Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Reliability Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation




Machine Design - Case No. 62: Drive Shaft Keyway Stress Concentration and Fatigue


Keyways are an important design option in power transmission. When all things are equal, the crack will more than likely initiate at the keyway. Make sure that the stress concentration factor for the keyway is figured into the design. And make sure you design for the weakpoint - torsional fatigue.

Shaft Cross Section View - Alternate Bending Load Fatigue

Figure 1

Keyway View - Crack Initiation Point

Figure 2


Figure 1 shows the cross section of a shaft. The crack initiated at the keyway approximately at location "A". It started in torsion at approximately a 45° incline with respect to the shaft axis, but quickly changed to rotational bending as the crack lined up perpendicular to the shaft axis (Figure 2). The direction of travel of the crack is shown in the picture to the left. The red dots indicate crack arrest points (or beach marks) in the progress of the crack.

When designing power transmission shafts it is very important to factor in stress concentration factors for the keyway. If you can avoid keyways by using some other method, such as for example a shrink fit, then do so; otherwise, treat keyways with great respect. This shaft lasted approximately 10 years and had over three hundred million revolutions on it. That doesn't sound bad, 10 years, but the shaft revolved at only 60 rpm. If it had revolved at lets say 600 rpm it may have only lasted one year!

A closer look at the crack initiation and progression can be seen in the Material Properties Case No. 63: Drive Shaft Torsion Fatigue versus Rotary Bending Fatigue article.

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