Abstract:
A hydrostatic power transmitting device in combination with a speed reducing apparatus together disposed within a common housing having an interior space divided by partitioning device into a first region in which the hydrostatic unit is disposed and a second region in which said speed reducer is accommodated. First and second regions are segregated from each other by the partitioning device being in the form of a flexible non-porous barrier. The elastically deformable partitioning device can therefore adjust its shape to take up any change in the amount of hydrostatic transmitting fluid in the first region due to temperature changes of the fluid and facilitates the regulation in depth of lubricant held by the second region. Thereby an initially low level of lubricant in the second region lessens the adverse effect of power-retarding drag losses, especially during cold weather winter operation, and a rising level of lubricant ensures good lubrication when temperatures are elevated and viscosity is low. Preferably the second region is sealed from the environment, and the above atmospheric positive pressure in the second chamber aided or induced by the expanding volume of fluid in the first region provides a net increase of pressure experienced by low-pressure side of the closed-loop circuit of the hydrostatic transmission.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to hydrostatic power transmitting and speed reducing equipment having independent sumps which are useful in many diverse applications, one being for instance, a vehicle drive-line of the type generally known as a hydrostatic transaxle. This invention is particularly concerned with an improved transmission or transaxle having a housing with an interior space divided by a partitioning device into a first internal volume for the hydrostatic transmission mechanism and its associated operating fluid and a second internal volume accommodating a speed reduction mechanism in the form of a lubricated gear train. 
     2. Description of Related Art 
     Hydrostatic transaxles are increasingly being used in lawn care and other outdoor power equipment duties such as snow-blowing and have become the preferred choice for power transmission drive lines; for example, in lawn and garden tractors with most employing a single hydraulic pump fluidly connected to a single hydraulic motor. Although in most instances single motor hydrostatic transmissions are coupled by speed reduction gearing to a mechanical differential, applications also exist where two hydraulic motors are used and where each hydraulic motor is connected by a respective gear train to axle output shafts. Furthermore, two hydraulic pumps can also be used with two such hydraulic motors to create a hydrostatic transmission for each drive wheel which can be useful for zero-turn radius vehicle applications. Occasionally, single motor hydrostatic transmissions are used without the addition of a mechanical differential, such that the hydraulic motor is coupled by speed reduction gearing to a single output shaft, and in these instances, the output shaft may be the axle driving one wheel of the vehicle or be arranged to drive the axle of the vehicle by an interconnecting chain drive. 
     All hydrostatic transmission require hydrostatic power transmission fluid in order to operate and the fluid acts as the medium to convey power between the pump and motor of the hydrostatic transmission. As the positive displacement fluid pumping mechanisms used by all hydrostatic transmissions and hydrostatic transaxles require careful and accurate manufacture to achieve the necessary close tolerance fits in order to minimize internal fluid leakage losses associated with high-pressure performance, a preferred practice is to prevent damaging contamination generated by general wear and tear in the power transmitting gear train from reaching the pressurized circuit of the hydrostatic transmission. By removing the chances for damaging particles of contamination from entering the hydrostatic pressurized circuit, especially important when sintered powder-metal gears are used in the gear train, a long and useful working life for the hydrostatic transmission can be expected. 
     Although by no means essential, it can nevertheless be desirable to position the hydrostatic mechanism in a fluid compartment which is physically separate from any adjacent compartments in which the gear train is located such that no exchange of fluid can take place and whereby damaging contamination in the gear train compartment remains confined to that compartment. Contamination containment by way of separate compartments is shown in U.S. Pat. No. 5,090,949 titled Variable Speed Transaxle, expressly incorporated herein by reference. Here. a bulkhead is provided in the housing which carries a shaft seal, the shaft seal operating on the interconnecting drive shaft which mechanically couples the motor of the hydrostatic transmission in the hydrostatic compartment to the first reduction gear of the gear train in the adjacent gear train compartment. Further quantifiable benefits are gained as the compartment providing the sump for the gear train need only contain the bare minimum quantity of oil to satisfy lubrication considerations. Thus by relying on what in effect is “splash lubrication”, expense is saved as the quantity of fluid needed is less and the efficiency of power transmission is improved as the associated drag losses of the fluid contacting the rotating gears is much less then with a sump carrying a full capacity of oil. 
     On the other hand, with some hydrostatic transaxles, the hydrostatic transmission is arranged to operate within the same oil bath as the speed reduction gearing (and mechanical differential when included) and such designs are commonly referred to as “common sump” types. Typically, the gear train and the hydrostatic transmission lie adjacent one another at the same elevation and the oil level in the sump is kept near to the brim to ensure that the hydrostatic components remain properly submerged at all times and also to avoid any ingestion of air. With a gear train operating submerged in the oil bath, power losses are greater due to the increase in fluid friction associated with the wetted area in contact with the oil than would be the case with the “splash lubrication” types mentioned earlier. Such gear drag losses can be especially noticeable in winter time when the gears are required to revolve from rest in a sump where the oil can be in an extremely viscous initial state, and the resulting higher than normal operational loads imposed on the components in the drive train are unavoidable. As it is not possible to select oils with different properties in the common sump design, a problem is posed as the optimum fluid type which would normally be selected as the preferred lubricant for a gearbox will have completely different characteristics as compared to the type of power transmission fluid most suited for the efficient operation of a hydrostatic transmission. Typically, a gear oil tends to be thicker with a high viscosity range whereas an automatic transmission fluid (“ATF”) tends to be much thinner with a lower viscosity curve. As the hydrostatic transmission normally prevails when a conflict in design arises, it is accepted that the gear train may be operated in a generally adverse environment of low viscosity fluid such that accelerated wear and resulting higher contamination levels are more likely. The common sump design has the further limitation in that grease cannot be employed as the lubricant for the gear train. For certain applications, grease can be a more economic choice of lubricant. 
