Abstract:
A hydrostatic transaxle assembly is drivingly connected to a power source and the power source is activated through an electric starter circuit. The hydrostatic transaxle includes an axial type hydrostatic transmission module having a pump which includes a plurality of axially arranged cylinders each including a piston therein. A tiltable swash plate includes a lateral surface rotatable within the hydrostatic transmission casing. The swash plate is in engagement with the plurality of pistons wherein tilting of the swash plate effectuates fluid displacement of the pump. A neutral switch is fixed relative to the transmission casing and includes a registering portion engaged with the swash plate to detect a neutral position and correspondingly allow starting of an electric starter through the starter circuit. A neutral adjustment assembly includes a control rod rotatably supported by the casing and a two piece shift lever is attached thereto. The two piece lever includes a first member attached to the control rod and a second member normally releasably fixed to the first member but selectively rotatable relative to the first member when loosened from the first member. A resilient biasing member has a first leg held substantially fixed to the casing and a second leg held substantially fixed to the second member of the shift lever to bias the first member of the shift lever into a neutral position and at least one fastener releasably fixes the first and second members together.

Description:
This is a Continuation-in-Part of U.S. patent application Ser. No. 09/498,692, filed Feb. 7, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to hydrostatic transaxles intended primarily for use in the lawn and garden industry on riding lawnmowers, lawn and garden tractors and the like, but may also be applied to larger implements and vehicles. 
     2. Description of the Related Art 
     Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically oil, and then back to rotary mechanical motion to rotate a pair of drive axles in order to drive the vehicle. The hydrostatic transmission controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Such transmissions have utilized radial piston pumps and motors, axial piston pumps and motors and hybrid transmissions wherein the pump may be of the radial piston design, for example, and motor formed as a gear pump. The speed of the output of the transmission is typically controlled by varying the eccentricity of the pump track ring or swash plate. 
     It is well known to provide a “neutral switch” for use with many vehicle types so that an operator is prevented from starting or activating the vehicle when the vehicle&#39;s transmission is engaged. Neutral switch applications, heretofore, include neutral switch placement, specifically the motion sensing portion thereof, external to a vehicle&#39;s transmission to monitor the neutral position secondarily through linkage positioned remotely respective of the pump. Problems arise when linkage becomes loose or worn or if the switch is damaged through the normal rigors of agricultural usage. One such problem involves “creep” i.e., slight movement of the vehicle when apparently in neutral, due to loose linkage between a shift lever and the internal componentry comprising the transmission. Often, the neutral switch contacts or registers off of loose linkage which allows activation of the power source even though the transmission remains engaged, albeit slightly. 
     In response to difficulties encountered with neutral switch placement exterior to the transmission, the neutral switch, or the motion sensing portion thereof, was installed internally for use with radial piston-type hydrostatic transmissions. Although the benefits of the neutral switch may be appreciated with radial piston hydrostatic transmissions, axial piston hydrostatic transmissions include certain advantages over their radial counterparts. One such advantage is reduction in overall transmission size which provides for a reduction in materials corresponding to a decrease in cost. A neutral switch adaptable to an axial piston hydrostatic transmission, capable of registering a neutral position internally to the transmission would be most desirable. Another problem incident with hydrostatic transmissions, specifically neutral arrangements and adjustability assemblies therefor, is the lack of a neutral adjustment assembly, externally accessible and low in cost. 
     The speed of a hydrostatic transmission is generally selectively controlled by an operator via a hand control or foot pedal control, for example, by varying the eccentricity of the pump track ring or swash plate. Hydrostatic transmissions do not always provide a true ‘neutral’ when first assembled (i.e., transmission fluid is pressurized by the pump, albeit minimally, which can cause rotation of the axle). Manufacturers confront this problem by providing a neutral adjustment mechanism, as part of the hydrostatic transmission, which cooperates with the swash plate such that a true neutral may be exacted after assembly. Some neutral adjustment mechanisms are provided internally within the hydrostatic transmission casing. Included in the group of neutral adjustment mechanisms, customarily used, is the extendible threaded linkage shaft. The linkage shaft allows an operator fine adjustment of the swash plate by extending or collapsing, via threaded engagement, the threaded shaft to effectuate the adjustment. 
