Hydrostatic displacement engine

A hydrostatic displacement engine having pistons which slide on the inside of a stroke ring. The eccentricity of the stroke ring, relative to the cylinder block, is variable. The interior of the housing is divided in two areas which are separated from each other preferably by a seal, namely in the area located radially outside the stroke ring and the area located radially within the stroke ring. The inner and outer areas serve as a leakage oil collection space and connects through a leakage oil line with a pressureless liquid container. Emptying in the area situated radially outside the stroke ring is a connection line which is supplied by a pressure supply. A pressure valve maintains the pressure in the outer area at a value of several bars.

BACKGROUND OF THE INVENTION 
The invention generally relates to a hydrostatic displacement engines such 
as radial piston engines (pump or motor) or vane engines (pump or motor). 
More particularly, the invention pertains to a hydrostatic displacement 
engine in which a rotating cylinder block contains several radially 
shiftable displacement elements that slide on a stroke ring. 
Prior engines of this type have been described, e.g., in the following 
documents: 
German patent disclosure 35 45 019, 
German patent disclosure 37 00 573. 
The radial piston pump known from the German patent disclosure 35 45 019 
has a rotatable cylinder block which is mounted on a stationary control 
journal. This journal is situated in a housing whose interior is sealed by 
a housing cover. Contained in the cylinder block are several radially 
movable pistons which, by way of a piston shoe each bear on the inside of 
a stroke ring. Although not immediately evident from the German patent 
disclosure, the stroke ring is movable transverse to the axis of rotation 
of the cylinder block in order to change the stroke of the piston. All 
areas of the housing interior, i.e., those areas situated within and 
outside the stroke ring, are connected with one another and connected with 
a low-pressure area (for instance in the form of a liquid container) by 
means of a leakage oil outlet. 
Radial piston engines of this design (working either as a pump or as a 
motor) and the vane engines operating according to a similar principle 
have been proven in practice. A problem with these displacement engines, 
however, is the noise which is occasionally experienced in their 
operation, and in particular the frequently observed change of noise. The 
noise intensity assumes elevated levels especially when the stroke ring is 
contained in an intermediate position, i.e., when the stroke ring bears on 
neither of its two limit stops. 
The problem underlying the invention is to introduce measures by which the 
operationally caused noise in the aforementioned hydrostatic displacement 
engines will be reduced, especially in an intermediate position of the 
stroke ring. 
SUMMARY OF THE INVENTION 
This problem is solved by the present invention. By adjusting a specific 
liquid pressure in the housing interior, radially outside the stroke ring, 
wherein the pressure ranges above atmospheric pressure, the noise 
propagation of a displacement engine may be considerably reduced. This 
applies primarily to the operating condition in which the stroke ring is 
contained in an intermediate position and in which an especially intensive 
noise has been experienced. Specifically, it is possible to keep the 
remaining noise intensity more uniform than heretofore. 
There are several options for establishing the desired seal between the 
area of the housing interior that is situated radially outside the stroke 
ring and the area situated radially within the stroke ring. According to 
one option, a ring-shaped sealing element is inserted on one of the two 
end faces of the stroke ring, between the stroke ring and the adjacent 
housing wall. This sealing element likewise forces the stroke ring with 
its other end face down on the adjacent housing wall, thus establishing a 
seal. This design has the advantage in that relatively coarse 
manufacturing tolerances can be permitted for the width of the stroke ring 
and for the spacing between the respective housing walls. Alternatively, 
the special sealing elements may be omitted. In this case, the end faces 
of the stroke ring bear directly on the adjacent housing walls. However, 
in this case, it is necessary to ensure that the stroke ring will continue 
to be movable, transverse to the axis of rotation of the cylinder block. 
Consequently, the components must be manufactured with great accuracy. 
According to one embodiment of the invention, a medium liquid pressure is 
located in the area of the housing interior that is situated radially 
outside the stroke ring. This pressure does not range halfway between the 
pressures that prevail in the high-pressure and low-pressure channels; 
instead, the pressure ranges several bars above atmospheric pressure or 
above the pressure that prevails in the area of the housing interior that 
is located radially within the stroke ring. 
