GB2140882A - Gear trains - Google Patents

Abstract

A gearbox for a truck comprises three layshafts 260,262,286 providing six transmission ratios. Each layshaft comprises a tubular outer shaft and a coaxial inner shaft with a multi-plate clutch 272,274,298 at one end. Input and output gears on each layshaft are in constant mesh with two coaxial central gear pairs on the input/output shaft axis of the gearbox. Two clutches interconnect the gears of the gear pairs for direct drive purposes. All three layshaft clutches are at one end of the gear train for servicing and access purposes. <IMAGE>

Description

SPECIFICATION Gear trains This invention relates to constant mesh change speed gear trains and to gearboxes incorporating such gear trains, for providing two or more drive transmission ratios through the gear train. The invention is particularly, but not exclusively, applicable to such gear trains for use in land vehicles, and a particular application of the invention is to gear trains for use in lorries or trucks and other highway vehicles.

The invention also provides a drive transmission incorporating such gear trains, which transmission may be provided with automatic or semi-automatic control systems for effecting ratio changing. Another aspect of the invention provides a vehicle, particularly a land vehicle such as a lorry or truck, incorporating such transmissions and/or gear trains.

In our published European patent applications, serial numbers EP 012,022 A and EP 012,023 A, we have described a geartrain comprising a drive input shaft having an associated drive input gear, and a drive output shaft having a corresponding drive output gear, and one or more layshafts each having layshaft input and output gears in direct constant mesh with the input shaft and output shaft gears respectively, and the or each layshaft also having clutch means operable to establish and interrupt the transmission of drive between the two input shaft gears, the clutch means being located at the end of its layshaft so as to be more accessible.

In our European patent application serial number EP 041,320 A we have described modifications to the gear train of our earlier applications, and have disclosed a drive transmission incorporating such modified gear train and provided with manual, semi-automatic or fully automatic control of ratio changing.

The gear trains, gearboxes and drive transmissions of our above-mentioned previous patent applications possess very significant technical advantages relating principally to simplicity of construction and consequent reduced manufacturing cost, robustness and reliability which lead to long service life, and ease of maintenance which greatly reduces vehicle "down time".

The specific embodiments of our prior patent applications have related principally to geartrain and transmissions for city buses and similar public service vehicles, whereas in the present application we are concerned more particularly though by no means exclusively, with the provision of gear trains, gearboxes, and transmissions particularly suitable for use in trucks and similar highway vehicles. We emphasise however that the present invention has many other applications.

In the case of city buses and the like, the favour able power to weight ratio of the vehicle is such that there is rarely a requirement for more than four selectable transmission ratios, and in some cases three such ratios and a torque convertor will suffice.

In the case of trucks however, the power to weight ratio is much less favourable and more transmission ratios are required, or at the very least, the ratios must be more widely spaced. In this connection a further factor is that where the engine or prime mover of the truck is turbocharged the torque output characteristics of the engine are modified so that the available torque is reduced at low engine speeds, and thus a vehicle having a turbocharged engine may well require one or two additional transmission ratios for use in moving off from rest and hill climbing.

A consequence of the much higher drive transmission ratios needed for the bottom (low speed) gears of a truck is the correspondingly high torque which is transmitted during use of such ratios. This high torque leads to a requirement for correspondingly large diameter clutches in the case of gear trains incorporating clutch-controlled layshafts. Incorporation of such large clutches leads to serious space considerations when designing a gearbox.

Also, the high transmission ratios needed by a truck for low speed work lead to the problem that, in the case of a constant mesh gear train, when the vehicle is travelling at maximum highway speed in the lowest transmission ratio (ietop gear), the rate of rotation of the gears associated with the highest transmission ratio (ie bottom gear) can be excessive and lead to undue energy losses from oil churning, if not to bearing failure.

Accordingly, there is a requirement for adaptation of the gear trains described in our prior patent applications to increase their versatility to meet the above-identified needs and related factors.

In US patent specification 3,064,488 (Lee) there is disclosed a constant mesh gear train having multiple clutches at one end of the gear train whereby a central assembly of equal diameter transfer gears may be driven at a series of speed ranges by selective actuation of the clutches. A further series of clutches at the other end of the gear train enables these speed ranges each to be divided into a set of transmission ratios. There is no disclosure in this prior specification of the simple concept of direct meshing engagement of the layshaft gears with the input and output shaft gears in the manner disclosed in our prior patent applications, and although the Lee specification discloses a gear train providing a wide spread of ratios, the gear train is too complex, cumbersome and costly and leads, for example, to the problem that some of the clutch drums are rotated much too fast.

