Manual displacement control

A manual displacement control is provided for varying the displacement of a hydraulic pump. The pump has a tiltable swashplate and is used in a hydraulic system which includes a control valve having an axially movable spool for metering control fluid to modulate the swashplate position. A displacement control linkage has a non-linear coupling element and a resilient override spring serially connected between the input lever and the valve spool. A torsional centering spring is connected between the input lever and the override spring to generate a feedback force which opposes a control force applied thereto and urges the input lever to a position representative of the no flow position of the pump swashplate. An adjustable connection is provided for centering the valve spool while maintaining a corresponding neutral position of the input lever and swashplate during assembly of the displacement control.

TECHNICAL FIELD 
This invention relates to a manual displacement control for a variable 
displacement hydraulic pump and, more particularly, to a versatile control 
linkage having improved performance and assembly capabilities. 
BACKGROUND ART 
Conventional variable displacement hydraulic pumps have a rotating cylinder 
block with axially movable pistons which engage a tiltable swashplate for 
varying the stroke of the pistons. The displacement of the hydraulic pump 
is proportional to the stroke of the pistons within the cylinder block 
and, therefore, the tilt angle of the swashplate. When the swashplate is 
not tilted with respect to a "no flow" position, the pistons are not 
stroked and the pump has a zero displacement. The zero displacement 
position of the swashplate represents a neutral mode of operation of the 
pump. 
In order to selectively prescribe the position of the swashplate, 
displacement controls are used to vary the swashplate position in response 
to a command input. Displacement controls take many forms, but in most 
cases they allow an operator to manually select a desired swashplate 
position and the corresponding hydraulic pump displacement. 
Many displacement controls include a fluid-metering control valve having an 
axially movable spool which is displaced in response to a command input. 
Displacement of the valve spool away from an axially centered position 
results in the interconnection of output ports formed on the valve with a 
source of pressurized control fluid, such that the control fluid 
appropriately is metered to a servo mechanism for effecting an angular 
displacement of the swashplate. A command input typically is transmitted 
to the control valve through an input linkage having a remote manual input 
lever, such as a pivoted hand lever or a foot pedal. Rotation of the input 
lever, such as the rotation of a foot pedal in response to the application 
of a control force, causes an axial displacement of the valve spool and a 
resultant change in the position of the swashplate. 
During the time that a control force is applied to the input lever, it is 
important that the control force be reacted so that a positive feedback 
force is developed to oppose rotation of the input lever and provide an 
operator with some measure of the amount of swashplate modulation which is 
occurring. The feedback force requirement commonly is satisfied by the use 
of a centering spring mounted on the control valve and interconnected 
between the valve spool and the control valve housing. Displacement of the 
valve spool generates an axial biasing force in the centering spring, such 
that the valve spool is biased toward a position representative of the no 
flow position of the swashplate. The centering force is transmitted 
through the input linkage to oppose the control force and urge the input 
lever to a position corresponding to the zero pump displacement position 
of the swashplate. 
In order to close the servo control loop between the control valve and the 
swashplate, a feedback linkage interconnects the swashplate with the valve 
spool and is operative to convert the relative displacement of the 
swashplate and valve spool to a feedback force which counteracts the 
control force acting on the spool and holds the valve spool in a centered, 
steady-state fluid metering position. 
It also is known to utilize a resilient override spring element in the 
input linkage to yieldably apply a force to the control valve in a manner 
which prevents an excessive manual force from being applied to the valve. 
In order to prevent excessive input force from being applied to the 
control valve during sudden control inputs, the override spring deflects 
and limits the force on the control valve. The force level at which the 
override spring deflects is a function of the stiffness and amount of 
initial compression of the override spring. 
Deflection of the override spring also permits continued displacement of 
the input lever when the valve spool has reached a physical travel limit. 
The spring preload force must be sufficiently low to prevent excessive 
feedback force from being transmitted to the lever once a travel limit is 
encountered. 
