THE FUTURE OF PREFORMED CABLE HARNESSESambientconditionsforelectricalcablesarebecomingtougher–forexampleforharnesseswhicharealready

beingmountednotjustalongsideenginesandtransmissionsbutincreasinglyalsoinelectrifiedpowertrains.

Thepreformedcableharnessprovidesaverygoodoptionformeetingtheserisingrequirements.althoughthe

productandtheprocesstechnologyareestablished,thereisneverthelessscopefordevelopmentandespecially

adaptiontohighertemperaturesandcriticalareasofuse,asLeonishows.

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46

reQuirements

The cable harness of a vehicle’s wiring system has to fulfil multifaceted require­ments and demands. An engine cable harness, for example, should be resistant to a variety of media such as fuels and oils, operate in a wide range of tempera­tures and differing climatic conditions and permanently withstand various me ­chanical stresses.

Other aspects such as defined, space­saving routing of cables, mounted at the engine. Costs have to be taken into account in addition to the operating con­ditions already at the development stage. The preformed cable harness provides a very good option for meeting these re ­quirements. Used in a vehicle for the first time in 1991, this technology is now established in mass­production vehicles and has its firm place as a problem solver for difficult applications. The most im ­portant features of preformed cable har­nesses include: : easy, rapid mounting (up to 50 % time

saving) : secure mounting (mounting errors

reduced by up to 90 %; up to 96 % less reworking)

: high packing density in the most limited spaces

: precise cable routing and positioning : optimum routing and fixing prevents

rattling and vibration stress

: protection of the cable against dirt, humidity, temperature, media

: integration of fixing elements possible : established in serial production at

facilities in Asia, Europe and America : preformed cable harnesses are devel­

oped mainly on customer­specific CAD systems and can thus safeguard the designs in early vehicle development phases, ❶.

deFinition

A cable harness is typically limp and has to be transformed into a specific, three­dimensional, preformed geometry using aids – for example cable conduits or tape. In contrast to this, the cable harness is preformed into a stable, three­dimensional geometry using a special material jacket­ing. The geometry nevertheless remains flexible to a certain degree for mounting purposes.

comparison With conventional caBle harnesses

The cables are coated with a polymeric foam. In this step the two components are mixed in a high pressure process and injected into the foam mould without pressure. This makes it possible to com­bine a large number of single wires in the most limited space, the bundled cables also permanently follow the prescribed

tino FranKisdirectorofelectromechanics

withintheresearchanddevelopmentatLeoni

Bordnetz-SystemeGmbHinKitzingen(Germany).

Karl-heinZ GraeFleadsthe“PreformedCable

Harnesses”developmentProjectatLeoniBordnetz-SystemeGmbH

inKitzingen(Germany).

dr. christian dreyerisChemistatLeoni

Bordnetz-SystemeGmbHinNuremberg(Germany).

aUTHOrS

❶Thedevelopmentofpreformedcableharnessexclusivelytakesplaceinclient-specificCadsystems

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routing, which is typically not the case with conventional cable harnesses. Fur­thermore, all required mounting elements can be integrated, which makes later assembly significantly easier. Both metal and plastic parts can be integrated.

By corresponding design, the cable harness’ properties can be altered in such a way that its flexibility is main­tained or avoided in a targeted way, which is not possible by such means with conventional cable harness jacket­ing (cable conduit, corrugated pipe, hose or tape). As the jacketing works not only around the cable bundle, but also com­pletely fills it out, ❷, this technology also provides the cables with protection against dirt, humidity and temperature influences and to a crucial extent reduces

vibration stress, which in the case of conventional technologies can cause the cable insulation to wear away.

Whereas on conventional cable har­nesses the position of the fixing elements and connectors is spatially not defined, and fitting can therefore lead to deviation and increased mounting times, the pre­formed cable harness is consistent in its layout and the position of all components.

