GB1573097A

GB1573097A - Layshaft gearboxes

Info

Publication number: GB1573097A
Application number: GB5014375A
Authority: GB
Original assignee: Rover Co Ltd
Current assignee: Rover Co Ltd
Priority date: 1976-12-15
Filing date: 1976-12-15
Publication date: 1980-08-13

Description

(54) LAYSHAFT GEARBOXES (71) We, B.L. CARS LIMITED, formerly British Leyland U.K. Limited, a British Com- pany of Leyland House, 174 Marylebone Road, London NW1 5AA, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to constantmesh layshaft gearboxes and more particularly to the provision of a synchromesh arrangement for them.

1) According to the present invention a constant mesh gearbox, with a mainshaft carrying gears which mesh with corresponding gears on a layshaft, has the following features: a) a duplicate mainshaft which carries gears which duplicate those on the mainshaft which also mesh with the corresponding gears on the layshaft; b) a coupling by means of which eadl of the mainshaft gears can be drivably connected to the mainshaft; c) a combined coupling and gear synchronising device by means of which each of the duplicate mainshaft gears can be drivably connected to the duplicate mainshaft; d) a control system which operates so that if no synchronism exists a sensor causes the closing of the engine throttle and the braking of the gearbox input shaft to cause synchronisation of the shafts when an upshift is required or opens the engine throttle to accelerate the input shaft and so cause shaft synchronisation when a downshift is required;and e) pairs of selector forks, each pair being associated with a coupling on the mainshaft and the corresponding coupling on the duplicate mainshaft, each pair of selector forks being movable to cause firstly engagement of a duplicate mainshaft gear with the duplicate mainshaft and then the corresponding mainshaft gear with the mainshaft, the latter occurring after the synchronising device has synchronised the speed of the duplicate mainshaft with the mainshaft.

2) A constant-mesh gear as in paragraph 1) wherein the sensor is connected to a baulking device so that movement of the selector forks can only be completed when the speeds of mainshaft and duplicate mainshaft are synchronised.

3) A constant-mesh gearbox as in paragraph 2) in which: a) the sensor comprises a differential gear assembly; b) the duplicate mainshaft is connected to one drive shaft of the differential gear through a slipping clutch; c) the mainshaft is connected to the other drive shaft of the differential gear; d) the differential casing or housing carries a sensor arm which is associated with first and second switches which control throttle opening and a gearbox input shaft brake respectively; e) when the speeds of the duplicate mainshaft and mainshaft are synchronised then the sensor arm is in a neutral position and both switches are open but when there is a speed differential between the mainshaft and the duplicate mainshaft the sensor arm is caused to move in one direction or the other to cause either the first or second switch respectively to be closed and thus either open the throttle or brake the gearbox input shaft respectively.

4) A constant-mesh gearbox as in paragraph 3) in which the slipping clutch is mounted outboard of the differential gear.

5) A constant-mesh gearbox as in paragraph 3) in which the slipping clutch is mounted between the differential gear and the duplicate mainshaft.

6) A constant-mesh gearbox as in any one 0: paragraphs 3) - 5) in which the baulk device comprises an arm connected to the sensor arm and having its other end or a detent carried by its other end engageable in a recess in the selector rod which carries the selector forks.

How the invention may be carried out will now be described by way of example only and with reference to the drawings accompanying the provisional specification in which: Figure 1 is a diagrammatic representation of a constant-mesh gearbox fitted with the present invention; Figure 2 shows in more detail the speed sensor shown in Figure 1 in one position; Figure 3 shows the speed sensor of Figure 2 in another position; The gearbox illustrated diagrammatically is a conventional constant-mesh layshaft gearbox having an input shaft 1, input gear 2, mainshaft 3 with mainshaft gears 4 (only one shown), and a layshaft 5 carrying a layshaft driving gear 6 and layshaft gears 7 (only one shown). A conventional gearbox of this type would normally be provided with synchromesh units on the mainshaft 3 which are movable axially by selector forks in order to firstly bring the mainshaft gear to be engaged into synchronism and then to cause it to be locked to the mainshaft. Drive would then be obtained through the input gear 2, the layshaft driving gear 6, the layshaft 5 and then the mainshaft 3 via the engaged mainshaft gear (for example gear 4) and its associated layshaft gear (for example gear 7).

In the present invention the normal synchronising couplings are replaced by a simple coupling 8 which does not effect synchronisation. However, each simple coupling 8 is connected via a double selector fork, or back-to-back pair of selector forks 9 to a corresponding combined coupling and synchronising unit 10 which is carried on the duplicate mainshaft 11.

