Axial-piston machine with inclinable control surface

An axial-piston machine of the type in which a prismatic housing, usually of rectangular or square cross section, receives a rotatable cylinder drum, the pistons of which bear against an inclinable control surface. The control surface is formed on a rocker or tilting box which can be tilted about an axis perpendicular to the axis of rotation of the drum. The rocker-setting elements are disposed between the housing bottom and the reaction surface of the rocker (against which the pistons bear), at the corners of a rectangle which closely circumscribes the orbit of the pistons. Consequently, the presence of the rocker-displacing means does not increase the dimensions of the machine over those necessary for a fixed control surface machine.

FIELD OF THE INVENTION 
The present invention relates to an axial-piston machine of the 
inclined-disk or swash-plate type in which a drum is formed with a 
plurality of cylinders each piston of which bears against a control 
surface which lies in a plane tiltable about an axis perpendicular to the 
axis of rotation of the drum. 
BACKGROUND OF THE INVENTION 
Axial-piston machines of the aforedescribed type are known in the art and 
can be used as hydrostatic motors or hydrostatic pumps, most often as 
variable-displacement pumps, the displacement being a function of the 
degree of tilt of the inclined disk (swash plate) or control surface. When 
in operation, the cylinder drum of the pump is rotated about its axis, 
e.g., by a shaft journaled in the housing, which can have a prismatic 
configuration, e.g., a square or rectangular cross section, so that the 
pistons of the drum are caused to move inwardly and outwardly as they 
orbit against the inclined plane. Means is customarily provided to 
displace the control surface about its pivot axis. 
In German open specification (Offenlegungsschrift) DT-OS No. 2,240,579, the 
control surface is formed as a flat face of a rocker which can be tilted 
about an axis parallel to the plane of the surface by a setting piston. In 
this construction, a setting cylinder is provided to either side of the 
pivot axis and the two setting cylinders lie in a common plane 
perpendicular to the pivot axis of the rocker, the setting pistons acting 
upon projections from the rocker. 
This construction in which the longitudinal median plane through the 
cylinder drum coincides with the plane of the setting cylinders, requires 
considerable space and increases the dimensions of the machine to 
accommodate the setting cylinders over the size which would be required 
simply to house the drum and rockers absent these cylinders. As a 
consequence, the machine is heavy and the cost of fabrication, in terms of 
additional material and the like, is high. 
It has also been proposed (see U.S. Pat. No. 3,779,137), to provide an 
axial-piston machine in which the setting cylinders can have their axes 
parallel to the axis of rotation of the setting drum. Another arrangement 
has the disadvantage that an articulated linkage is required between the 
piston of the setting cylinder and the rocker. Once again, the housing 
must be enlarged over the minimum prismatic housing accommodating the 
drum, to receive the actuating cylinders, their pistons and the respective 
linkages, if any. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide an 
axial-piston machine which is free from the disadvantages set forth above 
and affords effective control of the rocker in a minimum of space. 
Another object of the invention is to provide an axial-piston machine 
having a tilting box or rocker and an actuator therefor which functions 
more reliably than earlier systems. 
Still another object of the invention is to minimize the space required to 
accommodate an actuator for the tilting box or rocker of an axial-piston 
motor or pump. 
SUMMARY OF THE INVENTION 
These objects and others which will become more readily apparent 
hereinafter are attained, in accordance with the present invention, in an 
axial-piston machine which comprises a prismatic housing, generally of 
square cross section, a cylinder drum rotatable about an axis and a rocker 
or tilting box forming a planar control surface or swash plate against 
which the pistons of the drum bear. 
According to the present invention, the rocker or tilting-box actuator 
comprises four setting elements disposed at the corners of a rectangle 
(i.e., a square) and engaging the four corners of the rocker within the 
prismatic outline of the housing but, advantageously, with a radial 
spacing from the axis of rotation of the drum which is not substantiallly 
greater than the radius of the drum but is less than half the length of 
the diagonal through the housing. Thus, if the housing is prismatic and 
the drum is cylindrical, the rocker surface engaged by the pistons of the 
drum may also be rectangular so that the aforementioned corners of the 
rocker lie radially outwardly of the orbit of the pistons of the drum. 
