slot approaches the leading edge 
48, for example of the intake port, the initially communication between 
the piston bores and the intake passage 39 is through the flow path 50. 
Fluid flow through the flow path is restricted by the orifice 56 until the 
kidney slot opens directly into the second slot 53. At that time, the 
fluid flow through the flow path progressively increases until the kidney 
slots open directly into the intake port. This allows the fluid pressure 
in the piston bore to substantially equalize prior to the kidney slot 
opening into the intake port. The initial communication between the kidney 
slots and the discharge passage 47 occurs in a similar manner through the 
flow path 51. 
Referring now to the alternate embodiment of FIGS. 6 and 7, the initial 
communication between the piston bores and the intake passage is through 
the slots 53 and 52. It is readily apparent that the initial opening area 
between the respective kidney slot and the slot 53 is very small and 
progressively increases until the kidney slot becomes fully communicative 
with the slot 53 prior to the kidney slot opening into the intake port. 
The geometry of the flow path determines the characteristics of the piston 
pressure as initial communication is established between the piston bores 
and the intake and discharge passages. The term geometry includes sizes 
and shapes of the slots 52,53, the apertures 54,57, the size of the 
orifice 56, and so forth. Thus, it is readily apparent that the use of two 
stacked valve plates provide increased flexibility in selecting the 
geometry of the flow path to optimize the pressure characteristics for 
each hydraulic unit configuration. 
The pressure balancing devices 58,59 function by collecting any leakage 
flow that may leak from the high pressure discharge ports either inwardly 
or outwardly between the plate assembly and the barrel or head and 
distributes the pressure substantially evenly in annular paths prior to 
being discharged into the housing. 
Other aspects, objects, and advantages of this invention can be obtained 
from a study of the drawings, the disclosure, and the appended claims.