812 Lightweight shafting CV jointing and road wheels

Between the CVT and the tyre ground contact patch, there is considerable scope for weight saving and the reduction of rotational inertia in the drive-line. A development programme by GKN has led to substantial weight reductions in most elements of the drive-line for front-, rear- and all-wheel drive systems, Fig. 8.22. Reduction in weight has also had a knock-on effect in obtaining increased whirling speeds and therefore the ability to have prop-shaft UJs without the risk of vibration problems.

With the group's latest generation of half-shafts for both front- and rear-end application, there are 11 standardized sizes available. Lower weight is accompanied by higher resistance to strain, reduced vibration and easier assembly/repairability. In the case of prop-shafts, improved acoustics have been achieved by decoupling of torsional and axial vibration as well as less run-out resulting in reduced wear. Transmission efficiency of universal joints has also been increased and systems developed for providing controlled collapse of the shaft under front-end crash loading of the vehicle. A choice of steel, aluminium alloy and high strength composite systems allows tailoring shafts, and their couplings, for particular operational applications.

Steel shafts have been lightened by the use of high tensile materials along with the appropriate design modifications and revised joining methods. Aluminium alloy shafts can be up to 50% lighter than conventional steel shafts provided appropriate design modifications are made. Metal matrix/composite solutions, involving aluminium with ceramic inclusions, are also possible. Metal/ composite combination shafts have also been successfully exploited on applications where three-shaft assemblies have been replaced by two-shaft systems. After a first generation of composite shafts has proven the effectiveness of resin systems reinforced by glass and carbon fibre in achieving up to 70% weight reductions over traditional shaft assemblies, the group has now announced a second generation. This involves end fittings, as well as the shafts, in polymer construction and has resulted in overall weight savings of up to 75%. In this second generation, one approach has been to replace the Hookes-type universal joint with a composite disk UJ where only small angularity is required. In such cases a typical steel shaft system weighing 10 kg can be replaced by a firstgeneration assembly weighing 5 kg and a second-generation one of only 2.7 kg, (a). Other advantages of second-generation shafts include increased torque capacity, (b), 40% increase in static break value and a new resin system which provides 15% increase in fracture value at temperatures of120°C. Techniques of introducing damping into the fibre compound help acoustic performance.

In terms of assistance given by the prop-shaft to passenger protection in frontal impact, a key factor is the possible removal of the centre UJ/bearing where the shaft would potentially bend out of line on impact. By the shaft contributing to impact reaction, more protection can be provided to the footwell of the vehicle. By use of a drop-weight rig the group is able to produce crash-optimized cardan shafts in a variety of material combinations and geometrical configurations, (c). In the case of composite shafts, radially aligned reinforcing fibres can be used at the ends so that the end pieces are pushed into the main tube on initial impact such that they lie up against the joint shoulders. Thereafter the tube is split open on one or both sides with energy being reduced both by splitting and friction. With aluminium-alloy shafts, an axially weak point in the centre of the tube is the focal point; by using two different tube diameters either side of it, and carefully designing the transition area different impact characteristics can be obtained, by the smaller tube sliding inside the larger one. Special press fit connections of the end pieces can also be provided. The graphs in (d) and (e) show the critical shaft length and minimum required diameter plotted for different shaft materials, and fibre reinforcements, at a critical rotating speed of 7200 rpm. Fibre-reinforced types have strengths related to fibre orientation as shown in the second figure. As the shear strength

Traditional system

1st generation composite

140 120 100 80 60 40 20

Axial force generation percentage (x-axis) against fibre orientation angle in degrees (y-axis) at 685 Nm torque and 340 rpm speed with conventional joint (full-line) and low-friction joint (broken line)

140 120 100 80 60 40 20

Axial force generation percentage (x-axis) against fibre orientation angle in degrees (y-axis) at 685 Nm torque and 340 rpm speed with conventional joint (full-line) and low-friction joint (broken line)

Fig. 8.22 Lightweight drive-shaft assemblies: (a) shafts compared; (b) new-generation shaft characteristics; (c) joint performances; (d) modulus vs fibre angle; (e) Shaft diameter vs critical speed.

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of a shaft, with 0° fibre orientation, is comparatively low; (e) is based on a realistic orientation of 15°. Usually multi-layer composites are involved and the elastic behaviour of any defined combination of layers is generally derived from the properties of one unidirectional layer, defined by the modulus parallel and perpendicular to the fibres, one of the two Poisson's ratios and the inplane shear modulus.

The company also recently released a series of lightweight shafts incorporating some of the technologies just discussed as well as considerably redeveloped universal joints. These assemblies involve constant velocity joints of the AC (ball type) and GI (tripode type). All shafts involve a boron steel with deep-case hardening. Within the downsized AC joint, point loadings of up to 3 GPa are involved for which a new grease has been developed to ensure the required performance. Tribological engineering was used in the analysis and has shown areas of subsurface 'contact' stresses beyond conventional Hertzian stress theory, surface, finish effects.

8.12.1 STRESS DISTRIBUTION IN ROAD WHEELS

While considerable weight savings have been achieved by constructing road wheels in aluminium and magnesium alloys there is also scope for lightening conventionally fabricated wheels by understanding the magnitude and direction of their internal loading. For those contemplating the design of special road wheels, Fig. 8.23(a), it is useful to consider the forces on an elemental length of rim (b) when trying to determine rim stresses. The stress derived from standard solid mechanics theory is given by

Work due to Svenson13, (b), considered the imposition of stresses from both horizontal and vertical forces at the tyre/ground contact patch, leading to an expression for dynamic factoring of the statically derived stress values as

where C is the spring rate of the tyre.

The complex shape of the wheel disc of the conventional pressed variety makes it difficult to calculate the stress from simple theory. Empirical work carried out at MIRA has shown that bending moments imposed on wheel discs are increased not only by lateral weight transfer but also by fore-and-aft weight transfer, body roll and tyre distortion, as illustrated at (c). Analysis shows torque on a stub axle to be:

Ass A c c in cornering while torque on wheel is

MW = Vscosgld + Vssinglrsl static and MW = Vc cos g2d + Vc sin g2rc1 + Hrc in cornering.

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