This invention relates to a multi-range transmission, hereinafter referred to as being of the kind specified, having a first output element connectable to a first clutch in a torque transmitting path of a first range and a second output element connectable by a second clutch in a torque transmitting path of a second range and in which the speed of the first and second elements vary inversely so as to rotate synchronously at a range change condition.

At the range change condition, it is necessary to disengage the first clutch and to engage the second clutch and it is desirable to avoid interrupting the torque transmitted by the transmission and to avoid delay in range change.

An object of the invention is to provide a multi-range transmission of the kind specified in which the abovementioned problems are overcome or reduced.

According to one aspect of the invention, we provide a multi-range transmission of the kind specified comprising synchronicity sensing means to sense when said elements are rotating synchronously, first clutch operating means to engage the second clutch when the first and second elements are synchronous, torque adjustment means to adjust the torque transmitted by the second clutch to a predetermined value relative to the torque transmitted by the first clutch at synchronicity or at a predetermined speed difference prior to synchronicity and second clutch operating means to disengage the first clutch when the torque transmitted by the second clutch equals said predetermined value.

By "synchronous" we mean at the same speed or within a predetermined speed differential.

The transmission may comprise a power split transmission comprising a summing gear set having first and second input elements and said first and second output elements, an input member connected to said input elements

through a speed varying means to vary the speed of rotation of the first and second input elements to cause said inverse variation of the speed of rotation of said output elements.

The input member may be mechanically connected to said first input element and may be connected to the second input element through said speed varying means.

The speed varying means may comprise a hydrostatic transmission. The hydrostatic transmission may comprise a hydraulic pump device driven mechanically by said input member and a hydraulic motor drive hydraulically by said pump and having an output element drivingly connected to said second input element.

At least one of the hydraulic motor and pump may be of variable displacement. Preferably the pump is of variable displacement whilst the motor is of constant displacement.

The motor and the pump may be connected in a hydraulic circuit having a first branch extending between a first port of the pump and a first port of the motor and a second branch extending between a second port of the motor and a second port of the pump.

The volume and the direction of flow depend upon the setting of the variable displacement pump. For example, where the pump is a swash type plate pump, a "positive" swash plate angle may cause flow along the first branch A from the pump to the motor and return flow from the motor to the pump along the second branch B whereas negative swash plate angle may cause flow of fluid from the pump to the motor along the second branch and from the motor to the pump along the first branch.

The pressure obtaining in the first branch, hereinafter referred to as P _a may vary compared with the pressure P _b obtaining in the second branch, hereinafter referred to as P _b and the pressure difference is dependent upon the magnitude of the torque output of the motor.

For example, with positive swash plate angle and flow in branch A from pump to motor then P _a is greater than P _b whereas with negative swash plate angle and flow from the pump to the motor via branch B, P _b is greater than P _a .

Said torque adjustment means may therefore comprise means to sense a pressure difference between the pressure obtaining in the two branches of the hydraulic circuit at synchronicity or at a predetermined speed difference prior to synchronicity, means to determine a target pressure difference for drive transmission by said second clutch, and means to adjust the displacement of the pump to achieve said target pressure difference.

Means may be provided to maintain the value of the pressure difference at that sensed or at a predetermined relationship to that sensed whilst determining the target pressure difference and whilst engaging the second clutch.

Means may be provided to ascertain that the second clutch is engaged prior to performing the step of adjusting the displacement of the pump to achieve said target pressure difference.

Means may be provided to determine that said target pressure difference has been achieved prior to permitting release of the first clutch.

Means may be provided to determine that the first clutch has been released before permitting operation in the new range.

The control means may comprise a microprocessor.

An example of the invention will now be described with reference to the accompanying drawings, wherein:-

FIGURE 1 is a diagrammatic illustration of a transmission^ embodying the invention;

FIGURE 2 is another illustration of a transmission of Figure 1 but showing the gear ratios;

FIGURE 3 is a diagrammatic illustration showing the pressure difference between branches of the hydraulic circuit shown in Figures 1 and 2 with output torque of the transmission;

FIGURE 4 is a flow diagram of a programme executed by a microprocessor during use of the transmission embodying the invention.

