GB2241756A - Gear transmission system using synchronisation - Google Patents

Abstract

Control means receives signals representing gear box input and output speeds from sensors in the gearbox which indicate when two gears to be engaged are running at substantially the same speed, and initiates engagement in response to a gear selection signal. The control means preferably acts on a constant mesh gearbox in which ratio change is effected by means of a hydraulically actuated selector and clutch. The gearbox preferably has discrete gear sets such as a three ratio set and a high/low range set. The system may act to reduce engine fuel supply during shift to achieve synchronism. <IMAGE>

Description

IMPROVBMENTS IN GEAR TRANSMISSIONS This invention relates to gear transmissions. The invention is particularly applicable to constant mesh gear transmissions.

It is currently standard practice in utility vehicles, such as agricultural tractors which have to be capable of pulling heavy loads, for example in ploughing, to provide a syncro-mesh transmission gearbox having a series of hydraulically actuated clutches and/or torque convertors for changing between ranges of gear ratios and selecting between forward and reverse drive, as well as changing between gear ratios.

Syncro-mesh gearboxes are inherently complex. This is even more the case for the sophisticated gearboxes used in tractors and the like. Their main asset is that actually changing gear is very simple for the user.

However, they have various drawbacks besides being complex. One of these other drawbacks is that it is often necessary to change gear in a tractor while it is pulling a heavy load without the opportunity to release the load temporarily. Again, ploughing is an example of such an activity. This can be contrasted with a road vehicle in which it is usually possible to effect a gear change in a relatively more leisurely fashion once the main clutch is disengaged while the momentum of the vehicle all but maintains its road speed. With a tractor, it is important to be able to change gears and, in the process, lose as little ground speed as possible. Implicit in this is the need to effect the gear change as quickly as possible in order to avoid the loss of ground speed.

Before the development of syncro-mesh gearboxes so-called 'constant mesh' transmission gearboxes were used.

Usually, the driver has to "double de-clutch" to change gear in a conventional constant mesh gearbox. This has the effect of bringing the drive gear wheel and the driven gear wheel to substantially the same speed of rotation in order to effect a smooth change.

Furthermore, gear boxes having high and low ratio sets of gears cannot be shifted between the two sets whilst the vehicle is underway. The need for double de-clutching and halting the vehicle before changing between ranges have restricted the use of constant mesh gear boxes in agricultural tractors and the like.

Thus, since syncro-mesh gearboxes became available they have been preferred to constant mesh gearboxes.

According to the invention there is provided a transmission gear system comprising an input shaft, an output shaft, at least one set of gears including a drive gear and a driven gear, actuation means for engaging the gears to transmit rotation of the input shaft to the output shaft, control means responsive to a gear selection signal to enable the actuation means, speed sensing means arranged to provide signals related to the speed of rotation of each of the drive and driven gears, the control means being operable to enable the actuator means to engage the set of gears when their speeds of rotation are substantially the same.

Clearly, there will be a tolerance on the speeds at which the two gears to be meshed are rotating in which a smooth gear change can be effected. Thus, the word "substantially" should be read as providing a range of differential speeds of rotation during which engagement is allowed.

Preferably, the speed sensing means are magnetisable sensors or Hall effect devices which are energisable by the movement of the teeth of an associated gear wheel to emit the signal related to the speed of rotation of the associated gear. Many other types of motion sensors can also be employed as will apparent to the man skilled in the art.

If the speed of rotation of the two gears to be engaged is outside a predefined tolerance the control means may be operable to adjust the speed of an engine driving the input drive shaft in order to permit the gear change operation. The control means may be arranged to control the engine by overriding the normal speed demand signals given by a driver.

The invention is particularly, though not exclusively, applicable to land vehicles, such as tractors. The use of the control means to determine whether a gear change is possible and to perform the change also makes it desirable if those control means are able to determine also whether the speed of the vehicle is appropriate for a requested gear change. In this regard, the control means are arranged to receive a signal from further sensing means which signal is related to the speed of the vehicle. For example, the further sensing means may be arranged to detect the speed of rotation of the output shaft of the gear box which will be proportional to the overall speed of rotation of the road wheels.

