WO2007030840A1

WO2007030840A1 - Transmission ratio range extension

Info

Publication number: WO2007030840A1
Application number: PCT/ZA2006/000100
Authority: WO
Inventors: Johannes Jacobus Naude
Original assignee: Johannes Jacobus Naude
Priority date: 2005-09-09
Filing date: 2006-08-14
Publication date: 2007-03-15

Abstract

This invention relates to a method of extending the ratio range of a primary transmission with the method including the steps of: applying power from a prime mover to the primary transmission in a single first linear power path, shifting the primary transmission through its normal ratio range from its first to its terminal ratio, reversing the direction of the linear power path through a ratio range extension arrangement into a second linear power path through the primary transmission, shifting the primary transmission ratio from its terminal ratio back to its first ratio, and adding the inverse ratio range ratios to the normal ratio range ratios to provide an extended ratio range of the primary transmission which is in the same direction as the normal ratio range of the primary transmission.

Description

TRANSMISSION RATIO RANGE EXTENSION

FIELD OF THE INVENTION

[0001] This invention relates to a method of and an arrangement for increasing the ratio range of a primary transmission system.

BACKGROUND TO THE INVENTION

[0002] In the motor vehicle and, particularly in the truck industry a large number of gear ratios are generally required from a transmission system. The configurations of these transmission systems generally include a primary transmission which is coupled to a . high-low transmission. These transmission systems are generally tediously operated by shifting the primary transmission from its lowest gear to its highest, with the high-low transmission in low and then simultaneously shifting the primary transmission to its lowest gear and the high-low transmission to high, after which the primary transmission is shifted through its ratios to the highest gear. Alternatively, for each gear of the primary transmission a high and low option of the low-high transmission exists and the operation of this system will therefore be to first select the lowest gear in the primary transmission with the high-low transmission in low, with the next gear being selected only by shifting the high-low transmission to high, the following gear is then selected by shifting the primary transmission to the next gear and simultaneously shifting the high- low transmission to low.

[0003] The above transmission systems work well when using non-sequential transmissions where the ability to skip gears is possible so enabling any gear to be selected. For example the primary transmission may be shifted from its highest gear to its lowest without selecting the intermediate gears as is the case with power transmissions.

[0004] A problem, however, arises if the primary transmission is a continuously variable transmission (CVT) or sequential type transmission since neither can be shifted sequentially quickly through all of its gears.

[0005] A transmission system which, to an extent, solves this problem is described in the specification of American patent No. US 4,682,511. This transmission utilises planetary gearing power loops to act as power splits by the employment of a combination of clutches and gearbox variable gear sets. A major disadvantage to a transmission of this type is, however, that the percentage of power available that passes through the primary transmissions (in the above case a variable belt and pulley CVT) varies as the ratio of the CVT varies. In this situation where the ratio range of the primary transmission is decreased the planetary loops result in a true power split which causes only a percentage of the total power being transmitted to pass through the primary transmission. This type of power split is typically used in conjunction with friction drive CVT's such as belt and pulley systems, toroidal systems and hydrostatic CVTs as the primary transmission as these CVT's usually have a large ratio range but have low mechanical efficiencies. The percentage portion of the available power passing through the primary transmission reduces the percentage loss in the overall power transmission system but decreases the transmission range which in turn requires multiple ranges or modes of operation as is the case with the subject matter of patent No. US 4,682,511 which employs four modes of operation. The four modes of operation results in four distinct power/planetary loops acting as four power splits, to compensate for the ratio range reduction of the primary transmission (CVT), wherein the power from each mode flows simultaneously in a dual forward direction through the CVT as well as through the selected gearbox and engaged clutch to be combined in the common planetary gearing. Friction drive CVT's additional have to cope with the inherent torque limitations created by the friction drive interface which also makes power splits an attractive solution.

SUMMARY OF THE INVENTION

[0006] The term "linear power path" in this specification is to be taken to mean a power path which is not deviated by planetary gearing or the like into power loops or splits.

