Straight Bevel Fig. 26.35. Straight bevel The main features of the bevel type of gear is illustrated in Fig. 26.35. The tapered teeth, generated from the centre, are machined on the case-hardened steel gears and then ground together to form a 'mated pair'. The position of the crown wheel relative to the pinion determines the direction of rotation of the axle shaft. If the crown wheel is fitted on the wrong side, which is possible on some vehicles, then this provides one forward and several reverse ratios. For correct meshing and for setting the clearance between the teeth (backlash), adjusters in the form of distance pieces, shims or screwed rings are used. When backlash is too small, expansion results due to heat and wear is caused by lack of lubrication. On the other hand excessive backlash produces slackness and noise. Each manufacturer recommends a suitable backlash, but it is generally in the region of 0.15 mm for cars and 0.25 mm for heavy vehicles.
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Rear Axles Final-drive The rear axles final drive (i) transmits the drive through a angle of 90 degrees, and (ii) gears down the engine revolutions to provide a 'direct top' gearbox ratio. In the case of cars a final drive ratio of approximately 4 : 1 is used. Bevel or worn gears are employed to achieve the various functions of the final drive. 26.4.1. Bevel Gears Figure 26.34 illustrates the geometry of a bevel gear layout, which represents two friction cones 'A' forming the crown wheel and 'B' the pinion. For avoidance of slippage and wear, the apex of the pinion must coincide with the centre line of the crown wheel. The system with incorrectly positioned pinion causes unequal . peripheral speeds of the crown wheel and pinion. It is necessary to mount the gear in the correct position so that angle of the bevel is governed by the gear ratio. => Types of Bevel Gear :- 1. Straight Bevel 2. Sprial Bevel => Hypoid Gear => Worm and Wheel Drive :- 1. Bevel