In our International Patent Application No. PCT/ GB82/00346 (which was unpublished at the priority date of the present application) we have disclosed a multi-layshaft constant mesh change speed gear train which has primary layshafts and a range layshaft. Multi-plate clutches are provided for each primary layshaft at one end of the gear train to control ratio changing. The range layshaft splits the torque path at the output end of the gear train and has a corresponding layshaft clutch located approxi mately centrally along the length of the range layshaft.Moreover, the drive output from the range layshaft is transmitted back to the gear train output shaft through a relatively large final reduction gear on the latter shaft, this gear being necessarily positioned on the outer side of all the control clutches in the gear train and therefore somewhat restricting access to the clutches.

We have thus identified a requirementforimprovements as regardsthe gear train of our prior International application in respect ofthe general arrangement of the range layshaft, its control clutch and related aspects of the whole gear train layout, and the an aim of the present invention is to provide improvements in one or more of these respects.

According to the invention there is provided a constant mesh change speed gear train as defined in the accompanying claims.

The invention also provides a drive transmission incorporating a gear train as defined above. The transmission may be provided with automatic or semi-automatic control means for effecting ratio changing in accordance with monitored operational parameters of the gear train or of a transmission of a vehicle incorporating the same. The invention also provides a vehicle incorporating such transmission and/or gear train. The vehicle is preferably a lorry or truck.

The invention also provides any concept our feature or combination of features defined and/or described and/or illustrated in this application.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 7 shows an axial section through a gear train; and Figure 2 shows a diagrammatic reprentation of the geartrain of Figure 1.

The gear train of Figures 1 and 2 corresponds to that of our International application mentioned above, but the gear train includes several modifications, notably the following. Firstly, although one of the layshafts splits the torque path at the input end of the gear train and the other splits the torque path at the output end thereof, the clutch means of both layshafts are located at the same end of the gear train, remote from the input shaft.

As shown in the drawings, gear train 250 comprises an input shaft 252 having an input shaft gear 258.

Gear train 250 further comprises first and second primary layshafts 260 and 262 respectively, the layshafts each having layshaft input and output gears 264,266 and 268,270 respectively, together with clutch means 272,274 at one end of the layshaft and operable to establish and interrupt the transmission of drive between the layshaft input and output gears. The structure and arrangement of these primary layshafts is as described in the embodiment of Figures 1 to 7 of our prior international application, the disclosure and drawings of which is hereby incorporated in the present application, particularly Figures 5,6 and 7 thereof and the corresponding description.

Drive input to input shaft 252 is through range clutch means 276. In addition, direct top clutch means 278 is provided between the input shaft gear 254 and the output shaft gear 258 to enable a direct top gear transmission ratio to be provided, with no torque increase and no speed reduction.

Input shaft gear 254 comprises a cluster of three gears comprising first and second range gears 280 and 282 respectively, and between them a primary input gear 254 which is in constant direct meshing engagement with input gears 264 and 268 of the primary layshafts 260 and 262 respectively.

A range layshaft 286 is provided corresponding to the primary layshafts 260, 262 and having input and output gears 288 and 290 which are in direct constant mesh with the first and second range gears 280, 282.

Range layshaft 286 comprises an inner input shaft 294 and an outer tubular output shaft 296 arranged coaxially and with their respective input and output gears 288 and 290 mounted thereon at one end. At the other end is provided the range layshaft clutch means 298 to control the transmission of drive between the input and output gears 288 and 290.

All the clutch means 272, 274, 276 and 278 and 298 of gear train 250 are of the multiple interleaved plate kind as described in our international application.

The three layshaft clutch means 272, 274 and 298 are of the kind described in relation to Figures 5 to 7 of our international application, in which the layshaft inner input shaft (which carries the layshaft input gear) is coupled to the outer periphery of one set of clutch plates, and the layshaft tubular output shaft (which carries the layshaft output shaft) is coupled to the inner peripheries of the other set of clutch plates or elements, whereby the clutch housings 300 all rotate with the layshaft input shafts. It will be noted that the housing 302 of the direct top clutch means 278 is directly coupled to the output shaft 256 so that its maximum rate of rotation is engine speed.

