HYDRAULIC TRANSAXLE
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`CROSS REFERENCE TO RELATED APPLICATIONS
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`[0001]
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`This application is a continuation-in-part of US. Application No. 14/550,499,
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`filed November 21, 2014, which is a continuation-in-part of US. Application No.
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`14/179,334, filed February 12, 2014, which claims the benefit of US. Provisional
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`Application No. 61/843,165, filed July 5, 2013. The contents of each of these applications
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`are expressly incorporated herein by reference thereto in their entireties.
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`FIELD
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`[0002]
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`The present disclosure generally relates to a hydraulic transaxle, including an axial
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`piston hydraulic pump having a variable displacement and a transaxle casing
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`incorporating the hydraulic pump.
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`BACKGROUND
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`[0003]
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`As disclosed by US. 6,125,630 B, there is a well—known conventional hydraulic
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`transaxle serving as one of right and left transaxles carrying respective right and left
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`individual axles for driving a zero-turn vehicle, e.g., a lawn mower. The transaxle
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`includes an axial piston hydraulic pump having a variable displacement, and a transaxle
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`casing incorporating the hydraulic pump. The hydraulic pump includes a trunnion-type
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`movable swash plate for controlling its displacement.
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`The swash plate is formed
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`integrally with a pair of feet having a cylinder block therebetween. The swash plate
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`further includes a pair of trunnion shafts coaxial to each other. The trunnion shafts are
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`inserted into holes formed in opposite wall portions of the transaxle casing having the
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`hydraulic pump therebetween so as to be pivotally supported by the transaxle casing.
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`One of the pivotally supported trunnion shafts project outward from the transaxle casing
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`so as to be provided thereon with a control arm. Therefore, the swash plate is rotatable
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`centered on an axis of the trunnion shafts.
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`

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`-2-
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`[0004]
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`However, the swash plate is complex in shape and expensive because it is formed
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`integrally with the trunnion shafts projecting from the respective feet. Further, when the
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`swash plate is installed or removed into and from the transaxle casing, the swash plate
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`formed integrally with the trunnion shafts must be moved in the axial direction of the
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`trunnion shafts.
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`Therefore,
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`the transaxle casing has a large opening to allow the
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`movement of the swash plate in the axial direction of the trunnion shafts into and from an
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`inside space of the transaxle casing. After the swash plate is installed in the inside space
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`of the transaxle casing by journaling one trunnion shaft by a side wall of the transaxle
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`casing, a cover is detachably attached to cover the opening and to journal the other
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`trunnion shaft. Such a cover must be provided separately from basic housing members of
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`the transaxle casing for incorporating a hydrostatic transmission (hereinafter, referred to
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`as “‘HST”) including the hydraulic pump, an axle and gears interposed between the HST
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`and the axle, thereby increasing component parts in number and costs, and increasing
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`labor for installing and removing the swash plate into and from the transaxle casing.
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`SUlVflVLARY
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`[0005]
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`A hydraulic transaxle of the present application comprises an axial piston
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`hydraulic pump having a variable displacement, and a transaxle casing incorporating the
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`hydraulic pump. The hydraulic pump includes a movable swash plate and a pair of
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`trunnion shafts coaxial to each other. The pair of trunnion shafts are joined to the swash
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`plate separably from the swash plate. The swash plate is rotatable centered on a common
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`axis of the trunnion shafts to control
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`the displacement of the hydraulic pump. The
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`common axis of the trunnion shafts defines an axial direction of the trunnion shafts
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`perpendicular to a rotary axis of the hydraulic pump. The transaxle casing includes a pair
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`of side walls opposite each other in the axial direction of the trunnion shafts with respect
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`to the hydraulic pump. The transaxle casing includes a pair of casing holes each of which
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`penetrates each of the side walls between an inside of the transaxle casing and an outside
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`of the transaxle casing. The pair of trunnion shafts are passed through the respective
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`casing holes to be pivotally supported by the respective side walls of the transaxle casing.
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`The swash plate is formed with a pair of side portions opposite each other in the axial
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`direction of the trunnion shafts. The pair of side portions face the respective side walls in
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`the inside of the transaxle casing. The swash plate is formed with a pair of swash plate
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`