     The neutral adjustment offered by a threaded shaft tends to vibrate loose, and additionally, fine adjusting via a threaded shaft is often a cumbersome task since the shaft must be secured and the ends thereof extended. Internal type neutral adjustment mechanisms are also cumbersome since substantial disassembly of a vehicle is generally necessary to access the neutral adjustment device. What would be highly desirable is a neutral adjustment mechanism that is accessible subsequent to assembly of a vehicle and one which preserves a neutral setting once adjustment is finalized. Further, a neutral adjustment assembly which readily adapts to the structure of the existing neutral return assembly structure, mandating few additional parts and little if any additional machining, would be highly desirable. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art by providing a hydrostatic transaxle including a neutral switch, disposed within a transmission casing and registering off of the swash plate. Moreover, a neutral adjustment assembly is provided, to overcome further disadvantages plagued by the prior art, by providing a neutral adjustment assembly including an externally arranged two-piece shift lever structure facilitating neutral adjustment externally and within in proximity to the shift lever, which is particularly useful in foot pedal control applications. 
     The hydrostatic transaxle of the present invention is drivingly connected to a power source which is electrically activated through an ignition circuit and a variable displacement pump is fixed relative to the casing and driven by the power source. The pump includes a pump cylinder barrel which includes a plurality of parallel arranged axially reciprocable pistons. A swash plate is positioned between the pistons and the casing and includes a lateral surface rotatably engaged within a recess defined by the casing. Each piston is engaged with the swash plate whereby tilting of the swash plate effectuates a fluid displacement in the pump. A neutral switch includes a portion fixed to the casing and a registering portion internally positioned within the casing. The registering portion of the switch is positioned against the swash plate itself whereby pump displacement electrically deactivates the ignition circuit. 
     The present invention further provides a neutral adjustment assembly for use with a hydrostatic transmission including a casing enclosing the hydrostatic transmission and a control rod rotatably supported by said casing and including a portion positioned externally relative to said casing. A two member shift lever is provided which includes a first member attached to the externally positioned portion of the control rod and a second member adjustably fixed relative to the first member. A resilient member, having a first end held to the casing and a second end held to the second member of the shift lever wherein the resilient member urges the first member of the shift lever into a neutral position. At least one fastener is provided and releasably secures the first and second members together, wherein the first member is moveable relative to the second member through at least one slot formed in either the first or the second members of the shift lever. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of the embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a sectional view of a transaxle in accordance with one form of the present invention taken along a horizontal plane intersecting the axes of the axles; 
     FIG. 2 is a sectional view of the hydrostatic transmission taken along a vertical plane; 
     FIG. 3 is an elevational view partially in section showing the foot pedal shift embodiment of the hydrostatic transmission; 
     FIG. 4 is a perspective view of the pump and motor block assembly, partially in section, showing the neutral switch and a pair of fluid passageways common to the pump and motor; 
     FIG. 5 is a plan view of the swash plate; 
     FIG. 6 is a front elevational view of the swash plate of FIG. 5, viewed along line  6 — 6 ; 
     FIG. 7 is an end view of the swash plate of FIG. 5, viewed along line  7 — 7 ; 
     FIG. 8 is a partially fragmented end view of the swash plate of FIG. 5, viewed along line  8 — 8 ; 
     FIG. 9 is a plan view of the swash plate and neutral switch of FIG. 4 viewed from the top illustrating the switch in an electrically engaged position and the transmission in a neutral condition; 
     FIG. 10 is a fragmentary perspective view of the pump and motor block assembly showing the swash plate tilted and the neutral switch in an electrically disengaged position; 
     FIG. 11 is a plan view of the swash plate and neutral switch of FIG. 10 viewed from the top; 
     FIG. 12 is an electrical diagram of an ignition circuit showing the neutral switch in an electrically engaged state corresponding to the swash plate and neutral switch arrangement of FIGS. 4 and 9; 
