The present invention relates to gearboxes, particularly though not exclusively to automotive gearboxes.

Conventional gearboxes include an input or driving shaft, an output or driven shaft, which is coaxial with the input shaft, and a layshaft which is parallel to but spaced from the input and output shafts. The input shaft has a gear wheel rigidly connected to its end which is in mesh with a gear wheel rigidly connected to the layshaft. The output shaft and layshaft carry a number of pairs of meshing gear wheels, namely one pair for each gear. In a synchromesh gearbox, all the gear wheels on the layshaft are rigidly connected to it and are in permanent mesh with the other gear wheel of the pair on the output shaft. The gear wheels on the output shaft normally rotate freely with respect to it but may be selectively locked to rotate with it so that power is transmitted from the input shaft to the layshaft and then to the output shaft through the pair of meshing gear wheels of which one is locked to the output shaft.

Whilst automotive gearboxes commonly have four or five forward gear ratios, it is not uncommon for more gear ratios than this to be provided or desirable.

However, the fact that a meshing pair of gearwheels, provided on the output shaft and layshaft, respectively, is required for each gear ratio means that such a gearbox is inherently very large. It would, therefore, be desirable for a gearbox to be able to provide a number of gear ratios which exceeds the number of pairs of meshing gears on the output shaft and layshaft but no satisfactory gearbox of this type has been proposed.

The synchromesh system on conventional gearboxes includes a large number of components, namely hubs, sleeves, baulk rings, cones, forks, rods and the like and it would be desirable to reduce their number to reduce the cost and complexity of the gearbox.

Accordingly, it is the object of the invention to provide a gearbox in which the number of gear ratios exceed the number of pairs of meshing gears on the output shaft and layshaft, thereby reducing the size of the gearbox. It is a further object to provide a gearbox which includes a reduced number of components and is thus cheaper and simpler than conventional gearboxes.

According to the present invention a gearbox includes an input shaft, an output shaft substantially coaxial therewith and a layshaft parallel thereto but spaced therefrom, the input shaft carrying two or more gearwheels in mesh with respective gearwheels carried by the layshaft, the layshaft carrying further gearwheels which are in mesh with respective gearwheels carried by the output shaft, one of the gearwheels on the input shaft being rotationally coupled to a gearwheel on the output shaft to rotate therewith, the two rotationally coupled gearwheels on the input and output shafts and at least certain of the further gearwheels including a respective selectively operable rotary lock which is adapted to lock the gearwheel to the associated shaft so that it rotates therewith.

The coupling of one of two or more gearwheels on the input shaft to a gearwheel on the output shaft so that it rotates therewith in combination with the fact that all the gearwheels on the input shaft are also in mesh with respective gearwheels on the layshaft, both of the rotationally coupled

gearwheels on the layshaft and on the output shaft including a selectively operable rotary lock, imparts a considerable flexibility to the gearbox and permits it to provide a number of gear ratios in excess of the number of pairs of meshing gears on the output shaft and layshaft. Thus the power may be transmitted directly from one of the gearwheels on the input shaft to the layshaft and thence to the output shaft or alternatively it may be transmitted either directly to the output shaft, whereby the input shaft and output shaft rotate at the same speed, by rotationally locking the coupled gearwheels on the input and output shafts to their respective shafts. In a further possibility, in which the rotationally coupled gearwheel on the output shaft is unlocked and thus free to rotate with respect to it, the power is transmitted from the coupled gearwheel on the input shaft directly to the coupled gearwheels on the output shaft and thence to a gearwheel on the layshaft and subsequently back again to a further gearwheel on the output shaft.

One of the gearwheels on the input shaft may be rotationally fixedly connected thereto but the others will of course include respective selectively operable rotary locks.

Further features and details of the invention will be apparent from the following description of one specific embodiment in accordance with the invention which is given by way of example with reference to the accompanying drawings, in which: Figure 1 is a highly diagrammatic side view of an automotive gearbox showing only the gearwheels and the shafts carrying them;

Figure 2 is a longitudinal sectional view of the gearbox shown in Figure 1; Figure 3 is an exploded perspective view of one of the selectively lockable gearwheels and a portion of the shaft on which it is carried; Figure 4 is an axial sectional view of the gearwheel on the line Z-Z in Figure 5; Figure 5 is a radial sectional view of the gearwheel shown in Figure 3; and Figure 6 is a sectional view showing the selective rotary lock.

