The invention relates to a system for ensuring that the total force acting on a shifting mechanism that is used to change the gear ratio in a gearbox does not exceed a predetermined value.

In accordance with the prior art, the gear shifting mechanisms, such as the shift forks and the synchronisation mechanisms in manual gearboxes, are constructed so as to be able to withstand the forces to which they are subjected when operated manually. On operation of a manual gearbox, the forces employed by the driver when he operates the gear lever are transmitted by, for example, a mechanical or hydraulic transmission to the gear shifting mechanism in the gearbox.

In lorries with large motor torques available, the gearboxes will have to be constructed to withstand these motor torques, and this involves high shifting forces in order to change gear. The forces required to change gear are in some constructions so great that the driver needs assistance in the form of power assistance from some form of assistance system/servo system in order to be able to operate the gear lever in a comfortable manner. There are many different solutions which attempt to provide the necessary power assistance to ensure that a gear change can be made in a comfortable manner. In this connection, reference is made, for instance, to DE 2223881 which discloses examples of assistance systems/servo solutions.

Due to the large forces to which the gear shifting mechanism is subjected, it is necessary to manufacture the gear shifting mechanism parts in such manner that the construction withstands these forces. However, this need means that the gear shifting mechanism parts are expensive to manufacture and thus also expensive to purchase. There is therefore a desire on the part of gearbox manufacturers that measures should be taken to permit the gear change forces which occur in the gear shifting mechanism of the gearbox to be limited, so as to reduce the costs that are traditionally incurred in the manufacture of the parts of the gear shifting mechanism which are especially heavily loaded during the gear shift operation. If a solution is achieved in which the forces acting on the gear shifting mechanism are reduced, it will at the same time be possible to introduce cost-saving coatings or treatment methods for the synchronisation rings, so that considerable sums can be saved in production. One drawback of such a solution is that the gearboxes are then more marginally constructed, so that in practice they have lower tolerance for misuse than before. There is therefore a need for a solution which protects the gearbox whilst maintaining to the maximum possible extent the desired operating forces.

One solution to this problem in order to limit the forces acting on the gear shifting mechanism is to introduce a limit on the power assistance that is supplied by the assistance system. Therefore, in servo-assisted systems, a limit to the maximum power supplied by the servomechanism is introduced. In electronically controlled servo-assisted systems, this can be achieved by introducing a reduction of the servo assistance when the estimated total force on the gear shifting mechanism in the gearbox exceeds a predetermined value. This is done, e.g., in software.

However, it is an object of the present invention to provide a mechanical solution to the aforementioned problem that is independent of whether a mechanically or electrically controlled servo assistance system is used.

One approach in order to solve this problem is to obtain a condition-controlled limit to the assistance system power supply, the controlling conditions for limiting the power supply being set according to parameters such as the tolerance limit of the gear shifting mechanism etc. This can be solved by introducing a reducer which can work together with the servomechanism and which is capable of providing an altered power supply to the servomechanism.

The objects of the invention are obtained with the invention as disclosed in the independent patent claims, embodiments of the invention being disclosed in the dependent claims of the set of claims.

According to independent claim 1 , a system is provided to ensure that the total force acting on the gear shifting mechanism does not exceed a predetermined value. It is presupposed in this connection that the force transmitted from the gear lever to the gear shifting mechanism receives power assistance from an assistance system. The system is provided so that a first limit value is used to control the power assistance having a positive value, and that a second limit value is used to control the power assistance having a negative value. The power assistance provided by the assistance system has a value that holds the total force acting on the gear shifting mechanism constant or reduces the total force acting on the gear shifting mechanism, when the gear lever force is equal to or greater than a limit value (hereinafter referred to as the second limit value). The limit value is adjusted according to the durability of the gear shifting mechanism that is to be used and the other individual properties of the system.

