GB2237621A - Hydrostatic transmission - Google Patents

Abstract

A hydrostatic transmission includes a casing 1, 2 Fig. 1, containing rotary members 12 having radial cylinders 20 each with a piston 21 engaging a track ring 23 and constituting a pump 4 and a motor 5 respectively. Internal oil ducts 72, 80, 90, Fig. 7, are included for providing further temperature control of the main internal transmission component parts 14, 24, said ducts 72, 80, 90 being fed from an auxiliary fluid pump 8, 9, Fig. 1, the ducts 72, 80 being located in the pintle 14 to link the pintle ends and provide a cooling circuit. Further passages 30, 31, Fig 7, within the pintle provide the closed-loop fluid circuit for the transmission. <IMAGE>

Description

"ImProvements related to Rotary Hydrostatic Transmissions" This invention relates to rotary hydrostatic machines comprising a hydraulic pump and motor of the radial piston type.

It is an object of the invention to provide an improved hydrostatic machine which may be particularly suitable for example to high-duty hydrostatic transmissions such as are used in off-road vehicles or farm tractors and will avoid some of the problems experienced with existing transmissions.

In many such installations, compactness is of paramount importance and this invariably leads to difficulties in terms of heat generation within such transmissions causing poor operational efficiency and service life and it is an object of the present invention to avoid some of the problems experienced with existing transmissions.

The present invention relates to a hydrostatic transmission comprising a casing; an internal transverse partitioning wall dividing the casing into first and second chambers; a pintle fixedly and non-rotatably mounted in the partitioning wall, the pintle having first and second ends extending into said first and second chambers, respectively, and including at least four internal longitudinal hydraulic fluid passages, at least two of these internal longitudinal hydraulic fluid passages terminating in ports; a rotary cylinder member rotatably mounted on each first and second extending pintle ends, each cylinder member comprising a plurality of radially arranged cylinders and a plurality of pistons, a piston disposed in each cylinder, the cylinders successively connicating with a respective port during rotation of cylinder members; an annular cam track surrounding each cylinder member, respectively, pistons operatively connected to the cam track; an input rotary shaft and an output rotary shaft rotatably mounted in the casing and extending into said first and second chambers, respectively, each shaft is coupled to a respective cylinder member; and wherein the internal longitudinal hydraulic fluid passages other than those terminating in ports pass unobstructed through the pintle to hydraulically link the first and second ends of the pintle.

The pintle may be provided with one or more check valves communicating with the machine chamber either directly or via an auxiliary oil charge pump.

At present due to the above mentioned disadvantages of hydrostatic transmissions, standard gear shift transmissions are most commonly fitted to such vehicles, and therefore it is necessary for any improved hydrostatic transmission to fit into the same space envelope currently available in such vehicles.

For reasons of compactness it is therefore desirable to mount the pump and motor in a back-to-back coaxial configuration and as a result one of the problems associated in such layouts is that it is very difficult to keep the internal elements of the transmission sufficiently cool and so avoid complete seizure.

All existing radial piston transmissions utilising the back-to-back configuration where both pump and motor driving elements rotate on the same cylindrical pintle valve suffer a marked deterioration in performance and life expectancy when internal temperature rises above normal safe levels. This may occur when such transmissions are overloaded for longer periods than usual at high input speeds or when the usual means for cooling the transmission oil are no longer effective. Such high temperature conditions become especially serious for certain sensitive components such as the hydrostatic bearing of the pintle valve and the slipper shoes.

If the transmission is to be operated with its casing full of oil, the slipper shoes will remain sufficiently cool so long as the oil within the casing does not overheat. However the pintle is more prone to overheating due to its encapsulated position, as all heat must first be transferred to its surrounding components before it can pass through into the oil in the casing.

This effect of heat build-up in the pintle is further exaggerated in dry-case transmission applications, where a smaller volume of oil in the casing is used and remains in the sump or reservoir at the bottom of the transmission casing. As the oil is not in direct contact with the working elements of the transmission, such dry-case machines are more difficult to cool.

In this case the self-generated heat from the working components can only be removed by "splash" cooling, where oil from an auxiliary pump is allowed to flow over the rotating elements so as to -aw heat away. However, as the pintle valve is totall-I surrounded by other components, very little oil can reach it for cooling, and as a consequence it may become damaged.