Referring to Figures 1 and 2, there is shown one example of a transmission embodying the invention.

The transmission of Figures 1 and 2 incorporates a four branch differential gear 10 which comprises a planet carrier 30 which carries three first planet gears 31 and three second planet gears 32. The first planet gears are of constant diameter and mesh with a first sun gear 36 and with a portion of the second planet gears 32. A longitudinally adjacent portion of the second planet gears 32 meshes with a second sun gear 39. The second planet gears 32 are also in mesh with an annulus 40.

The carrier 30 acts as a first shaft of the controllable four branch differential gear 10, whilst the annulus 40 acts as a second shaft, the first sun gear 36 acts as a third shaft, and the second sun gear 39 acts as a fourth shaft.

The annulus 40 is connected to a hollow shaft 60 whilst the second sun gear 39 is carried by a further hollow shaft 61. The first sun gear 36 is connected to a shaft 37 whilst the carrier 30 is connected to a hollow shaft 29. The carrier 30 is provided with a peripheral gear 55.

The inner hollow shaft 61, is provided with a gear 62 which meshes with a second gear 63 of a range change gear set 64. The gear 63 is connectable by a fourth, or first output/range change, clutch 65 to a lay shaft 66 to which is fixed a first lay shaft gear 67 which meshes with a gear 68 fixed to the hollow shaft 69 which is mounted to rotate co-axially with the shaft 61. The shaft 69 is connected by gear 82 and gears 80 and 81 to forward and reverse output members 12F and 12R.

The outer tubular shaft 60 connected to the annulus 40 is connectable by a fifth or second output clutch 71 to the tubular shaft 69 whilst the inner shaft 61 is connectable by a sixth, or third output, clutch 70 to a tubular shaft 69.

The engine 11 is connected by an input member 13 to the transmission. The input member 13 is connected to an input gear 72 with meshes with a first input gear 73 and with a second input gear 74.

The second gear 74 meshes with a gear 76 which drives an input element 77 of a variable speed hydrostatic transmission 20. The hydrostatic transmission 20 may be of any suitable variable displacement type and comprises, in this example, a variable displacement swash plate pump and a swash plate motor. However, either or both of which may be of adjustable displacement so as to vary the speed and direction of rotation of the output element 22 of the motor relative to that of the input element 18 of the pump. If desired, instead of a hydrostatic transmission any other suitable (continuously) variable speed transmission may be used.

The shaft 37 connected to the first sun gear 36 is connected to a first gear 78 which meshes with a second gear 79 which is driven by a third gear 78 connected to the output element 22 of the transmission 20. The second gear 79 is connected by a first, or hydrostatic, clutch H to an intermediate gear 75 which meshes with the gear 55 of the carrier 30. The carrier 30 is connectable by a second, or forward, clutch F to a hollow shaft connected to the first input idler 73 whilst the second input idler gear 74 is connectable by a third or reverse clutch R to an intermediate gear 75.

In use, assuming that the vehicle is initially stationary, so that the forward and reverse output members 12R, F are stationary, the first clutch H is engaged and, the second and third clutches F and R are disengaged, whilst the first output clutch provided by the range change clutch 65 is engaged, whilst the second and third output clutches 71, 70 are disengaged.

The engine 11 drives the input element 77 of the hydrostatic transmission 20 via gears 72, 74 and 76. No drive is communicated to the four branch differential directly from the engine since the clutches F and R are disengaged. The output element 22 of the hydrostatic transmission is connected by gears 78, 79 and 78 to the first sun gear 36 and by the gears 78, 79, clutch H

and gears 75 and 55 to the carrier 30 and thus the carrier and the sun gear are rotated in the same direction at the same speed. Initially, since the swash plate angle is zero, no movement of the output element 22 takes place and the vehicle remains stationary. If it is desired to drive forwardly the swash plate angle is increased in a forward drive direction to cause the output element 22 to rotate in the forward direction, thus rotating the first sun gear 36 and carrier 55 in the same direction and at the same speed so that the differential 10 rotates as a "locked" unit and hence the two output shafts 60, 67 also rotate at the same speed as each other and at the same speed as the two input shafts 37, 29.

