M56

Design

Clutch, general




The main task of a clutch system is to transfer torque, disengage during shifting, allow comfortable starting and to act as an efficient vibration damper to eliminate noise and vibration in the chassis and drive train.
The clutch is between the engine and transmission. It consists mainly of a pressure plate and a clutch driven plate. The clutch transfers torque to and from the transmission and then on to drive the wheels.
The clutch is a single disc dry clutch. The clutch disc is connected to the transmission input shaft on the transmission.

Clutch driven plate




The clutch driven plate is the section that transfers the torque to the input shaft and into the transmission. It also plays a major role in ensuring gentle and comfortable starting.
The clutch driven plate has no damper and friction element. Instead these functions are integrated into the dual mass flywheel.
The clutch driven plate center is greased internally when the clutch driven plate is installed at the factory. This reduces the friction between the clutch driven plate and the input shaft to make shifting easier.
When replacing the clutch driven plate, the input shaft must be lubricated. See the instructions for replacing the clutch.

Concentric slave cylinder (CSC)




The clutch control mechanism is fully hydraulic. The slave cylinder is inside the clutch cover. There is no mechanical transfer (fork) inside the clutch cover from an external slave cylinder.
The concentric slave cylinder (CSC) means that the slave cylinder is integrated with the throwout bearing.
The concentric slave cylinder (CSC) is extremely efficient and reliable and the throwout bearing is accurately centered.
The unit is secured to the transmission cover using two screws. The bearings and friction surfaces do not need to be lubricated. The bearing on the concentric slave cylinder (CSC) is self-centering against the clutch diaphragm spring.
The hydraulic line from the master cylinder is connected to the concentric slave cylinder (CSC) via an adapter. There is a bleed nipple on the adapter.
There are ratios built-in to the system to reduce the force required to transfer torque:
- pedal effort
- hydraulic force
- clutch finger force.
The force is reduced from approximately 7800 N at the clutch to approximately 80 N at the pedal.
If the clutch pedal returns too quickly, the drive train could be damaged by the high torque. There is a shock load limiter in the clutch to avoid this. The shock load limiter chokes the flow between the concentric slave cylinder (CSC) and the master cylinder if there is a danger of the excessive torque. The shock load limiter is in the angle adapter, under the bleed nipple.
The concentric slave cylinder (CSC) is approximately 8 mm shorter for M66 than M56.

Dual mass flywheel




The dual mass flywheel reduces noise from the transmission and improves driveablity.
The dual mass flywheel also helps reduce the load on the crankshaft bearings. It also improves shift quality because the flywheel mass on the clutch driven plate is lower.
The component parts of the dual mass flywheel can turn in relation to each other. The amount by which they turn depends on the engine load. On a dual mass flywheel the component parts can turn by up to 60 degrees in both directions. A certain amount of turning can be felt between the component parts if a dual mass flywheel is turned.

Transmission, general




M56 is a five gear manual transmission. The transmission and final drive are integrated in the same housing. The pressed aluminum housing halves are called the transmission housing and the clutch cover.
The movements of the gear selector lever are transferred to the transmission via two heavy duty mechanical cables. This reduces the effort required when shifting and gives a distinct feel for each gear position. All gears are synchronized. The oil is synthetic so the transmission does not require oil changes.
In the transmission there are three short parallel shafts on which the gear wheels are mounted. There is also a differential with a ring wheel. The bearings for the shafts and differential in the housing sections are conical roller bearings. All idler wheels have needle bearings. Both output shafts are constantly engaged in the final drive.
There are four selector rods in the transmission. There are gear selector forks on three of these selector rods. These three can move axially in the transmission.
The gear shift selector on the control lever assembly which is turned by the selector sleeve, transfers the shift motion directly or indirectly via a flange or a lever. Each synchronizing hub is joined to its shaft by splines and rotates with it.
The fourth selector rod is joined to the lateral movement lever. These are connected to the gear selector lever by mechanical cables.

Transmission, internal components




- A: Primary shaft
- B: Lower output (secondary) shaft
- C: Upper output (secondary) shaft.





