Abstract:
A hydrostatic transmission connected by means of a power transmission shaft to a speed reducing transmission, the housing structure surrounding said transmissions defining a first and second internal volumes. The first internal volume providing a receptacle for the hydrostatic fluid and for containing the hydrostatic transmission components. The second internal volume, which may or may not contain the gear train components, operates as an overflow receiver for fluid transferred from the first internal volume. A duct disposed in the housing provides the only flow path between the volumes and any fluid volume change in the first internal volume due to temperature variation is translated in a change in depth in the overflow receiver. Preferably, the duct operates in accordance to the siphon principle and catering for fluid volume change internally is a significant improvement over current practice where reliance is placed on external paraphernalia to achieve this end. External devices as such can be prone to leakage whereas with the present invention, fluid expansion can be taken care of internally for an improved dependability in such hydrostatic transmission devices

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to stand-alone hydrostatic transmissions as well as combined hydrostatic and gear transmissions having housing structures provided with either independent or common sumps, such transmissions being usefully employed for many diverse applications such as vehicle drive lines of the type commonly referred to as hydrostatic transaxles.  
           [0003]    This invention is particularly concerned with an improved hydrostatic transmission or transaxle drive line disposed within a surrounding housing structure and where the interior space inside the housing can be said to be divided by structural walls or bulkheads into two distinct internal volumes. The first internal volume containing the hydrostatic transmission submerged in its operating fluid whereas the second internal volume, being either in the form of a spill over chamber or alternatively, a chamber containing a gear train, are arranged to be fluidly linked together at all times by a communication duct in the form of a siphon.  
           [0004]    2. Description of the Related Art  
           [0005]    Hydrostatic transmissions and transaxles are increasingly being used in the lawn care industry and for other outdoor power equipment duties such as snow-blowing. They 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 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.  
           [0006]    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 pressurised circuit of the hydrostatic transmission. By removing the chances for damaging particles of contamination from entering the hydrostatic pressurised 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.  
           [0007]    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 hydraulic motor of the hydrostatic transmission in the hydrostatic compartment to the first reduction gear of the gear train in the adjacent gear train compartment. As such, 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 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.  
           [0008]    On the other hand, with some hydrostatic transaxles, the hydrostatic transmission is arranged to operate within the very 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 in which 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 a 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.  
           [0009]    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 deg. C. and as hydrostatic transaxles can operate at below sub-zero ambient temperatures as well as on occasion above 100 deg. C. oil temperature, it is necessary to include an additional dead space volume of about 8% to allow for such volume expansion to occur without restriction over its initially contracted volume state. Accordingly, the fluid level in the sump rises and falls in relation to such temperature variation.  
           [0010]    Quite often, an external expansion tank has to be fitted to the transaxle housing to cater for such volume changes in the quantity of fluid held in the sump. Should the tank be vented to atmospheric and rely on gravity-fed to work, such an external expansion tank can be troublesome to include as it must then be situated directly above the transaxle itself. Frequently the space available under the frame of the vehicle is needed for rear-discharge ducts for the grass clippings, and therefore, little space remains between the chassis and the transaxle for an adequately sized header or expansion tank. Recent attempts to overcome this problem are disclosed in U.S. Pat. Nos. 6,073,443 and 6,185,936. Both patents show the use of a siphon to connect the internal chamber of a hydrostatic transaxle to an external tank, the siphon allowing the tank to be located to one side of the transaxle housing exterior and at an elevation below the fluid level in the internal chamber. Although this solution does overcome one problem, namely the lack of available height in the installation, such external tanks may be vunerable to being damaged, for instance by stones kicked up by the revolving grass mower blades puncturing the tank and allowing fluid to escape to the environment. Furthermore, during severe winter conditions, a start-up of the hydrostatic transmission in such conditions while fluid in the siphon is in a semi-frozen state may cause the rotary seals in the hydrostatic transaxle to blow out.  
           [0011]    There therefore is a need for a new solution to overcome the above mentioned disadvantages, and in particular there would be an advantage if the volume change in the fluid held in hydrostatic transaxle could be accommodated in a more protected and heat insulated environment inside in oppose to outside the transaxle housing. Thus a solution whereby the external expansion/header tank could be entirely eliminated would have the additional advantage of reduced cost.  
