Abstract:
A transmission casing design including externally mounted protrusions or devices on the casing for creating a durable, visible coding system. The casing may include a plurality of housing sections for mounting a hydrostatic transmission or other type of transmission. The protrusions may be machined or otherwise durably marked to create an information code.

Description:
This application is a divisional application of U.S. Ser. No. 09/196,182 filed Nov. 20, 1998 now U.S. Pat. No. 6,122,996 issued Sep. 26, 2000. 
    
    
     BACKGROUND 
     This invention relates to an improved design of a hydrostatic transmission (“HST”) and includes several novel features. Hydrostatic transmissions are well known in the art, and are more fully described in, e.g., U.S. Pat. No. 5,314,387, which is incorporated herein in its entirety. Many of the inventions described herein can also be adapted for use in an integrated hydrostatic transmission (“IHT”) incorporating output gearing and axles within a single housing. 
     In general, an HST has a hydraulic pump and a hydraulic motor mounted in a housing. The pump and motor are hydraulically linked trough a generally closed circuit, and both consist of a rotatable body with pistons mounted therein. Hydraulic fluid such as oil is maintained in the closed circuit, and the HST generally has a sump or reservoir with which the closed circuit can exchange oil. This sump may be formed by the housing itself. 
     The pump is usually driven by an external motive source such as pulleys or belts connected to an internal combustion engine. The pump pistons engage a moveable swash plate and, as the pump is rotated by an input source driven by the external engine, the pistons engage the swash plate. Other HST designs may use a radial piston or ball piston pump and motor design, but the general operation is similar, and this invention is not limited to use with a specific design. Movement of the pump pistons creates movement of the hydraulic fluid from the pump to the motor, causing rotation thereof. The motor pistons are engaged against a fixed plate, and rotation of the motor drives an output shaft engaged thereto. This output shaft may be linked to mechanical gearing and output axles, which may be internal to the HST housing, as in an IHT, or external thereto. 
     The pump/motor system is fully reversible in a standard HST. As the swash plate against which the pump pistons move is moved, the rotational direction of the motor can be changed. In addition, there is a “neutral” position where the pump pistons are not moved in an axial direction, so that rotation of the pump does not create any movement of the hydraulic fluid. 
     The HST closed circuit has two sides, namely a high pressure side in which oil is being pumped from the pump to the motor, and a low pressure or vacuum side, in which oil is being returned from the motor to the pump. When the swash plate angle is reversed, the flow out of the pump reverses so that the high pressure side of the circuit becomes the vacuum side and vice versa. This hydraulic circuit can be formed as porting formed within the HST housing, or internal to a center section on which the pump and motor are rotatably mounted, or in other ways known in the art. Check valves are often used to draw hydraulic fluid into the low pressure side to make up for fluid lost due to leakage, for example. Such check valves may be located so that they directly contact the porting or they may be located separate from the porting and connected through additional bores to the closed circuit. 
     There is a need to have a means to open, or bypass, this closed circuit in certain circumstances. For example, when the vehicle is stopped, the oil in the closed circuit provides hydraulic braking, making it impossible to manually move the vehicle. Mechanical bypass designs are known in the art and are described in, for example, U.S. Pat. No. 5,010,733. Such designs generally achieve bypass by opening the closed hydraulic circuit to the sump by, e.g. opening check valves in the circuit, or by opening a shunt between the high pressure and low pressure sides of the circuit. Such designs are generally complicated and add significantly to the cost of the unit. 
     SUMMARY OF THE INVENTION 
     This housing design is a significant improvement over current transaxle designs. Using a traditional transaxle design, it is very difficult to achieve rear discharge, as the input shaft is near the vehicle centerline. Some designs have attempted to overcome this problem by mounting the transaxle on the same deck as the engine, and using connecting chains to another axle on which the tires are mounted. Such a design adds significantly to the overall cost of the unit. 
     One aspect of this invention is the use of a housing formed of two pieces, generally divided along a vertical axis with respect to the orientation of the output axles. One section of the housing or casing is much narrower than the other housing to maintain clearance between the body of the transmission and the vehicle flame on one side, in order to accommodate a rear discharge chute. Many of the HST elements internal to the housing are contained in the larger of the two casing portions. In addition, the external housing elements are designed to conform as closely as possible to the shape of the internal IHT elements, so as to minimize the amount of material needed and the overall size of the unit. In essence, this design allows the main housing component to be offset to one side of the vehicle, while still maintaining the input shaft at or near the vehicle center line. Thus, the discharge chute parallels the vehicle frame, rising up slightly to clear the axle horn. 