     FIG. 13 is an electrical diagram of an ignition circuit showing the neutral switch in an electrically disengaged state corresponding to the swash plate and neutral switch arrangement of FIGS. 10 and 11; 
     FIG. 14 is an end elevation of one of the bearing strips; 
     FIG. 15 is a bottom view of the upper half casing of the hydrostatic transmission broken away showing one of the bearing strips; 
     FIG. 16 is a sectional view of the hydrostatic transmission shown in FIG. 1, taken along a vertical plane; 
     FIG. 17 is a perspective view of a pump and motor block assembly, partially in section, illustrating the return to neutral linkage assembly; 
     FIG. 18 is an exploded view of the return to neutral linkage assembly of FIG. 17; 
     FIG. 19 is a sectional view of the return to neutral linkage assembly of FIG. 2, viewed along line  19 — 19 ; 
     FIG. 20 is an elevational view of the return to neutral linkage assembly of FIG. 2, viewed along lines  20 — 20 ; and 
     FIG. 21 is a sectional view of the turn to neutral linkage assembly of FIG. 2, viewed along line  21 — 21 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, transaxle  20  is drivingly engaged with a power source, typically a gasoline powered engine (not shown), whereby transaxle  20  transfers power, generated from the power source, to a pair of respective drive axles  24  and  26  rotatably mounted within axle mechanism module  34 . Transaxle  20  includes hydrostatic transmission module  28  removably connected with axle mechanism module  34 . Axle mechanism module  34  includes casing  32  which encloses reduction gear train  40  and differential mechanism  42  supported by casing  32 . Output shaft  38  extends between hydrostatic transmission casing  30  and axle mechanism casing  32  and includes a mechanical disconnect mechanism  31  of the type disclosed in U.S. Pat. No. 5,701,738 assigned to the assignee of the present application and is expressly incorporated herein by reference. Mechanical disconnect mechanism  31  is provided to disengage axle mechanism module  34  from hydrostatic transmission module  28 . As is customary, output shaft  38  is engaged with reduction gear train  40  which, in turn, is engaged with differential mechanism  42  to provide power to drive axles  24  and  26 . Hydrostatic transmission module  28  includes pump  33  and motor  35  hydraulically connected through pump and motor block  44 . Pump and motor block  44  is fastened to upper casing half  37  of hydrostatic transmission casing  30  by a pair of threaded screws  39  (FIG.  16 ). 
     Transaxle  20  includes brake mechanism  21  having distal end  23  of shaft  38  splined to disc  25  which is engaged by a pair of friction pads (not shown) when brake lever  27  is rotated. A cast housing  19  supports the brake mechanism  21  and is mounted to axle casing by a pair of screws  17 . Brake mechanism  21  (FIG. 1) employs a self-adjustment feature comprising a self-adjusting nut  18  that accommodates for friction pad wear. The operation of the brake itself is well known and the self-adjustment mechanism is the subject of pending patent application Ser. No. 09/165,904, filed Oct. 2, 1998, and assigned to the assignee of the present application. This application is expressly incorporated herein by reference. 
     Referring to FIGS. 2,  3  and  16 , hydrostatic transmission module  28  includes casing halves  37  and  41  fastened together by screws  45  to form transmission casing  30 . Similarly, axle mechanism module  34  includes a pair of casing halves fastened together by a plurality of screws (not shown) to form axle mechanism module  34 . One of the casing halves is identified as  29  and is shown in FIG.  1 . The structure and operation of the modular transaxle is the subject of pending patent application Ser. No. 09/498,692, filed Feb. 7, 2000, and assigned to the assignee of the present application. This application is incorporated herein by reference. 
     Referring to FIGS. 2 and 16, hydrostatic transmission module  28  of transaxle  20 , includes hydrostatic pump and motor mechanism  36  which provides pump and motor block  44  rotatably supporting pump cylinder barrel  46  and motor cylinder barrel  48  (FIG.  4 ). Pump cylinder barrel  46  includes a plurality of axially arranged cylinders  50  each having piston  52  disposed therein. Pump cylinder barrel  46  and pistons  52  are common and interchangeable with motor cylinder barrel  48  and respective pistons  52  disposed therein to decrease costs associated with implementing separate components. Springs  16  are provided within each cylinder  50  and contact pistons  52  to provide continuous engagement of pistons  52  with the respective swash plate (FIG.  2 ). 
