Note: Descriptions are shown in the official language in which they were submitted.
CA 02391544 2002-06-25
Attorney Docket No. DHIOa6
IN THE UNITED STATES PATENT AND TRADEhIARh OFFICE
io APPLICATION FOR UNITED STATES LETTERS PATENT
TITLE OF THE INVENTION:
Axial PistonPump With Rocker Cam Counterbalance Feed
BACKGRQUND. OF TAE INVENTION:
This invention relates to a variable displacement, axial piston pump having a
rocker cam pivotally mounted in a rocker cradle formed in the pump housing to
thereby
permit the displacement of he pump to be changed. Suoh a pump includes a
rotatably
mounted pump barrel having a plurality of longitudinal bores each housing
apump
piston. Each piston has a shoe pivotally attached to a head end which projects
from the
barrel and is retained against a thrust urface formed on the rocker cam. A
prime mover
rotates the barrel causing the pistons to reciprocate and thereby pump fluid
as the piston
shoes slide over the thrust surface. The angle-of the rocker cam thrust
surface determines
the displacement of the pump: If the rocker cam is centred, i:e. off stroke,
the thrust
surface is perpendicular to the axes of the pistons and they do not
reciprocate: If the
rocker cam is not centered; I.e. on-stroke; the thrust surface is angled with
respect to the
ends of the shoes and the,piston reciprocates as the barrel is rotated.
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The rocker cam may be pivoted to change the d'csplacement of the pump by a
rotary servo controlled fluid motor as described in U.S, Patent Number
3,967,541
assigned to the predecessor in interest to the assignee of the present
invention and hereby
incorporated by reference: Alternatively, the,. pump may be put on-stroke by a
spring
s which biases the rocker cam to a full on-stroke; position as is common with
a pressure
compensated pump of the type disclosed in the instant application. One type of
pressure
compensated pump may be seen in U.S: Patent Number 4,289;452 which is hereby
incorporated by reference herein.
During the operation of an axial piston pump; forces caused by pumping working
1o pressure fluid are transmitted through ;the pi tons and the thrust surface
to the interface
between the rocker cam and the rocker cradle. As the pressure of the working
fluid
increases, the pumping forces applied to the rocker cam/rocker cradte
interface increase.
In order to overcome the pumping forces at the rocker cam/rocker cradle
interfaces, fluid
pockets are formed in either the :rocker :cradle or the rocker cam. Working
pressure fluid
15 is supplied through the piston shoes, the thrust surface and the rocker cam
to the fluid
pockets to provide a counterbalancing force to the pumping forces applied to
the rocker
cam/rocker cradle interface. The counterbalancing force greatly reduces the
force a
pressure compensator control or a rotary servo control must be capable of
exerting to_
change the displacement of the pump.
20 Because of the complexities involved in balancing the piston shoes, it is
desirable
to provide a means of applying working pressure fluid to the rocker cam/cradle
interface
fluid pockets which does not require feeding the fluid from the piston shoes
unto the
rocker cam.
2
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SUIVIiI~IARY OF THE INVENTION:
An axial piston pump for pumping fluid at a set working pressure which
comprising a housing, a barrel rotatably mounted in the housing, a plurality
of bores in..
the barrel; apiston mounted for: reciprocation in each of said bores, a shoe
mounted on
the end of each piston,: a rocker cam support in the housing, a rocker cam
having a pair of
spaced arcuate bearing surfaces pivotally mounted in complementary arcuate
bearing
surfaces formed in said rocker cam support, a thrust: surface on said rocker
cam, a
holddown for clamping said piston shoes against said thrust surface wherein
said shoes
to slide over said thrust surface when said barrel is rotated and the pistons
reciprocated in
their bores when the rocker cam thrust surface is angled to the axis of said
barrel,. a
counterbalance fluid 'pocket formed.in each of said rocker cam areuate bearing
surfaces; a
first fluid passage formed. in said housing connecEed to a source of working
pressure fluid
for conducting working pressure fluid to one of said counterbalance fluid
pockets and a
second fluid passage formed in said rocker cam connecting said one and the
other of said
counterbalance fluid pockets such that worlting pressure fluid supplied to
said one
counterbalance fluid pocket through said first fluid passage in said housing
is supplied:: to
said other counterbalance fluid pocket through said second fluid passage
formed in said
rocker cam.