The invention also extends to a constant mesh reduction gear arrangement for a land vehicle including the above system and also to a land vehicle having such a system.

As previously mentioned, gear boxes for land vehicles commonly have a forward/reverse selection arrangement which may include a pair of clutches or torque convertors each respectively engageable to configure the output shaft of a gear box to be driven in one sense in response to rotation of the input shaft in the other sense. It is therefore advantageous if the control means are operable to control actuation of the forward/reverse selection means, such as the clutches, in response to a forward/reverse request. In this regard, the speed of the vehicle is desirably sensed to determine whether or not it is excessive for executing a change of direction.

Gear boxes are often arranged to have a first part containing a selection of gears and a second part having a high and low ratio output. Thus, the range of gears provided by the set of gears is effectively doubled by the provision of a high/low ratio arrangement. It will be appreciated that this invention is also applicable to changing between high and low ratio gears as it is between gears in a set in substantially the same manner.

Various features of the invention are set forth in the accompanying claims.

The invention can be put into practice in various ways some of which will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a first embodiment of the invention.

Figure 2 is a side view of a gearbox according to a second embodiment of the invention; Figures 3, 4 and 5 are end views of details of the gearbox of Figure 2; and Figure 6 is a schematic block diagram of a controller for the gearbox of Figure 2.

A first embodiment of the present invention is illustrated in Figure 1. It is based on a tractor having a six speed gearbox 8. This gearbox can be considered as a three speed gearbox mated to another two speed high/low ratio gearbox to provide the six speeds in all. The gearbox is a constant mesh unit which engages the gears by moving dog-clutches on splined shafts. The gearbox is mounted in a tractor and is mated to a torque convertor to provide a disengageable drive transmission. The gearbox also has a hydraulic selection clutch arrangement which provides the forward and reverse options.

The engine speed control is governed by an electronic governor. The governor sets the engine to a particular engine speed limit. The input gear shaft is monitored to determine the speed of the engine since the torque convertor provides variable transmission from 2.8:1 to 1:1.

The gearbox is modified to include sensors for detecting the speed of the input shaft to the gearbox, the output shaft from the gearbox, the road speed and the position of the gear. A microprocessor control module 10 is programmed to control the gear change operation by sensing if the gears to be meshed are at substantially the same speed of rotation and enabling solenoid actuators to effect the change of gear. If the two gear wheels to be engaged are not running at the same speed, or within predetermined limits, the control module 10 is arranged to adjust the speed of the input shaft to the gearbox by adjusting the speed of the engine. Sensing, adjusting and shifting operations are all performed in a matter of fractions of a second and the smooth change is achieved without the time consuming effort of double de-clutching.

The measurement of the road speed is obtained from a speed sensor on an input gear of a differential unit mounted on the tractor. The electrical pulses produced by the sensor are then fed into the control module which measures the frequency of the pulses. The frequency of the pulses is obtained by measuring the time between subsequent rising edges of the waveform using a timer integrated circuit. To make sure that a slow pulse rate does not affect the operation of the system a second timer is constantly running whilst the system is waiting for a pulse. If the second timer expires the control module assumes that the shaft it is monitoring has stopped.

Using the speed of the shaft, the control module 10 can calculate road speed based on the ratio of the reduction gearbox 8 and an average tyre rolling circumference. The road speed is then used by the gear selection routine to determine if the tractor is travelling too fast or too slow to allow a requested gear selection.

The engine speed is deduced from a measurement of the speed of the input gear to the gearbox. The reason for this is that the engine speed and the input speed to the gearbox will not always be the same due to the variability of the drive provided by the torque convertor. The speed on the input gear is measured in substantially the same way as that for the road speed.

This calculation is used to determine the speeds of the first, second and third gears in the gearbox. This is used to gauge whether the speeds of selected gears are correctly synchronised and to permit or prevent a gear change from taking place.

