CONTROL VALVE FOR A VARIABLE DISPLACEMENT PUMP
This application is being filed on 05 March 2010, as a PCT
International Patent application in the name of Eaton Corporation, a U.S. national
corporation, applicant for the designation of all countries except the U.S., and
Navneet Gulati, a citizen of India, applicant for the designation of the U.S. only, and
claims priority to U.S. Utility Patent Application Serial No. 12/406,813 filed on 18
March 2009.
BACKGROUND
[0001] A variable displacement axial piston pump/motor includes a
swashplate against which axial pistons are slidably engaged. The swashplate is
adapted to pivot about an axis in order to increase or decrease the displacement of
the axial piston pump/motor.
[0002] Some axial piston pumps/motors include a controller that is adapted
to adjust the displacement of the swashplate in response to a pump/motor over-limit
condition (e.g., pressure, torque, etc.). These controllers typically provide flow to a
swashplate piston that is adapted to adjust the position of the swashplate relative to
the axis. However, accurate positioning of the swashplate in the axial piston
pump/motor in response to the over-limit condition can be difficult to attain.
SUMMARY
[0003] An aspect of the present disclosure relates to a fluid device having a
variable swashplate adapted for movement between a first position and a second
position. A control piston is adapted to selectively move the variable swashplate
between the first and second positions. A control valve is in fluid communication
with the control piston. The control valve includes a sleeve defining a spool bore, at
least one fluid inlet passage that is in fluid communication with a fluid source and at
least one control passage that is in fluid communication with the control piston. The
control fluid passage includes an opening at the spool bore. A spool is slidably
disposed in the spool bore of the sleeve. The spool includes a metering surface that
selectively communicates fluid between the fluid inlet passage and the control fluid
passage. The metering surface having a first end and an oppositely disposed second

end. The metering surface having a tapered surface disposed between the first and
second ends.
[0004] Another aspect of the present disclosure relates to a control valve of a
fluid device. The control valve includes a sleeve defining a spool bore and at least
one control passage. The control fluid passage has an opening at the spool bore.
The control valve further includes a spool slidably disposed in the spool bore of the
sleeve. The spool includes a metering surface having a first end, an oppositely
disposed second end and a tapered surface disposed between the first and second
ends. The tapered surface cooperates with the opening to define a variable orifice.
The tapered surface is adapted to provide a linear flow area per axial displacement
of the spool in the spool bore over a range of axial displacements of the spool in the
spool bore.
[0005] Another aspect of the present disclosure relates to a method to
compensate a fluid device in response to an over-limit condition. The method
includes providing the fluid device having a control valve in selective fluid
communication with a control piston that is adapted to adjust a displacement of the
fluid device. The control valve includes a spool having a metering surface and an
opening to a control fluid passage that is in fluid communication with the control
piston. The metering surface has a tapered surface. The method further includes
displacing the spool in the control valve to define a flow area between the tapered
surface and the opening, where fluid enters the control fluid passage through the
flow area.
[0006] A variety of additional aspects will be set forth in the description that
follows. These aspects can relate to individual features and to combinations of
features. It is to be understood that both the foregoing general description and the
following detailed description are exemplary and explanatory only and are not
restrictive of the broad concepts upon which the embodiments disclosed herein are
based.
DRAWINGS
[0007] FIG. 1 is a schematic representation of a fluid device having
exemplary features of aspects in accordance with the principles of the present
disclosure.