During operation of the pump, metal chips or particles may become lodged 
within the control valve and obstruct displacement of the valve spool. 
This is a particularly undesirable condition when the control valve is 
jammed in a position which holds the swashplate fixed in a maximum 
displacement condition. Input force can be applied rapidly to attempt to 
shear the particles and overcome the obstruction. 
The use of an input lever-centering spring and an override spring in series 
presents a problem. In known devices, the torque which is applied to the 
input lever during normal operation of the device, that is, with the 
override spring not deflected, is determined largely by the stiffness of 
the centering spring. Normally, the force generated by the centering 
spring is just high enough to overcome friction between the valve spool 
and the valve housing and to center the input lever. 
In some vehicle applications of hydrostatic pumps, however, it is desired 
to have a much higher centering spring torque on the input lever to 
provide greater force feedback to an operator of the input lever. If the 
centering spring stiffness is increased, the increased force level must be 
transmitted through the override spring, requiring that the torque level 
of the override spring be increased to prevent the override spring from 
deflecting during normal operation of the control. This requirement places 
a penalty on the control design in terms of the space required to house 
the override spring and in an increased input torque when the override 
spring deflects. 
Another problem which exists in known devices is the inability to easily 
align the centered position of the valve spool with the corresponding 
"neutral" positions of both the input lever and the swashplate. The input 
linkage and the feedback linkage commonly are connected to the valve spool 
by means of a summing link, with the feedback linkage including a first 
link pivotally connected between the summing link and the swashplate, and 
the input linkage including a second link pivoted between the input lever 
and the summing link. The summing link, in turn, is pivoted to the valve 
spool. In order for the centered position of the valve spool to align with 
the corresponding neutral positions of both the input lever and the 
swashplate, the pivotal connections of the first and second links and the 
valve spool must lie along the summing link when the valve spool is in the 
centered position. 
The alignment problem arises due to the fact that the centered position of 
the swashplate and the input lever are determined independently of the 
position of the summing link. The centered position of the swashplate is 
established during assembly of the pump and is maintained by a pair of 
swashplate centering springs symmetrically connected between the 
swashplate and the pump. The centered position of the input lever is also 
established by a centering spring, as previously described. Thus, the 
locations of the pivots on the first and second links specify the position 
of the summing link at the respective neutral positions of the input lever 
and swashplate. In order, then, to connect the summing link to the valve 
spool, it is necessary to adjust the axial position of the valve spool 
relative to the summing link and away from the centered spool position to 
line up the connection points on the valve spool and the summing link. 
One approach to solving this problem has been to connect the summing link 
with the centered valve spool prior to connecting the input linkage with 
the summing link. The position of the summing link at a neutral condition 
then is determined by the centered position of the first link and the 
valve spool. The centered position of the input lever then is adjusted to 
conform with the prescribed summing link geometry. This proposal generates 
additional problems, however, in applications in which the input linkage 
includes an intermediate coupling which defines a non-linear response 
characteristic relationship between displacements of the input lever and 
the valve spool. By varying the neutral position of the input shaft, which 
in turn drives the coupling, the neutral position of the input lever is 
offset from the center of the predetermined response curve and results in 
unacceptable operating characteristics of the pump control. 
This invention is directed toward overcoming the problems set forth above 
in a novel and useful way. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide an improved manual displacement 
control for a variable displacement hydraulic pump which provides an input 
lever centering force which is independent of the force characteristics of 
an associated override spring. An adjustable connection is provided to 
facilitate the precise assembly of an input lever with the pump swashplate 
and a fluid-metering valve spool. 
In an exemplary embodiment of the invention, a manual displacement control 
is provided for varying the position of a tiltable swashplate in a 
hydraulic pump. The pump is used in a hydraulic system which includes a 
source of control fluid, a servo mechanism coupled to the pump, and a 
control valve for metering control fluid to the servo mechanism to 
modulate the swashplate position. A resilient displacement control linkage 
transmits a command input from a manual input lever to an axially movable 
spool in the control valve. A feedback linkage interconnects the 
swashplate and the valve spool to transmit a feedback signal to the 
control valve and close the servo control loop. 
