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SIMPLE MECHANIC
Page 88
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byPreceptor Constant-mesh gearbox
A FAIR degree of skill and judgment is required from the idriver of a motor vehicle to obtain a silent gear change from a sliding mesh or "crash" gearbox, described in the last article. A development of the "crash" gearbox, which makes gear changing easier, is the constant-mesh type.
In the constant-mesh gearbox the 'layout of the shafts and pinions is very similar to that on a sliding mesh type. The output pinions on the mainshaft, however, are always in mesh with their corresponding pinions on the layshaft.
The main-shaft pinions revolve freely on bushes until the driver engages a gear when a dog clutch, which is splined to the mainshaft, engages with the selected pinion, locking it to the mainshaft to give the required gear ratio.
With this arrangement the pinion teeth are prevented from clashing and so becoming
damaged. All the dogs on one gear are engaged at the same time instead of only a pair of teeth, as on the sliding mesh box. The dog teeth must be synchronised before engagement can be made, but this is easier for the driver to achieve than with the -crashbox.
Most modern vehicles are fitted with synchromesh gearboxes where the sliding dog clutches and the corresponding gear pinions are provided with friction cones. Figure 1 shows a simplified sketch of a synchromesh unit for third and top gear. It will be seen that the device consists of an inner and
outer hub which are splined together. The inner hub is splined to the mainshaft and is free to move slightly along it. It is retained in the position shown by radial balls and springs.
When the driver starts to move the gear lever to engage a gear the friction cones are pressed together so that the speed of the hubs and the gear to be engaged are synchronised. Added pressure by the driver overcomes the holding action of the balls and springs, the outer hub moves over the dogs on the wheel and the gear is engaged.
Often a baulking device, to prevent too rapid movement of the gear lever, is provided; this prevents the driver -beating the conesand clashing the dogs. This generally consists of a floating ring which locks the sliding clutch member until the speeds are equalised.
Figure 2 shows a sectioned gearbox for a light van where top, third and second gears are provided with a synchromesh unit but where bottom gear is a sliding mesh type.
Figure 3 shows a partly dismantled synchromesh six-speed commercial gearbox.