The duplicate mainshaft 11 also carries duplicate main shaft gears 12 which correspond to each of the mainshaft gears 4 and like the latter mesh with the corresponding layshaft gears 7. Similarly gear 1 2a corresponds to the input gear 2 and both mesh with the layshaft gear 6.

Each doubleended selector fork 9 or pair of selector forks is connected to a conventional selector shaft or rod 13 which is movable by a gear change lever 14 in the normal way.

The right-hand end of the duplicate mainshaft 11 is connected to a speed sensor device 17 through a slipping clutch 18 (sensor clutch).

The speed sensor device comprises basically a conventional differential unit which consists of two pinions 19 and 20 carried on a pinion carrier 21 and two drive pinions (or side wheels) 23 and 24 which in a conventional differential would in fact be connected to output shafts, the input of the differential being through the pinion carrier 21. However, in this arrangement the differential is, as it were, inverted because the pinions 23 and 24 act as inputs and the pinion carrier acts as an output.

More specifically, the duplicate mainshaft 11 is connected to the input pinion 24, via the slipping clutch 18, and the mainshaft 3 is drivably connected to the input pinion 23 through transfer gears 25 and 26 so that in the latter case the input gear rotates in the opposite direction to the mainshaft and-at a speed ratio of 1:1.

The pinion carrier 21 is connected to a sensor arm 27 which will adopt a neutral position when the input speeds of the differential are equal, i.e. the pinions 23 and 24 are rotated at the same speed, but which will move in one direction or the other when there is a speed differential between these two input speeds, that is to say between the speed of rotation of the mainshaft and the duplicate mainshaft.

The sensor arm has two functions, the first being to control a baulk mechanism 41 which prevents movement of the gear selector rod 13 unless there is synchronisation, and the second being to control the engine throttle and a clutch brake 28 associated with the input shaft 1 of the gearbox during downshifts or upshifts respectively.

This control is effected by means of first and second electric switches 29 and 30 which are associated with the engine throttle and the brake 28 respectively.

The sensor arm 27 engages a shuttle member 31 which will be in the neutral position shown in Figure 1 when the speed of the mainshaft 3 equals the speed of the duplicate mainshaft 1 1 .

The electric switch 29 controls a first solenoid operated valve 32 and the electric switch 30 controls a second solenoid operated valve 33. The valve 32 controls the flow of compressed air or hydraulic fluid to a first piston and cylinder actuator 34 which is connected to a bell crank lever 35 so that the throttle linkage 36 of the vehicle can be operated to open the engine throttle and thus speed up the input shaft 1 of the gearbox. The second solenoid valve 33 controls the flow of compressed air or hydraulic fluid to a second piston and cylinder actuator 37 which is connected to a bell crank lever 38 whose other end is associated with the brake 28 so that operation of the actuator 37 will cause the brake 28 to be operated and thus retard the speed of rotation of the input shaft 1 of the gearbox.

The actuator 37 is connected to the bell crank lever 38 by a first lost motion connection 39 and the clutch pedal 40 of the vehicle is also connected to the bell crank lever 38 by a second lost motion connection 41. This latter arrangement enables the input shaft 1 of the gearbox to be braked by full depression of the clutch pedal for the purpose of engaging a gear from rest in a manner described in more detail later.

The operation of the sensor, i.e. inverted differential 17, is shown in more detail in Figure 2. A modified arrangement in which the slipping clutch 18 is located between the duplicate mainshaft 11 and the inverted differential 17 is shown in Figure 3.

Immediately prior to commencing an upshift the main shaft 3 and duplicate shaft 11 would be turning at some lower speed than the input shaft depending on the gear ratio.

The sensor 17 would be centralised, i.e. in its neutral position, the shift baulk 41 released, and the switches 29, 30 for the brake 28 and throttle operation open.

To move to neutral the driver releases the throttle, declutches, and moves the change speed lever 14 to the neutral position. Keeping the clutch disengaged, initial movement of the lever 14 to select direct drive brings the synchro cones of the unit 10 into engagement and starts to accelerate the duplicate mainshaft 11. This gives a speed difference between the pinions 23 and 24 which causes the sensor arm 27 to move off-centre thus engaging the shift baulk 41, closing the switch 30 to apply the brake 28, and also ensuring through the throttle over-ride system 35 that the engine throttle is kept closed.