The gearbox includes a rotatable input shaft 2, coaxial with which is an output shaft 4. The output shaft 4 carries a stub shaft 6 of reduced diameter on its end which is freely rotatably received in a circular section recess in the opposed end of the input shaft. Parallel to the shafts 2,4 and spaced from them is a layshaft 8. Fixedly attached to the input shaft 2 is a gearwheel A which is in permanent mesh with a gearwheel F which is carried by the layshaft 8 and is selectively lockable by a rotary lock which will be described below. The gearwheel F may thus selectively rotate with respect to the shaft 8 or be constrained to rotate with it. Also connected to the input shaft by a similar rotary lock is a further gearwheel B which is in permanent mesh with a gearwheel G which is carried by the layshaft and connected to it by a further rotary lock. The layshaft also carries three more gearwheels H, J and K, each of which is in permanent mesh with a respective gearwheel C, D and E carried by the output shaft. All of these gearwheels are connected to their respective shafts by further selective rotary locks with the exception of two of them, in this case the gearwheels D and E, which are rigidly connected to the output shaft. Gearwheel C on the output

shaft is permanently connected to gearwheel B on the input shaft by a connection 10 so that these two gearwheels always rotate at the same speed.

Each of the shafts 2,4 and 8 is hollow and also defines a number of pressurised oil passages 12, which is at least equal to the number of selectively lockable gearwheels carried by it. These passages are connected to a common relatively low pressure oil pump, which may be mechanically coupled to be driven by an element of the gearbox or the associated engine or may be driven by a separate motor, e. g. an electric motor. A filter, valves and, if required, a cooler may also be provided. The oil pump may also supply lubricating oil to wherever it is required thus permitting the gearbox to operate with a"dry sump", thereby eliminating the conventional parasitic power loss.

The selective rotary lock will now be described with reference to Figures 2 to 6 which relate equally to all of the gearwheels other than A, D and E but will be supposed to relate specifically to the gearwheel J. Keyed to the central shaft 8 to rotate with it is an annular hub 18 whose outer peripheral surface is stepped and thus constitutes a larger portion 20 and a circular portion of smaller diameter 22. The gearwheel J is annular and its internal surface is complementarily stepped. The smaller diameter portion 22 of the hub is spaced from the smaller diameter portion of the hole in the gearwheel by a needle bearing 24. The larger portion 22 of the hub is polygonal and thus has a number, in this case 32, of"flats", i. e. planar surfaces 26 on its outer periphery.

The planar surfaces 26 are associated in pairs which are inclined to one another at an oblique angle in the manner of a shallow V and together constitute a surface portion 27. Closely spaced, e. g. only by an oil film, from the central portion of each surface portion 27, i. e. aligned with the apex of the V, is a

rolling body 28, in this case a roller, which is also in rolling contact with the opposed internal cylindrical surface of the gearwheel J. The rolling bodies 28 are all retained in an annular cage 30 which maintains their spacing constant.

When the rolling bodies 28 are in alignment with the central portions of the surface portions 27, as shown in Figure 6, relative rotation of the gearwheel J and of the shaft 8, and the hub 18 keyed to it, is possible by rolling motion of the bodies 28 with respect to the two opposed surfaces with which it is substantially in contact.

The cage 30 is normally retained in the position shown in Figure 6, in which rotation of the gearwheel J relative to the shaft 8 is permitted, by the provision of two substantially diametrically opposed holes 32 formed in it into which respective diametrically opposed latches, in this case chamfered cylindrical locking pins 34, project. The holes 32 are oval, with their major axes extending in the peripheral direction, and are in fact slightly offset in the peripheral direction so that one locking pin 34 engages one end of the associated hole 32 and the other locking pin engages the same end of the associated hole. There is thus a clearance between one side of each locking pin and the adjacent side of the hole. Each locking pin 34 is integral with a respective piston 36 which is accommodated in a respective cylinder 38 and is urged in the radially outward direction by a respective spring 40. The space on the radially outer side of the piston 38 communicates via a passage 42 with a respective pressurised oil passage 12 which communicates with a source of pressurised oil via a valve (not shown) which is controlled by e. g. the engine management system of the vehicle.

As will be clear, when the locking pins 34 are engaged in the holes 32 rotation

of the cage 30 with respect to the shaft is prevented and thus rotation of the shaft 8 and hub 18 with respect to the gearwheel J is permitted. The rolling bodies thus roll in the space in which they are accommodated and the gearwheel J can therefore rotate or freewheel with respect to the shaft. However, if the locking pins are retracted from the holes, rotation of the cage and rolling bodies about the shaft is permitted. As this rotational movement occurs, each rolling body 28 moves a small distance along its associated surface portion 27. Since the spacing between the surface portions 27 and the opposed cylindrical surface of the gearwheel decreases progressively in each direction from the centre of the surface portions, the rolling bodies rapidly jam between these two surfaces.

This jamming constitutes a rotary lock between the hub and the gearwheel and the two then rotate together as a solid body. The spaces in which the rolling bodies are accommodated is so dimensioned that the cage and rolling bodies can only move a distance in the circumferential direction which is less than the clearance referred to above.