If the invention is to be used in a gear shifting mechanism which requires large forces to effect a shift of the gear ratio, an assistance system is used which provides power assistance that makes it easier for the driver to change gear, typically a so- called servo system. The system according to the invention as disclosed in independent patent claim 2 will have a two-part function wherein: - the power assistance provided by the assistance system has a value which increases the total force acting on the gear shifting mechanism, when the gear lever force is increased within a range in which the gear lever force is equal to or greater than the first limit value and less than the second limit value; and wherein - the power assistance provided by the assistance system has a value which holds the total force acting on the gear shifting mechanism constant or reduces the total force acting on the gear shifting mechanism, when the gear lever force is equal to or greater than the second limit value.

The system may also be controlled by setting the limit values for the total force acting on the gear shifting mechanism.

The term "signal transmission" is used below to describe the power transmission that takes place in the gear shifting system between the gear lever mechanism and the gear shifting mechanism and the assistance system, respectively. By transmission of signals in this connection is meant transmission of information about which input conditions apply to the gear shifting mechanism and the assistance system. The signal that is transmitted between the gear lever mechanism and the gear shifting mechanism and the assistance system, respectively, may thus be of different types. It may be a combined force and motion signal via a rod or cable, for example, if the gear shifting system is mechanical. The signal transmission may also be in the form of a pressure signal, for example, if the gear shifting system is a hydraulic system. The pressure signal transmission may, of course, also be carried out by pressure mediums other than hydraulic, for example, pneumatic. Alternatively, the signal transmission may be in the form of an electric, optical or electromagnetic signal. It should be mentioned that this same term "signal transmission" will also be used to describe the transmission of pressure, force and voltage conditions between other components in the system.

If a power-providing device, for example, a servomechanism, is included in the assistance system, the signal is transmitted from the gear lever to the power- providing device, and depending upon the value of the signal, power assistance is transmitted to the gear shifting mechanism. Alternatively, a signal can be transmitted from the assistance system which represents power assistance to a power-providing device that is connected to the gear shifting mechanism.

A signal that corresponds to the signal from the gear lever or that is derived from this signal is transmitted to a reducer that is provided so that, depending on the value of the incoming signal, it can adjust the value of a power supply to which the reducer is connected. This may be a power supply of different kinds such as pressurised fluid, a voltage source etc. The reducer can transmit a power supply to the servomechanism, or may alternatively transmit a signal that represents a power supply, for example, if the power supply is of a kind that must be converted in order to be compatible with the servomechanism.

In a preferred embodiment of the reducer, it comprises at least one measuring unit and at least one regulator, The measuring unit receives the signal from the gear lever, or the signal that is derived from the gear lever's signal, and controls the regulator on the basis of this signal. When the signal from the gear lever represents a value that is equal to or greater than the second limit value, the measuring unit controls the regulator so that the signal from the reducer to the servomechanism is or represents a reduced power supply.

When the signal from the gear lever represents a value that is less than the second limit value, but greater than or equal to the first limit value, the signal that is transmitted to the servomechanism from the reducer represents the power supply that the regulator receives from a source. When the signal from the gear lever represents a value that is equal to or greater than the second limit value, the power supply from the source is reduced in the regulator, and the value of the signal that is transmitted to the servomechanism from the reducer is reduced correspondingly.

In one embodiment of the invention, the power supply can be provided by a pressurised fluid, and the regulator is made having a inlet for conducting the pressurised fluid from the source to a cavity in the regulator. Arranged in the cavity is a preloaded regulator piston which is made having a bore for conducting the fluid to an outlet. Furthermore, the piston causes the opening of an exhaust port when moved in the direction of its preloading force. When the signal from the gear lever represents a value that is less than the second limit value, the pressure of the fluid exiting the outlet is essentially equal to the pressure of the fluid that is supplied to the regulator. When the signal from the gear lever represents a value that is equal to or greater than the second limit value, the exhaust port in the regulator is opened for discharge of fluid from the regulator and the pressure of the fluid that is passed out of the outlet is thus reduced in accordance with the value of the signal from the gear lever.

According to a further embodiment it is the case that when the signal from the gear lever represents a value that is equal to or greater than the second limit value, the measuring unit controls the regulator so that a force is provided in the regulator which counteracts/is greater than the preloading of the regulator piston, and optionally the preloading of the regulator piston valve seat that closes the exhaust port, the piston thereby opening for discharge of fluid through the exhaust port. The force that is provided in the regulator can be produced in different ways, and the measuring unit's control of the regulator can also be effected in different ways.

