High-performance Friction Elements
for Automatic Transmissions

Knowledge Library

whitepaper_Knowledge Library Reibelemente.indd 125.04.14 13:39

Knowledge Library

High-performance Friction Elements
for Automatic Transmissions
Modern drivetrains and new trends in transmission design require compact clutches that provide smooth shifting capability and high fuel-efficiency. Such clutches
need to be equipped with suitable friction enablers that can handle high temperatures, and facilitate stable friction characteristics as well as higher coefficients of friction. By optimising wet friction elements and offering design solutions, BorgWarnerprovides improvements in various aspects of clutch operation.
By Harald Merkel, Technical Specialist Friction Technology, BorgWarner

Trends and Challenges

Current global transmission design trends aretowards reduced package size or increased
torque density, improved transmission efficiency, improved shift quality, and increased durability. In addition, recent developments ofeight-speed to ten-speed automatic transmissions have created a need for friction materials
that can handle the increased power density
and increased energy levels. Along with the requirement of minimal dimensional changes in
the lining, smoother clutch engagement necessitates the friction torque curve to maintain apositive controllability (u-v relationship) throughout the lifetime of the transmission under various operational conditions, including higher
pressure applications.
The chemical and physical interactions of various fluids with friction materials play a key roBorgWarner Knowledge Library 201 4

le in affecting the key friction performance of a
wet clutch system. Wet friction elements areused in shifting or starting clutches inside conventional stepped automatic transmissions,dual clutch transmissions (DCT) and continuously variable transmissions (CVT) as wellas in torque converter lock-up clutches, transfer case clutches and disconnecting clutches inhybrid applications.
Various clutch designs make high demands on
applied wet friction elements, which not only
need to provide stable friction characteristics
and high temperature resistance but must also
be able to handle higher torque and/or limited
oil flow. Higher torque can be generated either
by the use of high friction coefficient materials
or by the use of higher unit loads by applying
higher pressure. The use of simulation tools
can help to optimise the friction plate design to1

reduce drag torque. New ideas for the coreplate design reduce axial length, weight andcost. [1 ]Development Steps in Friction Plate
DesignAccording to the specific requirements for eachapplication, the friction plate design is uniquefor every single application. The design process is started by the Statement of Requirements (SOR) from the customer. The first stepin this process is the verification of the geometrical layout by calculation of net pressure on
the basis of the required torque capacity. Thesecond step is the thermal calculation of theinterface and oil outlet temperature. The input
here is the shift cycle by the customer.
The third step is the drag torque calculation,using analytical, CFD and neural network simulation tools. The fourth step is the lifetime
prediction using the duty cycle of the customer
as input data.

Based on durability testing on different energy
levels, it is possible to predict the lifetime of thefriction system. The result of the whole process
is the definition of the friction plate design interms of the lining/friction material, groove
geometry and core plate geometry as well as
the definition of manufacturing in terms of segmenting and post processing.

High Surface Adsorption Capacity

Modern friction materials feature improved heat
resistance, thus enabling safer operation overthe entire lifetime, even at lower cooling flows.
However, the increased demands of newtransmission designs require further and continuous advancements, such as friction elements with high surface adsorption capacity
which readily adsorb the oil friction modifiers in
the automatic transmission fluid (ATF) strongly
on the surface while not being affected by degraded ATF.
To fulfil these requirements, BorgWarner has

Results of the durability test of the friction material during high energy launches
BorgWarner Knowledge Library 201 42

Results of the durability test of the friction material during high lining pressure shifts

developed a new friction material family. Onefriction material from this family is called BW
691 0, which is especially designed for wet
starting clutches, torque converter lock-up clutches, torque transfer clutches and hybrid disconnecting clutches. The material providesresistance to oil degradation and glazing, evenwith the use of modern low-viscosity oil. The
material’s ability to withstand high interface
temperatures and to maintain a stable and positive µ-v characteristic (shudder resistance) isan enabler for the Low Lube concept.
In this case, the reduction of the cooling oil flow
allows the use of more efficient pump systems
and the optimisation of the efficiency of thetransmission. In addition, the material makes it
possible to handle a higher surface pressure.
This provides the opportunity to reduce thenumber of surfaces or to reduce the friction
diameter. Vehicle measurements of shudder atacceleration with micro slip conditions verified
the advantage of this material type compared
to standard launch friction material.
BorgWarner Knowledge Library 201 4

Hot Spot Resistance

New automatic transmission architectures require a higher differential speed on the shifting
clutch elements. The emerging extreme shifting
conditions at 70 m/s in conjunction with very
low oil flow cause severe accumulation of hot
spots on separators.
The new BW 5000 material family has especially been developed for those applications.
This material is extremely elastic, has a uniform oil retention surface and a high-temperature fibrous surface. Target applications are
shifting clutches in new state-of-the art automatic transmissions.