           [0012]    Although it has been known for the housing for the hydrostatic transaxle to be manufactured slightly larger than is strictly necessary in order to enable an additional space or void to be inclided near the top to cater for the expanded volume of fluid, such a solution is not always practical. However, this solution will work well so long as the air present in the void does not become mixed in with the oil before the oil has sufficiently warmed to expel, through a breather, the air pocket from the void. Such a breather vent or passage is normally positioned at the highest position in the housing, and allows the free flow of atmospheric air in either direction from the void such that the fluid level in the sump can rise and fall depending on the temperature condition of the oil. Even so, it is difficult to completely eliminate the chances for mixing of the air and the oil and the risk is ever present so long as the revolving componentry of the hydrostatic transmission, such as the input drive shaft to the hydraulic pump or the ring gear of the differential, are able to break through the surface of the fluid. In practice, 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. Consequently as the oil warms up towards its normal operating temperature and before its expanded volume has yet to reach a maximum, the remaining void or space situated in close proximity with the highest positioned shaft or gear still contains some air, and as these revolving components break through the surface of the oil, the induced severe agitation is likely to led to air being pulled into the oil. Should such mixing occur to any great degree, it 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.  
           [0013]    A further problem can occur should the sump be insufficiently filled to 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.  
           [0014]    A typical problem encountered with vertical input shaft machines, should the oil level be lower then specified, 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 vapour 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.  
           [0015]    There therefore would be an advantage to be able to take care of volume changes in the hydrostatic transaxle without either recourse to using an inconveniently positioned external expansion tank or by having to rely on an internal dead space void above the fluid with its attendant risk in the formation of foam. There would be further advantage if environmental airborne contaminants as well as moisture be entirely prevented from entering the first internal volume containing the hydrostatic transmission components, or at the very least be allowed to be first absorbed in the fluid contained in the second internal volume in order to thereby slow and impede their progress towards entering the first internal volume.  
           [0016]    Hydrostatic transmissions 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 an advantage if the hydrostatic circuit could be pressurized without having to include an auxiliary pump.  
         SUMMARY OF THE INVENTION  
         [0017]    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 spill over chamber or the compartment used to house the gear train is sealed from the environment, and a rise in pressure in the spill over chamber or gear compartment aided or induced by the expanding volume of fluid in the hydrostatic compartment produces a net increase of pressure experienced by the low-pressure passage of the hydrostatic transmission.  
           [0018]    It is a further object of the invention to improve the running efficiency of the speed reduction gearing used in hydrostatic transaxles. To achieve this object, 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.  
           [0019]    One aspect of this invention is to make better use of the interior space inside the housing and thereby attend to fluid volume changes due to fluid temperature variation, and as such, a portion of the interior space inside the housing serves as an overflow receiver for the hydrostatic fluid in the first internal volume. Catering for fluid volume change internally is a significant improvement over current transaxle practice, as traditional transaxle practice is to rely on external paraphernalia to achieve this end. External devices as such can be prone to leakage and it is therefore a further object of this invention to provide a new and novel solution whereby a fluid expansion chamber is incorporated internally rather than externally in a hydrostatic transmission or a hydrostatic transaxle.  
           [0020]    As one example of the invention, an overflow receiver for the administration of volume changes in the first internal volume can be incorporated in a hydrostatic transmission of the stand-alone type. As often there are no gears needed in such stand-alone types, the overflow receiver as the second internal volume, is fluidly connected by the siphon to the first internal volume, so that expansion and contraction of fluid surrounding the hydrostatic transmission components can occur without restriction. The over-flow receiver may be vented to atmosphere or preferably, remains sealed such that fluid entering it from the first internal volume causes internal pressuization in the over-spill receiver as well as in the first internal volume and thereby enhancing the operational characteristics of the hydrostatic transmission.  
           [0021]    As a further example of the invention, the overflow is in the form of the gear compartment sump.  
           [0022]    In one form thereof, the invention is embodied as a hydrostatic and gear transmission having an integral or combined housing formation whereby the interior space provided by the housing formation can be said to comprise a first internal volume expressly used for the purpose of accommodating components comprising the hydrostatic transmission and a second internal volume expressly used for the purpose of accommodating components of the gear transmission. The first internal volume contains the hydrostatic transmission submerged in its operating fluid whereas the second internal volume provides a fluid sump to lubricate the speed reducing gearing. First and second internal volumes are arranged to be fluidly linked together at all times by a communication duct in the form of a siphon such that any change in the volume of the fluid held by the first internal volume due to temperature change is translated by a flow of fluid through the siphon to effect an equal but opposite volume change in the fluid held by the second internal volume. The gear compartment sump may be vented to atmosphere or preferably, remains sealed such that fluid entering produces internal pressuization of the first internal volume such that the operational characteristics of the hydrostatic transmission may be enhanced.  