In one implementation of the invention, the measuring unit comprises two preloaded pistons that are arranged in the cavity of the measuring unit. The piston rod of the first piston is placed in a though-going bore that extends through the second piston and the piston rod of the second piston. When the signal from the gear lever represents a value that is equal to or greater than the second limit value, a force against one of the two pistons is provided which counteracts/is greater than the sum of the preloading of the piston in question, the preloading of the regulator piston and optionally the preloading of the valve seat. Thus, the piston of the first measuring unit is displaced into abutment against the regulator piston which is also displaced so that the exhaust port is opened.

In the case where the reducer with measuring unit as described in the above paragraph is used in a hydraulic gear shifting system, the force exerted against one of the two pistons will be provided by the supply of fluid to the measuring unit in response to the gear lever force. On movement of the gear lever in one direction, fluid is transferred to the measuring unit so that it exerts pressure against the piston face of the first piston. On movement of the gear lever in the other direction, fluid is transferred to a space between the first and the second piston in the measuring unit so that the fluid exerts pressure against the piston face of the second piston.

In the case where the reducer is used in a mechanical gear shifting system, the reducer may, in a preferred embodiment, comprise two regulators and one measuring unit that is disposed between the two regulators. The measuring unit comprises two preloaded slides that are placed in a cavity in the measuring unit. Movement of the gear lever with a force that is equal to or greater than the second limit value in one direction or the other results in a force being exerted against one of the two slides that is greater than, the sum of the preloading of the slide in question, the preloading of a valve arranged in the cavity of the measuring unit, the preloading of the regulator piston in question, and optionally the preloading of the regulator piston valve seat. This results in an opening of the exhaust port with subsequent pressure regulation of the power supply into the servomechanism.

In what follows the premises of the invention, together with a functional description and two embodiments of the invention, will be explained with reference to the attached drawings, wherein:

Figure 1 shows the forces that arise in the gear shifting mechanism according to the prior art. Fig. 2 shows the forces that arise in the gear shifting mechanism according to the solution principle that forms the basis for the invention. Fig. 3 shows a block diagram of the solution principle according to the invention. Fig. 4 shows the invention used in a hydraulic gear shifting system. Fig. 5 shows a reducer that is used in the system in Figure 4. Figs. 6, 7 and 8 show the invention used in a mechanical gear system.

In Figure 1 the horizontal axis shows the force that the driver exerts on the gear lever. The vertical axis shows the force that is applied to the gear shifting mechanism. A standard system without power assistance from an assistance system will give a force on the gear selector mechanism in the gearbox as a function of the driver's force on the gear lever. This situation is shown by curve 2, where it is seen that the force acting on the gear selector mechanism increases proportionally with the driver's force on the gear lever. Curve 5 shows an example of how the power assistance from the assistance system will work. At low gear lever forces, the power assistance from the assistance system will increase as a function of the gear lever forces, and when maximum power assistance from the assistance system has been reached, the curve will flatten out and indicate constant supplementary power to the gear selector mechanism in the gearbox.

The total force that acts on the gear selector mechanism in the gearbox is shown by curve 6. This curve is the resultant of the gear lever force shown by curve 2 and the power assistance from the assistance system shown by curve 5.

Curve 1 shows the maximum force that the synchronisation rings of the gearbox can withstand. For gear lever forces that are greater than the force value at the point where the curve 6 intersects the curve 1 , an overloading of the gearbox synchronisation mechanism is a risk. This area is indicated by the reference numeral 7 in Figure 1.

To prevent the resultant shown by curve 6 from exceeding the maximum tolerance value of the gear mechanism/synchronisation mechanism, the aim of the present invention is to provide a system which, with the aid of suitable means, ensures that the total force acting on the gear mechanism is maintained below the maximum tolerance value.

A solution principle for such a system is shown in Figure 2. The horizontal axis shows the force that the driver exerts on the gear lever. The vertical axis shows the force that is applied to the gear shifting mechanism. Curve 2 shows, as in Figure 1 , a standard system without power assistance from an assistance system which will provide a force on the gear selector mechanism in the gearbox as a function of the driver's force on the gear lever.

