11 final drive

Final-driveor

Rear Axles Final-drive

Rear Axles Final-drive

The rear axles final drive(i) Transmits the drive through a angle of 90degrees, and

(ii) Gears down the engine revolutions to provide a ‘direct top’ gearbox ratio.

(iii) In the case of cars a final drive ratio of approximately 4 : 1 is used.

(iv) Bevel or worn gears are employed to achieve the various functions of the final drive.

TYPES OF FINAL DRIVES

1. Bevel Gears

– Straight Bevel.– Spiral Bevel.

2. Hypoid Gear

3. Worm and Wheel Drive

Bevel Gears

• 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.

• 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. Spiral Bevel.

Backlash In Gears• Backlash is most commonly created by cutting

the teeth deeper into the gears than the ideal depth.

• Another way of introducing backlash is by increasing the center distances between the gears.

• Standard practice is to make allowance for half the backlash in the tooth thickness of each gear.

Straight Bevel• 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.

• 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.

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.

• Figure 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.

Hypoid Gear

• This type of gear 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.

• 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.

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.

• It 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.