Table of Contents Table of Contents
Previous Page  52 / 102 Next Page
Information
Show Menu
Previous Page 52 / 102 Next Page
Page Background

A D V A N C E D M A T E R I A L S & P R O C E S S E S | O C T O B E R 2 0 1 5

5 2

12

FEATURE

DISCUSSION

Carburizing and oil quenching the 8620 steel gear in-

troduces compressive residual stresses in the gear surface,

which significantly reduces the magnitude of actual stresses

at the root fillet under torsional load. Using the local cylindri-

cal coordinate system described in Fig. 4b, predicted actual

stresses in the tangential direction of the root fillet under the

same torsional load of 3287 N·m are compared for the fol-

lowing three cases:

Case 1: Original gear size (4340 steel) without residual

stresses from heat treatment

Case 2: Reduced gear size (8620 steel) with residual

stresses from carburization and oil quench

Case 3: Reduced gear size (4340 steel) without residual

stresses from heat treatment

For Case 1, the highest stress at the root fillet is about

625 MPa, located at point A (Fig. 7a). For Case 2, the highest

stress is reduced to 600 MPa, and is located at point B, as

shown in Fig. 7b, moving slightly from the root toward the

gear tip, which is due to the nonuniformcompressive residu-

al stress at the root fillet. The combination of residual stress-

es and the applied stresses is shown in Fig. 7b. Based on the

logic described above, the benefit of compressive residual

stresses to fatigue performance is further improved by op-

timizing gear geometry and heat treatment, so the highest

applied tensile stress locationmatches the highest compres-

sive residual stress location after heat treatment.

During rotational bending, the history plot of tangen-

tial stresses at the most critical positions (points A and B in

Fig. 7) of the root fillet are compared in Fig. 8 for the three

cases. Either point A or B is used depending on which loca-

tion has the highest actual stress under load. Peak stress is

considered the main driver of bending fatigue failure. The

comparison shows the significant effect of compressive sur-

face residual stresses on bending fatigue performance.

CONCLUSIONS

The selection of gear material and heat treatment pro-

cess is critical to bending fatigue performance. Carburizing

and oil quenching gears made of a carburizing steel grade

generates compressive residual stresses in the surface of the

hardened case. These stresses benefit the high cycle bend-

ing fatigue performance of gears. The concept is validated

by both modeling and previous experiments. In this study,

the concept is further applied to reduce gear size without

reducing its torque load capacity. A mass or volume reduc-

tion of 44% is compensated for by taking advantage of the

compressive residual stresses generated by heat treatment.

Material selection is also critical; clean material is preferred

to reduce potential crack initiation sites under the hard-

ened case, where residual tension exists to balance surface

compression.

References

1. A. Banka, et al., Applying CFD to Characterize Gear

Response during Intensive Quenching Process,

Proc. 24th

HTS Conf. & Expo

, p 147–155, 2007.

2. D. Lohe, et al., Residual Stresses and Fatigue Behavior,

Handbook of Residual Stress and Deformation of Steel

,

ASM International, p 27–53, 2002.

(a)

(b)

Fig. 5 —

(a) Maximumprincipal stress generated in root fillet of

gear under torsion load for (a) original size gear, and (b) reduced

size gear.

(a)

(b)

Fig. 6 —

(a) Maximumprincipal stress generated in root fillet of

gear under rotation bending load of reduced size gear (with heat

treatment residual stresses in the left gear), and (b) zoomed-in

contour of contact teeth.