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Innovative clutch facing materials –Cool facings for hotapplications!

Robert FelgerChristian SpandernMartin HäßlerHans-Dieter Elison

More powerful engines, longer ratios and newlifetime specifications have led to the develop-ment and introduction of more complex clutchdesigns. Along with other important compo-nents, the facing is the heart of the clutch. Look-ing back at the history of the clutch facing, itbecomes apparent that the technologicaladvancement of the clutch has had very littleimpact on clutch facing technology. LuK hasbegun to tap this potential.

History ofthe clutch facingThe very first cars used the first dry-runningclutch. Facings made from beech and oak woodserved as friction material. The invention ofphenolic resin at the beginning of the 20th cen-tury laid the cornerstone for the facing technol-ogy customary today. Quickly, the advantagesof phenolic resin became apparent and wereput into use as a binding agent in brake liningsand clutch facings. For the first time, compo-nents of any shape could be made of an easilyformable material, which – after appropriatehardening – remained hard even under intenseheat.

Clutch facings as we know them today have beenmanufactured since around 1930. Already in the1930s, comprehensive material tests were per-formed on clutch facings on inertia test rigs

beyond 400°C. Fully synthetic polymers withexcellent final properties that can be processedefficiently are still the basis of the clutch facingtoday.

As in many other cases, when conventional tech-nologies reach their limits, new approachesneed to be found to surpass these limits. The following presentation shows the requirementprofile, the limits of the current state of the art,and provides a glimpse into the future of theclutch facing.

Clutch facing requirementsTemperature stability andoperational robustnessCurrent complex clutch designs allow for a clutchto last for the entire service life of a vehicle. Forthis to actually happen, the facing must handleall high-load situations without significant lossof lifetime.

The facing must survive occasional severeevents, while affecting friction properties, struc-tural integrity and remaining lifetime as little aspossible. The higher the temperature robust-ness, the greater is the operational robustnessof the facing. At the same time, more powerfulengines with only insignificantly changed instal-lation space conditions increase the power den-sity for the facings.

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Introduction

Figure 1 Friction coefficient test up to 470°C from 1940

Figure 2 Destruction of the facing material under theinfluence of heat

The current technology for dry-running clutchesis a facing made of an organic base material, aduroplastic/elastomer compound with a chemi-cal structure that changes significantly above320 °C and completely decomposes above 450 °C. This means that the wear curve of a fac-ing is a function of temperature and increasesexponentially with temperature.

LifetimeThe wear behavior of the facing is highly depend-ent on the temperature level, which varies great-ly during vehicle operation, the friction powerand the specific unit pressure.

An overwhelming proportion of the distance driv-en is covered at temperatures around or below100 °C, while peakloads can exceed 400 °C. The wearrates then increaseexponentially. A goodfacing has a low wearrate in a broad time-weighted spectrum ofthe different tempera-tures (Figure 3).

High clamp loads foraverage clutch sizeslead to clutches withhigher power density,which lead to a highspecific facing unitpressure.

When friction power is high, the friction surfacetemperature quickly increases to values highenough to damage the base material and thecoefficient of friction drops quickly. The slowheat conductivity of the facing causes initiallyonly the surface to be damaged, provided theheat generation is interrupted on time.

ComfortIn principle, facings can only be used if they con-tribute to damping the power train vibrationsthrough their friction coefficient over slip speedgradient and thus prevent judder. In the past,judder was one of the biggest clutch-relatedquality problems. Judder is noticed when thefirst natural frequency of the drive train is excit-ed so much that the longitudinal vibration of thevehicle can be felt. Even so-called unsusceptiblevehicles with above average power train damp-ing are prone to judder in some areas of the wideusage spectrum.

Structural integrityIn practice, facings are exposed to a wide spec-trum of centrifugal forces and temperatures.For a clutch to be able to function under all cir-cumstances, the facing must have a high struc-tural integrity. High-revving engines and wideratio transmissions lead to challenging burstspeed requirements.

The burst speed requirements and thus the ten-sile strength of the facing material is an impor-tant criterion for the clutch facing require-

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Figure 4 Requirements of the centrifugal force influence on the material compoundstrength

Figure 3 Facing wear as a function of temperature

ments. The burst speed must significantlyexceed the maximum engine speed becauseinadvertent mis-shifts, in connection with capa-ble transmission, lead to very high clutch discspeeds.

Occasional high temperatures can significantlyreduce the facing strength permanently. There-fore, state of the art facings must exhibit highmaterial strength even after taking the facing toits thermal limit.

Coefficient of frictionThe clamp load and coefficient of frictiondetermine the torque in the clutch. The higherthe coefficient of friction, the lower are theforces needed in the clutch, and in the actua-tion system. The goal in facing developmentis thus to achieve the highest possible coeffi-cient of friction while all other facing proper-ties remain acceptable. Theoretically, levelsbetween 0.1 (slide friction) and 1.0 (tyres) are possible. When the whole temperaturerange and all of the facing properties are con-sidered together, a lower boundary limit of 0.2is achievable. It is, however, important tomaintain a constant coefficient of friction.

Installation space and weightThe total wearreserve of the facingsconsists only of a fewmillimetres and thusmakes up onlyapproximately one-third of the facingthickness. The major-ity of the facing thick-ness is needed formounting the facings.

Good transmissionshiftability requires alow inertia from theclutch disc and thusthe facings. The goal isto keep the volumeand specific weight ofthe facings as low aspossible.

CostWhen considering large production series, thereare two drivers that determine the cost of the fac-ing: material expense and processing expense.On a more detailed level, however, there are twoadditional cost drivers to be taken into account:design effort for the clutch and the warrantycosts, which both are influenced significantly bythe facing properties.