It has been found that this arrangement of the setting elements at the four 
corners of the rocker, preferably at the vertices of a square as seen in 
section through the axis of rotation of the drum and closely 
circumscribing same, is highly advantageous. In this case, the setting 
elements can be disposed in the housing without increasing the dimensions 
thereof over those of a machine in which the swash plate or control 
surface is not tiltable with respect to the cylinder drum axis. Because 
the four setting elements engage the four corners of the rocker in 
force-transmitting relation and symmetrically on opposite sides of a 
median plane through the rocker and perpendicular to the tilting axis, the 
resultant of the setting force is applied along the median axis of the 
rocker bearing and no canting moment is applied to the rocker. 
According to a feature of the invention, all four of the setting elements 
are setting cylinders in each of which a piston is displaceable. 
Preferably if a cylinder receives a spring which bears upon the piston in 
the direction of the rocker and hence is effective parallel to the 
application of fluid pressure to the setting cylinder. This arrangement 
has been found to be most effective for servovalve control of the tilt of 
the rocker and hydraulic fluid can be admitted to the cylinders either 
through the rocker or through the housing. 
Alternatively, two of the setting elements on one side of the tilting axis 
of the rocker can be hydraulic cylinders of the type described while the 
other pair of units can be control springs which bias the rocker into one 
extreme tilted position. The hydraulic fluid feed to the cylinders can 
thus be a function of the main fluid pressure so that a power control is 
provided automatically, i.e., the rocker is tilted to a greater or lesser 
extent depending upon the pressure prevailing in the machine. 
Since, upon tilting of the rocker, the point of attack of the setting 
element upon the rocker describes an arcuate path, while the pistons of 
the setting units generally are received within cylinders of a rectilinear 
configuration, it is advantageous to compensate for the different 
movements in the following manner: 
(a) the setting cylinders are articulated or pivotally mounted; or 
(b) the setting cylinders are rigidly fixed in the housing while the 
pistons have spheroidal portions sealingly cooperating with the 
rectilinear walls of the cylinders to permit tilting of the pistons 
relative to the cylinder axis; or 
(c) the cylinder is fixed in the housing and the pistons can undergo only 
rectilinear movement within their cylinders. In the last-mentioned case, a 
slide shoe or other slide device is provided between each setting piston 
and the rocker so that the end of the setting piston can shift with 
respect to the opposing surface of the rocker. 
Best results have been obtained when the hydraulic fluid supply to the 
setting cylinders is effected through the rocker. In this case, the 
setting cylinder can be articulated to the rocker and can have the pistons 
engage the housing, or the piston can be provided with a passage 
communicating with a bore in the rocker and with the working chamber in 
the setting cylinder which is connected to the housing. 
This has an advantage that the rocker can be provided directly with a 
follower-type control by, for example, disposing a slide valve along a 
flank of the rocker and moving the slide valve with respect to the ports 
in the lateral flank of the rocker. Alternatively, the ports of the rocker 
may move relative to the valve structure to open or close a fluid passage 
to or from the rocker and hence the setting cylinder. Since the setting 
pressure can be relatively small, the leakage losses are not significant 
and, even where they occur, do not involve any significant energy loss. 
According to a particularly advantageous embodiment of the invention, each 
of the setting pistons has a ball head swingable in a spacer plate which 
is, in turn, connected to the inclinable control surface of the rocker. 
The setting cylinder can either be swingable in the housing or the piston 
can be provided with a spheroidal part enabling each piston to tilt in the 
respective cylinder. Since the ball seat for the head of the setting 
piston is not formed directly in the rocker but is constituted by the 
spacer plate, the structure can be made significantly less costly since 
machining a socket in the rocker directly is avoided, especially since the 
inclinable control surface of the rocker is generally hardened and 
superfinished. 
The spacer plate permits bearing metal or like material to be used for the 
socket, which can consist of sintered steel particles, wear-resistant 
steels or the like. This reduces the cost of manufacture and permits 
replacement of a spacer plate when the latter wears out without a 
replacement of the entire rocker. 
It should be apparent that this arrangement also has numerous other 
advantages. For example, if the dimensions of the rocker must be limited, 
the spacer plates can project beyond the edges of the rocker and thereby 
increase the distance between the point of attack of the setting cylinder 
upon the rocker and the cylinder-drum axis. When the ball socket is formed 
directly in the rocker, such outward spacing of the point of attack is 
limited by the boundaries of the rocker. Of course, the greater the 
distance between the point of attack and the tilting axis of the rocker, 
the greater is the lever arm effectiveness for tilting the rocker and the 
the smaller can be the setting piston diameter for application of a given 
torque thereto. 
Furthermore, the spacer plate has the additional function that it can be 
used to hold the plate by which the heads of the working pistons are 
retained against the inclinable control surface, i.e., the plate which 
prevents the slide shoes of these pistons from withdrawing from the 
control surface. In this case, the thickness of the spacer plate must be 
reduced to the height of the slide shoes. 