           [0023]    Regardless whether the second internal volume be so configured as to be able to accommodate the gear train or not, it is to be preferred that the first region should remain completely full of hydrostatic fluid at all times.  
           [0024]    According to the invention from another aspect, the interior space inside the housing can be said to be divided by structural walls or bulkheads to form these two distinct internal volumes.  
           [0025]    Since the overflow receiver serves to receive displaced fluid from the first internal volume containing the hydrostatic transmission, there is little possibility for fluid from the first internal volume to escape into the environment. It is also an object of the invention to provide a simple contamination trap juxtapose the open-end of the siphon duct in the second internal volume so to reduce the likelihood of contamination from being able to enter the first internal volume and damaging the hydrostatic transmission.  
           [0026]    In the detailed description and drawings which follow, the internal fluid expansion chamber for a hydrostatic transmission is shown in one form for both the first and second embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    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:  
         [0028]    [0028]FIG. 1 is a side view of a hydrostatic transaxle in accordance with a first embodiment of the present invention.  
         [0029]    [0029]FIG. 2 is a plan view of the hydrostatic transaxle of FIG. 1 along the section line I-I.  
         [0030]    [0030]FIG. 3 is a further sectioned view of the hydrostatic transaxle on line II-II of FIG. 2.  
         [0031]    [0031]FIG. 4 is a plan view of the interior of a hydrostatic transmission in accordance with a second embodiment of the present invention.  
         [0032]    [0032]FIG. 5 is a section taken along line III-III of FIG. 4.  
         [0033]    [0033]FIG. 6 is a plan view of the interior of a hydrostatic transaxle in accordance with a third embodiment of the present invention.  
         [0034]    [0034]FIG. 7 is a section taken along line IV-IV of FIG. 6.  
         [0035]    [0035]FIG. 8 is a section taken along line IV-IV of FIG. 6 and showing one form of contamination trap.  
         [0036]    [0036]FIG. 9 is a plan view of the interior of a hydrostatic transmission and gear transmission combination in accordance with a fourth embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0037]    For the embodiment of the invention depicted by FIGS.  1  to  3 , 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 . Input drive-shaft  4  is connected either directly, or via a belt, to a prime mover such as an internal combustion engine. Parting-plane  5  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. The structure of the housing may also, just to cite one other alternative example, have one or more parting-planes disposed perpendicular with respect to the axis of the axle shafts. Irrespective of whichever housing arrangement is chosen, the housing has to accommodate both the hydrostatic transmission as well as the gear transmission in the form of a lubricated gear train. The gear train is disposed in a gear sump containing either oil or grease as the lubricant for the gears, and preferably, the lubricant for the gears is prevented from being able to mix with the hydrostatic fluid of the hydrostatic transmission. Plugs  15 ,  16  are provided in the housing in order for hydrostatic fluid and gear lubricant into the inserted.  
         [0038]    [0038]FIG. 2 shows the interior of the hydrostatic transaxle with internal elements comprising the hydrostatic and the gear transmission positioned in place in housing element  3 . At the housing interface between housing elements  2 ,  3  which happens in these embodiments to be conincident with parting-plane  5  and best seen in FIG. 3, a gasket seal or preferably a liquid gasket sealant is applied to cover the engaging surfaces so to produce a sealingly tight contact once upper housing element  2  is located and secured to housing element  3 . A plurality of bolts or screws  6  being used to secure the housing elements  2 ,  3  together.  
         [0039]    Thus, in the assembled condition or state, housing elements  2 ,  3  can be said to divide the interior space into three distinct regions, one region expressly used for a hydrostatic transmission denoted by the reference numeral  10 ; another region expressly used for a gear transmission denoted by the reference numeral  11 ; and a third region which according to both the first and second embodiments of this invention, is the fluid expansion chamber which here is in the form of the overflow receiver as denoted by the reference numeral  12 .  
         [0040]    According to both first and second embodiments of the invention, the overflow receiver in this particular form of internal fluid expansion chamber for the hydrostatic transmission has the sole function for taking care of any fluid volume changes occurring in the region containing the hydrostatic transmission. However, it may be helpful at this time to note that according to both the third and fourth embodiment of the invention, there the overflow receiver in an alternative form of internal fluid expansion chamber has a dual function: firstly, as it is the gear compartment sump it must therefore contain both gearing and lubricant, and secondly, it must carry any fluid displaced from the region containing the hydrostatic transmission due to fluid volume changes occuring in that region.  