[0007] A method of extending the ratio range of a primary transmission according to the invention includes the steps of:

(a) applying power from a prime mover to the primary transmission in a single first linear power path,

(b) shifting the primary transmission through its norma! ratio range from its first to its terminal ratio,

(c) reversing the direction of the linear power path through a ratio range extension arrangement into a second linear power path through the primary transmission, (d) shifting the primary transmission ratio from its terminal ratio back to its first ratio, and

(e) adding the inverse ratio range ratios to the normal ratio range ratios to provide an extended ratio range of the primary transmission which is in the same direction as the normal ratio range of the primary transmission.

[0008] The primary transmission the ratio range extension arrangement may each have distinct power input and output shafts with the method including the steps of :

applying power in the first linear power path from the ratio range extension arrangement input shaft to the primary transmission input shaft,

shifting the primary transmission through its normal ratio range to its terminal ratio while applying power from its output shaft to the extended ratio range arrangement output shaft,

activating switch means on the primary transmission reaching its terminal ratio to cause the power to be switched in its second linear power path, from the extended ratio range input shaft to the primary transmission output shaft,

shifting the primary transmission ratio from its terminal ratio to its first ratio and adding the inverse ratio ranges to the primary transmission normal ratio ranges while applying power to the extended ratio range arrangement output shaft.

[0009] The switching means may be clutches with: a clutch of a first pair of clutches is located in the first power path between the ratio range extension arrangement input shaft and the input shaft to the primary transmission and the second between the primary transmission output shaft and the ratio range extension arrangement output shaft,

a clutch of a second pair of clutches is located in the second linear power path between the ratio range extension arrangement input shaft and the primary transmission output shaft and the second between the primary transmission input shaft and the ratio range extension arrangement output shaft,

the method includes the steps of activating the first pair of clutches, while the second pair are disengaged, to supply power in the first linear power path from the ratio range extension arrangement input shaft to its output shaft, and

on the primary transmission being switched from its terminal ratio to the first ratio of the extended ratio range, switching the clutch activation from the first to the second pair of clutches, while disengaging the first pair of clutches, to supply power through the second linear power path from the ratio range extension arrangement input shaft to its output shaft.

[0010] The clutches of both pairs may have toothed engaging components and the method could include the step of synchronising the rate of rotation of the toothed components of each clutch under full power to facilitate engagement of the clutch components. [0011] A transmission ratio range extension arrangement according to the invention may include:

a primary transmission having an input and output shaft,

the ratio range extension arrangement may include:

first and second fixed gearing endless drive train arrangements,

a power input shaft which is drivingly engaged with the first drive train arrangement and is, in use, driven by a prime mover,

a power output shaft which is driven by the second drive train,

coupling means in each drive train arrangement for selectively coupling one or the other of the primary transmission input or output shafts to the first drive train and the remaining shaft to the second drive train,

switch means which, in a first mode of operation, in use, activates the coupling means to couple the primary transmission input shaft to the first drive train and its output shaft to the second drive train to cause power to flow, in a first linear power path, from the ratio range extension arrangement input shaft, through the primary transmission in its normal mode of operation, and to the ratio range extension arrangement output shaft, and

in a second mode of operation to which it is switched, on the primary transmission being switched from its terminal ratio to the first ratio of the extended transmission ratio range to couple the primary transmission output shaft to the first drive train and its input shaft to the second drive train to cause power to flow, in a second linear power path, from the ratio range extension arrangement input shaft, in a reverse direction through the primary transmission, and to the ratio range extension arrangement output shaft in the extended mode of operation of the primary transmission.

[0012] The coupling means, in both drive train arrangements, may comprise:

two clutch elements which are freely rotatable by the drive trains in fixed positions on the primary transmission input and output shafts, and a clutch component which is located on and rotatable by each of the primary transmission shafts between the drive train clutch elements of the two drive trains with the switching means causing one of the clutch components on one shaft to be coupled to the clutch element on one of the primary transmission shafts and the other clutch component to be coupled to the other primary transmission shaft and on switching of the switch means causing the coupling order of the clutch components to be reversed.