The displacement control linkage has a non-linear coupling element and a 
resilient override spring serially connected between the input lever and 
the feedback means. An input link is driven by the lever and provides a 
displacement input to the non-linear control element. The non-linear 
coupling has an output link which is displaced in non-linear proportion to 
the input displacement. The override spring resiliently couples the output 
link with an intermediate link which, in turn, is connected to the valve 
spool to apply a yieldable force to the control valve in response to a 
displacement of the input lever. 
A torsional centering spring is connected between the input lever and the 
override spring and generates a feedback force which opposes a control 
force applied thereto and urges the input lever to a position 
representative of the no flow position of the pump swashplate. Because the 
centering spring is positioned upstream of the override spring, it is 
possible to vary the feedback centering force applied to the input lever 
independently of the stiffness of the override spring. 
An adjustable connection means is provided on the control valve for 
centering the valve spool while maintaining a corresponding neutral 
position of the input lever and swashplate during assembly of the 
displacement control. The adjustable connection means is constructed such 
that the zero or centered position of the input lever can be precisely 
aligned with the centered position of the non-linear coupling to ensure 
that manual displacements of the input lever cause the desired 
displacement of the output link of the non-linear coupling. 
The adjustable connection means includes a threaded connector positioned 
between the valve spool and the displacement control linkage. A connector 
block is coupled to a threaded portion of the valve spool and is connected 
to the displacement control linkage. The valve spool has a flat end 
portion which engages a slotted rotatable element positioned at one end of 
the valve housing. An adjuster screw projects from the slotted element and 
extends through the valve housing, such that rotation of the adjuster 
screw causes relative axial displacement between the valve spool and the 
connector block to vary the relative axial position of the valve spool and 
the control linkage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring first to FIG. 1, a hydrostatic transmission, generally designated 
10, includes a reversible variable displacement hydraulic unit 12 and a 
fixed displacement hydraulic unit 14. Hydraulic units 12 and 14 are axial 
piston devices, with hydraulic unit 12 functioning as a pump and hydraulic 
unit 14 functioning as a motor. The transmission is adapted to be driven 
by a prime mover or engine (not shown) through an input shaft 16 connected 
to pump 12. The pump is connected to motor 14 by main loop fluid lines 18 
and 20 to transmit power from input shaft 16 to motor output shaft 22. An 
angularly positioned swash plate 24 on the pump modifies both the amount 
of flow and the direction of flow of the pump output in a known manner. 
Lines 18 and 20 are provided with high pressure relief valves 25, and a 
shuttle valve 26 is connected to a pressure relief valve 28. Swashplate 
centering springs 29 bias the swashplate toward a no flow, zero 
displacement position and will be clearly understood from the following 
discussion. 
As is known, the transmission is provided with a charge pump 30 which is 
driven by input shaft 16. The output of charge pump 30 is controlled by a 
charge pressure relief valve 32 and flows through check valve 34 or 36 to 
either line 18 or 20, depending on which is at low pressure, to replenish 
lost hydraulic fluid to the transmission main loop. The output of the 
charge pump also is directed to a control fluid line 38. 
Variable displacement pump 12 is provided with a displacement control valve 
40 and servo means, generally indicated at 42, for varying the position of 
swashplate 24. Control valve 40 includes a stationary valve body 44 which 
defines an internal axial valve bore 46. An opening 48 is formed in the 
valve body to communicate control fluid in line 38 with the valve bore 
through a flow restricting orifice 50. The valve body also has a pair of 
openings 52 and 54 communicating with servo means 42 by servo lines 56 and 
58, respectively. A movable valve spool 60 is located within valve bore 46 
and may be axially displaced with respect to the valve bore to control 
flow to servo means 42. Stops 61 are positioned at each end of the valve 
body to limit displacement of valve spool 60, and a relatively light 
anti-backlash spring 62 applies an axial biasing force tending to oppose 
small deviations of the valve spool away from externally commanded 
positions. Also connected to the control valve is a tank line 63 in fluid 
communication with a tank or reservoir 64 at atmospheric pressure. 
