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Friction elements - Product Profile

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BORGWARNER’S LATEST FRICTION PLATES ARE IMPROVING AUTOMATIC TRANSMISSION EFFICIENCY

In the development of new vehicle components, manufacturers are obliged to consider customers’ needs, such as fuel efficiency and shifting

comfort. Current trends in automatic transmission design take these considerations into account and focus on reduced package size, increased

torque density and durability as well as improved efficiency and shift quality. Suitable friction enablers with great reliability, even at high temperatures, are essential for this.

Today, modern 8- to 10-speed transmissions require friction materials that can handle increased power density and higher energy levels. In conjunction with the necessity for minimal dimensional changes in the lining, smoother clutch engagement demands a consistent positive ?-v relationship of the friction torque curve throughout the entire life of the transmission under various operational conditions. The friction performance of a wet-clutch system is affected by the chemical and physical interactions of various fluids with friction materials. Wet friction

elements are used in shifting or starting clutches and have to provide stable friction characteristics as well as high temperature resistance.

They must be able to handle limited oil flow and higher torque, which can be generated using higher unit loads or materials with high friction

coefficients. In addition, simulation tools help to reduce drag torque by optimizing the friction plate design.

Being based on specific requirements, the design of the friction plate is unique for each application. On receipt of the Statement of Requirements (SOR) from the customer, the first steps in the design process comprise the verification of the geometric layout by calculating the net pressure

on the basis of the required torque capacity and the thermal calculation of the interface and oil outlet temperature according to the shift cycle specified by the customer. After drag torque calculation using analytical, CFD and neural network simulation tools, the fourth step covers a lifetime prediction based on duty-cycle data and durability testing at different energy levels. All steps result in the definition of the friction plate design with regard to lining/friction material, groove geometry, core plate geometry and manufacturing in terms of segmenting and post processing.

Modern friction materials provide improved heat resistance even at lower cooling flows, thus facilitating safer operation over the entire lifespan. Also, they necessitate further and continuous advance, such as friction elements with a high surface-adsorption capacity. These readily adsorb the oil friction modifiers in the automatic transmission fluid (ATF) while not being affected by degraded ATFs.

BorgWarner’s newly developed family of friction materials helps to fulfill these requirements. Specifically designed for wet starting clutches,

torque converter lock-up clutches, torque transfer clutches and hybrid disconnecting clutches, the BW 6910 friction material provides

resistance to oil degradation and glazing, withstands high interface temperatures and maintains a stable and positive ?-v characteristic.

It thus enables a low-lube concept to be used, and the reduced cooling oil flow permits the use of more efficient pump systems and optimizes

transmission efficiency. In addition, the friction material facilitates the handling of higher surface pressures and a reduction in the number

of surfaces or the friction diameter. Vehicle measurements of shudder at acceleration under micro slip conditions verified the advantages

of this material in contrast to standard launch friction material.

Recent automatic transmission architectures demand a higher differential speed on the shifting clutch elements. Extreme shifting conditions at 70m/s and an extremely low oil flow can cause a severe accumulation of hot spots on separators. Specifically for shifting clutches in modern automatic transmissions, BorgWarner developed the new BW 5000 friction material family, which is extremely elastic, has a uniform oil retention surface and features a high-temperature fibrous surface.

Reducing drag losses inside a wet clutch plays an important role. In a dual-clutch transmission, for instance, drag losses can be classified into three sections: losses occurring during pre-selection, idle-D losses and drag losses at the seal rings or bearings. Nevertheless, most of these

negative effects can be reduced or avoided by optimizing the software of the transmission control unit or via implementation of an engine stop/start system. In addition, an optimized clutch design as well as improvements in the applied friction material are additional possibilities

for reducing drag torque.

Various calculation tools can be used to predict drag torque reliably. Analytical modeling, for example, uses a calculation program with adjustment based on actual measurements. The neural network method applies artificial intelligence that depends on previously collected

data, while a third method using CFD software performs exact calculations of fluid behavior inside the clutch. The latest calculations resulted in

the following friction material design solutions for effectively reducing total drag torque: waved friction plates, waved separator plates, an

optimized groove pattern, active separation and two-step lining.

The core plate design offers further potential for optimization. One new concept is the ‘hemmed spline’ design, in which a doublefolded core plate steel in the spline area increases the spline contact area, thus allowing the clutch pack length to be reduced without reducing the contact area of the splines. In general, hemmed spline design enables a reduction in weight, axial space and material costs.

A further modification method is the segmentation of the core plates. This facilitates the more efficient use of the core plate steel and is

therefore a reasonable cost reduction effort. BorgWarner started series production with an OEM customer in 2012.

Recent transmission design trends necessitate improvements in friction materials. Choosing the appropriate friction product depends on the

individual application and must be predicted using simulation tools in close cooperation with transmission and vehicle manufacturers. These

simulations help to predict the drag torque and enable optimization of the friction plate layers during the design phase.

BorgWarner’s BW 6910 friction material allows clutch systems with a low-lube strategy, provides a high torque density and improves

durability as well as NVH robustness. Also, advanced friction materials, like the BW 5000 family, allow shifting at high differential speeds and prevent the severe accumulation of hot spots on separators. Drag torque, packaging and costs can be further improved by the use of new design concepts, such as the hemmed spline design and segmented core plates.

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