[0013] The ratio range extension arrangement input shaft may include fixed rotatable means for driving the first drive train and its output shaft includes fixed rotatable means which is driven by the second drive train with each of the drive trains including an endless drive belt which, in use, rotates the rotatable shaft components in it.

[0014] The rotatable shaft components are sprockets and the endless belt may be a chain which is engaged with the sprocket teeth. [0015] The rotatable means on the extended ratio range input and output shafts and the clutch elements of both drive trains may be gears which are gear rotated by further meshed gears in a gear drive train.

[0016] The primary transmission may be a sequentially operable incremental CVT. Alternatively, the primary transmission could be any suitable transmission or transmission system.

BRIEF DESCRIPTION QF THE DRAWINGS

[0017] The method and arrangement of this invention for increasing the ratio range of a primary transmission is now described by way of non-limiting examples only with reterence to the drawings in which:

[0018] Figure 1 is a basic schematic diagram which illustrates the operation of the invention,

[0019] Figure 2 is a graph illustrating the operation of the extended transmission arrangement of the invention,

[0020] Figures 3 and 4 are identical block diagrams which together illustrate the operation of the transmission ratio extension of the invention,

[0021] Figures 5 and 6 are isometric views from above and one side of the basic components of a practical example of the ratio extension arrangement of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Figure 1 illustrates the basic concept of the extended transmission ratio range arrangement of this invention. The drawing shows a primary transmission, which in this example of the invention, is a sequentially operable CVT 10 and, for the purpose of explanation only, an imaginary dotted image 12 of the CVT 10 which is arranged with its low ratio end in a back-to-back relationship with the CVT 10.

[0023] The CVT 10 has, as shown in the drawing, a normal ratio range through gears 1 to 32 while the extended ratio range of the imaginary CVT 12 is through extended gears 33 to 64.

[0024] The normal ratio range A of the CVT 10 is illustrated in the Figure 2 graph where, assuming a constant input speed of rotation, it is shown that the ratio sequentially decreases in substantially uniform steps from gear number one at the beginning of the normal ratio range of CVT 10 to the normal range gear number 32.

[0025] The next gear is switched from gear number 32, as shown in Figure 1 , to the imaginary CVT 12 extended gear number 33, as seen in the Figure 2 graph B, and from there through the extended dotted line gear range of the machine in an increase (1/the ratios of the CVT10) to its transmission gear number 64.

[0026] The extended gear ratios, 33 to 64, are added in steps, in the same direction, to the ratio of gears 32 to 64 to result in the overall transmission ratio graph C of Figure 2.

[0027] The above description of operation of the transmission ratio range extension of the invention is highly simplistic and the description of the imaginary ratio extension CVT 12 is in practice achieved, as illustrated in Figure 1, by simply reversing the solid line CVT 10 output between gear numbers 32 and 33, with extended gear 33 in the reversed direction then factually being the inverse of the normal gear 32 of the CVT 10, as described below in more detail with reference to Figures 3 to 6.

[0028] An example of the ratio range extension arrangement of the invention is shown in Figures 3 and 4 to each comprise a primary transmission CVT 10, a first fixed gearing drive train 14(a) and 16(a), a second fixed gear drive train 14(b) and 16(b) and clutches 18, 20, 22 and 24. These components are connected as shown in the two diagrams with the arrangement additionally including a power input 26 from a prime mover and an output 28 to whatever is to be driven by the transmission.

[0029] It is important to note that the fixed gearing drive train 14(a) and 16(a) as well as 14(b) and 16(b) are non-variable and in no way multiply or otherwise alter the power flow paths through them and serve only as fixed direct drive train transmission arrangements as will be described with reference to Figures 5 and 6.

[0030] Figure 3 illustrates, by means of the solid line arrows, a first linear power path through the conventional unextended ratio arrangement of the CVT 10 and Figure 4 the extended range dotted line arrow linear power path through the CVT 10.

[0031] In the normal unextended mode of operation of the CVT 10, as shown in Figure 3, with power, solid line arrows, applied at a fixed speed of rotation to the input 26, the clutches 18 and 20 are engaged while the clutches 22 and 24 remain disengaged, to direct the drive power in its first linear power path through; the fixed gearing 14(a), the engaged clutch 18 to the CVT 10 input, through all of the ratios of the CVT ratio range, through the engaged clutch 20 and the fixed gearing 16(a) to the transmission output 28.

