Structural dynamics optimised to meet design requirements

This project involved the optimisation of structural dynamics to avoid resonance frequencies and mode shapes, which caused high stress concentrations that consequently initiated fracture mechanics and finally fatigue cracks. The latter causing oil leaks.

All this due to inadequate material selection and design scope creep.

Predecessor

The predecessor of this engine for which this pan was made, had an aluminium sump. Sturdy enough to hold the engine weight when placed on a level surface, and easy to cast.

But aluminium had an acoustic quality, which attenuates engine noise - reduces engine noise, for noise, vibration and harshness (NVH ) reasons, the gauge used for refining a vehicle.

But it was costly and heavier than this pan.

Sandwiched polymer

So it was decided to fabricate an oil pan using a sandwich composite, metal - rubber - metal. This would provide damping to the pan when vibrating under the engine vibrations. Which was an excellent idea.

Manufacturing process

However, the idea was thwarted by the manufacturing process, which will also explain the final shape of the pan.

When forming the oil pan shape, a deep drawing process was used. (this also explains the reason why the sandwich material is not adequate for forming.)

When formed, the inner metal sheet at a bend would follow the tool shape, and the outer metal sheet would follow the inner metal sheet shape, albeit for the rubber between the two.

The rubber did not provide resistance under bending, and therefore the two sheets made contact on the radius of a bend, causing a stress concentration.

This stress concentration was aggravated by the number of pans pressed before oiling the tool.

The tool was oiled every 100 cycles, which was inadequate for repeatability of the shape of the finished product. Increasing the stress of the bend, as the tool lost lubricity, as the metal was under more strain during the deep draw forming process of sheet metal.

This caused the oil pans to crack at random engine hours during engine endurance testing.

The engine hours completed before cracking was indirectly proportional to the number of the pan after oiling the tool, i.e. if the pan was formed just be fore oiling the tool, the pan competed the least hours on the engine endurance test.

The solution

The solution was to revert back to a single sheet of metal, using the same tool; as the tool couldn't be changed or the pan redesigned, so the pan kept its basic shape.

The pan also required stiffening in certain areas due to the now undamped resonances and corresponding mode shapes, hence the swages in the pan for stiffening up the oil pan.

The disc shape in the pan is for the oil pickup pipe that's a conduit to the oil pump.

The moral of this case study

The sandwiched metal-polymer-metal composite sheet is useful for vibration damping, but in this case it was not.

The purpose of the swages is to stiffen the base of the oil pan, consequently increasing the natural frequencies and changing the mode shapes of the pan, and in this case raising the natural frequencies outside of the maximum operating speed of the engine; for the second order of a diesel engine (the worst vibration), 4600rpm/30 = 153Hz.

Vibration Control

Vibration control is most effective in a design with continuous model updating of the finite element analysis model using test data, this is an iterative process to confirm that the design will be successful.

Design for structural dynamic optimisation

In this case, the base of the pan could have been domed, i.e. no flat regions which flex easily under vibration; increasing stiffness of the base.

However, as an after thought, it would not have been possible, as the tool was already made, as a new tool would have been required and at an increased cost to manufacturing.

Therefore, only small modifications could be accommodated, hence so many swages.

New technology vs proven reliability

New material and new technology can be harnessed for good intentions, but a deep understanding in vibration, sheet metal forming, stress and optimised structural dynamics is also required; however, this combination of knowledge and experience is rare.

Further Information

For more information or help with a retrospective noise assessment, please call us on 01908 643433.