Atty. Docket No. 0666.5620004
`(KK-2332)
`
`CONTROL IVIECHANISM FOR STEPLESS TRANSMISSION
`
`Inventors:
`
`Jun MAT SUURA
`
`Nobuhisa KAMIKAWA
`
`Syouto YOSHIIVIOTO
`Koji IWAKI
`Masaru IIDA
`
`Toshifumi YASUDA
`
`CROSS-REFERENCE TO RELATED APPLICATIONS
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`[0001]
`
`The present application claims priority to Japanese Patent Applications No.
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`2014-161495, filed on August 7, 2014, and No. 20l4-l62729, filed on August 8, 2014. Further,
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`the present application is a continuation-in-part application of US. patent application Ser. No.
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`15/650,358, filed on July 14, 2017.
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`BACKGROUND OF THE INVENTION
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`Field of the Invention
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`[0002]
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`The present invention relates to a control mechanism for controlling the output rotary
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`speed and direction of a hydrostatic transmission (hereinafter, “HST”). Especially, the control
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`mechanism is a servomechanism including a servo unit that combines a telescopic (linearly
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`movable) actuator and a valve.
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`Related Art
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`[0003]
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`As disclosed by JP 2002-250437 A (hereinafter, “Dl”), JP HO6-lOO278 B (hereinafter,
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`“D2”), and JP 2013-096449 A (hereinafter, “D3”), an HST used as a transmission for a tractor
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`is usually disposed in a casing that also serves as a vehicle body frame (chassis) of the tractor.
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`Also, as disclosed by D1, D2 and D3, it is well-known that a piston in a hydraulic cylinder
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`serves as the actuator for controlling a position of a movable swash plate of the HST.
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`In this
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`regard, a trunnion shaft serving as a pivot of the movable swash plate is connected to the piston.
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`Especially, each of the hydraulic cylinders disclosed by D2 and D3 is configured as a
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`servomechanism.
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`

`

`Atty. Docket No. 0666.5620004
`(xx—233.2)
`
`[0004]
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`With regard to connection of the actuator to the trunnion shaft, in DI, the trunnion
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`shaft has a tip projecting outward from the casing, and an arm is fixed on the tip of the trunnion
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`shaft, and is connected via a link to a tip of a piston rod extended from the hydraulic cylinder
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`disposed outside of the casing. Similarly, in D2, an arm is fixed on a tip of the trunnion shaft
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`projecting outward from the casing. However, a tip of the arm is directly connected to the
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`piston in the hydraulic cylinder without a link. In D3, a part of the casing serves as a housing
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`of the hydraulic cylinder. The piston in the hydraulic cylinder formed in the casing is directly
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`connected to the movable swash plate of the HST in the casing so that the HST and the
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`hydraulic cylinder are assembled together in the casing.
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`[0005]
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`To achieve a simple and compact vehicle body frame, some tractor makers desire the
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`HST to be minimized for its arrangement in the casing serving as the vehicle body frame, and
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`the actuator to be separated from the HST so as to enable its external attachment on the outside
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`of the casing. From this viewpoint, the structure disclosed by D3 does not meet these desires.
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`[0006]
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`If it is premised that the HST is disposed inside of the casing, and the actuator outside
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`of the casing, it is desirable in assembility, maintenanceability, and reduction of parts count
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`and costs that the actuator can be easily connected or disconnected to and from the tip of the
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`trunnion shaft projecting outward from the casing. A space under the step of the tractor on a
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`right or left side of the casing is suggested as one of appropriate spaces for arrangement of
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`such an actuator. However, if the actuator is to be disposed under the step, the actuator must
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`be disposed at a considerably low position so that a sufficient vertical gap between the actuator
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`and the step above the actuator is ensured to facilitate works for connecting or disconnecting
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`the actuator to and from the trunnion shaft, and to prevent the attached actuator from
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`interfering with the step. Such a low position becomes considerably lower than the trunnion
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`shaft. Therefore, as taught or suggested by D1 and D2, a link or an arm is needed to connect
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`the trunnion shaft to the actuator.
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`[0007]
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`If the actuator is a telescopic actuator, such as a piston rod of a hydraulic cylinder, the
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`movement of the actuator is linear while the movement of the arm that rotates centered on the
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`trunnion shaft or the like is circular. Therefore, such a differential movement between the arm
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`and the actuator should be considered when they are connected to each other. This differential
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`movement becomes larger as the distance between the trunnion shaft and the actuator becomes
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`larger.
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`In other words, if a large vertical gap between the actuator and the trunnion shaft is
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`-2-
`
`