         [0041]    For the purpose of definition therefore, that region in the housing in which the components of the hydrostatic transmission  10  are disposed is the first internal volume denoted by reference numeral  13 , and this is true for all four embodiments of the present invention. As such, the first internal volume  13  can be said to be the receptacle for the hydrostatic fluid surrounding the components of the hydrostatic transmission  10 .  
         [0042]    Therefore components of the hydrostatic transmission  10  are contained in first internal volume  13  and the second internal volume, as overflow receiver  12 , receives and supplies fluid, as and when necessary, from the first internal volume  13  via an intercommunication duct  9  having interior bore  19  and where duct  9  preferably operates in accordance to the siphon principle. As shown, overflow receiver  12  is conveniently located in relatively close proximity to first internal volume  13 .  
         [0043]    With this as well as in the second embodiment, hydrostatic power transmission fluid is associated with both second internal volumes  13 ,  12  whereas only gear lubricant is associated with the separate region in which the gear transmission  11  is located. This separate region for the purposes of defintion will now be referred to as the gear compartment sump and is denoted by the reference numeral  14 .The gear transmission  11  which is required in order to mechanically couple the hydraulic motor of the hydrostatic transmission to the output shaft or shafts such as axle output shafts  7 , 8 , may, when necessary, include a mechanical differential  25 .  
         [0044]    Hydrostatic transmission  10  is comprised of at least one hydraulic pump  22  fluidly coupled to at least one hydraulic motor  23 , 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 . To control the speed and forward/reverse direction of the vehicle in which this hydrostatic transaxle is fitted, as shown in FIG. 1, a protruding speed control-shaft  14  from housing element  2  is provided, the control-shaft being journalled in the housing and connecting internally inside the unit with the tiltable swash-plate  32  of the hydraulic pump  22 . Fluid passages  25 ,  26  are provided 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  in order that the hydrostatic transmission  10  can recover any fluid loss during operation because of high-pressure leakage.  
         [0045]    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. 3) 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. 3 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.  
         [0046]    As drive shaft  40  must pass from the motor  23  in the first internal volume  13  and connect with gear train  11  in the gear compartment sump  14  in order for the transfer of power between motor  23  and gear train  11 , a shaft seal  45  is needed so that hydrostatic fluid is prevented from escaping first internal volume  13  to mix with the gear lubricant contained in the gear compartment sump  14 . When the first internal volume is intended to operate under pressurized conditions, it is preferable that good quality shaft seal is used such as the well known types manufactured by the company Freudenberg.  
         [0047]    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  10  can be connected to the first speed reducing gear  43  of the gear train  11 . Gear  43  meshes with gear  55  fixed to intermediate shaft  56  to cause rotation of said shaft  56 . Intermediate shaft  56  is supported by bearings  57 ,  58  in the housing elements  2 ,  3  and has a further gear  59  attached to it, gear  59  meshing with ring gear  60  of the differential assembly  25 . The differential assembly  25  includes four internal gears, three being visible in FIG. 2 and numbered  71 ,  72 ,  73 ,  74  and where gears  72 ,  73  are fixed on respective 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, as there are applications where no such differentialled action is required, 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.  
         [0048]    Once assembly of the hydrostatic transaxle  1  has been completed, the unit can be placed upside down so that first internal volume  13  can be filled with power transmission fluid through hole  17 . At this time, both plug  15 ,  16  have been removed from the housing so that respective holes  17 ,  18  are open. Once first internal volume  13  is full of fluid, excess fluid is transferred through the interior  19  of duct  9  and enters the overflow receiver  12  which is observed as escaping fluid from hole  18 . At this point, plug  15  is attached to housing element  3  and tightened on threaded hole  17 . This stops any further flow of fluid through the duct  9  and the unit can be moved back to its upright position. Then once sufficient fluid has been added through hole  18  into the overflow receiver  12  to ensure the entrance  62  of duct  9  is submerged below the oil level shown as  64 , plug  16  can be attached to housing element  2  and tightened on threaded hole  18 . It should be noted the exact fluid levels as shown in these embodiments of the invention, for instance, upper fluid level  65  and lower fluid level  64  in FIG. 3., are for purely illustrative purposes only, in order to show that the fluid level can change during operation of the hydrostatic transmission.  