Referring more particularly to Figure 11, drive input to the gear train is through a resilient coupling device 304 providing some torsional damping but zero slip. Drive take-up from rest is by means of the layshaft clutch means.

Range clutch means 276 has a housing 306 with first range gear 280 directly mounted on it. Drive outputfrom clutch means 276 is through an inner shaft 308 on which the primary input gear 284 and the second range gear 282 are directly mounted and which is coupled to one set of the clutch plates of direct top clutch means 278 at the inner peripheries of the plates, while the housing 302 thereof is directly coupled to output shaft gear 258.

The primary layshafts 260 and 262 are not seen in Figure 11 but are disposed on opposite sides of the input/output shaft axis 312 and with their axes approximately in a common plane with the latter axis.

An oil feed is provided to the clutch means of the primary layshafts, as mentioned in our priorapplica- tion. These clutch means are used for retardation purposes by partial engagement of one or both of the clutch means when not used for drive transmission purposes. Retardation using both the primary clurch means would be possible in the case where the direct top clutch means 278 is in its drive engaged condition. Likewise, an oil feed to the range layshaft clutch means 298 is provided by means of an oil inlet 314 and an oil duct 316 extending lengthwise of the range layshaft.

Drive outputfrom gear train 250 is through an output flange 318.

The drive transmission ratios provided by gear train 250 may be from a 1 to 1 ratio in direct drive top gear to a 1 to 7 ratio or lower for crawling purposes, hill starts and off-highway use.

All mesh points in the gear train are arranged in the case of a drive transmission for a truck, to provide torque increase. In the case of the highest transmission ratio of, say, 1 to 7, there are four mesh points to achieve this ratio. In practice,.the mesh point between the range layshaft output gear 290 and the second range gear 282 usually provides only a modest torque increase, depending on the exact requirements of the particular vehicle in question. It is not essential to provide torque increase at this mesh point.

In use, the gear train 250 is operated insubstantial- liy the same manner as the gear train of the embodiment of Figures 1 to 7 of our international application. It provides six transmission ratios including a direct drive top gear. Automatic ratio changing is provided by means of conventional equipment monitoring vehicle operating parameters. This same control gear also provides for partial engagement of the primary layshaft clutches, and if desired, also the range layshaft clutch (during operation in direct top gear), to provide retardation.

Servicing of the three layshaft clutches is readily performed by removal of an end cover 330 or by means of access openings (not shown) therein.

Among the advantages provided by the abovedescribed embodiment are the simple layout and structure of the gear train and the provision of the three layshaft clutches together at one end of the gear train where they are accessible by means of an end cover. In the situation where direct top gear is engaged, all three layshaft clutches can be used, if desired, for retardation purposes by partial engagement of the clutch means by appropriate actuation of the clutch control system.

The gear train retains the main advantages of the basic features of our multi-layshaft gear train arrangement as described and claimed in our patent applications prior to our international applications, while extending the application of the gear train to meet the requirements of truck and related operations by providing a relatively large number of transmission ratios which can be widely spaced.

Moreover, despite the provision of transmission ratios as high as 1 to 7 (for very low speed work) the kinetic energy of rotating parts within the gearbox during high speed road operation by a truck is kept to a minimum by virtue of the arrangement of the clutches on the primary and range layshafts and by virtue of the direct connection of the layshaft input shafts through the clutch housings or spiders to the outer set of clutch plates, and by virtue of the provision (for at least the highest transmission ratio) of a graded series of four torque-increasing mesh points whereby the torque increase at the last mesh point is only modest.

As a result, the gear train is able to meet the requirements for a six or more ratio truck transmis sion, while offering the facility for automatic control of ratio changing, which has hitherto not been available for truck transmissions. Moreover, the transmission offers facilities for an inbuilt retarder at negligible extra cost. It ia a well known fact that a transmission offering these facilities is not currently available to the automotive industry. Moreover, these facilities are provided by the above embodiment in a transmission package in which the first gear primary layshaft control clutch can be used for drive take-up purposes, whereby the expense of a torque converter or fluid coupling is avoided.

Among modification which could be made in the above embodiment without departing from the scope of the invention are the following: 1. The use of only one primary layshaft, or three or more such layshafts, in combination with the range layshaft. In the former case, four transmission ratios are obtainable if a direct drive top gear is provided.

