Page 130 - Gear Technology Solutions

In the case of the cut gear, the center contact stress reduces smoothly in all four
directions as shown by the color change, from bright red to orange, yellow, green
and blue.

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Figure 17: Stress comparison, Coniflex Pro cut versus forged
(input torque = 1000Nm), [7]

A realistic operating torque of 1000Nm was applied in a second calculation. The
bending stress results to the left in Figure 17 show a 13% to 17% advantage of
the cut side gear. It is noticeable by the red patch inside of the web area that the
web has a high contribution on transmitting the torque. Also, the contact stress to
the right in Figure 17 shows up to 65% higher values for the forged gear. The
especially the high value of 3753N/mm² in connection with the high bending stress
in the same area will result in high sub surface stresses which can cause case
crushing. Case crushing often leads to flank fracture.

Also, for 1000Nm torque, the area with high surface stress of the forged version
extends to the topland and the toe boundary. In case of the cut gear, there is also
a smooth stress reduction towards the tooth boundaries for 1000Nm input torque.

In a third step, the unrealistic high input torque of 5000Nm was applied to the cut
and forged side gear. Such extremely high torque will cause stresses in the root
as well as on the surface which are well above the allowable material properties,
yet it is used in test rigs to simulate shock loads and to record how many
revolutions (or fractions thereof) the gearset will sustain. The fracture which
consequently will occur is then analyzed with practical conclusions for the survival
of the differential during a lifetime load collective.

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