GB2161553A

GB2161553A - Synchronised mechanical-hydrostatic transmission

Info

Publication number: GB2161553A
Application number: GB08417674A
Authority: GB
Inventors: Bo Hedlund
Original assignee: STENSELE MEK VERKSTAD AB; Stensele Mekaniska Verkstad AB
Current assignee: STENSELE MEK VERKSTAD AB; Stensele Mekaniska Verkstad AB
Priority date: 1983-01-13
Filing date: 1984-07-11
Publication date: 1986-01-15
Also published as: GB2161553B; GB8417674D0

Abstract

A synchronised mechanical-hydrostatic transmission for a vehicle with infinitely variable drive ratio including a hydrostatic transmission (3,4) and a following mechanical gearbox (5) coupled thereto. The hydrostatic transmission includes a hydraulic transmission motor (4) with variable displacement which is connected to the mechanical gearbox (5). The mechanical gearbox has at least two alternatively- selectable gear ratio stages, and the gear ratio is changed automatically by selector valve means (17) and clutches (11,11') when the displacement of the hydraulic motor (4) reaches its minimum value, under control of a speed-control valve (16). Means (18,21) is provided for altering the displacement of the hydraulic motor (4) on alteration of the gear ratio of the mechanical gearbox to an extent corresponding to the gear ratio change, so as to synchronise the output shaft (7) of the hydrostatic transmission to the mechanical gearbox. <IMAGE>

Description

SPECIFICATION Synchronised mechanical-hydrostatic transmission The present invention relates to a synchron ized mechanical-hydrostatic transmission with infinitely variable drive ratio, including a hydrostatic transmission and a mechanical gearbox coupled to and preferably following after the hydrostatic gear.

The need for large propulsion power, i.e.

large transmitted torque, for a vehicle at low speeds, and high rotational speed at high road speeds can not be satisfied with hydrostatic transmission as known up to now, which include variable displaement hydraulic pumps and hydraulic motors. Available components for such hydrostatic transmissions limit both transmitted torque and permitted rotational speed, and it is not possible to cover a sufficiently large speed range solely with variable displacement pump and motor units. Up to now a mechanical gearbox has therefore been used, which due to price and the known art has been of a fairly simple type with different fixed gear ratios. It has thus been necessary to stop the vehicle to change from terrain gear, to road speed gear (from low to high gear). It is possible, for example, to have a gear for the speed range of 0-15 km/h and a gear for the range of 0-30 km/h.Such previously known gearboxes are thus arranged with two or more speed ranges, all of which start from zero speed, however.

It is obvious that there is a considerable disadvantage in needing to stop the vehicle to alter the gear ratio, and this has been a hindrance in use.

With transmissions of the hydrodynamic or torque converter type, there is continuous slip between the input and output shafts. The slip increases for increasing load and the rotational speed is thus lowered on the output shaft. In order to provide a hydrodynamic transmission with a sufficiently large speed range the converter has been provided with a following mechanical gear constructed such that gear changing can take place while the vehicle is being driven. In such a case the mechanical gearbox is of the so-called power-shift type which means that at the gear-changing instant two gear ratios are simultaneously engaged during successive transfer of the mesh via slip clutches. The momentary rotational speed change which will be the result in the gear changing instant is taken up by the slip in the converter, and the vehicle continues without any notable jerk in speed.Gear changing is carried out manually by the driver and always acts solely on the mechanical gearbox, the hydrodynamic part of the transmission adjusting itself automatically to the new rotational speed. The consequence will thus be a lowering of the input rotation speed to the converter and thereby also that of the driving unit, if a change up is carried out. For changing down these rotational speeds are of course increased.

Attempts have also been made with hydrostatic transmission to change speed with a power-shift gearbox so that a sufficiently large speed range can be obtained, but the inelastic hydrostatic gear does not function as a damper in changing speed, and a heavy jerk occurs, which feels uncomfortable and results in large stresses on both driver and vehicle.

