GB2100373A

GB2100373A - Power transmission mechanisms

Info

Publication number: GB2100373A
Application number: GB8216574A
Authority: GB
Original assignee: BROWN GEAR INDUSTRIES DAVID Ltd; David Brown Gear Industries Ltd
Current assignee: BROWN GEAR INDUSTRIES DAVID Ltd; David Brown Gear Industries Ltd
Priority date: 1981-06-12
Filing date: 1982-06-08
Publication date: 1982-12-22
Also published as: GB2100373B

Abstract

A variable-ratio power transmission mechanism 10 having two output shafts 16, 17 for driving respective wheels or tracks at opposite sides of a heavy off-the-road vehicle, and having a steering mechanism for varying the relative rotational speeds of said shafts, is known. Where the steering mechanism includes a null shaft 30 which is not driven when straight-line travel is required, said shaft has been driveable mechanically from the input of the power transmission mechanism 10 by two-speed gearing in series with gearing capable of rotating said shaft in either direction. This arrangement requires three dynamically-engageable clutches, and has no differential lock capability. In a simpler arrangement having such a capability, the null shaft 30 is driveable mechanically from the input of the power transmission mechanism 10 by a single dynamically- and progressively-engageable clutch 71 in series with gearing 50-58 capable of rotating said shaft in either direction when said clutch is engaged and of positively locking said shaft against rotation when said clutch is disengaged and both forward and reverse clutches 57, 58 are engaged. <IMAGE>

Description

SPECIFICATION Power transmission mechanisms The invention relates to power transmission mechanisms, and more particularly to a steering mechanism for power transmission mechanisms of the type having two output shafts adapted to be connected to respective propulsion means disposed at opposite sides of a vehicle.

Throughout this specification, the expression "propulsion means" is intended to denote wheeis or tracks, and the expression "fixed to" is intended to mean "integral with or rigidly secured on".

It is known to steer a vehicle having a power transmission mechanism of the type referred to by varying the relative rotational speeds of the output shafts by an amount which is independent of the gear ratio being transmitted by the power transmission mechanism. This gives a larger turning circle in the higher gear ratios, which is in accord with the requirements of the vehicle for stable steering conditions.

In one known arrangement for effecting this, which is disclosed in the U.K. Patent Application No. 41025/78 (Publication No. 2006899A) of ourselves, G. A. Shipley and J. McKnight, a steering mechanism includes two eipcyclic gears driveably interposed between the power transmission mechanism and the respective output shafts, a single null shaft driveably connected by an odd number of gears to a reaction member of one of the epicyclic gears and by an even number of gears to a reaction member of the other of the epicyclic gears, and a drive taken from the input side of the power transmission mechanism comprising a mechanical two-speed gear, constituted by an epicyclic reduction gear lockable by a clutch to give a direct drive, in series with a mechanical gear selectively capable of rotating the null shaft in either direction or of not rotating the null shaft, the last-mentioned gear being constituted by two gear trains having odd and even numbers of gears respectively and two progressively engageable clutches interposed in the respective gear trains and selectively operable so that either or neither of said clutches is engaged. Because the driving shaft of the last-mentioned gear is driven continuously by the two-speed gear, it is not possible to engage both of the progressively engageable clutches simultaneously, and the steering mechanism therefore does not have a differential lock capability, that is to say there is no way of positively interconnecting the output shafts to ensure that they both rotate at precisely the same speed even though one may encounter more drive resistance than the other due to roughness or slipperiness of the terrain being traversed by the vehicle.Furthermore, all three of the clutches referred to in this paragraph need to be engaged dynamically, that is to say when some of their components are in motion under load, and must be designed with this in mind.

The object of the present invention is to provide a power transmission mechanism of the type hereinbefore referred to with a steering mechanism which has a differential lock capability and is nevertheless structurally simpler than that of the afore-mentioned patent application with only one clutch which needs to be engaged dynamically.

According to the invention, in a power transmission mechanism of the type having two output shafts adapted to be connected to respective propulsion means disposed at opposite sides of a vehicle, a steering mechanism for simultaneously causing the rotational speed of either one of said shafts to increase and that of the other of said shafts to decrease includes two epicyclic gears driveably interposed between the power transmission mechanism and the respective output shafts, a single null shaft driveably connected by an odd number of gears to a reaction member of one of the epicyclic gears and by an even number of gears to a reaction member of the other of the epicyclic gears, and a drive taken from the input side of the power transmission mechanism comprising a progressively engageable clutch in series with a gear mechanism selectively capable of rotating the null shaft in either direction or of locking the null shaft against rotation.

A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing of a power transmission mechanism having a steering mechanism.

Referring now to the drawing, a variable-ratio power transmission mechanism 10 takes its input drive by way of a bevel wheel 11 and pinion 12 and a cylindrical pinion 13 and wheel 14 from a conventional hydrodynamic torque converter 1 5 having an input shaft 9 adapted to be connected to an engine, and has two output shafts 1 6 and 1 7 adapted to be connected to respective propulsion means disposed at opposite sides of a vehicle. The torque converter 1 5 can be locked up to give a direct drive by engaging an associated multi-plate friction clutch 18, and includes a freewheel (one-way) clutch 19, in known manner.

A steering mechanism for varying the relative rotational speeds of the output shafts 1 6 and 1 7 includes two epicyclic gears 20 and 21 driveably interposed between the power transmission mechanism 10 and the respective output shafts.