         [0049]    In this invention, the form of the siphon duct  9  used is arranged to have entrance  61  close to the upper interior surface  63  in first internal volume  13  and from there extending downwards before turning horizontally to be parallel and coincident with respect to parting-plane  5 . The horizontal portion of the duct  9  being arranged to locate in semi-cylindrical surfaces  66 ,  67  provided in respective housing elements  2 ,  3  to form a corridor  68  denoted in FIG. 2. between respective internal volumes  13 ,  12  for the passage of the duct  9 . An anaerobic sealing product is applied at the interface of the siphon portion engaging with semi-cylindrical surfaces  66 ,  67  so ensuring that the only way for fluid transfer to occur between first and second internal volumes  13 ,  12  is through the interior of the duct  9 .  
         [0050]    As the duct  9  protrudes through into second internal volume  12 , it is arranged to turn upwards in a direction towards plug  16  before turning downwards and extending towards lower interior wall  69  of overflow receiver  12 . Preferably, entrance  62  of duct  9  should remain submerged at all times below fluid level  64 .  
         [0051]    The first internal volume should remain completely full of hydrostatic operating fluid during the service life of the hydrostatic unit, and preferably at a sufficiently high level so that entrance  61  of siphon  9  remains submerged. The first internal volume is arranged to communicate through a siphon duct to the overflow receiver  12  in order that volume changes in the first region cause the level of fluid in the spill-over chamber to rise and fall. During operation of the hydrostatic transmission  10 , as the fluid contained in the first internal volume  13  warms up and expands, the expansion in the fluid causes a flow of the excess fluid through the siphon duct  9  into overflow receiver  12 . The initial level  64  of fluid in overflow receiver  12  rises and denoted by the high fluid level  65 , and because in this embodiment, plug  16  seals overflow receiver  12  from the environment, the air inside above the fluid level  65  is compressed with the result that the pressure level in the fluid in the first internal volume  13  is also increased such that the abilty for make-up fluid to be taken into the closed-loop circuit  26 ,  26  of the hydrostatic transmission  10  through the check-valves  28 ,  29  is enhanced thereby improving the operating characteristics of the hydraulic pump  22  and motor  23 .  
         [0052]    In order to minimize the actual volume size required for the overflow receiver, preferred practice is to include a fluid barrier such as the aforementioned shaft-seal between the first internal volume and the gear compartment sump. If on the other hand sufficient interior space could be found enabling a larger overflow receiver to be incorporated within the housing, then the embodiment of the present invention described above could be modified whereby the gear and hydrostatic compartments would be fluidly connected together alone the lines of the “common sump” hydrostatic transaxles mentioned earlier. In this case, there would be no need to include a fluid barrier to prevent hydrostatic fluid in the first internal volume from mixing with the gear lubricant in the gear compartment sump provided the larger overflow receiver has sufficent increased capacity to accommodate the potentially greater displaced volume of fluid from the “common sump”, The second embodiment of the invention shown as FIGS. 4 &amp; 5 has been included in order to show the concept of using an internal expansion chamber in the form of an overflow receiver can equally be applied with merit to stand-alone hydrostatic transmissions. As many components relating to the hydrostatic transmission remain substantially similar to those already described for the first embodiment, for convenience sake, they are numbered to carry the same reference numerals as have been designated in the first embodiment. Here the housing comprising elements  80 ,  81  form a first internal volume  82  to surround the components of the pump  22  and motor  23  and a second internal volume  83  that acts as the overflow receiver. Corridor passage  84  allows the passage of duct  85  such that first and second internal volumes  82 ,  83  are thereby in fluid communication. Although it is a preferable but not an essential feature of the invention for the second internal volume to operate under slight pressure, a breather could be used in place of the sealing plug in order for over-spill chamber  12  to remain at normal atmospheric conditions. 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 overflow receiver, or alternatively and as shown in this embodiment, a plug  86  with a small vent hole  87  could be used. It would further be possible to manuafacture vent hole  87  as a very small diameter hole to be a throttle to operate as a pressure restriction valve in order that during operation, a slight pressure build-up, perhaps by only one or two psi above atmospheric pressure, would occur in both first and second internal volumes  82 ,  83  and in effect, create a positive head of pressure in the hydrostatic closed-loop circuit, analogous to using a charge pump to boost the pressure on the intake line of pump  22 .  