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BRIEF DESCRIPTLON OF THE DRAWINGS:
Figure 1 is a perspective view of a rocker cam type pump which incorporates
the
instant invention looking 'at the intake arid discharge ports;
Figure 2 is a side view, partially sectioned of the pump in Figure l;
Figure 3 is an axial sectional view of the pump taken along line 3-3 of Figure
2;
Figure 4 is a sectional view :of the portion of the pump housing having a
rocker
cradle formed therein;
Figure 5 is a perspective view of the purrap with the housing removed to
illustrate
to the rocker cam and other internal parts;
Figure 6 is a perspective view of a rocker cam; and
Figure ? is a rear view partially in section of the rockercam depicted in
Figure 6.
DESCRIPTION OF THE PREFERRED ElVIBOIDMENT:
Referring to Figs. T through 5 of the drawings, a pressure compensated,
variable
displacement, axial piston pump IO having a rocker cam pivotally mounted in a
cam
support or cradle may be seen to include a central housing 12, having a
mounting piiot
. end 14 and a port cap 16: at the other end. Bolts 17 connect port cap 16 to
housing 12.
Housing 12: defines a cavity which houses a rotatable barrel 18 mounted on a
2o drive shaft 20. The inner end of drive shaft 20 is supported in a bearing
22 mounted in
the port cap 16. Drive shaft 20 also is supported in a bearing 24 mounted
within housing
12 and has a. splined drive end 26 which projects outwardly of housing 12:
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Barrel 18 has a plurality of bores 28' equally spaced circumferentiaily about
its
rotational axis. Each bore 28 contains a piston 30 having;a ball shaped head
32. A shoe
34 is swedged onto head' 32 of piston 30 such that the shoe can pivot about
the end of
the piston. Each of the shoes is clamped against: a flat thrust plate or swash
plate surface
36 formed on the face of a pivotal rocker cam 38 utilizing a conventional shoe
retainer
assembly of he type described in detail in U.S. Patent Number 3,904,318
assigned to the
predecessor in interest of the assignee of the subject invention.
Turning to Figs. 2 through 6, it maybe seen that rocker cam 38 has a pair of
arcuate bearing surfaces 40 which are received in complementary arcuate
bearing
to surfaces 42 which comprise a rocker cam support or cradle 44 firmed in
mounting pilot
end 14 in housing 12. Rocker cam 38 pivots about a fixed axis perpendicular to
the axis
of rotation of barrel 18 to change the displacement of pump 10. In operation,
the prime
mover; not shown, affixed to spline drive end 26 rotates drive shaft 20 and
barrel 18
within housing 12. When thrustsurface 36 on rocker cam 38 is perpendicular to
the axis
of rotation of barrel 18, rotation of barrel l8.will cause the shoes to slide
across the
surface of thrust surface 36 but no pumping action will occur inasmuch as-the
pistons 30
will not reciprocate within bores 28. In other words; when thrust surface 36
is
perpendicular to the axis of drive shaft 20, the pump is in a position of
minimum fluid
displacement. As rocker cam 38 and thrust surface 36 are inclined from this
position,
2o the pistons 30 will reciprocate within bores 28 as shoes 34 slide over the
surface of thrust
plate 36. As the pistons 30 move inwardly of bores 28 i.e. away from port
plate 46, low
pressure fluid is drawn into cylinder bores 28 from inlet poet 48. As piston
shoes 34 slide
across thrust surface 36 and move toward port: plate 46, high pressure fluid
is expelled
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through outlet port 50. It should be noted that fluid displacement increases
as the angle
of inclination of thrust surface 36 increases. Referring to Fig: 3, it may be
seen that
rocker cam 38 and thrust surface 36 are shown in a position of maximum fluid
displacement. Rocker Cain 38 may be pivoted clockwise to reduce the
displacement of
pump 10. Although, :pump 10 of the instant embodiment is depicted as a
pressure
compensated pump which does not cross ' center, the instant invention
described
hereinbelow applies equally to a rocker cam type variable displacement axial
piston
where rocker cam 38 may be pivoted clockwise across center such that the
intake and
exhaust ports are reversed and the device is providing maximum fluid
displacement in the
opposite direction. Such a pump may be seen in U:S: Patent Number 5,076,145
assigned
to the predecessor in interest of the subject invention. The instant invention
also applies
equally to a rocker cam type, variable displacement pump having a rotary servo
or linear
servo type control.
In the instant embodiment, in which pump 1U is depicted as a pressure
compensated device; a piston 52 is slidably mounted in a bore 54 formed in a
cylinder 56
rigidly mounted within port cap 16. A spring 58 around cylinder 56 biases
piston S2
against a button 60 mounted on one side of rocker cam 38 to force the rocker
cam to
pivot to a position of maximum fluid displacement. A sttoking piston 62 is
slidab~y
mounted in a bore 64 of a cylinder 66 rigidly secured in port cap 16 at a
position within
pump housing 12 diametrically opposite that of biasing piston 52. Stroking
piston b2
engages a button 68 mounted in-rocker cam 38 at a position diametrically
opposite that of
button 60.
