33 Gear synchronization and engagement

The gearbox basically consists of an input shaft driven by the engine crankshaft by way of the clutch and an output shaft coupled indirectly either

Fig. 3.4 Five speed and reverse single stage synchromesh gearbox with integral final drive (transaxle unit)

through the propellor shaft or intermediate gears to the final drive. Between these two shafts are pairs of gear wheels of different size meshed together.

If the gearbox is in neutral, only one of these pairs of gears is actually attached rigidly to one of these shafts while the other is free to revolve on the second shaft at some speed determined by the existing speeds of the input and output drive shafts.

To engage any gear ratio the input shaft has to be disengaged from the engine crankshaft via the clutch to release the input shaft drive. It is then only the angular momentum of the input shaft, clutch drive plate and gear wheels which keeps them revolving. The technique of good gear changing is to be able to judge the speeds at which the dog teeth of both the gear wheel selected and output shaft are rotating at a uniform speed, at which point in time the dog clutch sleeve is pushed over so that both sets of teeth engage and mesh gently without grating.

Because it is difficult to know exactly when to make the gear change a device known as the syn-chromesh is utilized. Its function is to apply a friction clutch braking action between the engaging gear and drive hub of the output shaft so that their speeds will be unified before permitting the dog teeth of both members to contact.

Synchromesh devices use a multiplate clutch or a conical clutch to equalise the input and output rotating members of the gearbox when the process of gear changing is taking place. Except for special applications, such as in some splitter and range change auxiliary gearboxes, the conical clutch method of synchronization is generally employed.

With the conical clutch method of producing silent gear change, the male and female cone members are brought together to produce a synchronizing frictional torque of sufficient magnitude so that one or both of the input and output members' rotational speed or speeds adjust automatically until they revolve as one. Once this speed uniformity has been achieved, the end thrust applied to the dog clutch sleeve is permitted to nudge the chamfered dog teeth of both members into alignment, thereby enabling the two sets of teeth to slide quietly into engagement.

3.3.1 Non-positive constant load synchromesh unit (Fig. 3.5(a, b and c))

When the gear stick is in the neutral position the spring loaded balls trapped between the inner and outer hub are seated in the circumferential groove formed across the middle of the internal dog teeth (Fig. 3.5(a)). As the driver begins to shift the gear stick into say top gear (towards the left), the outer and inner synchromesh hubs move as one due to the radial spring loading of the balls along the splines formed on the main shaft until the female cone of the outer hub contacts the male cone of the first motion gear (Fig. 3.5(b)). When the pair of conical faces contact, frictional torque will be generated due to the combination of the axial thrust and the difference in relative speed ofboth input and output shaft members. If sufficient axial thrust is applied to the outer hub, the balls will be depressed inwards against the radial loading of the springs. Immedi ately the balls are pushed out of their groove, the chamfered edges of the outer hub's internal teeth will then be able to align with the corresponding teeth spacing on the first motion gear. Both sets of teeth will now be able to mesh so that the outer hub can be moved into the fully engaged position (Fig. 3.5(c)).

Note the bronze female cone insert frictional face is not smooth, but consists of a series of tramline grooves which assist in cutting away the oil film so that a much larger synchronizing torque will be generated to speed up the process.

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