     Under normal atmospheric conditions, hydraulic fluids contain about 9% by volume of dissolved air which has virtually no effect on the physical properties of the fluid and therefore does not lead to any reduction in the performance of the system. However, should any appreciable quantity of undissolved air be present, the fluid will be prone to foaming problems, especially should the fluid experience excessive agitation, for instance, by any revolving elements such as gears being operated in only a partially submerged condition in the fluid sump. If such foaming occurs, it will rapidly lead to the destruction of the hydrostatic transmission. 
     It is also a physical characteristic of the fluid to expand and contract in volume in relation to changes in its temperature. In general terms, the volume of oil increases by about 0.7% for every increase in temperature of 10° C., and as hydrostatic transaxles can operate at below sub-zero ambient temperatures as well as on occasion above 100° C. oil temperature, it is necessary to include an additional dead space volume of about 8% to allow for such volume expansion over its initially contracted volume state. Accordingly, the fluid level in the sump rises and falls in relation to such temperature variation. 
     Quite often, an external expansion tank is fitted to the transaxle housing to allow for such expansion and contraction of the hydrostatic fluid. However, an external expansion tank can be troublesome as it is most often situated directly above the transaxle where little space exists. Frequently the space available under the frame of the vehicle is needed for rear-discharge ducts for the grass clippings. Therefore, there is often an advantage in casting the housing such that an additional space or void can be incorporated internally such that the need for an external expansion tank is avoided. Incorporating a breather vent in the housing directly above the void will allow the free flow of atmospheric air in either direction depending on temperature condition of the oil, and usually such a breather vent is positioned near or adjacent to where the largest gear resides, most often the ring gear of the differential. This works well so long as the air present in the void does not become too mixed up with the oil by rotating elements such as gears before the oil has sufficiently warmed to expel the air pocket from the void. Furthermore, as more oil has to be carried in a common sump transaxle as compared to a design having separate and distinct chambers for the hydro and gearing as mentioned earlier, a larger dead space volume has to be included to take care of the resulting increased volume expansion. Often, as the oil warms up towards its normal operating temperature, its expanded volume is not yet at a maximum, and, consequently, the remaining void or space situated in close proximity with the highest positioned gear still contains some air. This can cause considerable trouble as the gear, as its breaks through the surface of the oil, induces excessive agitation in the fluid, and the resulting mixture of air and oil in the sump can lead to foaming of the oil. Should such mixing occur to any great degree, this can be detrimental to the performance of the hydrostatic transmission as well as result in cavitation erosion on the load carrying bearing surfaces accompanied by pressure shocks and noises. The problem is further compounded should the undissolved air in the form of foam escape via the breather to pollute the environment. 
     A further problem can occur should the sump not be filled with the correct level of oil, as too low a level of oil can later cause the oil to aerate and foam when the transaxle is operated, whereas too much oil can result in it being expelled to the environment via the breather passage once it has expanded due to temperature rise. A typical problem encountered with vertical input shaft machines should the oil level be low is premature failure of the related bearing or seal due to a lack of lubrication. Furthermore, such naturally vented aspirated hydrostatic transaxles once left to cool after use in humid atmospheric conditions, draw moist air through the breather as soon as the oil begins to contract in volume and often this results in mist in the form of condensation of water vapor forming on the walls of the sump. Such entrained moisture, if not at once expelled as steam by the hot oil when the transaxle is once more in use, can even in small quantities over a period of time accelerate sludging of the oil by forming emulsions and by promoting the coagulation of insolubles such as dust particles that are also drawn through the breather as particles of solid matter as the unit cools after use. In general, air entering the sump causes the gradual oxidation of the oil and this deterioration in the lubricating properties of the oil ultimately lowers the life span of the hydrostatic transmission. Such a deterioration in the quality of the fluid can be rectified by oil changes at regular service intervals, but to undertake this is both costly and complicated to do, due to the nature of the construction of such transaxles. 
     Since the early 1960&#39;s a number of solutions have been developed for the protection of fluid in a hydraulic reservoir from such problems associated with contaminated atmospheric environments. One such solution was a pliable device called the “Fawcett Breather Bag.” The Fawcett breather bag, being a permanent flexible non-porous barrier, has the physical appearance of a synthetic rubber bag fully enclosed except for a metal stem giving access to the bag interior. As stated in its brochure, the Fawcett breather bag prevents atmospheric air and its associated contaminants from contacting the fluid in the reservoir. However, the Fawcett breather bag does not solve the problem of air trapped in the space between the bag and the fluid from getting mixed into the oil as undissolved air. 