However many of the cooling problems associated wit prior transmissions are easily overcome with the disclosed features of the present invention through the inclusion of special flow passages within the pintle and track ring elements so that the cool oil from the auxiliary pump can be constantly circulated through these said components when the hydrostatic transmission is in full operation as well as when the bulk of engine power passes through the mechanical drive link rather then through the hydrostatic transmission.

The invention may be performed in various ways and one specific embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional side elevation through a radial piston transmission according to the invention, Figure 2 is a cross-section on the line I-I in Fig.1.

Figure 3 is a cross-section on the line III-III in Fig.2.

Figure 4 is a cross-section on the line II-II in Fig.1.

Figure 5 is a cross-section on the line VI-VI in Fig.4.

Figure 6 is a cross-section on the line IV-IV in Fig.4.

Figure 7 is a cross-section on the line V-V in Fig.6.

In this example the invention is applied to a rotary hydrostatic transmission for a medium to heavy duty farm tractor.

The transmission illustrated in Fig. 1 comprises two main case covers 1, 2 and a central sandwich plate 3 and these contain the rotary hydrostatic pump 4 and the rotary hydrostatic motor 5. The engine of the vehicle is coupled to drive an external input drive shaft 6 at one end of the casing. An external output drive shaft 7 at the other end of the casing coupled to the vehicles driving wheels via differential and reduction gearing. The input shaft 6 may be connected via gear train 8, 9 to drive a secondary by-pass shaft 10 that for reasons of compactness is shown passing through the hollow pivot pin 11 of the transmission.

The pump 4 and motor 5 of the transmission are generally of similar construction. Each comprises a rotary cylinder block 12 attached to a ported sleeve 13 mounted to rotate on one end of a common fixed pintle 14, which is rigidly secured in a central sandwich plate 3 acting as an internal partition between the case covers 1,2. Each cylinder block 12 is formed with a number of radial bores 20 each containing a piston 21 attached to a slipper 22, that act against a surrounding annular track ring 24.

Preferably the track ring 24 has a hardened ring 23 inserted into its bore.

Ideally the two track rings 24 are pivotally mounted on a common transverse pivot pin 11 and where the track ring of the pump 4 is connected through the movable hinge pins 25, 26 to a control lever 27 that is linked to the manual transmission ratio control lever in the cab of the vehicle, and where the motor 5 track ring 24 remains fixed in an offset position.

Thus by turning the control lever 27 the eccentricity of the pump 4 track ring 24 pivotting on hollow pin 11 is altered in relation to the fixed pintle 14, thereby changing the volumetric capacity of the pump and hence the speed ratio of the complete transmission.

The fixed pintle 14 has two internal longitudinal passages 30,31. Passage 31 is shown in Fig. 1 opening at opposite ends into an arcuated ports 32,33 which communicate with respective cylinder bores 20 of the rotating cylinder blocks 12. The second internal longitudinal passage 30 in the pintle 14 is shown in Fig. 4 and acts as the fluid return passage and thereby together with passage 31 forms the closed-loop fluid circuit for the transmission.

At the end of the pintle 14 there are provided one or more check valves 34 arranged to allow additional fluid to be drawn into the oil circuit 30,31 for "make-up" purposes so to replenish any fluid lost by leakage.

A separate casing 40 incorporating two spur gears 8, 9 shown in Fig. 2 may be attached to the case cover 1 of the transmission. Gear 8 is keyed to the input drive shaft 6 and meshes with the larger gear 9 locked onto the bypass-shaft 10.

The two gears 8, 9 may also conveniently be used as the auxiliary oil pump for providing "make-up" fluid of the main oil circuit as well as for oil filtration and cooling.

Input drive shaft 6, which is supported in ball bearings 41, connects with the rotary cylinder member 12 by way of the drive coupling element 35 as shown in Fig.1.

If the vehicle operator wishes to move the vehicle from rest, he moves the lever from the neutral position into forward drive. Therefore the transmission ratio is altered as the track ring 24 is moved to an eccentric position relative to the fixed pintle 14. Up to the point of full eccentricity, all power is transmitted hydraulically to shaft 7 of the hydrostatic transmission.

During this period, clutch 50 remains disengaged from the transmission output shaft 7, and therefore the by-pass shaft 10 does not transmit any power through the gears 51, 52.

