STEPPED RATIO-CHANGE GEARBOX

Abstract: Heavy-duty vehicles such as agricultural tractors fre­ quently have a stepped ratio-change gearbox in series with one or more additional gearboxes one of which is a multi-range and reverse gearbox. To minimise the size, weight and cost of a gearbox providing four ratios, only three gear pairs are em­ ployed. The gearbox has co-axial input and output shafts (11, and 12), a parallel layshaft (13) with three integral gears (25, 28 and 31), two gears (26 and 29) respectively rotateable on and clutchable to the input and output shafts (11 and 12) and mesh­ ing constantly with two of the gears (28 and 31) on the layshaft (13), and a gear (14) rotateable on the input and output shafts (11 and 12), clutchable to either or both of these shafts, and meshing constantly with the other gear (25) on the layshaft (13).

Description: Stepped Ratio-Change Gearbox This invention relates to a stepped ratio-change gearbox, particularly but not exclusively for use in vehicles in which event it may operate in series with one or more additional gearboxes one of which includes reverse gearing. The object of the present invention is to provide a stepped ratio-change gearbox which includes a small number of toothed gears in relation to the number of gear ratios it provides thus minimising its size, weight and cost. According to the invention, a stepped ratio-change gearbox comprising a casing, co-axial input and output shafts, and a layshaft parallel to said shafts, is characterised by a gear co-axial with the input and output shafts, freely rotateable relative to and capable of being selectively driveably connected to each of said shafts, and meshing constantly with a gear fixed to the layshaft. A preferred embodiment of the invention will now he described, by way of example only, with reference to the accompanying drawing which is an axial section through a stepped ratio-change gearbox. referring now to the drawing, a gearbox providing four different speed ratios is adapted to he operatively connected in series with a gearbox (not shown) providing three forward speed ranges and one reverse speed range in an agricultural tractor. If required, there can be operatively connected between the two gearboxes an epicyclic gear unit (not shown) capable of selectively transmitting a direct drive or a low speed ratio drive. The four-ratio gearbox is operable hydraulically while the tractor is on the move, and the epicylic gear unit (if fitted) and the range gearbox are operable manually while the tractor is stationary. The four-ratio gearbox comprises a casing 10, only portions of which are shown, in which there are journal led a tubular input shaft 11, a tubular output shaft 12 co-axial with the shaft 11, and a layshaft 13 parallel to the shafts 11 and 12. Also journalled in the casing 10 is a helically toothed gear 14 formed integrally at the mid-point of a short sleeve 15 of relatively large diameter which co-axially surrounds the adjacent ends of the shafts 11 and 12. The sleeve 15 is supported in two bearings 16 and 17 located in aligned bores 18 and 19 in respective parallel webs 20 and 21 formed integrally with the casing 10 near the mid-point of its length, and the adjacent ends of the shafts 11 and 12 are supported in respective bearings 22 and 23 located in the bore 24 of the sleeve 15. The gear 14 is thus disposed between the webs 20 and 21, and meshes constantly with a gear 25 of almost the same pitch circle diameter which is formed integrally on the layshaft 13 near the mid-point of its length. A helically-toothed pinion 26 is rotateably mounted on a needle bearing 27 on the input shaft 11 and meshes constantly with a wheel 28 formed integrally on the layshaft 13, and a helically-toothed wheel 29 is rotateably mounted on a needle bearing 30 on the output shaft 12 and meshes constantly with a pinion 31 formed integrally on the layshaft 13. The ratio of the pitch circle diameter of the pinion 26 to that of the wheel 28 with which it meshes is substantially diff erent from the ratio of the pitch circle diameter of the pinion 31 to that of the wheel 29 with which it meshes, and said ratios are so chosen, in conjunction with the ratio of the pitch circle diameters of the gears 14 and 25, that the four overall speed ratios provided by the gearbox are arranged substantially in a geometric progression. ()ne synchromesh clutch indicated generally at 32 includes a hub 33 which is driveably connected by splines 34 to the input shaft 11 between the pinion 26 andthe gear 14, and another synchromesh clutch indicated generally at 35 includes a hub 36 which is driveably connected by splines 37 to the output shaft 12 between the wheel 29 and the gear 14. The synchromesh clutch 32 includes an axially slideable internally toothed sleeve 38 shown in neutral position and engageable selectively with a toothed clutch element 39 driveably connected to the adjacent end of the pinion 26 by means of a gear-type coupling 53 and with a toothed clutch element 40 driveably connected to the adjacent end of the sleeve 15 bv means of a gear-type coupling 41. Similarly the synchromesh clutch 35 includes an axially slideable internally toothed sleeve 42 shown in neutral position and engageable selectively with a toothed clutch element 43 driveably connected to the adjacent end of the wheel 29 by means of a gear-type counling 54 and with a toothed clutch element 44 driveably connected to the adjacent end of the sleeve 15 by means of a gear-type coupling 45. The collplings 41 and 45 are necessary to permit assembly, whereas the couplings 53 and 54 help to achieve minimal length of the gearbox. acti of the synchromesh clutches 32 and 35 also includes conventional cone-clutch means 46 for effecting speed synchronisation between the hub 33 or 36 and the appropriate toothed clutch element 39, 40, !+3, or 44 prior to their interconnection by the internally toothed sleeve 38 or 42. Respective hydraulically operable selector forks (not shown) engage in annular grooves 47 and 4# in the peripheries of the internally toothed sleeves 38 and 2. A power take-off shaft 49 passing co-axially through the tubular input and olltmlt shafts 11 and 12 is journalled in a nlain hearing 90 in the output shaft 12 and en named by an oil seal 51 mounted in a collar 52 on the input shaft 11. In operation, first (low) speed ratio is obtained by clutching the pinion 26 to the input shaft 11 and the wheel 29 to the output shaft 12, the gear 14 thus rotating idly; second speed ratio is obtained by shifting the internally toothed sleeve 38 of the synchromesh clutch 32 so as to release the pinion 26 from the input shaft 11 alld driveably connect to said shaft the gear 14; third speed ratio is obtained by shifting the internally toothed sleeves 38 and 42 of both synchromesh clutches 32 and 35 so that the pinion 26 is driveably connected to the input shaft 11 and the gear 14 is driveably connected to the output shaft 12; and fourth (high) speed ratio is obtained by shifting the internally toothed sleeve 38 of the synchromesh clutch 32 so as to release the pinion 26 from the input shaft 11 and driveably connect to said shaft the gear 14, thereby utilising said gear to directly connect the input and output shafts 11 and 12 whilst the layshaft 13 rotates idly. It will be understood that one or more of the gears 25, 28 and 31 on the layshaft 13 can be rigidly secured thereto instead of being formed integrally thereon, and the expression "fixed to" is used herein to cover either arrangement. The selector forks can equally well be manually operable, the power take-off shaft 49 can be omitted and the input and output shafts 11 and 12 made solid, and the clutches need not be of synchromesh type.

