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 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 applications. 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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