GB2146101A - Variable speed/torque hydraulic transmission system - Google Patents

Abstract

An automotive hydrostatic transmission comprises for each driven wheel a set of fixed displacement hydraulic motors (H1-H4) coupled to a common hub gear (J) and receiving fluid from a fixed displacement pump (B) via a valve (F) operable to selectively isolate one or more of the motors. Cavitation in the isolated motor(s) is avoided by directing fluid thereto from the outlet of an active motor. The valve (F) may be operated manually or automatically, and hydrostatic braking is provided by a valve (L) in the motor return lines. <IMAGE>

Description

SPECIFICATION Variable speed/torque hydraulic transmission systems In conventional hydrostatic transmission systems, since wheel motors have a fixed oil displacement per revolution the oil flow into them is increased in porportion to the speed at which they rotate. Large bore diameters are necessary to create the required torque under load, therefore high "gear" conditions de mand a high oil flow from the pump. On the other hand, the system must be designed for very high pressures which are required only for low "gear" high torque conditions, mak ing both pump and motors mechanically uneconomical, and the whole system must be designed for both extreme conditions.Further, the pump must be variable in delivery per revolution, resulting in the entire system being very costly to manufacture and the range of speed that is practicable is severely restricted, in particular when a road vehicle is considered.

The object of the present invention is to provide a system which can effect a wide range of speed/torque variation to a final drive or shaft from a supply of fluid under moderate pressure. This supply of fluid will be divided into one or more branches or arteries and arranged such that each branch will deliver a proportion of the fluid under pressure in such a way as to cause rotation of the final drive member or shaft. The number of such branches or arteries that distribute the supply of fluid under pressure may be varied to suit the speed/torque mode of the final drive or shaft, since the greater the number of branches supplying oil under pressure to the mechanism, the greater will be the torque and inversely the speed transmitted to the final drive member or shaft.

This system may be used to drive any member, but is particularly suited to drive a road vehicle.

Such a system when used to drive a vehicle provides a vast reduction in mechanical components over conventional vehicles, such as clutch, gearbox, propeller shaft, reduction and differential drives, axle unit, drive shafts and braking system. Moreover, the system according to the present invention allows the use of low or medium fluid pressure and flow as opposed to the conventional hydrostatic transmission drive.

The accompanying drawings show schematically in Figure 1 the prior art hydrostatic system, and in Figure 2A shows a specific embodyment of the system according to the present invention as adapted for a light road vehicle.

In Fig. 2A prime mover 'A' drives pump 'B' and in a 'neutral' condition it's output will be recirculated to the fluid reservoir 'E' by the valve 'C'. The fluid reservoir is vented to atmosphere. When drive is required valve 'C' is operated to close pipe 'D' to the reservoir 'E' and direct the fluid to enter the control valve 'F'. The control valve lever 'G' is positioned manually to open all ports 1, 2, 3, and 4, thus allowing the fluid under pressure from pump 'B' to enter and drive all wheel motors H1, H2, H3 and H4. Fig. 2A shows only one drive wheel. It will be understood that the pipe from each port will be branched so as to feed also a corresponding motor on the other drive wheel (not shown for clarity). Pinions are fixed to the hydraulic motor shafts, which pinions are meshed into the hub gear 'J' of the drive shaft to transmit drive to the wheels of the vehicle.

As the vehicle is accelerated by increasing the speed of prime mover 'A' and thus the speed of pump 'B', the momentum gained by the vehicle will result in a drop in pressure to the system as the torque requirement is reduced. At this point, one hydraulic motor (per wheel) is isolated from flow from the pump by changing the position of control valve lever 'G' to close port 4; thus only ports 1, 2, and 3 are supplying the entire pump output to three instead of four hydraulic motors (per wheel). This condition will result in a further increase in speed and pressure to the system.

As the vehicle is further accelerated a drop in pressure will again occur and fluid to a second hydraulic motor will be isolated from the pump output as previously described. A third hydraulic motor is similarly isolated as the speed is increased further. Thus the entire fluid output from pump 'B' is supplied to one motor only (per wheel) to maintain a high cruising speed. Thus it will be seen that as compared with known hydrostatic systems (see Fig. 1), the entire pump capacity with regard to both flow and pressure is utilised regardless of load conditions, i.e. initial torque demands. gradients and cruising speed.This feature is of great importance since it allows a fixed delivery pump to be utilised which will operate at moderate pressures and displacement, and also allows other components pipes, valves, connections and fittings to be made to a less demanding specification, particularly when it is envisaged to use the system initially for a small light car.

It will be noted that when a hydraulic motor is isolated from the pump as described above, it will be driven as a pump by the hub gear 'J'. In this condition it will receive fluid via the control valve 'F' from the output of a hydraulic motor that is not isolated. This feature is of great importance since it will prevent any risk of cavitation to the input side of any hydraulic motor when isolated. Fig. 2B shows a method of achieving this conditon by utilising two separate control valves F1 and F2. This arrangement shows (for clarity) the system for three speed operation; hydraulic motors 1 and 3 are shown as under pressure from the pump at all times and hydraulic motors 2 and 4 isolated in sequence to effect speed/torque changes to the final drive or shaft.In this arrangement, hydraulic motors 1, 3 and 4 are shown to be under pressure flow from the pump to drive the system, and hydraulic motor No. 2 is shown to be isolated by movement of control valve F1; a similar movement to control valve lever F2 will isolate motor No. 4 in the same way. Compensating port 'P' will balance any discrepancy between the output flow from hydraulic motor No. 1 and the input drawn in by hydraulic motor No. 2; thus isolated hydraulic motors wll rotate as free running pumps without load, being driven by the hub gear. Other means of isolation may be utilised, but essentially only those motors not isolated will receive the full fluid flow from the pump 'B'.