[0032] In the Figure 4 extended mode of operation of the CVT 10 the clutches 18 and 20 are disengaged and the clutches 22 and 24 engaged, without any perceptible interruption of the machine operation, with the second linear power path, dotted line arrows, now passing through the fixed gearing 14(b) and 16(b) and the engaged clutches 22 and 24 to the transmission output 28 in the opposite direction inverse ratios through the CVT 10 to that of the Figure 3 linear power path, as mentioned with reference to Figure 1.

[0033] Figures 5 and 6 illustrate the practical essentials of the Figures 3 and 4 CVT 10 together withrits ratio range extension arrangement. In Figures 5 and 6, the fixed gearing drive train arrangements 14(a) and 14(b) and 16(a) and 16(b) are shown as composite first input and second output fixed gearing drive train arrangements 14 and 16 respectively.

[0034] The first drive train arrangement 14 is shown in Figures 4 and 5 to be composed of an input drive sprocket 30, two sprockets 32 and 34 and an endless chain 36 which drives the sprockets 32 and 34 from the input drive sprocket 30. The second drive train arrangement 16 includes a power output sprocket 38, two sprockets 40 and 42 and an endless chain 44 which concomitantly drives the sprockets.

[0035] In the normal unextended ratio range of the CVT 10, as shown in Figure 5, the input to the CVT is through a rotatable input shaft 46 and the CVT power output through an output shaft 48. [0036] The sprockets 34 and 42 as well as the sprockets 32 and 40 are freely rotatable in fixed positions on the CVT 10 shafts 46 and 48. The shafts are splined over their lengths between the sprockets which they carry.

[0037] Dog clutch components 50 and 52 are slidably located on the splined portions of the CVT shafts 46 and 48. The sides of the sprockets 32, 34, 40 and 42 which face the dog clutch components 50 and 52 carry dog teeth which are engageable with those of the dog clutch components.

[0038] The sprocket 34 together with the clutch component 50 defines the clutch 18 of Figures 3 and 4 with the remaining clutches being defined in the drawings as follows: clutch 20 by the sprocket 40 and clutch member 52, clutch 22 by the sprocket 32 and the clutch member 52 and clutch 24 by the sprocket 42 and the clutch member 50. The two clutch components 50 and 52 are switched from one sprocket to the other on the shafts which carry the components between their positions shown in Figures 4 and 5 on the transmission being shifted from the gear 32 to 33 by any suitable electric, hydraulic or pneumatic high speed switching arrangement.

[0039] For purposes of the following examples of the extended ratio range transmission of the invention the reference is made to Table 1 and Tables 2 to 9. In the following tables it is assumed that the primary transmission CVT 10 is that which forms the subject matter of our WO 2005/036028 publication which includes 32 ratios and operates on a sequential basis with the description of the CVT and its operation described in the publication being incorporated herein by reference. [0040] Table 1 - Gear ratios of CVT 10:

[0041] Table 1 provides the ratios of the primary transmission CVT10, defined as the speed of input shaft 46 rotation divided by the speed of rotation of the output shaft 48 through each of the 32 ratios of the CVT 10, as shown in the left hand gear column, during normal operation of the CVT. The extended gear ratios 33 to 64 are shown in the second gear column wherein the linear power path through the CVT 10 is reversed and the ratios are defined as the speed of the output shaft 48 rotation divided by the speed of rotation of the input shaft 46 and therefore 1 /(normal ratio) to conform to the reversed direction of linear power path flow through the CVT in its extended mode of operation. From Table 1 and the graph C of Figure 2 it is seen that the ratio, during normal operation, through gears 1 to 32, of the primary transmission decreases and while in the extended mode, gears 33 to 64, the now inverse primary transmission ratio continues to decrease up to 0.346 in gear 64 in the same direction as in normal transmission as a result of the reversed direction of linear power path flow through the CVT 10.

