Trade Press Conference K 2004 s1

Trade press conference K 2004

on June 22 and 23, 2004, in Ludwigshafen

The long road to success

First thermoplastic truck-engine oil sump

Report by Willi Bartholomeyzik,

Sales Engineering Plastics

In the autumn of 2003, after more than four years in development, a part finally went into production, whose shape, colour and function is nothing to write home about. On the other hand, given the exceptionally tough engineering requirements that had to be met, it represents a real innovation. The oil-sump for Mercedes Actros truck engines is the first in the world to be made from a thermoplastic. The sump is a large component, usually made from either light alloy or SMC. The efforts of the part's developers and manufacturer, Kunststofftechnik Sachsen (KTSN), were honoured last July when the sump won the Grand Innovation Award 2003, presented by the Society of Plastics Engineers (SPE). BASF contributed to this success story by providing not only Ultramid® nylon moulding compound but also considerable engineering expertise (figure 1).

Requirements

The main purpose of the oil sump is to contain the engine oil (approx. 39 litres), while providing a tight seal around the crankcase. The sump needed to withstand very high static loads, equivalent to the weight of the engine. This is because, in the case of the Actros truck, the engine is rested on the sump while being installed in the vehicle. In addition, the capacity of the new sump had to be increased by some 30%. This was only possible by introducing bulges into the sump's side walls, which protrude well beyond the outline of the flange.

Other requirements that had to be met within the agreed cost framework included weight reduction, optimization of the shape, reduction in vibration and an effective gasket system against leakage. The type of effort that was necessary to achieve these goals can be seen from the following examples taken from the part's development and optimization phases.

Simulations

Computer-aided engineering methods were employed in the early development phase to simulate the moulding process and analyze the part’s structural static and dynamic characteristics.

Starting with a wire-frame model, a mould-fill analysis was carried out and the fibre orientation and resultant warpage determined. The simulation revealed that the two longer side-walls would warp inwards – a fault that even optimized processing conditions (eg, different mould core temperatures) could not rectify. To counter this, two cross struts were introduced inside the sump to buttress the side walls. The flatness of the flange face was judged not to be critical from the results of this simulation – something that was confirmed later on the moulded part (figure 2).

The simulation of static internal pressure shows how the base of the sump bulges outwards. This deformation causes large stress peaks to appear around the periphery (due to a hinge effect). Such stresses can only be reduced to an acceptable level by an optimized rib pattern (figure 3).

The oil sump experiences the greatest static load when the fully assembled engine, including gear-box, is set down on a surface (figure 4). The centre of gravity is such that the total weight of nearly two tonnes is borne solely by one edge of the sump. Most of the load is transmitted by the stiff regions at the corners, where very high stress peaks occur. Only by fitting a load-distributing rubber foot is it possible to reduce the stress peaks to within the permitted limits (figure 5).

The investigation of the part’s dynamic properties mainly concentrated on modal analysis – the determination of the characteristic shapes of the vibrating structure. Here, computational methods were used to determine the resonant frequencies.

Experimental methods

The vibrational behaviour of the sump was also investigated experimentally. Examples of the thermoset production part, cast from glass-fibre reinforced nylon, were used for this purpose. The sump was excited by striking it at specific points on its surface with a hammer fitted with a force transducer. A set of electronics then worked out the location of abnormally large vibrations (eigenforms) and the associated resonant frequencies (eigenfrequencies) (figure 6).

The eigenforms of smaller areas of the sump can also be identified by placing the measurement points closer together.

At the next stage of development, the vibrational and acoustic properties of a near-production part were investigated more closely by means of Speckle interferometry, a laser technique for visualizing the individual resonance states or eigenmodes. In contrast to the previous hammer-impulse technique – in which many eigenmodes are stimulated simultaneously – a device known as an electro-dynamic shaker was used to stimulate the sump at a specific frequency within the frequency range being investigated. The pictures generated show the resonance states as a series of interference fringes spaced at 0.25 mm. The beauty of this method is that the part’s vibrational and acoustic characteristics can be quickly optimized at attaching stiffening ribs to the surface of the sump (figure 7).

P 254e

Leak and internal-pressure tests were carried out to check the reliability of the crankcase seal. A customer-specific test, the so-called “set-down test”, confirmed the sump’s ability to carry the full weight of engine and gearbox. The curve in the figure shows how the sump responds to a static load up to the point of failure (figure 8).

Experimental modal analysis, noise level measurement, engine test-bed trials, passing-vehicle noise tests, stone impact tests, static and dynamic bursting tests all verified the computational predictions. Compliance testing of prototype parts was carried out by BASF in collaboration with the customer and vehicle manufacturer throughout the development process.