Accordingly, when the valves controlling the passages 42 are opened, the oil pressure acts on the radially outer surfaces of the pistons 36 thus causing them and thus the locking pins 34 to move radially inwards against the force of the springs 40. The pins 34 are thus retracted from the holes 32 and rotation of the cage 30 about the hub is thus possible to a limited extent. However, as soon as such motion occurs, the gearwheel is locked to the hub and the two rotate as a solid body. Accordingly, when the pins 34 are retracted the gearwheel is effectively locked to the shaft.

In the specific embodiment, gearwheel A has 17 teeth, B has 33 teeth, C 48 teeth, D 36 teeth, E 44 teeth, F 54 teeth, G 40 teeth, H 22 teeth, J 32 teeth and K

28 teeth. Although there are only three gearwheels on the output shaft in mesh with gearwheels on the layshaft and five meshing pairs of gears in all, the gearbox can provide a total of nine forward ratios.

When the gearbox shown in the drawings is in neutral, all the rotary locks are released, that is to say all the locking pins 34 are located in the associated holes 32 in the cages 30. Accordingly, gearwheel A rotates gearwheel F but this is not locked to the layshaft and the layshaft therefore does not rotate. The input shaft is also not locked to gearwheel B. The output shaft therefore does not rotate.

In first gear, the rotary locks on gearwheels F, H and C are locked and the remainder released. Power therefore flows from the input shaft to the output shaft through gearwheels A, F, H and C.

In second gear, the rotary locks on gearwheels F and K are locked and power therefore flows through gearwheels A, F, K and E.

In third gear, the rotary locks on gearwheels F and J are locked and power flows through gearwheels A, F, J and D.

In fourth gear, the rotary locks on gearwheels F, G and C are locked and power flows through gearwheels A, F, G and B and thence to the output shaft via gearwheel C by virtue of the fact that gearwheels B and C are rigidly connected together.

In fifth gear, the rotary locks on gearwheels B, G and K are locked and power

flows through gearwheels B, G, K and E.

In sixth gear, the rotary locks on gearwheels B, G and J are locked and power flows through gearwheels B, G, J and D.

In seventh gear, the rotary locks on gearwheels B and C are locked which means that the input shaft and output shaft are effectively connected together and the two shafts rotate at the same speed.

In eighth gear, the rotary locks on gearwheels B, H and K are locked and power flows through gearwheels B and C, due to the fact that they are connected together, and thence through gearwheels H, K and E.

In ninth gear, the rotary locks on gearwheels B, H and J are locked and power flows through gearwheels B, C, H, J and D.

When changing gear, it is fundamental that all the upshift gearwheels are rotating more rapidly and all the downshift gearwheels are rotating more slowly than the gearwheel currently in use. To lock an upshift gearwheel to its shaft its locking pins are retracted from the holes in the associated cage by applying hydraulic pressure to the associated pistons. This permits the cage and the rolling bodies to rotate slightly with respect to the hub and thus to lock the gearwheel to the shaft. Immediately prior to this, the hydraulic pressure applied to the pistons of the gearwheel currently in use is relieved. The associated locking pins therefore move radially outwardly under the action of the spring and centrifugal forces. However, depending on the direction in which the cage moved initially to lock the gearwheel to the shaft, only one of the pins will be

able to engage in its associated hole. This will ensure that the gearwheel can continue to transmit drive torque but can freewheel in one direction and thus be overdriven by the higher ratio gearwheel. The gearwheel is then overdriven by the higher ratio gearwheel which permits the other locking pin to engage in its associated hole. The gearwheel is then unlocked from the associated shaft and drive torque is transmitted only through the new gearwheel. There is no need to remove or reduce engine power whilst the gear change process is taking place.

It will be appreciated that numerous modifications may be effected to the embodiment described above. Thus whilst both cylinders of the two locking pins associated with each gearwheel will normally be connected to the same oil passage and will thus be pressurised or relieved in synchronism, it may be desirable for the two locking pins of each gearwheel to be separately controllable. This would permit the driving characteristics of a vehicle incorporating the gearbox to be selectively temporarily altered, e. g. to permit freewheeling of one gearwheel in one direction for a brief period of time. The spaces in which the rolling bodies are accommodated have been described as tapering in thickness in the radial direction in both circumferential directions of the shaft. This will ensure that the gearwheels lock to the associated shaft in both directions. However, a similar effect can be achieved by dimensioning certain of the spaces so that they taper in one direction only and the remaining spaces so that they taper in the other direction. This means that certain of the rolling bodies will lock the gearwheel to the shaft in one direction of rotation and the remainder will lock the gearwheel to the shaft in the other direction of rotation.

Whilst the gearwheels illustrated in the drawings are spur gears, it will be appreciated that other types of gearwheel may be used, such as helical gearwheels.