`

`Atty. Docket No. 0666.5620004
`(KK-233.2)
`
`desired to ensure the facility in work for connecting and disconnecting the actuator to and from
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`the trunnion shaft, there should be any configuration for absorbing the differential movement
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`between the arm and the actuator that may become large because of the large vertical gap.
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`[0008]
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`In this regard, Dl discloses a structure that the cylinder serving as the actuator is
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`pivotally connected at the tip of the piston rod thereof to the arm, and at a cylinder bottom
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`thereof to a part of the vehicle (e.g., a frame). During the telescopic movement of the piston
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`rod, the entire cylinder rotates centered on the pivot at the cylinder bottom according to
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`rotation of the arm. D2 discloses a structure that a groove or a slot is provided in a portion of
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`the piston of the servomechanism to the tip of the arm so as to absorb the rotation of the tip of
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`the arm during the sliding movement of the piston.
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`[0009]
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`However, in the case of DI, the part of the vehicle pivotally supporting the cylinder
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`bottom is weighed on or loaded eccentrically because it cantilevers the cylinder, so that it is
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`liable to a twisting stress due to the rotation of the cylinder. If the actuator is assembled with a
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`valve so as to constitute a servo unit, especially, if the valve is a large and heavy solenoid valve,
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`the part pivoting the cylinder bottom further tends to be twisted. In the case of D2, the portion
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`of the piston engaged with the tip of the arm is complicated in structure.
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`It needs accurate
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`dimensions to surely absorb the differential movement between the piston and the arm while
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`ensuring a stable telescopic movement of the piston and a stable rotation of the arm. Further,
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`the slot or groove has to be formed. As a result, the cost of the configuration of D2 becomes
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`great. Moreover, in the case of D2, the actuator is a twin rod piston. While the piston engages
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`with the tip of the arm, the actuator unit including the valve and the piston has to be supported
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`by attaching both tips of the piston rods to the casing of the HST, thereby increasing the
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`number of positions and processes for attaching or detaching the actuator unit.
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`[0010]
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`Moreover,
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`it is desirable that the servo unit is minimized as much as possible.
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`However,
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`the actuator needs a biasing means that biases the actuator to a position
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`corresponding to a neutral position of the movable swash plate, i.e., a neutral position of the
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`HST. The existence of the biasing means hinders the servo unit from being minimized.
`
`

`

`Atty. Docket No. 0666.5620004
`(KK-233.2)
`
`SUlVllVlARY OF THE INVENTION
`
`[0011]
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`An object of the invention is to downsize a servo unit and further to facilitate
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`attachment and detachment of the servo unit, in the case where an HST control mechanism
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`such as a servo mechanism to be provided separately from an HST includes the servo unit
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`constituted by the combination of an actuator and valves.
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`[0012]
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`A control mechanism for stepless transmission according to the invention is a control
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`mechanism connected to a manipulation lever in a stepless transmission and disposed outside
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`a housing of the transmission, and includes: a) a piston rod connected to the manipulation
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`lever; b) a piston provided on the piston rod coaXially with the piston rod, c) a cylinder case
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`provided with a cylinder adapted to house the piston rod and the piston such that the piston rod
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`and the piston are displaceable in an aXial direction, the piston and the cylinder forming a first
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`fluid chamber to be supplied with a hydraulic fluid for withdrawing the piston rod from the
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`cylinder and a second fluid chamber to be supplied with the hydraulic fluid for introducing the
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`piston rod into the cylinder; d) a spring adapted to bias the manipulation lever in a neutral
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`direction coaXially with the piston rod, e) a proportional pressure control valve adapted to
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`selectively supply the hydraulic fluid to the first fluid chamber or the second fluid chamber,
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`the proportional pressure control valves being mounted to the cylinder case, and f) a pivot
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`shaft adapted to support the cylinder case oscillatably with respect to the housing.
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`[0013]
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`Thus, the servo unit is pivotally supported through the pivot shaft, which can ensure
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`the integrity of the servo unit constituted by the combination of the actuator and the valves, can
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`enhance the assemblability, and also can ensure easiness of attachment and detachment of the
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`servo unit.
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`[0014]
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`Preferably,
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`in the control mechanism for stepless transmission,
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`the proportional
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`pressure control valve is composed of a first valve configured to supply the hydraulic fluid to
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`the first fluid chamber and a second valve adapted to supply the hydraulic fluid to the second
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`fluid chamber,and an aXis center of the piston rod, an aXis center of displacement of a valve
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`body of the first valve and an aXis center of displacement of a valve body of the second valve
`
`form a triangle shape, when viewed in the aXial direction of the piston rod.
`
`

