SEMI-PLUGGED STAR GEROTOR AND METHOD OF ASSEMBLING THE SAME
TECHNICAL FIELD
[0001] The present invention relates generally to a gerotor assembly for use within a
fluid control device, and in particular to a semi-plugged star gerotor and a method of
assembling the same.
BACKGROUND
[0002] Star gerotors are positive-displacement fluid pumping devices having meshed
inner and outer rotors. The inner and outer rotors are typically referred to as a star
member and a ring member, respectively. Each rotor has a fixed center point that is
eccentric with respect to the center point of the other rotor. The star member has n teeth,
and is circumscribed by the ring member having (« + 1) lobes. Rotation of one rotor
drives the other, with a low relative speed maintained between the two rotors. The
volume defined between the mating teeth/lobes of the engaged rotors creates a vacuum
during gerotor rotation, and thus a resultant suction or intake stage for each revolution of
the gerotor.
[0003] A steering control unit (SCU) of a hydrostatic power steering system is one
type of fluid control device that commonly uses a star gerotor in its construction. An
SCU may experience slip between its rotating gerotor members and a stationary member,
e.g., an end cap which is secured adjacent to the gerotor. For example, when a steering
cylinder controlled via a valve housing section of the SCU reaches the limit of its range
of travel, a steering wheel controlled via the SCU may still rotate beyond this limit. Such
additional rotation is often a result of internal fluid leakage between the star member and
an adjacent surface of the stationary end cap.
SUMMARY
[0004] A gerotor assembly is provided herein for use with a fluid control device such
as the SCU noted above. The gerotor assembly disclosed herein is semi-plugged, i.e., a
hybrid between a solid plug-style star seal design and a conventional sealing ring, as set
forth in detail below. The gerotor assembly includes a star member, a ring member, an
annular plug member, and an o-ring. The star member has («) teeth, and defines a center
opening of a first diameter. The center opening is in fluid communication with a lowpressure
fluid reservoir when the gerotor assembly is installed in the fluid control device.
The ring member circumscribes the star member, and has (n+1) lobes that mesh with the
(n) teeth, as is well understood in the art of gerotors.
[0005] The ring member is configured to define, in conjunction with a stationary end
cap of the fluid control device, a high-pressure fluid channel, i.e., a fluid channel that is
connectable to a high-pressure fluid supply. The annular plug member is circumscribed
by the star member, and defines a center bore of a second diameter that is smaller than
the first diameter. The o-ring is positioned between the star member and the annular plug
member. The annular plug member is thus configured to form a semi-plugged fluid seal
against the stationary end cap of the fluid control device, with various performance
benefits as explained below.
[0006] A fluid control device is also disclosed. The fluid control device includes a
gerotor star member, a gerotor ring member, an annular plug member, an o-ring, and a
valve housing section. The star member defines a center opening of a first diameter, with
the center opening in fluid communication with a low-pressure fluid reservoir. The ring
member circumscribes the star member, and has (n+1) lobes that engage with the (n)
teeth of the star member. The plug member is circumscribed by the star member, and
defines a center bore of a second diameter less than the first diameter.
[0007] The center bore is in fluid communication with the low-pressure fluid
reservoir via the center opening. The o-ring is positioned between the star member and
the annular plug member. The o-ring is in fluid communication with the high-pressure
fluid reservoir via a high-pressure fluid channel, and with the low-pressure fluid reservoir
via the center opening. The valve housing section has a stationary end cap and a wear
plate, with the end cap positioned immediately adjacent to the annular plug member to
define the high-pressure fluid channel in conjunction with the star member. The highpressure
fluid channel is in fluid communication with a high-pressure fluid reservoir.
[0008] A method is also disclosed herein, including providing a gerotor star member
defining an annular shelf and a center opening of a first diameter, and circumscribing the
star member with a gerotor ring member such that (n+1) lobes of the ring member
engage with n) teeth of the star member. The method includes positioning an o-ring on a
surface of the star member, and providing an annular plug member that defines a center
bore of a diameter less than the first diameter. The annular plug member is placed on the
o-ring such that the annular plug member is circumscribed by the star member to thereby
form the gerotor assembly.
