MMT6
Function
Mechanical transmission cables
The shift cable:
- is pulled when 1st, 3rd 5th and back-up gears are selected
- is pushed when 2nd, 4th and 6th gears are engaged.
Mechanical transmission cable for lateral travel:
- is pushed when the gear selector lever is moved between 1st and 2nd gears. Gives maximum lateral movement when the lever is moved to back-up gear
- is depressed when the gear selector lever is moved to positions below 5th and 6th gears.
Power flow
1st gear
The schematic illustration above displays the power flow.
Note! The three shafts are displayed from their actual positions so that their design appears more clear.
When 1st gear is selected, the coupling sleeve for 1st and 2nd gears is moved by a gear selector fork along the synchronizing hub towards the idler wheel for 1st gear. The coupling sleeve and synchronizing hub lock the idler wheel for 1st gear at the lower output (secondary) shaft.
The engine torque is transferred from the clutch via the input shaft to 1st gear and the idler wheel for 1st gear, through the coupling sleeve and via the secondary shaft on to the final drive gear and out to the final drive.
2nd gear
The schematic illustration above displays the power flow.
Note! The three shafts are displayed from their actual positions so that their design appears more clear.
When 2nd gear is selected, the coupling sleeve for 1st and 2nd gears is moved by a gear selector fork along the synchronizing hub towards the idler wheel for 2nd gear. The coupling sleeve and synchronizing hub lock the idler wheel for 2nd gear at the lower output (secondary) shaft.
The engine torque is transferred from the clutch via the input shaft to 2nd gear and the idler wheel for 2nd gear, through the coupling sleeve and via the secondary shaft on to the final drive gear and out to the final drive.
3rd gear
The schematic illustration above displays the power flow.
Note! The three shafts are displayed from their actual positions so that their design appears more clear.
When 3rd gear is selected, the coupling sleeve for 3rd and 4th gears is moved by a gear selector fork along the synchronizing hub towards the idler wheel for 3rd gear. The coupling sleeve and synchronizing hub lock the idler wheel for 3rd gear at the primary shaft.
The engine torque is transferred from the clutch via the input shaft, through the coupling sleeve on the primary shaft to the idler wheel for 3rd gear. It is then transferred to the gear for 3rd gear on the lower output (secondary) shaft, and on to the final drive gear and out to the final drive.
4th gear
The schematic illustration above displays the power flow.
Note! The three shafts are displayed from their actual positions so that their design appears more clear.
When 4th gear is selected, the coupling sleeve for 3rd and 4th gears is moved by a gear selector fork along the synchronizing hub towards the idler wheel for 4th gear. The coupling sleeve and synchronizing hub lock the idler wheel for 4th gear at the primary shaft.
The engine torque is transferred from the clutch via the input shaft, through the coupling sleeve on the primary shaft to the idler wheel for 4th gear. From there it is transferred to the gear wheel for 4th gear on the lower output (secondary) shaft and via the driven gear to the final drive.
5th gear
The schematic illustration above displays the power flow.
Note! The three shafts are displayed from their actual positions so that their design appears more clear.
When 5th gear is selected, the coupling sleeve for 5th and 6th gears is moved by a gear selector fork along the synchronizing hub towards the idler wheel for 5th gear. The coupling sleeve and synchronizing hub lock the idler wheel for 5th gear at the upper output (secondary) shaft.
The engine torque is transferred from the clutch via the input shaft, to 5th gear on the primary shaft for the idler wheel for 5th gear on the upper secondary shaft, through the coupling sleeve and via the final drive gear and out to the final drive.
6th gear
The schematic illustration above displays the power flow.
Note! The three shafts are displayed from their actual positions so that their design appears more clear.
The gear wheel for 4th and 6th gears does not only drive the idler wheel for 4th gear but also drives the idler wheel for 6th gear.
When 6th gear is selected, the coupling sleeve for 5th and 6th gears is moved by a gear selector fork along the synchronizing hub towards the idler wheel for 6th gear. The coupling sleeve and synchronizing hub lock the idler wheel for 6th gear at the upper output (secondary) shaft.
The engine torque is transferred from the clutch via the input shaft, to 5th gear on the primary shaft for the idler wheel for 5th gear on the upper secondary shaft, through the coupling sleeve and via the final drive gear and out to the final drive.
Back-up gear
The schematic illustration above displays the power flow.
Note! The three shafts are displayed from their actual positions so that their design appears more clear.
