Machine Drawing

(avery) #1

Part Drawings 363


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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.

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