5111 Description of transmission system

The system being described is broadly based on the ZF Man Tip Matic/ZF AS Tronic 12 speed twin countershaft three speed constant mesh gearbox with a front mounted two speed 'splitter' gear change and a rear positioned single stage two speed epicyclic gear 'range' change; however, the basic concept has been modified and considerably simplified in this text.

Gear changes are achieved by four pneumatically operated power cylinders and pistons which are attached to the ends of the three selector rods, there being one power cylinder and piston for each of the splitter and range selector rods and two for the three speed and reverse constant mesh two piece selector rod. Gear shifts are actuated by inlet and exhaust solenoid control valves which supply and release air to the various shift power cylinders as required (see Fig. 5.48).

A multiplate transmission brake with its inlet and exhaust solenoid control valves are provided to shorten the slow down period of the clutch, input shaft and twin countershaft assembly during the gear change process.

A single plate dry friction clutch is employed but instead of having a conventional clutch pedal to control the engagement and disengagement of the power flow, a pneumatic operated clutch actuator with inlet and exhaust solenoid control valves are used. Thus the manual foot control needed for driving away from rest and changing gear is eliminated.

Gradual engagement of the power flow via the clutch when pulling away from a standstill and smooth gear shift changes are achieved via the wheel speed and engine speed sensors, air pressure sensors and the electronic diesel control unit (EDCU): this being part of the diesel engine management equipment, they all feed signals to the electronic transmission control unit (ETCU). This information is then processed so that commands to the various solenoid control valves can be made to produce the appropriate air pressure delivery and release to meet the changing starting and driving conditions likely to be experienced by a transmission system. A gear selector switch control stick provides the driver with a hand control which instructs the electronic transmission control unit (ETCU) to make an up and down gear shift when prevailing engine torque and road resistance conditions are matched.

5.11.2 Splitter gear change stage (Fig. 5.47) Power flows via the clutch and input shaft to the splitter synchromesh dog clutch. The splitter syn-chromesh dog clutch can engage either the left or right hand matching dog clutch teeth on the central splitter gear mounted on the input shaft to obtain a low splitter gear ratio, or to the central third gear

Input

Power flow path

Output

Low range

High range

Input

Power flow path

Output

Low range

High range

Range H selector rod and fork

Selector rod and fork

1 and R shift power cylinder

Fig. 5.47 Twin countershaft 12 speed constant mesh gearbox with synchromesh two speed splitter and range changes

- 3-2 Selector rod and fork Splitter -selector rod and fork

Range H selector rod and fork

Selector rod and fork

1 and R shift power cylinder

Fig. 5.47 Twin countershaft 12 speed constant mesh gearbox with synchromesh two speed splitter and range changes mounted on the mainshaft to obtain the high splitter gear ratio. Power is now able to pass via the twin countershafts to each of the mainshaft constant mesh central gears by way of the constant mesh gears 1, 2, 3 and R.

5.11.3 Constant mesh 1-2-3 and R gear stage

The selection and engagement of one of the sliding dog clutch set of teeth either with R, 1, 2 or 3 floating mainshaft central constant mesh gears permits the drive path to flow from the twin countershaft gears via the mainshaft to the epicylic range change single stage gear train.

5.11.4 Range change gear stage (Fig. 5.47)

Low range gear selection With the synchromesh dog clutch hub moved to the left-hand side, the internal toothed annular gear (A) will be held stationary; the drive from floating mainshaft is therefore compelled to pass from the central sun gear (S) to the output shaft via the planet gear carrier (CP) (see Fig. 5.47). Now since the annular gear is held stationary, the planet gears (P) are forced to rotate on their axes and also to roll around the internal teeth of the annular gear (A), consequently the planet carrier (CP) and output shaft will now rotate at a lower speed than that of the sun gear (S) input.

High range gear selection With the syncromesh dog clutch hub moved to the right-hand side, the annular gear (A) becomes fixed to the output shaft, therefore the drive to the planet gears (P) via the floating mainshaft and sun gear (S) now divides between the planet gear carrier (CP) and the annular gear carrier (CA) which are both fixed to the output shaft (see Fig. 5.47). As a result the planet gears (P) are prevented from rotating on their axes so that while the epicyclic gear train is compelled to revolve as one rigid mass, it therefore provides a one-to-one gear ratio stage.