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`-3-
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`holes in the respective side portions. The pair of swash plate holes are open to the pair of
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`casing holes, respectively. Proximal end portions of the respective trunnion shafts are
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`inserted into the respective swash plate holes to be engaged to the swash plate unrotatably,
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`relative to the swash plate. A distal end portion of one of the trunnion shafts projects
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`from the corresponding casing hole to the outside of the transaxle casing so as to be
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`operatively connected to an operation device for controlling the displacement of the
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`hydraulic pump.
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`[0006]
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`Accordingly, the swash plate separated from the pair of trunnion shafts can be
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`easily installed into the transaxle casing, and can be easily removed from the transaxle
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`casing, because the movement direction of the swash plate to be installed or removed into
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`and from the transaxle casing is the same as a movement direction of main component
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`parts of the hydraulic pump, such as a cylinder block, to be installed or removed into and
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`from the transaxle casing. In this regard, due to the separation of the trunnion shafts from
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`the swash plate, the swash plate can easily be installed or removed into and from the
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`transaxle casing independently of the insertion or withdrawal of the trunnion shafts into
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`and from the case holes of the transaxle casing. The transaxle casing does not need an
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`opening for allowing movement of the swash plate in the axial direction of the trunnion
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`shafts. Therefore, the transaxle casing does not need a cover for covering the opening,
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`thereby minimizing the number of housing members for constituting the transaxle casing.
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`[0007]
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`In some embodiments, the pair of side portions of the swash plate are formed with
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`respective distal end surfaces that are adjacent to inside surfaces of the respective side
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`walls of the transaxle casing so that a width of the swash plate between the distal end
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`surfaces in the axial direction of the trunnion shafts is equal to or slightly less than a
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`distance between the inside surfaces of the side walls of the transaxle casing in the axial
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`direction of the trunnion shafts.
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`[0008]
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`Therefore, the swash plate and the transaxle casing are minimized in the axial
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`direction of the trunnion shafts so as to minimize the hydraulic transaxle.
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`[0009]
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`In some embodiments, the transaxle casing includes at least two housing members
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`joined to each other at a joint plane parallel to the common axis of the trunnion shafts.
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`One of the at least two housing members is formed integrally with the pair of side walls
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`having the respective casing holes.
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`

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`-4-
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`[0010]
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`Therefore, due to the ab ove—mentioned separation of the trunnion shafts from each
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`other and the swash plate, only the one of the at least two housing members is formed
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`with both the side walls having the casing holes, thereby reducing the number of parts and
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`costs,
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`in comparison with the conventional transaxle casing that has the opening for
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`allowing the movement of the swash plate formed integrally with the trunnion shafts for
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`installing or removing the swash plate into and from the inside space of the transaxle
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`casing so as to need the cover for covering the opening and journaling one of the trunnion
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`shafts.
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`[0011]
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`In some embodiments, the hydraulic transaXle further comprises a control arm and
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`a neutral biasing mechanism for biasing the swash plate to a neutral position for the
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`hydraulic pump.
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`The control arm is provided on an outside end portion of a
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`corresponding one of the side walls of the transaXle casing to be fixed on the distal end
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`portion of the one of the trunnion shafts so as to operatively connect the swash plate to
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`the operation device. The trunnion shafts are identical in shape to each other. The neutral
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`biasing mechanism is provided on an outside end portion of the other of the side walls of
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`the transaXle casing to be provided on a distal end portion of the other of the trunnion
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`shafts.
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`[0012]
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`The pair of trunnion shafts identical
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`in shape to each other reduce costs for
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`manufacturing the trunnion shafts. The neutral biasing mechanism can be easily provided
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`or removed on and from the outside end portion of the side wall of the transaXle casing
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`journaling the other trunnion shaft than the trunnion shaft having the control arm thereon.
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`[0013]
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`These and other features and advantages will appear more fully from the
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`following detailed description with reference to the attached drawings.
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`BRIEF DESCRIPTION OF THE DRAWWGS
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`[0014]
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`Fig.
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`l
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`is a schematic plan view of a lawn mower lOO serving as a zero-turn
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`vehicle equipped with right and left transaXles IR and IL and with right and left control
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`levers lOlR and lOlL for controlling respective transaxles 1R and IL.
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`[0015]
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`Fig. 2 is a schematic plan view of a lawn mower 300 serving as a zero-turn
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`vehicle equipped with right and left transaXles 1R and 1L and with a steering wheel 301
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`and a speed control pedal 306 for controlling right and left transaxles 1R and 1L.
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`