Drive is transmitted from the inner shaft 61 via gears 62 and 63 and first output clutch 65 to lay shaft 66 and via gears 67 and 68 to shaft 69 and thence via gears 82 and 80, 81 to output members 12F and 12R.

At maximum swash plate angle the gear set 10 gears are arranged to be driven at the speed of rotation of the input member 13, but in the reverse direction thereto, so that the second clutch F may be engaged and then the first clutch H allowed to become disengaged in a similar manner to previously described embodiments.

Once the second clutch F has been engaged the swash plate angle may be reduced and hence the speed of rotation of the sun gear 36 decreased whilst the speed of rotation of the carrier 30 is maintained at the same "engine" speed and as a result the speed of rotation of the sun gear 39 increases and this increase in speed of rotation is transmitted to the output members 12F and 12R through the first output/lay shaft clutch 65 as in purely hydrostatic drive.

Continuing faster rotation of the output members 12F and R continues as the swash plate angle is reduced to zero and then increased in the reverse direction to cause the first sun gear 36 first to become stationary, when the swash plate angle is zero, and then to rotate at increasing speed in the reverse direction.

At maximum swash plate angle in the reverse direction the annulus 40 is rotating at its slowest speed whilst the second sun gear 39 is rotating at its fastest speed. The ratio of these two speeds is Rl.

The lay shaft gear ratio also substantially equals Rl and thus at this point the speed of rotation of the gear 68 and hence of the shaft 69 is substantially the same as the speed of rotation of the annulus 40 and thus the second output clutch 71 may be engaged and then the first output/range change clutch 65 can become disengaged.

As the swash plate angle is reduced to zero the sun gear speed in the reverse direction decreases, then becomes stationary and then increases again in the forward direction as the swash plate angle is increased in the positive direction, thus speeding the rotation of the annulus 40 and hence of the shaft 69 and hence of the output members 12F and R via gears 82, 80 and 81.

When the swash plate angle is at its maximum in a forward direction the annulus 40 and second sun gear 39 are rotating at the same speed and hence the third output clutch 70 can be engaged and then the second output clutch 71 becomes disengaged. As the swash plate angle is reduced first to zero and then to maximum in the reverse direction, increase in speed of rotation of the second sun gear 39 occurs. In this case drive is transmitted via the clutch 70 to shaft 69 and then via gears 82, 80 and 81 to output members 12F and R.

When it is desired to operate in the reverse direction then, with first clutch H engaged and second and third clutches F and R disengaged movement of the swash plate in the reverse direction causes reverse direction of rotation of the gear set 10 and hence reverse drive is transmitted to the output members 81F and R. When the swash plate angle reaches its maximum in the reverse direction then, as in the case of the first embodiment, the carrier 30 is rotating at the same speed as the input member 13 but in the same direction thereas and hence the gear 74 is also rotating at the same speed and direction as the gear 75 so that the reverse clutch 8R can be engaged and then the hydrostatic drive clutch 8H can be disengaged. Thereafter the transmission operates in an exactly similar manner, but in the reverse direction to that described hereinbefore for forward drive.

The hereinbefore described transmission therefore provides forward or reverse drive up to a hydrostatic range and three compound ranges thus

providing maximum torque at zero vehicle speed as the vehicle starts to move in either forward or reverse.

Of course, if desired, the highest range in reverse may be omitted when it is not required to drive a vehicle in reverse in the speed range which would be provided by the highest speed range. This is provided simply by inhibiting engagement of third clutch 70 and preventing disengagement of clutch 71 when clutch 8R is engaged, in the control means of the transmission.

Figure 2 shows the number of teeth provided on the various gears of the embodiment shown in Figure 1. In this example at maximum forward swash plate angle the slowest speed of annulus 40 is 0.448A rpm where Rpm is the speed of rotation of the input member 13, whilst the speed of rotation of the second sun gear 39 is then at its fastest, namely 2.234A. The ratio of the two speeds is 4.99.