Curve 3 shows an example of a solution in which desired protection against overloading of the gear shifting mechanism is introduced by adjusting the power assistance from an assistance system. As curve 3 illustrates, this is done by reducing the power assistance from the assistance system when the gear lever force reaches a certain limit value. This solution principle results in the gear shifting mechanism being loaded with the resultant (illustrated by curve 4) of the gear lever force (illustrated by curve 2) and the power assistance from the assistance system (illustrated by curve). It is seen from Fig. 2 that at low gear lever forces, the power assistance from the assistance system will increase as a function of the gear lever forces. At a certain limit value, whether this is a limit value for the gear lever force or a limit value for the total force on the gear shifting mechanism, the power assistance from the assistance system will be reduced with increasing gear lever force, so that the total force on the gear shifting mechanism never exceeds the maximum force allowed. The maximum total force allowed on the gear shifting mechanism is shown by curve 1.

It is thus seen from Fig. 2 that the power assistance from the assistance system has a magnitude that increases the total force acting on the gear shifting mechanism when the gear lever force is increased within a range in which the gear lever force is equal to or greater than a first limit value and less than a second limit value. Also, the power assistance from the assistance system has a magnitude that holds the total force acting on the gear shifting mechanism constant, or reduces the total force acting on the gear shifting mechanism, when the gear lever force is equal to or greater than the second limit value.

The value of the maximum total force allowed on the gear shifting mechanism, and the first and second limit values may vary depending on what type of system is used. In the example shown in Figure 1 , the second limit value of the gear lever force will be in the range where it is seen that the power assistance from the assistance system has a reducing effect (see curve 2) on the total force shown by curve 4. The first limit value of the gear lever force will also be less than the second limit value, and in the example shown in Figure 2, the first limit value will be found in the range in which the power assistance from the assistance system (curve 2) begins to make a positive contribution to the total force on the gear shifting mechanism (curve 4).

It is seen from Fig. 2 that if the gear lever force is further increased beyond the range in which the assistance system provides power assistance, the increase in gear lever forces will gradually result in the maximum tolerance force (curve 1) being exceeded. For safety reasons, it may in some cases be desirable to provide the system so that it is possible for the driver to make a gear change at such high gear shifting forces, even though the high gear lever force then results in the maximum tolerance force of the gear shifting mechanism being exceeded, and there being a danger of damage to the gear shifting mechanism.

In Figure 3, a functional description of the system according to the invention is illustrated in the form of a block diagram.

Arrow 6a illustrates the force that the driver applies to the gear lever. The gear lever mechanism transfers the driver's force load on the gear lever to a signal 6b. The signal 6b may be a combined force and motion via rod or cable or the like. Optionally, it may be a hydraulic or pneumatic pressure or an electrical signal.

In most cases, the signal from the gear lever mechanism will actuate the gearbox gear shifting mechanism 3 directly by signal 6c. In the illustrated case, a servo assistance is included in the assistance system, and the signal from the gear lever mechanism controls a servomechanism 2. The servomechanism provides power assistance, represented as signal 2a, to the gearbox gear shifting mechanism. Signal 2a and signal 6c represent the total force applied to the gear shifting mechanism in the gearbox.

The servomechanism 2 receives a signal 5b which is or represents a power supply. The signal 5b may, for example, be a pneumatic or hydraulic pressure, or alternatively a voltage supply signal.

The assistance system further comprises a reducer which is shown within the marked area 1. The reducer comprises a regulator 5 and a measuring unit 4 which receives desired gear change force signal from the gear lever mechanism via signal 6b. The measuring unit 4 controls the regulator 5 which is provided so that a power supply 5a can be adjusted to a desired level 5b, depending on the input signal 6b to the measuring unit 4. A power source provides the power supply 5a, the power source, for example, being an air pressure supply, supply of hydraulic pressure, voltage supply etc.

