The ability to control the effective friction coefficient between sliding surfaces is a problem of significant interest in automotive applications. For reducing friction, lubricants or different material combinations are typically used. In this research, the role of ultrasonic vibrations on the friction coefficient between sliding surfaces is investigated, with the goal of being able to control the friction coefficient in an automotive seat belt system by modulating the vibrations at the interface between the D-ring and seat belt webbing. These ultrasonic vibrations are generated using piezoelectric materials that respond mechanically to an electrical input. To that end, a systematic approach is developed, with the help of experiments and models, to predict and characterize the frictional force between sliding surfaces in the presence of ultrasonic vibrations under various controlling parameters.
The applied ultrasonic vibrations may be tangential, perpendicular or out-of-plane to the direction of sliding velocity. For rigid surfaces in contact, maximum friction reduction has been reported in the case of tangential vibrations. It has been shown that the extent of friction reduction depends on the ratio of the velocity of the ultrasonic vibrations to the sliding velocity. A series of experiments over a wide range of loads and speeds are designed to characterize the friction reduction effect between solid-solid contacts and Hertzian contacts in the case of seat belts. Using Coulomb and Dahl friction models, the mechanism of friction reduction in the presence of ultrasonic vibrations is studied.
System level analytical modeling is presented which consists of a single degree-of-freedom model with LuGre friction at the sliding interface. By controlling parameters such as load, system stiffness, contact stiffness and the control force generated by the piezoelectric stack, characterization plots are obtained which can help optimize design parameters of ultrasonic lubrication systems.
In summary, this research investigates the potential of ultrasonic vibrations in actively controlling friction in automotive seat belt systems and other systems in which the use of lubricants is undesirable. For a given ultrasonic power, the extent of reduction decreases at higher speeds and loads. Active control of friction would help improve the performance, efficiency and lifetime of general sliding mechanisms.