This results in a reduction of assembly times, error sources and thereby in lower reworking costs. Especially in the case of cable harnesses under heavy stress in commercial vehicles this can yield savings in guarantee costs by reducing field failures. An important factor in cost assessment is comparison of the system costs, meaning consideration of the assembly costs of the

corresponding vehicle, any reworking costs and guarantee costs in addition to the costs of the cable harness, ❸.

Further developments and Future areas oF application

Leoni has already been working, in its Wiring Systems Division, for many years on the development of new polymeric foam materials for preformed cable har­nesses. The Euro 6 and US 10 compliant engines that will be coming on the market soon must meet the new emission stand­ards required by law. Achieving this in ­volves, among other means, the use of new exhaust gas after­treatment methods that raise engine temperature. Cable har­nesses that are mounted alongside and

❷Foam-coatedcablebundle

❸Costcomparisonofconventionalcableharnessversuspreformedcableharness

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-001

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even in the engine therefore have to with­stand higher operating temperatures than up to now. The polyurethane foams which are currently used are designed for a con­stant operating temperature of approx. 120 °C (248 °F). The new Euro 6 and US 10 engines exceed this temperature limit. New options are therefore needed. In addition to the solutions currently established on the market, new, higher thermo­stable foam materials make it possible to preserve the advantages of the preformed cable harness. In this further development, Leoni is pur­suing the aim of continuing to use proven process and tool technologies so that existing plants can in the future be used with new foam materials as well.

Together with material suppliers, the Company is developing foam materials based on new polyurethanes and silicones as well as qualifying them for use in cars and commercial vehicles. This requires extensive and exhaustive tests of the new materials’ thermal stability; resistance to the various media with which the cable harnesses may come into contact presents a particular challenge in this respect. One of the strictest requirements involves the resistance to hot engine oils. At Leoni test­ing of foam materials that are sufficiently stable even after 3000 hours in 140 °C hot oils is now under progress, 4.

Applications in transmissions require stability to even higher temperatures than in the engine. It has up to now not been possible to use conventional PUR systems in such cases. Very high temperatures also occur in the proximity of the exhaust sys­tem, where costly screening and heat shields are used at present. The higher the ther­mal stability of a polymer material is, the higher its price will also be. For these rea­sons Leoni is currently working on three classes of high temperature materials so that optimum cost effectiveness, depend­ing on the respective application or oper­ating temperature, can always be found: : For the lower temperature range (130­

135 °C; 266­275 °F), the favoured ma ­terial is an economical, flexible PUR system that can also be used as a pass­through. This material is currently at the qualification stage at Leoni and the customer.

: For the medium temperature range (150 °C; 302 °F), a more rigid material system that is also based on polyurethane is currently at the evaluation stage.

: High­priced silicone foams are an obvi­ous choice for high temperatures from 180 to 200 °C (356­392 °F). However, silicone foams require a significantly modified process technology compared with that established for polyurethane foams. To date, it has been proven that silicone foams can in principle be used in cable harnesses. A range of tests will still have to be run before they can be used in mass production.

Especially on commercial vehicles nu ­merous cable harnesses are in sight, be cause the engines are not completely cov­ered. Coloured design foams are of interest for such and similar applications. The usa­bility in principle of coloured foam systems and foams with design aspects such as cor­porate logos is currently being studied.

Foam coatings made of renewable raw materials (polyols made of lactose, glycer­ine and plant oils) as well as recyclables (polyols based on recycled PUR and PET) make a contribution in the “green car” field. These materials are at present still significantly more expensive than petro­chemically made PUR foams. However, as production quantities become larger the prices of such “green materials” can be expected to drop significantly.

Prefromed cable harnesses are up to now established particularly in the com­mercial vehicle segment. The develop­ments presented here and the associated, addition al application options suggest that preformed cable harnesses will in the future also be more frequently chosen over conventional systems on cars as well.

❹example:cableharnessafter3000 hoursinanoven(top)at150 °C(302 °F)andafter616 hoursstoredinoil(bottom)at140 °C(284 °F)

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