There are now various possibilities for building up the "halfway" pressure 
in the area of the housing interior that is situated radially outside the 
stroke ring. One possibility is that leakage fluid flows without specific 
additional measures from the high-pressure area into the radially outer 
area of the housing interior, thus creating the "halfway" pressure. In 
order to limit this "halfway" pressure to 2-4 bars, for example, the 
radially outer area can be connected with the radially inner area by way 
of a hydraulic resistance, e.g. a choke. 
To keep the "halfway" pressure safely at the desired value, however, it is 
preferable to allow the fluid to flow deliberately from a pressure supply 
into the radially outer area. It is advantageous to connect the two areas 
that lie radially outside and radially inside the stroke ring with one 
another by way of a pressure valve. All embodiments of the invention 
presuppose that the area situated inside the stroke ring (as so far the 
entire housing interior) is connected to a low-pressure area, for instance 
an external non-pressurized liquid container or to the low-pressure 
channel contained in the housing. 
As an external pressure supply for the radially outer area, the leakage oil 
space of a control unit pertaining to the displacement engine may 
preferably be used. This control unit, which may be a pressure controller, 
is directly attached to the housing of the displacement engine for the 
adjustment of the individually desired eccentricity (spacing between 
stroke ring axis and cylinder block axis of rotation). In other words, the 
servo oil accruing in the controller and/or the leakage oil is passed into 
the area of the housing interior that is situated radially outside the 
stroke ring. Thus, the "halfway" pressure will build up to the level of, 
e.g., 2 to 4 bars, depending on the setting of the pressure valve. 
However, the area of the housing interior that is located radially outside 
the stroke ring can also be connected to any other external pressure 
supply. Another possibility is the use of the low-pressure channel 
contained in the housing as a pressure supply. This is possible whenever 
the displacement engine is part of a closed hydraulic system where a 
"halfway" pressure prevails in the low-pressure channel. 
Other options are available if at least approximately atmospheric pressure 
prevails in the low-pressure channel. In this case, the area of the 
housing interior that is situated radially inside the stroke ring is 
connected directly with the suction channel. Or, only the outlet of the 
pressure valve is connected with the suction channel.

Corresponding reference characters indicate corresponding parts throughout 
the several views. The exemplification set out herein illustrates one 
preferred embodiment of the invention, in one form, and such 
exemplifications are not to be construed as limiting the scope of the 
invention in any manner. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The essential and already known components of the illustrated radial piston 
engine (for instance radial piston pump) are the following: a housing 10, 
in which a fixed control journal 11 is inserted, is sealed by a housing 
cover 12. A drive shaft 13 including an antifriction bearing 14 and a 
drive shaft seal 15 is mounted in housing cover 12. The drive shaft 
connects through a clutch 16 with the cylinder block 17, which is 
rotatably mounted on control journal 11. Contained in cylinder block 17, 
in a star-shaped arrangement, are seven pistons 18. Other numbers of 
pistons are possible. Each of these pistons 18 is hinged to a piston shoe 
19. The stroke ring 20 assumes an eccentric position in housing 10 
relative to the control journal 11. Piston shoes 19 slide along the inner 
cylinder surface of stroke ring 20. The magnitude of the eccentricity e 
between control journal 11 and stroke ring 20 is variable by shifting 
stroke ring 20 with the aid of servo pistons 21 and 22. The working fluid 
circuit comprises a low-pressure channel 23 and a high-pressure channel 24 
that extend through housing 10 and control journal 11. A control unit for 
adjustment of the liquid pressure acting on servo pistons 21 and 22 has 
been omitted in FIG. 2; however, refer to FIG. 3. 
The area 25 of the housing interior situated radially inside stroke ring 
20, in which the movable parts 13, 14 and 16-19 rotate, connects by way of 
a leakage oil channel 29 with a low-pressure area, e.g. with a 
pressureless oil container 9 (illustrated only symbolically). Thus, in 
this radially inner area 25, there prevails a pressure that amounts to 
about between zero and 1 bar. 