[0009] The above features and advantages and other features and advantages of the
present invention are readily apparent from the following detailed description of the best
modes for carrying out the invention when taken in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1 is a schematic illustration of a fluid control device using a semiplugged
gerotor assembly of the type disclosed herein;
[0011] FIG. 2 is a schematic plan view illustration of the present gerotor assembly;
[0012] FIG. 3 is a partial schematic cross-sectional side view illustration of a portion
of the fluid control device shown in FIG. 2, including the gerotor assembly and a
stationary end cap of the fluid control device of FIG. 1; and
[0013] FIG. 4 is a schematic cross-sectional illustration of a portion of the fluid
control device shown in FIG. 1, including the portion shown in FIG. 3.
DESCRIPTION
[0014] Referring to the Figures, wherein like reference numbers correspond to similar
components, FIG. 1 is a schematic illustration of a fluid control device 11. The fluid
control device 11 includes a semi-plugged gerotor assembly 13. The gerotor assembly 13
has an annular plug member 18 forming a star seal. As described in detail below, the
annular plug member 18 is configured to reduce internal fluid leakage within the fluid
control device 11 in which the plug member 18 is installed.
[0015] In one possible embodiment, the fluid control device 11 may be configured as
a steering control unit (SCU) for use in a hydrostatic power steering system. The gerotor
assembly 13 may be included as part of an SCU to reduce undesirable steering wheel
rotation while reducing friction losses relative to conventional designs, thereby increasing
energy efficiency. Other fluidic systems in which fluid leakage from a high-pressure side
to a low-pressure side is a critical design concern, such as fluid motors pumping systems,
may likewise benefit from use of the semi-plugged gerotor assembly 13 and its annular
plug member 18 as described herein.
[0016] Within the fluid control device 11 shown in FIG. 1, the gerotor assembly 13
may be bolted or otherwise securely fastened to a valve housing section 70, e.g., via bolts
17. A stationary wear plate 80 is disposed between the gerotor assembly 13 and the valve
housing section 70. The gerotor assembly 13 is positioned between the wear plate 80 and
a stationary end cap 24. The valve housing section 70 may define various fluid ports,
including a fluid inlet port 72, a fluid return port 74, and various cylinder control ports,
e.g., control ports 76 and 78, either on one side of the valve housing section 70 or
distributed as shown. A fluid device subassembly 10 is formed by the gerotor assembly
13 and the end cap 24, and is described below with reference to FIGS. 3 and 4.
[0017] Although not show in FIG. 1 for simplicity, the interior of the valve housing
section 70 defines a bore containing any required valves and associated control devices
for actuating the device being controlled, e.g., a rotatable spool and a cooperating,
relatively rotatable follow up valve member, as is well understood in the art. The follow
up valve member may be driven using a main drive shaft (not shown), with the main
shaft splined to and rotatable in conjunction with the semi-plugged gerotor assembly 13.
[0018] Referring to FIG. 2, the gerotor assembly 13 includes an internally-toothed
outer rotor, which is referred to hereinafter as a ring member 12. The gerotor assembly
13 further includes an externally-toothed inner rotor, i.e., a star member 14. The star
member 14 is eccentrically disposed within the ring member 12 for orbital and rotational
movement therein. Both the star member 14 and the ring member 12 may be constructed
of steel, powder metal, or another suitable metallic material.
[0019] The star member 14 defines an annular, axial wall 62. The axial wall 62
defines a center opening (arrow 20) as shown in FIG. 4. The star member 14 may
includes splines 22 (see FIGS. 3 and 4) to allow the star member 14 to engage with
mating splines of a main drive shaft (not shown) positioned within the valve housing
section 70 of FIG. 1. As understood in the art of gerotors, a plurality (n) of teeth 15 of
the star member 14 mesh with or engage with a larger plurality n + 1) of teeth or lobes
2 1 of the ring member 12 to define multiple fluid volume chambers (arrows 23). The
fluid volume chambers (arrows 23) are in fluid communication with the valve housing
section 70 of FIG. 1 through passages (not shown) defined by the wear plate 80 shown in
the same Figure.