Servo means 42 includes a servo cylinder 66 having a movable piston 68 
coupled with swashplate 24 by means of a mechanical servo link 70. Servo 
piston 68 is displaced axially within the servo cylinder in response to a 
differential pressure applied across opposite ends 72 and 74 of the piston 
through fluid lines 56 and 58, respectively. 
Swashplate 24 is provided with feedback means 76 which includes a feedback 
link 78 connected to servo link 70 and a summing link 80 connected at one 
end to the feedback link and connected at another end to a link 82 which, 
in turn, is connected to valve spool 60 by an adjustable connector means 
83, the details of which will be described below. A control input force 
yieldably is applied to the control valve by a resilient displacement 
control linkage, generally designated 84, for prescribing the axial 
position of valve spool 60. When an axial displacement is imparted to 
valve spool 60, fluid flow is initiated from control fluid line 38 to one 
of the servo lines 56 or 58 and thus to servo cylinder 66 to cause angular 
displacement of swashplate 24. The angular swashplate displacement imparts 
a corresponding movement to feedback linkage 76 which, in turn, closes the 
control loop by accurately maintaining a steady state fluid metering 
position of the valve spool such that a sufficient flow is supplied to the 
servo cylinder for maintaining swashplate 24 in an angular position 
corresponding to the manual command input. 
Displacement control linkage 84 now will be described. As shown generally 
in FIG. 1, the displacement control linkage includes a manual input means 
including an input lever 86 pivotally mounted by a pin 88 and connected to 
summing link 80 by a resilient linkage 90. Linkage 90 has a non-linear 
coupling 92 interconnected between a first spring 94 and a second spring 
96. First spring 94 is connected to the input lever and the non-linear 
coupling by a rotatable link 100. Spring 94 is a coiled torsional spring 
and has a pair of upstanding legs 136 and 138, with leg 136 contacting 
fixed pin 102. Rotatable link 100 contacts leg 138 of the spring, such 
that rotation of input lever 86 and link 100 distorts the spring 94 and 
generates a biasing return torque against the input lever. Spring 96 is 
connected operatively to non-linear coupling 92 by a rotatable link 104 
and is coupled with control valve 40 by an interconnected pair of 
rotatable input links 106 and 108. Input link 108 is connected directly to 
summing link 80. 
Non-linear coupling 92 is illustrated in detail in FIG. 2 and includes a 
first rotatable camming member 110 having an integral input shaft 112 for 
rotation with link 100 and an axially extending cam follower 114. A second 
rotatable member 116 is supported rotatably by a fixed pivot 118 and has 
an arcuate cam input slot 120 for receiving cam follower 114 on camming 
member 110. The second rotatable member also has a generally oval cam 
output slot 122. A third rotatable camming member 124 has a cam follower 
126 extending into cam output slot 122, with output link 104 projecting in 
colinear relation with cam follower 126. A cylindrical hub portion 128 is 
formed integrally on camming member 124 and is coaxial with the axis of 
rotation 130 of the input shaft 112. 
The above construction of coupling 92 can be understood to provide a 
non-linear relationship between an angular input displacement .phi..sub.1 
of input link 100 and an angular output displacement .phi..sub.2 of output 
link 104 in the following manner. When input shaft 112 is rotated by lever 
86, cam follower 114 moves within slot 120 to cause a rotation of camming 
member 116 about pivot 118. Due to the coupling between cam follower 126 
and camming member 116, rotation of the camming member 124 and output link 
104 results. The relationship between the rotation of input shaft 112 and 
link 104 results directly from the geometry of cam input slot 120 and the 
location of cam followers 114 15 and 126a. 