When the sensor arm 27 strikes its limit stop, slip at the clutch 18 commences, and rises to a speed equal to the difference between the gearbox input 1 and output 5 speeds as indicated by the speeds of the duplicate mainshaft 11 and mainshaft 3 respectively. Whilst the sensor arm 27 remains off-centre the travel of the selector rod 13 is limited by the shift baulk 41 to only engagement of the toothed coupling of the synchroniser 10 on the duplicate mainshaft 11, the brake 28 is applied to reduce the speed of the input shaft 1, and the throttle 36 is closed. As the speed of the input shaft 1 falls slip at the clutch 18 decreases until the point of synchronism of the input and output shafts, when slip at the clutch 18 ceases. The sensor arm 27 is still off-centre, the shift baulk is still engaged and the brake switch 30 remains closed. Thus the brake 28 is still applied and deceleration of the gearbox input shaft 1 continues until the difference in the speed of the sensor pinions 23 and 24 causes the sensor arm to move to the centre position, when the shift baulk 41 and brake 28 will be released. A speed difference now exists between the coupling and gear being engaged, but the shift can be completed, the transmission clutch engaged, and power reapplied.

Immediately prior to commencing a downshift from direct drive to the next lower gear the main and duplicate mainshafts 3 and 11 would be running at the same speed as the gears box input shaft. The sensor 17 would be centralised, the shift baulk 41 released and the switches 30 and 29 for the brake and throttle operation open.

To select neutral the driver releases the throttle pedal 40, declutches, moves the gear lever 14 to the neutral position and reengages the clutch. Initial movement of the lever 14 to select the next lower gear brings the synchro cones in the unit 10 into engagement and starts to decelerate the duplicate shaft 11. This gives a speed difference between the sensor pinions 23 and 24 which causes the sensor arm 27 to move off-centre, engaging the shift baulk 41, and closing the switch 29 to open the engine throttle 36 through the throttle over-ride system 35.

When the sensor arm 27 strikes its limit stop slip at the sensor clutch 18 commences and rises to a speed equal to the difference between the gearbox input and mainshaft speeds. Whilst the sensor arm 27 remains off centre the travel of the selector rod 13 is limited, by the shift baulk 41 to engagement of the toothed coupling 10 on the duplicate mainshaft 11, and the engine throttle 36 is kept open to increase the speed of the input shaft 1. As the speed of the input shaft 1 increases, slip at the sensor clutch 18 decreases until the point of synchronism of the gear 4 and engaging coupling 8, when slip at the sensor clutch 18 ceases.

The sensor arm 27 is still off centre and the throttle switch 29 remains closed. Thus the engine throttle is still open and acceleration. of the input shaft 1 and the duplicate shaft 11 continues until the difference in the speed of the sensor pinions 23 and 24 causes the sensor arm 27 to move to the centre position, when the shift baulk 41 will be released and the throttle over-ride 35 cancelled.

A speed difference now exists between the coupling and gear being engaged, but the shift can be completed. It may be necessary to disengage the clutch whilst completing the shift.

Engaging a gear for starting can be regarded as an up-shift, in that the gearbox mainshaft 3 would be stationary and the input shaft 1, before the shift, would be at idling speed. So te engage the gear the input shaft 1 and layshaft ' would need to be brought to rest and would require the application of the brake 28.

The brake 28 could be applied depressing the clutch pedal to its extreme travel, and the selection of the gear made in a normal way.

If the shift or selection is made using the sensor system to control the brake 28, continued deceleration after synchronism coul not be effected, and so the brake switch 30 would remain closed and the shift baulk 41 would still be engaged. This would prevent engagement of the gear, but careful design of the baulk mechanism 41 could probably be applied so that a little extra effort on the change speed lever would overcome the baulk 41 and at the same time would centralise the sensor and release the brake 28.

Such a scheme would, due to the special baulk detent on the low gear selector and baull rods, prevent engagement of the higher (lower numerical) gears for starting, and so prevent abuse of the transmission clutch.

The synchromesh arrangement of the present invention has various advantages. Because synchronisation is concerned with the rotating masses on the duplicate mainshaft only, and is completely independent of the main gearbox and transmission inertias, the shifting loads will be lower than those for normal synchro-mesh.

The speed differences resulting from centralising of the sensor arm 27 should assist in avoiding "end on" tooth contacts during final engagement of the gear couplings when completing a shift.

The system should be particularly suitable for gearboxes with wide ratio steps, say 1:6 upwards, and for high torque capacity units, where, due to the larger rotating masses involved, conventional synchro-mesh may not be satisfactory, i.e. it is particularly useful in gearboxes used in trucks or buses.

The system should be easily adaptable for pre-selection and automatic shifting.