     A further object of the invention is to provide an HST having an improved swash plate mounted on at least one trunnion which is secured to the transmission casing, to offer lower control moments for the swash plate. This design offers improved control of the swash plate, which is particularly important for use of a foot control mechanism. 
     This invention also addresses the shortcomings in prior HST bypass designs, as an improved mechanical bypass system for a hydrostatic transmission is disclosed herein. One particular improvement of this design is in the tolerances allowed, as this design reduces or eliminates many of the tolerance issues which arise from known bypass designs. This invention uses a filter housing secured to the bottom of the center section indirectly by the check plugs, and a filter secured to the filter housing. The bypass actuator is mounted inside the filter housing and is actuated by means of a bypass rod which can extend outside the housing of the hydrostatic transmission to be operated by the user. Rotation of the rod causes the actuator to engage the check balls to unseat them from the check plug and allow the unit to enter the bypass mode. Other embodiments include use with an HST where the hydraulic porting is integrally formed with the transmission housing and the filter housing and filter are thus secured directly to the transmission housing. 
     A further object of this invention is to provide an improved and novel design of a center section for an HST, whereby the output shaft of the hydrostatic motor is secured at least partially by the center section and is positioned so that the axis of the output shaft is located below the plane of the surface on which the hydrostatic pump is mounted on the center section. The benefits of this arrangement include, among other things, a reduced height of the pump, motor and center section, which can reduce the overall height of the unit and/or provide more flexibility for mounting other HST elements. The horizontal mounting of the center section also allows for the use of the vertical split line as disclosed herein and the unique arrangement of the HST elements within the housing units. 
     A further object of this invention is to provide an improved and novel expansion chamber that can be bolted or otherwise secured to the HST and which prevents leakage or spillage of the hydraulic fluid therefrom. In a preferred embodiment this chamber is external to the housing and includes an internal tube extending from the top of the tank to the bottom, although variations on this design will be obvious to one of skill in the art. The use of an external tank allows for use of a smaller transmission housing, and reduces the possibilities of leakage due to gear splash and oil movement at various operating angles. The internal tube provides siphoning action which allows for, among other things, greater flexibility in the location of the tank. 
     A further feature disclosed herein in one embodiment is an improved design of a friction pack which enables the vehicle user to maintain the position of the pump swash plate, and thus the speed and direction of the vehicle. Friction packs have been known for years in connection with HSTs and have been shown in, for example, U.S. Pat. No. 5,201,692. The improved design shown in the figures affords additional benefits that will be discussed herein. 
     A further embodiment of this invention provides a clip assembly secured to the ends of the axle horns to prevent excessive wear on the die cast transmission housing due to contact with the wheels. A pair of wheels are mounted at the ends of the axles and secured thereto by means of a retaining ring or other mechanism at the end of each axle. Many vehicle manufacturers will install washers on the axles between the wheels and the housing in order to space and locate the wheels. During operation of the vehicle, the wheels or the washers, as the case may be, can be forced into contact with the die cast aluminum housing, which can result in damage to the housing and oil seal. A clip composed of a material such as spring steel can be secured at the end of the housing to provide the necessary wear surface and prevent direct contact between the die cast housing and the wheels or washers. 
     There is also a need in the industry for being able to review a unit and readily determine information about the unit, such as its place and date of manufacture or similar information. At the present time, such information is generally placed on a unit by means of a label. This creates additional costs in both parts and assembly, and placement of a label on such a unit is made difficult by the obvious problem of oil present on the unit during the assembly process. The present invention in one embodiment solves this problem by use of a “information pad” comprising a series of protrusions on the external housing of the unit, which may be machined or left in the natural state, to create a variety of patterns. These patterns can be used as a code for any information the manufacturer may wish to include. The cost of machining for small external pieces is relatively small, and once the unit is so coded, the code will always be visible and accessible. 
     Other benefits and objects of this invention are disclosed herein and will be obvious to readers of ordinary skill in the art. The features disclosed herein can be combined to create a unique hydrostatic transmission design; it is understood, however, that such features are unique in their own right and can be used independently with other transmission designs, as will be obvious to one of ordinary skill in the art. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an external housing for an integrated hydrostatic transmission in accordance with the present invention. 