Claims: Claims: 1. A stepped ratio-change gearbox comprising a casing (10), co-axial input and output shafts (11 and 12), and a layshaft (13) parallel to said shafts, characterised by a gear (14) co-axial with the input and output shafts (11 and 12), freely rotateable relative to and capable of being selectively driveably connected to each of said shafts, and meshing constantly with a gear (25) fixed to the layshaft (13). 2. A stepped ratio-change gearbox according to claim 1, further comprising a pinion (26) freely rotateable on the input shaft (11) and meshing constantly with a wheel (28) fixed to the layshaft (13), a wheel (29) freely rotateable on the output shaft (12) and meshing constantly with a pinion (31) fixed to the layshaft (13), a clutch device associated with the input shaft (11) and capable of selectively driveably connecting to said shaft either the pinion (26) thereon or the gear (14) co-axial with the input and output shafts (11 and 12), and a clutch device associated with the output shaft (12) and capable of selectively driveably connecting to said shaft either the wheel (29) thereon or the gear (14) co-axial with the input and output shafts (11 and 12). 3. A stepped ratio-change gearbox according to claim 2, wherein the clutch devices are synchromesh clutches (32 and 35). 4. A stepped ratio-change gearbox according to claim 2 or claim 3 wherein elements (40 and 44) of the clutch devices are connected by means of gear-type couplings (41 and 45) to the respective ends of the gear (14) co-axial with the input and output shafts (11 and 12). 5. A stepped ratio-change gearbox according to any one of claims 2 to 4, wherein elements (39 and 43) of the clutch devices are respectively connected by means of gear-type couplings (53 and 54 to the pinion (26) freely rotateable on the input shaft (11) and to the wheel (29) freely rotateable on the output shaft (12). 6. A stepped ratio-change gearbox according to any oneof the preceding claims, wherein the gear (14) co-axial with the input and output shafts (11 and 12) is journalled in the casing (10), and the adjacent ends of said shafts are each journal led in said gear. 7. A stepped ratio-change gearbox according to claim 6, wherein the gear (14) co-axial with the input and output shafts (11 and 12) is journalled in two parallel webs (20 and 21) of the casing (10). 8. A stepped ratio-change gearbox according to any one of the preceding claims, wherein the input and output shafts (11 and 12) are tubular and a power take-off shaft (It7) passes co-axially through them.

Inventors: HODGSON, ERIC CHARLES, WEYMAN, ROGER MICHAEL
Applicants: J.I. CASE EUROPE LIMITED
Application year: 1983
Main classification: F16H3
IPC codes: F16H3