When back-up gear is selected, the coupling sleeve for back-up gear is moved by a gear selector fork along the synchronizing hub towards the idler wheel for back-up gear. The coupling sleeve and synchronizing hub lock the idler wheel for back-up gear at the upper secondary shaft.
Engine torque is transferred from the clutch via the input shaft through 1st gear, gear wheel on the primary shaft to the idler wheel of 1st gear, which is used as back-up intermediate gear on the lower secondary shaft. The torque is transferred to the back-up idler wheel on the upper output (secondary) shaft, through the coupling sleeve and via the final drive gear and out to the final drive.
The solution of combining the 1st gear idler wheel and the intermediate gear for back-up means that no separate shaft is required for back up gear. Back up gear is synchronized.
Shifting mechanism
The transfer from the control unit to the selector forks is divided between two locations in the transmission:
- The upper gear selector affects back-up gear and 5th and 6th gears
- The lower gear selector affects 1st, 2nd 3rd, and 4th gears.
Four gear selector forks transfer the movement from the selector unit to the relevant coupling sleeve.
The gear selectors are connected to the gear selector forks which are in the shift unit.
Two carrier plates are used to transfer the torque from the longitudinal movement lever to the axially sliding gear selectors.
The carrier plates move vertically between four possible positions. The different gears can be activated in the different positions.
- In the uppermost position the upper gear selector activates back-up gear
- In the next highest position the lower gear selector activates 1st and 2nd gears
- In the next lowest position the lower gear selector activates 3rd and 4th gears
- In the lowest position the upper gear selector activates 5th and 6th gears.
The longitudinal movement lever affects the gear selector so that the correct gear is engaged. The control unit also contains a spring which helps return the gear selector lever to the neutral position.
Differential
When driving straight ahead, the ring gear and differential housing rotate at the same speed as the drive shafts and the driving wheels.
When cornering, the differential gears rotate to compensate for the different speeds of the wheels. Because the small side gears are rotating on the shaft journal, the drive shafts can rotate at different speeds. Power is transferred from the differential housing to the drive shafts via the small side gears in the same way as when driving straight ahead. Both drive wheels still have the same driven power.
Clutch, general
The main tasks of a clutch system are to:
- transfer torque
- disengage during shifting
- provide a comfortable start
- act as an effective shock absorber to eliminate noise and vibration in the chassis and drive train.
Disengagement of the clutch driven plate when shifting is a very important part of this functionality. When the driver presses the clutch pedal, the throwout bearing moves a given distance towards the diaphragm spring and disengages the clutch driven plate. Complete disengagement takes place during the last quarter of clutch pedal travel.
The clutch driven plate is trapped between the flywheel and pressure plate to transfer the torque. The torque is transferred via the clutch to the input shaft.
The torque is transferred across the crankshaft via the flywheel to the clutch driven plate. Half of the torque is transferred via the clutch screw to the clutch housing through lifting springs, over to the pressure plate and to the clutch driven plate. The clutch driven plate then transfers the torque via its hub and splines to the input shaft.
During down-shifting the rotation speed of the clutch driven plate increases. It decreases during up-shifting. The engine speed (RPM) is then synchronized with the vehicle speed when the clutch is released. If there is no disengagement during shifting, there would be abnormal synchronization wear in the transmission.
The clutch is defined by the three characteristic curves:
- pressure plate pressure against the clutch driven plate
- diaphragm spring pressure against the throwout bearing
- the lift of the pressure plate.
The pressure of the pressure plate increases as the clutch driven plate wears. This increase in pressure increases the pressure at the diaphragm spring and with it pedal effort as the clutch driven plate wears. The diaphragm spring moves backwards as the clutch driven plate wears. The clutches are guaranteed to tolerate wear of 1.5 mm to the facing.
Hydraulic clutch control mechanism
In order to change gears, the transmission must be disengaged from the engine. This is done by the clutch pedal. Fluid is transferred from the master cylinder through a pipe to the slave cylinder by depressing the clutch pedal. This then acts on the clutch, disengaging the clutch driven plate.
Sound and vibration is also transferred from the engine. The pipe and hose are balanced so that they absorb vibration.
Self-adjusting clutch (XTend)
When the facing on a clutch driven plate wears, the normal working position of the pressure plate and the disengagement position increase.
On the self-adjusting clutch an adjuster ring responds to the wear of the clutch driven plate. As wear increases, the adjustment ring moves automatically slightly at the next disengagement and adjusts the bearing position of the diaphragm spring. This means that the operating area of the pressure plate, and therefore engagement pressure, remain constant.