         [0053]    In the case of some output speed reducing gearing being included in the housing package and located between the hydraulic motor and the output shaft in a modified form of “stand alone” hydrostatic transmission, it would be possible for the gear compartment to be fluidly connected to the hydrostatic compartment provided sufficient interior space can still be found enabling a larger overflow receiver to be incorporated within the housing. However, in order to keep the volume size for the overflow receiver to a minimum and minimize contamination problems, it is still to be preferred to install a seal as a fluid barrier on the shaft connecting the hydraulic motor to the gear train.  
         [0054]    The third embodiment of the present invention as shown in FIGS.  6  to  8  offers an alternative solution for the location of an internal expansion chamber and contrasts with the hydrostatic transaxle already described as the first embodiment in that here the gear compartment sump performs the dual role for not only containing the gear train but also acting as the overspill receiver for receiving and supplying fluid displaced from the first internal volume containing the hydrostatic transmission. Therefore, in this embodiment, as it is now gear compartment sump that performs the duty as the overspill receiver, the gear compartment sump is to be here designated as being the second internal volume of the present invention. It is therefore a requirement in this embodiment that a fluid barrier be provided in order to prevent fluid in respective first and second internal volumes intermixing in any way other then by way of the permeable siphon duct.  
         [0055]    As many components relating to the hydrostatic transmission as well as the reduction gearing remain substantially similar to those already described for the first embodiment, for convenience sake, they are numbered to carry the same reference numerals as have been designated in the first embodiment.  
         [0056]    The hydrostatic transaxle  90  is shown having a two-piece housing construction  91 ,  92 A formed to include a first internal volume  93  for the components of the hydrostatic transmission  10 ; a second internal volume  94  for the components of the gear train  11 ; and where here the position of corridor passage  95  in wall  42  of housing  92 A allows the siphon duct  96  to hydraulically links first  93  and second  94  internal volumes together. As such, an opening  44  is provided in wall  42  in the housing  92 A into which rotary-seal  45  is disposed, and seal  45  disposed in opening  44  and engaging drive-shaft  40  of the hydraulic motor  23  thereby provides the required fluid barrier at this junction between first and second internal volumes  93 ,  94 . As a result of any volume change experienced in the fluid capacity of the first internal volume  93  due to temperature change, the fluid within first internal volume  93  has the only recourse to pass through the siphon  96  to effect an equal but opposite volume change of the fluid lubricant held in the second internal volume which for this embodiment is the gear compartment sump  94 .  
         [0057]    Thereby, the fluid surrounding the hydrostatic transmission components in the first internal volume is allowed to freely mix with the fluid carried in the gear compartment sump by free passage through the siphon duct, and provided that no reliance is being placed on using an external expansion tank, it is most beneficial that second internal volume  94  for the speed reducing apparatus  11  be only partially filled with fluid. With second internal volume  94  only partially filled with fluid when cold denoted by reference numeral  97  as the lower fluid level, there is thereby provided an additional volume space denoted as air pocket  98  within the housing to take care of the expansion in both first  93  and second  94  internal volumes. As warming of the fluid takes place in first internal volume  93 , the displaced fluid from first internal volume  93  flows through duct  96  to arrive in the second internal volume, causing a rise in the upper surface level of fluid and a corresponding decrease in the size of air pocket  98 . Air pocket  98  will continue to deminish in volume size until such time that steady state conditions have been reached in the unit, the level of fluid within second internal chamber  94  having then reached a maximum as shown as fluid surface level  99 .  
         [0058]    As soon as the fluid in first and second internal volumes  93 ,  94  warms up and the fluid level begins to rise from the initial lower level  97  towards upper level  99 , air pocket  98  in second internal volume  94  becomes subject to an increase in pressure above atmospheric and continues to diminish in volume size so long as plug  100  is effective in preventing the escape of air fom pocket  98 . In this respect, it is equally important that effective seals are used elsewhere in the unit transaxle such as the well-known shaft seals manufactured by the company Freudenberg. Compression of air pocket  98  produces an equal rise in pressure in the first internal volume  93  such that the check-valves  28 ,  29  operate more efficiently in replenishing lost fluid from the closed-loop circuit passages  25 ,  36 , and a corresponding decrease in operational noise emanating from the hydrostatic transmission  10  is noted.  
         [0059]    Once the unit begins to cool, the size of air pocket  98  begins to increase as the fluid surrounding the components of the hydrostatic transmission  10  contracts in volume and draws fluid back through duct  96  from the second internal chamber  94 . As this happens, there is a corresponding fall in the level of fluid held in the second internal volume  94 .  
