Organic Drivetrain Friction Elements

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Organic Drivetrain Friction Elements
Demanding requirements and emerging trends for modern transmissions mean thatnew developments in wet friction elements are much needed
By Harald Merkel, Technical Specialist at BorgWarner Transmission Systems

Introduction

The recent global automotive market trends formodern drivetrains require compact, smoothshifting and highly fuel-efficient designs fortransmission technologies. High-performance
friction solutions need to withstand high temperatures and feature stable friction characteristics in combination with higher coefficients offriction, while further preventing hot-spotting.
Automatic transmissions with eight to ten gears
triggered the development of friction materials
that can handle the increased power density andenergy levels. In addition, smoother clutch engagement necessitates a consistent positive µ-v
relationship of the friction torque curve undervarious operational conditions over the transmission's entire lifespan. Wet friction elements
are used in dual-clutch transmissions (DCT),

Figure 1 . BorgWarner’s optimized friction
elements provide high torque density, low lube
strategy and improved durability as well as NVH
robustness
BorgWarner Knowledge Library 201 5

continuously variable transmissions (CVT) andmultistep automatic transmissions, as well as
in torque converter lock-up clutches, transfer
case clutches, and disconnecting clutches inhybrid applications. These different clutch designs, one of which can be seen in Figure 1 ,place high demands on applied wet friction
elements, which have to provide stable friction
characteristics as well as high temperature resistance, while handling higher torque and/orlimited oil flow.

Friction Component Design

The friction plate design is unique to each individual application and is modeled according to
its specific requirements. The design process
then starts with the statement of requirementsfrom the customer, followed by the verificationof the geometrical layout by calculating the netpressure on the basis of the required torquecapacity. Subsequently, the thermal calculationof the interface and oil outlet temperature basedon the input of the shift cycle, as well as dragtorque calculation, are carried out using simulation tools.
The last step is to make a lifetime prediction on
the basis of durability testing on different energy levels. The whole process provides thedefinition of the friction plate design in terms of
the lining/friction material, groove geometry
and core plate geometry, as well as further
providing definition of manufacturing with regard to segmenting and post-processing.

Friction Material Development

BorgWarner has developed a friction materialfamily with a high surface adsorption capacity1enabler for shudder resistance over the wholelifetime. The extremely elastic BW 5000 friction
material features a uniform oil retention andhigh-temperature fibrous surface. It was developed for applications with a high shiftingenergy and extreme shifting conditions at differential speeds of up to 1 00m/s. It can also
handle low cooling oil flow, which causes severe accumulations of hot spots on separators.
Since the new transmission architectures require a higher differential speed of the shifting
clutch elements, the BW 5000 is used for shifting
clutches in state-of-the-art automatic transmissions.
Figure 2. BorgWarner developed innovative
groove designs and uses advanced friction materials for a stable friction coefficient and improved temperature resistanceto fulfill the requirements of current transmission designs. The material BW 691 0 is used forwet starting clutches, torque converter lock-up
clutches, and hybrid disconnecting clutches. It
provides resistance to oil degradation and glazing, as well as a higher-pressure capability.
Additionally, it features high interface temperature resistance and is able to maintain a positiveµ-v characteristic. These abilities qualify thematerial, which is illustrated in Figure 2, as an

Drag Torque Reduction

Most negative effects causing drag losses in a
wet clutch can be reduced or avoided by using
a stop/start system or by using software optimizations in the transmission control unit (TCU).
Nevertheless, improvements to the clutch design are needed to further reduce drag losses.
Various calculation tools such as analyticalmodeling, the neural network method and CFD
software are used for reliable drag torque prediction. The calculations resulted in five different friction material design solutions to effectively reduce drag torque depending on the application in hand.
One solution is the two-step lining, a concept
which makes use of two different friction mater-Figure 3. Illustration of the segmented core plate
BorgWarner Knowledge Library 201 52Figure 4. High-efficient dual clutch designials on the friction surface, including one basic
friction material appropriate to the specific application and one very elastic material for increased thickness. This elastic material causes
an active separation process and prevents thefriction surface from experiencing any suction
to the separator surface when the clutch isopen. The result is improved controllability at
low temperature and low pressure, which enables less tumbling at high engine speeds andoffers significant torque improvements compared with a wave plate design.

New Design Solutions

Modifications of the core plate design, such as
the hemmed spline or the segmented coreplate design, allow further improvements tobe made. The hemmed spline design features
a double-folded core plate steel in the spline
area, which can be used to decrease the lengthof the clutch pack without reducing the contact
area of the splines. In addition, it offers thepossibility of increasing the spline contact areaof existing clutches without added clutch packlength, and it helps to avoid expensive heat
treatment such as nitriding when hammering
problems occur. The segmented core plate deBorgWarner Knowledge Library 201 4

sign, which is illustrated in Figure 3, allows fora better utilization of the core plate steel material to reduce costs.
Because of recent industry trends, BorgWarnerhas improved its friction materials and friction
element designs, as can be seen in Figure 4,
working in close cooperation with the transmission and vehicle manufacturers using simulation tools to optimize the performance for the applications involved. For this reason, BorgWarnerhas developed the friction materials BW 691 0and BW 5000 for improved durability and NVH
robustness. The hemmed spline design helps
to reduce axial space and weight. The segmented core plate design enables lower material
costs, while maintaining the same functionality.
In addition, the two-step lining concept is afurther possibility for improving drag torque,packaging and costs.

Contact
Email: technology@borgwarner.com
For more information please visit
borgwarner.com
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