[0008] FIG. 2 is a schematic representation of the fluid device of FIG. 1
showing a swashplate in a second position.
[0009] FIG. 3 is an enlarged schematic representation of a control system of
the fluid device of FIG. 1.
[0010] FIG. 4 is an enlarged schematic representation of the control system
of FIG. 3.
[0011] FIG. 5 is a schematic representation of a spool suitable for use in the
control system of FIG. 3.
[0012] FIG. 6 is an exemplary graphical representation of flow area of the
control valve of FIG. 3 versus the axial displacement of a spool of the control valve.
[0013] FIG. 7 is an enlarged view of the graph of FIG. 6.
[0014] FIG. 8 is an exemplary embodiment of the control valve shown
schematically in FIG. 3.
[0015] FIG. 9 is a cross-sectional view of the control valve taken on line 9-9
of FIG. 7.
[0016] FIG. 10 is an exemplary graphical representation of gain of the
control valve of FIG. 3 versus the axial displacement of the spool.
DETAILED DESCRIPTION
[0017] Reference will now be made in detail to the exemplary aspects of the
present disclosure that are illustrated in the accompanying drawings. Wherever
possible, the same reference numbers will be used throughout the drawings to refer
to the same or like structure.
[0018] Referring to FIGS. 1 and 2, a fluid device, generally designated 10, is
shown. The fluid device 10 includes a housing, generally designated 12, defining a
pumping chamber 14. A rotating group, generally designated 16, is disposed in the
pumping chamber 14 of the housing 12. The rotating group 16 is adapted to rotate
about a rotational axis 18. In the depicted example of FIG. 1, the rotational axis 18
is offset from the longitudinal axis 20 of the fluid device 10.
[0019] The rotating group 16 is engaged to an input shaft 22. In one aspect
of the present disclosure, the rotating group 16 includes a plurality of internal splines
that are engaged to a plurality of external splines disposed on the input shaft 22.

[0020] In one aspect of the present disclosure, the rotating group 16 includes
a cylinder barrel, generally designated 28, defining a plurality of cylinder bores 30.
A plurality of pistons 32 is adapted to reciprocate in the plurality of cylinder bores
30 when the cylinder barrel 28 is rotated about the rotating axis 18 and the fluid
device 10 is at some displacement other than zero. The plurality of cylinder bores
30 and the plurality of pistons 32 cooperatively define a plurality of volume
chambers 34. When the displacement of the fluid device 10 is as some displacement
other than zero, at least one of the plurality of volume chambers 34 contracts while
at least one of the plurality of volume chambers 34 expands. Fluid enters the
expanding volume chambers 34 and is expelled from the contracting volume
chambers 34 during rotation of the rotating group 16.
[0021] The plurality of pistons 32 includes axial ends 36 that are engaged
with a plurality of slippers 38. The plurality of slippers 38 is disposed against a first
surface 40 of a rotationally-stationary swashplate 42. As the rotating group 16
rotates about the rotational axis 18, the slippers 38 slide about the first surface 40 of
the swashplate 42.
[0022] While the swashplate 42 is rotationally stationary with respect to the
rotating axis 18, the position of the swashplate 42 is variable. In one aspect of the
present disclosure, the swashplate 42 is adapted to tilt or pivot about a transverse
axis 44 (shown as an X in FIG. 1) in order to increase or decrease the displacement
of the fluid device 10. In one aspect of the present disclosure, the transverse axis 44
is generally perpendicular to the rotational axis 18 of the rotating group 16. As the
displacement of the fluid device 10 increases, the amount of fluid that enters and is
expelled from the rotating group 16 increases. As the displacement of the fluid
device 10 decreases, the amount of fluid that enters and is expelled from the rotating
group 16 decreases.
[0023] The swashplate 42 is movable between a first position (shown in FIG.
1) and a second position (shown in FIG. 2). In one aspect of the present disclosure,
the first position is a full displacement or full stroke position. In another aspect of
the present disclosure, the second position is a neutral position. In the neutral
position, the first surface 40 of the swashplate 42 is generally perpendicular to the
rotational axis 18 of the rotating group 16. In this position, the amount of fluid
displaced by the rotating group 16 per revolution is about zero in3/rev.