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`-5-
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`[0016]
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`Fig. 3 is a plan view of a transaxle 1 representing each of right and left transaxles
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`[0017]
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`[0018]
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`IR and IL of a zero—turn vehicle.
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`Fig. 4 is a bottom view of transaxle 1.
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`Fig. 5 is a side view of transaxle 1 when viewed from a side toward a distal end of
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`an axle 2 projecting outward from a transaxle casing 10 of transaxle l.
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`[0019]
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`Fig. 6 is a cross sectional side view of transaxle 1 taken along A-A arrows of Fig.
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`[0020]
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`3.
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`3.
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`Fig. 7 is a cross sectional front view of transaxle 1 taken along B-B arrows of Fig.
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`[0021]
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`Fig. 8 is the same cross sectional front view of transaxle l as Fig. 7 except for an
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`alternative arrangement of a hydrostatic stepless transmission (hereinafter referred to as
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`HST) control arm 46 and a neutral return spring 47.
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`[0022]
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`Fig. 9 is a cross sectional front view of transaxle 1 taken along C-C arrows of Fig.
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`[0023]
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`3.
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`3.
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`Fig. 10 is a cross sectional rear view of transaxle 1 taken along D-D arrows of Fig.
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`[0024]
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`Fig. 11 is a cross sectional rear view of transaxle 1 taken along E-E arrows of Fig.
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`3 wherein an HST 20 and relevant arms are omitted.
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`[0025]
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`Fig. 12 is a cross sectional rear view of transaxle 1 taken along F—F arrows of Fig.
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`[0026]
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`Fig. 13 is the same cross sectional rear view of transaxle 1 as Fig. 12 except for an
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`alternative reduction gear train 703.
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`[0027]
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`Fig. l4(a) is a sectional side view of a trunnion shaft 45 and an eccentric pin 49
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`showing neutral return spring 47 when a movable swash plate 44 of HST 20 is disposed at
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`a neutral position.
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`[0028]
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`Fig.
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`l4(b) is a sectional side view of trunnion shaft 45 and eccentric pin 49
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`showing neutral return spring 47 when movable swash plate 44 of HST 20 is not disposed
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`at the neutral position.
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`Fig. 15 is a perspective view ofa center section 30 for HST 20.
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`Fig. 16 is a plan view of center section 30.
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`Fig. 17 is a bottom view of center section 30.
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`Fig. 18 is a rear view of center section 30.
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`[0029]
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`[0030]
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`[0031]
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`[0032]
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`