`

`Atty. Docket No. 0666.5620004
`(xx—233.2)
`
`[0015]
`
`This can reduce the radial size of a portion of the servo unit where the actuator and the
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`valves are disposed, which can downsize the hydraulic transmission control mechanism,
`
`thereby preventing it from interfering with obstructions existing around the housing.
`
`[0016]
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`Further, preferably, in the control mechanism for stepless transmission, the spring is
`
`arranged in the axial direction such that a coil axis center of the spring and the axis center of
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`the piston rod are substantially coincident with each other.
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`[0017]
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`This can reduce the radial size of a portion of the servo unit where the neutral spring is
`
`disposed, which can downsize the hydraulic transmission control mechanism,
`
`thereby
`
`preventing it from interfering with obstructions existing around the housing.
`
`[0018]
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`Further, preferably, in the control mechanism for stepless transmission, the piston rod,
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`the first valve, and the second valve are disposed such that the axial direction of the piston rod,
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`a direction of displacement of a valve body of the first valve and a direction of displacement of
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`a valve body of the second valve are parallel with each other, and the spring is disposed on one
`
`end of the cylinder case in the axial direction of the piston rod, and the first valve and the
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`second valve are disposed on the other end of the cylinder case on an opposite side to the
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`spring, in the axial direction of the piston rod.
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`[0019]
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`This can reduce the radial size of the servo unit constituted by the actuator, the valves
`
`and the neutral spring which are integrated, which can downsize the hydraulic transmission
`
`control mechanism.
`
`[0020]
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`Further,
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`in the control mechanism for stepless transmission further includes a
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`mounting part having the pivot shaft such that the mounting part is oscillatable and is
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`prevented from being disengaged from the housing, wherein the mounting part is detachably
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`mounted to the cylinder case.
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`[0021]
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`This enables easily coping with changes of the specifications of the control mechanism
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`for stepless transmission, through replacement of the mounting part.
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`[0022]
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`Further, preferably, in the control mechanism for stepless transmission, the cylinder
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`case is mounted to the mounting part such that an axis line of the piston rod and an axis line of
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`the oscillatable shaft portion are overlapped with each other in a plan view.
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`[0023]
`
`This allows the cylinder case to smoothly oscillate during stretching and shrinking of
`
`the piston rod, thereby efficiently moving the manipulation lever.
`
`

`

`Atty. Docket No. 06665620004
`(KK-233.2)
`
`[0024]
`
`Further, preferably, in the control mechanism for stepless transmission, the piston rod
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`is provided with a thread portion at a tip end portion, is secured, through screwing, to the
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`manipulation lever connecting portion, and is provided with a flat portion in at least a portion
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`of an outer peripheral surface.
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`[0025]
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`This enables changing the position of the manipulation lever connecting portion by
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`rotating a tool fitted to the flat portion, thereby easily and accurately adjusting the neutral
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`position.
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`[0026]
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`These and other objects, features and advantages of the invention will appear more
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`fully from the following detailed description of the invention with reference to the attached
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`drawings.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`[0027]
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`Fig. l is a side view of a vehicle frame casing 11 to which a servo set 40 according to a
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`first embodiment is attached.
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`[0028]
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`Fig. 2 is a sectional side view of vehicle frame casing 11 showing a structure of an HST
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`[0029]
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`[0030]
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`[0031]
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`[0032]
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`[0033]
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`[0034]
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`[0035]
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`[0036]
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`[0037]
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`[0038]
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`[0039]
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`[0040]
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`l therein.
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`Fig. 3 is a perspective view of a servo unit 60.
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`Fig. 4 is a side view of servo unit 60.
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`Fig. 5 is a cross sectional view taken along V-V line of Fig. 4.
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`Fig. 6 is a cross sectional view taken along VI—VI line of Fig. 4.
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`Fig. 7 is a cross sectional view taken along VII-VII line of Fig. 6.
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`Fig. 8 is a cross sectional view taken along VIII-VIII line of Fig. 4.
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`Fig. 9 is a cross sectional view taken along IX-IX line of Fig. 4.
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`Fig. 10 is a cross sectional view taken along X-X line of Fig. 4.
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`Fig. 11 is a cross sectional view taken along XI-XI line of Figs. 6 and 7.
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`Fig. 12 is a sectional side view of a neutral returning unit 80.
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`Fig. 13 is a hydraulic circuit diagram for supplying fluid to HST l and servo set 40.
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`Fig. 14 is a sectional side view of HST l disposed in vehicle frame casing 11 to be
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`provided with a servo unit 30 according to a second embodiment.
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`

