LEEVENSPARK

Worm and Wheel Drive Since this drive is expensive, it is rarely used nowadays as a final drive on light vehicles, but is still used on heavy vehicles. However, this type of gear has a number of other applications on motor vehicles. Various arrangements, illustrated in Fig. 26.38, can be employed to provide a very quiet and long-lasting gear, but efficiency is less than the bevel (94 percent against 98 percent). This type of gear provides a large reduction in a small space. The worm may be mounted below (under-slung) or above (overhead) the wheel. An hour-glass or Hindley worm embraces more teeth than the straight worm but adjustment becomes more critical. Fig. 26.38. Worm drives. The sliding action of the worm causes friction, which is reduced by using a worm wheel of phosphor-bronze and a worm of case-hardened steel, but still the unit becomes hot. A large, well-cooled sump is incorporated to reduce oxidation of the oil at high temperature, which otherwise causes the oil to thicken.

Hypoid Gear This type of gear (Fig. 26.37) is the commonly used now a days. The pinion axis of this gear is offset to the centre line of the crown wheel. Although the gear can be placed above or below the centre, but in cars it is always placed below to allow for a lower propeller shaft so that a reduction in the tunnel height is possible. Pinion offset can vary with the application, but an offset of one-fifth the wheel diametre is commonly used. If the axis is lowered, the tooth pitch of the pinion increases, so that for a given ratio, the pinion diameter can be larger (30 percent for normal offset). This enables the use of a stronger gear specifically on commercial vehicles. Fig. 26.37. Hypoid bevel. A hypoid is considered to be halfway between a normal bevel and a worm drive. In the former case a rolling action occurs, whereas the latter case is totally sliding. An increase in the sliding motion in the hypoid gear reduces meshing noise, but the high temperature and pressure of the o

Spiral Bevel Although the straight bevel is cheaper and mechanically efficient, the meshing of the gears causes an unwanted noise, which has been reduced by introducing a helical form of tooth. It is impossible to generate a helix on a tapered pinion, so the gear is called as a spiral bevel. Figure 26.36 illustrates the construction of the gear, A number of teeth are generated from the centre of the crown wheel, and form a left-handed spiral in the case of the pinion. This direction provides a large outward thrust on the drive and a smaller inward thrust on the over-run so that wear of the pinion bearing increases the backlash instead of causing seizure of the gear. Fig. 26.36. Spiral bevel. Since the crown wheel teeth are inclined to the pinion, the tooth pressures are much higher. The gear oil with no additives, and high-viscosity, suitable for the straight bevel type, is not satisfactory when used in spiral bevel units. The oil film brakes down under the high loads, causing rapid we

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

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. Fig. 26.34. Friction cones representing bevel gear drive Types of Bevel Gear Straight Bevel The main features of the bev