[0042] From Table 1 it is important to note that the normal ratio transmission range of the CVT10 together with its extended ratio range has an overall CVT 10 ratio range of 2.89/1.17 - 2.47 or 0.857/0.346 = 2.47: The percentage of gear ratio change between gears 1 and 64 is shown in the two percentage ratio change columns of the Table.

[0043] As is mentioned above the range extended transmission system includes a first non-extended linear power path through the transmission from the power input 26 to the power output 28 as illustrated in Figures 3 and 5 and in a reversed direction second linear power path between the input and output as illustrated in Figures 4 and 6.

[0044] It is to be appreciated that, when transmitting power to the transmission via the first linear power path, the transmission output sprocket 38 will increase in rpm as the ratio in the primary transmission CVT 10 is shifted from gear number 1 to gear number 32 in Table 1. However, when transmitting power via the second linear power path (gears 33 to 64) in Table 1 the transmission output sprocket 38 will continue to increase in rpm as the ratios in the extended transmission range is shifted from gear number 32 back to gear number 1. [0045] The reason for the above is the fact that power is transmitted in different directions through the CVT 10 in the first and second linear power paths as shown by the solid line arrows in Figure 5 and the dotted line arrows in Figure 6.

[0046] In the above manner the ratio range of the CVT 10 may sequentially be doubled in number but raised to the power of 2 in range by selecting the sizes of all of the sprocket gears in such a way that when the CVT 10 is in gear number 32 while transmitting power through the first linear power path (Figures 3 and 5) the overall ratio (rpm of input sprocket 30 divided by the rpm of the output sprocket 38) will be substantially the same when transmitting power through the extended second linear power path (Figures 4 and 6). The rpm of sprockets 32 and 40 and sprockets 42 and 34 are likewise substantially the same during power transmission through the two linear power paths when the CVT is in gear number 32 and shifts to gear number 33 which facilitates almost instantaneous engagement of the dog clutch components with the relatively slowly rotating sprockets with which they are engageable without imposing shock loads on the clutch components or causing a perceptible interruption of power flow through the system.

[0047] The above shifting strategy eliminates the need for the CVT 10 to downshift back to gear number 1 which would have been the case in a conventional primary transmission coupled with a high-low transmission.

[0048] Examples are now provided of actual sprocket gear sizes (teeth numbers) and calculated rpm through both the first and the second linear power paths for a constant input rpm of 1500 rpm applied to the input sprocket 30. The conditions for the CVT 10 in gear numbers 1, 10, 20 and 32 in both linear paths of power transmission are shown in the correct sequence in Tables 2 to 9 to sequentially decrease the overall ratio (rpm of input sprocket 30 divided by rpm of output sprocket 38). The rpm's of the sprockets 30, 32, 34, 40, 42 and 38 are presented in both their first and second paths of power transmission in Tables 2 to 5 and 6 to 9 respectively.

[0049] Table 2 :

CVT 10

Gear no

1

Power transmission via first path (solid arrows in Figure 5 )

Sprocket gear Teeth RPM Dog clutch 50 engaged with sprocket gear 34

30 25.00 1500

34 21.00 1786 Dog clutch 52 engaged with sprocket gear 40

40 25.00 618

38 43.00 359

Overall ratio(30/38) - -

4.17

[0050] Table 3 :

CVT 10

Gear no

10

Power transmission via first path (solid arrows in Figure 5 )

Sprocket gear Teeth RPM Dog clutch 50 engaged with sprocket gear 34

30 25.00 1500

34 21.00 1786 Dog clutch 52 engaged with sprocket gear 40

40 25.00 748

38 43.00 435

( Overall ratio(30/38)

3.45 [0051] Table 4 :

CVT 10

Gear no

20

Power transmission via first path (solid arrows in Figure 5 )

Sprocket gea Teeth RPM Dog clutch 50 engaged with sprocket gear 34

30 25.00 1500

34 21.00 1786 Dog clutch 52 engaged with sprocket gear 40

40 25.00 974

38 43.00 566

Overall ratio(30/38)

2.65

[0052] Tables 5 and 6 :

CVT 10

Gear no

[0053] Table 7 :

