Quenching friction-induced oscillations by the use of high-frequency excitation

Dry friction is present in many technical systems and is the reason for a variety of undesired phenomena. The property of non-smoothness and a negative slope at low relative velocities of the friction force may cause friction induced oscillations or stick-slip motion. One attempt to quench these oscillations are superposed high-frequency vibrations. Thus, the effective friction characteristic is smoothed and undesired friction induced oscillations can be quenched. For systems with one degree of freedom, this method is investigated by simulations and experiments, which show good accordance. Systems with multiple degrees of freedom and multiple friction contacts however show a largely richer dynamic behavior. Also, the influence of high-frequency excitation is not investigated yet. To improve the comprehension of such systems, numerical and analytical approaches are inquired. Applied procedures are the excitation of single masses and investigating the effect on the whole system. Furthermore, the influence of the direction of the excitation and the importance of which masses are excited is examined.

Superposed oscillations in revolutional joints with dry friction

Along with roller bearings, plain bearings are the type of bearings that are used most in technical systems. By the use of different lubricants and pairing materials, the bearings can be adapted to many different requirements. When lubricants are not deployable because of environmental restrictions, dry bearings have to be used. In such systems, undesired effects like stick-slip-motion or break-away can occur due to dry friction. Especially in positioning facilities, where small tolerances and high precision are required, this can be problematic. There are several approaches to compensate effects caused by dry friction and research still goes on. Especially in control technology, many sophisticated concepts are developed to obtain the desired system dynamics.

Another approach to influence the dynamical behavior of the system is the superposition of high-frequency oscillations. These vibrations can be in plane of the contact surface or normal to it. In-plane oscillations can be divided into transversal and longitudinal motion. The superposed oscillations have a smoothing effect on the system dynamics, which can yield a damping effect that can even suppress friction-induced oscillations. This topic has been subject of research in recent years and many papers were published, where theoretical and experimental results are discussed.

To contribute to the understanding and application of this effect, a model of a novel revolution joint with dry friction and axial excitation is built and investigated. A plain bearing rotates around a bolt, which cannot rotate and is excited to axial oscillations. Due to the rotational motion of the bearing, there is permanent sliding in the contact area between plain bearing and the bolt and no such effects like stick-slip-motion or break-away occur. Furthermore, this can lead to a reduction of the torque that is required to realize the rotation of the bearing.  However, the friction power leads to a heat input into the bolt which results in thermal expansion and a changed frictional behavior in the contact. To investigate the system dynamics and to analyze the influence of the  heat generation, the equation for heat conduction, the equation for the displacement field in the bolt and the equation of motion of the plain bearing have to be evaluated and solved. Numerical simulations show, that a stationary state is reached, where the bolt has a constant, elevated temperature and oscillates at a constant amplitude. The thermal expansion of the bolt results in an increased friction force, which limits the oscillation amplitude. This also results in a greater torque to realize the rotation of the plain bearing. Under certain parameters, this thermal property can even quench the friction reducing effect of superposed vibrations.

To validate this investigation, a test rig is constructed, where an electric motor realizes the rotation and piezo actuators are used for the axial excitation. The drive torque is measured by a sensor and the temperature at the contact area is measured by a pyrometer. The goal is to get insights in the relation between high-frequency excitation, heat generation and the friction torque.

Contact: Prof. A. Fidlin, S. Keller