[0020] The bolts 17 of FIG. 1 pass through the stationary end cap 24 (see FIG. 1) and
through a plurality of bolt holes 25 defined by the ring member 12 in order to fasten the
semi-plugged gerotor assembly 13 to the valve housing section 70 shown in FIG. 1.
Within the star member 14, the axial wall 62 intersects a radial floor 60 (see FIGS. 3 and
4) to thereby form a radial shelf, with the position of the radial shelf generally indicated
by reference number 44 in FIG. 3. As used herein, the term "axial wall" refers to a wall
extending in the same direction as the axis of rotation of the star member 14, and the term
"radial floor" refers to a floor extending in a direction perpendicular to the same axis.
[0021] The annular plug member 18 has a bore wall 19 forming a center bore as
indicated by arrow 27. The annular plug member 18 is positioned on the radial floor 60
shown in FIGS. 3 and 4. When the semi-plugged gerotor assembly 13 is installed in the
fluid control device 11 of FIG. 1 or another suitable device, a dynamic fluid seal is
formed between the annular plug member 18 and the stationary end cap 24 shown in that
Figure. Both the structure and the function of the annular plug member 18 are described
in detail with reference to FIGS. 3 and 4.
[0022] Assembly to various levels may be accomplished by circumscribing the star
member 14 with the ring member 12 such that the lobes 2 1 of the ring member 12 engage
with the teeth 5 of the star member 14. The o-ring 16 is positioned on the radial shelf 44
(see FIGS. 3 and 4) of the star member 14. The annular plug member 18 is then placed
on the o-ring 16 and the radial shelf 44. Subsequently connecting the assembled gerotor
assembly 13 to the stationary end cap 24 defines a high-pressure fluid channel (arrow 82
of FIG. 4) between the star member 14 and the end cap 24. The center opening (arrow
20) is then connected to a low-pressure fluid reservoir 40 as shown in FIG. 4, and the
fluid channel (arrow 82) of FIG. 4 is connected to a high-pressure fluid reservoir 30.
[0023] Referring to FIG. 3, a partial cross-sectional side view is shown of the fluid
device subassembly 10 of FIG. 1. FIG. 3 is not intended to be drawn to scale with
respect to FIGS. 1, 2, or 4, but rather to provide a close-up view of certain internal
structural portions of the fluid device subassembly 10. When the semi-plugged gerotor
assembly 13 is connected to the stationary end cap 24, a high-pressure fluid channel is
defined between an upper surface 52 of the star member 14 and the underside 50 of the
end cap 24. High-pressure fluid (arrow 31) enters the fluid channel, which is indicated
by arrow 82 in FIG. 4, causing sealing to occur as explained below with reference to FIG.
4.
[0024] The axial wall 62 and the radial floor 60 of the star member 14 form the radial
shelf 44, on which an o-ring 16 is disposed. The o-ring 16 forms a fluid seal between the
star member 14 and the annular plug member 18. The o-ring 16 may be constructed of a
suitable wear-resistant elastomeric material having a hardness level sufficient for
resisting extrusion in pressurized operation. In one embodiment, the o-ring 16 is
provided with a hardness level of at least approximately 90 durometer on the ASTM
D2240 type D scale, i.e., 90D hardness. Suitable materials at this hardness level may
include, without being limited to, Nitrile Butadiene Rubber (NBR), Hydrogenated NBR
(FfNBR), polyurethane, etc.
[0025] The annular plug member 18 is used to form a seal against an underside 50 of
the end cap 24, and may be constructed of steel, powder metal, high hardness resin-based
materials, or other suitable materials. The annular plug member 18, which has a
generally L-shaped cross section as shown, includes a first surface 66 and a second
surface 68, which are perpendicular with respect to each other to form a circumferential
notch 85 facing the annular radial shelf 44. The first surface 66 and a second surface 68
are both in direct contact with the o-ring 16, which is disposed at least partially within the
circumferential notch 85. A third surface 69 of the annular plug member 18 is in direct
frictional contact with the underside 50 of the end cap 24. As used herein, the term
"underside" refers to the particular major surface or side of the end cap 24 that is
positioned immediately adjacent to the star member 14 within the fluid control device 1
(see FIG. 1) in which the star member 14 is used.