The graph in FIG. 3 illustrates the relationship between the input 
displacement .phi..sub.1 of link 100 and the output displacement 
.phi..sub.2 of link 104. The relatively low slope of the curve indicated 
at "A" represents the capability of non-linear coupling 92 to provide 
relatively small angular displacements of link 104 in response to input 
displacement .phi..sub.1 of link 100 at small values of .phi..sub.1, that 
is, near zero displacement of lever 86. Similarly, the relatively large 
slope of the curve indicated at "B" indicates the capability of providing 
large output displacement .phi..sub.2 of link 104 in response to input 
displacements .phi..sub.1 of link 100 at larger displacements of lever 86. 
Referring back to FIG. 1, spring 96 also is a coiled torsional spring with 
the coiled portion thereof surrounding cylindrical hub 128 projecting from 
non-linear coupling 92. Spring 96 has a pair of upstanding legs 132 and 
134 which form a bifurcated connection between rotatable links 104 and 
106, respectively. Spring 96 has an initial torsional preload prescribed 
by the distance between legs 132 and 134 and the inherent spring 
stiffness, such that initial movement of lever 86 results in the coupled 
rotation of links 104 and 106, with rotation of link 106 being opposed by 
a reaction force transmitted by summing link 80. The reaction force 
opposing the motion of the link 106 and the input force encouraging motion 
of link 104 act oppositely against legs 132 and 134 to establish a 
differential force couple which imparts a torque to spring 96. When the 
reaction force is great enough that the induced spring torque exceeds the 
initial torsional preload, spring 96 deflects to permit link 104 and input 
lever 86 to move independently of link 106. Such a condition occurs, for 
example, when valve spool 60 reaches a travel stop 61 and lever 86 is 
still further rotated. Rather than transmit sudden excessive force to the 
control valve, spring 96 operates to temporarily decouple links 104 and 
106 until the input force/reaction force differential is reduced. 
The graph shown in FIG. 4 illustrates the relationship between angular 
displacement .phi..sub.2 of link 104 and the resulting angular 
displacement .phi..sub.3 of link 106. In the range of operation in which 
the output displacement of link 104 extends between values "C" and "D", 
spring 96 has a linear response characteristic such that displacement 
.phi..sub.3 of link 106 occurs simultaneously with displacement 
.phi..sub.2 of link 104. When valve spool 60 encounters one of the valve 
stops 61 or otherwise is obstructed, link 106 is prevented from any 
further displacement in response to continued rotation of link 104. This 
condition is illustrated in FIG. 4 at "F" where displacement .phi..sub.3 
maintains a constant value and torsional spring 96 deflects in response to 
further output displacement of link 104. At this condition, input lever 86 
can be moved independently of the pump swashplate position and spring 96 
thus provides an input override function. 
The relationship between swashplate angle .phi..sub.5 and the angular 
displacement .phi..sub.3 of link 106 is illustrated in the graph of FIG. 
5. As indicated in the graph at "G" and "H", swashplate 24 responds in 
substantially linear proportion to the angular displacement of link 106 in 
an operating range slightly off-center, that is, slightly offset from the 
zero position of link 106. The offset, generally indicated "I", represents 
a deadband range of operation of the swashplate in which deflection of 
lever 86 produces no change in position of the swashplate. The deadband 
range of operation of the swashplate corresponds to the range of initial 
movement of control valve spool 60 in which control fluid is not yet 
communicated to either of servo lines 56 or 58. 
It is believed that operation of the displacement control linkage can be 
readily understood from the foregoing description and can be summarized as 
follows. In the absence of a control force applied to lever 86, swashplate 
24 is maintained in a zero displacement position by centering springs 29 
and pump 12 provides no flow to motor 14, such that the transmission 10 is 
in a neutral or idle position with no power being transmitted from input 
shaft 16 to motor output shaft 22. 