     FIG. 2 is an exploded view of a center section and bypass mechanism in accordance with the present invention. 
     FIG. 3 is a sectional side view along the lines  3 — 3  in FIG. 5 showing a hydrostatic transmission incorporating a center section and bypass mechanism in accordance with the present invention, and showing a different embodiment of a torque bracket. 
     FIG. 4 is a detailed sectional view of the center section and bypass unit shown in FIG.  3 . 
     FIG. 5 is a top view of the transmission in accordance with one embodiment of the present invention. 
     FIG. 6 is a cross-sectional side view of a transmission in accordance with one embodiment of the present invention, along the lines  6 — 6  in FIG.  5 . 
     FIG. 7 is the same cross-sectional side view of a transmission as shown in FIG. 6, in accordance with another embodiment of the present invention, showing the differential block as shown in FIG.  9 . 
     FIG. 8 is a side sectional view along the lines  8 — 8  of FIG.  5 . 
     FIG. 9 is a side view of the transmission with one portion of the casing removed. 
     FIG. 10 is a side view of the transmission, with one portion of the casing and the bevel gears of the differential removed. 
     FIG. 10-A is a side view of the transmission similar to that shown in FIG. 10, with a different embodiment of the differential. 
     FIG. 11 is a side view of the center section hydrostatic pump and motor and swash plate of the subject invention. 
     FIG. 12 is a perspective view of the swash plate of the subject invention. 
     FIG. 13 is another perspective view of the swash plate of the subject invention. 
     FIG. 14 is a sectional side view of the external expansion chamber of the subject invention. 
     FIG. 14-A is a sectional side view of the external expansion chamber as shown in FIG. 14, and also showing portions of the transmission housing. 
     FIG. 15 is a partial side view of a portion of a transmission and locking clip incorporating an embodiment of this invention. 
     FIG. 16 is an end view of the transmission housing and locking clip shown in FIG.  15 . 
     FIG. 17 is a perspective view of the center section. 
     FIG. 18 is an expanded view of the floating friction pack in accordance with one embodiment of this invention. 
     FIG. 19 is a view of the floating friction pack of FIG. 18 mounted on the transmission housing. 
     FIG. 20 is a partial sectional view of the floating friction pack as shown in FIG.  19 . 
     FIG. 21 is a rear view of a tractor using a transmission in accordance with one embodiment of the present invention. 
     FIG. 22 is a side view of the external housing, showing a second embodiment of the return to neutral feature of the present invention. 
     FIG. 22-A is another side view of the external casing design. 
     FIG. 23 is a top view of an alternative embodiment of the external housing for a hydrostatic transmission, without the external controls. 
     FIG. 24 is a top view of another alternative embodiment of the external housing for a hydrostatic transmission, without the external controls. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The figures herein, and in particular, FIGS. 1,  3 ,  5 ,  6  and  9  illustrate an IHT configured with a vertically split housing with main casing  21  and side casing  22 . The arrangement of these housing elements are a key feature of the design, but certain embodiments of this invention do not require any specific housing configuration, and other housing configurations can be accommodated therewith. All specifics of an IHT are not shown in these figures, as the general operation of an IHT is known in the art. In general, where different embodiments of the various elements of the transmission are shown in different figures, like numerals designate like elements. 
     Pump  11  is disposed on center section  10  and receives input shaft  24 , which communicates with and is driven by a vehicle engine (not shown). Center section  10  includes internal porting  25  that hydraulically connects pump  11  comprising pump cylinder block  17  and pump pistons  28  and a hydraulic motor comprising cylinder block  27  and motor pistons  32 . Pump pistons  28  engage adjustable swash plate  23  to create pressure within internal porting  25 . As shown in, e.g., FIGS. 3 and 8, pistons  28  generally include a spring  124  mounted therein and piston washer  125  placed in the top of piston  28  to prevent damage to the piston by spring  124 . 
     Casings  21  and  22  form an internal sump or reservoir  43  external to center section  10 . Motor cylinder block  27  is connected to and drives output shaft  66 , which in turn drives various reduction gears, including gear  67 , gear  69 , gear  70  and differential  68  including bull gear  72 . Differential  68  is in turn operatively connected to the output drive axles  90 A and  90 B of the vehicle. 