Another gear speed measurement takes place on an input gear to the high/low ratio two speed part of the gearbox described above. Similarly this allows the speed of the high/low ratio gears to be measured.

Thus, changing between the high and low ratio ranges is effected in substantially the same manner as selecting one of the three gears in the main gearbox.

The gears to be meshed have to be running at substantially the same speed before a change can be effected. The control of the engine is based on the knowledge of the speed of rotation of the input shaft to the gearbox and the current speed of rotation demanded by the driver. The engine is accelerated or decelerated until the change over speed is reached.

The engine is then kept at the change over speed until the gear change is completed. The only processing required by the control module is that of the comparison of the real engine speed with the change over speed in order that the real engine speed can be altered accordingly. The output of the control module is transmitted to an engine control device. This may be an electronic governor as mentioned above or a fuel injection system 12 or part of a full engine management system.

The gear engagement function of the control module keeps a record of which gear is currently engaged and which is required so that it can operate the correct gear change solenoids to accomplish the change. The procedure also controls the torque convertor or clutch drive transmission to disengage it before the gearbox is placed in neutral. Once the gearbox is in neutral, the drive transmission is re-engaged and control for the gear selection handed over to the previously described synchronising procedure before engagement of the selected gear is performed.

A similar procedure can be used to effect change from the high to the low ratio gear sets.

A watch-dog arrangement is provided as part of the control module 10 which shuts the system down should a terminal fault be detected. The watch-dog is a re-triggerable monostable which is prevented from shutting the system down by having its address written to at regular intervals. It has a timer attached to it and if it is not written to by the time the timer runs out the processor must have ceased operation or the control module has developed a fault. In the event of a fault situation the watch-dog places the system in such a condition that it is not a danger to the operator or the unit. Once the watch-dog has been activated the drive is placed in neutral.

The gear synchronisation routine of the control module has inputs of the speeds of the shafts to be synchronised. The routine has to decide whether the speed of the drive shaft is too high or too low for the smooth engagement to take place. If the engine of the tractor should fail or the tractor should come to a halt for any other unscheduled reason the watch-dog timer defaults the system to leave the gearbox in neutral.

Physical faults in the gearbox with which the control module has to concern itself are excessive gearbox temperature and low hydraulic pressure. Sensors for this are mounted in strategic locations in the gearbox and the output of them is checked by the control module. If a fault occurs, an alarm is sounded and a warning light is illuminated.

The forward/reverse selection of the gearbox is also controlled by the control module using a hydraulic selector arrangement.

The control module requires three inputs to perform its forward/reverse selection function, namely the position of the control stick, the position of the hydraulic clutch and the speed of travel. When a control module is to execute a change of direction, i.e. the appropriate control stick is moved, it has to determine whether the tractor is travelling in the direction in which has been requested to move. In this case the request is ignored. If a request is made at a tractor speed of over 4.827km/hr, the request will also be ignored. If a valid request for a change of direction is received, the currently engaged selector clutch is disengaged and the programmed engagement of the other selector clutch is carried out. As before, smooth operation is assured by the programmed control of the hydraulic pressure regulating valve determining the position of the selector clutch plates.

The control module knows the position of the gearbox due to the use of several position sensors inside the gearbox. Preferably, the position sensor should be of a sealed non-contact type that will not wear out or be affected by oil or metal swarf inside the gearbox. The position sensors interact with the slidable selector rails. The control module senses which gear is currently selected by the output of these sensors. The sensors can also be utilised to make sure that when a gear change has been initiated that the selector rails assume the correct positions. If the sensor does not change to the correct value for the gear currently selected then the control module can assume that an error has occurred.

The use of position sensors allows the control electronics to detect if a mechanical fault in the gear box which prevents use of a particular gear or gears has occurred. The module can make allowances for this and not use the affected gears until the fault has been rectified.

The tractor driver selects the gear he wants by using a twist grip on top of the forward and reverse selection lever. The twist grip has seven positions and a button on the top. The button is used by the operator to launch a gear change request. The selected gear is dialled up on the twist grip. But only after the button is pressed and released will the control module check to see if the request which has been made is valid or not.