     An alternative solution marketed by the Swiss company Angst+Pfister does however directly address this problem. Sales brochures of that product show an assortment of different breather bags designs, some of which have overcome the problem of air entrapment in the tank including one type shown formed in the shape of a bellows mounted externally to the top of the reservoir tank. A similar design of bellows is disclosed in U.S. Pat. No. 4,987,796 which is expressly incorporated by reference herein. This particular bellows differs in that it operates in an inverted sense and is mounted internally in the fluid reservoir to one side of the housing. With such a corrugated configuration exposed to the environment, it could be prone to clogging in dirty environments once there a sufficient accumulation of airborne debris settled at the bottom of the folds which would hamper and impede its natural free movement. However, neither bellows type or for that matter the Fawcett breather bag solves the practical problem should too much fluid be inadvertently poured into the reservoir such that the expansion volume is insufficient to allow for full fluid expansion. Once pliable devices such as these have deformed to their maximum extent, any further expansion of the fluid will cause the pressure in the reservoir to rise to the point where the fluid will leak at the point of least resistance. Such leakage, quite likely to occur at the interface between the pliable device and the housing, is polluting for the environment and would especially be a problem with the pliable device shown in U.S. Pat. No. 4,987,796 as its location is below the uppermost oil surface. Gradual leakage could furthermore take place should there be any manufacturing defects or imperfections on the surface to which the pliable device is engaged. 
     There therefore would be an advantage to be able to take care of volume changes in the hydrostatic compartment without recourse to using an external expansion tank or reliance on an externally vented bellows apparatus. 
     Hydrostatic transmissions also tend to be quieter in operation and work more efficiently and effectively when the fluid within the low-pressure side of the closed-loop circuit is charged or boosted from an auxiliary pump. The addition of such an auxiliary pump increases the manufacturing cost of a hydrostatic transmission and often requires a higher power output from the engine in order to drive both the auxiliary pump and the main pump of the hydrostatic transmission. There would therefore be a further advantage if the hydrostatic circuit could be pressurized without having to include an auxiliary pump. 
     SUMMARY OF THE INVENTION 
     It is one of the objects of this invention to create a positive head on the hydrostatic fluid entering the low-pressure passage of the hydrostatic transmission without recourse to using a charge pump. Preferably the compartment containing the gear train is sealed from the environment, and rising in pressure in the gear compartment aided or induced by the expanding volume of fluid in the hydrostatic compartment produces a net increase of pressure experienced by low-pressure passage of the hydrostatic transmission. 
     According to a preferred embodiment of the invention, the surface level of lubricant in the gear sump is automatically adjusted in direct proportion to the operational temperature of the fluid contained within the hydrostatic chamber. Having initially a low level of lubricant in the gear sump on the one hand lessens the adverse effect of power-retarding drag losses, especially during cold weather winter operation, whereas on the other hand, a rising level of lubricant in the gear sump can ensure good lubrication even when temperatures are elevated and viscosity is low. It is therefore a still further object of the invention to enhance the operational characteristics for the hydrostatic transmission by performance matching with respect to the operation of the speed reduction assembly irrespective of the temperature conditions in the environment. 
     In one form thereof, the invention is embodied as a hydrostatic and gear transmission having an integrated or combined housing formation whereby the interior space provided by the housing formation can be said to divided by a deformable nonpermeable partitioning device into a region expressly used for the purpose of accommodating components comprising the hydrostatic transmission and a further region expressly used for the purpose of accommodating components of the gear transmission. The first region is completely filled with hydrostatic fluid and hermetically sealed from both the gear lubricant contained in the second region and the ambient air atmosphere environment surrounding the housing, and where any volume change in the fluid capacity of said first region due to temperature change is assimilated by the partitioning device to effect an equal but opposite volume change in said second region. The partitioning device should be pliable with the inherent characteristic of being easily elastically deformable to take up a change in the amount of hydrostatic transmission fluid in the first region, for instance, due to temperature changes of the fluid, and thereby facilitates the regulation in depth of lubricant held by the second region. Such elastic deformation of the partitioning device can occur for instance, as a result of an increase in fluid pressure above atmospheric pressure within the first region caused by the hydrostatic fluid expanding in volume and exerting a force on the partitioning device. 
     According to the invention in an another aspect, the housing may include an internal wall structure or bulkhead having an aperture positioned directly adjacent both the first and second regions. The partitioning device is arranged to reside juxtapose the aperture in a manner whereby to the one side of the partitioning device lies the region containing hydrostatic power transmission fluid and to the opposite side lies the lubricant for the speed reduction apparatus that may or may not also contain a mechanical differential. The hydrostatic region preferably remains full to capacity at all times with power transmission fluid whereas the region containing the speed reducing device need only be with lubricant to a certain level that does not necessarily correspond with its full capacity. In the practical operating spectrum intended for the invention, the partitioning device preferably has an initial position set at about 0° C., which corresponds to a contracted volume state of the hydrostatic fluid in the hydrostatic region and the lowest level of lubricant in the gear region, and a final position state set at about 110° C., which corresponds to the maximum expanded volume state of the hydrostatic fluid in the hydrostatic region and the highest level of lubricant in the gear region. 
     It is a still further preferred feature of the invention to situate the partitioning device such that its expanding and contracting motion occurs substantially along a vertical axis with respect to the ground to cause a change in the level of lubricant in the gear sump, and for fluid on the one side of the partitioning device to be in effect counterbalanced by lubricant on the opposite side. 