The variable-ratio input member of the epicyclic gear 20 is an annular gear 22, the output member thereof is a planet carrier 23, and a planet pinions 24 journalled in said carrier mesh with both the annular gear 22 and a sun gear 25. Similarly, the variable-ratio input member of the epicyclic gear 21 is an annular gear 26, the output member thereof is a planet carrier 27, and planet pinions 28 journalled in said carrier mesh with both the annular gear 26 and a sun gear 29.A null shaft 30 is driveably connected, by way of a gear 32 fixed to one of its ends and one idler gear 33, to a gear 31 co-axially fixed to the sun gear 25 of the epicyclic gear 20 and, by way of a gear 35 fixed to its other end and two idler gears 36 and 37, to a gear 34 co-axially fixed to the sun gear 29 of the epicyclic gear 21, so as to apply equal and opposite reaction forces to the sun gears 25 and 29 in known manner.

A mechanical drive is provided for rotating the null shaft 30 in either direction so as to effect steering of the vehicle by varying the relative rotational speeds of the output shafts 1 6 and 1 7.

Said drive is taken from the pinion 13, that is to say from the input side of the power transmission mechanism 10, by way of an idler pinion 38 and a gear wheel 39 fixed to a clutch shaft 70. Said shaft is capable of being driveably connected by means of a multi-plate friction clutch 71, which in known manner is engaged progressively by hydraulic means and disengaged by spring means, to a sleeve 72 which is co-axial therewith and constitutes the driving shaft of a gear mechanism selectively capable of rotating the null shaft 30 in either direction or of locking the null shaft 30 against rotation.

Said gear mechanism comprises two aiternative power paths having the same ratio and disposed in parallel between the driving shaft 72 and a driven shaft 49 thereof. One of said paths includes a pinion 50 fixed to the driving shaft 72 and meshing with a wheel 51 fixed to a first sleeve 52 co-axially surrounding one zone of the driven shaft 49, and the other of said paths includes a pinion 53 fixed to the driving shaft 72 and meshing with an idler pinion 54 which in turn meshes with a wheel 55 fixed to a second sleeve 56 co-axially surrounding another zone of the driven shaft 49. The sleeves 52 and 56 are driveably connectible to the driven shaft 49 by respective multi-plate friction clutches 57 and 58 each of which is engaged hydraulically and disengaged by spring means.A gear 59 fixed to the driven shaft 49 meshes constantly with a gear 60 driveably connected to the null shaft 30 by means of a gear-type coupling 61.

In operation, when the vehicle is required to move straight ahead, both of the friction clutches 57 and 58 associated with the driven shaft 49 are engaged contemporaneously whilst the friction clutch 71 associated with the driving shaft 72 is fully disengaged. Accordingly, the clutches 57 and 58 need only be engaged statically, that is to say when all their components are stationary. The null shaft 30 and the sun gears 25 and 29 are then positively locked against rotation so that the output shafts 1 6 and 1 7 are forced to rotate at precisely the same speed even though one may encounter more drive resistance than the other, thereby achieving a differential lock effect.When it is desired to steer the vehicle to either side, the appropriate one of the clutches 57 and 58 is disengaged and oil under pressure is then supplied at a controlled rate to the friction clutch 71. The last-mentioned clutch is therefore engaged dynamically and progressively so that the null shaft 30 is caused to rotate in one direction at an increasing rate, resulting in an increasing steering action due to an increasing variation in the relative rotational speeds of the sun gears 25 and 29 of the epicyclic gears 20 and 21, until said clutch becomes fully engaged and maximum steering action is consequently attained. Thus the progressive engagement of the friction clutch 71 gives a variable turning circle approximating to the characteristics of a wheeled vehicle with conventional steering-wheel controlled steering means separate from its power transmission mechanism. When the vehicle is again required to move straight ahead, the clutch 71 is fully disengaged and said one of the clutches 57 and 58 is then statically reengaged.

In a modification, the driven shaft and the null shaft are one and the same, instead of being separate shafts continuously interconnected by a pair of gears.

Claims

1. In a power transmission mechanism of the type having two output shafts adapted to be connected to respective propulsion means disposed at opposite sides of a vehicle, a steering mechanism for simultaneously causing the rotational speed of either one of said shafts to increase and that of the other of said shafts to decrease includes two epicyclic gears driveably interposed between the power transmission mechanism and the respective output shafts, a single null shaft driveably connected by an odd number of gears to a reaction member of one of the epicyclic gears and by an even number of gears to a reaction member of the other of the epicyclic gears, and a drive taken from the input side of the power transmission mechanism comprising a progressively engageable clutch in series with a gear mechanism selectively capable of rotating the null shaft in either direction or of locking the null shaft against rotation.

2. A steering mechanism according to claim 1, wherein the gear mechanism comprises a driving shaft; a driven shaft; two gear trains having the same ratio, but odd and even numbers of gears respectively, disposed in parallel between said shafts; and two clutches interposed in the respective gear trains and selectively operable so that both of said clutches are engaged contemporaneously whilst the progressively engageable clutch is fully disengaged, or the lastmentioned clutch is progressively engaged and subsequently disengaged whilse either one of said two clutches is disengaged.

3. A steering mechanism according to claim 2, wherein the driven shaft of the gear mechanism is continuously driveably connected to the null shaft by a gear train.

4. A steering mechanism according to claim 3, wherein the gear train drives the null shaft by way of a gear-type coupling.

5. A steering mechanism according to claim 2, wherein the driven shaft of the gear mechanism constitutes the null shaft.

6. A steering mechanism constructed, arranged and adapted to operate substantially as hereinbefore described with reference to, and as illustrated by, the accompanying drawings.