An object of the present invention is to eliminate the disadvantages in the prior art that have been discussed above.

The condition for a jerk-free gear change between two variable-speed gearboxes, a hydrostatic and a mechanical gearbox in the case in question, is that the output shaft from the one gearbox is always given the same rotational speed that is to be obtained by the input shaft in the other gearbox, the two shafts being coupled together. In other words, the gear change must be synchronized.

The present invention comprises a seriescoupled combination of a hydrostatic transmission and a mechanical gearbox with at least two alternatively-selectable gear ratios or 'speeds', in which on the occurrence of each change of 'speed' of the mechanical gearbox a compensating change in the displacement of a variable-displacement component of the hydrostatic transmission is made.

In this way any sudden alteration in the overall velocity ratio of the combination can be avoided and a jerk-free synchronised change of ratio of the mechanical gearbox can be made.

In one form of the present invention, a synchronised mechanical-hydrostatic transmission with infinitely-variable velocity ratio between upper and lower limits comprises a hydrostatic transmission which includes a hydraulic pump component whose fluid output drives a hydraulic motor component, both of these two components being of positive-displacement type, and at least one of them being of infinitely-variable displacement type and being provided with displacement control means for varying its displacement between upper and lower limits, and a mechanical gearbox mechanically connected in series with the hydrostatic transmission, the gearbox having at least two selectable gear ratios, and a gear ratio selector means responsive to the operation of the displacement control means and arranged to change the gear ratio of the mechanical gearbox by a predetermined step when the displacement of the said component reaches a predetermined threshold value, and synchronising means arranged to alter the displacement of the said component from the threshold value in immediate response to each operation of the gear ratio selector means to change the gear ratio, the sense and amount of the said alteration in displacement being such as to compensate for the effect of that change in gear ratio on the overall velocity ratio of the mechanical-hydrostatic transmission.

Preferably the mechanical gearbox is coupled to the hydrostatic transmission on the output side of the latter, i.e. the mechanical gearbox "follows" the hydrostatic transmission.

In one arrangement of the invention, the hydraulic motor of the hydrostatic transmission may be the component of infinitely-variable-displacement type which is controlled by the displacement control means. However in a preferred embodiment of the invention the pump of the hydrostatic transmission is also of infinitely-variable displacement type and is also controlled by the displacement control means in sequence with that of the hydraulic motor.

Preferably the synchronising means acts on the displacement of the hydraulic motor in the compensating manner referred to above.

It is also preferred that the said upper and lower limits of displacement of the component of the hydrostatic transmission on which the synchronising means acts, be chosen to correspond to the limits of the compensating alteration in displacement effected by the synchronising means on each change in gear ratio.

From yet another aspect, the present invention comprises a synchronised mechanicalhydrostatic transmission with infinitely-variable velocity ratio, comprising a hydrostatic transmission including a positive displacement hydraulic motor of variable displacement type which is mechanically connected to a following mechanical gearbox having at least two predetermined selectable gear ratios, the hydraulic motor being provided with displacement control means for varying its displacement between maximum and minimum values and the mechanical gearbox having a gear ratio selector valve means operated by a control pressure in response to the operation of the displacement control means and arranged to change the gear ratio of the gear box by a predetermined step, respectively downwards (for higher output speed) when the displacement of the hydraulic motor reaches its maximum value, and upwards (for lower output speed) when the displacement of the hydraulic motor reaches its minimum value, and synchronising means arranged to alter the displacement of the hydraulic motor from its respective maximum or minimum value, in the opposite sense to that of its preceding approach to that maximum or minimum value, in immediate response to each operation of the gear ratio selector means to change the gear ratio, the amount of the said alteration in displacement being such as to change the rotational speed of the output shaft of the hydraulic motor in correspondence with the change in gear ratio of the mechanical gearbox.