CVT 10

Gear no

43

Power transmission via second path (dotted arrows in Figure 6 )

Sprocket gea Teeth RPM Dog clutch 50 engaged with sprocket gear 42

30 25 1500

32 25 1500 Dog clutch 52 engaged with sprocket gear 32

42 22 2750

38 43 1407

Overall ratio(30/38)

1.07

[0054] Table 8 :

CVT 10

Gear no

53

Power transmission via second path (dotted arrows in Figure 6 )

Sprocket gear Teeth RPM Dog clutch 50 engaged with sprocket gear 42

30 25 1500

32 25 1500 Dog clutch 52 engaged with sprocket gear 32

42 22 3583

38 43 1833

Overall ratio(30/38)

0 82

[0055] Table 9 :

CVT 10

Gear no

64

Power transmission via second path (dotted arrows in Figure 6 )

Sprocket gear Teeth RPM Dog clutch 50 engaged with sprocket gear 42

30 25 1500

32 25 1500 Dog clutch 52 engaged with sprocket gear 32

42 22 4333

38 43 2217

Overall ratιo(30/38)

0.68

[0056] From the above table scenario with the CVT 10 in gear number 32 the dotted lines indicate, in Tables 5 and 6, an rpm difference between sprocket gears 40 and 32 of only 2% while the dashed solid line illustrates the same percentage difference between sprocket gears 34 and 42. The overall ratio difference, indicated by the solid line, indicates a difference of only 0.61% between the first and second linear power paths. This is only an example and is not optimised for a certain criteria. For example, the above differences can completely be eliminated by the optimisation of the CVT 10 ratios in conjunction with the various sprocket gear sizes.

[0057] It is to be noted that by means of the above process the overall ratio decreases from 4.17 (CVT 10 in gear number 1 transmitting power through the first linear power path) to 0.68 (CVT 10 in gear number 64 transmitting power through the second linear power path) results in a ratio range of 4.17/0.68 = 6.13, whereas the overall ratio during the first power path transmission only decreased from 4.17 to 1.69. The ratio range of the GVT 10 is therefore 4.17/1.69 = 2.47. The ratio range extension of the transmission of the invention thus substantially increases the primary transmission CVT 10 ratio range to the power of 2, that is 2.47² = 6.10 « 6.13 which is the general trend in the CVT 10 transmission of the invention to increase the primary transmission ratio range by power 2.

[0058] As mentioned above the application of the above range extension arrangement of this invention is intended for use with the incremental CVT as disclosed in our WO 2005/036028 publication. This 32 speed incremental CVT, shifts ratios under full power (no power interruptions), and is a positive drive (a non-friction drive), which has been proven to have an efficiency which is comparable to that of a conventional manual transmission.

[0059] Since all power flows through the primary transmission of the invention through both the first and second linear power paths, the abovementioned high transmission efficiency will be maintained through both the extended and normal ratio range of the invention, if this invention were applied to the above incremental CVT.

[0060] Existing friction-based CVT's (belt and pulley as well as toroidal) may also benefit from the range extension arrangement of the invention by being employed as the prime transmission of the system of this invention. For example an existing belt and pulley CVT may be operated in such as way that the chain or belt only operates on larger radii on the conical surfaces where the torque capabilities and efficiencies are higher. The resultant drop in ratio range (typically the square root of the normal ratio) can then be extended via the range extension method of this invention, back through the original CVT ratio range in a reverse direction.

[0061] The result of the above would be that existing friction CVT's could substantially increase their torque, power capabilities and efficiencies without any changes to existing transmission systems by only adding to them two chain or gear sets without further ratio varying gearbox devices to implement the range extension of this invention.

[0062] The invention is not limited to the precise details as herein described. For example the sprockets and chains of the first and second drive trains 14 and 16 of Figures 5 and 6 could be replaced by meshed gears and the dog clutch arrangements could be replaced by multiple plate clutches. Additionally, the primary transmission need not necessarily be that described in our WO2005/036028 publication and could be any suitable transmission or transmission system.