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`[0033]
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`[0034]
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`[0035]
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`—6—
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`Fig. 19 is a sectional plan view of center section 30.
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`Fig. 20 is a sectional side view of center section 30.
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`Fig. 21 is a side view of an alternative transaxle 1A serving as right transaxle 1R
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`that represents the pair of right and left transaxles 1R and 1L, including a sectional side
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`view of an upper portion of a reduction gear train 170 taken along N—N arrows of Fig. 28.
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`[0036]
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`Fig. 22 is a sectional bottom view of transaxle 1A taken along G—G arrows of Fig.
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`21, showing bottom views of axle 2, an HST 120, reduction gear train 170 and a gear
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`locking system 180 in a transaxle casing 110 from which a lower transaxle housing 112
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`has been removed.
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`[0037]
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`Fig. 23 is a sectional side view of transaxle 1A taken along H—H arrows of Fig. 22,
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`showing sectional side views of axle 2, HST 120, reduction gear train 170 and a gear
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`locking system 180A as an embodiment of gear locking system 180.
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`[0038]
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`Fig. 24 is a sectional front view of transaxle 1A taken along 1—1 arrows of Fig. 22,
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`showing a sectional font view ofHST 120.
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`[0039]
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`Fig. 25 is a fragmentary sectional side view of transaxle 1A taken along J-J arrows
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`of Fig. 22, showing a sectional side view of HST 120.
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`[0040]
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`Fig. 26 is a fragmentary sectional front view of transaxle 1A taken along K-K
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`arrows of Fig. 22, showing front views of charge check and bypass valves 121 having
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`function as bypass valves in HST 120 and a bypass valve operation mechanism 127.
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`[0041]
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`Fig. 27 is a fragmentary sectional plan view of transaxle 1A taken along L—L
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`arrows of Fig. 23, showing front views of charge check 121 having the function as bypass
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`valves in HST 120 and bypass valve operation mechanism 127.
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`[0042]
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`Fig. 28 is a plan view of reduction gear train 170 in transaxle casing 110 of
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`transaxle 1A from which a gear top cover 113 has been removed, showing a gear locking
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`system 180B serving as another embodiment of gear locking system 180.
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`[0043]
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`Fig. 29(a) is a sectional side view of gear locking system 180A when a locking
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`pawl 181A engages with bevel pinion 71, Fig. 29(b) is a sectional side view of gear
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`locking system 180A when locking pawl 181A disengages from bevel pinion 71, and Fig.
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`29(c) illustrates gear locking system 180A when viewed in the axial direction of a motor
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`shaft 51.
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`[0044]
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`Fig. 30(a) is a sectional side view of gear locking system 180B when a locking
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`pawl 181B engages with bevel pinion 71 via an engagement plate 182, Fig. 30(b) is a
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`

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`-7-
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`sectional side view of gear locking system 180B when locking pawl 181B disengages
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`from bevel pinion 71 and engagement plate 182, and Fig. 30(c) illustrates gear locking
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`system 180B when viewed in the axial direction of motor shaft 51.
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`[0045]
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`Fig. 31 is a fragmentary sectional rear view of transaXle 1A taken along M-M
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`arrows of Fig. 28, showing a sectional rear view of reduction gear train 170 provided with
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`a bevel gear shaft support structure shown in Figs. 21 and 28.
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`[0046]
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`Fig. 32 is a fragmentary sectional rear view of transaXle 1A taken along M-M
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`arrows of Fig. 28, showing a sectional rear view of reduction gear train 170 provided with
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`an alternative bevel gear shaft support structure.
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`[0047]
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`Fig. 33 is a sectional side view of a transaXle 1Aa including HST 120, showing a
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`sectional side view of a center section 30A and charge check valve 121 having the
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`function of bypass valves.
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`[0048]
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`Fig. 34 is a perspective bottom view of transaxle 1Aa from which a lower
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`transaxle housing 1 12 has been removed.
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`[0049]
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`Fig. 35 is another perspective bottom view of transaxle 1Aa from which lower
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`transaXle housing 112 and a filter 26 have been removed.
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`[0050]
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`Fig. 36 is a perspective plan view of a front portion of transaXle 1Aa with a pump
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`shaft 41 from which an input pulley 14 and a cooling fan 15 have been removed.
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`Fig. 37 is a perspective front view of center section 30A.
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`Fig. 38(a) is a schematic plan view of a bypass operation arm 128A and stopper
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`[0051]
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`[0052]
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`edges llla-l 1 1c.
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`[0053]
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`Fig. 38(b) is another schematic plan view of a bypass operation arm 128A and
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`stopper edges 1 l la-l 1 1c.
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`[0054]
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`Fig. 38(c) is another schematic plan view of a bypass operation arm 128A and
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`stopper edges llla-111c.
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`[0055]
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`Fig. 38(d) is another schematic plan view of a bypass operation arm 128A and
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`stopper edges 111a-111c.
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`[0056]
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`Fig. 39 is a fragmentary sectional side view of transaXle 1Aa, showing a sectional
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`side view of a breathing pipe 115.
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`[0057]
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`Fig. 40 is a sectional side view of an alternative transaxle 1Ab including HST 120
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`provided with a charge pump mechanism.
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`