If the signal 6b is greater than or equal to the first limit value, the power supply 5b that is delivered to the servomechanism 2 will essentially correspond to the power supply 5a into the regulator. Under these conditions, the servomechanism provides power assistance which increases the total force on the gear shifting mechanism, and which thus makes changing gear easier for the driver of the vehicle. If the signal 6b is equal to or exceeds the second limit value, the measuring unit 4 will control the regulator so that the power supply 5a to the regulator is adjusted down in the regulator, and a reduced power supply 5b to the servomechanism is obtained. It is thus ensured that the power assistance 2a provided by the servomechanism to the gear shifting mechanism is limited when the force on the gear lever is greater than the second limit value. Thus, it is ensured that the total force on'the gear shifting mechanism is maintained at less than or equal to a pre-set value, thereby avoiding a possible overloading of the shifting mechanism.

The solution principle illustrated in Fig. 3 is, in Fig. 4, implemented in a hydraulic gear shifting system with pneumatic servo assistance. In the example shown in Figure 4, the reducer 1 is shown made having a regulator part 5, here shown as a pneumatic pressure reduction valve 5, which is controlled by a hydraulic measuring unit 4. Of course, other types of fluid may also be used as pressure medium in the regulator, and for that matter also in the measuring unit. The detailed construction of the reducer is shown most clearly in Figure 5. This embodiment of the solution principle will be described below with reference to both Figure 4 and Figure 5.

The measuring unit 5 is comprised of a cavity 12 that is formed in the reducer 1 , where in the cavity 12 there is arranged a first preloaded piston 13 and a second preloaded piston 14. It is seen from Figures 4 and 5 that the first piston 13 is arranged with its piston rod projecting through a bore in the second piston 14, so that the ends of the piston rods of both pistons are placed at a similar distance from the regulator piston 15. It is also seen in Figures 4 and 5 that the first and the second piston 13, 14 are preloaded by springs 13a, 14a.

When a gear change is made, hydraulic fluid is transferred through transfer lines to the servomechanism 2 and to the measuring unit 4. In this embodiment, the servomechanism 2 comprises a valve unit and a servo actuator. If the gear lever is moved in the direction of the arrow A, hydraulic fluid is transferred through the lines 10 to the servomechanism 2 and to the area of the cavity that is above the piston face of the piston 13b. If the gear lever is moved in the direction of the arrow B, hydraulic fluid is transferred through lines 11 to the servomechanism 2 and to the area that is above the piston face 14b of the second piston 14.

The pressure that acts against the respective piston faces in each of the two cases is dependent upon the force that the driver exerts on the gear lever. To obtain displacement of one piston or the other, the force that acts on the pistons 13 and 14 must have a magnitude and direction that are capable of overcoming the preloading force of the springs 13a and 14a, respectively.

If the force that is exerted against the piston faces 13b, 14b is less than the second limit value, sufficient displacement of the respective pistons is not obtained in the reducer to enable the valve seat 19 of the regulator piston to be opened. A fluid path is established from a source 16 to the valve unit that is connected to the servomechanism. Pressurised fluid is passed from the source 16 into an inlet 16a in the regulator and then through a bore 17 formed in the regulator piston 15, out through an outlet 21 and then into the valve unit. In this way, it is ensured that the servomechanism is supplied with a pressure substantially equivalent to the pressure of the fluid in the source 16. The pressure supply from the reducer 1 to the servomechanism 2 is determining for the power assistance provided by the servomechanism to the gear shifting mechanism 3. In this case, the pressure supply into the servomechanism 2 provides power assistance that helps to increase the total force acting on the gear shifting mechanism 3.

In the regulator part 5, the regulator piston 15 is shown preloaded by a spring 15a. If the force acting on one of the pistons 13, 14 is greater than the second limit value, this will result in the end of the associated piston rod being moved into abutment against the regulator piston 15. In this case, the piston 13 or 14 transfers a force to the regulator piston 15 which counteracts the preloading of the regulator piston 15.

