332 Positive baulk ring synchromesh unit

The gearbox mainshaft rotates at propellor shaft speed and, with the clutch disengaged, the first motion shaft gear, layshaft cluster gears, and mainshaft gears rotate freely.

Drive torque will be transmitted when a gear wheel is positively locked to the mainshaft. This is achieved by means of the outer synchromesh hub internal teeth which slide over the inner synchro-mesh hub splines (Fig. 3.6(a)) until they engage with dog teeth formed on the constant mesh gear wheel being selected.

When selecting and engaging a particular gear ratio, the gear stick slides the synchromesh outer hub in the direction of the chosen gear (towards the left). Because the shift plate is held radially outwards by the two energizing ring type springs and the raised middle hump of the plate rests in the groove formed on the inside of the hub, the end of the shift plate contacts the baulking ring and pushes it towards and over the conical surface, forming part of the constant mesh gear wheel (Fig. 3.6(b)).

The frictional grip between the male and female conical members of the gear wheel and baulking ring and the difference in speed will cause the baulking ring to be dragged around relative to the inner hub and shift plate within the limits of the clearance between the shift plate width and that of the recessed slot in the baulking ring. Owing to the designed width of the shift plate slot in the baulking ring, the teeth on the baulking ring are now out of alignment with those on the outer hub by approximately half a tooth width, so that the chamfered faces of the teeth of the baulking ring and outer hub bear upon each other.

As the baulking ring is in contact with the gear cone and the outer hub, the force exerted by the driver on the gear stick presses the baulking ring female cone hard against the male cone of the gear. Frictional torque between the two surfaces will eventually cause these two members to equalize

Constant Load Synchro Hub Engagement
Fig. 3.5 Non-positive constant load synchromesh unit
Constant Load Synchromesh Device
Fig. 3.6 (a-c) Positive baulk ring synchromesh unit

their speeds. Until this takes place, full engagement of the gear and outer hub dog teeth is prevented by the out of alignment position of the baulking ring teeth. When the gear wheel and main shaft have unified their speeds, the synchronizing torque will have fallen to zero so that the baulking ring is no longer dragged out of alignment. Therefore the outer hub can now overcome the baulk and follow through to make a positive engagement between hub and gear (Fig. 3.6(c)). It should be understood that the function of the shift plate and springs is to transmit just sufficient axial load to ensure a rapid bringing together of the mating cones so that the baulking ring dog teeth immediately misalign with their corresponding outer hub teeth. Once the cone faces contact, they generate their own friction torque which is sufficient to flick the baulking ring over, relative to the outer hub. Thus the chamfers of both sets of teeth contact and oppose further outer hub axial movement towards the gear dog teeth.

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