         [0060]    The fluid level in the gear compartment sump can therefore remain low when the unit is cold, for instance during winter start-up conditions, and then only increases as the unit warms up ensuring the gear train remain well lubricated even as the fluid viscosity falls. Such auto-levelling in the surface level of gear lubricating fluid can ensure that the hydrostatic transaxle has the best operating efficiency possible.  
         [0061]    It is therefore a feature of this embodiment that the fluid lubricating the gear train  11  can flow in either direction through duct  96  depending whether the temperature conditions experienced by the machine is rising or falling.  
         [0062]    Although as set forth in the example described above, first and second internal volumes  93 ,  94  as well as whatever remaining void left of air pocket  98 , are subject to pressurization due to the fluid carried by the housing being in its expanded state, it is nevertheless not intended to limit the invention in this way. For instance, air pocket  98  could alternatively be pressurized by an inert gas such as nitrogen at the factory once the hydrostatic transaxle has been assembled and filled with operating fluid such that the first and second internal volumes remain pressurized even when the unit is cold and the fluid contained within remains at its minimum volume expanded state.  
         [0063]    Furthermore, there may be certain applications where it is still advantageous for the hydrostatic transaxle to operate with the gear compartment sump substantially or completely full of lubricating fluid even when the operating temperatures remain low. In this respect, the embodiment of the invention described as FIGS.  6  to  8  may be modified to include an external expansion tank fitted directly to the housing or by a pipe connection in place of plug  100 . Expanding fluid in the first internal volume would be forced through the duct to enter the second internal volume such that the additional would be displaced to the external expansion tank. 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 feature of the invention to confine all natural variations in fluid volume carried in the first and second internal volumes  93 ,  94  to within the interior of the transaxle  90 .  
         [0064]    As the third embodiment of the invention relies on fluid held by the gear compartment sump to replenish any contraction in fluid volume held by the first internal volume due to falling temperature, there would be an advantage if apparatus could be included inside second internal volume for the removal of foreign bodies suspended in the fluid. Fig. 8  illustrates one form of contamination settlement trap  105  for the removal of such foreign bodies before they can be ingestion by the siphon duct.  
         [0065]    The lower housing element  92 B is shown cast wih a sediment and contamination trap arrowed as  105  comprising two wells  101 ,  102 , the inner well  101  is where the open end  103  of siphon duct  96  protrudes into its interior  106  and where a wire gauze strainer in the shape of a hollow disc  104  seats near to the top of the well  101  to prevent larger-sized particulates from entering interior  106  of well  101 . The outer well  101  contains one or more magnets  107  which act to attract any ferrous particles of contamination that might be otherwise suspended in the hydrostatic fluid, and where a further wire gauze strainer  108  located above magnet  107  prevents fluid motion from rotating gears  59 ,  60  from disturbing any foreign bodies that settle as sediment in well  102 .  
         [0066]    In the case of hydrostatic and gear reduction transmissions which although connected together in some manner do not share a common housing, the present invention may be adapted so that the first internal volume surrounding the components of the hydrostatic transmission is in fluid communication to the second internal volume surrounding the gear train by a modified form of siphon duct, and where a portion of the siphon duct lies external of the housing constructions in order to be able to span the gap at the interface between the hydrostatic and gear transmissions. As a result of any volume change experienced in the fluid capacity held in the first internal volume due to a change in temperature, the displaced fluid from the first internal volume flows through the siphon duct to effect an equal but opposite volume change in the level of fluid held by the second internal volume. Therefore, according to the fourth embodiment of the invention shown as FIG. 9, housing  109  provides an interior into which components of the hydrostatic transmission  110  are located, the interior defining the first internal volume  111 . Adjacent housing  109  is housing  115 , housing  115  provides an interior into which various components of the transaxle and gear train are disposed such as axle shaft  116 , the interior defining the second internal volume  120 .  
         [0067]    As shown, one open end  121  of the siphon duct  123  is arranged to be close to the upper interior surface  125  in first internal volume  111  and extends downwards before turning horizontally to exit housing  109  at  127 . Siphon duct  123  then extends to span the distance between housings  109 ,  115  to enter housing  115  at  128 . Respective seals  129 ,  130  are shown surrounding the siphon  123  at the exit  127  and entry  128  points to ensure at leak-free joint.  
