334Split baulk pin synchromesh unit

The synchronizing assembly is composed of two thick bronze synchronizing rings with tapered female conical bores, and situated between them is a hardened steel drive hub internally splined with external dog teeth at each end (Fig. 3.8(a)). Three shouldered pins, each with a groove around its centre, hold the bronze synchronizing cone rings apart. Alternating with the shouldered pins on the same pitch circle are diametrically split pins, the ends of which fit into blind bores machined in the synchronizing cone rings. The pin halves are sprung apart, so that a chamfered groove around the middle of each half pin registers with a chamfered hole in the drive hub.

If the gearbox is in the neutral position, both sets of shouldered and split pins are situated with their grooves aligned with the central drive hub (Fig. 3.8(a and b)).

When an axial load is applied to the drive hub by the gear stick, it moves over (in this case to the left) until the synchronizing ring is forced against the adjacent first motion gear cone. The friction (synchronizing) torque generated between the rubbing tapered surfaces drags the bronze synchronizing ring relative to the mainshaft and drive hub until the grooves in the shouldered pins are wedged against the chamfered edges of the drive hub (Fig. 3.8(c)) so that further axial movement is baulked.

Immediately the input and output shaft speeds are similar, that is, synchronization has been achieved, the springs in the split pins are able to expand and centralize the shouldered pins relative to the chamfered holes in the drive hub. The drive hub can now ride out of the grooves formed around the split pins, thus permitting the drive hub to shift further over until the internal and external dog teeth of both gear wheel hub mesh and fully engage (Fig. 3.8(d)).

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