The maximum and minimum values of the displacement of the hydraulic motor are preferably chosen in proportion to the gear ratio alteration step of the mechanical gearbox, whereby each said alteration in displacement effected by the synchronising means ends at a minimum or maximum value of the displacement.

The invention may be carried into practice in various ways, but one specific embodiment thereof, and a modification of that embodiment, will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows diagramatically a vehicle transmission including a combined hydrostatic transmission and mechanical gearbox embodying the invention, and Figure 2 shows a modified form of the speed control valve in the transmission of Fig.

1.

A vehicle transmission in its entirety is illustrated in Fig. 1, and consists of a diesel engine 1 driving via a shaft 6 a hydraulic pump 2 for servo pressure, and a hydraulic transmission pump 3 via a shaft extension 6', this pump having adjustable displacement and a flow that can be directed both on the pressure and return sides to a hydraulic transmission motor 4, which also has adjustable displacement. The pump 3 and motor 4 together form a hydrostatic transmission of infinitely-variable velocity ratio. Via a shaft 7 the hydraulic transmission motor 4 is rigidly connected to the input shaft 7' of a mechanical gearbox 5, the gearbox being of the socalled power-shift type, and in this embodiment constructed with two gear ratios. Via shaft 8 the mechanical gearbox 5 is connected to a power consumer, in this case the driving wheels of a land vehicle.

Synchronised gear change can take place if the common shaft 7,7 in Fig. 1 for both the hydrostatic gear 3,4 and the mechanical gear 5 can be provided with the conditions for having the same rotational speed alteration.

This is possible if the alterations in gear ratio are equal and opposite, and the gear changes take place simultaneously, in both gearboxes.

Since both the hydrostatic gearbox 3,4 and the mechanical gearbox 5 can change gears while full torque is being transmitted, the transmission in its entirety can change gear 'infinitely variably' during the transmission of full power in a vehicle.

In order to simplify the description so that it solely relates to the gear changing function, it is assumed that the diesel engine 1 of the vehicle operates at constant rotational speed in an embodiment for complete vehicle power transmission. A speed regulation device of known type causes fuel supply to be adjusted to the power the diesel engine 1 is to supply.

Only one embodiment of the control of the components included in the transmission is described, in this case a substantially hydraulic control system.

To keep the continued description as simple as possible, consider now the initial situation of a stationary vehicle with the transmission illustrated in Fig. 1, the diesel engine 1 being assumed, as mentioned, to operate at constant revolutions and at such a level that increased fuel supply allows full torque takeoff.

In the initial situation, the displacement of the hydraulic pump 3 is zero, resulting in that the hydraulic motor 4, with its displacement initially set at a maximum, does not rotate, which in turn naturally means that the vehicle is stationary. To put the vehicle into motion, a speed control valve 1 6 is actuated, e.g. by amechanical foot pedal 24, and proportional to the pedal position the valve is caused to supply control pressure from the servo pump 2 to the displacement setting means 1 9 of the hydraulic pump 3 and the displacement setting means 20 of the hydraulic motor 4 and to the setting means 26 for the gear selector valve 1 7 of the mechanical gearbox 5.In the initial situation, the gear selector valve 1 7 directs servo pressure to an engaging means 12' for the hydraulic clutch 11' of the mechanical gearbox 5, the clutch 11' thus connecting a gear wheel 9' to the output shaft 8 and by the meshing of this gear wheel 9' with a gear wheel 10 on the input shaft 7' is achieved that the gearbox 5 operates with its higher gear ratio (low speed gear).

The servo pressure is maintained constant with the aid of a pressure regulating valve 15, and the servo oil from the tank 1 3 is cleaned in a filter 14.

