Diesel Engine are a source of significant vibrations. When designing brackets and fittings that are connected to, or near the engine, care must be taken to avoid introducing stress concentrations.

Sharp Corners

With the increased use of 3D CAD packages (SolidWorks in the case of Penke) to create sheet metal components, with the aim of simplifying fabrication, care must be taken to understand how to develop the parts to avoid introducing stress concentrators.

In the above example, SolidWorks has been used to create a “rip” in the material to the allow the bent section to be folded. At the bottom of the rip is a rectangular cut (highlighted in red). The part was fitted to a rail car, which had a design life of 20 years, the part cracked within the first year. It is expected that if the part was made with an appropriate radius the part would have lasted significantly longer.

Truck OEM’s know about vibration and cracking. If you are trying to figure out how your bracket should be designed, go out into a truck bay and look around.

Abrupt Changes in Section

There are a number of issues with the part shown in the photo below. Firstly, it was designed to use PFC which has an increased amount of material in the area of the radius, the part was however manufactured from a laser cut and bent part.

The second issue with the part, was the sharp corner drawn on the cut out circled in red, this could have easily been designed with a 5mm radius. The other issue with the original design, was the unnecessarily abrupt change in section, as can be seen on the right-hand side view, the final (and durable) design has a gradual change in section and no sharp edges.

Other causes and mitigations:

In some instances, it is not immediately clear why a part is cracking.

Is the part assembled correctly, in its relaxed state is it free from stress? Sometimes bolted components look like they fit but when the bolts are undone, a gap appears. Make sure parts that are fitted to an engine installation are correctly manufactured and fit as designed.

Is a process introducing an unwanted stress? Is the toe of a weld the cause a stress concentration? Maybe think outside the box. Is it necessary to stress relieve a part prior to installing it? There are a number of options – thermal stress relief, shot peening, shot blasting, sometimes the answer isn’t immediately obvious.