Design-Construction

Introduction

Once in a while we come across something which is guaranteed to fail before it even exists. It means: wrong design. This is the story of an ill-fated shaft. It happened in 2006. The shaft was driven by a gearbox. It was a shiny brand-new shaft and it failed after only 7 days in service. The remains are shown in Figure 1 below. Why did it fail?

Fig. 1. Debris of the ill-fated shaft.

Hardening and Tempering

Figure 2 shows the same debris from different angle. Red arrows indicate the origin of fracture. According to the source, the shaft was hardened and tempered before it was used. I don’t really understand why the hardening and tempering were necessary. They suspected that the hardening is the reason why the shaft failed. Various theories of advanced material were soon thrown in by engineers and material experts. Some even went so far as to discuss how the heat treatment turns martensite into cementite and many other fancy rocket-science phrases. Looking at the picture, I found very basic design mistake which I am very sure is the main reason why the shaft failed.

Fig. 2. Debris of shaft from different angle.

Stress Concentration

Let’s go back to a simple concept in strength of material: stress concentration. The concept of stress concentration is taught to every mechanical and material engineering sophomore. For readers who are not familiar with this phrase, just imagine stress concentration is the notches on the edge of your candy wrap. See Figure 3 for illustration. Without the notches, it is almost impossible to rip open the candy wrap with bare hand. The notches amplify the stress on very small area on the edge and thus induce failure on the wrap. That’s stress concentration in action.

Fig. 3. Ripping open candy wrap with the help of stress concentration.

The Fatal Mistake

Figure 4 below shows the schematic of the above mentioned shaft. The design is a complete disaster because it induces the stress concentration by making a sharp geometry transition on the neck of the shaft, just like the candy wrap. The red lines on Figure 4 are virtual stress flow inside the shaft material. The flow turns drastically near the neck transition and creates stress concentration.

To make things even worse, the designer put the keyway very close to the neck as shown in bottom schematic in Figure 4. This heavily raises the stress concentration due to terrible geometry distortion. That is the reason why the shaft was ill-fated and condemned to fail since it was still on the drawing board. In Figure 1 and 2, you can see that the fracture started exactly from the predicted point.

Fig. 4 (Top) Stress flow inside the shaft; (Bottom) Keyway is too close to the neck and produces higher stress concentration.

The Right Thing

Now we have seen where things go wrong: (1) drastic neck transition; and (2) keyway is too close to the neck transition. Figure 5 below shows comparison between the ill-fated shaft (top) and how it should be (bottom). A good shaft design has tapered neck to accommodate smooth transition of the stress flow. Keyway is moved backward so it is not too close to the neck transition. This produces a smooth stress flow lines and low stress concentration.

Fig. 5. Comparison between: (top) poorly-designed shaft; and (bottom) well-designed shaft.

Figure 6 below shows 2 gearbox-driven shafts with keys attached to the keyways. They are part of LIMOV BOG-80T, heavy-duty Bogie capacity 80 ton. The picture was taken in 2005. It was among first line of LIMOV engineering products. Neck transition is indicated by red arrows. Note that the keyway has sufficient distance from the neck and the neck is well-tapered to provide smooth geometry transition.

Fig. 6. Gearbox-driven shafts of LIMOV BOG-80T.

Design with Visualization

Real engineers design with visualization and intuition first, and check the calculation later. Operators design with numbers and calculation first, and never visualize nor have the engineering intuition. Stress concentration is a very simple concept. Visualizing the stress flow is also very simple thing to do. They are so simple that they are often underestimated. Next time, when something goes wrong, go back to these basic principle first before you go too far with rocket-science theory such as martensite or cementite . I wish you strong and reliable works always.

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