     Swash block or swash plate  56 , positioned between pump cylinder barrel  46  and casing half  37  of transmission casing  30 , includes arcuate axial surface  94  which engages, and is tiltable within, casing half  37 . The path of rotation or tilt of swash plate  56  is illustrated by arrows  51  in FIG. 16. A pair of arcuate low friction bearing strips  63  (FIGS. 14 and 15) are captured within casing half  37  and provide a guide for tilting axial surface  94  of swash plate  56  (FIGS.  2  and  16 ). Referring to FIGS.  2  and  14 - 16 , each bearing strip  63  includes arcuate concave surface  67 , which is engaged with arcuate surface  94  of swash block  56 , and an arcuate convex surface  69 , which abuts respective arcuate recess  75  (FIGS. 2 and 16) formed in transmission casing half  37 . Each recess  75  in casing half  37  includes protrusion  91  which extends within aperture  93  defined in bearing strip  63  (FIG.  15 ). These bearing strips are the subject of pending patent application Ser. No. 09/498,692, filed Feb. 7, 2000, and assigned to the assignee of the present application. The disclosure of this patent application is expressly incorporated herein by reference. Swash plate  56  includes second lateral surface  96 , positioned overlaying and substantially opposite respective of first lateral surface  94 . Second lateral surface  96  includes bore  59  and counterbore  61  (FIGS. 2 and 8) therein, which receives thrust bearing  54  (FIG.  2 ). 
     Referring to FIG. 2, thrust bearing  54  is axially arranged, respective of pump cylinder barrel  46 , and is in contact with ends of pistons  52 . Thrust bearing  54  comprises a pair of grooved plates or races  53 ,  55  which capture therebetween a plurality of ball bearings  57  fitted within grooves formed in plates  53  and  55 . Thrust bearing  54  fits snugly within swash plate  56 , specifically plate  53  of thrust bearing  54  engages bore  59  of swash plate  56  and plate  55  freely rotates within counterbore  61  (FIG.  2 ). 
     In operation, pump cylinder barrel  46  is driven by the power source (not shown) through input shaft  22 . Typically, input shaft  22  includes a first end attached to sheave or pulley  65  (FIGS. 3 and 16) which is belt driven by the power source. Fan  58  (FIG.  3 ), included to provide convective cooling to the transmission, and pulley  65  are axially abutted and keyed to shaft  22 , as is customary. The other end of input shaft  22  includes splined portion  60  which engages matching splined portion  62  formed within pump cylinder barrel  46  (FIG.  2 ). Referring to FIGS. 4 and 10, swash plate  56 , is manually or selectively controlled by shift lever  64  located externally to transmission casing  30 . Movement of shift lever  64  causes swash plate  56  to tilt which initiates fluid displacement within pump cylinder barrel  46 . The fluid displaced by pump  33  hydraulically connects motor  35  through a pair of arcuate passageways  78  and  80  formed within pump and motor block  44 . Motor cylinder barrel  48  comprises outwardly thrusting pistons  52  which contact an inclined and fixed swash plate  126  causing rotation of the motor cylinder barrel  48  (FIGS. 1,  4  and  10 ). Motor cylinder barrel&#39;s  48  rotation is transferred to rotation of drive axles  24  and  26  through reduction gear train  40  and differential mechanism  42 . 
     Referring to FIG. 2, the shift assembly will be described. A two-part shift lever includes shift lever  64  and adjustable member  186  and shift lever  64  is attached to rotatable control rod  66  (FIG. 2) by screw  68 , external to casing  30  (FIG.  3 ). Control arm  70  includes first end  71  attaching to control rod  66  and a second end  73  extending outwardly and generally perpendicular from control rod  66 . Second end  73  of control arm  70  pivots respective of control rod  66  when control rod  66  is rotated. Pin  72  attaches to second end  73  of control arm  70  and extends into slot  76  disposed on periphery  77  of swash plate  56 . Friction roller  74  fits over pin  72  and freely rotates about pin  72  to engage with slot  76  of swash plate  56 . Selectively positioning control lever  64 , for example, by an operator depressing a foot pedal or fender control lever linked thereto by way of linkage, causes swash plate  56  to tilt, and in turn, pistons  52 , orbiting about input shaft  22 , reciprocate thereby causing fluid to become pressurized within cylinder  50  by reciprocating pistons  52 . Notably, switch  82  is threadably connected to transmission casing  30  and includes registering portion  85  extending internally within interior  90  of transmission casing  30  to prevent non-neutral startup as described below (FIG.  2 ). 