`

`Atty. Docket No. 0666.5620004
`(KK-233.2)
`
`[0041]
`
`Fig. 15 is a cross sectional view taken along XV-XV line of Fig. 14 as a sectional rear
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`view of HST 1 provided with servo unit 30 according to the second embodiment.
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`[0042]
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`Fig. 16 is a sectional side view of a portion of servo unit 30 including a hydraulic
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`cylinder 32.
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`Fig. 17 is a hydraulic circuit diagram for supplying fluid to HST l and servo unit 30.
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`Fig. 18 is a sectional rear view of a servo unit 30A.
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`Fig. 19 is a sectional rear view of a servo unit 30B.
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`Fig. 20 is a sectional rear view of a servo unit 30C.
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`Fig. 21 is a sectional rear view of a servo unit 30D.
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`Fig. 22 is a sectional rear view of a servo unit 30E.
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`Fig. 23 is a sectional side view of servo unit 30E.
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`Fig. 24 is a hydraulic circuit diagram for supplying fluid to HST l and a servo unit 90.
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`Fig. 25 is a side view of servo set 200.
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`Fig. 26 is a rear view of servo set 200.
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`Fig. 27 is a front view of servo set 200.
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`Fig. 28 is a perspective view of servo set 200.
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`Fig. 29 is a cross-sectional view taken along A-A line of Fig. 25.
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`Fig. 30 is a cross-sectional view taken along B-B line of Figs. 25 and 26.
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`Fig. 31 is a cross-sectional view taken along C-C line of Fig. 26.
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`Fig. 32 is a cross-sectional view taken along D-D line of Fig. 26.
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`Fig. 33 is a side view illustrating a state of mounting of servo set 200 to vehicle frame
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`[0043]
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`[0044]
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`[0045]
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`[0046]
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`[0047]
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`[0048]
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`[0049]
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`[0050]
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`[0051]
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`[0052]
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`[0053]
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`[0054]
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`[0055]
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`[0056]
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`[0057]
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`[0058]
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`[0059]
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`casing 11.
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`[0060]
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`Fig. 34 is a perspective assembly view illustrating the state of mounting of servo set
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`200 to vehicle frame casing 11.
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`[0061]
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`Fig. 35 is a front-portion cross-sectional view illustrating a portion for pivotally
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`supporting servo set 200 on vehicle frame casing 11.
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`[0062]
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`Fig. 36 is a hydraulic circuit diagram for supplying fluid to HSTl and servo set 200.
`
`