The system of the present invention can also be used for tilting the 
swingable housing of a drive flange machine, the cylinder being received 
in the tiltable housing and the piston rods of the cylinder bearing 
against a fixed portion thereof in which the drive flange is journaled.

SPECIFIC DESCRIPTION 
The axial-piston machine illustrated in FIG. 1 comprises a housing 1 in 
which the drive shaft 2 is journaled, the drive shaft being connected to a 
cylinder drum which has not been illustrated in this Figure. The housing 1 
is provided with a cylindrical concave surface 3 whose axis lies 
perpendicular to the axis of shaft 2 and of the cylinder drum. Within this 
cylindrical concavity, a rocker 5 is tiltable by means of a bearing 4 in 
the form of a roller band, i.e., a cage provided with roller bearings as 
can be seen diagrammatically in FIG. 1. The left-hand face of the rocker 5 
forms a control disk surface 6 against which the slide shoes of the 
pistons of the cylinder drum bear. 
A passage 7 represents the hydraulic fluid connections to the cylinder 
drum, these connections being provided in the usual manner to supply 
hydraulic fluid to the cylinders of the drum and remove hydraulic fluid 
under pressure therefrom. 
The housing 1 is also provided with two threaded bores 8 into each of which 
a threaded head 9 is screwed, each threaded head 9 forming part of a 
setting cylinder 10. The two bores and setting cylinders illustrated in 
FIG. 1 represent the oppositely effective setting cylinders on one side of 
the rocker. A second such set of setting cylinders is provided on the 
opposite side of the rocker so that the setting cylinders lie at the 
corners of a square as has been illustrated in FIG. 2. 
In each of the setting cylinders 10, a setting piston 11 is shiftable, the 
free ends of the setting pistons 11 being each formed with a spheroidal 
portion 12 sealingly engaging the wall of the respective setting cylinder 
10. Hence the pistons 11 can be tilted to a limited extent. 
A neck 13 of each of the pistons 11 carries a ball head 14 which is 
received with freedom of pivotal movement in a ball seat 15 of the rocker 
5. Within each setting cylinder 10 and each setting piston 11, a 
compression 16 is disposed to urge the piston in the direction of the 
rocker. The housing bottom 17 is held by four anchoring screws 18 against 
the housing 1. Each threaded head 9 is provided with a slot 19 adapted to 
accommodate a screw driver so that the respective cylinder 10 can be 
mounted in the threaded bores 8 into which the bolts 18 are also screwed. 
Alternatively, or in addition, the cylinders 10 can have hexagonal 
(nut-shaped) outer peripheries which can be gripped by a wrench and are 
held in position by housing plates 24 described below. 
In the rocker 5, each of the ball sets 15 is connected via a passage 20 
with a bore 21 which opens in an end face of the cylindrical segmental 
rocker 5 and serves to supply hydraulic fluid to and remove hydraulic 
fluid from the setting cylinders 10. 
Against the lateral face of the rocker 5, a valve slider can lie to form a 
flat control valve therewith. The control valve has not been illustrated 
in these Figures although it may be similar to the arrangement illustrated 
in FIG. 13 and described hereinafter. For example, the lower bore 21 may 
be supplied with hydraulic fluid while the hydraulic fluid is released 
from the upper bore 21. In this case, the two upper cylinders 10 are 
supplied with fluid while fluid is discharged from the two lower cylinders 
and the rocker 5 is caused to rotate in the clockwise sense along the 
cylindrical seat 3. The rocker 5 can thus be shifted from its solid-line 
position as illustrated in FIG. 1 to the dot-dash-line position shown. The 
prestress of the springs 16 can be varied by screwing the heads 9 more or 
less deeply into the bores 8 and thereby shifting the cylinders 10 toward 
or away from the rocker 5. 
In the embodiment illustrated in FIG. 2, the setting cylinders, instead of 
being formed individually, are constituted as bores 25 in corresponding 
corner pieces 26 of walls 23 of the housing. The cylinders 25 receive 
pistons 11 with neck portions 13 as described in connection with FIG. 1. 
The housing is closed by a pair of cover plates 24. When the latter are 
used with the embodiment of FIG. 1, they can hold the hexagonal portions 
of the setting cylinders against rotation as has been described. 