     As shown in FIGS. 2,  11  and  17 , center section  10  has a motor running surface  12  and a pump running surface  14 , on which motor cylinder block  27  and pump cylinder block  17  are respectively mounted for rotation. Center section  10  acts as, among other things, a mounting unit for the pump and motor of the hydrostatic transmission. 
     One goal of the invention is to minimize the effort required to manufacture such an HST, and to minimize the number of fasteners needed. Furthermore, the use of the horizontal connections between center section  10  and casing  21  allows for the vertical split configuration shown herein, with most of the HST elements being located in main casing section  21 . 
     As shown in FIGS. 2 and 17 center section  10  can be secured to main casing  21  through bolt openings  15 . Since these bolts are horizontal with respect to the HST as it is in use, the design uses stops  45  and  45   b  on center section  12  to contact main casing  21 . Motor running surface  12  is formed as an integral part of center section  10  and includes sides  12   a  shaped so as to fit in a bore in main casing  21  in such a manner as to allow free communication of the hydraulic oil between the area surrounding the motor and the internal sump formed by the housing sections. The interaction of side  12   a  and stops  45   a  and  45   b  of center section  10  with main casing  21  supports the center section in the vertical direction and prevents rotation of center section  10  caused by torque in the system. 
     The hydraulic circuit is integrally formed as porting  25  in center section  10 , although other alternative embodiments could be used. Such a hydraulic circuit generally has a high pressure side and a low pressure, or vacuum, side. Arcuate ports  13   a  and  13   b  are formed in motor running surface  12  and arcuate ports  13   c  and  13   d  are formed in pump running surface  14 , and each such port corresponds to either the high pressure or low pressure sides of the hydraulic circuit. Check openings  16   a  and  16   b  are formed in center section  10  and are similarly correlated to the respective sides of the circuit. As shown most clearly in FIG. 4, check plugs  18  are threaded into the check openings  16 , or may be fitted therein through other methods, and act to secure check balls  20 . The operation of check plug systems is generally known in the art and is disclosed in U.S. Pat. No. 5,546,752, which is incorporated herein in its entirety. Check openings  16  are formed on what is generally referred to, for ease of reference, as the bottom of center section  10  although it is understood that the orientation is not so limited. 
     As shown in FIG. 2, a feature of this invention is the use of a separate filter housing  30 , which is mounted adjacent to check plugs  18  at the bottom of center section  10 . In the preferred embodiment, filter housing  30  is secured to the bottom of the center section  10  by washers  33  when check plugs  18  are screwed into openings  16 , and o-rings  36  are used to assist in securing check plug  18  and to create a seal. Filter  34 , which preferably is a 100 mesh filter, can be secured to filter housing  30  using flexible plastic snaps  35  which are integrally formed with filter housing  30 . Snaps  35  then extend through corresponding openings  37  formed on filter  34 . This allows filter  34  to be connected to housing  30  without the use of separate fasteners to minimize cost and assembly time. Other known methods of connecting filter  34  to housing  30 , such as use of fasteners or tabs formed on filter  34 , could also be used. It is also understood that the bypass mechanism disclosed herein is not specifically limited to the shape or design of the center section or check plug mechanisms disclosed, but could also be easily used with other center section or check plug designs, or even with units which do not use a center section, but have the porting mounted elsewhere in the unit such as integrally formed with the housing. 
     As shown in FIGS. 3 and 4, check balls  20  are mounted in internal chambers  19  of check plugs  18 . A seat is formed with openings  38  so that when a ball  20  is seated, no fluid can pass through opening  38 . Bypass actuator  40  is mounted through use of spring  41  on tab  42  of filter housing  30 , and use of guide pins  44  on filter  34 . Projections  46  are formed with actuator  40  to contact balls  20  when actuator  40  is forced in that direction. Check plugs  18  may include bleeds  48  to allow discharge of fluid under high pressure. Bleeds are generally known in the art and provide a smoother transition when starting the vehicle or changing direction, e.g. from forward to reverse, and can also provide cooling for the hydraulic circuit. 
     In the preferred embodiment, shield  50  is secured through use of guide pins  44  and bypass actuator  40  and is positioned to block the high pressure flow of fluid from bleeds  48 , in order to prevent the high pressure flow from contacting and damaging mesh filter  34 . Flange  47  is formed on shield  50  for the purpose of providing additional bending strength to the member. Other methods of strengthening shield  50  could also be used. A shield mechanism could take different shapes and could also be integrally formed as part of actuator  40  and/or projections  46 . 