Conflicting goalsfor facing developmentOrganic friction facings represent a satisfactorycompromise of the required properties. A compro-mise, because some tribological and structuralfacing properties are diametrically opposing. Inaddition, there are interactions between all facingproperties. That means that when one property isoptimised, all of the others change with it.

The macroscopic properties of a facing are theresult of the interaction of its material contents.A clutch facing recipe contains up to 25 differentraw materials. The many raw materials create acomplex multi-component system with evenmore interactions.

The large number of raw materials and theamount of possible interactions leads to exten-sive testing and requires to work systematically.

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Figure 5 Conflicting goals in facing development

Actual facing solutionsBase technology-wovenfacingsApart from meeting certain basic requirements,cost is of primary concern for normal applica-tions. State of the art Fare facings made from awoven composite of resin-impregnated yarn.

The development goal is to find a balanced com-promise of all facing requirements.

Woven facing with functional divisionThe result of the facing development is a bal-anced compromise of all of the facing's technicalproperties. In order to achieve a larger degree offreedom in the development of clutch facings, afacing with two functional areas can be con-ceived.

The base layer of the facing serves as the basisfor structural integrity and strength and is opti-mised in this regard. The friction layer of thefacing can be tribologically optimised for long

lifetime and coeffi-cient of friction per-formance. A thin sur-face coating ensuresa high initial coeffi-cient of friction andalleviates the needfor break-in.

Strength and wearbehaviour are influ-enced overwhelm-ingly by the types offibres used. Bothlayers differ withregard to the typesof yarn and carryingfibre structures,whereas the samebonding agent withthe same fillers isused. Material flowof the agent betweenthe layers improvesthe bond betweenthe layers.

The material C3002achieves with thismulti-layer technolo-gy a high degree ofstrength, even aftercontinuous highthermal load, whilealso having verygood tribologicalproperties.

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Figure 7 Structure of the LuK C3002 facing as a wound-type facing in two layers

Figure 6 Production cycle of a wound-type facing

Dry powder mix facing withfunctional divisionThe idea of dividing the clutch facing into differ-ent functional areas was first launched withLuK's dry powder mix facing RCF-1. This facinghas been produced by LuK for many years and isa proven technology in millions of vehicles.

The dry mix organically bonded clutch facinghas the same chemical structure as the wovenclutch facing. There are two principle differ-ences. The reinforcing fibres are random-orient-ed in the facing instead of being incorporatedstraight in the form of yarns. In addition, theduroplast-elastomer compound consists of apowder mix instead of a liquid solution. A steelplate in the facing ensures structural integrityand strength.

Slim Disc conceptThe high power density, particularly for the east-west power train applications, requires clutcheswith compact dimensions and high thermalcapacity. In many cases, the thickness of thepressure plate represents a compromisebetween the necessary thermal capacity andavailable space. Thinner clutch discs and thusfacings contribute to a sturdier clutch due tothicker pressure plates.

The development of LuK's “Slim Disc” designtakes these requirements into consideration.“Slim Disc” combines the steel plate reinforcedmass-pressed facing technology developed byLuK with a new type of mounting concept. Thefacings hold their parallel shape through theentire temperature spectrum. This is achieved by tabs between the sides of the facing padretainers.

With the same wear allowance, approximately 2mm of installation space can be gained. As aresult, thicker pressure plates and flywheels,thus increased thermal capacity, are possible,which contributes significantly to a more robustclutch.

High temperature conceptSlim Disc HPFWith the development of the new HPF (HighPerformance Facing) material system based on a special temperature-resistant Duroplastand the previously described disc design, LuK has succeeded in creating a clutch discthat functions reliably at high temperatureswhile still fulfilling all other facing require-ments.

The brittleness and the process technologyfavours producing the facing material in the formof facing pads. As with the Slim Disc design,dimensional stability at high temperatures is

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Figure 9 Slim disc design with inside radially mountedfacing rivets

Figure 10 Advantage of the new high-temperature HPFmaterial

Figure 8 Structure of the LuK RCF-1 facing with steelretainers

achieved using a tabconnection and thepads are mountedradially on the insideof the clutch disc.

In addition to aclutch that is ther-mally optimised bya pressure plate with higher thermalcapacity, the HPFconcept allows formuch higher tem-perature levels onthe friction surfacesand in the ambient.The clutch facing material is no longer the thermally limiting factor.

Other facing materialsCerametallic materialsCerametallic materials are used in tractors andheavy construction machinery and if outstand-ing temperature resistance and high frictioncoefficients are desired. Serious weaknesses inlaunch behaviour and opposing material wearlimit its applications.

Carbon fibre compoundsCarbon fibre compounds exhibit a very interest-ing property. The friction coefficient increasesover temperature, particularly above 300 °C.The cost situation and the wear behaviouragainst conventional materials are, however,problematic. Therefore, carbon fibre com-pounds are only used in aeronautic and racingapplications.

Ceramic materialsCeramic materials based on silicates, carbidesor oxides are still in the experimental stage.Ceramic materials are so far used primarilyas replacements for cast iron as friction sur-face, while the wearing friction element stillremains an organic material compound. Inter-esting new applications using inorganic fric-tion elements still show too much abrasivewear.

SummaryLuK can offer a broad range of facing technolo-gies, from the widely used base technology tovery economical new solutions to high-end solu-tions for very demanding applications. LuK canprovide a tailor made facing technology for everyvehicle application.

OutlookThe development of clutches and facings hasbecome more closely interconnected and theinterdisciplinary cooperation of chemical andmechanical engineers has created new solutionswith concrete potential for mass-production.

More concrete ideas are in early developmentstages and are bound to lead to further techni-cally and economically interesting clutch facingalternatives.

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Figure 11 Structure of the Slim Face HPF high-temperature concept

Figure 12 Product range of LuK clutch facings