     Switch  82  is a conventional limit-type switch and includes threaded housing portion  84  engaged with threaded portion  86  of casing  30  and registering portion  85  which includes ball or roller  89  retractably engaged with swash plate  56 . As shown in FIG. 2, roller  89  is disposed on an outermost extent of portion  84  of switch  82  and in direct engagement with a surface discontinuity such as groove  92  formed in swash plate  56  (FIG.  8 ). Groove  92 , formed in periphery  77  of swash plate  56 , extends from first lateral surface  94  of swash plate  56  to second lateral surface  96  of swash plate  56  (FIGS.  2  and  8 ). Groove  92  is located on periphery  77  of swash plate  56  and has a generally semi-circular cross-section  100  (FIGS. 5,  10  and  11 ). As best seen in FIGS. 4 and 10, swash plate  56  has an oval slot  98 , positioned generally centered respective of first lateral surface  94  of swash plate  56 , which receives input shaft  22 , extended through oval slot  98 . As best seen in FIG. 4, first lateral surface  94  of swash plate  56  is arcuate and overlays second lateral surface  96  of swash plate  56 . Located opposite groove  92  is slot  76  (FIG. 7) having a substantially rectangular cross-section  110  which receives the shift lever linkage. As best seen in FIG. 5, swash plate  56  is generally rectangular and its periphery  77  includes a pair of respective end faces  112 ,  114  and a pair of respective side faces  111 ,  113 . 
     Referring to FIGS. 4 and 10, rotation of control rod  66  through, for example, user displacement of shift lever  64 , causes arcuate lateral surface  94  of swash plate  56  to rotate, guided by contact between lateral surface  94  and bearing strips  63  (FIG.  16 ). Lateral surface  94  of swash plate  56  includes a pair of arcuate surfaces  132  and  134  (FIGS. 5-8) which are engaged with respective arcuate surfaces  65  of bearing strips  63  respectively, (FIGS.  14  and  15 ). Bearing strips  63  may be formed, for example, from a DELRIN and TEFLON composite, comprising about 20% TEFLON. Alternatively, bearing strips  63  may be manufactured from a like material which exhibits suitable durability and low friction characteristics. Referring to FIG. 16, end faces  112 ,  114  of periphery  77  of swash plate  56  rock or tilt, respective of face  115  of pump cylinder barrel  46 , illustrated by arrows  51 , to produce fluid displacement of pump  33 . In contrast, movement of side faces  111  and  113  (FIGS. 4-6 and  9 - 11 ) of periphery  77  may be best described as rotation in a single plane substantially perpendicular to face  115  of pump cylinder barrel  46  (FIG.  2 ). 
     Referring to FIGS. 1 and 4, pump and motor block  44  is a two piece assembly which includes pump block  47  and motor block  124  fastened together by a pair of screws  49  which thread into upper half  37  of casing  30  (FIG.  16 ). The two piece pump and motor block is the subject of pending patent application Ser. No. 09/498,666 filed Feb. 7, 2000, and assigned to the assignee of the present application. The disclosure of this application is expressly incorporated herein by reference. Referring to FIG. 4, pump and motor block  44  provide a pair of arcuate pump openings  116  and  118  in fluid communication with arcuate motor openings  120  and  122 , respectively, through respective passageways  78 ,  80 . Passageways  78  and  80  are formed in pump and motor block  44 , and comprise two continuous and separate passageways fluidly connecting pump  33  to the motor  35 . Referring to FIG. 2, hydraulic fluid accumulates within casing half  41  which is drawn in through filter  125  and enters lower portion  127  of pump and motor block  44  through ports (not shown). The ports are fluidly connected to respective passageways  78  and  80 . 
     Referring to FIG. 4, passageway  78 , of pump and motor block  44 , includes passageway  117  formed in pump block  47  mating with passageway  79  formed in motor block  124 . Similarly, passageway  80 , includes passageway  119  formed in pump block  47  mating with passageway  81  formed in motor block  124 . Passageways  117  and  119  in pump block may be machined by drilling cross holes and thereafter providing plugs  121  to close the drilled hole entrances (FIG.  16 ). However, passageways  117  and  119  may also be formed by providing foam cores during the casting process to eliminate additional machining and plugging of the pump block  47 . Passageways  79  and  81  in motor block  124  (FIG. 4) may be formed contemporaneously with motor block  124  through, for example, a powder metal manufacturing process. 
     Continuous passageways  78  and  80 , in pump and motor block  44 , are in hydraulic communication with pump cylinder barrel  46  through arcuate openings  116  and  118 , respectively. Similarly, passageways  78  and  80  are in hydraulic communication with motor cylinder barrel  48  through respective arcuate openings  120  and  122  formed in motor block  124 . Arcuate pump openings  116 ,  118 , and additionally, arcuate motor openings  120 ,  122 , are machined to provide a suitably precise tolerance of corresponding opening to respective orifice  123  (FIG. 2) within pump cylinder barrel  46  and motor cylinder barrel  48 . 