Claims

1. A method of extending the ratio range of a primary transmission characterised in that the method includes the steps of: applying power from a prime mover to the primary transmission in a single first linear power path, shifting the primary transmission through its normal ratio range from its first to its terminal ratio, reversing the direction of the linear power path through a ratio range extension arrangement into a second linear power path through the primary transmission, shifting the primary transmission ratio from its terminal ratio back to its first ratio, and

adding the inverse ratio range ratios to the normal ratio range ratios to provide an extended ratio range of the primary transmission which is in the same direction as the normal ratio range of the primary transmission.

2. A method as claimed in claim 1 wherein the primary transmission and the ratio range extension arrangement each have distinct power input and output shafts with the method including the steps of : applying power in the first linear power path from the ratio range extension arrangement input shaft to the primary transmission input shaft, shifting the primary transmission through its normal ratio range to its terminal ratio while applying power from its output shaft to the extended ratio range arrangement output shaft, activating switch means on the primary transmission reaching its terminal ratio to cause the power to be switched in its second linear power path, from the extended ratio range input shaft to the primary transmission output shaft,

3. A method as claimed in claim 2 wherein: the switching means are clutches, a clutch of a first pair of clutches is located in the first power path between the ratio range extension arrangement input shaft and the input shaft to the primary transmission and the second between the primary transmission output shaft and the ratio range extension arrangement output shaft, a clutch of a second pair of clutches is located in the second linear power path between the ratio range extension arrangement input shaft and the primary transmission output shaft and the second between the primary transmission input shaft and the ratio range extension arrangement output shaft, the method includes the steps of activating the first pair of clutches, while the second pair are disengaged, to supply power in the first linear power path from the ratio range extension arrangement input shaft to its output shaft, and

4. A method as claimed in claim 3 wherein the clutches of both pairs have toothed engaging components and the method includes the step of synchronising the rate of rotation of the toothed components of each clutch under full power to facilitate engagement of the clutch components.

5. A transmission ratio range extension arrangement including a primary transmission having an input and out shaft, characterised in that, the ratio range extension arrangement includes: first and second fixed gearing endless drive train arrangements, a power input shaft which is drivingly engaged with the first drive train arrangement and is, in use, driven by a prime mover, a power output shaft which is driven by the second drive train, coupling means in each drive train arrangement for selectively coupling one or the other of the primary transmission input or output shafts to the first drive train and the remaining shaft to the second drive train, switch means which, in a first mode of operation, in use, activates the coupling means to couple the primary transmission input shaft to the first drive train and its output shaft to the second drive train to cause power to flow, in a first linear power path, from the ratio range extension arrangement input shaft, through the primary transmission in its normal mode of operation, and to the ratio range extension arrangement output shaft, and

6. A transmission ratio range extension arrangement as claimed in claim 5 wherein the coupling means, in both drive train arrangements, comprises: two clutch elements which are freely rotatable by the drive trains in fixed positions on the primary transmission input and output shafts, and a clutch component which is located on and rotatable by each of the primary transmission shafts between the drive train clutch elements of the two drive trains with the switching means causing one of the clutch components on one shaft to be coupled to the clutch element on one of the primary transmission shafts and the other clutch component to be coupled to the other primary transmission shaft and on switching of the switch means causing the coupling order of the clutch components to be reversed.

7. A transmission ratio range extension arrangement as claimed in claim 6 wherein the ratio range extension arrangement input shaft includes fixed rotatable means for driving the first drive train and its output shaft includes a fixed rotatable means which is driven by the second drive train with each of the drive trains including an endless drive belt which, in use, rotates the rotatable shaft components in it.

8. A transmission ratio range extension arrangement as claimed in claim 7 wherein the rotatable shaft components are sprockets and the endless belt is a chain which is engaged with the sprocket teeth.

9. A transmission ratio range extension arrangement as claimed in claim 7 wherein rotatable means on the extended ratio range input and output shafts and the clutch elements of both drive trains are gears which are gear rotated by further meshed gears in a gear drive train.

10. A transmission ratio range extension arrangement as claimed in any one of the above claims wherein the primary transmission is a sequentially operable incremental CVT.