Download - 11 final drive
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Final-driveor
Rear Axles Final-drive
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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.
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TYPES OF FINAL DRIVES
1. Bevel Gears
– Straight Bevel.– Spiral Bevel.
2. Hypoid Gear
3. Worm and Wheel Drive
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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.
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Types of Bevel Gear.
1. Straight Bevel.2. Spiral Bevel.
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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.
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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.
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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.
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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.
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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.
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