[0024] Referring now to FIGS. 1-4, the fluid device 10 further includes a
control system 46. In one aspect of the present disclosure, the control system 46 is
adapted to adjust the displacement of the fluid device 10 based on the output torque
of the fluid device 10. If the output torque of the fluid device 10 exceeds a limit, the
control system 46 reduces the displacement of the fluid device 10 (or destrokes the
fluid device 10) to bring the output torque of the fluid device 10 within an
acceptable range.
[0025] In one aspect of the present disclosure, the control system 46 includes
a controller assembly 50 that is adapted to adjust the position of the swashplate 42
between the first and second positions. The controller assembly 50 includes a
control piston 52 and a control valve 54 that is in fluid communication with the
control piston 52.
[0026] The control piston 52 is slidably disposed in a piston bore 56 of the
housing 12. The control piston 52 includes a first axial end portion 58 and a second
axial end portion 60. The control piston 52 is disposed in the piston bore 56 such
that the first axial end portion 58 of the control piston 52 is adjacent to the first
surface 40 of the swashplate 42. In one aspect of the present disclosure, the first
axial end portion 58 of the control piston 52 is immediately adjacent to the first
surface 40 of the swashplate 42. In one aspect of the present disclosure, the control
piston 52 is adapted to extend from the piston bore 56 in response to fluid
communicated to the second axial end portion 60 of the control piston 52 through
the control valve 54. As the control piston 52 extends from the piston bore 56, the
first axial end portion 58 acts against the first surface 40 of the swashplate 42 and
causes the swashplate 42 to pivot toward the second position.
[0027] In another aspect of the present disclosure, a spring 62 is disposed in
the housing 12 such that the spring 62 is adjacent to a second surface 64 of the
swashplate 42, which is oppositely disposed from the first surface 40. The spring 62
biases the control piston 52 to the retracted position when fluid is not being
communicated to control piston 52 and biases the swashplate 42 to the first position.
[0028] Referring now to FIGS. 3-5, the control valve 54 includes a spool 70
that is slidably disposed in a spool bore 72, In one aspect of the present disclosure,
the spool bore 72 is defined by a sleeve 74 of the control valve 54. In one aspect of
the present disclosure, the control valve 54 defines at least one fluid inlet passage 76
that is in fluid communication with a fluid source (e.g., a fluid discharge port of the

fluid device 10, etc.) and at least one control fluid passage 78 that is in fluid
communication with the piston bore 56. The fluid inlet passage 76 includes an inlet
opening 80 at the spool bore 72 while the control fluid passage 78 includes an
opening 82 at the spool bore 72. In one aspect of the present disclosure, there are at
least two control fluid passages 78 with each control fluid passage 78 having the
opening 82. In another aspect of the present disclosure, there are at least four
control fluid passages 78. In another aspect of the present disclosure, the opening 82
at the spool bore 72 is generally circular in shape.
[0029] The spool 70 includes a first axial end 83 and an oppositely disposed
second axial end 84. The spool 70 further includes a metering surface 86 that is
disposed between the first and second axial ends 83, 84. The metering surface 86 is
adapted to selectively block fluid communication between the fluid inlet passage 76
and the control fluid passage 78. It will be understood, however, that the term
"block" as used herein allows for leakage across the metering surface 86 of the spool
70 as a result of clearances between the spool 70 and the spool bore 72.
[0030] The metering surface 86 extends between a first end 88 and a second
end 90. The metering surface 86 includes an outer surface 91 that is generally
cylindrical in shape.
[0031] In one aspect of the present disclosure, the spool 70 is biased by a
spring 92 to a first position in which the fluid inlet passage 76 is blocked from fluid
communication with the control fluid passage 78 by the metering surface 86. In the
depicted schematic of FIGS. 3 and 4, the spring 92 acts against the second axial end
84ofthespool70.
[0032] The pressure of the fluid from the fluid source (e.g., the discharge
port of the fluid device 10) acts on the spool 70 in the spool bore 72 in a direction
opposite from the direction of the force applied to the spool 70 from the spring 92.
When the pressure of the fluid from the fluid source increases such that the force
applied to the spool 70 by the fluid is greater than the force applied to the spool 70
by the spring 92, the spool 70 is axially displaced from the first position in the spool
bore 72. As the spool 70 is axially displaced from the first position in the spool bore
72, the metering surface 86 at least partially uncovers the opening 82 of the control
fluid passage 78. As the metering surface 86 uncovers the opening 82, the spool 70
allows for fluid communication between the fluid inlet passage 76 and the control
fluid passage 78. As the pressure of the fluid source increases, the spool 70 is