     Actuation tab  51  is formed on actuator  40  and extends through an opening in filter  34  to contact paddle  53  of bypass actuator rod  52 , which acts as a cam. Spring  41  acts to hold actuator  40  and projections  46  in the “disengaged” position shown most clearly in FIG.  4 . When rod  52  is rotated, paddle  53  engages tab  51  and forces actuator  50  away from filter  34  and in a direction towards check balls  20 , overcoming the bias force of spring  41 . In this filly engaged position, projections  46  engage check balls  20  to push them off the seats and into internal chamber  19  to allow discharge of fluid from check plugs  18 , thus placing the unit in bypass. 
     Rotation of rod  52  back to its original position will take paddle  53  off of tab  51 , and the bias force of spring  41  will force actuator  40  off balls  20  to take the unit out of the bypass mode. Bypass rod  52  is rotated by means of an external arm  54 , as shown in FIGS. 6,  8  and  9  or it may be activated by other methods as known in the art. External arm  54  contacts tapered flat  91  formed on rod  52  and may be secured by means of a push-on nut. End  92  of rod  52  may rest in the housing or could be otherwise secured for rotation. End  92  of bypass rod  52  can rest in a slot  89  formed in main casing  21 , as shown most clearly in FIG. 9, where the other elements of the bypass have been removed, in order to facilitate ease of manufacture. As shown in FIG. 8, rod  52  is held in slot  89  by the lower side surface  12   a  of motor running surface  12 . As an alternative embodiment, the diameter of motor running surface  12  could be increased, and the rear side of surface  12  could have a bore formed in it or otherwise have an opening created to hold end  92  of rod  52 . 
     Other methods of actuating the bypass could also be used, such as a member extending directly through the bottom of casing  21 , which could directly engage tab  51 . Magnet  55  can optionally be secured on rod  52  by means of a tab, for example. This magnet functions as a washer to assist in maintaining rod  52  in the housing, while also acting to filter loose metal parts from the hydraulic fluid. It is understood that such a bypass design could be used with a variety of hydrostatic transmission designs. 
     As shown in FIGS. 1,  3 ,  5  and  6 , the transmission housing includes main casing  21  and side casing  22 , which are secured by bolts  31  along a vertical flange  61  defining a split line. The benefit of this arrangement is shown most clearly in FIG. 21, where the arrangement of main casing  21  and side casing  22  allows for a central location of input shaft  24  so that it can engage the driving linkage (not shown) without any modification of the tractor design, while still allowing use of a rear discharge chute  99 . 
     Input shaft  24  is powered by an external motive force (not shown) to power hydrostatic pump  11 . Input shaft  24  extends through an opening formed in casing  21 , and is supported therein by ball bearing  101 . Seal  122  and retaining ring  123  act to prevent leakage. Shaft  24  also extends through swash plate  23  and swash plate thrust bearing  29 . 
     As shown in FIG. 9, motor shaft  66  is drivingly engaged to gear  67 , which in turn is engaged to gear  69 . Gears  67  and  69  are mounted entirely within main casing  21 . Gear  71  is rotatably mounted on intermediate (or jack) shaft  70 . Gear  69  includes gear teeth on its internal diameter sized to correspond with the teeth of gear  71 , such that gear  71  fits inside and drives gear  69 . Gear  71  is also engaged to differential bull (or spur) gear  72 . A cross shaft  74  is mounted in bull gear  72  and has a pair of planet bevel gears  75  mounted thereon. Gear  71  and bull gear  72  are mounted such that the plane of flange  61 , i.e. the parting line between the two housing casings  21  and  22 , passes therethrough. As shown in FIG. 6, axle bevel gears  77  are engaged to axles  90 A and B and to the differential. 
     One of the benefits of the current design is that it provides a significantly smaller external housing for an HST than is generally provided by the prior art designs. As shown most clearly in FIGS. 1,  5  and  8 , the external housing is shaped to conform to the shape of the internal IHT components. This minimizes the amount of material needed, which reduces cost and weight. Such a design does present potential concerns for strength due to the smaller amount of material used. Therefore, a plurality of support ribs  104 , including flying rib  105 , are formed on the external surfaces of casings  21  and  22  to provide additional support for the housing. 