`

`DETAILED DESCRIPTION OF THE INVENTION
`
`Atty. Docket No. 0666.5620004
`(xx—233.2)
`
`[0063]
`
`A first embodiment about an HST control mechanism (servomechanism) serving as an
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`actuator device for controlling a movable swash plate of an HST (Hydro-Static-Transmission)
`
`shown as an example of a continuously variable transmission capable of switching forward
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`and reverse will be described with reference to Fig. 1 to 13. Referring to Figs. 1 and 2, an HST
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`1, a vehicle frame casing 11 and an axle casing 12 will be described. Vehicle frame casing 11
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`is a casing formed to serve as a vehicle body frame (chassis). Vehicle frame casing 11
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`incorporates HST 1 serving as a unit for changing a speed of a vehicle such as a tractor.
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`Vehicle frame casing 11 has front and rear open ends. One of the front and rear end open ends
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`of HST 1 (in this embodiment, the front open end) is an open end 11a, which is joined to an
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`end portion (in this embodiment, a rear end portion) of an engine 10 (see Fig. 13). The other of
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`the front and rear ends of HST 1 is an open end 11b, which is joined to an end portion (in this
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`embodiment, a front end portion) of axle casing 12. Hereinafter, positions and directions of all
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`component elements of the first embodiment and a later-discussed second embodiment are
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`defined on the assumption that vehicle frame casing 11 has front open end 11a and rear open
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`end 11b.
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`[0064]
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`Axle casing 12 journals an axle (not shown) and incorporates a transmission for
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`driving the axle. This transmission transmits power outputted from HST 1 to the axles. The
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`transmission may include a speed-changing mechanism (e.g., a gear transmission).
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`In this
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`case, HST 1 serves as a main speed-changing transmission, and the transmission in axle casing
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`12 serves as a sub speed-changing transmission.
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`[0065]
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`Vehicle frame casing 11 and axle casing 12 joined to each other serve as a vehicle body
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`frame, which is disposed at a laterally middle portion of a vehicle such as a tractor, for
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`example. Referring to Fig. 1 (and Fig. 2 illustrating another embodiment), this vehicle body
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`frame is fixedly provided on right and left ends thereof with left and right steps 13 and 14
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`made of horizontal plates, which are used for an operator’s getting on and off the vehicle, and
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`as a foot rest for an operator sitting on a seat. A servo set (servomechanism) 40 for controlling
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`a later-discussed movable swash plate of HST 1 is disposed under one of steps 13 and 14 (in
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`this embodiment, step 14. See Fig. 15 or others illustrating the second embodiment).
`
`

`

`Atty. Docket No. 06665620004
`(xx—233.2)
`
`[0066]
`
`HST 1 includes a hydraulic pump 2, a hydraulic motor 3, a center section 4, an HST
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`housing 5, a charge pump 20 and others, which are assembled together into a unit defined as
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`HST 1.
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`In this embodiment, center section 4 is shaped as a vertical plate, which is fixed in
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`vehicle frame casing 11 so as to cover rear open end 1 lb of vehicle frame casing 11. A vertical
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`front surface of center section 4 is formed with a pump mounting surface and a motor
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`mounting surface, which are vertically juxtaposed so that one is above the other.
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`In this
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`embodiment, the pump mounting surface is disposed above the motor mounting surface. A
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`cylinder block 2a of hydraulic pump 2 is rotatably slidably mounted onto the pump mounting
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`surface, and a cylinder block 3a of hydraulic motor 3 onto the motor mounting surface. A
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`valve plate may be interposed between each cylinder block 2a or 3a and the pump or motor
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`mounting surface. HST housing 5 is extended forward from center section 4 so as to cover
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`hydraulic pump 2 and hydraulic motor 3 at upper, lower, right, left and front sides of hydraulic
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`pump 2 and motor 3.
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`[0067]
`
`Referring to Fig. 2, a front wall of HST housing 5 is disposed forward from hydraulic
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`pump 2 and motor 3. A movable swash plate 6 of hydraulic pump 2 is disposed adjacent to an
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`upper portion of the front wall of HST housing 5. Movable swash plate 6 is a trunnion-type
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`movable swash plate having later-discussed lateral trunnion shafts pivoted by right and left
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`side walls of HST housing 5, and abuts against heads (front ends) of plungers 2b projecting
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`forward from cylinder block 2a. A later-discussed pump shaft 2c is extended forward from
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`movable swash plate 6, is journalled by an upper portion of the front wall of HST housing 5,
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`and is extended forward from HST housing 5 to as to be drivingly connected to a
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`later-discussed flywheel lOb. On the other hand, a fixed swash plate 6 of hydraulic motor 3 is
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`fixed to a lower portion of the front wall of HST housing 5 so as to abut against heads of
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`plungers 3b projecting forward from cylinder block 3a.
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`[0068]
`
`Movable swash plate 6 includes right and left trunnion shafts serving as a fulcrum axis
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`for rotation of movable swash plate 6. Right and left trunnion shafts are journalled by right
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`and left walls of HST housing 5. Of the trunnion shafts, one trunnion shaft 6b projects
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`outward from HST housing 5, and has an end portion projecting outward from a right or left
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`(in this embodiment, right) outer side surface llc of vehicle frame casing 11 just below step 14.
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`A first arm 107 is fixed at a top portion thereof to the end portion of trunnion shaft 6b. First
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`arm 107 is extended downward along the outer side surface of vehicle frame casing 11, and
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`-9-
`
`