[0026] The star member 14, the annular plug member 18, and the o-ring 16 rotate
together with respect to the stationary end cap 24. The center section or internal diameter
(ID) of the star member 14 defined by an inner wall 42 is connected to a low-pressure
fluid reservoir 40, and the all other sides of the star member 14 are connected to a highpressure
fluid reservoir 30. Both of the reservoirs 30 and 40 are shown schematically in
FIG. 4. The terms "low" and "high" are relative fluid pressures. In one embodiment,
"low pressure" may be approximately 0 to approximately 40 bar, while "high pressure" is
any pressure in excess of 40 bar. In another embodiment, 70 to 150 bar may be used as a
high pressure range, although high pressure could vastly exceed 150 bar depending on the
application. The placement and use of the annular plug member 18 and the o-ring 16 as
described herein helps to reduce leakage of high-pressure fluid (arrow 31) to the lowpressure
fluid reservoir 40 of FIG. 4.
[0027] Referring to FIG. 4 in conjunction with FIG. 3, the stationary end cap 24
extends to include the ring member 12 of FIG. 2, and is therefore shown in broken line
form in FIGS. 3 and 4. The inner wall 42 of the star member 14 defines the center
opening (arrow 20) of FIG. 4. The center opening (arrow 20) is in fluid communication
with the low-pressure fluid reservoir 40 of FIG. 3, such that low-pressure fluid (arrow 41)
is in communication with the o-ring 16, the annular plug member 18, and the end cap 24
via the center opening (arrow 20). The o-ring 16 may be preloaded to form a sufficient
seal against the star member 14 and the annular plug member 18.
[0028] The area of contact between the annular plug member 18 and the end cap 24
should be sufficiently large so as to reduce leakage past the end cap 24, the star member
14, and the o-ring 16 from the high-pressure side to the low-pressure side, and yet small
enough to minimize friction losses. Thus, the annular plug member 18 forms only a
partial plug, i.e., the term "semi-plugged" as used herein. In one embodiment, the
diameter of the center bore as defined by the bore wall 19 of the annular plug member 18
is between approximately 60% to approximately 75% of the outer diameter (OD) of the
annular plug member 18.
[0029] As noted above, the fluid device subassembly 10 shown in FIG. 4 is in fluid
communication with high-pressure fluid (arrows 31) delivered from the high-pressure
fluid reservoir 30. A high-pressure fluid channel (arrow 82), as shown in FIG. 4, is
defined between the underside 50 of the end cap 24 and an upper surface 52 of the star
member 14 as noted above, with the surfaces 50 and 52 being adjacent to each other.
[0030] The o-ring 16 is in fluid communication with the high-pressure fluid reservoir
30 of FIG. 4 via the high-pressure fluid channel (arrow 82), and with the low-pressure
fluid reservoir 40 via the center opening (arrow 20) of the star member 14. The size of a
gap (arrows 84 of FIG. 3) between an underside 64 of the annular plug member 18 and
the radial floor 60 of the star member 14 should be minimized to prevent extrusion of the
o-ring 16 to the low-pressure side during operation.
[0031] In operation, high-pressure fluid (arrows 31) enters the high-pressure fluid
channel (arrow 82) and pushes against the o-ring 16. This forces the annular plug
member 18 into frictional contact with the stationary end cap 24. Fluid leakage from the
high-pressure side to the low-pressure side may occur between the o-ring 16 and the star
member 14, between the o-ring 16 and the annular plug member 18, and/or between the
annular plug member and the end cap 24.
[0032] However, since the annular plug member 18 is only semi-plugged, as that
term is used herein, a relatively large contact area remains present between the annular
plug member 18 and the stationary end cap 24. Fluid leakage is reduced from highpressure
side to the low-pressure side relative to conventional gerotor star seal designs.