The information, such as the gear selected, whether high or low ratio is being used, any error signals etc., can be displayed on a display device. This can be a separate stand alone display system or may be integrated into an EIC (Electronic Instrument Cluster).

The control module is connected to the external display device by means of a serial communications port. The display is updated after the successful completion of the engagement procedure or detection of an error occurs.

A second embodiment of the invention is illustrated in Figures 2 to 6 of the drawings. The block diagram of Figure 1 and the features described above are also generally applicable to this embodiment.

Figure 2 illustrates a six-speed constant mesh transmission gearbox. The gearbox has an input shaft 50 coupled to the input of a forward and reverse double acting hydraulic clutch 52.

By actuating selection of a forward portion 52A of the clutch 52 the drive is transmitted through a drive shaft 54 to a gear wheel 56 mounted on its end remote from the clutch 52. The gear wheel 56 engages a further gear wheel 58 which is connected to part of a selector clamping clutch 60. The selector clutch 60 is a hydraulically actuated multiplate double acting selector clutch, having two clutch halves 60A and 60B selectable by hydraulic actuation to drive a main shaft 62 or, alternatively, a hollow countershaft 64 through which the main shaft 62 extends. The main shaft 62 is engageable to be driven by the first part 60A of the selector clutch 60 while the counter shaft 64 is engageable to be driven by the corresponding second part 60B of the selector clutch 60. Each clutch is normally disengaged.Hydraulic actuating fluid is provided through respective input ports 66 and 68 by which either clutch may be selected by a controlling microcontroller through pulse width modulated solenoid valves in a manner to be described later.

By selecting clutch part 52B of the clutch 52 reverse drive is transmitted to the gear wheel 56 by means of a further gear wheel 69 mounted on the drive shaft 54.

The reverse portion 52B engages a drive gear 70 commonly mounted with a driven gear 72 on a lay shaft 74. The driven gear 72 engages a reverse idler gear 76 (this is best illustrated in Figure 3). The reverse idler 76 drives the drive shaft 54 in the opposite sense through the gear wheel 69.

The gear 58 connected with the clutch part 60B, is also mounted on a splined portion of a clutch shaft 78 which is selectively engageable with the main shaft 62 through the clutch portion 60A.

A first group of drive gears 80, 82, 84, corresponding to first, third and fifth gears in the gearbox range, are mounted on the counter shaft 64 while a second group of drive gears 86, 88 and 90 corresponding to gears second, fourth and sixth, are mounted on the independently rotatable main shaft 62.

The drive gears 80 to 90 are each respectively meshed with a corresponding drivengear wheel 92 to 102 on an output shaft 104. Each pair of engaged gears 80/92, 82/94, 84/96, etc., represents a cooperating set of gears providing a transmission path from the selector clutch 60 to the output shaft 104.

Each gear wheel 92 to 102 is engageable to rotate with the output shaft 104 by means of a dog clutch 106 to 110. Each dog clutch 106 to 110 is of largely conventional design and is disposed between first and third; fifth and second; and fourth and sixth gears, respectively. A dog collar part 112 of each dog clutch is slidable along the output shaft 104 to engage a selected to rotate with the output shaft in known manner. As is also known, the collar 112 is shifted along the output shaft 104 by means of a selector fork 114. On dog clutches 106 and 110 in Figure 2, these forks are shown in part only for the sake of clarity.

One end of each fork 114 opposite from its tines 116 is mounted on an associated selector shaft 118, 119, 120, (this is best illustrated in Figures 4 and 5). It will be noted that each selector shaft is mounted at a different level, with reference to a view perpendicular to the plane of the drawing, but which are aligned according to the view of Figure 2.

Each fork 114 is held axially in position on its selector shaft by means of a pair of axially spaced retaining bushes 122. Between each bush 122 and the part of each fork embracing the selector shaft on which each is mounted is a buffer spring 124.

