Page 116 - Gear Technology Solutions

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9 Coniflex Pro - New Process for Straight Bevel Gear
Manufacturing

9.1 Why a new Straight Bevel Gear Process?

The manufacturing of differential gears went away from the traditional Revacycle
broaching process to forging more than 30 years ago. These forged differentials
were made for vehicles with internal combustion engines. The relative motion of
the gears was low and basically occurred only while driving around a bend [1].
Most electric vehicles have one electric motor per driven axle which transmits
motion and torque through a single or two speed transmission to the wheels.
However, between the final drive gear of the transmission and the drive shafts to
the wheels a differential is required. The differential gears are subjected to the
peak torques electric motors can provide, which can be a multiple of the maximal
torque of a combustion engine of a comparable vehicle. Also, differential gear
noise is a concern because some advanced eDrive designs have operating
conditions with multiple times higher relative motion between the differential gears
compared to the traditional differentials.

This paper introduces a new cutting process for straight bevel gears which roll
quieter and offer a higher power density compared to conventionally cut or forged
straight bevel gears.

9.2 Forged Differential Gears

Finish forged differential gears are a good solution for the mass-produced
differential of cars and trucks with internal combustion engines. Differentials have
generally a high-power density because they must fit inside of the differential cage
which is inside the final drive gear. The size of a transmission depends therefore
on the size of the differential unit because the transmission is built around it. The
severe constraint in size led the designer of forged differential gears to introduce
stiffening webs in the root at toe and heel which, in addition to a certain tooth
stiffening, also allows the outer diameter to be reduced and still maintains an
acceptable wall thickness between toe and bore (see Figure 1). The stiffening
webs give a higher root bending strength at moderate loads; however, they also
constrain the tooth bending and therefore cause increased sub surface stresses
below the area of the webs which in the case of high loads or shock loads can
lead to cracks and flank fracture. The increased tooth stiffness also prevents the
desirable small amounts of tooth bending which allows the neighboring teeth to
provide a load sharing which reduces the root bending stress.

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