     Referring to FIGS. 2-4 and  16 , swash plate  56  is illustrated in a “neutral position” respective of face  115  of pump cylinder barrel  46 . The neutral position corresponds to pistons  52  within pump cylinder barrel  46  being substantially equally extended, so as not to cause reciprocation thereof. The neutral position coincides with an insignificant fluid displacement of pump  33 . It may be seen that plate  55  of thrust bearing  54  is substantially coplanar with face  115  of pump cylinder barrel  46  when pump and motor mechanism  36  is in the neutral position (FIGS.  2  and  16 ). Neutral switch  82  is electrically engaged so that an operator may energize the power source without concern of activating an engaged transmission. Referring to FIG. 12, shown is a typical ignition circuit  150  corresponding to hydrostatic pump and motor mechanism  36  in the neutral position as shown in FIGS. 4 and 9. Ignition circuit  150  includes normally closed switch  82  electrically connected to starter  152 , ignition switch  154  and battery  156 . Typically, an operator activates or closes switch  154  by turning an ignition key (not shown) which provides power, via battery  156  to starter  152 , to start power source (not shown), such as a combustion engine. 
     Referring to FIGS. 4 and 9, registering portion  85  of switch  82 , positioned internally relative to hydrostatic transmission housing (FIG.  2 ), includes semi-spherical roller  89  which extends into groove  92  provided on side face  113  of periphery  77  of swash plate  56  to close switch  82 . It is envisioned that the extending portion of the switch may contact groove  92 , or alternatively, a gap may exist between roller  89  and groove  92  to provide equally suitable electrical connection of switch  82 . 
     Referring to FIGS. 10 and 11, hydrostatic pump and motor mechanism  36  is shown in an “engaged position” corresponding to a significant displacement of fluid from pump cylinder barrel  46  to motor cylinder barrel  48 . As best shown in FIG. 10, swash plate  56  is tilted, respective of face  46  of pump cylinder  46 , which sequentially forces each piston  52  to reciprocate as pump cylinder barrel  46  rotates. Plate  55  of thrust bearing  54  is at an angle respective of face  115  of pump cylinder barrel  46 . Neutral switch  82  is electrically disconnected (FIG.  13 ), thus an operator is prevented from activating the power source. Neutral switch  82  is engaged with side face  113  of periphery  77  of swash plate  56  and roller  89  is displaced or retracted into housing portion  84  of switch  82 . End face  112  of periphery  77  of swash plate  56  is tilted toward face  115  of pump cylinder barrel  46  and pistons  52  are urged to reciprocate within their respective cylinder  50  as pump cylinder barrel  46  rotates, driven by input shaft  22 . Correspondingly, roller  89  of switch  82  is retracted into a body of switch  82  which provides an electrical deactivation or “open” switch  82  (FIG.  13 ). Swash plate  56 , tilted in either direction  51  (FIG.  16 ), results in roller  89  traversing groove  92  to deactivate switch  82 . 
     Referring to FIG. 13, shown is ignition circuit  150 , which corresponds to hydrostatic pump and motor mechanism  36  in an engaged position as shown in FIGS. 10 and 11 and switch  82  is mechanically engaged with side face  113  of periphery  77  of swash plate  56 , corresponding to a significant displacement of fluid from pump  33  to motor  35 . Thus, switch  82  is disengaged and activation of ignition switch  154 , by itself, will not provide power via battery  156 , to starter  152 . 
     Referring to FIGS. 2,  17 , and  18 , neutral adjustment assembly  158  will now be described. Shift lever  64  is attached to rotatable control rod  66  and includes a square aperture  160  which corresponds to, and is engaged with, square portion  162  formed on an end of control rod  66 . Threaded fastener  68  is threaded into control rod  66  which rigidly attaches shift lever  64  to control rod  66  (FIGS.  3  and  18 ). Spacer  166  is provided with clearance hole  170  and outer surface  172  of control rod  66  is extended through hole  170  of spacer  166 . Torsion spring  168  is substantially concentrically positioned relative to spacer  166 , restrained by outer surface  176  of spacer  166 . Torsion spring  168  is provided with a pair of outwardly extended and substantially parallel legs  178  and  180 . First and second legs  178  and  180  of torsion spring  168  are in continuous engagement with a pair of posts. First post  182  extends from casing  30  and post  184  extends inwardly toward casing and is rigidly fastened to second member  186  of the two-piece shift lever. In the exemplary embodiment, second member  186  of the two-piece shift lever consists of an adjustable plate. 