`

`Atty. Docket No. 0666.5620004
`(xx—233.2)
`
`has a lower end portion to which a rear end portion 82b of a rod 82 extended rearward from a
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`cylindrical member 81 of a neutral returning unit 80.
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`[0069]
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`A second arm 109 is disposed rearward from first arm 107 outside of vehicle frame
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`casing 11, and is extended along outer side surface 11c of vehicle frame casing 11. Second
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`arm 109 has a pivot shaft 109a at a vertically intermediate portion thereof so as to be pivoted
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`onto outer side surface 11c of vehicle frame casing 11 via pivot shaft 109a. Second arm 109 is
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`pivotally connected at a top portion thereof to a vertically intermediate portion of first arm 107
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`via a link 108. A tip (i.e., rear end) of a piston rod 33a extended rearward from a hydraulic
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`cylinder 32 of a servo unit 60 is pivotally connected to a lower end portion of second arm 109.
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`Servo unit 60 and a neutral returning unit 80 are assembled together so as to constitute a servo
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`set (i.e., servomechanism) 40 for controlling movable swash plate 6 of hydraulic pump 2.
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`In
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`this regard, fist arm 107, link 108 and second arm 109 serve as a rotary member, to which a
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`piston rod 33a of a piston 33 serving as a telescopic actuator of servo unit 60 and a rear end
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`portion 82b of a rod 82 serving as a telescopic member of neutral returning unit 80 are
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`pivotally connected. Neutral returning unit 80 includes a neutral biasing spring 83 (the neutral
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`spring) serving as a biasing device. Neutral biasing spring 83 biases rod 82 toward its neutral
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`position corresponding to a neutral position of movable swash plate 6.
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`[0070]
`
`Due to a fore-and-aft telescopic movement of piston rod 33a of servo unit 60, second
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`arm 109 rotates centered on pivot shaft 109a. Accordingly, first arm 107 connected to second
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`arm 109 via link 108 rotates centered on an axis of trunnion shaft 6b, thereby rotating movable
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`swash plate 6 of hydraulic pump 2, because trunnion shaft 6b of movable swash plate 6 is fixed
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`to first arm 107. Further, due to the telescopic movement of piston rod 33a and the rotation of
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`second arm 109, first arm 107 rotates so as to move rod 82 telescopically against the biasing
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`force of neutral biasing spring 83.
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`In servo unit 60, proportional pressure control valves 35
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`and 36 can be controlled to apply an operation force to piston rod 33a. Once piston rod 33a is
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`released from the operation force, rod 82 returns to the neutral position defined by the biasing
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`force of neutral biasing spring 83, thereby returning first arm 107 and movable swash plate 6
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`to their neutral positions. Due to the movement of first arm 107 to its neutral position, second
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`arm 109 is rotated via link 108 so that piston rod 33a returns to a position in the telescopic
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`movement direction (fore-and-aft direction) of piston rod 33a corresponding to the neutral
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`position of movable swash plate 6 with trunnion shafts 6a.
`
`-10-
`
`