However when full eccentricity is reached, the vehicle operator may wish to engage mechanical through-drive by engaging clutch 50 so that engine power is now via bypass-shaft 10 and through gears 51,52 to output shaft 7.

This is only possible when the transmission motor 5 is rotating at the same speed as the transmission pump A t and when the gears 51,52 are of the same ratio as gears 8, 9.

The clutch 50 is engaged through straight mechanical, electrical or hydraulic means acting on lever 53 which pivots on pin 54 allowing the springs 55 acting on the flange member 56 to force friction discs 57, 58 together onto the end face of gear 51.

As a result, a direct connection between shaft 10 and gear 51 is made and therefore almost all the power (except for small losses in rotating the hydraulic elements) passes through the transmission mechanically and no longer hydraulically.

Both friction discs 57,58 are splined to the spigoted end of shaft 10 and the complete clutch 50 assembly and gears 51,52 are contained within the end housing 60.

Under these conditions the transmission pump 4 rotary member 12 is still being driven by way of shaft 6, and as the track ring 24 is set at full eccentricity, the pistons 21 will still be pumping oil into the hydraulic motor 5. However the working elements of both pump and motor do still generate some heat and if this cannot be extracted sufficiently quickly, the rotating elements of the transmission may overheat resulting in damage to the bearing surfaces. Therefore the present disclosed improvement allows for cooling oil to be pumped through the pintle and track ring elements thereby largely preventing such overheating.

However because the transmission motor 5 rotary member 12 is also driven by shaft 7 through gears 51,52 at the same speed as its opposite pump 4 rotary member 12, the overall effect is that both rotary members are not transmitting any power between them.

Even so, in order for this to be the case, it is necessary for the displacement of both pump and motor to be near identical.

This can be done by including a screw adjustment on the motor track ring, so that under test prior to sale, the transmission motor can be adjusted in displacement until it matches the pump displacement.

Under operation, a further advantage of the transmission is that as soon as the vehicle operator reduces the eccentricity of the pump 4 track ring 24 by way of altering the setting of the transmission control lever, a linkage disengages the clutch in the axle, so returning all drive power back through the hydraulic transmission circuit. Another advantage of the transmission is that as both rotary members are always in rotation during mechanical through-drive, that at the moment when this link is broken, and all the power is transmitted back through the hydraulic circuit, no serious jerk occurs to the vehicle. This would not be the case if the rotary members 12 were stationary at the moment when power is to be passed through them.

In order to reduce the control forces of the transmission, it is preferable that a servo ram system operating off the high pressure oil circuit is included. This is shown in Fig. 4 where pressure oil is ducts out from the pintle oil gallery 31 through a passage 100 which not only leads to a high pressure relief valve 101, but also to passage 102 in the valve plug 103, which thereby provides a path for the oil to enter cylinder 105 of the servo ram 106, this ram 106 acts against a stop 107 located on the track ring 24. An opposing servo ram 110 linked hydraulically from pintle oil gallery 30 by passage 112 becomes "active" when said oil gallery 30 is under high pressure.These rams help reduce the required control forces as the operator alters the transmission ratio, while also allowing for smoother transition from mechanical into hydraulic drive. The valve plug 103 is designed so that it can be locked in either one or two positions depending on the desired output rotation of the transmission. In its second position (not shown), pressure oil in gallery 31 is directed to the opposite servo ram 110 via passage 112 located in valve plug 103. Pin 111 locks the valve plug 103 in place in the sandwich plate 3.

In a hydrostatic transmission of the type disclosed that includes a mechanical through-drive and clutch, it becomes convenient to use gears 8,9 as an oil pump for ancillary purposes.

The meshing gears 8,9 act to draw cool oil up through a feed passage 64 from the reservoir 65 to the suction cavity 66 of the gear pump as shown in Figs. 2 and 3. The oil is delivered at 67 and flows through the small internal passages 68,69,70,71 in the sandwich plate 3 and into the pintle cooling circuit shown in Fig. 5.

As the oil flows to either ends 73,74 of the pintle 14 through the axial passage 72, it will absorb some of the heat contained within the pintle 14 member. The oil flows to both cavities 75,76 formed between the pintle 14 ends 73,74 and the drive coupling 35. Two small seals 77 are positioned at each end of the drive coupling 35 to prevent oil from the cavities 75,76 seeping out into the interior of the housing.