further displaced in the spool bore 72 so that the metering surface 86 uncovers more
of the opening 82. In one aspect of the present disclosure, the spool 70 is displaced
to a second position in which the opening 82 is fully uncovered.
[0033] The metering surface 86 of the spool 70 and the opening 82 of the
control fluid passage 78 cooperatively define a variable orifice 94. The variable
orifice 94 defines a variable flow area through which fluid can pass into the control
fluid passage 78. With the spool 70 in the first position, the flow area of the variable
orifice 94 is zero. As the spool 70 is axially displaced in the spool bore 72 away
from the first position, the flow area of the variable orifice 94 increases. The size of
the flow area of the variable orifice 94 affects the volumetric flow rate Q of fluid
passing through the control fluid passage 78 to the control piston 52.
[0034] The volumetric flow rate Q is characterized by the following
equation:
where Q is the volumetric flow rate of fluid passing through the variable orifice 94
to the control piston 52, Cd is a discharge coefficient, p is the density of the fluid, ΔP
is the pressure differential across the flow area, A is the flow area of the variable
orifice 94 through which the fluid passes. The stability of the control system 46 is
directly dependent on the volumetric flow rate of the fluid from the control valve 54
to the control piston 52.
[0035] In the present disclosure, the term "stability" refers to a generally
oscillation-free response of the swashplate 42, which is adapted to provide a
predictable response of the control system 46 to over-limit conditions (e.g.,
exceeding torque limit, pressure limit, etc.) of the fluid device 10. For example, if
pressurized fluid from the inlet fluid passage 76 overcomes the force of the spring
92 acting on the spool 70 thereby opening the control fluid passage 78 and if the
flow area of the variable orifice 94 is too large, the volumetric flow rate Q of the
fluid passing through the flow area of the variable orifice 94 will be too high. As a
result, the control piston 52 will respond too quickly to the fluid passing through the
control fluid passage 78, which may cause the control piston 52 to overcompensate
for the fluid provided through the control fluid passage 78 and thereby over adjust
the swashplate 42. Following this over-adjustment, the extra fluid in the piston bore

56 will be drained in an attempt to position the swashplate 42 to the desired position.
If, on the other hand, the flow area of the variable orifice 94 is too small, the
volumetric flow rate Q of the fluid passing through the flow area of the variable
orifice 94 will be too low. As a result, the control piston 52 will respond too slowly
to the over-limit condition.
[0036] In addition to the size of the flow area of the variable orifice 94, the
stability of the fluid device 10 is also affected by the temperature of the fluid. As
the temperature of the fluid increases, the viscosity of the fluid decreases. As the
viscosity of the fluid decreases, the volume of fluid that can flow through the flow
area of the variable orifice 94 during a given time interval (At) increases. As the
volume of fluid flowing through the flow area of the variable orifice 94 increases,
the response rate of the control piston 52 increases. In some situations, this
increased response rate may result in the fluid device 10 becoming unstable.
[0037] In one aspect of the present disclosure, the control system 46 is
stabilized by providing a tapered surface 96 at a leading edge portion 98 of the
metering surface 86 of the spool 70. In one aspect of the present disclosure, the
tapered surface 96 of the metering surface 86 of the spool 70 reduces the risk of
instability of the control system 46 when fluid (e.g., hydraulic fluid, oil, etc.) at high
temperatures (e.g., >140 degrees F) is used in the fluid device 10.
[0038] The tapered surface 96 of the metering surface 86 of the spool 70 is
adapted to cooperate with the opening 82 of the control fluid passage 78 to define a
flow area that reduces flow to the control piston 52 at small axial displacements of
the spool 70 as compared to a flow area defined by the opening 82 and a metering
surface of a spool without a tapered surface 96. The tapered surface 96 and the
opening 82 cooperate to define a generally linear gain (shown in FIG. 10) of the
control system 46 for small axial displacements of the spool 70, where the gain of
the control system 46 is defined by the flow area divided by the axial displacement
of the spool 70. In one aspect of the present disclosure, the tapered surface 96 and
the opening 82 cooperate to define a generally constant gain of the control system 46
for small axial displacements of the spool 70.
[0039] The tapered surface 96 extends a length / from a first edge 100 to a
second edge 102, which is disposed between the first end 88 and the second end 90
of the metering surface 86. In the depicted examples of FIGS. 1-5, the first edge 100
is disposed on the first end 88 of the metering surface 86. In one aspect of the