5.11.5 Clutch engagement and disengagement

With the ignition switched on and the first gear selected the clutch will automatically and progressively take up the drive as the driver depresses the accelerator pedal. The three basic factors which determine the smooth engagement of the transmission drive are vehicle load, which includes pulling away from a standstill and any road gradient, vehicle speed and engine speed. Thus the vehicle's resistance to move is monitored in terms of engine load by the electronic diesel control unit 'EDCU' which is part of the diesel engine's fuel injection equipment, and engine speed is also monitored by the EDCU, whereas vehicle speed or wheel speed is monitored by the wheel brake speed sensors. These three factors are continuously being monitored, the information is then passed on to the electronic transmission control unit 'ETCU' which processes it so that commands can be transferred in the form of electric current to the inlet and exhaust clutch actuator solenoid control valves.

Engagement and disengagement of clutch when pulling away from a standstill (Fig. 5.48) With the vehicle stationary, the ignition switched on and first gear selected, the informed ETCU energizes and opens the clutch solenoid inlet control valve whereas the exhaust control valve remains closed (see Fig. 5.48). Compressed air now enters the clutch cylinder actuator, this pushes the piston and rod outwards causing the clutch lever to pivot and to pull back the clutch release bearing and sleeve. As a result the clutch drive disc plate and input shaft to the gearbox will be disengaged from the engine. As the driver depresses the accelerator pedal the engine speed commences to increase (monitored by the engine speed sensor), the ETCU now progressively de-energizes the solenoid controlled clutch inlet valve and conversely energizes the solenoid controlled exhaust valve. The steady release of air from the clutch actuator cylinder now permits the clutch lever, release bearing and sleeve to move towards the engagement position where the friction drive plate is progressively squeezed between the flywheel and the clutch pressure plate. At this stage the transmission drive can be partially or fully taken up depending upon the combination of engine speed, load and wheel speed.

As soon as the engine speed drops below some predetermined value the ETCU reacts by de-energizing and closing the clutch exhaust valve and energizing and opening the clutch inlet valve, thus compressed air will again enter the clutch actuator cylinder thereby causing the friction clutch drive plate to once more disengage.

Note a built-in automatic clutch re-adjustment device and wear travel sensor is normally incorporated within the clutch unit.

Engagement and disengagement of the clutch during a gear change (Fig. 5.48) When the driver moves the gear selector stick into another gear position

Exhaust valve qpen_ Inlet valve closed (IVC)

Selector rod

Selector — fork

Constant mesh 1-R shift solenoid control valves 1-R shift

Exhaust valve

Exhaust valve qpen_ Inlet valve closed (IVC)

Selector rod

Selector — fork

Exhaust valve

Constant mesh 1-R shift solenoid control valves 1-R shift

Clutch actuator solenoid control valves clutch

Clutch actuator cylinder

Exhaust valve

Fig. 5.48 A simplified electro/pneumatic gear shift and clutch control

Clutch actuator solenoid control valves clutch

Clutch actuator cylinder

Exhaust valve

Fig. 5.48 A simplified electro/pneumatic gear shift and clutch control with the vehicle moving forwards, the ETCU immediately signals the clutch solenoid control valves to operate so that the compressed air can bring about the disengagement and then engagement of the clutch drive plate for sufficient time (programmed time setting) for the gear shift to take place (see Fig. 5.48). This is achieved in the first phase by de-energizing and closing the clutch solenoid exhaust valve and correspondingly energizing and opening the inlet valve, thus permitting the compressed air to enter the clutch actuator cylinder and to release the clutch. The second phase de-energizes and closes the inlet valve and then energizes and opens the exhaust valve so that the clutch release mechanism allows the clutch to engage the transmission drive.

5.11.6 Transmission brake (Figs 5.47 and 5.48) This is a compressed air operated multiplate brake. Its purpose is to rapidly reduce the free spin speed of the driven disc plate, input shaft and twin countershaft masses when the clutch is disengaged thus enabling fast and smooth gear shifts to be made.

When a gear shift change is about to be made the driver moves the gear selector stick to a new position. This is signalled to the ETCU, and one outcome is that the transmission brake solenoid control inlet valve is energized to open (see Fig. 5.48). It thus permits compressed air to enter the piston chamber and thereby to squeeze together the friction disc plate so that the freely spinning countershafts are quickly dragged down to the main shaft's speed, see Fig. 5.47. Once the central gears wedged in between the twin countershafts have unified their speed with that of the mainshaft, then at this point the appropriate constant mesh dog clutch can easily slide into mesh with it adjacent central gear dog teeth. Immediately after the gear shift the transmission brake inlet valve closes and the exhaust valve opens to release the compressed air from the multiplate clutch cylinder thereby preventing excessive binding and strain imposed to the friction plates and assembly.