On increased actuation of the speed control valve 16, the pressure level of the control pressure to the displacement setting means 1 9 and 20 is increased. The setting means 1 9 is adjusted to react first, and thus progressively increases the displacement of the hydraulic pump 3, with the resulting increased flow from the pump causing the hydraulic motor 4 to rotate, and thereby causing the vehicle to move with increasing speed. When the control pressure has reached the level where the displacement of the hydraulic pump 3 has arrived at its maximum, the setting means 20 begins to reduce displacement of the hydraulic motor 4, and the shaft 7,7' and the shaft 8 further increase their rotational speed. When the hydraulic motor 4 has been brought to its minimum displacement the speed of the shaft 7,7' cannot be increased further.For the case where this speed range is insufficient, as discussed above, the setting means 26 of the gear selector valve 1 7 has been adjusted so as to change, at this level in the value of the control pressure, the operative position of the valve so that the servo pressure acting on the setting means 12' and clutch 11' is relieved by being diverted to a tank 1 3. The engagement of the gear wheel 9' to the shaft 8 is thus released, simultaneously as the servo pressure from pump 2 is diverted to actuate another setting means 1 2 and another clutch 11 associated with the gearbox 5 so that a gear wheel 9 is connected to the shaft 7,7', whereby the output shaft 8 can be driven via a gear wheel 10' at the lower gear ratio (high speed gear) of the gearbox 5.

This method would give the shaft 8 a momentary rotational speed increase. However, during the gear change instant the servo pressure also actuates another setting means 21 on the hydraulic motor 4 via a pressure regulation valve 18 with exactly the pressure required for counteracting the reduction by the setting means 20 of the hydraulic motor displacement, so that the displacement increases to an extent such that the rotational speed of the motor 4 and of the shaft 7,7' is instead reduced by an amount corresponding to the difference in gear ratio between high and low speed range in the mechanical gearbox 5. In the described embodiment the maximum and minimum displacements of the hydraulic motor 4 have been selected in a proportion such that the latter exactly corresponds to the gear ratio change in the mechanical gearbox 5.

By a further increase in the control pressure to the setting means 20 the displacement of the hydraulic motor 4 can once again be reduced and its rotational speed increased, thereby to further increase the rotational speed of the output shaft 8.

When the hydraulic motor 4 has reached its minimum displacement, increase in speed can be continued by increasing the rotational speed of the diesel engine 1.

When it is subsequently decided to lower the speed, this is done first by lowering the rotational speed of the engine 1 to its normal operational speed, after which the control pressure is lowered progressively by the valve 1 6 and the transmission goes through the above-described sequences in reverse order to lower the speed of the shaft 8, first by increasing the displacement of the hydraulic motor 4 and thereafter by changing down when the control pressure falls below that for maintaining the switching position of the gear selector valve 17, and thus the clutches 11,11' of the mechanical gearbox 5 change engagement positions and the counterpressure for the setting means 21 is drained off.

The displacement of the hydraulic motor 4 is thus decreased to meet the rotational speed increase requirement of the shaft 7,7' at the gear changing instant from high to low gear.

Finally the displacement of the hydraulic motor 4 is once again increased to the maximum value and the displacement of the hydraulic pump 3 is decreased to zero, whereby the vehicle once again becomes stationary.

In the embodiment described above the setting means 19, 20 and 21 are suitably of the servo valve type.

At several places in Fig. 1 there are illustrated tanks 1 3 included in the transmission for enabling by-passing or draining of the servo pressure at different points in the speed change sequence as described above.

In Fig. 2 there is illustrated an alternative form of control for the speed control valve 16'. In this embodiment speed control takes place electrohydraulically by the use of a regulating potentiometer 22, which is suitably operated by a foot pedal to control an electrically operated valve 16'.

To enable vehicle travel in both directions, the flow from the hydraulic pump 3 to the hydraulic motor 4 can be conventionally caused to change direction. This can be done, for example, by the pump itself being formed to supply its flow in a selected one of two opposite directions, or by a valve being placed between the hydraulic pump and hydraulic motor for chaning the direction of the pressure medium, whereby the whole transmission can be driven both forwards and backwards in an equivalent manner.