Drag Torque Reduction

Reducing drag losses plays an important rolewith wet clutch systems. There are various parameters that can cause drag losses inside a
wet clutch. For example, in a dual clutch transmission, drag losses can be classified into
three main categories: during pre-selection, id3

Analytical calculation of drag torque reduction

le-D losses and drag losses to the seal rings orbearings.
The cooling oil temperature plays an importantrole, too. However, most of these negative effects can be reduced or avoided by the use ofsoftware optimisations in the transmission control unit (TCU) or by implementation of an engine start-stop system [2]. Nevertheless, clutch
design optimisation and improvements of theapplied friction material are further possibilities
for reducing drag torque.
For this purpose, there are different calculation
tools that can be used for reliable drag torqueprediction. Analytical modelling uses a calculation program with adjustment by actual measurements. The neural network method uses
artificial intelligence for drag torque prediction
on the basis of previously collected measurement data. A third method is the use of CFD
software to perform exact calculations of fluid
behaviour inside the clutch. Recent calculatiBorgWarner Knowledge Library 201 4

Illustration of the two-step lining design

ons finally resulted in the following different
friction material design solutions – dependingon the respective application – to effectively
reduce drag torque:• waved friction plates• waved separator plates4crease the spline contact area. This can beused to reduce the length of the clutch packwithout reducing the contact area of splines.
In an existing clutch, it is possible to increase
the contact area without increasing the lengthof the clutch pack, or to avoid expensive heat
treatment like nitriding when hammering problems occur. In summary, the Hemmed Spline
design helps reduce weight, axial space andmaterial cost while providing the same spline
contact area.
Illustration of the Hemmed Spline design• optimised groove pattern• active separation• two-step lining.
The two-step lining is a new concept that uses
two different friction materials on the friction
surface. A basic friction material appropriate to
the specific application complemented by additional areas with a second very elastic material
with increased thickness. When the clutch isopen, the elastic lining element with an additional height causes an active separation process
and prevents the friction surface from experiencing any suction to the separator surface.
This concept leads to perceptible drag torqueimprovements compared to a waved plate design. It also improves controllability at low temperature and low pressure and results in lesstumbling at high engine speeds.

Core Plate Modifications

Apart from the friction plate design, there isfurther potential for optimisation by modifying
the core plate design. One new concept is theHemmed Spline design. Here, the core platesteel in the spline area is folded double to inBorgWarner Knowledge Library 201 4

A further modification method is the segmentation of the core plates. This new technology allows a much better utilisation of the core platesteel material, which is a reasonable contribution to cost reduction efforts. BorgWarner hasdeveloped a series production process andstarted series production with an OEM customer in 201 2.

Conclusion

Recent trends in automatic transmission design also require improvements to the friction
materials used. The choice of the appropriate
friction product depends on the respective application and must be predicted using simulation tools in close cooperation with transmission
and vehicle manufacturers to guarantee an optimum result. These simulations help to reliably
predict the drag torque and allow optimisationsof the friction plate layers during the designphase. The new BorgWarner BW 691 0 friction
material enables clutch systems with a hightorque density and a low lube strategy, improved durability and improved NVH robustness tobe produced. A reduced cooling oil flow (lowlube strategy) additionally helps to optimise theefficiency of the transmission. What is more,
optimised friction materials such as the BW
5000 family allow shifts to be performed at highdifferential speeds without a severe accumula5

Illustration of the segmented core platetion of hot spots on the separators. Further
possibilities to improve drag torque, packagingand costs are new design concepts such as
the Two Step Lining, a Hemmed Spline design
and Segmented Core Plates.

References[1 ] Lam, R.; Dong, F.: New Generation Friction
Products and Technologies, 1 st CTI
Symposium, Shanghai 201 2[2] Moser, A.; Saxena, V.; Schäfer, M.; Rischel,M.: Insight on wet clutch efficiency, 1 0th CTI
Symposium, Berlin 2011

Contact

Email: technology@borgwarner.com
For more information, please visit
borgwarner.com
BorgWarner Knowledge Library 201 46