present disclosure, the length / is greater than 0.010 inches. In another aspect of the
present disclosure, the length / is greater than or equal to 2% of the outer diameter of
the metering surface 86. In another aspect of the present disclosure, the length / is in
the range of about 2% to about 5% of the outer diameter of the metering surface 86.
[0040] The tapered surface 96 includes an angle 6. The angle 0 of the
tapered surface 92 flares outwardly in a direction from the first edge 100 to the
second edge 102 so that the outer diameter of the tapered surface 96 at the first edge
100 is less than the outer diameter of the tapered surface 96 at the second edge 102.
[0041] The angle is an oblique angle. In one aspect of the present
disclosure, the angle can be calculated using the following equation 104:

where θ is the angle of the taper surface 96, n is the number of openings 82 in the
spool bore 72, r is the radius of each of the openings 82, D is the diameter of the
metering surface 86 of the spool 70, and / is the axial length of the tapered surface
96. In one aspect of the present disclosure, the angle θ is less than 30 degrees.
[0042] Referring now to FIGS. 6 and 7, a graph of the flow area of the
variable orifice 94 versus the axial position of the spool 70 in the spool bore 72 is
shown. The graph includes two curves. A first curve 106 plots the flow area of the
variable orifice 94 versus the axial position of the spool 70, where the spool 70
includes the tapered surface 96. A second curve 108 plots the flow area of the
variable orifice 94 versus the axial position of the spool 70, where the spool 70 does
not include the tapered surface 96.
[0043] As shown in the graph, the spool 70 with the tapered surface 96
reduces the flow area of the variable orifice 94 during an initial axial displacement
(i.e., measured from the edge of the opening 82 to the second edge 102 of the
tapered surface 96) of the spool 70 as compared to the spool 70 without the tapered
surface 96. This reduction in flow area of the variable orifice 94 reduces the risk of
a high volumetric flow rate Q being provided to the control piston 52 as a result of a
small displacement of the spool 70.
[0044] During the initial displacement of the spool 70 with the tapered
surface 96, the flow area of the variable orifice 94 is equal to the area defined
between the edge of the opening 82 and the tapered surface 96 of the spool 70

provided that the angle θ is less than or equal to the angle calculated using equation
104. As this area is less than the area of the opening 82 uncovered by the spool 70,
the risk of a high volumetric flow rate of fluid being communicated to the control
piston 52 is reduced. If the angle θ is greater than the angle calculated using
equation 104, the flow area of the variable orifice 94 will be generally equal to the
area of the opening 82 that is uncovered by the spool 70 and will be generally equal
to the spool 70 without the tapered surface 96.
[0045] After the spool 70 has been displaced a distance greater than the axial
length / of the tapered surface 96, the flow area of the variable orifice 94 is generally
equal to the area of the opening 82 that is uncovered by the spool 70. During this
displacement region, the tapered surface 96 has limited affect on the flow area of the
variable orifice 94.
[0046] In one aspect of the present disclosure, the axial length / of the
tapered surface 96 is less than or equal to the diameter of the opening 82 of the
control fluid passage 78. In another aspect of the present disclosure, the axial length
/ of the tapered surface 96 of the spool 70 is less than or equal to 10% of the
diameter of the opening 82. In another aspect of the present disclosure, the axial
length / of the tapered surface 96 of the spool 70 is less than or equal to 5% of the
diameter of the opening 82. In another aspect of the present disclosure, the axial
length / of the tapered surface 96 of the spool 70 is less than or equal to 0.030
inches. In another aspect of the present disclosure, the axial length / of the tapered
surface 96 of the spool 70 is less than or equal to 0.020 inches.
[0047] Referring now to FIGS. 8 and 9, an exemplary control valve 54 is
shown. The control valve 54 includes the spool 70 disposed in the spool bore 72 of
the sleeve 74. The sleeve 74 defines the fluid inlet passage 76 and the control fluid
passage 78 that is in fluid communication with the piston bore 56. The fluid inlet
passage 76 includes the inlet opening 80 at the spool bore 72 while the control fluid
passage 78 includes the opening 82 at the spool bore 72.
[0048] The spool 70 includes the metering surface 86. The metering surface
86 includes the tapered surface 96. In the depicted example of FIGS. 8 and 9, the
metering surface 86 further includes a groove 110 that extends circumferentially
around the metering surface 86. The groove 110 is disposed between the tapered
surface 96 and the second end 90 of the metering surface 86. The groove 110 is