The sequence-controlled synchronized gearchange transmission described above thus works entirely infinitely variably over its entire speed range. Its essential function consists in, as described above, changing the displacement of the hydraulic motor 4 at the same instant and in the same proportion as the mechanical gear 5 is changed. By the known power-shift function and since displacement of the hydraulic motor 4 can be changed during full torque transfer, there is obtained continuity in the entire power transmission.

Only one embodiment has been described above. It is obviously possible to provide the mechanical gearbox 5 with more than two gear ratios, and for the sake of simplicity, with preferably the same step between each gear ratio, thus enabling a simple repetition of the hydraulic motor displacement alteration for each change in gear ratio of the mechanical gearbox.

If the displacement of the hydraulic motor is given fixed adjustments whose maximum and minimum values are in proportion to the mechanical gear ratio steps, the control pressure can be given comfortable overlaping values for ensuring stable change points. It is obvious, however, that it is theoretically possible to control the whole hydrostatic gear in balance with the mechanical gear ratio steps.

By the introduction of a sensor (not shown in the Figures) for the transmission pressure between hydraulic pump 3 and motor 4, this sensor being caused to conventionally counteract the build-up of control pressure, it is possible to limit the take-off power so that overloading of the diesel engine is avoided.

Furthermore, if the rotational speed of the diesel engine is controlled, as mentioned above, to increase further at the maximum transmission speed, the speed range of the vehicle can also be increased.

The transmission also functions for optional rotational speed of the diesel engine, which gives the possibility of separately regulating the engine to work at its most economical revolutions for each transmission stage.

Claims

1. A synchronised mechanical-hydrostatic transmission with infinitely-variable velocity ratio between upper and lower limits, comprising a hydrostatic transmission which includes a hydraulic pump component whose fluid output drives a hydraulic motor component, both of those two components being of positivedisplacement type, and at least one of them being of infinitely-variable displacement type and being provided with displacement control means for varying its displacement between upper and lower limits, and a mechanical gearbox mechanically connected in series with the hydrostatic transmission, the gearbox having at least two selectable gear ratios, and a gear ratio selector means responsive to the operation of the displacement control means and arranged to change the gear ratio of the mechanical gearbox by a predetermined step when the displacement of the said component reaches a predetermined threshold value, and synchronising means arranged to alter the displacement of the said component from the threshold value in immediate response to each operation of the gear ratio selector means to change the gear ratio, the sense and amount of the said alteration in displacement being such as to compensate for the effect of that change in gear ratio, on the overall velocity ratio of the mechanical-hydrostatic transmission.

2. A mechanical-hydrostatic transmission as claimed in Claim 1, in which the mechani cal gearbox is coupled to the hydrostatic transmission, on the output side of the latter.

3. A mechanical-hydrostatic transmission as claimed in Claim 1 or Claim 2, in which the hydraulic motor of the hydrostatic transmission is of infinitely-variable-displace ment type and is controlled by the said dis placement control means.

4. A mechanical-hydrostatic transmission as claimed in Claim 3, in which the pump of the hydrostatic transmission is also of infin itely-variable-displacement type whose dis placement is also controlled by the said dis placement control means in sequence with that of the hydraulic motor.

5. A mechanical-hydrostatic transmission as claimed in Claim 3 or Claim 4, in which the synchronising means acts on the displace ment of the hydraulic motor in the said compensating manner.

6. A mechanical-hydrostatic transmission as claimed in any one of the preceding claims, in which the said upper and lower limits of displacement of the said component of the hydrostatic transmission are chosen to correspond to the limits of the compensating alteration in displacement effected by the synchronising means on each change in gear ratio.

