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
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.
K1and K2are 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,
For first gear, G2and 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, G2and F2are
slid towards right. In this case the drive will be from
B via, U1, U4, U5,
D, F2 are splines to the main shaft.