Rotation of lever 86 about pivot pin 88 induces an angular input 
displacement .phi..sub.1 of link 100 and a resulting angular displacement 
.phi..sub.2 of link 104 through non-linear coupling 92. The practical 
significance of the response characteristics of the non-linear coupling 
illustrated in FIG. 3 is that for initial displacement of input lever 86 
with the swashplate in the neutral position, link 104 is relatively 
nonresponsive and therefore can be prescribed with high resolution. More 
specifically, small displacements of link 100 produce even smaller 
displacements of output link 104. Displacements of swashplate 24 about the 
neutral position thus can be prescribed with a high degree of accuracy. 
This capability is particularly advantageous when transmission 10 is 
utilized in the drive train of a vehicle and it is desired to propel the 
vehicle at a low speed. At larger swashplate angles, where the vehicle is 
operating at relatively high speed, it is important that the swashplate 
respond quickly to command inputs. The increased slope "B" of the curve in 
FIG. 3 reflects such responsiveness for relatively large values of input 
displacement .phi..sub.1. 
Displacement of link 104 induces a force which is applied yieldably to the 
valve spool 60 by links 106 and 108. As described above, override spring 
96 resiliently couples rotatable links 104 and 106 to translate 
displacement of input lever 86 into axial displacement of the valve spool. 
When a reaction force opposing displacement of the control valve spool 
cooperates with the input force to generate a torque on the spring in 
excess of the spring torsional preload, the spring deflects to allow 
independent motion of link 106. Thus, the value of the reaction force 
required to decouple the input lever from the swashplate is established by 
the initial preload of the override spring. For rapidly applied control 
inputs, the valve spool may engage the stops quite suddenly, with the 
override spring deflecting to effectively decouple the input lever and the 
valve spool and allow continued rotation of the lever. In this situation, 
it is desirable to prevent excessive reaction force from being transmitted 
to the input means, and the override spring preload accordingly is 
selected to achieve this end. 
In addition to the above mentioned case in which a reaction force 
sufficient to decouple the input lever from the swashplate is generated 
when valve spool 60 encounters a stop 61, it is possible for an excessive 
reaction force to result when the valve spool otherwise becomes jammed in 
valve body 46. Small metal chips and other particles may become lodged in 
the control valve and prevent the valve spool from moving, causing the 
swashplate to be held in a fixed position. This situation is particularly 
undesirable when the swashplate is held in a wide-open or maximum 
displacement position. 
In order to overcome the obstruction, a command input force is applied to 
lever 86 and transmitted through the displacement control linkage to move 
the valve spool and shear the particle within the valve body. The input 
force is applied to leg 132 of the override spring, and a reaction force 
transmitted through the feedback linkage is applied oppositely to leg 134 
of the spring. As the input lever is displaced continuously, the input 
force attempting to shear the particle and the resulting reaction force 
gradually increase. When the differential force acting on the legs of the 
override spring exceeds the preload force of the spring, the spring 
deflects and the input lever becomes ineffective in increasing the chip 
shearing force. Because the torque preload of the spring is determinative 
in establishing the maximum input force level, the spring preload, 
therefore, also limits the amount of force which can be generated to 
overcome an obstruction of the valve spool. The spring must be 
appropriately selected to permit shearing metal chips of a maximum 
expected size while preventing excessive feedback force from being 
transmitted to the input lever when the valve spool engages a stop. 
FIG. 6 illustrates the relationship between displacement .phi..sub.1 of 
input link 96 and the angular displacement .phi..sub.s of swashplate 24, 
and represents the cumulative response characteristics of the control 
elements described with respect to FIGS. 3 through 5. As shown in FIG. 6, 
swashplate 24 initially is nonresponsive to small input displacements of 
link 100 near the neutral position. The on-center nonresponsiveness is 
contributed by the deadband response illustrated in FIG. 5 and is desired 
to assure that small unintended displacements of manual lever 86 will 
result in no displacement of swashplate 24 with the pump in neutral and to 
stabilize the swashplate about the zero displacement position. 