     As another embodiment, the housing could be constructed without the flying ribs as shown in FIG. 23, where main casing  221  and side casing  222  are formed without the ribs, and axles  290 A and  290 B extend from the casings  221  and  222 . The internal configuration of such a unit could be substantially the same as that shown in other embodiments herein, and input shaft  224  could be used to drive a pump in the manner described above. In such an embodiment the die cast aluminum of the housing would necessarily be enhanced in certain areas to increase the strength of the unit. This embodiment would improve the cooling of the unit, as air flow is maximized over the primary heat generating surfaces. 
     A further embodiment is shown in FIG. 24, where the housing consisting of main casing  321  and side casing  322  have been further reduced in size, so that axles  390 A and  390 B are rotatably supported therein but significant portions of said axles extend outside of the casings and are supported at the ends thereof by bearing pillow blocks  300 A and  300 B. The bearing pillow blocks  300 A and  300 B would then be mounted to the frame of the vehicle. 
     Axles  90 A and  90 B extend from their respective housings. As shown in FIG. 10, which shows the differential with the bevel gears removed, lobed bearings  78  act to secure bevel gears  77  and axles  90 A and  90 B, while solid bearings  79  provide support at the ends of the axles. The use of lobed bearings  78  allows transfer of hydraulic oil from the main casing to the internal chambers  88   a  and  88   b  of the axle horns, and the bearings include a clocking mechanism  80  to prevent rotation of the bearings  78  and the wear inherent in such rotation. 
     As shown in FIGS. 3,  9  and  11 , pump  11  is rotatably mounted on center section  10 . Hydrostatic transmissions in the past have generally used cradle mounted swash plates mounted directly on the housing. In the preferred embodiment of the present invention, the speed and direction of the hydrostatic transmission maybe changed by use of moveable swash plate  23 , which is mounted on trunnions  26   a  and  26   b  secured to casings  22  and  21 , respectively. As shown also in FIGS. 8 and 18, trunnion  26   a  includes a step  93  to act as an oil seal surface with trunnion seal  94  of casing  22 , and flats  49  extend outside casing  22  to engage control arm  108 . 
     Bolt  97  extends through opening  121  formed in control arm  108  and is threaded or otherwise secured directly into trunnion  26   a.  Opening  121  preferably has flat sides with a radius formed to improve stability of control arm  108 . In the preferred embodiment, friction bearings  130  interface between main casing  21  and trunnions  26   a  and  26   b.  It is understood that trunnions  26   a  and/or  26   b  could also run directly on the housing elements without the need for a friction bearing. 
     Center section  10 , pump cylinder block  17  and motor cylinder block  27  are mounted completely within the main casing  21 . Swash plate  23  crosses the parting line  61  of main casing  21  and side casing  22 , with the portion of the swash plate  23  that supports the pump block  11  within the main casing  21 , and trunnion  26  of swash plate  23  extends across the parting line or flange  61  to interface with side casing  22 . Swash plate  23  is supported by main casing  21  at one end, and by side casing  22  at the other end. 
     As shown in FIGS. 3 and 11, pump cylinder block  17  includes a plurality of pump pistons  28 , which engage thrust bearing  29  mounted inside swash plate  23 . Motor cylinder block  27  houses motor pistons  32 , which engage a fixed angle thrust bearing  39  secured in main casing  21 . 
     Swash plate  23  includes opening  76  formed therein for input shaft  24  to extend therethrough. As shown most clearly in FIG. 12, opening  76  includes a plurality of notches  76   a  formed therein to provide necessary clearance for input shaft  24 . Swash plate also is shaped to include a plurality of notches  81 , which can be used for clamping swash plate  23  during machining thereof. The location of notches  81  provides the optimal clamping location to avoid flexing the material during machining. A further benefit of notches  81 , and particularly the notches adjacent to trunnion  26   a  is to provide additional clearance inside the housing. As shown in, for example, FIG. 9, the location of notch  81  avoids contact of swash plate  23  with gear  69  during certain swash orientations. 
     Motor shaft  66  also crosses the parting line of main casing  21  and side casing  22 . One end of motor shaft  66  is supported by center section  10 , and the other end is supported by and extends out of side casing  22 , and includes a spline  66   a  for mounting to a conventional brake mechanism. Motor shaft  66  is mounted below the running surface  14  of center section  10  and parallel thereto, to reduce the height of these hydrostatic components. 