`

`—8—
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`[0058]
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`Fig. 41 is a sectional plan view of a lower transaxle housing 112A of transaxle
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`1Ab, showing a plan view partly in section of the charge pump mechanism.
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`[0059]
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`Fig. 42 is a bottom view of a center section 303 used for transaxle lAb, showing
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`the charge pump mechanism drawn in phantom lines as fitted to center section 30B.
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`[0060]
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`Fig. 43 is a hydraulic circuit diagram of a zero-turn vehicle in which transaxle
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`lAa having no charge pump mechanism serves as the left transaxle and transaxle 1Ab
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`having the charge pump mechanism serves as the right transaxle.
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`DETAILED DESCRIPTION
`
`[0061]
`
`Referring to Figs.
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`1 and 2, each of lawn mowers (hereinafter simply referred to as
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`“vehicles”) 100 and 300 serving as typical zero-turn vehicles is equipped with right and
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`left transaxle IR and IL carrying respective right and left axles 2R and 2L. Each of
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`vehicles 100 and 300 includes a vehicle body frame 4 having right and left parallel
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`sideboards 4a extended in the fore—and—aft horizontal direction thereof. Vehicle body
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`frame 4 supports right and left transaxles 1R and 1L so that transaxle casings 10 of right
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`and left transaxles IR and 1L are disposed in the inside of vehicle body frame 4, i.e.,
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`between right and left sideboards 4a when viewed in plan. Right axle 2R is extended
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`rightward from transaxle casing 10 of right transaxle 1R to the outside of right sideboard
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`4a of vehicle body frame 4 so as to be fixed at a distal end thereof to right drive wheel 3R.
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`Left axle 2L is extended leftward from transaxle casing 10 of left
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`transaxle 1L to the
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`outside of left sideboard 4a of vehicle body frame 4 so as to be fixed at a distal end
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`thereof to left drive wheel 3L.
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`[0062]
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`Right and left drive wheels 3R and 3L serve as right wheels of each of vehicles
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`100 and 300 so as to be disposed on the right and left outsides of a rear portion of vehicle
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`body frame 4. A front end portion of vehicle body frame 4 supports castors (or castor) 5
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`serving as front wheels (or a front wheel) of vehicle 100 or 300. A mower unit 6 is
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`disposed below a fore-and-aft middle portion of vehicle body frame 4 between front
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`wheels 5 and rear wheels 3R and 3L. Further, each of vehicles 100 and 300 is equipped
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`with a prime mover (not shown) such as an engine whose output power is transmitted via
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`a belt (not shown) or the like to input pulleys 14 of respective right and left transaxles 1R
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`and IL.
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`

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`-9-
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`[0063]
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`Referring to Fig. 1, vehicle 100 is provided with right and left control levers 101R
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`and 101L. Link rods 102 are extended rearward from respective right and left control
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`levers 101R and lOlL to respective transaxle casings 10 of right and left transaxles IR
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`and 1L. Therefore, right and left link rods 102 extended from respective control levers
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`101R and lOlL are disposed along respective right and left sideboards 4a of vehicle body
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`frame 4. When viewed in plan, right and left link rods 102 are extended in the fore-and-
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`aft direction along respective right and left Sideboards 4a. As discussed later, transaxle
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`casing 10 of each of right and left transaxles IR and IL can be selectively provided with
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`HST control arm 46 on either the right or left proximal side surface thereof or the right or
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`left distal side surface thereof. Therefore, in the present embodiment, transaxles IR and
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`IL in vehicle 100 are provided with respective HST control arms 46 on the distal side
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`surfaces of respective transaxle casings 10 thereof because right and left link rods 102 are
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`rather close to the distal sides of vehicle 100. Further, a later-discussed neutral return
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`spring 47 is provided on the right or left side surface of transaxle casing 10 of each of
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`right and left transaxles IR and IL opposite HST control arm 46. Therefore, whether
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`neutral returning spring 47 is provided on the proximal or distal side surface of transaxle
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`casing 10 depends on whether HST control arm 46 is provided on the proximal or distal
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`side surface of transaxle casing 10.
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`[0064]
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`Right control lever 101R is operable to change the tilt angle and direction of a
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`later—discussed movable swash plate 44 of an HST 20 of right transaxle 1R so as to
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`change the rotary speed and direction of right rear wheel 3R. Left control lever 101L is
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`operable to change the tilt angle and direction of movable swash plate 44 of HST 20 of
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`left transaxle 1L so as to change the rotary speed and direction of left rear wheel 3L.
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`When right and left control
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`levers 101R and lOlL are synchronously manipulated,
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`movable swash plates 44 of HSTs 20 of right and left transaxles 1R and IL are
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`synchronously controlled to change the straight travel speed or direction of vehicle 100.
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`When right and left control
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`levers 101R and 101L are independently manipulated,
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`movable swash plates 44 of HSTs 20 of right and left transaxles 1R and IL are
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`differentially controlled so as to control the turn direction, angle and speed of vehicle 100.
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`[0065]
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`Alternatively, referring to Fig. 2, vehicle 300 is provided with a steering wheel
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`301 that is connected to a sector gear 303 via a stem 302. Right and left link rods 304 are
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`extended from sector gear 303 to a zero-tum control unit 305, and right and left rods 307
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`