As the displacement of link 100 increases, the swashplate becomes 
increasingly responsive to displacements of lever 86. Due to the nonlinear 
response characteristics of coupling 92, precise control of the swashplate 
is obtained for small displacement away from the neutral position, 
providing the capability of "inching" the vehicle in which the 
transmission is utilized at a very low speed. At higher vehicle speeds, 
where inching is not required, the slope of the response curve increases 
and swashplate position becomes more sensitive to displacement of lever 
86. 
Spring 94 will be understood to provide an important centering function of 
the input link 100 and input lever 86. In some applications of hydrostatic 
transmissions, it is desired to have a relatively high centering torque on 
the input lever such that a large feedback force is supplied to an 
operator. Because the centering spring 94 is positioned upstream of the 
override spring 96, that is, between the override spring and the input 
lever, the centering force is not transmitted through the override spring, 
and the override spring can be sized independently of the centering torque 
requirements. 
In order to maintain the symmetric response characteristics represented in 
the graph of FIG. 6, it is necessary that valve spool 60, feedback linkage 
76, and displacement control linkage 84 be interconnected with the various 
links being accurately aligned to provide the appropriate kinematic 
relationships between input lever 86, valve spool 60 and swashplate 24. In 
other words, input lever 86 must be positioned in an orientation 
corresponding to a neutral flow position of the pump swashplate when the 
valve spool is centered in the control valve and does not supply any fluid 
to servo means 42. Although it is theoretically possible to manufacture 
the various links such that the apparatus will always be properly 
assembled, inevitable manufacturing tolerances can stack up or accumulate 
to prevent a reliably proper assembly. As described immediately below, 
adjustable connector means 83 provides the capability of precisely 
assembling the linkage in a novel manner. 
Referring again to FIG. 1, feedback link 78 connects swashplate 24 with 
summing link 80 at a pivoting connection 140. Input link 108 connects 
displacement control linkage 84 with the summing link at a pivoting 
connection 142. The centered position of swashplate 24, as established by 
centering springs 29, thus establishes the location of connection 140 when 
the swashplate is in a neutral position. Similarly, the centered position 
of input lever 86, as established by centering spring 94, establishes the 
location of connection 142 when the input lever is in a position 
representative of the neutral position of the swashplate. The prescribed 
positions of connections 140 and 142 therefore establish the position of 
summing link 80 when the pump is at a no flow condition. Summing link 80 
is connected to the valve spool at a pivoting connection 144. It is a 
principal function of adjustable connector means 83 to permit the pivotal 
connection between the "centered" summing link 80 and the valve spool 
while the valve spool is centered within control valve 40. Structure 
adapted to provide such a function is illustrated in FIG. 7. 
As shown in FIG. 7, valve spool 60 has a reduced diameter portion 142 which 
terminates in a flat end 15. Threads 146 are formed on the periphery of 
valve spool portion 143 and adjustably engage mating threads 148 formed on 
the inner wall of a cylindrical connector block 150. Relative rotation of 
the connector block and the valve spool thereby results in the axial 
advancement of the connector block along the spool. 
A pin 152 projects transversely through one sidewall of the connector block 
and engages one end of a connector link 154. The connector link is joined 
at an opposite end to summing link 80 by a pin 156. Pin 156 thus embodies 
the pivotal connection 144 illustrated in FIG. 1. Rotation of the valve 
spool relative to the connector block, and corresponding axial 
displacement of the valve spool, provides the means for adjusting the 
position of the valve spool relative to the summing link. 
In order to rotate the valve spool relative to the connector block, end 145 
of the valve spool is received in a slotted coupling 158. The slotted 
coupling is supported rotatably at one end of valve body 44 and has an 
axially opening slot 160 and a threaded adjuster screw 162 projecting 
through the valve housing. Flat end 145 is constrained rotatably in slot 
160 but is free to move axially therein, such that axial displacement of 
the valve spool is not restricted. Rotation of the slotted coupling and 
the valve spool is performed by turning the adjuster screw 162. A lock nut 
164 secures the angular position of the slotted coupling and the valve 
spool. 