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In a pressure compensated pump it is necessary to reduce the displacement of
the
pump when the pressure of the discharge fluid becomes' excessive. When this
condition
occurs, pressure fluid is supplied to the end of stroking piston 62 to force
it to move
outwardly of bore 64 and 'thereby cause rocker cam 38 to pivot cloclewise (as
viewed in .
Fi?ure 3) towards a position of reduced fluid displacement. Stroking piston 62
will
continue to pivot rocker cam 38 until such time as the discharge pressure of
working fluid
falls below a maximum setting. When: this occurs, pressure fluid no longer is
supplied to
stroking piston 62 and biasing spring 58 moves stroking piston 52 outwardly to
thereby
pivot rocker cam 38 in a counterclockwise direction and thereby increase the
to displacement of the pump. Inasmuch as the instant invention is for- any
type of rocker
cam type pump independent of its displacement control, a further description
of the
pressure compensated mechanism of pump 10 is not required.
As mentioned above, when rocker cam 38 is pivoted Counterclockwise
sufficiently to cause working pressure fluid to be expelled from pump 10 at a
relatively
high pressure, large pumping forces are exerted through pistons 30 to rocker
cam 38.
These forces are transmitted through the complementary arcuate bearing
surfaces 40 and
42 into rocker cam support 44: The large pumping forces cause large friction
forces to
occur at the interface of rocker cam bearing surfaces 40 and rocker support
bearing
surfaces 42 fo make movement of rocker cam 38 within rocker support 44 very
difficult.
In an attempt to reduce the friction forces between rocker cam 38 and rocker
support ~4
plane bushins 7U are inserted between rocker cam arcuate bearing surfaces 40
and
rocker support arcuate bearing surfaces 42 as depicted in Fig: 2; While plain
bushings 70
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reduce the aforementioned frictional forces to some extent, they are
inadequate by
themselves to reduce the frictional forces to a satisfactory level.
Accordingly; working pressure fluid is supplied to counterbalance pockets 72
and
74 formed in the rear faces ~6 of rocker cam 38 as depicted in Figures in 2 ,'
S and 6. The .
areas of the counterbalance pockets 72 and 74 are designed- such that when
they receive
working pressure fluid they reduce the force required to pivot rocker cam 38
within cam
support 44 to within desirable levels. Heretofore, working pressure fluid has
'been
supplied to counterbalance pockets in rocker cam where the working pressure
fluid
source is a pumping piston and fluid is supplied to the piston shoe and
thereafter to bores
in the thrust plate which bores connect to the counterbalance pockets.
Applicant has discovered a unique means for supplying working pressure fluid
to
the counterbalanced pockets 72 and 74 formed in the rear face 76 of rocker cam
38 where'
the fluid source is'in the housing 12.
Turning to the Figures 2 and 4 through 7; it may be seen that a fluid passage
78
connected to a sources not shown, of working pressure fluid is formed in
housing 12.
FIuid passage 78 opens into a fluid passage 80 formed in housing l2 one end of
which is
closed by a plug 82 which may be replaced with a ensor or other device
utilizing
working pressure fluid for control purposes.
A hollow roll pin 84 is mounted in a central bore of plain bushing 70, in cam
support arcuate bearing surface 42 and in a corresponding bore in housing 12.
Roll pin
84 serves two purposes. It anchors plain bushing 70 on cam support or cradle
44 and it
intersects fluid passage 80 to thereby connect that passage to a fluid passage
86 formed in
rocker cam 38 and in arcuate cam surface 40. Fluid passage 86 intersects an
angled fluid
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passage 88 formed in rocker cam 38 and closed by a plug 90: Fluid passage 88
intersects
an oppositedly angled passage 92 closed at one end by a plug 94, The fluid
passage 96
which parallels fluid passage 86 has one end which intersects fluid passage 92
at a right
angle and another end v~hich opens into fluid pocket 74 formed in rear. face
76 of rocker
cam 38. Turning to Fig. 2, it may be een that a roll pin 98 anchors plain
bushing 70 to
cam support surface 44.
Thus, it rnay be een that the instant invention provides a means of taking
source
of working pressure fluid in the Bump housing and supplying the fluid to the
counterbalance pockets 72 and 74 formed in the rear face 76 of rocker cam 38
which
to pockets are connected through passages' formed in rocker cam 38. This
greatly
simplifies supplying pressure fluid to'the counterbalance pockets 72 and 74.
We claim:
