ANALYSIS OF A CRACKED SPLINE GEAR
Posted on July 1st, 2019 by Met-Tech
ANALYSIS OF A CRACKED SPLINED COUPLING
Summary:
A fractured spline gear coupling was received for analysis to investigate the cause of failure. Results indicated the coupling fractured in fatigue, initiated at the bottom corner of the keyway from the ID of the coupling. The keyway exhibited significant yielding which widened the keyway, indicating the strength of the material was insufficient for the loads carried. The spline gear teeth also exhibited evidence of significant deformation and wear. The keyway exhibited a sharp corner radius that was a significant stress raiser and provided a site for fatigue initiation.
The spline gear coupling was manufactured from a non-US standard grade of chromium-nickel, low carbon steel. Metallographic analysis of the fractured coupling revealed a soft matrix of pearlite and ferrite. Rockwell hardness testing revealed a soft hardness of 88 Rockwell B (HRB) indicating the part had not been heat treated.
The wear resistance and strength of the material can be significantly increased by carburizing, quenching and tempering the part. It is surprising that the part was not supplied in this condition and it may have inadvertently missed heat treatment. Increasing the radius at the keyway bottom edges would also minimize stress concentrations in the region of fatigue crack initiation.
ANALYSIS:
History and Visual Examination
A fractured spline gear coupling for a motor shaft was received for analysis to investigate the cause of failure. Figure 1 provides an overview of the coupling as received for analysis. It was reported that the coupling was installed near the top of the filter shaft and was inserted into a gearbox motor. The service history was unknown, but time in service was thought to be greater than three years.
Figures 2 and 3 present close-up views of the spline gear coupling. The coupling had cracked in a longitudinal manner to just over half its length. The crack arrested at the termination of the spline gear teeth contact witness marks. Significant wear was observed on the splines.
A close-up end view of the cracked coupling appears in Figure 4. The crack origin was at the bottom edge of a keyway and the crack propagated to the root of one of the spline teeth. The coupling was sectioned in a transverse manner across the tip of the crack at the end of the spline gear engagement region. The crack fell into two sections revealing the fracture surface morphology (Figure 5).
A close-up view of the remaining portion of the coupling is displayed in Figure 6. The keyway had been enlarged, due to heavy deformation. The opened crack fracture surface was examined at elevated magnifications using a scanning electron microscope (SEM).
A low magnification SEM view of the opened fracture face is detailed in Figure 7. Fracture surface features indicated the fracture initiated in the bottom corner edge radius of the keyway and propagated in fatigue before overload propagation to a spline root. An increased magnification SEM view of the fracture surface (Figure 8) more clearly resolved the fracture origin, fatigue, and overload zones.
A high magnification SEM view of the fracture origin is shown in Figure 9. The surface is rubbed, masking fracture surface details. High magnification SEM views of the fracture surface, a short distance from the fracture origin area, revealed fatigue striations as exhibited in Figures 10 and 11. A high magnification SEM view of the region of final overload (Figure 12) exhibited dimpled surface features of microvoid coalescence, indicative of ductile overload.
A transverse cross-section was extracted through the fracture initiation area of the failed spline gear coupling. The cross-section was prepared for metallographic analysis in accordance with ASTM E3. Etching with 2% Nital was performed in accordance with ASTM E407 to reveal the microstructure. The cross-section was examined using an optical microscope in accordance with ASTM E883.
Figure 13 provides a low magnification photomicrograph of the cross-section at the fracture origin at the edge of the keyway bottom corner radius. (reference Figure 13). The keyway and splines were loaded beyond the yield strength of the coupling material.
A second transverse cross-section was taken on the remaining portion of the keyway (reference Figure 6) which contained a portion of the crack tip. The resulting cross-section was prepared for metallographic analysis.
A low magnification, un-etched view of the cross-section is provided in Figure 16. The fatigue crack at the keyway bottom corner radius was observed. A small secondary crack was observed at the root of the adjacent spline. Increasing magnification optical microscopic views of the crack in the keyway corner radius appear in Figures 17 and 18. Deformation of the keyway side surface microstructure was observed. A core microstructure of soft pearlite and ferrite was noted.
An optical microscopic view of the shallow crack at the spline root is exhibited in Figure 19. Significant grain deformation was observed in the spline root microstructure.
Hardness Testing
Rockwell hardness testing was performed in accordance with ASTM E18. Four separate readings were taken at the approximate mid-radius of the transverse cross-section for an average hardness value of 88 HRB.
CHEMICAL ANALYSIS
Chemical analysis of the fractured spline gear coupling was performed using an optical emission spectrometer in general accordance with ASTM E415. The spline coupling was manufactured from a non-US standard grade of chromium-nickel, low carbon, alloy steel. This type of material is typically carburized for spline tooth coupling applications. Results of the composition analysis are provided in Table 1.
Table 1
Blade Base Metal Composition Analysis Results
|
Element |
Blade |
Assembly Weld |
|
|
Carbon |
0.16 |
Carbon |
|
|
Manganese |
0.99 |
Manganese |
|
|
Phosphorous |
0.005 |
Phosphorous |
|
|
Sulfur |
0.026 |
Sulfur |
|
|
Silicon |
0.24 |
Silicon |
|
|
Nickel |
0.87 |
Nickel |
|
|
Chromium |
1.09 |
Chromium |
|
|
Molybdenum |
0.03 |
Molybdenum |
|
CONCLUSIONS:
The coupling fractured in fatigue, initiated at sharp corner radius at the bottom of the keyway on the ID of the coupling.
Significant deformation yielding was observed on the sidewall of the keyway.
The spline gear teeth exhibited significant yielding deformation and wear.
Metallographic analysis of the fractured coupling revealed a soft microstructure of pearlite and ferrite.
The spline gear coupling was manufactured from a non-US standard grade of chromium-nickel, low carbon, alloy steel. This would be considered a carburizing grade; however, the coupling was not carburized or heat-treated by quenching and tempering.
Rockwell hardness testing revealed a soft hardness of 88 HRB, indicating the part had not been heat treated.
Carburizing followed by quenching and tempering of the part would significantly increase the fatigue strength and wear resistance of the part.
Increasing the radius at the keyway edges would minimize stress concentrations in the region of fatigue crack initiation.
IMAGES:
Figure 1: An overview of the cracked spline gear coupling as received for analysis. A longitudinal crack (arrow) extends approximately one half the length of the coupling.
Figure 2: A close-up view of the cracked spline gear coupling. The longitudinal crack (arrows) arrested at the end of the spline teeth contact witness marks.
Figure 3: A close-up view of the cracked spline gear coupling. Significant wear was observed on the splines. Deformation ridges are observed at the end of the contact witness marks.
Figure 4: A close-up end view of the cracked coupling. The crack origin (arrow) is at the bottom edge of a keyway and the crack propagated to the root of one of the spline teeth.
Figure 5: An overview of the opened crack mating fracture surfaces and an end view of the remaining portion of the coupling, which shows the keyway cross-section.
Figure 6: A close-up view of the remaining portion of the coupling. The keyway had been enlarged (arrows), due to heavy deformation yielding.
Figure 7: A low magnification SEM view of the opened fracture face. Fracture features indicated the fracture initiated in the bottom corner of the keyway (arrow) and propagated in fatigue before overload propagation to a spline root. (SEM Photo, Mag. 10X)
Figure 8: An increased magnification SEM view of the fracture surface more clearly resolves the fracture origin (arrow), fatigue, and overload zones. (SEM Photo, Mag. 40X)
Figure 9: A high magnification SEM view of the fracture origin at the edge of the fracture face. The surface is rubbed, masking fracture surface details. (SEM Photo, Mag. 1,000X)
Figure 10: A high magnification SEM view of the fracture surface a short distance from the fracture origin revealed fatigue striations (faint curved parallel markings). (SEM Photo, Mag. 4,000X)
Figure 11: A second high magnification SEM view of the fracture surface, some distance from the fracture origin, revealed additional fatigue striations. (SEM Photo, Mag. 4,000X)
Figure 12: A high magnification SEM view of the region of final overload exhibits dimpled features of microvoid coalescence, indicative of ductile overload. (SEM Photo, Mag. 2,000X)
Figure 13: A low magnification photomicrograph of the transverse cross-section, through one side of the opened crack, at the region of fracture initiation at the edge of the keyway bottom corner. (Mag. 15X)
Figure 14: A high magnification optical view of the cross-section at the fracture origin revealed grain distortion indicative of yielding deformation in the sidewall of the keyway. (Mag. 200X)
Figure 15: A high magnification view of grain deformation observed on a lower flank of a spline tooth. (boxed area from Figure 13) (Mag. 200X)
Figure 16: A low magnification, un-etched view of the transverse cross-section through the intact keyway bottom corner. The fatigue crack emanated from the sharp keyway corner radius. A small secondary crack was observed at the root of the adjacent spline. (Mag. 15X)
Figure 17: An increased magnification optical microscopic view of the crack origin (arrow) region in the keyway bottom corner radius. (Mag. 50X)
Figure 18: A high magnification optical microscopic view of the crack origin in the keyway corner radius. A core microstructure of soft pearlite and ferrite was observed. The grains are deformed along the keyway edge. (Mag. 200X)
Figure 19: An optical microscopic view of the shallow crack at the spline root. Significant grain deformation was observed in the spline root. (Mag. 100X)