Interconnection of valve spool 60 and summing link 80 can be summarized as 
follows. With both feedback link 76 and input link 108 connected to the 
summing link and the valve spool disconnected from the summing link, 
centering springs 29 and 94 center swashplate 24 and input lever 86, 
respectively, and thereby establish the location of pivotal connection 144 
on the summing link. Valve spool 60 then is manually displaced within the 
valve housing until the end of coupling link 154 is aligned with the 
pivotal connection 144. Pin 156 then is threaded through the coupling link 
and the summing link to interconnect the valve spool and the summing link. 
At this point, valve spool 60 conceivably is displaced slightly from a 
centered position within bore 46. 
In order to move the valve spool to a centered position within housing 44 
whereat the flow of pressurized control fluid to servo means 42 is 
shunted, adjuster screw 162 is rotated manually. Because valve spool end 
145 is constrained within slotted coupling 158, rotation of the adjuster 
screw and the slotted coupling applies a torque to the valve spool. 
Centering springs 29 and 94 maintain the position of the swashplate and 
input lever, respectively, such that summing link 80 is fixed in a neutral 
position while torque is applied to the valve spool. Connector block 150 
is constrained from moving by the fixed summing link, and the rotation of 
the valve spool induced by the applied torque causes the valve spool to 
advance axially through the connector block. The adjuster screw is rotated 
continuously until the valve spool is centered within the housing. It 
should be understood that the valve spool can be axially advanced in two 
opposite directions, depending on the direction which the adjuster screw 
is rotated. 
In practice, centering of the valve spool is accomplished by operating the 
transmission with the input lever in the position corresponding to the 
neutral position of the swashplate. If hydraulic control fluid accumulates 
in servo cylinder 46, indicating that the valve spool is not centered 
accurately within the valve housing, the adjuster screw is rotated and the 
transmission again is operated. This process is repeated until no control 
fluid accumulates in the servo cylinder. 
Because the width of the lands formed on the valve spool is greater than 
the width of the openings 52 and 54 (see FIG. 7) in the control valve, the 
"centered" position of the valve spool actually corresponds to a range of 
positions in which control fluid is shunted from the servo cylinder. This 
range of positions can be considered a "neutral band". Rotation of the 
adjuster screw thus advances the valve spool through the control valve to 
establish a first axial limit on the neutral band of valve spool positions 
in which no control fluid is metered to the servo cylinder. 
To establish the second limit of the neutral band, the adjuster screw then 
is rotated further in the first direction while the transmission is 
operating in neutral, that is, with the input lever in a position 
corresponding to the no flow position of the swashplate. While the valve 
spool is advancing through the range of shunting positions, no control 
fluid accumulates in the servo cylinder. When the valve spool reaches the 
opposite limit of the neutral band, control fluid appears in the servo 
cylinder. At that point, the total angular displacement of the adjuster 
screw required to axially advance the valve spool through the neutral band 
is established. 
To precisely center the valve spool in the neutral band, the adjuster screw 
first is rotated to the position corresponding to the first limit of the 
neutral band determined by the above procedure. The adjuster screw then is 
rotated an amount calculated as one half the total rotation of the screw 
required to advance the valve spool through the entire neutral band to 
place the valve spool at the exact center of the neutral band. Because the 
summing link remains stationary during the adjustment process, the input 
lever and swashplate remain in a neutral position while the valve spool is 
centered precisely. 
It will be understood that the invention may be embodied in other specific 
forms without departing from the spirit or central characteristics 
thereof. The present examples and embodiments, therefore, are to be 
considered in all respects as illustrative and not restrictive, and the 
invention is not to be limited to the details given herein.