7. A synchronised mechanical-hydrostatic transmission with infinitely-variable velocity ratio, comprising a hydrostatic transmission, including a positive-displacement hydraulic motor of variable displacement type which is mechanically connected to a following mechanical gearbox having at least two predetermined selectable gear ratios, the hydraulic motor being provided with displacement control means for varying its displacement between maximum and minimum values and the mechanical gearbox having a gear ratio selector valve means operated by a control pressure in response to the operation of the displacement control means and arranged to change the gear ratio of the gearbox by a predetermined step, respectively downwards (for higher output speed) when the displacement of the hydraulic motor reaches its maximum value, and upwards (for lower output speed) when the displacement of the hydraulic motor reaches its minimum value, and synchronising means arranged to alter the displacement of the hydraulic motor from its respective maximum or minimum value, in the opposite sense to that of its preceding approach to that maximum or minimum value, in immediate response to each operation of the gear ratio selector means to change the gear ratio, the amount of the said alteration in displacement being such as to change the rotational speed of the output shaft of the hydraulic motor in correspondence with the changes in gear ratio of the mechanical gearbox.

8. A mechanical-hydrostatic transmission as claimed in Claim 7, in which the maximum and minimum values of the displacement of the hydraulic motor are chosen in proportion to the gear ratio alterations step of the mechanical gearbox, whereby each said alteration in displacement effected by the synchronising means ends at a minimum or maximum value of the displacement.

9. A mechanical-hydrostatic transmission as claimed in Claim 7 or Claim 8, in which the hydrostatic transmission also includes a positive-displacement hydraulic pump of infinitely-variable-displacement type which is hydraulically-connected to the hydraulic motor.

10. A mechanical-hydrostatic transmission as claimed in any one of Claims 1 to 6 and 9, which includes a speed control valve arranged to regulate controllably a control pressure operatively applied to the displacement-adjusting means of the pump and of the motor and to the gear ratio selector means of the mechanical gearbox.

11. A mechanical-hydrostatic transmission as claimed in Claim 10, in which the speed control valve is operated mechanically as by a foot pedal, or electrohydraulically with the aid of a controlling potentiometer.

1 2. A mechanical-hydrostatic transmission as claimed in Claim 10 or Claim 11, in which the progressive actuation of the speed-control valve in the sense to progressively increase the control pressure and controlled speed operates first on the displacement-adjustment means of the hydraulic pump so as to increase its displacement to a maximum value, and then operates on the displacement-adjusting means of the hydraulic motor to reduce its displacement.

1 3. A mechanical-hydrostatic transmission as claimed in Claim 7 or in Claim 8 or in Claim 9, or in any one of Claims 10 to 1 2 when dependent on Claim 7, in which the gear ratio selector valve means is arranged to select a high ratio (low speed gear) of the mechanical gearbox when the control pressure is below a predetermined threshold value, and a low ratio (high speed gear) when the control pressure exceeds that threshold value.

1 4. A mechanical-hydrostatic transmission as claimed in Claims 1 2 and 13, in which the threshold value is that corresponding to the minimum value of displacement of the hydraulic motor.

1 5. A mechanical-hydrostatic transmission as claimed in any one of Claims 1 to 6 and 9, or in any of Claims 10 to 14 when dependent on Claim 9, for use in a vehicle, and in which the hydraulic pump is arranged to supply fluid pressure flow to the motor selectively in opposite directions along interconnecting hydraulic conduits, respectively for forwarnd and reverse drive to the vehicle.

16. A mechanical-hydrostatic transmission as claimed in any one of Claims 10 to 12, or in any one of Claims 1 3 to 1 5 in combination with Claim 10, including a discrete hydraulic pump for servo pressure arranged to provide the said control pressure.

1 7. A mechanical-hydrostatic transmission as claimed in any one of the preceding claims, in which the mechanical gearbox has more than two selectable gear ratios, with equal ratio steps between them, and in which the synchronising means is arranged to alter the displacement of the said component or motor in an amount corresponding to the respective change in gear ratio at each change of gear ratio.

1 8. A mechanical-hydrostatic transmission, substantially as specifically described herein with reference to Fig. 1 of the accompanying drawings, with or without the modification of Fig. 2.