WHAT WE CLAIM IS: 1. A constant-mesh gearbox with a mainshaft carrying gears which mesh with corresponding gears on a layshaft, which has the following features: a) a duplicate mainshaft which carries gears which duplicate those on the mainshaft which also mesh with the corresponding gears on the layshaft; b) a coupling by means of which each of the mainshaft gears can be drivably connected to the mainshaft; c) a combined coupling and gear synchronising device by means of which each of the duplicate mainshaft gears can be drivably connected to the duplicate mainshaft; d) a control system which operates so that if the main shaft and the duplicate mainshaft are not synchronised a sensor causes the closing of the engine throttle and the braking of the gearbox input shaft to cause synchronisation of the shafts when an upshift is required, or opens the engine throttle to accelerate the input shaft and so cause shaft synchronisation when a downshift is required; and e) pairs of selector forks, each pair being associated with a coupling on the mainshaft and the corresponding coupling on the duplicate mainshaft, each pair of selector forks being movable to cause firstly engagement of a duplicate mainshaft gear with the duplicate mainshaft and then the corresponding mainshaft gear with the mainshaft, the latter occurring after the shaft synchronising device has synchronised the speed of the duplicate mainshaft with the mainshaft.

2. A constant-mesh gearbox as claimed in Claim 1 wherein the sensor is connected to a baulking device so that movement of the selector forks can only be completed when the speeds of the mainshaft and duplicate mainshaft are synchronised.

3. A constant-mesh gearbox as claimed in Claim 2 in which: a) the sensor comprises a differential gear assembly; b) the duplicate mainshaft is connected to one drive shaft of the differential gear through a slipping clutch; c) the mainshaft is connected to the other drive shaft of the differential gear; d) the differential casing or housing carries a sensor arm which is associated with first and second switches which control throttle opening and a gear input shaft brake respectively; e) when the speeds of the duplicate mainshaft and mainshaft are synchronised then the sensor arm is in a neutral position and both switches are open but when there is a speed differential between the mainshaft and the duplicate mainshaft the sensor arm is caused to move in one direction or the other to cause either the first or second switch respectively to be closed and thus either open the throttle or brake the gearbox input shaft respectively.

4. A constant-mesh gearbox as claimed in Claim 3 in which the slipping clutch is mounted outboard of the differential gear.

5. A constant-mesh gearbox as claimed in Claim 3 in which the slipping clutch is mounted between the differential gear and the duplicate mainshaft.

6. A constant-mesh gearbox as in any one of Claims 3-5 in which the baulk device comprises an arm connected to the sensor arm and having its other end or a detent carried by its other end engageable in a recess in the selector rod which carries the selector forks.

7. A constant-mesh gearbox substantially as hereinbefore described with reference to and as shown in the drawings accompanying the pro visional specification.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. shifting loads will be lower than those for normal synchro-mesh. The speed differences resulting from centralising of the sensor arm 27 should assist in avoiding "end on" tooth contacts during final engagement of the gear couplings when completing a shift. The system should be particularly suitable for gearboxes with wide ratio steps, say 1:6 upwards, and for high torque capacity units, where, due to the larger rotating masses involved, conventional synchro-mesh may not be satisfactory, i.e. it is particularly useful in gearboxes used in trucks or buses. The system should be easily adaptable for pre-selection and automatic shifting. WHAT WE CLAIM IS:

1. A constant-mesh gearbox with a mainshaft carrying gears which mesh with corresponding gears on a layshaft, which has the following features: a) a duplicate mainshaft which carries gears which duplicate those on the mainshaft which also mesh with the corresponding gears on the layshaft; b) a coupling by means of which each of the mainshaft gears can be drivably connected to the mainshaft; c) a combined coupling and gear synchronising device by means of which each of the duplicate mainshaft gears can be drivably connected to the duplicate mainshaft; d) a control system which operates so that if the main shaft and the duplicate mainshaft are not synchronised a sensor causes the closing of the engine throttle and the braking of the gearbox input shaft to cause synchronisation of the shafts when an upshift is required, or opens the engine throttle to accelerate the input shaft and so cause shaft synchronisation when a downshift is required; and e) pairs of selector forks, each pair being associated with a coupling on the mainshaft and the corresponding coupling on the duplicate mainshaft, each pair of selector forks being movable to cause firstly engagement of a duplicate mainshaft gear with the duplicate mainshaft and then the corresponding mainshaft gear with the mainshaft, the latter occurring after the shaft synchronising device has synchronised the speed of the duplicate mainshaft with the mainshaft.