         [0068]    As the siphon duct  123  protrudes through into second internal volume  120 , it is arranged to turn upwards in a direction towards plug  150  before turning downwards and extending towards lower interior wall  140 . Preferably, open end  132  of siphon duct  120  should remain submerged at all times below fluid level  133 . That portion of siphon duct  123  which extends from exit  127  to entrance  128  should, preferably, be thermally insulated in order to negate the chill factor, especially prevalent in winter conditions.  
         [0069]    An alternative form of sediment trap is disclosed in this embodiment and which comprises a baffle plate  136  which is held at a fixed angle in the housing  115  in order that any foreign bodies suspended in the hydrostatic fluid can settle out at the bottom of the second internal volume. The baffle  136  includes one or more holes  137  which allow the fluid below and above the baffle plate to freely mix. When the hydraulic fluid in both first and second internal volumes  111 ,  120  warms up, the increased volume of fluid causes the surface level to rise to level marked as  134 . Once the unit cools down, the fluid level drops back towards  133 . As the level drops below hole  137 , and once the sediment has settled out, a substantial amount of the sediment shown as  160  will accumulate at the bottom upper side of the baffle  136  and not at the lower side of the baffle where the entrance to siphon duct  123 . The accumulation of sediment  160  could be attracted to surround a magnet placed at the bottom upper side of baffle  136  at  160 , and a steel baffle in this case has the advantage that it will become thus slightly magnetized with the result that the vast majority of ferrous contaminant suspended in the fluid, for instance, worn from the powder-metal gearing, would become locked-up on the magnetized baffle plate surface as well as the magnet. Hence, quantities of foreign bodies suspended in the fluid in second internal volume  120  that could pass through siphon duct  123  to enter the first internal volume  111  would be small and pose little risk in reducing the useful working life of the hydrostatic transmission  111 .  
         [0070]    Although neither of the last two embodiments have shown a contamination trap in the form of a cylindrical oil filter attached over the entrance end of the siphon duct, such a filter could be usefully deployed in place of the apparatus already mentioned above. 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.  
         [0071]    With respect to the first and second embodiments of the invention, during periods when the hydrostatic fluid within the first internal volume remains cold in at its most contracted state, the fluid level in the overflow receiver need only be as high as necessary to ensure that the siphon vent will operate properly once operation of the hydrostatic transmission commences. With respect to the third and fourth embodiments, the level of fluid held by the second internal volume need only to be sufficient to ensure the gears and support bearings remain sufficiently lubricated. Furthermore, although it is to be preferred for the expanding volume of hydrostatic fluid to cause a pressure increase in both the overflow receiver as well as in the first internal volume, it is not intended to limit this invention in this way. Consequently, many advantages of the present invention are still prevent even when the the second internal volume is arranged to be naturally vented to atmopheric conditions. It should also be noted that although the hydrostatic and gear train components are preferably kept apart within the same housing structure, the invention can be modified such that both hydrostatic transmission and gear train components reside within a common sump, and where in this modification, the overflow be sized sufficiently to enable it to accommodate a greater displacement in volume of fluid from the sump.  
         [0072]    The internal fluid expansion chamber of the present invention responds to changed conditions in the environment with far more precision and rapidity than known solutions relying on external devices, especially as there is a more rapid transfer of heat within the housing such that surges in fluid movement are lessened with the result that seal failures and other fluid leakage to the environment are far less likely.  
         [0073]    It should be noted that this invention does not preclude the use and incorporation of an input shaft driven “charging” pump for the closed-loop hydrostatic circuit. Such a charge-pump could still be included for operation within the first internal volume and its operation would not detract from the advantage of the present invention. In the case of a such an auxiliary pump being required as a hydraulic power take-off, the return line from hydraulic power take-off would preferably be connected to the overflow receiver which would be very adapatable for carrying varying quantities of fluid, depending on what demands were imposed by the duty cycle of the hydraulic power take-off. Similarly, an externally exposed flexible membrain of the type known as a bellows could also be incorporated with advantage, for example by attachment to the housing so that one one side it is exposed to the atmophere whereas on the opposite side it is disposed to the overflow receiver. Such a beloows would act as a barrier in preventing airborne contamination from the environment from being ingressed in the hydrostatic fluid of the hydrostatic transmission.  
         [0074]    In accordance with the patent statutes, we have described the principles of construction and operation of our invention, and while we have endeavoured to set forth the best embodiments thereof, we 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.