     Filling the hydrostatic chamber with power transmission fluid can be time consuming at the factory, especially as there are often air pockets remaining which are difficult to remove without first operating the hydrostatic transmission. Such air pockets are normally not a problem for hydrostatic units fitted with breathers, as such trapped air can eventually escape. However, when a hydrostatic transmission has to operate without a breather, any such trapped air, if present, will remain incarcerated inside the hydrostatic chamber and is likely to cause poor operational performance and objectionable noise. What is therefore needed is a new solution that will not only ensure that air pockets are easily eliminated at the factory but also allow the fluid level to be easily re-checked in the field. According to a preferred embodiment of the invention, the partitioning device is fastened to the housing before the hydrostatic chamber is filled with fluid. It is therefore a preferred feature of the invention to provide a fluid filling plug on the exterior of the housing enabling the hydrostatic chamber to be exposed for fluid level inspection and for the partitioning device to be correctly positioned. Correct positioning of the partitioning device can be achieved by blowing compressed air through the hole for the bung into the gear chamber before the filling plug is fastened to the housing thereby closing off the hydrostatic chamber and thereby setting the position of the partitioning device. If necessary, once the hydrostatic machine has been factory tested to ensure it functions as intended, the screw plug on the housing which closes the hydrostatic chamber can be removed to allow any remaining air that may have floated to the surface to escape to atmosphere as well as allowing the topping-up of fluid if it should be required. Compressed air can again be blown into the gear chamber to correctly re-position the partitioning device before the filling plug is tightened to seal against the housing. Even so, should subsequent checks be necessary, the fluid level can be checked by any service agent who has the correct indication depth stick and access to a compressed air appliance. 
     Any noticeable leakage of lubricant to the environment is unacceptable and according to the preferred embodiment of the invention, any slight leakage of fluid from the hydrostatic chamber, for instance due to a manufacturing defect at interface between the housing and the partitioning device or initial overfilling of the chamber, can be captured internally. Therefore, according to the invention in another aspect, a leakage capturing system in the form of the gear train compartment is provided for the collection of unintentional discharges of fluid from the hydrostatic compartment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above mentioned and other novel features and objects of the invention, and the manner of attaining them, may be performed in various ways and will now be described by way of examples with reference to the accompanying drawings, in which: 
     FIG. 1 is an external side view from one side of an embodiment of a hydrostatic transaxle according to the invention. 
     FIG. 2 is a plan view along the section line I—I in FIG. 1 to show the interior of the lower case housing element and a shaft seal element. 
     FIG. 3 is a plan view of the hydrostatic transaxle of FIG. 1 along the section line II—II. 
     FIG. 4 is a further sectioned view of the hydrostatic transaxle on line III—III of FIG.  3 . 
     FIG. 5 is a sectional view of the housing elements of FIG. 4 with the internal elements for the hydrostatic and gear transmission removed. 
     FIG. 6 is a further embodiment of the invention disclosing an alternative type of partitioning device. 
     FIG. 7 is a part-sectional view on line IV—IV of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     For the embodiment of the invention depicted by FIGS. 1 to  5 , the hydrostatic and gear transmission is in the form of a hydrostatic transaxle designated by the numeral  1  and has by way of example a housing structure comprising an upper cover housing element  2  joined to a lower case housing element  3  along parting-plane  5 . An input drive-shaft  4  is included which is rotatably supported in the housing structure as is shown protruding from the upper cover housing element  2 . Parting-plane  5  is here shown coincident with the axis for the output axle shafts  7 ,  8 , but could also be positioned offset to one side of the shafts  7 ,  8  axis in a parallel but not coincident relationship. Alternatively, the structure of the housing may also for instance have the parting-plane arranged in a manner whereby it is situated perpendicular with respect to the axis of the axle shafts. Regardless of where the parting-plane lies, for purposes of filling the hydrostatic transaxle with hydrostatic power transmission fluid as well as lubricant for the mechanical transmission, plug  15  and bung  16  are provided, both preferably at or near the top of the housing as shown in FIG.  1 . 
     FIG. 3 shows the interior of upper housing element  2  with all internal elements comprising the hydrostatic and gear transmission positioned in place, and FIG. 2 shows the interior of lower housing element  3 . A gasket seal or preferably a liquid gasket sealant is applied to cover those engaging surfaces in the housing so to produce a sealingly tight contact once housing elements  2 ,  3  are joined, and where a plurality of bolts or screws  6  are used to secure the housing elements  2 ,  3  together. The housing elements  2 ,  3  when thus combined together provide an interior space as best seen in FIG. 3 as distinct regions marked as  11   a  and  12 , and where region  11   a  is the hydrostatic chamber and contains only hydrostatic power transmission fluid whereas region  12  is the gear chamber and contains only gear lubricant. Region  11   a  therefore houses the hydrostatic transmission depicted by arrow  9 , whereas speed reducing gearing  10 , and, when needed, a mechanical differential  13 , reside in region  12 . 