If the check valves 34 are positioned at the ends of the main transmission oil galleries 30-, 31 at the pintle 14 end 73, a small proportion of the cooling oil delivered from the auxiliary gear pump 8,9 into cavities 75,76 will also provide the necessary make-up oil for the transmission oil circuit 30,31.

The remaining oil flows out of the pintle 14 through axial passages 80, and through a number of passages 81,82,83 in the sandwich plate 3 and into a cavity 90 formed between the track ring 24 and the inserted ring 23 as shown in Figs. 4, 6 and 7.

This oil further absorbs the heat generated on the slipper engagement surface and passes out from cavity 90 by way of hole 92 trilled from one end of the track ring 24 to break into the cavity 90. Hole 92 connects with passages 93,95,96 formed inside the sandwich plate 3. A fluid seal 120 is used to prevent oil leakage at the interface between hole 92 and passage 93. As a consequence the cooling oil falls from passage 96 into the reservoir 65 where it can dissipate its heat energy before re-entering the auxiliary pump of the transmission.

Preferably the oil flow from the gear pump is directed through a filter and radiator type oil cooler before it enters the cooling oil circuit 72,80 in the pintle.

A part of the subject matter described above is also contained in co-pending patent application 8719975, where a by-pass shaft is provided within the case structure of the hydrostatic transmission. The input end of the by-pass shaft is rotatably driven by gearing from the input drive shaft of the hydrostatic transmission, and where the output end of the by-pass shaft can be coupled through further gearing to the output drive shaft of the hydrostatic transmission by means of a clutch when required. As a result, the hydraulic fluid circuit of the hydrostatic transmission is no-longer operative in transmitting power between the input and output drive shafts.

Claims (9)

"CLAIMS"

1. A hydrostatic transmission comprising a casing; an internal transverse partitioning wall dividing the casing into first and second chambers; a pintle fixedly and non-rotatably mounted in the partitioning wall, the pintle having first and second ends extending into said first and second chambers, respectively, and including at least four internal longitudinal hydraulic fluid passages, at least two of these internal longitudinal hydraulic fluid passages terminating in ports; a rotary cylinder member rotatably mounted on each first and second extending pintle ends, each cylinder member comprising a plurality of radially arranged cylinders and a plurality of pistons, a piston disposed in each cylinder, the cylinders successively communicating with a respective port during rotation of cylinder members; an annular cam track surrounding each cylinder member, respectively, pistons operatively connected to the cam track; an input rotary shaft and an output rotary shaft rotatably mounted in the casing and extending into said first and second chambers, respectively, each shaft is coupled to a respective cylinder member; and wherein the internal longitudinal hydraulic fluid passages other than those terminating in ports pass unobstructed through the pintle to hydraulically link the first and second ends of the pintle.

2. A hydrostatic transmission according to claim 1 further including radial fluid transfer ducts in the partitioning wall which are in communication with the internal longitudinal hydraulic fluid passages which pass through the pintle.

3. A hydrostatic transmission according to claim 2 wherein the radial fluid transfer ducts are in communication with an auxiliary hydraulic pump.

4. A hydrostatic transmission according to claim 1 wherein each annular cam track has an internal fluid cavity following a path between an inner and outer diameter of the annular cam ring, the ends of the cavity terminating in fluid entry and exit ports.

5. A hydrostatic transmission according to claim 4 wherein the internal fluid. cavity of the annular cam track follows a circular path.

6. A hydrostatic transmission according to claim 4 wherein each end face of the partstioning wall is in fluid communication with a respective cam track.

7. A hydrostatic transmission according to claim 6 wherein each end face of partitioning wall includes a fluid supply port and a fluid return port.

8. A hydrostatic transmission according to claim 7 wherein one end face in each annular track ring includes a fluid entry port and a fluid exit port, fluid entry port fluidly connected to cooperate with fluid supply port, and fluid exit port fluidly connected to cooperate with fluid return port.

9. A hydrostatic transmission according to c'laim 8 wherein the annular cam track includes an internal fluid cavity which follows a path between the inner and outer diameter of the annular cam ring, the ends of the cavity terminating in fluid entry and exit ports, respectively, with the fluid entry and exit ports arranged for cooperation with fluid supply and return ports in each end face of the partitioning wall.