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Mechanical Fatigue of a Stainless Steel Shaft

ENVIRONMENT:

Manufacturing Plant

EQUIPMENT:

Type 316 Stainless Steel

MATERIAL:

4 years

FAILURE:

Mechanical Fatigue

 

Summary

A failed shaft was submitted for a failure analysis. Examination of the fracture surface revealed characteristics consistent with mechanical fatigue failure. These characteristics were typical fatigue marks associated with a uniformly loaded shaft with moderate to severe stress concentration subjected to rotating-bending fatigue. No indication of chemical interaction was present.  

 

Background

The shaft bearings were receiving lubrication and there were no signs of heat stress. The bearings were not hot to the touch at the time of the failure. 

The shaft was noticed to be recessed approximately 1” on the sheave after failure. The sheave was flush with the end of the shaft after installation. The shaft had its speed increased on the last installation. The shaft was “clean” where the sheave slipped. The sheave was held in place with a key and tapered bushing.  

The shaft had been reported to be at least 4 years old. The bearings were last replaced 4 years ago. The material of construction of the shaft was 316 stainless steel. The shaft was threaded in the area of the failure to accept a washer and lock nut to hold the bearing in place. The bearing was shrink fitted to the shaft. 

The grinder was powered by a 150 horsepower motor operating at 1775 rpm. Prior to the failure, current draw on the motor was normal. 

 

Description of Material

CTL Sample #1

This piece was approximately 6” long. The shaft was 2 Ό” in diameter at the end where the fracture occurred. A damaged bearing was in place on the shaft next to the fracture, Figure 1. The opposite end of the shaft was saw cut through a taper. The shaft was approximately 4” in diameter at this end.

 CTL Sample #2

This piece was approximately 9” long with a noticeable bend on the end (»30°) where the fracture occurred, Figure 2. The fractured end was threaded with the retaining nut still on the shaft but cracked and pushed out of place. The shaft was 2 Ό” in diameter.

 

Figure 1. Failed shaft with damaged bearing in place.

Figure 2.  Matching section of failed shaft to Figure 1.

 
Findings

Approximately 75% of the fracture surface appeared relatively smooth and associated with striations of a propagating ductile fatigue crack. The remaining 25% of the surface had a rough texture associated with the final brittle fracture of the shaft. Noticeable deformation of the shaft was present in the area of final failure. 

On the fracture surface, several crack initiation sites are present, with one main crack that led to failure.  Beachmarks are present on the main crack typical of a propagating fatigue crack. The cracks appear to have initiated in the shear groove next to the bearing. The area of final failure is approximately at a 15° offset from the beach marks associated with the main crack. 

Micro-cracks are present in the 1/8-inch radius shear groove, Figure 3. Some of these micro-cracks are associated with crack initiation sites visible on the fracture surface.

 

Figure 3.  Micro-cracks in shear groove (10X Original Magnification).

 

Discussion

The observed fracture surfaces have characteristics consistent with mechanical fatigue failure. These characteristics are striations and beachmarks associated with a uniformly loaded shaft with moderate to severe stress concentration subjected to rotating-bending fatigue. No indication of chemical interaction was present.  

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