5.11.7 Splitter gear shifts (Fig. 5.48)

The splitter gear shift between low and high gear ratio takes place though a synchromesh type dog clutch device. Note for all the gear changes taking place in the gearbox, the splitter gears are constantly shifted from low to high going up the gear ratios or from high to low going down the gear ratios. With ignition switched on and the gear selector stick positioned say in low gear, the ETCU signals the splitter solenoid control to close and open the inlet and exhaust valves respectively for the high splitter gear solenoid control, and at the same time to close and open the exhaust and inlet valves respectively for the low splitter gear solenoid control (see Fig. 5.48). The splitter shift power cylinder will now operate, compressed air will be released from the left-hand side and simultaneously compressed air will be introduced to the right-hand side of the splitter shift power cylinder; the piston and selector rod now smoothly shift to the low splitter gear position. Conversely if high splitter gear was to be selected, the reverse would happen to the solenoid control valves with regards to their opening and closing so that the piston and selector rod would in this case move to the right.

5.11.8 Range gear shifts (Figs 5.47 and 5.48)

The range gear shift takes place though a single stage epicyclic gear train and operates also via a synchromesh type dog clutch mechanism.

Going though the normal gear change sequence from 1 to 12 the first six gear ratios one to six are obtained with the range gear shift in the low position and from seventh to twelfth gear in high range shift position, see Fig. 5.47.

With the ignition switched on and the gear selector stick moved to gear ratios between one and six the low range gear shift will be selected, the ETCU activates the range shift solenoid control valves such that the high range inlet and exhaust valves are closed, and opened respectively, whereas the low range inlet valve is opened and exhaust valve is closed, see Fig. 5.48. Hence compressed air is exhausted from the left hand side of the range shift power cylinder and exposed to fresh compressed air on the right-hand side. Subsequently the piston and selector rod is able to quickly shift to the low range position.

A similar sequence of events takes place if the high range gear shift is required except the opening and closing of the valves will be opposite to that needed for the low range shift.

5.11.9 Constant mesh three speed and reverse gear shift (Figs 5.47 and 5.48)

These gear shifts cover the middle section of the gearbox which involves the engagement and disengagement of the various central mainshaft constant mesh gears via a pair of sliding dog clutches. There is a dog clutch for engagement and disengagement for gears 1-R and similarly a second dog clutch for gears 2-3.

To go though the complete gear ratio steps, the range shift is put initially into 'low', then the splitter gear shifts are moved alternatively into low and high as the constant mesh dog clutch gears are shifted progressively up; this is again repeated but the second time with the range shift in high (see Fig. 5.47). This can be presented as range gear shifted into 'low', 1 gear constant mesh low and high splitter, 2 gear constant mesh low and high splitter, and 3 gear constant mesh low and high splitter gear; at this point the range gear is shifted into 'high' and the whole sequence is repeated,

1 constant mesh gear low and high splitter,

2 constant mesh gear low and high splitter and finally third constant mesh gear low and high splitter; thus twelve gear ratios are produced thus:

First six overall gear ratios = splitter gear (L and H)S x constant mesh gears (1, 2 and 3) x range gear low (LR)

Second six overall gear ratios = splitter gear (L and H) x constant mesh gears (1, 2 and 3) x range gear high (HR).

12 OGR = HS x CM (3) x HR High range where OGR = overall gear ratio

CM = constant mesh gear ratio LS/HS = low or high splitter gear ratio LS/HR = low or high range gear ratio

Assume that the ignition is switched on and the vehicle is being driven forwards in low splitter and low range shift gear positions (see Fig. 5.48). To engage one of the three forward constant mesh gears, for example, the second gear, then the gear selector stick is moved into 3 gear position (low splitter, low range 2 gear). Immediately the ETCU signals the constant mesh 3-2 shift solenoid control valves by energizing the 2 constant mesh solenoid control so that its inlet valve opens and its exhaust valve closes; at the same time, the 3 constant mesh solenoid control is de-energized so that its inlet valve closes and the exhaust valve opens (see Fig. 5.48). Accordingly, the 2-3 shift power cylinder will be exhausted of compressed air on the right-hand side, while compressed air is delivered to the left-hand side of the cylinder, the difference in force between the two sides of the piston will therefore shift the 3-2 piston and selector rod into the second gear position. It should be remembered that during this time period, the clutch will have separated the engine drive from the transmission and that the transmission brake will have slowed the twin countershafts sufficiently for the constant mesh central gear being selected to equalize its speed with the mainshaft speed so that a clean engagement takes place. If first gear was then to be selected, the constant mesh 3-2 shift solenoid control valves would both close their exhaust valves so that compressed air enters from both ends of the 2-3 shift power cylinder, it therefore moves the piston and selector rod into the neutral position before the 1-R shift solenoid control valves are allowed to operate.

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