`

`-10-
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`are extended from zero—turn control unit 305 to respective transaxle casings 10 of right
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`and left transaxles 1R and 1L. A speed control pedal 306 is operatively connected to
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`zero-tum control unit 305. Due to the depression of speed control pedal 306, right and
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`left link rods 307 are pulled or pushed synchronously in the same direction and to the
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`same degree to rotate respective HST control arms 46, thereby controlling the travel
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`speed of vehicle 300 in either the forward or backward direction. Due to the rotation of
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`steering wheel 301, right and left link rods 307 are independently pushed or pulled so as
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`to differentially rotate right and left HST control arms 46, thereby turning vehicle 300.
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`Typical zero-turn control unit 305 is configured as disclosed by US 4,875,536 A, for
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`example.
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`[0066]
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`In this regard, zero—turn control unit 305 is disposed at the lateral middle portion
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`of vehicle 300 between right and left sideboards 4a of vehicle body frame 4 so that right
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`and left link rods 307 extended from zero-turn control unit 305 are disposed inward from
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`respective right and left sideboards 4a so as to be rather distantly inward from respective
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`right and left sideboards 4a. Therefore, transaxles IR and 1L in vehicle 300 are provided
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`with respective HST control arms 46 on the proximal side surfaces of respective transaxle
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`casings 10 thereof.
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`[0067]
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`In each of vehicles 100 and 300, right and left transaxles 1R and lL are arranged
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`to have respective axles 2R and 2L being coaxial to each other.
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`Incidentally, right and
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`left transaxles 1R and lL are arranged so that later—discussed HSTs 20 operatively
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`connected to respective HST control arms 46 are disposed in respective front portions of
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`transaxle casings 10 of transaxles lR and 1L and so that axles 2R and 2L are supported
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`by respective rear portions of transaxle casings 10 of transaxles 1R and lL rearward from
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`HSTs 20. Alternatively, either or both of transaxles 1R and IL may be disposed to have
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`HST 20 rearward from axle 2R or 2L.
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`[0068]
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`Referring to Figs. 3 to 20, a transaxle l carrying an axle 2 will be described on the
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`assumption that it represents right transaxle 1R carrying right axle 2R and left transaxle
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`lL carrying left axle 2L such as those of zero—turn vehicle 100 or 300. Transaxle 1
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`includes a transaxle casing 10, an HST 20 disposed in a front portion of transaxle casing
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`10, an axle 2 (serving as either right axle 2R or left axle 2L of vehicle 100 or 300)
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`journalled by a rear portion of transaxle casing 10, and a reduction gear train 70 disposed
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`in the rear portion of transaxle casing 10 so as to drivingly connect HST 20 to axle 2.
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`

