2. 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.
3. TYPES OF FINAL DRIVES
1. Bevel Gears
– Straight Bevel.
– Spiral Bevel.
2. Hypoid Gear
3. Worm and Wheel Drive
4. 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.
7. 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.
8. 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.
10. 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.
12. 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.
14. 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.