Drive Axle Gears

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					                                      DRIVE AXLE GEARING

The primary function of the drive gears employed in
most rear wheel drive axles is; to provide a torque
increase, while changing the path of power to the drive
wheels of the vehicle. For this reason the majority of
the drive gears used in rear axles, are "Bevel" types.
Bevel type drive gears permit the power flow to turn a
corner, usually at right angles or at 90 degrees from the
gears input. The small driving gear is known as the
"Pinion” or "Drive Pinion” gear, while the larger driven
gear is often called the "Ring" or "Crown" gear.

                                                       BEVEL GEARS

                       The gear teeth of the plain bevel gear are straight cut and the drive pinion is
                       mounted on the centreline of the ring gear. Because of its inherent noise
                       problem and lower strength, this gear type is normally used in low speed or
                       lighter load applications.

                           SPIRAL BEVEL

Spiral-bevel gearing was developed to meet the needs of higher speed
and heavier loads. The gear teeth are cut obliquely on the angular
tooth faces, to form curved or spiral-shaped gear teeth on the ring and
drive pinion gears. Similar to the plain bevel gear set, the pinion is
mounted on the centre-line of the ring gear. Spiral-bevel gears are
smoother, quieter and stronger than the plain bevel type.

                                         HYPOID GEARING

Hypoid gearing looks much like the spiral-bevel gear system, in that the gear teeth are spiral-shaped as
                               well. The major difference is that it employs an offset mounted drive
                               pinion to ring gear arrangement. Normally, pinions are mounted below
                               the centreline of the ring gear. This feature has allowed the vehicle's
                               centre of gravity to be lowered, thereby providing greater stability in
                               turns. The drive pinion can also be meshed more deeply into the ring
                               gear, providing the gear set with more strength, along with quieter and
                               smoother operation. This design of drive gear also created a problem
                               that normal gear lubricants could not handle, namely "Extreme Pressure"
                               at the gear teeth. Hypoid gearing is considered to be the strongest type
of bevel gear used in the automotive field. Hypoid gears usually require a lubricant that contains (EP)
or extreme pressure additives, to prevent metal-to-metal contact of the pinion and ring gears.
                                         AMBOID GEARING

                                   Amboid gears also employ an offset pinion to ring gear mounting
                                   arrangement, but the pinion gear is mounted above the ring gear's
                                   centreline. Notice that the drive pinion's gear teeth are cut at a
                                   similar angle to the spiral-bevel and hypoid gears, but the ring gear
                                   teeth are cut in the opposite direction.
                                   Amboid gears are not widely used and are currently employed only
                                   in the rear rear axle of some models of tandem. They were primarily
                                   introduced to lessen the operating angles on some inter-axle
                                   driveshaft systems. Take note that the "Normal" drive side of the
                                   ring gear teeth is on the "Concave" portion of the ring gear, not on
                                   the "Convex" side as is the case for conventional "Hypoid" or
                                   Spiral-Bevel" gearing.

                                       DURAPOID GEARING

A number of years ago a variation of Spiral Bevel gearing was developed to increase strength. This
gear system is known as the "Durapoid" type drive gear and has its drive pinion mounted on the
centreline of the ring gear. The major difference between spiral bevel and " DuraPoid" gears is their
                               tooth shape. The Durapoid gear tooth design is shaped to eliminate the
                               localized stress concentrations that are present in conventional drive
                               gears. This design prevents tooth end-loads, thereby providing more
                                strength and longer gear life.
                                When comparing the drive pinion gear teeth of Spiral-bevel and Durapoid
                                gear types, notice that that the gear teeth are symmetrical on the Spiral-
   Figure 1                     bevel type, while the durapoid's
                                 pinion teeth are not symmetrical.
The spiral-bevel gear set on the drive side (Point A) (Fig 1) and
coast side (Point B) of the pinion teeth are symmetrical. The
Durapoid gear set by comparison on the drive side (Point C) has
an almost sheer tooth face, while the coast side (Point D) uses a
buttress support shape to provide increased strength.
Conventional spiral-bevel gears normally have tooth contact
patterns that shift towards the "heel" area of the ring gear as
loads are increased, while "Durapoid" gears have patterns that
shift equally towards both heel and toe on the ring gear as loads
are increased. Conventional bevel gearing usually requires the
gear tooth Contact bearing to start at the "Toe" end of the ring
gear teeth, while the Durapoid gear set has a centrally located
tooth contact bearing.
                                         HYPOID GENEROID

Just as the "Durapoid" type drive gear set was an improvement of
"Spiral-Bevel" gearing, an improved "Hypoid" gear was
developed and is known as the "Hypoid-Generoid" gear type.
Hypoid-Generoid gearing uses an offset pinion to ring gear
mounting arrangement; it basically provides the same advantages
that Durapoid gears did for Spiral-Bevel types.


Worm drive gears are actually an inclined plane or screw mechanism, capable of large speed reductions
and great torque multiplication in a compact space. The "Worm" pinion has teeth that are cut much
like the threads of a capscrew, while its mating "Crown" or Ring gear employ gear teeth that are curved
     at their tips to allow larger contact areas. The Worm or pinion is mounted at the outer edge of the
                                    Crown gear, usually as a "Top-Mounted" or "Underslung"
                                    arrangement. The worm pinion is normally, a hardened steel
                                    component, while the Crown or Worm Wheel is manufactured from a
                                    soft metal such as "Bronze" or "Aluminum". Worm drive axle gears
                                    are not in common use to-day and are used in heavy duty, special

                                        STRAIGHT CUT SPUR GEARS

Straight cut spur gearing use straight cut gear teeth, cut parallel to the axis of
rotation. These gears usually have one or two pairs of teeth in mesh at one time.
The straight cut spur gear is considered to be noisy and weaker than other gear
types, but has the advantage of low axial or thrust loading. In drive axles, straight
cut spur gears have been used to provide the second reduction in "Double
Reduction" type drive axles.
                                        HELICAL CUT GEARS

                              The gears of a helical cut are mounted on parallel shafts, but the teeth are
                              cut obliquely across the gears perimeter on a "Helix" angle. Compared to
                              straight cut spur gearing, helical gears provide a smoother and quieter
                              operation. Additionally, their angular contact is longer than equivalent
                              width straight cut gears, thus providing greater strength.

                             The major disadvantage of helical gears is the angular side thrusts,
                             resulting in higher thrust or axial loads that must be absorbed by the
bearings supporting the gear shafts. Helical gearing has been popularly used in double reduction drive
axle systems, providing the second or first gear reductions.

                                       HERRINGBONE GEARS

Herringbone gears are actually a "Double Helical" cut
gear arrangement, with the tooth angles reversed on
opposite sides. This results in a cancellation of the thrust
loads produced in helical gearing. Each gears double
helix is usually separated at the center by a narrow gap or
groove, to maintain better alignment and to avoid
pressurization of the lube oil at the apex of the teeth.
Though not commonly used today, they have been used
to provide the second reduction in double reduction drive
axle systems.

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