Each selector shaft is moved axially by a double-acting hydraulic piston cylinder actuator 126 connected to one end which is able to urge the fork, and hence the dog clutch collar 112, in either axial direction in order to select which of its pairs of gears are to be engaged with the output shaft. The buffer springs 124 allow for any misalignment of the dog clutch when a gear is selected. In the event of misalignment the dogs will not engage until relative rotation between the two dog bearing collars has taken place. However, the hydraulically applied selecting force is absorbed by one of the springs on either side of the fork until alignment is achieved when the spring force will snap the dog collars into engagement.

The double-acting hydraulic actuator 126 is connected to on/off solenoid command valves through connectors 128 and 130. The end of the piston part of each actuator 126 remote from the connection to its selector shaft is connected to a position sensor 132 which is arranged to relay information on which gears are engaged back to the microcontroller.

Once a gear has been selected the hydraulic actuating pressure can be removed from the piston and cylinder actuator 126. Each shaft is held in a selected position by means of a set of three annular channels in positions which correspond to the two engaged and the central neutral position of the engaging fork. Thus, the actuating force of the selector shaft has only to be applied momentarily. In each position a resiliently mounted detent ball, projecting from a recess in a mounting for the selector shaft holds the selector shaft in position. The annular channels are denoted by the numeral 134 in Figure 2 and the projecting balls by the numeral 136.

Turning now to Figure 6, the microcontroller 140 is a 16 bit microprocessor provided with an 8K random access memory(RAM), 32K read only memory (ROM) and a 2K erasable programmable read only memory (EPROM). In this particular embodiment, the EPROM is provided in order to be able to alter programming easily. However, a microcontroller for a particular application is likely to be provided only with ROM containing the appropriate programme.

Many of the facilities and inputs to the microcontroller are the same as those referred to in relation to Figure 1.

The processor has an internal watchdog which will reset if it is not accessed at least once every 500 microseconds. The circuits peripheral to the processor have an independent watchdog routine associated with them which is programmed into the microcontroller 140.

This will turn off the various outputs from the microcontroller in the event of failure of the processor.

Similarly, there is a safety cut-out feature including a relay which allows power to the external equipment, such as the solenoids, but is cut off in the event of system failure.

The microprocessor is programmed to execute a diagnostic check of the system on start-up and to inhibit further operation if a fault is detected.

The microcontroller has an input port which is capable of handling analogue signals. The analogue input signals are converted to a digital representation by an on-board analogue to digital convertor.

The communications port provided allows the microcontroller to convey and receive relevant information from other microprocessor-based devices such as engine management systems or instrument clusters. In a more advanced system the controller may be connected to a network system comprising many other controllers.

The solenoid valve referred to above must be actuated either to open gradually, in the case of clutches where a high degree of control is required, or on an on/off basis when a snap action is required, in the case of the double acting dog clutch actuators.

Pulse width modulation outputs are connected to high speed output lines of the processor. The output can be programmed to produce a pulse width output of up to 2500 Hz with 300 quantisation levels. For example, if a 6 amp solenoid is connected to the output, the current in the solenoid could be controlled in 2 milliamp steps.

All the outputs have a circuit which allows them to be checked by the processor. This is done by switching on each output momentarily. When the output is switched on 12 volts is present on the drain of a pulse width modulation field effect transistor. This is chopped down to 5 volts by a resistor network and a zener diode. Checking at this voltage is carried out to see whether the solenoid is switched on. It should be noted that this is not considered to be dangerous since the checking routine for each output will take approximately one microsecond.

On/off field effect transistor outputs share the same circuitry as those of the pulse width modulating field effect transistor outputs, including the same fault detection systems as the pulse width modulation outputs. The main difference is that they can only be switched on or off and not pulsed. The safety cut-out is an on/off output. If it is not turned on at regular intervals, it will turn the output off. If this is connected to an external relay which powers the solenoid, then in the event that the cut-out trips, all power to the solenoids will be cut. This can also be done deliberately in the event of a fault occurring by means of a main cut-out switch.