     In the preferred embodiment, housing casings  21  and  22  include a plurality of through holes  102  formed therein to be used to secure the transmission to a vehicle frame. These holes can be sized as needed for the application, and the number of holes can be increased or decreased. In addition to securing the transmission to the vehicle frame through bolt holes  102 , there is a need to secure the unit against rotation caused by the torque created by the unit. It is known to attach torque brackets to a vehicle and to secure them in some manner to the housing. One feature of this housing design is that the bolts  31  securing main casing  21  to side casing  22  extend all the way through both casings, as shown by way of example in FIG.  5 . Bolts  31  are sized to be long enough so that torque bracket  135  can be directly mounted on bolts  31 , which allows torque bracket  135  to be secured directly to the transmission housing during assembly of the transmission. This eliminates the need for separate attachment means, such as bolt holes being formed in the housing or stud  86  as shown in FIGS.  22  and  22 -A, thus lowering the manufacturing costs. It also eliminates the need for a separate assembly step to secure torque bracket  135  to the transmission when the transmission is mounted on the vehicle. 
     An oil fill port  106  is formed in main casing  21 , although it could be mounted elsewhere on the unit, and is used to fill the transmission as needed. 
     A further novel feature of one embodiment of the invention is in the design of the external expansion tank for hydraulic fluid. As shown most clearly in FIGS. 6,  14  and  14 -A, expansion tank  56  is secured to the main casing  21  and is shaped to fit securely against main casing  21 . Tab  132  extends from tank  56  and is secured to housing by use of fastener  133 , which is preferably a screw. Because tank  56  is shaped to conform to the shape of transmission main casing  21 , fastener  133  and fitting  58  are sufficient to hold it to the transmission. 
     Tube  57 , which may be composed of rubber, is inserted inside tank  56  and secured to fitting  58  and is sized to fit as close to the bottom of tank  56  as possible. Tank  56 , which may be composed of high density polyethylene, includes projection  59  having an opening formed therein extending therefrom and matching up to boss  73  extending from main casing  21 . Fitting  58  is mounted from the inside of main casing  21  and extends into the opening of projection  59 , and o-rings  63  act to prevent leakage of hydraulic fluid. Fitting  58  includes a barb-type end extending into tube  57  to provide an air-tight connection, and provides an internal passage  58   a  connecting passageway  58   b  to the internal volume of the transmission. In the preferred embodiment an internal hex is used to drive fitting  58  into main casing  21 . As shown in FIG. 9, a through hole  134  is formed in main casing  21  to connect to expansion tank  26  and fitting  58  is threaded therein. In the preferred embodiment, through hole  134  should be mounted as high in the unit as possible to maximize oil fill capacity and allow for the siphoning action of tube  57 . Having the tube at the highest point is also preferred to prevent excessive drainage of oil from the sump in the event an air leak develops. 
     Air vent  62  is formed in the tank  56  and is covered by cap  65 . A unique feature is the use of an additional flexible cap  64  which acts to prevent water and other foreign contaminants from entering the tank  56  during operation or cleaning of the vehicle. Flexible cap  64  is shaped to conform to the external configuration of tank  56  and cover cap  65  in its entirety. The use of a flexible material such as nitrile for cap  64  forms enough contact with the external housing to prevent water from entering the system; in a preferred embodiment a small groove may be formed in cap  64  to allow improved air ventilation but still keep the system essentially water-tight. 
     During use of the hydrostatic transmission, as the hydraulic oil expands through heating it will flow through fitting  58  into tube  57  and thus into tank  56 . As the oil cools and contracts, it will be drawn back in the reverse flow from tank  56  into the main housing. The placement of the open end of tube  57  adjacent the bottom of tank  56  prevents the hydraulic fluid from exiting the air vent  62  at the top of the tank regardless of the orientation of the unit during operation, thus eliminating the leakage problems inherent in other prior external tank designs. 
     As shown in FIGS.  1  and  1 -A, axles  90 A and  90 B extend outwardly from axle housings  21  and  22  respectively. Vehicle wheels (not shown) may be secured to each of said axles  90 A and  90 B through standard means such as a retaining ring (not shown) at the ends thereof, and as discussed above, washers (not shown) may be mounted between the wheel and the housing. In order to prevent contact of the wheels or the washers with transmission casings  21  and  22 , the present invention discloses use of a clip  82  to be secured on either end of the transmission. Clip  82  is preferably composed of spring steel, although other materials may be used, and such a clip could be used on any type of axle housing to prevent contact between such a housing and vehicle wheels. 
