This type of gear box is similar to the constant mesh
type in that all the gears on the main shaft are in constant mesh with
the corresponding gears on the lay shaft. The gears on the lay shaft are
fixed to it while those on the main shaft are free to rotate on the
same. Its working is also similar to the constant mesh type, but in the
former there is one definite improvement over the latter. This is the
provision of synchromesh device which avoids the necessity of double
declutching. The parts which ultimately are to be engaged are first
brought into frictional contact which equalizes their speed, after which
these may be engaged smoothly.
Figure shows the construction and working of a synchromesh
gear box. In most of the cars, however the synchromesh devices are mot
fitted to all the gears as is shown in this figure. They are fitted only
on the high gears and on the low and reverse gears ordinary dog
clutches are only provided. This is done to reduce the cost.
Construction And Working Of Synchromesh Gear Box
Construction And Working Of Synchromesh Gear Box
In the above figure is the engine shaft, Gears B, C, D, E
are free on the main shaft and are always in mesh with corresponding
gears in the lay shaft. Thus all the gears on main shaft as well as on
lay shaft continue to rotate so long as shaft A is rotating. Menders F1
and F2 are free to slide on splines on the main shaft. G1 and G2 are ring shaped members having internal teeth fit onto the external teeth members F1
and F2 respectively. K1 and K2
are dog teeth on B and D respectively and these also fit onto the teeth of G1 and G2. S1and
S2 are the forks. T1and
T2 are the ball supported by springs. These tend to prevent the sliding of members G1
(G2)
on F1
(F2)
.however, when the force applied in G1 (G2)
through fork S1 (S2)
exceeds a certain value, the balls are overcome and member G1
(G2)
slides over F1
(F2)
. There are usually six of these balls symmetrically placed circumferentially in one synchromesh device.
M1, M2,
N1, N2,
P1, P2,
R1, R2

are the frictional surfaces.
The working of the gear box is as follows .for direct gear, member G1
and hence member F1 (through spring –loaded balls) is slid towards left till comes M1
and M2 rub and friction makes their speed equal. Further pushing the member G1
to left causes it to override the balls and get engaged with dogs K1.
Now the drive to the main shaft is direct from B via F1
and the splines. We have to give sufficient time for synchronization of speeds, otherwise clash may result.

For the second gear the members F1
and G1 are slid to the right so that finally the internal teeth on G1
are engaged with L1.then the drive
to main shaft will be from B via U1,
U2, C, F1
and splines.
For first gear, G2 and F2 are moved towards right. In this case the drive will be from B via U1,
U3, D, F2

and splines to the main shaft.
For reverse, G2 and F2 are slid towards right. In this case
the drive will be from B via, U1,
U4, U5,
D, F2
are splines to the main shaft.

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