Rena Hecht Basch

The effect of metal fibers on the friction performance of automotive brake friction materials

Abstract This study investigates the effect of different metallic fibers upon friction and wear performance of various brake friction couples. Based on a simple experimental formulation, friction materials with different metal fibers (Cu, steel, or Al) were fabricated and then evaluated using a small-scale friction tester. Two different counter disks (gray cast iron and aluminum metal matrix composite (Al-MMC)) were employed for friction tests. The friction tests were carried out at two different temperature ranges: ambient and elevated temperatures. Results from ambient temperature tests revealed that the friction materials with Cu fibers showed a pronounced negative μ – ν (friction coefficient versus sliding velocity) relation when the friction material was rubbed against gray cast iron disks, implying that stick–slip may occur at low speeds. The negative μ – ν relation was not observed when the friction material with Cu fibers was rubbed against the Al-MMC counter surface. On the other hand, elevated temperature tests showed that the friction material with Cu fibers exhibited better fade resistance than the others. The test results also showed that the friction material with steel fibers was not compatible with Al-MMC disks due to severe material transfer and erratic friction behavior during sliding at elevated temperatures.

Tribological study of gray cast iron with automotive brake linings: The effect of rotor microstructure

Experimental studies of friction characteristics were conducted using gray cast iron and automotive brake linings. The gray iron samples were manufactured to have different microstructures by changing the carbon equivalent and cooling speeds of melts and two different types of non-commercial brake linings (non-steel and steel-containing linings) were used as a counter material. Friction tests were performed on a pad-on-disk type tribotester and particular emphases were given to the effect of graphite flakes and ferrite in the gray iron disks on fade phenomena and the level of the coefficient of friction. Results showed that the coefficient of friction increased with the amount of graphite flakes on the gray iron and the effect was more pronounced in the case of using steel-containing linings. The amount of ferrite phase on the disk surface showed little influence on the coefficient of friction. Fade resistance of non-steel linings was improved with the increase of graphite flakes on the disk surface and steel-containing linings showed good fade resistance regardless of graphite contents in the gray iron disks.

A reduced-scale brake dynamometer for friction characterization

Abstract Friction behavior is a critical factor in brake system design and performance. For up-front design and system modeling it is desirable to describe the frictional behavior of a brake lining as a function of the local conditions such as contact pressure, temperature, and sliding speed. Typically, frictional performance is assessed using brake dynamometer testing of full-scale hardware, and the average friction value is then used for the remaining brake system development. This traditional approach yields a hardware-dependent, average friction coefficient that is unavailable in advance of component testing, ruling out true up-front design and leading to redundant lining screening tests. To address this problem, a reduced-scale inertial brake dynamometer was developed to determine the frictional characteristics of lining materials. Design of a reduced-scale dynamometer began with the choice of a scaling relation. In this case, the energy input per unit contact area was held constant between full-scale and reduced-scale hardware. All linear variables were thereby scaled by the square root of the scaling factor, while the pressure, temperature, sliding velocity, and deceleration were kept constant. Experimental validation of the scaling relations and the reduced-scale dynamometer focused on comparisons with full-scale dynamometer data, particularly the friction coefficient. If similar trends are observed between reduced-scale and full-scale testing, the reduced-scale dynamometer will become an important tool in the up-front design and modeling of brake systems.

Multibody System Modeling of Leaf Springs

Leaf springs are essential elements in the suspension systems of vehicles including sport utility vehicles, trucks, and railroad vehicles. Accurate modeling of the leaf springs is necessary in evaluating ride comfort, braking performance, vibration characteristics, and stability. In order to accurately model the deformations and vibrations of the leaf springs, nonlinear finite-element procedures, which account for the dynamic coupling between different modes of displacement, are employed. Two finite-element methods that take into account the effect of the distributed inertia and elasticity are discussed in this investigation to model the dynamics of leaf springs. The first is based on a floating frame of reference formulation, while the second is an absolute nodal coordinate formulation. The floating frame of reference formulation allows for using a reduced-order model by employing component mode synthesis techniques, while the absolute nodal coordinate formulation enables more detailed finite-element models for the large deformation of very flexible leaf springs. Methods for modeling the contact and friction between the leaves of the spring are discussed. A comparison is also presented between the results obtained using the proposed method and simplified approaches presented in the literature. While there are many issues that can be important in leaf spring modeling, the analysis presented in this paper is focused on a few key issues that include the computer implementation, the effect of the dynamic load on the spring stiffness, the selection of the vibration modes in the reduced-order model, and the effect of the structural damping on the response of the leaf spring.

High Temperature Wear Properties of Multiphase Composite: The Role of Transfer Film

The role of transfer film on high temperature wear properties of a multiphase composite for a brake friction material was investigated using a pad-on-disk type tribometer. A novolac resin-bonded composite based on a simple formulation with 6 ingredients (aramid pulp, cashew dust, Cu fiber, graphite, potassium titanate, and zirconium silicate) was used in this study. Results showed that the wear properties of the composite were significantly affected by the temperature at the frictioninterface when the transfer film was present on the counter face during sliding. In particular, the transfer film on the disk surface was well developed at approximately 200°C, resulting in theimproved wear resistance. It suggested that the transfer film on the disk surface effectively prevented direct contacts of the composite onto the counterface. On the other hand, no apparent relationship between transfer film thickness and friction coefficient was found in this experiment.

Kinematic Analysis of Brake-Induced Vehicle Steering Wheel Vibration

Vehicle steering comfort is strongly influenced by chassis system sensitivity to brake-induced vibration. If drivers feel vibration on the steering wheel during a braking event, it negatively impacts their satisfaction with a vehicle’s performance and quality. The suspension is the primary chassis sub-system that transmits vibration from the brakes to the rest of vehicle. Since the suspension system is directly linked with the brake, tire/wheel assembly, sub-frame and steering system, its coupled vibration is very complicated. The current paper presents a sensitivity study of chassis system vibration distribution, a vibration reduction proposal with traditional vibration approach and a vibration reduction proposal with a kinematic design strategy. A system integration approach is used to derive an improvement strategy that can potentially make a vehicle insensitive to the vibration caused by torque variation of the brakes. The approach links suspension, steering and sub-frame vibration characteristics, kinematics modeling and robust design principles in a systematic fashion. The concept and method have been successfully demonstrated on a typical mid-size passenger car.Copyright

A nonlinear finite element model for leaf springs

This paper presents a nonlinear finite element model for the leaf spring that can be used in multibody applications and vehicle dynamic simulations. The floating frame of reference formulation is used in this investigation to model leaf spring nonlinear dynamics. This formulation accounts for the coupling between different modes of deformation as well as the nonlinear coupling between the rigid body motion and the elastic deformation. By employing component mode synthesis techniques, a reduced order model is obtained for the leaf spring while maintaining a good degree of accuracy. The inertia shape integrals can be calculated once in advance using a preprocessor and then stored to be used to automatically generate the nonlinear equations of motion of the leaf spring. The use of a preprocessor to evaluate the inertia shape integrals before the dynamic simulation leads to considerable saving in CPU time and allows the utilization of existing finite element computer codes to obtain the data required for the flexible body simulation. This reduced order model is implemented in a general multibody algorithm in order to examine the effectiveness and robustness of the proposed techniques. As an application, the wind-up deformation of the front suspension system of a typical sport utility vehicle under severe braking condition is investigated.© 2003 ASME