     FIG. 1 shows the transaxle with both clips  82  in place. Clip  82  can be secured to main casing  21  and side casing  22  through use of guide pin  84 , which can be integrally formed with the housing as cast, in the preferred embodiment, or can be separate members secured to the housing in known manners. Guide pin  84  engages slot  85  in clip  82  to assist in easily locating and mounting clip  82 . Clip  82  could also be secured through other methods known in the art and still accomplish the same functional benefits. Pads  83  may also be formed on main casing  21 , as cast, in order to prevent rotation of clip  82  under torque, to protect pin  84  from damage. This allows for a clip  82  having a generally square or rectangular shape, as depicted, to keep costs lower. Other methods of preventing rotation of clip  82  could also be used, such as shaping clip  82  to fit the housing thrust surface  97 . The curvature of clip  82  as shown in FIG. 15 aids in assembly of clip  82  to casings  21  and  22 . 
     As shown in FIGS. 18,  19 ,  20 ,  22  and  22 -A, an optional friction pack feature of the present invention includes a control arm  108  having an arcuate slot  110  formed therein. Carriage bolt  111  extends through arcuate slot  110  and engages nut  112 , and is not secured to side casing  22 . Friction packs  114   a  and  114   b  are mounted on bolt  111  and engage control arm  108 . Packs  114   a  and  114   b  can be manufactured from a generally flexible material such as acetal and washer  115  acts to maintain rigidity against packs  114   a.  Spacer  116 , spring  117  and washer  118  are also mounted on one end of the bolt  111  to maintain the proper level of friction. 
     Drag link stud  120  is threaded directly into side casing  22 , and extends through openings in drag link  119 , friction packs  114   a  and  114   b  and washer  115  as well as the arcuate slot  110  in control arm  108 . Arcuate slot  110  acts as an external means for limiting the movement of control arm  108  to limit movement of the internal trunnion mounted swash plate. 
     The entire assembly can thus move within arcuate slot  110  on stud  120 . Opening  113  can be used to attach control arm  108  to external linkages (not shown) of the vehicle. FIGS.  22  and  22 -A show different embodiments of the external linkages of the transmission, including for example the friction pack. 
     A further embodiment of the differential including differential block  95  is shown in FIG. 9, where like numerals designate like elements. As is known in the art, bull or spur gears such as gear  72  used in differentials must be properly positioned and must be of sufficient strength to withstand the inherent forces. One method known in the art is to maintain the bull gear as a generally solid piece with openings formed therein as needed. However, such a gear is undesirable as it adds to the weight of the unit and the manufacture of such a solid gear as a powdered metal part requires a significantly larger press machine, thus increasing manufacturing costs. The use of block  95  allows the use of a bull gear  72  having fairly large opening  96  therein to reduce the amount of material. Block  95  is held in slots  98  formed in bull gear  72  and acts to position bull gear  72 . FIG. 10-A shows a more standard arrangement of a differential block  107  in the transmission. 
     Another optional feature of the invention is the use of an external means for recording information directly on the housing in an inexpensive and durable manner. As shown in FIGS. 1,  5   14 -A and  22 , information pads  140  consist of a series of projections formed on main casing  21  and side casing  22 . The location of such a pad  140  is not critical, and pad  140  could also be formed on only one of the casings  21  and  22  instead of both. In the preferred embodiment, eight individual units are formed, and during the machining process, one or more of these individual units may be machined to encode any information the manufacturer wishes to include through the pattern of machined and un-machined projections. The use of eight individual units obviously offers a large number of coding possibilities, and the number of projections may be increased or decreased as needed. 
     As shown in FIG. 8, motor shaft  66  extends out of side casing  22 , and is supported therein by friction bearings  141 . Spline  66   a  engages brake disk  142 . Brake arm  144  is retained by castle nut  146  and bias is provided by spring  148 . As is known in the art, movement of brake arm  144  will cause yoke  149  to engage disk  142 , inhibiting the rotation thereof and thereby inhibiting the rotation of motor shaft  66 , slowing the vehicle. There is also a “return to neutral” feature disclosed in certain of the embodiments of the invention. FIGS. 1 and 22 show return arm  150  which engages ball bearing  152 . Adjusting puck  154  is secured at the base of return arm  150 . 
     It is to be understood that the above description of the invention should not be used to limit the invention, as other embodiments and uses of the various features of this invention will be obvious to one skilled in the art. This invention should be read as limited by the scope of its claims only.