374Lay cock simple gear train overdrive

Description (Fig. 3.29) The overdrive unit is attached to the rear of the gearbox and it consists of a constant mesh helical toothed epicyclic gear train which has a central sunwheel meshing with three planet gears which also mesh with an internally toothed annulus gear. The planet gears are supported on a carrier driven by the input shaft whilst the annulus is attached to the output shaft via a carrier forming an integral part of both members. A double cone clutch selects the different ratios; when engaged one side of the clutch provides direct drive and when the other side is used, overdrive.

Direct drive (Fig. 3.29) Direct drive is obtained when the inner cone clutch engages with the outer cone of the annular gear. Power will then be conveyed via the unidirectional clutch to the output shaft by means of the rollers which are driven up inclined ramps and wedged between the inner and outer clutch members. When the vehicle overruns the engine, the output shaft will try to run faster than the input shaft and so tend to release the unidirectional clutch rollers, but this is prevented by the inner cone clutch locking the sun gear to the annulus, thereby jamming the sun, planet and annular epicyclic gear train so that they cannot revolve relative to each other.

Engagement of the inner cone clutch to the external cone surface of the annulus gear is provided by four stationary thrust springs (only one shown) which are free to exert their axial load against a thrust plate. This in turn transfers thrust by way of a ball bearing to the rotating cone clutch support member splined to the sun gear sleeve. This overrun and reverse torque will be transmitted between the engine and transmission in direct drive.

Owing to the helical cut teeth of the gear wheels, an end thrust exists between the planet gears and the sun gear during overrun and reverse which tends to push the latter rearwards. Therefore, additional clamping load between the cone clutch faces is necessary.

Overdrive (Fig. 3.29) When overdrive is engaged, the cone clutch, which is supported on the splined sleeve of the sun gear, is moved over so that its outer friction facing is in contact with the internal cone brake attached to the casing. Consequently the sun gear is held stationary. With the sun gear held still and the input shaft and planet carrier rotating, the planet gears are forced to rotate about their own axes and at the same time roll around the fixed sun gear, with the result that the annulus gear is driven at a faster speed than the input shaft. This causes the unidirectional clutch outer member (annular carrier) to overrun the inner member (planet carrier) so that the wedged rollers on their ramps are released. Pulling the cone clutch away from the annulus cone and into fric-tional contact with the brake casing cone against the axial load of the six thrust springs is achieved by means of hydraulic oil pressure. This pressure acts upon two slave pistons (only one shown) when a valve is opened by operating the driver controlled selector switch.

The outward movement of the slave pistons, due to the hydraulic pressure, draws the stationary thrust plate, ball bearing and rotating clutch member away from the annular cone and into engagement with the outer brake cone, thereby locking the sun gear to the casing. Of the helix angle of the gear teeth, the torque reaction tends to push the sun gear forward so that extra end

Fig. 3.29 Laycock single epicycle overdrive

thrust is necessary to maintain sufficient clamping thrust between the frictional faces of the cones in the brake position.

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