Pulse measurement inputs are designed to be connected to the speed sensors. In Figure 2 these are denoted by the numerals 140 and 142. They are respectively mounted to sense the motion of the gear wheel 71 on the input shaft and the 6th driven gear wheel 102 on the output shaft. These sensors produce a saw-tooth pattern wave, the frequency of which is proportional to the number of teeth on a gear and the speed of rotation. The sensors are mounted on the gear box shell adjacent the periphery of each gear wheel to be monitored.

A conditioning circuit converts the saw-tooth wave into a square wave which the microcontroller can read.

Incorporated into this circuit is a system which detects if a wire on the sensor is broken. Each of the pulse inputs has one of these sensor circuits. The outputs of the circuits are multiplexed on an analogue input which allows all the states of the inputs to be checked. The outputs from the sensor circuits are multiplexed onto high speed input lines to the microcontroller.

The overall safety strategy of the design is to allow the system to recover smoothly from any fault which might occur. The system is designed so that if the microcontroller fails, the system will cut power to any solenoids, which will leave the unit in a safe condition. If a wiring fault occurs either at power-up or during system operation, the execution of the software will be halted and the operator alerted to the fault. Protection for reverse and two-battery connection are also provided.

As the microcontroller is arranged to govern the actuation of the various clutch solenoid valves, it is preferable for it to be able to calibrate the different pressures at which each clutch begins to engage.

Clearly, the hydraulic force required for the same degree of engagement of, for example, the multiple plate selector clutch will depend on the gear engaged and the load on the output shaft. Thus, the microcontroller is programmed with a calibration routine which allows it to monitor the engagement of the gears to produce a working range of actuating pressures for each gear engaged to reflect the anticipated loads. The calibration routine can be run when the apparatus is installed and following maintenance which may have altered the characteristics of the solenoid valve, clutch or gears, etc. When the working range is determined, an average working pressure is determined. This is used by the hydraulic system as the actuating pressure.

In an agricultural tractor, the input shaft of a gearbox according to this embodiment of the invention is connected to a conventional 'dry' plate clutch.

However, gear changes, changes between forward and reverse and, in other embodiments, changes in gear ranges can all be effected after neutralising, i.e.

isolating the drive unit from the gears, by disengaging the selector clutch. Because this clutch now has to absorb the load energy, it is of the multiplate type.

Once both parts of the selector clutch have been disengaged, it is possible to make the appropriate gear, range or direction change as required. The microcontroller senses the speeds of the gears to be engaged by means of the speed sensors mounted on the gearbox casing and adjusts the engine speed as necessary for effecting a change or prevents a change from taking place if the gear change selected is in compatible with the prevailing conditions.

Particularly advantageously when a hydraulic selector clutch and a split counter shaft/main shaft arrangement are used, it is possible to execute a so-called 'power shift' between gears without disengaging the drive transmission between the input and output shafts during the operation. The microcontroller is programmed to pre-engage the gears of adjacent ratio to that engaged.

For example, if third gear is selected on the counter shaft, fourth gear can be preselected by engagement of its associated dog clutch but allowed to rotate on the main shaft as it is not engaged by the selector clutch.

The actuation of the two parts of the selector clutch can be arranged to overlap so that the pair are engaged together briefly during a gear shift (e.g. from third to fourth). The hydraulic clutches used can absorb the differential torque between them for the brief period while change over from one part of the selector clutch to the other takes place. While the gear change executed by neutralising the gearbox takes place very quickly (in the order of tens of milliseconds) the power shift is still quicker as the gear to change to is preselected-only clutch changeover is required.As mentioned above, the need for a quick and reliable change while the gearbox is under a constant load is very important and the ability to execute a power shift without de-coupling the engine from the tractor wheels by means of the double acting selector clutch and counter shaft/main shaft arrangement is very desirable.

In an agricultural tractor the microcontroller is also provided with an 'inching' as well as a 'normal' clutch mode of operation. The purpose of this is to provide fine control of clutch engagement during a delicate manoeuvre. In the inching mode the range of the solenoid actuating signal relayed from the microcontroller covers a proportionally smaller range of values for a given movement of the foot pedal. The driver is provided with a switch to change between modes as required.

