Part Drawings 363
dharm
d:\N-Design\Des19-1.pm5
is in two main parts. A sleeve with internal cones [Fig. 19.6 (c)] slides inside the toothed outer
ring, which forms the dogs to match the gear wheel cones with the parts rotating at the same
speed. The spring loaded outer ring of the collar is pushed forward for the dogs to mesh.
When the collar is pushed towards the gear wheel with which it is to mesh, a conical ring
on the gear wheel in front of the dogs comes into contact with the surface of a matching conical
hole in the collar. The friction between the conical surfaces, brings the free-running gear wheel
up or down to the speed of the output shaft. The collar continues to move along and the pair of dogs
slide smoothly into mesh. However, if the gear lever is moved too fast, the gears will clash.
A typical automobile gear box consists of a cast iron or an aluminium housing, four shafts,
bearings, gears, synchronising device and a shifting mechanism. Figure 19.6 shows the assembly
of such a gear box, partially sectioned. This gear box provides four forwardes speeds of the ratios
4:1, 2.4:1, 1.4:1 and 1:1 and a reverse speed.
19.2.6.5 Working
Figure 19.6 (a) shows the gear box in its neutral position. The housing 1 is made of aluminium
alloy and a supporting plate 2 is fixed to it for supporting the reverse gear shaft 5 at one end; the
other end being located in the rib provided inside the housing. The input shaft 3 is supported by a
ball bearing 7. One end of the output shaft is supported by a ball bearing, while the other end is
located with free running fit, inside the bore (φ 15) provided at the inner end of the input shaft. The
intermediate shaft 4 is supported both sides by the ball bearings.
The gears B, C and E are keyed in position on the intermediate shaft. The gear G is
integral with the shaft. The gear A is keyed onto the input shaft. The gears D and F are constantly
in mesh with the gears C and E but free to rotate on the output shaft when not engaged. The gear
H is integral with the toothed ring 8 and slides on the sleeve 11 when operated by the fork 14. The
sleeve 11 is splined to the output shaft and has external splines also on which the toothed ring 8
along with gear H slides on a single collar.
Similarly, the sleeve 10 is splined to the output shaft at its inner end, on which is fixed the
toothed ring 9 with internal splines and slides over the sleeve when operated by the fork 13. Three
spring loaded balls 12 are provided between the sleeve and the ring to keep them together as a
single collar during free running and also when engaged with the toothed dogs [Fig. 19.6 (c)]. The
bushes 15 act as bearings for the reverse gear shaft.
When the input shaft rotates, power is transmitted to the intermediate shaft continuously
through the herring-bone gears A and B. Now, depending on the position of the forks 13 and 14
and the corresponding collars, different speeds are obtained. When the fork 13 is in neutral
position and fork 14 is moved until the spur gear H engages with G, then, due to two step
reduction of speed, the lower speed ratio 4:1 is obtained at the output shaft. To obtain the second
gear, the fork 14 is moved to the left until the collar completes the meshing and engages with the
dog teeth. This operation arrests the independent free rotation of gear F, by engaging the dog
teeth with the toothed ring, and the output shaft through the sleeve which is splined on it. The
gear E on the intermediate shaft transmits power to the output shaft through gear F which is the
second gear with a ratio 2.4:1.
To obtain the third gear, the gears in mesh are A-B, and C-D with the collar consisting of
sleeve 10 and toothed ring 9 engaged with dog teeth on gear D; the speed ratio being 1.4:1. When
the toothed ring 9 engages with the dog teeth on gear wheel A, mounted on input shaft, the
transmission is established directly to the output shaft; resulting in top speed ratio of 1:1.