adapted for pressuring balancing the spool 70 in a radial direction in the spool bore
72.
[0049] Referring now to FIG. 10, a method for determining the dimensions
of the parameters of the tapered surface 96 will be described. The values of the
parameters, such as the number of metering holes n, the radius of the metering holes
r and the diameter D of the spool 70 are determined. In one aspect of the present
disclosure, these values are determined using a root-locus approach. In another
aspect of the present disclosure, these values are determined using a loop-shaping
approach.
[0050] A gain 112 versus axial displacement of the spool 70 is graphed. As
previously provided, gain 112 is the measure of flow area of the variable orifice 94
versus the axial displacement of the spool 70. The flow area is calculated for a spool
70 without a tapered surface 96. The gain 112 for the spool 70 without the tapered
surface 96 is shown in FIG. 10. The gain 112 includes a first portion 114 and a
second portion 116. The first portion 114 is nonlinear. The second portion 116 is
nonlinear although the best curve fit through the second portion is a straight line.
The length / of the tapered surface 96 is determined at the location where the first
and second portions intersect. In the example shown in FIG. 10, this location is at a
spool position of 0.020 inches.
[0051] The angle 6 for the tapered surface 96 is calculated using the equation
104. The angle 8 of the tapered surface 96 is then cut into the spool 70 such that the
angle 6 is less than or equal to the value provided by the equation 104. With the
angle 6 of the tapered surface 96 less than or equal to the value provided by the
equation 104, the metered surface 86 has a gain 118 (shown in FIG. 10). The gain
118 includes a first portion 120 and a second portion 122. The first portion 120 is
generally linear over a range of axial displacements of the spool 70. In one aspect of
the present disclosure, the range of axial displacements of the spool 70 over which
the first portion 120 is generally linear is equal to the axial length I of the tapered
surface 96. In one aspect of the present disclosure, the gain 120 is generally
constant over a range of axial displacements of the spool 70. This generally constant
gain 120 provides a linear increase in volumetric flow rate Q as the spool 70 is
displaced in the spool bore 72. In one aspect of the present disclosure, the range of
axial displacements of the spool 70 over which the first portion 120 is generally
constant is equal to the axial length / of the tapered surface 96.

[0052] Various modifications and alterations of this disclosure will become
apparent to those skilled in the art without departing from the scope and spirit of this
disclosure, and it should be understood that the scope of this disclosure is not to be
unduly limited to the illustrative embodiments set forth herein.

We claim:

1. A fluid device comprising:
a variable swashplate adapted for movement between a first position and a
second position;
a control piston adapted to selectively move the variable swashplate between
the first and second positions;
a control valve in fluid communication with the control piston, the control
valve including:
a sleeve defining a spool bore, at least one fluid inlet passage that is
in fluid communication with a fluid source and at least one
control fluid passage that is in fluid communication with the
control piston, the control fluid passage having an opening at
the spool bore; and
a spool slidably disposed in the spool bore of the sleeve, the spool
including a metering surface that selectively communicates
fluid between the fluid inlet passage and the control fluid
passage, the metering surface having a first end and an
oppositely disposed second end, the metering surface having a
tapered surface disposed between the first and second ends.
2. The fluid device of claim 1, wherein the tapered surface includes an angle
that flares outwardly from the first end to the second end.
3. The fluid device of claim 2, wherein the angle is a function of at least one
dimension of the opening.
4. The fluid device of claim 2, wherein the angle is less than or equal to
where r is the radius of the opening, n is the number of
control fluid passages disposed in the sleeve, D is a diameter of the spool, and / is an
axial length of the tapered surface on the metering surface.