Since the machine is generally produced with a rectangular housing 
configuration, at least approximately, and the cover plates 24 are 
rectangular, the arrangement of the setting cylinders 25 in the four 
corners of the structure does not increase the size for a given diameter 
of the cylinder drum. Hence the size of the housing is generally 
determined by the diameter of the cylinder drum 27. The axes of the 
setting cylinders 25 thus lie at the corners of a square which 
circumscribes the cylinder drum. Naturally, when only two setting 
cylinders are provided at two of the vertices of a square, e.g., above and 
below the cylinder drum axis as shown in FIG. 1, the arrangement also does 
not increase the dimensions of the housing which again is determined by 
the rectangular configuration necessary to enclose the cylindrical drum. 
In the embodiment of FIGS. 3 and 4, the drive shaft 2 is journaled in the 
housing 31 and is connected with the cylinder drum 27. In the four corners 
of the housing 31, there are provided corner members 32 in which the 
setting cylinders 33 are formed. In each of the setting cylinders 33, a 
setting piston 34 is shiftable and is provided with a ball head 36 which 
is received in a sliding shoe 37 pressed against the flat surface 6 of the 
rocker 35, this surface also forming the inclined or inclinable plane 
against which the pistons of the cylinder drum bear. The rocker 35 
corresponds to the rocker 5 with the sole difference that the rocker 35 is 
not formed directly with the ball seats 15 for the heads of the setting 
pistons. The supply of hydraulic fluid to the working compartment of the 
setting cylinders 33 is effected via the passages 21 and 20 as previously 
described via fluid cushion compartments 37a in the shoes 37 and bores 38 
in the setting pistons 34. Within each of the setting pistons 34 a 
compression spring 40 is disposed which bears against a spring plate 41 
centering the spring within the cylinder 33. The housing 31 is here also 
closed by cover plates 24. Here again the setting cylinders are disposed 
within the normal rectangular machine contour at the vertices of a square 
which generally circumscribes the orbit of the pistons of the axial-piston 
cylinder drum 27. 
The embodiment of FIG. 5 differs from that of FIG. 1 only in that the 
setting pistons 42 have a different configuration from that of the setting 
pistons 11. The setting pistons 42 are provided with generally spheroidal 
sealing portions 43 which permit slight tilting of the pistons 42 and 
sealingly engage the cylinders 10. These sealing portions 43 are connected 
by piston rods 44 with ball heads 45 which are pivotal within the ball 
seats 15 of the rocker 5. In this embodiment, compression springs bearing 
upon the pistons 42 are eliminated and the ball heads 45 are held against 
the rocker 45 by retaining plates 46 which can be bolted to the rocker. 
The embodiment of FIG. 6 uses the same pistons 42 as has been described in 
connection with the embodiment of FIG. 5, although the setting cylinders 
are of the configuration of those of FIG. 4. In each of the embodiments of 
FIGS. 1 through 6, four setting cylinders are preferably provided at the 
vertices of a square. However, in the embodiment of FIGS. 7 and 8, only 
two setting cylinders 50 are provided. In each of these setting cylinders 
50, a ball-shaped control piston 51 is shiftable by hydraulic fluid and is 
connected by a piston rod 52 with a ball head 53 held in a ball seat 54 of 
the rocker 55. The setting cylinders 50 are connected by bores 56 in the 
housing bottom 57 which is held against the cylinder housing 59 by bolts 
58. Hydraulic fluid under pressure is supplied to the bores 56 from the 
working fluid passage 60. 
While the two control pistons 51 are effective at one side of the tilting 
axis 61 of the rocker 55, the opposite side of the rocker is engaged by 
two control springs 62 which bear against spring plates 63 whose ball 
heads 64 are received in ball sockets of the rocker 55. The springs 62 are 
seated against spring plates 65 which are centered within bores 67 formed 
in the housing 59. The bores 67 also receive the springs 62. The cylinder 
drum 27 is rotatable in the housing 59 which can be provided with an 
appropriate recess to accommodate this cylinder drum. The housing is 
completed by cover plates 24 as previously described. 
In this embodiment, the springs 62 act counter to the hydraulic force in 
the cylinders 50 and thus to swing the rocker 55 in the counterclockwise 
sense. When the hydraulic fluid under pressure is applied through the 
passage 60 and bores 56 to the cylinders 50, the rocker 55 is swung in the 
clockwise sense, e.g., to the position illustrated in dot-dash lines. 
In the embodiment of FIG. 9, the housing 71 corresponds to the housing 1 of 
FIG. 1 and can be provided with a housing bottom 7 which has not been 
illustrated. The cylinders 10 have threaded heads 9 received in bores in 
the housing bottom 7 and the pistons 11 are connected by necks 13 to ball 
heads 14 provided with bores 22 communicating with the interiors of the 
cylinders 10 as described in connection with FIG. 1. 