     As best seen in FIGS. 2 and 19, post  184  is positioned above post  182 , however it will be understood that post  184  is rotatable, rotating respectively with shift lever  64  while post  182  remains fixed with casing  30  and extends through large through hole  185 . Adjustable plate  186  fastens to shift lever  64  by means of a pair of threaded fasteners  188  and  189  which respectively engage holes  190  and  192  of shift lever  64  (FIG.  19 ). Unlike clearance hole  193  in second member or adjustable plate  186 , slot  194  in adjustable plate  186  facilitates positional rotation of shift lever  64  relative to adjustable plate  186 , which is best seen in FIGS. 20 and 21. Slot  194  is defined by slot wall  191  which includes a pair of arcuate stops  195 ,  197  (FIG.  18 ). Stops  195 ,  197  limit the adjustability of neutral adjustment assembly  158 , i.e., rotation of shift lever  64  relative to adjustable plate  186 . Located at a lowermost portion of shift lever  64  is through hole  196 , which receives foot pedal linkage (not shown) such as, for example, cable linkage. 
     In operation, as an operator rotates shift lever  64 , via a foot pedal (not shown) or other control typically known to those having ordinary skill in the art. In turn, shift lever  64  is thereby rotated, for example, in a first direction as indicated by arrow  202  (FIGS.  17  and  19 ). In this first position, post  184 , extending from adjustable plate  186 , rotates relative to control rod  66  and is in continuous contact with inner portion  198  of second spring leg  180  (FIG.  19 ). It may be seen that, while inner portion  198  of leg  180  is in continuous and rotating contact with pin  184 , inner portion  200  of second leg  178  is in stationary and in continuous contact with post  182 . 
     Conversely, operator movement of shift lever  64 , for example, via pedal depression as was previously mentioned in a second direction (e.g., reverse) as indicated by arrow  204 , is now described. First leg  178  of torsion spring  168  is caused to rotate, rather than second leg  180  of torsion spring  168 , as previously described relative to movement of shift lever  64  as indicated by arrow  202 . Specifically, when an operator rotates shift lever  64  as indicated by arrow  204 , post  184  is in continuous contact with inner portion  200  of leg  178  and rotates relative to control rod  66 . Second leg  180  of torsion spring  168  is in stationary and in continuous contact with post  182 , extended from casing  30 . Since first and second legs  178  and  180  of spring  168  rotate relative to each other, an oppositional force is developed which counters motion of the moving leg to thereby return shift lever to a neutral position. This general type of spring return action for neutral holding has been used for a number of years on hydrostatic transmissions. 
     Neutral adjustment mechanism  158  of the present invention includes shift lever  64  and adjustable plate  186 . Shift lever  64 , as best seen in FIG.  20  and corresponding to rotation direction  204 , is rotatable relative to adjustable plate  186  at a maximum angle from the vertical  195 , which is, for example, 2.344°. Additionally, shift lever  64  may be rotatably adjusted in direction indicated by arrow  202 , an additional 2.344° (not shown). Therefore, shift lever  64  includes a range of rotational adjustability which spans 4.688°. 
     In operation, shift lever  64  pivots about threaded fastener  188  and slot  194  in adjustable plate  186  accommodates for rotation of shift lever  64  relative to adjustable plate  186 . Through hole  185  in shift lever  64  also accommodates for the shift lever&#39;s positional rotation relative to adjustable plate  186  and specifically the positioning of post  184  extending through hole  185  of shift lever  64 . It will be understood by those having ordinary skill in the art that when fine adjustment of the hydrostatic transmission assembly is warranted, to obtain a neutral position therefor, threaded fastener  188  and  189  are loosened enough that shift lever  64  is rotatable relative to adjustable plate  186 . Thereafter, a neutral position for shift lever  64  is located and threaded fasteners  188  and  189  are tightened to rigidly affix adjustable plate  186  to shift lever  64 . 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.