Additionally, since the contact area between the annular plug member 18 and the end cap
24 is relatively small in the present semi-plugged design relative to a solid-plug design,
frictional losses are concurrently reduced in this area. Overall efficiency is thereby
increased.
[0033] While the best modes for carrying out the invention have been described in
detail, those familiar with the art to which this invention relates will recognize various
alternative designs and embodiments for practicing the invention within the scope of the
appended claims.
CLAIMS
1. A gerotor assembly (13) for a fluid control device ( 1 1),
comprising:
a star member (14) having a number (n) of teeth (15), wherein the star
member (14) defines a center opening (20) of a first diameter which is connectable to a
low-pressure fluid reservoir (40);
a ring member (12) circumscribing the star member (14), and having a
number ( + ) of lobes (21) that mesh with the number (n) of teeth, wherein the ring
member (12) is configured to define, in conjunction with a stationary end cap (24) of the
fluid control device ( 11), a fluid channel (82) that is connectable to a high-pressure fluid
supply (30);
an annular gerotor plug member (18) that is circumscribed by the star
member (14), wherein the annular plug member (18) defines a center bore (27) of a
second diameter less than the first diameter; and
an o-ring (16) positioned between the star member (14) and the plug
member (18);
wherein the plug member ( 18) is configured to form a fluid seal against
the stationary end cap (24) of the fluid control device ( 1 1).
2. The gerotor assembly ( 13) of claim 1, wherein the star member
(14) defines a radial shelf (44), and wherein the o-ring (16) is positioned on the radial
shelf (44).
3. The gerotor assembly (13) of claim 3, wherein the plug member
(18) defines a circumferential notch (85) on a surface of the plug member (18) facing the
radial shelf (44), and wherein the o-ring (16) is disposed at least partially within the
circumferential notch (85).
4. The gerotor assembly (13) of claim 1, wherein the diameter of the
center bore (27) is approximately 60 percent to approximately 75 percent of the outer
diameter of the plug member (18).
5. A fluid control device ( 1 1) comprising:
a gerotor star member (14) defining a center opening (20) of a first
diameter, wherein the center opening (20) is in fluid communication with a low-pressure
fluid reservoir (40);
a gerotor ring member (12) circumscribing the star member (14), and
having a number + l ) of lobes (21) that engage with a number (n) of teeth (15) of the
star member (14);
an annular plug member (18) that is circumscribed by the star member
(14), wherein the annular plug member (18) defines a center bore (27) of a second
diameter less than the first diameter, and wherein the center bore (27) is in fluid
communication with the low-pressure fluid reservoir (40) via the center opening (20);
an o-ring (16) positioned between the star member (14) and the annular
plug member (18), wherein the o-ring (16) is in fluid communication with the highpressure
fluid reservoir (30) via a high-pressure fluid channel (82), and with the lowpressure
fluid reservoir (40) via the center opening (20); and
a valve housing section (70) having a stationary end cap (24) and a wear
plate (80), wherein the end cap (24) is positioned immediately adjacent to the annular
plug member ( 18) to thereby define the high-pressure fluid channel (82) in conjunction
with the star member (14), and wherein the high-pressure fluid channel (82) is in fluid
communication with a high-pressure fluid reservoir (30).
6. The fluid control device ( 11) of claim 5, wherein the star member
(14) defines a radial shelf (44), and wherein the o-ring (16) is disposed on the radial shelf
(44).
7. The fluid control device ( 11) of claim 6, wherein the annular plug
member (18) defines a circumferential notch (85) on a surface (66, 68) of the annular
plug member (18) facing the radial shelf (44), and wherein the o-ring (16) is disposed at
least partially within the circumferential notch (85).
8. The fluid control device ( 11) of claim 5, wherein the diameter of
the center bore (27) is approximately 60 percent to approximately 75 percent of the outer
diameter of the plug member (18).
9. The fluid control device ( 11) of claim 5, wherein the fluid control
device ( 11) is configured as a steering control unit for a hydrostatic power steering
system.
10. The fluid control device ( 11) of claim 5, wherein the o-ring (16) is
constructed of a wear-resistant elastomeric material having a hardness level of at least
approximately 90 durometer,