`

`Atty. Docket No. 0666.5620004
`(xx—233.2)
`
`[0071]
`
`Hydraulic pump 2 includes a fore-and-aft horizontal pump shaft 2c serving as a rotary
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`axis of cylinder block 2a. Hydraulic motor 3 includes a fore-and-aft horizontal motor shaft 3c
`
`serving as a rotary axis of cylinder block 3a. Pump shaft 2c and motor shaft 3c are journalled
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`by center section 4. Rear end portions of pump shaft 2c and motor shaft 3c project rearward
`
`from center section 4 into axle casing 12. In axle casing 12, a front end portion of a PTO shaft
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`or a front end portion of a PTO transmission shaft 15 interlocking with the PTO shaft is
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`connected via a coupling 15a to the rear end portion of pump shaft 2c. On the other hand, in
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`axle casing 12, a front end portion of an input shaft 16 of the transmission in axle casing 12 is
`
`connected via a coupling 16a to the rear end portion of motor shaft 3c serving as an output
`
`shaft of HST 1.
`
`[0072]
`
`In vehicle frame casing 11, a gear casing 21 is fixed to a front end of HST housing 5. A
`
`front portion of pump shaft 2c projects forward from the front wall of HST housing 5 through
`
`gear casing 21. After engine 10 is joined to front open end 11a of vehicle frame casing 11, a
`
`fore-and-aft horizontal engine output shaft 10a of engine 10 is extended into vehicle frame
`
`casing 11 via front open end 11a, so that a flywheel 10b provided at a rear end portion of
`
`engine 10 is disposed in a front inside space in vehicle body frame 11. Pump shaft 2c serving
`
`as an input shaft of HST 1 projects forward from gear casing 21 as mentioned above, so that a
`
`front end portion of pump shaft 2c is connected to flywheel 10b via a damper 10c.
`
`[0073]
`
`In gear casing 21, pump shaft 2c is journalled at a fore-and-aft intermediate portion
`
`thereof, and a charge pump driving shaft 24 is extended parallel to pump shaft 2c and is
`
`journalled.
`
`In gear casing 21, a gear 22 is fixed on the fore-and-aft intermediate portion of
`
`pump shaft 2c, and a gear 23 is fixed on charge pump driving shaft 24. Gears 22 and 23 mesh
`
`with each other so as to constitute a gear train for transmitting power from pump shaft 2c to
`
`charge pump driving shaft 24. A pump housing 26 incorporating charge pump 20 is fixed to a
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`front end of gear casing 21 so as to extend parallel to the front portion of pump shaft 2c
`
`projecting forward from gear casing 21. Charge pump driving shaft 24 projects forward from
`
`gear casing 21 into pump housing 25. In pump housing 25, a pump driven shaft 20c parallel to
`
`charge pump driving shaft 24 is journalled.
`
`In pump housing 25, a pump gear 20a fixed on
`
`charge pump driving shaft 24 and a pump gear 20b fixed on pump driven shaft 20c mesh with
`
`each other so as to constitute a gear pump serving as charge pump 20.
`
`-11-
`
`

`

`Atty. Docket No. 06665620004
`(KK-233.2)
`
`[0074]
`
`Referring to Figs. 2 and 13, a hydraulic fluid supply system for supplying fluid from
`
`charge pump 20 to HST 1 will be described. Charge pump 20 sucks fluid from a fluid sump in
`
`axle casing 12. In this regard, aXle casing 12 has a fluid discharge port P1. Fluid of the fluid
`
`sump in axle casing 12 is taken out of aXle casing 12 via fluid discharge port P1 and is sucked
`
`to a suction port of charge pump 20 disposed in vehicle frame casing 11 via a fluid passage L1.
`
`Fluid passage L1 may be formed within a wall of vehicle frame casing 11, or may be made of
`
`pipes disposed outside of casings 11 and 12, for example.
`
`[0075]
`
`Fluid delivered from a delivery port of charge pump 20 is taken out of vehicle frame
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`casing 11 again, and is supplied into a charge fluid passage 4a formed in center section 4 via a
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`line filter 26, a fluid passage L2 and a charge port P2.
`
`In the embodiment shown in Fig. 2,
`
`charge fluid passage 4a has an open end serving as charge port P2 at a rear end surface of
`
`center section 4, and fluid passage L2 is connected to charge port P2.
`
`[0076]
`
`A relief valve 28 is provided in center section 4 so as to adjust a hydraulic pressure of
`
`fluid in charge fluid passage 4a of center section 4. Fluid released from relief valve 28 is
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`drained into a chamber in HST housing 5 forward from center section 4 via a drain fluid
`
`passage 4b formed in center section 4. Referring to Fig. 1, HST housing 5 has a drain port for
`
`draining fluid from the inside of HST housing 5. This drain port is normally plugged with a
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`drain cap 29, as shown in Fig. 2. Fluid from the drain port is drained to the fluid sump in HST
`
`housing 5 or vehicle frame casing 11. This fluid sump may be fluidly connected to a fluid
`
`sump in aXle casing 12 via a fluid passage L5, as shown in Fig. 13.
`
`[0077]
`
`Referring to Fig. 13, center section 4 is formed therein with a pair of main fluid
`
`passages MLl and ML2 that are interposed between hydraulic pump 2 and hydraulic motor 3.
`
`Center section 4 is provided therein with a pair of charge valve units CV1 and CV2. The fluid
`
`in charge fluid passage 4a has a hydraulic pressure regulated by relief valve 28. When main
`
`fluid passage MLl is hydraulically depressed, a check valve 45 in charge valve unit CV1 is
`
`opened so that the fluid in charge fluid passage 4a is supplied to main fluid passage MLl via
`
`opened check valve 45 in charge valve unit CV1. When main fluid passage ML2 is
`
`hydraulically depressed, a check valve 45 in charge valve unit CV2 is opened so that the fluid
`
`in charge fluid passage 4a is supplied to main fluid passage ML2 via opened check valve 45 in
`
`charge valve unit CV2. Each of charge valve units CV1 and CV2 includes a relief valve 46
`
`-12-
`
`