According to one aspect of the invention, area 26 of the housing interior 
situated radially outside stroke ring 20 is separated from the remaining 
housing interior, i.e., from the radially inner area 25. To that end, 
according to FIGS. 1 and 2, one of the end faces of stroke ring 20 is 
provided with an annular groove 30 in which an axially movable ring seal 
31 is contained. Stroke ring 20 and ring seal 31 are spread apart in an 
axial direction with the aid of at least one elastic element 32, (e.g. an 
O-ring). As a result, stroke ring 20 continuously bears in sealing fashion 
with its one end face on surface 40 of housing 10. On the opposite end 
face of stroke ring 20, ring seal 31 constantly bears in sealing fashion 
on surface 41 of housing cover 12. It is essential that in the radially 
outer area 26 of the interior, a "halfway" pressure of about 2 to 4 bars 
builds up. This can be brought about, e.g., in that leakage fluid 
penetrates from a pressure space 54, which is later discussed (refer to 
FIG. 3), along the servo piston 21 into outer area 26. Preferably, 
however, outer area 26 is connected with any pressure supply through 
channel 33 so that the buildup of the "halfway" pressure in area 26 will 
take place as quickly as possible. 
A certain pressure difference is suitably maintained at all times between 
the pressure that prevails in radially outer area 26 and the pressure in 
radially inner area 25. To maintain this pressure difference of, e.g., 2 
to 4 bars, area 26 communicates through bores 36 and 38 with leakage oil 
channel 29. Additionally, as illustrated schematically in FIG. 1, bore 36 
accommodates a check valve 37 which opens only when the pressure 
difference exceeds the desired value. Instead of check valve 37, any other 
suitable pressure valve may be provided, e.g. a pressure relief valve. 
Housing 10 possesses on its inside, in known fashion, two guide surfaces 42 
for guidance of stroke ring 20. These guide surfaces divide the radially 
outer area 26 of the housing interior in two chambers which are connected 
with each other through channels 45. In the illustrated example, 
connecting channels 45 have been machined into housing 10. However, 
similar connecting channels could be provided also in stroke ring 20. 
In FIG. 6, a connecting channel 33' from low pressure channel 23 to one of 
the connecting channels 45 is additionally illustrated by dash-dot lines. 
This arrangement is an alternative to the aforementioned channel 33 of 
FIG. 2. The arrangement with connecting channel 33' is applicable whenever 
in low-pressure channel 23 there exists a "halfway" liquid pressure which 
then propagates into area 26 of the housing interior that is situated 
radially outside the stroke 20. 
FIG. 3 shows a supplement to FIG. 2. In FIG. 3, part of housing 10 is 
shown, in addition to part of stroke ring 20 and servo piston 21. A 
control unit 50, which is schematically illustrated, is attached to 
housing 10. A movable valve body 51 of a pressure control valve is 
contained in this control unit. Valve body 51 connects a channel 53, which 
extends to pressure space 54 of servo piston 21 with either a channel 52 
that is connected to high-pressure channel 24 (FIG. 1) or with a relief 
channel 59 that empties into the interior 56 of the control unit. This 
controls the servo force of servo piston 21 acting on stroke ring 20, and 
thus the magnitude of eccentricity e (FIG. 2). A servo force acting, for 
instance mechanically, on valve body 51 is symbolically illustrated by 
arrow 55. The servo fluid and/or leakage fluid accumulates in interior 56 
up to a certain pressure, whereupon the fluid propagates through channel 
33 and into the radially outer area 26 of housing 10, where it is limited 
by pressure valve 37 (FIG. 1). 
FIG. 4 shows an alternative embodiment to FIG. 1. Illustrated are parts of 
housing 10 and housing cover 12. The stroke ring is now marked 20'; it is 
divided into a principal partial ring 20a and an auxiliary partial ring 
20b. Ring 20b forms a radially outer part of the entire stroke ring 20' 
and is axially movable on principal partial ring 20a. A spring element, 
for instance O-ring 20c, is inserted under prestress between the two 
partial rings. The two partial rings 20a and 20b are thereby spread apart 
in axial direction so that the two partial rings will bear in sealing 
fashion on adjacent housing faces 40 or 41. 
FIG. 5 shows another alternative embodiment to FIGS. 1 and 4. Here, stroke 
ring 20" is now designed as a one-piece part without a specific sealing 
element. Its two end faces bear in sealing fashion on adjacent housing 
walls 40 and 41. 
While this invention has been described as having a preferred design, the 
present invention can be further modified within the spirit and scope of 
this disclosure. This application is therefore intended to cover any 
variations, uses, or adaptations of the invention using its general 
principles. Further, this application is intended to cover such departures 
from the present disclosure as come within known or customary practice in 
the art to which this invention pertains and which fall within the limits 
of the appended claims.