`

`-11-
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`[0069]
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`Transaxle casing 10 includes a transaxle housing 11, a lower cover 12 made of a
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`metal plate, and a top cover 13. Transaxle housing 11 serving as a main body of transaxle
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`casing 10 is entirely formed by casting. A downwardly open chamber 10a is formed in
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`transaxle housing 11 so as to extend from a front end of transaxle housing 11 to a rear end
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`of transaxle housing ll. HST 20 is disposed in a front half portion of chamber 10a in
`
`transaxle housing 11, and reduction gear train 70 is disposed in a rear half portion of
`
`chamber 10a in transaxle housing 11.
`
`[0070]
`
`Transaxle housing 11 is formed with bosses lla, 11m and lln having respective
`
`vertical
`
`through holes through which respective bolts are passed to fasten transaxle
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`housing 11 to vehicle body frame 4 of vehicle 100 or 300. Front and rear bosses lla are
`
`formed on front and rear ends of transaxle housing 11. A substantially laterally
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`symmetric main body of transaxle housing 11 is formed by casting with a main mold (not
`
`shown), and an axle support portion lli of transaxle housing 11 for joumaling axle 2 is
`
`formed by casting with a sub mold (not shown) combined with the main mold so as to be
`
`joined to the main body of transaxle housing 11. Whether the sub mold is disposed
`
`rightward or leftward of the main mold can be selected so that axle support portion lli
`
`can be selectively formed on either the right rear portion of the main body or the left rear
`
`portion of the main body.
`
`In this regard, transaxle 1 illustrated in Figs. 3 to 13 serves as
`
`right transaxle 1R carrying right axle 2R so that axle support portion lli extends
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`rightward to support right axle 2R as drawn in solid lines in Figs. 3 and 4 and others.
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`Alternatively, transaxle housing 11 may have axle support portion lli extended leftward
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`to support left axle 2L as drawn in phantom lines in Figs. 3 and 4 and others so as to
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`constitute left transaxle lL carrying left axle 2L. Whether axle support portion lli
`
`extends rightward or leftward, front and rear bosses lln are formed on axle support
`
`portion lli, and front and rear bosses llm are formed on a right or left side of transaxle
`
`housing 11 opposite axle support portion lli in the lateral direction.
`
`[0071]
`
`Lower cover 12 is fastened by bolts 16 to a bottom edge of transaxle housing 11
`
`so as to cover the bottom opening of chamber 10a in transaxle housing 11 surrounded by
`
`the bottom edge of transaxle housing 11. A ceiling wall of transaxle housing 11 has a top
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`opening 11 g in a rear top portion thereof defining the rear half portion of chamber lOa
`
`incorporating reduction gear train 70. Top cover 13 is fastened by bolts 17 to the edge
`
`

`

`-12-
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`portion of transaxle housing 11 surrounding top opening 11g so as to cover an upper
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`portion of reduction gear train 70 in transaxle housing 11.
`
`[0072]
`
`HST 20 includes a center section 30, a hydraulic pump 40 and a hydraulic motor
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`50. Referring to Figs. 15 to 20, center section 30 is entirely formed by casting so as to
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`have a horizontal discoid pump port block 3 l , an upwardly slant discoid motor port block
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`32, a bypass valve block 33 and plural (in this embodiment, four) bolt bosses 34 and 35.
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`Center section 30 will be described on the assumption that motor port block 32 is joined
`
`to pump port block 31 so as to extend rearwardly upward from a rear end of pump port
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`block 31.
`
`[0073]
`
`As shown in Figs. 15, 16, 19 and 20, pump port block 31 is formed therein with
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`right and left pump kidney ports 31b and 31c and a pump shaft hole 31d between pump
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`kidney ports 3 lb and 31c. A horizontal pump mounting surface 31a is formed on a top
`
`end of pump port block 31. Pump kidney ports 31b and 31c and pump shaft hole 31d are
`
`extended vertically upward so as to be open at horizontal pump mounting surface 3 l a.
`
`[0074]
`
`As shown in Fig. 17 and others, a horizontal filter mounting surface 31e is formed
`
`on a bottom end of pump port block 31. A circumferential area of filter mounting surface
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`31e is provided to contact a top edge of a cylindrical filter 26 (see Fig. 6 and others).
`
`Plural
`
`(in this embodiment,
`
`three) filter retainers 31i project downward from filter
`
`mounting surface 31e and are formed and aligned along the circumferential area of filter
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`mounting surface 31e so as to fit an upper portion of a peripheral side surface of filter 26
`
`whose top edge contacts the circumferential area of filter mounting surface 3le. Pump
`
`shaft hole 31d is extended vertically downward so as to be open at a central portion of
`
`filter mounting surface 3le. A charge port gallery 31h is open at a front portion of filter
`
`mounting surface 31e between pump shaft hole 31d and the circumferential area of filter
`
`mounting surface 31e. Right and left charge ports 31f and 31g are open at respective
`
`bottom ends thereof to charge port gallery 31h.
`
`[0075]
`
`In this regard, when HST 20 is disposed in transaxle casing 10, filter 26 contacting
`
`filter mounting surface 31e of center section 30 is submerged in a fluid sump in chamber
`
`10a so that fluid is filtered by filter 26 when the fluid penetrates filter 26 from the fluid
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`sump in chamber 10a to the inside space of filter 26. The fluid in the inside of filter 26
`
`constantly enters charge port gallery 31h and charge ports 31f and 31g.
`
`