`

`Atty. Docket No. 0666.5620004
`(xx—233.2)
`
`that bypasses corresponding check valve 45 so as to regulate the hydraulic pressure in
`
`corresponding main fluid passage ML1 or ML2.
`
`[0078]
`
`One of charge valve units CV1 and CV2 includes an orifice 47 bypassing
`
`corresponding check valve 45 so that orifice 47 functions to expand a neutral region of HST 1.
`
`If main fluid passage ML2 is designated to have a higher hydraulic pressure than that of main
`
`fluid passage MLl during backward traveling of the vehicle, preferably, charge valve unit
`
`CV2 for supplying fluid to main fluid passage ML2 has orifice 47.
`
`[0079]
`
`Referring to the hydraulic circuit diagram of Fig. 13, a general configuration of servo
`
`set 40 and a fluid supply system for supplying fluid from charge pump 20 to servo unit 60 of
`
`servo set 40. A delivery fluid passage from charge pump 20 passes line filter 26 and then is
`
`bifurcated into fluid passages L2 and L3. Fluid passage L2 is extended into HST 1. Fluid
`
`passage L3 is extended to servo unit 60. In this way, charge pump 20 supplies hydraulic fluid
`
`to both HST 1 and servo unit 60 for controlling movable swash plate 6 of HST 1.
`
`[0080]
`
`Servo unit 60 includes a hydraulic cylinder 32 and proportional pressure control valves
`
`35 and 36. Piston 33 serving as the actuator for controlling the position of movable swash
`
`plate 6 is slidably fitted in hydraulic cylinder 32. Opposite piston rods 33a and 33b are
`
`extended from piston 33 and project outward from opposite ends of hydraulic cylinder 32. The
`
`tip of piston rod 33a is pivotally connected to second arm 109 as mentioned above.
`
`[0081]
`
`Proportional pressure control valve 35 is adapted to supply or discharge fluid to and
`
`from a fluid chamber 32a formed in hydraulic cylinder 32 on one side of piston 33.
`
`Proportional pressure control valve 36 is adapted to supply or discharge fluid to and from a
`
`fluid chamber 32b formed in hydraulic cylinder 32 on the other side of piston 33. Proportional
`
`pressure control valves 35 and 36 are proportional solenoid valves provided with respective
`
`proportional solenoids 35a and 36a. Proportional pressure control valve 35 has a valve port
`
`35d fluidly connected to fluid chamber 32a in hydraulic cylinder 32 via a hydraulic fluid
`
`passage 43. Proportional pressure control valve 36 has a valve port 36d fluidly connected to
`
`fluid chamber 32b in hydraulic cylinder 32 via a hydraulic fluid passage 44. One of
`
`proportional pressure control valves 35 and 36 is designated so as to be excited during forward
`
`traveling of the vehicle, and the other of proportional pressure control valves 35 and 36 so as to
`
`be excited during backward traveling of the vehicle. Either proportional pressure control
`
`-13-
`
`

`

`Atty. Docket No. 0666.5620004
`(KK-233.2)
`
`valve 35 or 36, which is excited, supplies fluid from its valve port 35