25.10.
Gear Synchronization and Engagement
The gearbox primarily contains an input shaft and an output shaft. The input shaft is driven by the engine crankshaft through the clutch and the output shaft is coupled indirectly either through the propeller shaft or intermediate gears to the final drive. Pairs of gear wheels of different size are in mesh between these two shafts. In the neutral position of the gea~box only
Fig. 25.20. Five speed and reverse single stage synchromesh gearbox with integral final drive.
one of these pairs of gears is actually attached rigidly to one of these shafts while the other is free to revolve on the secondary shaft at some speed based on existing speeds of the input and output drive shafts.
To engage any gear, first the input shaft is disengaged from the engine crankshaft. But the angular momentum of the input shaft, clutch drive plate and gear wheels keeps them revolving. Then the gear changing technique must judge the speeds of the dog teeth of both the gear wheels selected and output shaft. When they rotate at a unfiorm speed, the dog clutch sleeve is pushed over so that both sets of teeth engage and mesh gently without grating. The synchromesh incorporated in the system applies a friction clutch braking action between the engaging gear and drive hub of the output shaft to unity their speeds before permitting the dog teeth of both members to engage.
Synchromesh devices utilise a multi-plate clutch or a conical clutch to equalize the speeds of the input and the output rotating members of the gearbox during the process of gear changing. The conical clutch method of synchronization is generally used for producing silent gear change. In this method, the male and female cone members are brought together to produce a synchronizÂing frictional torque of sufficient magnitudes to automatically adjust speeds of both the input and output members until they revolve as one. Once this speed uniformity is attained, the end thrust applied to the dog clutch sleeve permits to mesh quietly the chamfered dog teeth of both members into alignment.
25.10.1.
Non-positive Constant Load Synchromesh Unit
In the neutral position of the gear, the spring loaded balls trapped between the inner and outer hub sit in the circumferential groove formed across the middle of the internal dog teeth (Fig. 25.21A). If the gear stick is shifted into say top gear, 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. 25.21B). During the contact of the pair of conical faces frictional torque is generated as a result of the combination of the axial thrust as well as the differences in relative speed of the input and output shaft members. When the axial thrust applied to the outer hub becomes sufficient, the balls are depressed inwards against the radial loading of the springs. ConsequentÂly the balls are pushed out of their groove, and the chamfered edges of the outer hubs internal teeth align with the corresponding teeth spacing on the first motion gear. Both sets of teeth now mesh and the outer hub is moved into the fully engaged position (Fig. 25.21C). The bronze female cone frictional face is not smooth and contains a series of tramline grooves to help in cutting away the oil film so that a much larger synchronizing torque is generated to speed up the process.
25.10.2.
Positive Bulk Ring Synchromesh Unit
When the gearbox main shaft rotates at the speed of the propeller shaft and, the clutch is disengaged, the primary shaft gear, lay-shaft cluster gears, and main shaft gears rotate freely. Drive torque is transmitted when a gear wheel is positively locked to the main shaft by sliding the outer synchromesh hub internal teeth over the inner synchromesh hub splines (Fig. 25.22A) 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 so that the end of the shift plate contacts the baulking ring and pushes it towards and over the conical surfaces forming part of the constant mesh gear wheel (Fig. 25.22B).
Fig. 25.21. Non-positive constant load synchromesh unit. A. Neutral disengaged position. B. Synchronization position. C. Engaged position.
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 equalises the speeds of these two members. Until this takes place, full engagement of the gear and outer hub dog teeth is not established because the baulking ring teeth are out of alignment with those on the outer hub by approximately half a tooth width. When the gear wheel and main shaft speeds are unified, the synchronizing torque falls 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 the hub and gear (Fig. 22.22C). The function of the shift plate and springs is to transmit just enough axial loads to ensure a rapid union of the mating cones so that immediate misalignment of the baulking ring dog teeth with their corresponding outer hub teeth takes place. Once the contact of cone faces is established, their own friction torque is generated, which is sufficient to flick the baulking ring over, relative to the outer hub. Then the chamfers of both sets of teeth contact and oppose further outer hub axial movement towards the gear dog teeth.
25.10.3.
Positive Baulk Pin Synchromesh Unit
If the selector fork synchronizing sleeve is moved to the left (Fig. 25.23A and B), the female (internal) cone is forced to move into contact with the male (external) cone on the drive gear. Consequently, frictional torque synchronizes (unifies) the input and output speeds. Before synchronization, the collars on the three thrust pins (only one shown) are pressed hard into the enlarged position of the slots (Fig. 25.23C) in the synchronizing sleeve because of the frictional drag due to dissimilar speeds. Under these conditions, unless extreme pressure is exerted, the dog teeth cannot be crushed by forcing the collars into the narrow portion of the slots. Once the speeds of the synchromesh hub and drive gear are equaled (synchronized) the collars tend to float in the enlarged portion of the slots, so that only the pressure of the spring loaded balls to be overcome. The collars then slide easily into the narrow portion of the slots (Fig. 25.23D)
Fig. 25.22. Positive baulk ring synchromesh unit. A. Disengaged position. B. Synchronization position. C. Engaged position.
allowing the synchronizer hub dog teeth to shift in to mesh with the dog teeth on the diving gear.