2. The use of hydraulic control of the layshaft clutches instead of the pneumatic system used in the above embodiment.

3. The use of non-co-axial input and output shafts and the use of a single gear for the input gear and/or a single gear for the output gear.

4. One of the range gears may be constituted by the gear (or gears) which mesh directly with the primary layshaft input gears.

5. As regards transmission ratios, in the above embodiment the difference in transmission ratio between top and bottom gears is a factor of about 7 to 1 and in this case the advantages described above provided by the combination of the range layshaft and the clutch arrangement are particularly significant. However, these advantages are appreciable at ratio difference factors of as low as 5 to 1 and even 4.5 to 1. Such ratios may of course be provided.

6. The first and second range gears need not be on the same shaft nor co-axial.

7. The use of a further gear on the central change speed gear assembly, so as to provide another gear pair, together with a further layshaft having its own input gears in constant mesh therewith. Thus, for example, with three central gear pairs meshing with the gears of four layshafts (one each for two of the gear pairs and two for the other one) a total of twelve transmission ratios can be obtained.

Claims (8)

1. A multi-layshaft constant mesh change speed gear train of the kind comprising at least two layshaft end clutch means each located at one end of a respective coaxial assembly of an inner layshaft portion within an outer tubular layshaft portion, said layshaft clutch means being located within a casing of said gear train and including liquid cooled friction elements, the clutch means being operable to estab lish and interrupt the transmission of drive between said layshaft outer and inner portions; said gear train further comprising a change speed gear assembly, and layshaft gears on said inner and outer layshaft portions in constant mesh with said change speed gear assembly;;characterised in that said change speed gear assembly comprises a coaxial assembly of at least first, second and third independently rotatable change speed gears each providing speed change with respect to its constant meshing engagement with said layshaft gears, said first, second and third change speed gears providing first and second pairs of change speed gears comprising said first and second and said second and third gears respectively; and said layshafts each have their own layshaft input and output gears in direct constant mesh with said pairs of change speed gears, said first pair of change speed gears meshing with the input and output gears of one layshaft and the second pair of change speed gears meshing with the input and output gears of the other layshaft, the arrangement being such that drive can pass with speed change through said second change speed gear between said layshaft output and input gears meshing therewith from any one such layshaft output gear to any such layshaft input gear;; direct drive clutch means being provided between said first and second change speed gears and between said second and third change speed gears to provide a direct drive path therebetween in addition to the drive path therebetween provided by said layshafts whereby at least four drive paths through the change speed gear assembly providing at least four different transmission ratios of the gear train are available from said two layshafts; and said layshaft end clutch means controlling the transmission of torque between said first and second pairs of change speed gears, are both located at the same end of the gear train.

2. The gear train of claim 1 characterised in that said lasyshaft input and output gears of each of said layshafts are mounted respectively on said inner and outer shaft portions, and said layshaft end clutch means comprise first and second assemblies of interleved clutch elements having drive connection means at their outer and inner peripheries respectively, and said first assembly of clutch elements is directly connected to the layshaft inner shaft inner shaft portion and hence to the layshaft input gear.

3. The gear train of any preceding claim characterised in that drive input and drive output shafts for the gear train extend through the casing, said drive input and drive output shafts being arranged co-axially with said change speed gear assembly.

4. The gear train of claim 3 characterised in that said layshaft end clutch means are located at or in the region of the end of said gear casing remote from said drive input shaft.

5. The gear train of any preceding claim characterised by a third layshaft having layshaft input and output gears in direct constant mesh with said second pair of change speed gears, whereby at least six drive paths through the change speed gear assembly are available providing at least six different transmission ratios.

6. A drive transmission comprising the gear train of any preceding claim characterised by a torsionally damped mechanical drive input coupling to transmit drive to the gear train, said layshaft end clutch means being multi-plate interleaved clutch assemblies, and the arrangement being such that the drive take-up from rest is provided by progressive engagement of one of said layshaft clutch means, and automatic or semi-automatic control means being provided to effect changes of transmission ratio by actuation of said clutch means in accordance with monitored operational parameters of the gear train or transmission or of a vehicle driven thereby.

7. The gear train of claim 6 characterised in that said control means for said layshaft end clutch means is operable to effect partial engagement of at least one of said layshaft end clutch means in addition to the clutch means engaged for drive transmission purposes at a given time, whereby retardation of the gear train is effected.

8. A multi-layshaft constant mesh change speed gear train substantially as described herein with reference to the accompanying drawings.