The lay shaft ratio is 47 _. x £7 = 5.01

21 21

Because 5.01 is not equal to 4.99 synchronicity does not occur at precisely minus 100% swash angle but it is sufficiently close for practical purposes.

Set out below are the ratio ranges provided in the various ranges described hereinbefore. In the example illustrated there is no fourth reverse range and the lowest forward and reverse ranges are referred to as range 2 because in the embodiment to be described hereinafter an intermediate range is provided which is lower than range 2 and since in that embodiment ranges 2, 3 and 4 are the same as in the present embodiment, for consistency the above mentioned range number nomenclature has been adopted. Reverse range 3 ratio:- -3.31:1 → -1.48:1

Reverse range 2 -7.42:1 → -3.31: 1

Hydrostatic -7.42:1 → +7.42:1

Forward range 2 +7.42:1 -> +3.31:1

Forward range 3 +3.31:1 → + 1.48:1

Forward range 4 + 1.48:1 → + 0.66:1

In order to achieve a smooth transition between the various ranges without interruption in torque transmission, a pressure transducer P, is provided

in a first branch A of a hydraulic circuit between the pump 19 and the motor 21 and a second transducer P ₂ in the branch B of the circuit. The transducers Pj and P ₂ provide a signal dependent upon pressure in the respective branch, on lines L,, L- ₂ to a central processing unit CPU. In addition, the engine 11 and the output element 22 of the motor 21 are provided with a respective speed sensor S _} , S ₂ which provides signals, indicative of the speed of rotation of their associated element, on lines SL, and SL ₂ respectively to the CPU. Because the components on opposite sides of the clutches which provide range change are rotated in a fixed and known relationship to the engine and the motor their speed of rotation is calculated by the C.P.U.

The CPU is connected by line P to a swash plate control mechanism 90. The CPU is also connected by lines C, - C ₆ to solenoid operated valves of hydraulic means M for engaging the clutches. In the present embodiment each clutch is engaged by a hydraulic ram unit which, when energised, moves the clutch into engagement against a spring bias and, when de-energised permits the clutch to be disengaged by the spring bias.

When the speed of rotation of the respective pair of members approaches synchronicity as hereindefined, i.e. synchronicity or a predetermined speed difference e.g. not more than ± 10%, a range change condition exists. The following sequence of operations takes place to ensure smooth transfer of torque transmittal from an existing range to a new range.

Initially, either when precise synchronicity is detected by the pair of speed sensors SL^ - SL--> or when the speed differential between the respective pair of members is less than the predetermined value, the CPU determines the pressure difference P _a - P _b as detected by the sensors P, or P ₂ in the branches A and B of the circuit.

The CPU then sends a signal to the swash plate control 90 so as to maintain this pressure difference for the time being. In addition, the CPU determines, for example either by calculation or by using a look-up table, a target pressure to be attained during initial operation in the new range.

Then a signal sent on the appropriate line C, - C ₆ for the clutch of the new range for it to be engaged.

Suitable sensors (not shown) are provided so as to determine whether or not the new clutch is properly engaged and a suitable confirmation signal is sent to the CPU.

The CPU then sends a signal to the swash plate control 90 to cause the swash plate angle to be adjusted to bring the pressure difference to the target pressure or with a predetermined value thereof.

The CPU then checks whether or not the target pressure range has been achieved, and, if it has, it sends a signal to the appropriate line C, - C ₆ to the clutch of the old range to cause it to be released. Since at this stage the torque is adjusted so as to be equal to the target pressure range for the new range, the old clutch will be unloaded or substantially unloaded and therefore can move out of engagement under spring pressure.

The CPU then checks that the clutch has been disengaged and, assuming it has, normal operation of the transmission is permitted.

The target pressure, in the present example, is determined empirically on the basis of tests but it may be calculated on the basis of the pressure difference and gear ratio and the ratio of the variable speed transmission.

Figure 3 illustrates the variation in pressure difference whilst Figure 4 shows a flow diagram of the programme executed by the CPU which, preferably, is a microprocessor controlled device of known type.

If desired, the CPU may cause the swash ramp to be slowed as synchronicity of the respective pair of elements is approached.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.