     Comparing FIGS. 4 &amp; 5, it will however become immediately apparent that prior to assembly of the internal elements into the housing, the initially empty housing elements as shown in FIG. 5 would appear to allow region a to communicate with region  12 . Such communication is present as corridor passage  64  in the upper housing element  2  which allows the connection of region  11   a  to a further region here called the “mutual region  11   b ”, whereas for the lower housing element  3  best seen in FIGS. 2 &amp; 5, region  12  is connected by channel  55  to mutual region  11   b . Were the hydrostatic and gear transmission element to be installed in such a housing framework, then a flow of fluid and lubricant would occur in either direction between regions  11   a ,  12 . However, according to an important feature of the present invention, mutual region  11   b  of FIG. 5 becomes partitioned in a manner whereby hydrostatic fluid as well as gear lubricant can reside side by side in region  11   b  without contacting one another. In effect, the partitioning device of this invention can be said to segregate hydrostatic fluid on the one side and hydrostatic power transmission fluid on the opposite side in mutual region  11   b  while still allowing expansion and contraction in volumes to occur between the two regions  11   a ,  12 . Needless to point out, the lubricant on opposite sides of the partitioning device may be the same material, although as explained above there are certain advantages in using different lubricants each suited to its particular environment. 
     Also, as a point of lexicon, although the present specification and claims speak of adjacent gear and hydrostatic chambers, and although the invention certainly embraces embodiments in which such chambers are immediately adjacent, FIGS. 4 and 5 illustrate that other embodiments of the invention might also be considered to comprise a third intermediate chamber (the region  11   b ) accommodating the partitioning and expansion device. 
     Once assembly of the hydrostatic transaxle is complete, region  11   a  can be filled with power transmission fluid until full, before plug  15  is tightened down on compression seal washer  17  to produce a pressure tight seal on the upper housing element  2 . Similarly, once region  12  has been filled with sufficient lubricant, a simple rubber bung is employed to seal off region  12 . 
     Hydrostatic transmission  9  is comprised of at least one hydraulic pump fluidly coupled to at least one hydraulic motor, and where respective cylinder-barrels shown as  20 ,  21  of the hydrostatic-transmission pump  22  and motor  23  are mounted perpendicular to one another such that the rotating axis of the pump cylinder-barrel  20  is vertical and arranged parallel and co-axial with respect to the input-drive shaft  4  to which it is fixed for rotation whereas the rotating axis of the motor cylinder-barrel  21  is parallel with respect to the rotating axis of the axle-shafts  7 ,  8 . A control-shaft  14 , in this embodiment shown located in the upper cover housing element  2 , allows the operator to effect changes in the displacement of the hydrostatic transmission  9  for the purpose of controlling the speed of the vehicle. Fluid passages  25 ,  26  are provided in a fluid distributor member  27  which act to fluidly couple the pump  22  to the motor  23  as is well known in the art and commonly referred to as a closed loop fluid circuit. A respective check-valve  28 ,  29  is included for each passage  25 ,  26  to allow the admittance of make-up fluid into passages  25 ,  26  from region  11   a  in order that the hydrostatic transmission  9  can recover any fluid loss during operation because of high-pressure leakage. 
     The cylinder-barrel  20  of the pump  22  is provided with a plurality of axial cylinder-bores  30 , each bore  30  containing a respective piston  31  and where each piston  31  is being axially urged outwards by a spring (not shown) located behind the piston  31  in the bore  30  to engage a swash-plate  32 . Each cylinder-bore  30  is arranged to communicate in sequence with a pair of arcuate-shaped ports (although not visible they are generally the same as those arcuate-shaped ports  38 ,  39  shown for the motor  23  in FIG. 4) on the fluid distributor member  27  that connect with respective passages  25 ,  26 . The cylinder-barrel  21  of the motor  23  is almost in all respects identical to that of the pump, and carries with it a series of axially sliding pistons  35  which are operatively connected to the operational surface  36  of an inclined thrust plate  37 . FIG. 4 shows the pair of arcuate-shapes ports  38 ,  39  used for transferring fluid from passages  25 ,  26  to the cylinder-barrel  21  of the motor  23 . Cylinder-barrel  21  is fixedly attached to drive shaft  40  and because of the piston  35  reaction on inclined thrust plate  37 , an angular driving moment is created on the cylinder-barrel  21  which is then caused to revolve. 
     As drive shaft  40  must pass from hydrostatic region  11   a  to gear region  12  in order to transfer power from the hydraulic motor  23  to the gear train  10 , best seen in FIG. 2, semi-circular opening  42  is provided in internal bulkhead wall  44  of housing element  3  (and a matching opening  46  is provided in the opposite bulkhead of housing element  2  as shown in FIG. 3) for the purpose of supporting a shaft seal  45 . Without such a shaft seal  45 , regions  11   a ,  12  would in effect be in fluid communication. Therefore to better illustrate this point, the interior view of lower housing element  3  in FIG. 2 has the addition of shaft seal  45  positioned in that portion of the bulkhead wall  44  that exists in this particular housing element. 
     When the hydrostatic compartment region  11   a  is to operate under pressurized conditions, it is preferable that good quality shaft seals are to be used such as the well known types manufactured by the company Freudenberg. Similarly, the gear region under such conditions should also preferably be fitted with good quality seals over shafts protruding out from the region  12  such as, for example, axle shafts  7 ,  8 . Although as shown, shaft seal  45  is preferably of the double-lip type, single lip seals may also be employed depending on pressure conditions present within regions  11   a ,  12 . 