The regulator piston 15 is thus displaced downwards into abutment against the valve seat 19 of the regulator piston which is preloaded by a spring 19a, and furthermore the force is sufficient to counteract the preloading of the spring 19a so that the spring is compressed. Thus, the valve seat 19 of the regulator piston is opened and this allows a discharge of fluid to the exhaust port 20. How large an opening is obtained when the valve seat 19 of the regulator piston is opened, and thus how much fluid is passed out of the fluid path between the source 16 and the valve unit, is therefore dependent upon the force acting on the piston faces 13b or 14b. The greater the force the driver exerts on the gear lever, the greater the force that acts on the piston faces 13b or 14b, and this means that a greater drawing off of fluid through the exhaust port 20 is also obtained. The result is then a pressure supply that is down-regulated compared with the pressure of the fluid in the source 16, and power assistance from the servomechanism which ensures that the total force acting on the gear shifting mechanism is maintained below a pre-determined value

In Figures 6-8 the solution principle that is illustrated in Figure 3 is implemented in a mechanical gear shifting system with pneumatic servo assistance. Figure 6 shows an overview of the mechanical gear shifting system. The term "mechanical gear shifting system" is used here to mean a cable or rod transmission between gear lever and gearbox. Figures 7 and 8 show a reducer Ia for use in such a mechanical gear shifting system. The reducer I a comprises a measuring unit 4 that is disposed between two regulators 5a and 5b. The reducer's measuring unit in this case consists of a servo valve. However, the servo actuator is placed outside the reducer as in the embodiment in Figures 4-5. For the sake of simplicity, the term servomechanism 2 is used to refer to the servo actuator in the following.

The regulators 5a, 5b essentially resemble the regulator shown in Figures 4 and 5. Therefore, to a large extent the same reference numerals have been used as for the regulator in Figure 4. It is seen that the measuring unit 4' has a different structure than the measuring unit shown in Figure 4. The measuring unit 4' is double acting (works in both directions) and comprises two slides 22a, 22b that are preloaded by springs 22e and 22d. The force from the gear lever is transferred to the servomechanism 2 and the reducer Ia by a balanced lever (not shown). The lever has a point of action in the measuring unit 4' of the reducer at an actuating pin 12 that exerts a force on one slide or the other. Depending on whether the gear lever is moved in a direction corresponding to A or B as shown in Figure 4, the force will act on either the slide 22a or the slide 22b. The descriptions of the mode of operation and the structure will thus be identical for both sides of the reducer.

If the system is inoperative, fluid (air) is passed from the source into the regulator part through the inlet 16a, 16b. The outlet 21a, 21b leads to the side of the servomechanism 2 which on increase of pressure will move the actuator in the same direction as the force from the gear lever indicates. The exhaust ports 20 and 24 are used to release air from the system.

In this case, the outlet 21 a, 21b for transfer of pressurised air to the servomechanism 2 is directly connected to the exhaust port 24 via exhaust port valve seat 26 and a bore 25 inside the slide 22a, 22b. The system as shown in Figures 7 and 8 is shown in its unloaded state.

When the gear lever force is equal to or greater than the first limit value, but less than the second limit value, the measuring unit is activated in that the actuating pin 12 exerts a force that presses on one of the slides 22a, 22b and moves it down some distance so that the spring 22c, 22d which provides preloading of the slide 22a, 22b is compressed to a certain extent. This in turn means that the force required to move the slide is a function of spring stiffness and movement path. When the slide 22a, 22b is moved some distance, the exhaust port valve seat 26 will seal against the valve 27. This means that the outlet 21 to the servomechanism is no longer vented to the exhaust port 24. Further movement of the slide will therefore also move the valve 27, which in turn compresses the valve spring 28, and also opens for passage of fluid through the supply valve seat 29. The fluid from the source will be passed in through the inlet 16a, 16b, through the bore 17 in the regulator piston 15, then through duct 23 and then into the supply valve seat 29 via slots in the valve seat 30 where the valve seat 30 rests against the regulator piston 15. These slots are not shown in the drawing.

The fluid passage that is produced by the valve 27 being moved in the direction of the supply valve seat 29, is facilitated either in that there is a possibility for movement of the valve 27 relative to a pin 31 that is received in a recess in the valve 27 and the regulator piston 15, or that the valve 27 and the pin 31 are moved relative to the regulator piston 15. Alternatively, the valve 27, the connecting pin 31 and the regulator piston 15 are moved downwards together, the distance between the regulator piston valve seat 19 and the regulator piston 15 permitting this without it resulting in the opening of the regulator piston valve seat 19.