It will be appreciated that the significant advantages of the present invention, allow the simpler and more reliable constant mesh gearbox to be used for both fast and easily executed gear changes. This has created a considerable number of possibilities for the advantageous use of the constant mesh gearbox.

A further optional arrangement for the gearbox according to this invention is to program the controller to change gear automatically. By sensing the load on the engine either directly or as indicated by a drop in engine speed for a given throttle setting, the microcontroller can be arranged to change gear up or down as appropriate without a specific command to do so. In this regard torque convertors can be used in place of the selector clutch.

Claims (20)

1. A transmission gear system comprising an input shaft, an output shaft, at least one set of gears including a drive gear and a driven gear, actuator means for engaging the gears to transmit rotation of the input shaft to the output shaft, control means responsive to a gear selection signal to enable the actuator means, speed sensing means arranged to provide signals related to the speed of rotation of each of the drive and driven gears, the control means being operable to enable the actuator means to engage the set of gears when their speeds of rotation are substantially the same.

2. A system as claimed in claim 1 including a range of sets of gears, for example constant mesh reduction gears, and clutch means arranged between the input shaft and the range of sets of gears.

3. A system as claimed in claim 2 in which one of the drive and driven gears of each set in the range are grouped such that those drive gears of sets of adjacent ratio in the range are mounted to rotate on shafts independently of one another, the clutch means operable to connect the input shaft with either of the said grouped sets of gears.

4. A system as claimed in claim 3 in which the driven gears are mounted on the output shaft and the drive gears are grouped on separate shafts.

5. A system as claimed in claim 4 in which the separate shafts are arranged to rotate coaxially.

6. A system as claimed in claim 4 or 5 in which the control means are arranged to select two sets of gears of adjacent ratio for engagement simultaneously, only one of the sets of gears being connected clutch means transmission between the input and output shafts.

7. A system as claimed in claim 5 in which the separate shafts comprise a main shaft and a hollow counter shaft through which the main shaft extends.

8. A system as claimed in claim 4, 5, 6 or 7 in which the clutch means are a first double clutch selectively operable to connect the input shaft to one or the other of the separate shafts.

9. A system as claimed in any preceding claim in which the actuator means are a dog clutch.

10. A system as claimed in claim 9 in which the or each dog clutch comprises an actuating member arranged to effect engagement of the dog clutch, the member being mounted on an axially movable selector shaft, responsive to selection commands from the control means, and the actuating means being resiliently restrained on the selector shaft to allow limited axial movement in relation to the selector shaft.

11. A system as claimed in claim 9 in which the resilient restraint is provided by springs.

12. A system as claimed in claim 10 or 11 in which the selector shaft is actuated for axial movement by a double acting piston and cylinder device in response to the selection command from the control means.

13. A system as claimed in claim 10, 11 or 12 in which the selector shaft is provided with holding means for maintaining the selector shaft in a gear engaging position.

14. A system as claimed in any preceding claim including second double clutch means and a reverse gear train, the clutch being selectively operable, in response to signals from the control means, to connect the output shaft to rotate in one sense relative to the input shaft and to connect the output, through the reverse gear train, to rotate in the opposite sense thereto.

15. A system as claimed in any preceding claim in which the speed sensing means are a Hall effect or magnetically sensitive device arranged to provide a pulse output as the teeth of an associated gear wheel pass the device, the rate of the pulse produced being related to the speed of rotation of the gear.

16. A system as claimed in any preceding claim in which the control means include a microprocessor.

17. A vehicle including a system as claimed in any preceding claim.

18. A tractor including a system as claimed in any of claims 1 to 16.

19. A vehicle as claimed in claim 17 including an engine and means for governing the speed of the engine, the control means being operable to adjust the speed of the engine such that the speeds of rotation of the drive and driven gears are substantially the same prior to enablement of the actuator means.

20. A constant mesh gear system substantially as specifically described herein with reference to Figure 1 or Figures 2 to 6 of the accompanying drawings.