The gear wheels are on three short and parallel shafts. These are the input shaft and two output (secondary) shafts which are both constantly engaged in the gear on the final drive. This means only two meshings for each forward gear. Despite having three shafts, this gives the same efficiency as two shaft transmissions.
The design using three short shafts gives the shafts rigidity which minimizes the risk of bending under high loads and consequent incorrect meshing.
All gears, including back-up (reverse), are synchronized. This means that back-up (reverse) gear can be selected without uncomfortable scraping noises from the gear wheels, even if the gear wheels in the transmission are not completely still on engagement.
The gear wheels, idler wheels and synchronizing units are on each shaft as follows:
- Primary shaftconsists of an input shaft with the gears for 1st and 2nd as well as the gears for 5th, the idler wheel for 4th gear, the synchronizing unit for 3rd and 4th and the idler wheel for 3rd gear
- Lower output (secondary) shaftconsists of an output shaft for gears 1, 2, 3, 4, the gear wheel for final drive, the idler wheel for 1st gear (also operates as the intermediate gear for back-up (reverse) gear), the synchronizing unit for 1st-2nd, the idler wheel for 2nd gear, and the gear wheels for 3rd and 4th
- Upper output (secondary) shaftconsists of an output shaft for gear 5 and back-up (reverse), gear wheel for the final drive for back-up gear, the idler wheel for back-up gear, the synchronizing unit for back-up and 5th gear and the idler wheel for 5th gear.
The teeth are treated after hardening so that they work as quietly as possible. By the use of tall, narrow teeth and optimized engagement angles a high number of teeth can engage. Back-up (reverse) gear also has helical teeth and operates as quietly as the other gears.
The ring wheel and gear wheel for final drive are on the differential housing. The differential housing includes the differential gears (with and without driveshaft splines) and a planetary shaft with lock.

Gear selector




The transmission has four selector rods. There are gear selector forks on three of these selector rods. The shift motion is transferred by double mechanical cables from the gear selector lever to the transmission. The selector pin on the differential gear allows for longitudinal shift movements. The gear selector and gearshift gate are on the input gear selector rod which is connected to the lever for lateral movement. The back-up (reverse) inhibitor prevents back-up (reverse) gear from being accidentally selected when shifting from 5th to 4th gear.
The mechanical transmission cable for lateral travel is adjustable. The mechanical transmission cable for longitudinal travel is not adjustable. If there is a fault or damage in/to one of the mechanical cables, both must be replaced at the same time.
The inner gear selector control in the transmission is in sliding bearings which are lubricated by the transmission fluid.

Back-up (reverse) inhibitor
There is a spring-loaded back-up (reverse) inhibitor connected to the gearshift gate which prevents the gear selector lever from moving to back-up (reverse) gear from 5th gear. The back-up (reverse) inhibitor normally rests on a lug on the gearshift gate.

Differential




The differential distributes power equally between the drive wheels, even if they are rotating at different speeds.
The differential consists of the differential housing, large and small differential gears, shaft journals and a thrust washer.

Synchronization
The synchronizing units are positioned on the shafts in the transmission as follows:
- The synchronizing unit for 1st - 2nd gear is on the lower output (secondary) shaft (output shaft for gears 1, 2, 3, 4)
- The synchronizing unit for 3rd - 4th gear is on the primary shaft
- The synchronizing unit for 5th/back-up (reverse) gear is on the upper output (secondary) shaft (output shaft for 5th and back-up (reverse) gear).
The synchronizing hub assembly consists of a coupling sleeve, flange and the hub. Each synchronizing hub is joined to its shaft by splines and rotates with it.
There are idler wheels on both sides of each synchronizing hub. The idler wheels rotate freely on the shaft. Each idler wheel is constantly engaged in its gear.
The synchronizing rings expand when heated by the same amount as the components they are in contact with. As a result no safety margin is required to counter expansion.
The idler wheels for single and double synchronization have no cones. An inner ring on the synchronizer unit performs this function instead.
The synchronizer unit is a modular system. This makes it easy to upgrade from a single synchronizer to double synchronization by replacing the synchronizer kit.




Single synchronization, which has one friction surface, is on:
- back-up (reverse) gear
- 4th gear
- 5th gear.
The outer ring (to the left in the illustration) is manufactured in pressed steel. The inner ring is pressed steel and the friction surface is coated with sintered brass.




Double synchronization, two friction surfaces, is on:
- 3rd gear.
The outer ring (to the left in the illustration) is manufactured in pressed steel. The sealing ring is pressed steel and the friction surfaces are coated with sintered brass. The inner ring is machined steel.




Triple synchronization, three friction surfaces, is on:
- 1st gear
- 2nd gear.
The outer ring (to the left in the illustration) is manufactured in pressed steel. The sealing ring is pressed steel and the friction surfaces are coated with sintered brass. The inner ring is pressed steel and the inner friction surface is coated with sintered brass.