Fluid will thus flow out to the servomechanism 2 and make it easier for the driver to move it. When the servomechanism 2 moves towards the position indicated by the gear lever, the distance between desired position (the gear lever) and real position (servomechanism 2) will be reduced, and the lever mechanism that couples the regulator to the servomechanism 2 will ensure that once desired position has been reached, the displacement of the slide 22a, 22b is drawn back towards its original position. In this way, the described valve mechanism will act as a position regulator.

The regulator piston 15 can be actuated by the pin 31 that is received in a recess in the valve 27, the pin 31 also optionally being fastened to the valve. The pin 31 is not fixed in the regulator piston 15, but can be moved independently thereof. As has also been mentioned above, the valve 27 moves as a result of the fact that the force exerted on the gear lever represents a counter-force that is greater than the preloading of the slide spring 22c, 22d. Furthermore, the top part of the regulator piston 15 may be supplied with pressurised fluid via the duct 23. The supply from the source that is brought in through the inlet 16a, 16b comes into the regulator piston 15 to a cavity halfway between two slide seals as shown in Figures 6 and 7. The net force from the supply from the source against the regulator piston 15 is virtually zero. The underside of the large piston area of the regulator piston 15 is vented to exhaust port 20 via duct 32, so that there is never a build-up of pressure there.

The sum of the forces from the gear lever that are transferred via the actuating pin 12, and the force component from the pressure in the duct 23 against the top of the regulator piston 15 will seek to press the regulator piston 15 downwards, thus compressing the spring 15a. On a certain force from the gear lever mechanism, depending upon the spring stiffness of the slide spring 22c or 22d + valve spring 28 + spring 15a for the regulator piston, the regulator piston 15 will abut against the regulator piston valve seat 19 and thus close off the supply from the source through the inlet 16a, 16b.

If the driver stops the movement of the gear lever there, the servomechanism 2 will use some of the air from the outlet 21a or 21b, and thus the air pressure above the regulator piston 15 will drop, and the piston will be pushed back by the spring 15a until the regulator piston 15 is lifted from the regulator piston valve seat 19 and again lets fluid into the measuring unit/servo valve. (The same applies in the case of the embodiment shown in Figs. 4 and 5).

If, on the other hand, the driver seeks to increase the force on the gear lever further so that the gear lever force is equal to or greater than the second limit value, the regulator part 5 is actuated in order to obtain a down regulation of the pressure of the fluid that is passed into the servomechanism 2. In this case, the regulator piston 5 is pressed even further down, and the regulator piston valve seat 19 is pressed downwards in that the spring 19a is compressed. The fluid that is passed into the servo valve/measuring unit 4' through duct 23 will then be vented to the exhaust port 20. A balance between the pressure on the upper side of the regulator piston 15, the force the driver exerts on the gear lever mechanism that is transferred by the actuating pin 12 and the counter-force from the spring to the regulator piston ensures that the unit pin 31 , the regulator piston 15, the regulator piston 15 spring, the regulator piston valve seat 19 and the associated spring 19 together work as a pressure regulator and regulate down the supply pressure that is to be delivered to the servo valve/measuring unit 4'. The drawing off of fluid via the exhaust port 20, and the subsequent reduced pressure in fluid that is then passed to the servomechanism 2 through the outlet 21a, 21b, will result in the total force acting on the gear shifting mechanism being held constant or reduced. The pressure regulation of the fluid that is delivered to the measuring unit/servo valve and thence to the servomechanism 2 is so adapted that when the force that the driver exerts on the gear mechanism in excess of an initial dead band increases, the pressure of the fluid into the servo valve will drop proportionally. This means that the greater the force with which the driver changes gear, the smaller the servo boost the servo valve contributes, when the gear lever force is above the second limit value. This will continue until the servo unit is no longer capable of providing power assistance. In this way, the total force acting on the gear shifting mechanism from the servomechanism 2 and the gear lever force will be prevented from exceeding a predetermined maximum value.

It is of course possible to use means other than springs to provide the preloading of the various pistons that are used in the reducer 1 and Ia.