19-10-2012, 10:40 AM
POLYMER MATRIX COMPOSITES IN DRIVELINE APPLICATIONS
POLYMER MATRIX.PDF (Size: 354.18 KB / Downloads: 96)
ABSTRACT
GKN have been delivering carbon composite propeller shafts to production car applications since
1988. The technical drivers for use of composite material in this application are very powerful; the
weight saving in particular can be considerable. Despite this, the overwhelming majority of automotive
propshafts continue to be produced from steel.
This paper presents a review of GKN’s experience of some important commercial and technical issues
(weight analysis, crash performance, NVH, joint connections, recycling) applicable to composite
propshafts. GKN remain committed to the benefits of composites in driveline applications and are
actively developing new technologies to strengthen the commercial potential. The paper presents a
description of such developments, including work on composite couplings for propeller shafts, which
bring a further weight saving. An ultimate lean weight propshaft has been demonstrated with a mass
of 2.7kg for the complete system; the steel alternative had a mass of 10kg.
INTRODUCTION
Over 70% of rear-wheel-drive or four-wheel-drive passenger cars are built with multi-section propeller
shafts which require one or more support bearings under the floor of the passenger compartment The basic attraction of composite materials for propeller shaft applications is that they make it possible to increase the shaft length, which is otherwise constrained by bending resonance. For many vehicles,
a one-piece composite shaft may replace a two-piece steel shaft, which simplifies both the shaft and
installation in the vehicle.
GKN COMPOSITE PROPSHAFT APPLICATIONS
Renault Espace Quadra
The Renault Espace Quadra, launched in 1988, was the pioneering application for composite
propshafts in production vehicles.
A one-piece composite shaft was specified, in place of the alternative two-piece steel shaft solution.
The majority of Renault Espace production was front wheel driven vehicles; use of a composite shaft
for the four wheel drive versions reduced the enginering modifications required for the floorpan. The
floor was in any case sensitive to noise and vibration inputs, which were improved by the absence of
a propshaft centre support bearing. The composite propeller shaft system weighed 5kg, compared to
10 kg for the two-piece steel alternative. The vehicle remained in production until 1996, at which time
the Quadra version was deleted from the product range. This was a consequence of other engineering
changes which led to the orientation of the engine becoming transverse instead of longitudinal, and four
wheel drive was then no longer practical.
Renault Safrane Quadra
The Renault Safrane Quadra was introduced in 1992 with a two-piece shaft (front steel / rear
composite) in place of a three-piece steel alternative. The high bending mode frequency of the
composite section (>200 Hz) improved the vehicle NVH, and there was an overall weight saving of
40%. Small numbers of this vehicle, approximately 500 per annum, were delivered until 1996.
Toyota Mark II
Development of the composite propeller shaft for the Toyota Mark II is described in reference (1). The
shaft incorporates a novel technology for making the connections between the composite tube and the
steel couplings at the ends. The Renault applications described make use of a bonded connection,
whereas an interference connection method was developed for the Toyota Mark II. The ends of the
composite tube were reinforced by hoop-wound carbon fibre, with steel end parts inserted by force
to overcome a designed interference.
The Toyota Mark II is a rear wheel drive luxury car, and the one-piece composite shaft solution was
used in place of a two-piece steel alternative for a segment of production from 1993 to 1996. There
was improved NVH performance and crash behaviour, and a weight saving of 6 kg (50%).
CRASH PERFORMANCE OF COMPOSITE PROPSHAFTS
Increasing public interest in safe vehicles is encouraging car manufacturers and their suppliers to design
components and systems that will perform well in a crash (2). The propeller shaft in rear- and fourwheel-
drive cars is a good example of this. In a frontal crash, the propeller shaft transmits forces from
the engine / gearbox unit to the rear axle. Many vehicles today have a two-piece propeller shaft that can buckle at the centre bearing in any direction, depending on the joint position at impact. It is
therefore virtually impossible to predict the axial force and the energy absorbed by the shaft in a crash.
This is illustrated in Figure 4, contrasted with the behaviour of a propeller shaft with a defined axial
collapse mode.
The target for crash-optimized propeller shafts is to achieve a defined behaviour of axial force and
displacement during an impact, and consequently controlled energy absorption. Not all vehicle
platforms require the same behaviour, the Audi Quattro described earlier required collapse at very low
axial force so that the differential was decoupled from the engine / gearbox unit. In other applications,
the propeller shaft has been designed to meet a specific energy absorption characteristic so that loads
in the front footwell area of the passenger cell are reduced.
To specifically test and optimise propeller shafts for crash performance, GKN have designed and built
a dedicated test rig. A falling weight of up to 480kg, dropped from a height of up to 3.5 metres, allows
a maximum impact energy of 16,500 Joules to be realised.
PRODUCT COSTS
Material and manufacturing costs have been recognised as the key issue restraining the take-up of
composite propeller shafts from the earliest stages of development and examination. Kliger et al (3)
presented a review of composite shaft concepts in 1980 in which they examined the potential of
alternative manufacture processes to yield cost effective product. It is interesting to compare the
findings at that time with the current position, and projections into the future.
Material Costs
Carbon fibre cost is key to the viability of composite driveshafts. In reference (3), a mean price of
£20.50 / kg was taken for carbon fibre (with £0.75 / kg for glass fibre and £1.30 / kg for an epoxy
resin system). By the mid 1990's, the benchmark price for carbon fibre had reduced to £17 / kg whilst
those for glass fibre and epoxy resin had doubled. There have since been undertakings from USA
suppliers of high filament count carbon fibre to meet prices of £7 / kg. These pricing trends, and future
projections, have a profound effect on the attractiveness of the product.
COMPOSITE COUPLINGS
70% of the overall propshaft weight is accounted for in a conventional propshaft by the mass of the end
connections and joints or couplings. When the steel tube is replaced by composite, it is the minor share
of the propshaft weight which is reduced. To achieve the ultimate lean-weight propeller shaft, it is
necessary to develop low weight end fittings. GKN have published work in this area (4), which has
led to the ultra-low weight propeller shaft proposal shown in Figure 5. This system has an overall
weight of 2.5kg, replacing a conventional two-piece steel propeller shaft sytem weighing 10kg.
End connections have been developed which comprise an injection-moulded 3 arm flange (in place of
conventional flanges produced from cast iron or steel) and the GKN Composite Disc Joint provides
the function of a constant velocity joint or Hookes joint in a flexible element with a mass of just 55
grammes. The composite disc is restricted to applications with small working angles, but is appropriate
for a significant share of the propeller shaft market. Recent further developments in technology of the
Composite Disc Joint are described in reference (5).
CONCLUSIONS
The potential for carbon fibre composites in automotive propeller shafts as a means of achieving
substantial weight reduction has long been recognised, and has been demonstrated in small volume
applications since 1988.
The main barrier to large scale penetration of the market has been product cost, but industrial
developments in recent years offer the prospect of substantial reductions. Having achieved competitive
cost, the next most significant barrier has been methods for recycling, but it is shown that a costeffective
solution to this issue also exists.
GKN remain committed to the benefits of composites in driveline applications, and are actively
developing the technologies needed to realise their full commercial potential.