     Drive shaft  40  supported in the housing by at least one bearing  41  passes through seal  45  so that the motor  23  of the hydrostatic transmission  9  can be connected to the first speed reducing gear  43  of the gear train  10 . 
     Rotation of gear  43  is transmitted by further gears  50 ,  51 ,  52  to the internal gears of the differential  13  assembly and finally to axle shafts  7 , 8 . The inclusion of a differential assembly is important as it allows normal differentiation between the left and right drive wheels of the vehicle and helps prevent lawn damage especially when tight turns are undertaken. However, there are applications where no such differentialled action is required, and, in these instances, a single axle shaft may be used instead of the two as shown in this embodiment. In the case of a single axle shaft, this shaft can be arranged to extend outwardly on one or both sides from the housing. 
     Although this embodiment uses a simple rubber bung  16  to shut off region  12 , a threaded plug could also be used instead if so desired, which would be tightened in similar fashion to plug  15  to compress a bonded sealing washer on the housing. Although it is a preferable but not an essential feature of the invention that region  12  should be pressurized, for non-pressurized applications, a breather could be used in place of bung  16 . Such a breather could be for instance, of the type having an internal sintered filter which would prevent larger sized particles of solid matter from entering region  12 , or alternatively, be just a small vent somewhere at the top of the housing above region  12  so that region  12  would be atmospherically aspirated. It would also be possible to include a restrictor or even a one-way air valve in place of bung  12  where the restrictor would also allow region  12  to become slightly pressurized during machine operation. 
     Region  11   a  for the hydrostatic power transmitting device is preferably filled to capacity with power transmission fluid and remains completely full at all times. In contrast and provided that no reliance is being placed on using an external expansion tank, it is most beneficial that region  12  for the speed reducing apparatus be only partially filled with gear lubricant. With region  12  only partially full with lubricant when cold, there is thereby provided an additional volume space within the housing to not only take care of the expansion of the gear lubricant itself but also, as will be explained later, the unavoidable expansion in volume that will occur as the temperature of the power transmission fluid in region  11   a  increases. 
     The surface level of lubricant within region  12  has not been shown in the drawings as it is variable depending on temperature, but it would be preferable for the level of lubricant to be sufficiently high to keep essential elements such as shaft  49  and the shaft bearings  53 ,  54  well lubricated even when the unit is stone cold. Preferably, channel  55  remains flooded at all times. 
     As best seen in FIGS. 4 &amp; 5, upper cover housing element  2  has been constructed to include an integral bulkhead wall  60  that projects downwards from the upper interior surface of the horizontal exterior wall marked  61  in a direction towards the parting-plane  5  to join surface  63  on the lower case housing element  3 . Wall  61  is punctured at one location by machined hole  62  which is then threaded to accept fluid filling plug  15 , and this machining operation by removing a section of the bulkhead wall  60  thereby provides the corridor passage  64  which communicates region  11   a  to the neighboring mutual region  11   b . 
     By way of example, with this housing package comprising housing elements  2 ,  3 , a number of interior and external walls such as  44 ,  60  and  61  are provided which form a structural boundary surrounding the hydrostatic region  11   a  as well as the mutual region  11   b  and which would remain intact or complete were it not for the inclusion of an internally disposed opening  65  as shown in FIG.  5 . Opening  65  intersects the underside surface  66  of the upper housing element  2  and thereby communicates mutual region  11   b  with channel  55 . 
     The invention as here described preferably includes such an opening  65  in at least one of the interior wall bulkheads  60  in order that partitioning device  70  can be positioned over opening  65  before the housing elements  2 ,  3  are joined together during the assembly stage of machine building. With partitioning device  70  in place in mutual region  11   b  as shown in FIG. 4, it acts in dividing mutual region  11   b  into an upper pocket void  11   c  and a lower pocket void  11   d . 
     As pocket void  11   c  is connected by corridor passage  64  to region  11   a , it is also full of hydrostatic fluid, whereas only gear lubricant in region  12  is able to flow into pocket void  11   d  via channel  55 . Regardless of the type or design of housing package chosen to surround the hydrostatic transmission and the gear transmission, the interior space provided for the transmission elements by being divided by the partitioning device  70  as an integral part of the interior housing structure creates independent respective regions for the hydrostatic and gear transmission as mentioned earlier. 
     The hydrostatic region and the gear region are kept apart by the partitioning device  70  which as a non-porous barrier, performs to segregate the regions such that gear lubricant in contact on the one side is prevented from mixing with the hydrostatic fluid on the opposite side. Partitioning device  70  should exhibit the required degree of flexibility needed and may be fabricated in a variety of alternative shapes to suit the preference of the manufacturer, and may for instance be of bellows; bladder; diaphragm, or breather bag like construction. Whereas on the one hand the partitioning device  70  must have a pliable and deformable disposition, its affiliated static housing structure on the other hand must remain solid. To contrast with the interior walls or bulkheads, the partitioning device  70  could be said to be a deformable portion of the internal bulkhead. 
     Although less preferred at present, it is also possible that partitioning device  70  may be formed in whole or in part of porous material, or may include a porous element in the nature of a permeable membrane, to permit controlled passage of lubricating fluid between the hydrostatic transmission and gearing chambers. The passage of fluid through such a porous element should not prevent the partitioning element  70  from deforming in response to expansion of oil in the hydro chamber, and could serve as a pressure relief valve to prevent damage to the partitioning member when it has deformed to its maximum extent. 
