Automatic gearbox (Stepspeed Em-CVT) (Description): Description and operation (page 2 of 3)

Torsion damper

The transmission is driven from the engine via a torsion damper. The torsion damper is attached to the flywheel with six bolts and is constructed similar to a conventional clutch drive plate, but without the clutch lining. The torsion damper has a splined hub which engages with the gearbox input shaft. The hub is located on an inner plate which contains compression springs. Engine power is transmitted from the flywheel and damper attachment to the hub via the compression springs which absorb torsional vibrations from the engine and provide a smooth power delivery to the gearbox.

Planetary gear set

The planetary gear set enables the gearbox to provide a rotational output to the drive shafts in two directions to provide the vehicle with forward and reverse selections.

Engine torque is transmitted from the engine and the torsional damper to the input shaft which is attached to the planet carrier.

When forward is selected, the carrier is connected directly to the sunwheel by the drive clutch. The epicyclic gear set rotates as one unit and engine torque is passed directly to the primary pulley.

When reverse is selected, the annulus of the planetary gear set is held stationary by the reverse clutch. Three pairs of planet gears then drive the sunwheel in the opposite direction rotating the primary pulley in the reverse direction.

Clutches

Each of the two clutches comprise a multiplate wet clutch pack. Each pack has three friction plates providing six friction surfaces. Hydraulic pressure controls the clutches to allow the vehicle to move away smoothly regardless of the degree of throttle opening. Fluid from the fluid cooler is directed to the clutch plates to prevent overheating of the friction surfaces.

Pulleys and steel belt

The major drive components of the gearbox are a pair of vee shaped pulleys and a steel drive belt. Each pulley comprises one fixed sheave and one moveable sheave. Both moveable sheaves are positioned diagonally opposite each other to prevent misalignment of the belt during shift changes. Each moveable sheave is operated by a hydraulic cylinder and piston, with hydraulic pressure controlled by the hydraulic control unit. The moveable sheaves are located on ball splines which prevents them rotating in relation to the fixed sheaves.

Rotation of the planetary gear set rotates the primary pulley. The V-belt transfers the primary pulley rotation to the secondary pulley whose torque and speed is controlled by the position of the V-belt on the two pulleys.

A 24 mm wide steel, push type V-belt is used to transfer engine torque between the two pulleys. The belt is cooled and lubricated by a fluid jet.

The belt comprises two steel bands each constructed from ten steel strips. The steel bands contain approximately 350 steel segments which abut each other to allow the belt to transmit torque by compression. The belt has several different thicknesses of steel segments which reduce the noise of the segments contacting the pulleys by changing the harmonic frequencies.

Pinion shaft

The pinion shaft, which is supported on two tapered bearings, provides location for two gears which provide a two step helical gear reduction between the secondary pulley and the differential crownwheel and provide the correct rotational direction of the drive shafts.