In this embodiment, however, the rocker 75 is provided with bores 21 and 20 
similar to those of the rocker 5 with the sole distinction that the rocker 
75 does not have ball sockets 15. Consequently, the inclined or inclinable 
surface 6, which forms the control surface for the axial pistons of the 
drum, carries spacer plates 72 which can be screwed to the surface 6 and 
in which the sockets 73 are formed to pivotally receive the ball heads 14. 
The spacer plates 72 are provided with bores 76 which communicate with the 
bores 20. 
The ball heads 14 are held in the respective sockets 73 by respective 
retaining plates 77 which, in turn, can be bolted onto the spacer plates 
72. The surfaces of the retaining plates 77 turned toward the ball heads 
14 may be spheroidally concave or conical and can be provided with slots 
78 through which the necks 13 extend to facilitate mounting. 
The sliding shoes of the axial pistons of the drum, which engage the 
surface 6, can be held in place by a retainer in the form of a ring or 
plate (as shown at 150 in FIG. 10). In this case, the retaining plates 77 
can be replaced by the retaining plate which also serves to hold the 
sliding shoes against the surface 6. Hence the sliding shoes should have a 
thickness no greater than that of the spacer plates 72. When, however, the 
thickness of the shoes is greater, i.e., corresponds to the thickness of 
the spacer plate 72 plus the thickness of the retaining plate 77, the 
plate holding the slide shoes against the surfaces 6 can be simply screwed 
to the retaining plate 77. 
From FIG. 9 it will be apparent that the socket 73 in the spacer plate 72 
can have a greater distance from the pivot axis 61 than sockets which are 
formed directly in the surface 6. 
Referring now to FIG. 10, it will be seen that each cylinder drum 27 can be 
provided with an array of cylinder bores 100, each of which is provided 
with the usual piston 101 having a ball head 102 received in a ball socket 
103 of a slide shoe 104 which may be held by a plate of the type described 
against the surface 6 of the rocker 105, the latter representing the 
rockers shown in all of the embodiments previously described. Each of the 
rockers may be provided, as has been illustrated for the rocker 105, with 
a bore 106 through which the shaft 2 can pass with ample clearance to 
permit tilting of the rocker. The cylinders 100 communicate via ports 107 
with hydraulic fluid inlets or outlets formed in the bottom of the housing 
against which the cylinder 27 bears. The configuration of these ports is 
conventional in the art and requires no amplification here. 
As has been illustrated in FIG. 11, each rocker 105 with its bore 106 
controls the pistons 101 which orbit in a circle represented in dot-dash 
lines at 108, the ports 109 which are symbolic or representative of the 
ports communicating with the setting cylinders in the embodiments of FIGS. 
1-6 and 9, being disposed at the vertices of a square circumscribing the 
orbit 108. The ports 21 in each of the embodiments illustrated can open at 
a lateral face 110 of the rocker 105 as shown in FIG. 12, preferably in 
arc-segmental grooves 111 and 112. These grooves may be selectively 
aligned with a discharge port 113 in a slide valve 114 having the 
additional bores 115 and 116 which supply hydraulic fluid to the grooves 
111 and 112. Thus, when port 113 communicates with groove 111 and port 116 
with groove 112, hydraulic fluid is trained to the reservoir 117 from 
groove 111 which hydraulic fluid is fed under pressure from a pump 118 to 
the groove 112. When the slide valve 114 is shifted to the right, the port 
115 communicates with the groove 111 while the port 113 communicates with 
the groove 112 to reverse the flow of hydraulic fluid to the setting 
cylinders and thereby reverse the position of the rocker. 
The slide valve 114 illustrated in FIG. 13 is merely representative of any 
valve system which can perform a similar function. Advantageously, the 
port 113 is connected by a passage 120 to a chamber 121 communicating via 
line 122 with the reservoir 117. Similarly, the ports 115 and 116 
communicate via passages 123 and 124 with a chamber 125 connected by line 
126 to the pressure side of the pump 118. An actuator for the slide 114 
can be the rod represented at 130, the latter being displaced by a 
servomechanism not shown or by hand. The arc segmental grooves 111 and 112 
have centers of curvature on the pivot axis 135 of the rocker 105 so that 
they remain in registry with the ports 115 and 113 or 113 and 116 in all 
angular positions of the rocker 105 once the valve 114 has been positioned 
to supply fluid to one of the grooves and remove fluid from the other.