5. The fluid device of claim 2, wherein the angle of the tapered surface is less
than 30 degrees.
6. The fluid device of claim 2, wherein the angle of the tapered surface is
adapted to provide a linear gain over a range of axial displacements of the spool.
7. The fluid device of claim 6, wherein the angle of the tapered surface is
adapted to provide a constant gain over the range of axial displacements of the
spool.
8. The fluid device of claim 1, wherein the opening is generally circular.
9. The fluid device of claim 1, wherein the metering surface defines a groove
that is adapted to pressure balance the spool in the spool bore in a radial direction.
10. A control valve of a fluid device comprising:
a sleeve defining a spool bore and at least one control fluid passage, the
control fluid passage having an opening at the spool bore; and
a spool slidably disposed in the spool bore of the sleeve, the spool including
a metering surface having a first end, an oppositely disposed second end and a
tapered surface disposed between the first and second ends, the tapered surface
cooperating with the opening to define a variable orifice, wherein the tapered surface
is adapted to provide a linear flow area per axial displacement of the spool in the
spool bore over a range of axial displacements of the spool in the spool bore.
11. The control valve of claim 10, wherein the tapered surface is adapted to
provide a constant flow area per axial displacement of the spool in the spool bore
over a range of axial displacements of the spool in the spool bore.
12. The control valve of claim 10, wherein the tapered surface includes an
oblique angle.
13. The control valve of claim 12, wherein the angle is a function of at least one
dimension of the opening.

14. The control valve of claim 13, wherein the angle is less than or equal to
where r is the radius of the opening, n is the number of
control fluid passages disposed in the sleeve, D is a diameter of the spool, and / is an
axial length of the tapered surface on the metering surface.
15. A method for compensating a fluid device in response to an over-limit
condition, the method comprising:
providing the fluid device having a control valve in selective fluid
communication with a control piston adapted to adjust a displacement of the fluid
device, the control valve including a spool having a metering surface and an opening
to a control fluid passage that is in fluid communication with the control piston, the
metering surface having a tapered surface; and
displacing the spool in the control valve to define a flow area between the
tapered surface and the opening, wherein fluid enters the control fluid passage
through the flow area.
16. The method of claim 15, wherein the tapered surface includes an oblique
angle.
17. The method of claim 16, wherein the angle is less than or equal to
where r is the radius of the opening, n is the number of
control fluid passages disposed in the control valve, D is a diameter of the spool, and
/ is an axial length of the tapered surface on the metering surface.
18. The method of claim 16, wherein the angle of the tapered surface is adapted
to provide a flow area per axial displacement of the spool over a range of axial
displacements of the spool that is linear.

19. The method of claim 16, wherein an upper limit of the range of axial
displacement of the spool is less than or equal to a value that is 20% of the diameter
of the spool.
20. The method of claim 16, wherein the angle of the tapered surface is adapted
to provide a flow area per axial displacement of the spool over a range of axial
displacements of the spool that is constant.

A fluid device includes a variable swashplate adapted for movement between
a first position and a second position. A control piston is adapted to selectively
move the variable swashplate between the first and second positions. A control
valve is in fluid communication with the control piston. The control valve includes
a sleeve defining a spool bore, at least one fluid inlet passage in fluid
communication with a fluid source and at least one control passage in fluid
communication with the control piston. The control fluid passage includes an
opening at the spool bore. A spool is disposed in the spool bore of the sleeve. The
spool includes a metering surface that selectively communicates fluid between the
fluid inlet passage and the control fluid passage. The metering surface has a first
end and a second end. The metering surface having a tapered surface disposed
between the first and second ends.