`

`-13-
`
`[0076]
`
`As shown in Figs. 19 and 20 and others, right and left main fluid passages 36 and
`
`37 are formed in pump port block 31 so as to extend in the horizontal
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`fore—and—aft
`
`direction. Front portions of main fluid passages 36 and 37 are diametrically expanded to
`
`serve as charge valve chambers 36a and 37a that are open outward at the front end of
`
`pump port block 31. Right kidney port 3 lb is joined at a bottom thereof to a top portion
`
`of right main fluid passage 36 and right charge port 31f is joined at a top thereof to a
`
`bottom portion of right charge valve chamber 36a. Similarly, left kidney port 31c is
`
`joined to left main fluid passage 37, and left charge port 3 lg is joined to left charge valve
`
`chamber 3 7a.
`
`[0077]
`
`As shown in Fig. 19, right and left charge check valves 21 are disposed in
`
`respective right and left charge valve chambers 36a and 37a. Each charge check valve 21
`
`includes a valve casing 22, a ball valve member 23, a pressure pin 24 and a spring 25.
`
`Valve casing 22 is fitted in each of charge valve chambers 36a and 37a so as to serve as a
`
`plug closing the outward opening of each of charge valve chambers 36a and 37a. Valve
`
`casing 22 is formed therein with a valve port 22a that is constantly open to each of charge
`
`ports 31f and 31g. Ball valve member 23 is biased by spring 25 so as to block valve port
`
`22a from each of fluid passages 36 and 37. Charge check valve 21 is configured so that
`
`when either main fluid passage 36 or 37 is hydraulically depressed and the hydraulic
`
`pressure therein is lower than that in valve port 22a, the differential pressure of fluid
`
`between valve port 22a and hydraulically depressed main fluid passage 36 or 37 thrusts
`
`ball valve member 23 against spring 25 so as to open valve port 22a to hydraulically
`
`depressed main fluid passage 36 or 37, whereby fluid passed through a later-discussed
`
`filter 26 is supplied to main fluid passage 36 or 37.
`
`[0078]
`
`Motor port block 32 is formed therein with right and left motor kidney ports 32b
`
`and 32c and a motor shaft hole 32d between motor kidney ports 32b and 32c. A slant
`
`upper end of motor port block 32 is formed as a rearwardly downward slant motor
`
`mounting surface 32a. Right and left motor kidney ports 32b and 32c and motor shaft
`
`hole 32d are extended rearwardly upward so as to be open at motor mounting surface 32a.
`
`Right and left main fluid passages 36 and 37 are extended rearward into motor port block
`
`32 so as to be joined at rear ends thereof to respective right and left motor kidney ports
`
`32b and 32c.
`
`

`

`-14-
`
`[0079]
`
`Bypass valve block 33 projects downward from motor port block 32 so as to
`
`extend in the lateral horizontal portion. A lateral horizontal bypass valve hole 33a is
`
`formed through bypass valve block 33. Right and left ends of bypass valve holes 33a are
`
`open outward at right and left vertical end surfaces of bypass valve block 33. Right and
`
`left vertical bypass ports 33b and 33c are extended downward from respective right and
`
`left motor kidney ports 32b and 32c so as to cross bypass valve hole 33a and are further
`
`extended downward from bypass valve hole 33a so as to be open outward at a horizontal
`
`bottom surface of bypass valve block 33.
`
`[0080]
`
`In this regard, as shown in Fig. 9, a rotary bypass valve 27 having right and left
`
`diametric valve passages 27a and 27b is fitted in bypass valve hole 33a