     It is purposely arranged that where partitioning device  70  does come into non-moving contact with the static and therefore nondeforming housing structure of the machine wall (for instance bulkheads  44 ,  60 ), it occurs at its flange-like circular lip  71  which is arranged to engage recessed seat  69  provided in housing element  2 . A hollow disc  72  is then placed over the lip  71  and once screws  73  have been sufficiently tightened, lip  71  is slightly deformed on recess  69  thereby isolating region  11   a ,  11   b  from region  12 . Once partitioning device  70  has been fixed in this manner to housing element  2 , assembly of the machine can continue and housing elements  2 ,  3  can subsequently be joined together and fastened by screws  6 . 
     From the point of contact at recess  69 , partitioning device  70  is shown to be in a fully extended condition which corresponds to the volume of fluid within chamber  11   a , corridor passage  64 , and pocket void  11   c  being at a minimum value (i.e. when the fluid is cold). Although it is preferable but not essential for partitioning device  70  be provided with an inherent ability to deform on its own accord to comply as required to changes in volume in the respective regions, a tensioning member could be included to bias partitioning device  70  in one direction, for this embodiment in a direction towards its retracted state. As shown in FIG. 4, pocket void  11   d  on the underside of partitioning device  70  is at its greatest value and contains gear lubricant. However, as soon as the hydrostatic power transmitting device  9  is operated, the power transmitting fluid begins to warms up and there is a corresponding increase in the volume of hydrostatic fluid in  11   a ,  64 ,  11   c . The hydrostatic fluid in contact with partitioning device  70  begins to exert a force against partitioning device  70  causing it to retract in a direction towards recess  69 , and there a corresponding decrease in the size of pocket void  11   d  resulting in a displacement of gear lubricant to region  12  via channel  55 . The size of pocket void  11   d  will continue to decrease until such time that steady state conditions have been reached. As pocket void  11   d  may then only contain the bare minimum of gear lubricant, the volume having been displaced and induced a rise in level of lubricant surrounding the gearing can ensure the fall in viscosity due to elevated temperature operation is less serious than would otherwise be the case. Thus, this invention can provide auto-levelling in the surface level of gear lubricant within the second region irrespective of the pressure conditions within the second region. 
     Once the unit begins to cool down and the size of pocket void lid begins to increase, lubricant flows in the reverse direction through channel  55  and there is a decrease in depth of lubricant bathing the gears. As the lubricant for the gear train remains correspondingly low in level when the unit is cold, power losses are minimized during start-up and for general winter operation. It is therefore a feature of this embodiment that lubricant for the gears can flow in either direction along channel  55  depending whether the temperature conditions experienced by the machine are rising or falling. When region  12  experiences positive or above atmospheric pressure, it will also influence the magnitude of the positive head of the hydrostatic power transmission fluid to such an extent that not only do the check-valves  28 ,  29  operate more efficiently in replenishing lost fluid from the closed-loop circuit passages  25 ,  36 , but also the hydrostatic transmission operates with less noise. 
     Although as described both hydrostatic region  11   a  as well as gear region  12  are internally pressurized, it is nevertheless not intended to limit the invention in this way. For instance, region  12  could be pressurized by an inert gas such as nitrogen at the factory once the fluid and lubricant has been poured into the respective regions. 
     In the event that the unit contains a slight imperfection, for instance a barely visible scratch on the surface of recess  69  which interfaces with the lip  71  of partitioning device  70 , the very small amount of fluid lost during the life span of the machine by such leakage from region  11   a  would be captured by region  12  which acts in this respect as a safety receiver. 
     Although the type of lubricant used for lubricating the gear train can be grease, the invention performs better when a gear oil is used as it is likely to react more rapidly to temperature changes in the machine. Furthermore, gear oil would have the additional advantage of being able to more readily wash away any debris that may on occasion lodge itself on the surface of the partitioning device  70 . It is therefore preferable but by no means essential to arrange that partitioning device  70  moves in a vertical rather than a horizontal fashion, as well if possible, in as close a location as possible to the filling plug  15 . Some of the advantages in including a partitioning device  70  as described are: 
     a) Ease of filling hydrostatic region  11   a ,  11   b  with fluid (the nature springiness and pliability of partitioning device  70  will mean it can set its own correct initial position when filled with cold oil at the factory assembly stage unlike earlier known devices); 
     b) Ease of removing any trapped air after assembly; 
     c) Simple check for inspection for the correct height setting of the partitioning device  70  by insertion of depth probe through hole  62  while using compressed air through breather vent  16  for vertical adjustment; 
     d) Ease of maintenance in the topping up of fluid in hydrostatic region  11   a ,  11   b  by a service agent; 
     Once all has been checked to be satisfactory, filling plug  15  is placed into threaded hole  62  is and tightened down to compress the bonded seal washer  17  so to shut off region  11   a ,  11   c  from the outer environment. 
     A ring or horseshoe magnetic  76  disposed in recess  75  provided at the bottom of housing element  3  as shown in FIG. 4 has been included to attract any ferrous particles of contamination that might be otherwise suspended in the hydrostatic fluid. A gauze  77  located above magnet  76  prevents fluid motion within region  11   a  from disturbing any contamination that might have settled in recess  75 . 