It will be obvious that the number of wheel motors in operation at any one time will depend upon load conditions - for starting from rest, acceleration and gradients as a gearbox operates in a conventional road vehicle.

It will be understood that to obtain ideal speed ratios the size (number of teeth) of each individual pinion may vary and need not necessarily be all of the same size as shown in Fig. 2A. The hub of gear ratio 'R' and the number of hydraulic motors (not necessarily four as shown in Fig. 2A) may be varied to suit an operating pressure below that normally associated with prior art systems, thus obviating the need for motors of a high demand of manufacture and specification. Variations to a common principle used to generate pressure as a pump, shown at 'S' Fig. 3, may be adapted for the motors-fluid under pressure introduced at it's input will cause the shaft to rotate. In other modes as described previously, they will be driven as pumps by the hub gear.Alternatively, the branched fluid under pressure at each place of the mechanism may act upon a movable member or vane connected to the final drive shaft in such a way as to cause rotation of the shaft as shown at 'T' Fig. 3; the operating principle will be unchanged from the description as shown in Fig. 2A.

To effect braking the following actions will occur: 1). Valve 'C' will be operated by the brake pedal to recirculate the pump output to the reservoir 'E', as in the no-drive neutral conditon.

2). All hydraulic motors will be driven as pumps by the hub gear, drawing their fluid from the resevoir.

3). The outlets from all hydraulic motors will be returned to the reservoir 'E' via brake valve 'L', which valve is also operated by the brake pedal. This brake valve 'L' will restrict or completely cut off this return flow, thus creating "hydraulic lock" to each hydraulic motor, and multiplied by the pinion/hub gear ratio will effectively brake the vehicle and control the descent when negotiating steep hills.

When reverse is required, the fluid flow is reversed to the motors, either by control valve 'F' or by a separate valve arrangement (not shown) to supply the fluid to the hydraulic motors in a reverse direction.

It will be understood that the changes in fluid pressure which occur when hydraulic motrs need to be brought into or out of operation may be used to control the lever 'G' of the control valve to provide automatic operation, or the pressure changes may be used to automatically control the system by electrical devices.

The maximum torque at the driven member or shaft will be generated when all the hydraulic motors are under pressure. and in this condition the speed/torque changes may be brought about by utilising the fluid under pressure from a variable source.

It will be noted that differential drive between the two driving shafts will be apparent.

Commercial Aspect of the Invention It is felt that a gap exists for a small light commuter car, perhaps for one person only, but having a performance and degree of presentation comparable to larger vehicles in general use.

To manufacture such a car on conventional lines would result in insufficient price advantage over the more general vehicle in current use since essentially the same transmission and braking components would be common to both types of vehicle.

It is claimed that the present invention can be adapted to fill this gap by offering a significant price advantage over other systems.

Claims (12)

1. A mechanism that can provide a range of speed, and inversely torque variations to a final drive from a specific flow rate of fluid under pressure or pump to the mechanism, as opposed to other forms of hydrostatic transmission.

The flow of fluid under pressure to the mechanism will be divided into branches or arteries, and each of these branches or arteries may be connected by valve arrangement to deliver a proportion of the fluid under pressure to specific places of the mechanism that will individually transmit torque to the final drive. Such torque generated at each individual place of the mechanism will be additive. By preventing the flow of fluid under pressure toone or more of such individual places by blanking it's (or their) branch or artery by valve control, a greater proportion of the fluid under pressure will flow into those that are not blanked, and which are transmitting torque to the final drive; thus a corresponding increase in speed (and inversely the torque) of he final drive will result.

When all of the fluid under pressure is entering one only place of the mechanism, (the others being blanked as described), maximum speed to the final drive will occur, and conversely, when all branches or arteries are open to deliver the fluid under pressure to all places of the mechanism, torque at the final drive will be at maximum. Thus the speed of rotation of the final drive of the mechanism may be varied to suit torque demands without necessarily altering the flow rate of the fluid under pressure to the mechanism. It will be understood that in each mode of operation speed to the final drive may be varied by changing the flow rate of fluid to the mechanism.

2. A mechanism as in Claim 1 in which the individual drive places are spaced around the final drive or shaft.

3. A mechanism as in Claim 1 in which the individual drive places are co-axial.

4. A mechanism as in Claims 2 and 3 in which the individual drive places are hydraulic motors.

5. A mechanism as in Claim 2 in which the individual drive places apply rotation directly to a rotating member of the mechanism of fluid pressure reacting upon vanes or movable members.

6. A mechanism as in Claims 1, 2, 3, 4 and 5 in which the fluid under pressure is supplied by a fixed delivery pump (per revolution).

7. A mechanism as in Claims 1, 2, 3, 4 and 5 in which fluid under pressure is supplied by a variable displacement pump (per revolution).

8. A mechanism as in Claim 4 in which the hydraulic motors will be driven as pumps free of pressure by the final drive when not providing torque to the mechanism.

9. A mechanism as in Claim 8 in which the motor input port is directly coupled to it's output flow when being driven as a pump by the final drive.

1 0. A mechanism as in Claim 4 in which the hydraulic motors can drive in one direction only, and their drive member connecting them to he final drive will free wheel when they are not delivering torque to the final drive.

11. A mechanism as in Claims 1, 2, 3, 4, 5, 6, 7, 8 and 9 in which the fluid flow is reversed to cause the final drive to counter rotate.

12. A mechanism as in Claims 1, 2, 3, 4, 5, 6, 7, 8 and 9 wherein the outward flow of fluid from all drive places is restricted or blanked off to control overrun or effectively brake the mechanism by creating hydraulic lock.

1 3. A variable speed/torque hydraulic mechanism as described herein with reference to Figures 2A, 2B, 3S and 3T of the description.