     A second embodiment differs in two main respects from the first embodiment, and the following description is directed principally to the main points of difference. Furthermore, as most internal components remain substantially similar to those already described for the first embodiment, for convenience sake, many that are here numbered carry the same reference numerals as have been designated in the first embodiment. 
     As shown in FIGS. 6 &amp; 7, the housing structure surrounding the components of the hydrostatic power transmitting device  9  comprises housing elements  80 ,  81  whereas housing elements  80 ,  82  surround the speed reducing apparatus that may be in the form of a gear train  10  and additional mechanical differential unit  13 . Although as shown, lower housing element  80  is both common to the hydrostatic transmission  9  as well as the differential gear train  9 ,  10 , it could be modified to two separate housing elements if so desired. Similarly, upper housing elements  81 ,  82  could be combined into a single housing element. Fluid passages  25 ,  26  connecting the pump  22  to the motor  23  are formed integrally in housing element  81 . 
     The partitioning device denoted for this embodiment by reference numeral  85  allows the housing structure to be divided into a hydrostatic region  11   a ,  11   c  and a gear region  12 ,  11   d . In this embodiment, partitioning device  85  resides closer to axle shaft  8  to one side of the differential unit  13 . As best seen in FIG. 7, partitioning device  85  has an inverted orientation in contrast with the first embodiment, and is provided with a flanged lip  86  which is circular except for being radially extended to one side as denoted by reference numeral  87 , and from where it extends radially to curl over a large tube  88  which protects it from contacting revolving axle shaft  8  to enter recess  89  provided in housing element  80  to hook under small tube  90 . Once all components of the hydrostatic power transmitting device  9  and speed reducing apparatus  10  have been assembled into place, the upper two housing elements  81 ,  82  can be lowered and attached to common housing element  80  and where they engage small tube  90  to compress and deform partitioning device  85  at this location. Elsewhere, over its remaining circumferential length, flanged lip  86  is trapped between housing elements  80 ,  82  so eliminating the hollow disc as is used for the first embodiment. 
     Small tube  90  connects chamber  11   a  to pocket void  11   c  whereas longitudinal groove  92  etched on the axle shaft bearing surface  91  of the lower housing element  80 , connects pocket void  11   d  to chamber  12 . Hydrostatic power transmission fluid within region  11   a  is thereby able flow freely in either direction through small tube  90  into variable-volume pocket void  11   c  depending on operating temperature of the machine, whereas longitudinal groove  92 , taking the place of the channel used in the first embodiment, allows the free flow of lubricant in both directions between gear region  12  and variable-volume pocket void  11   d.    
     FIG. 7 shows partitioning device  85  being in its fully expanded condition which corresponds to when the volume of hydrostatic fluid within region  11   a  is at a maximum when the machine is operated at an elevated temperature. Once cooling occurs, hydrostatic fluid and gear lubricant is contracted and the inverted partitioning device  85  moves upwards drawing in through groove  92  lubricant from region  12  to enter at its underside pocket void  11   d.    
     In such examples of pressurized or semi-pressurized gear sumps as described, the partitioning member may alternatively be positioned in a horizontal fashion rather than vertical, preferably but not essentially arranged to remain below the surface of the lubricant at all times. 
     Although not shown in these embodiments, an air trap can be included at the top of the hydrostatic region in the housing so that any air not expelled during unit fill-up at the factory can become lodged in the trap. 
     For certain applications, it may be desirable for the machine to operate with region  12  full to capacity with lubricant. It should therefore be noted that this invention may be modified, for example, by including an external expansion tank which would be connected by a pipe to region  12 , and where the aforementioned rubber bung  16  would be discarded and replaced by such a piped connection. The external expansion tank could be atmospherically vented or for that matter encased to become pressurized with or without reliance on being charged by an inert gas such as nitrogen. Even so, for many applications, the addition of such an external expansion tank would be impractical as little space exists under the frame of a vehicle for such a tank to be located, and that therefore, it is preferable although not essential to confine all natural variations in fluid volume to within region  12 . 
     In the case of separately located hydrostatic and gear reduction transmissions or for that matter units that are not connected together by a common housing, the partitioning device, being disposed in the gear transmission chamber, can be fluidly connected to the hydrostatic chamber by means of a pipe. With the hydrostatic chamber and well as the volume contained in the pipe full of fluid in contact with one side of the partitioning device, an increased volume of fluid in the hydrostatic chamber due to temperature rise forces the partitioning device to displace an equal volume of lubricant in the gear chamber. Alternatively, the partitioning device could be disposed in the tube that serves as the communication passage between the hydrostatic and gear transmissions. 
     Although neither of these two embodiments have showing an oil filter for the hydrostatic transmission, a filter strainer may be usefully deployed if so desired, to filter the make-up fluid entering the check valves members. Furthermore, although an axial piston hydrostatic machine has been described, this invention is also applicable to any type or form of hydrostatic power transmitting machine as well as for that matter, other forms or types of speed reduction apparatus. 
     In accordance with the patent statutes, I have described the principles of construction and operation of my invention, and while I have endeavored to set forth the best embodiments thereof, I desire to have it understood that obvious changes may be made within the scope of the following claims without departing from the spirit of our invention.