Sliding bearing with adjustable friction properties T. Engel, A. Lechler, A. Verl (2)* Institute for Control Engineering of Machine Tools and Manufacturing Units (ISW) – University of Stuttgart, Germany 1. Introduction The friction characteristics of bearing systems are usually defined by the material and lubricants within the contact region. Roller bearings, sliding bearings and fluido static bearings are state of the art. Sliding bearings have good damping and stiffness properties but suffer from high stiction and nonlinear friction characteristics [1]. Roller bearings have much lower friction and stiction but poor damping properties, lower stiffness and vibrations from compres- sing and decompressing of rolling elements. Furthermore, roller bearings are often mounted with wipers to protect the bearing from dust and fluids, hence stiction is reintroduced by wiper setups [2]. The idea of a bearing with adjustable friction properties comes from industrial processes where ultrasonics are used to reduce the process forces, like in ultrasonic cutting, milling and drawing [3]. Theoretical models for the effect are given in [4] explaining that the high frequency microscopic motion is averaging the friction on a microscopic scale resulting in a loss of friction on the macroscopic scale. Section 2 gives a short review of the important oscillation parameters from the theoretical friction reduction model. The design approach to combine a sliding bearing with ultrasonics is described in Section 3. Experimental results of the oscillation and friction properties are presented in Section 4. 2. Friction theory and parameters of an oscillating slider The effect of high-frequency oscillations reducing the macro- scopic friction can be described using the coulomb friction model. In this model, the friction force depends on the normal force F N and an experimentally determined friction coefficient m. With the direction of friction opposing the relative velocity v rel between the surfaces, the friction force F R becomes F R ¼ signðv rel ÞmF N : (1) Through oscillation of the slider the relative velocity becomes a superposition of the constant feed velocity v f and an oscillating velocity v s . For a harmonic oscillation with an amplitude of x s at the frequency f s , the relative velocity of the motion becomes v rel ðtÞ ¼ v f þ v s ðtÞ ¼ v f þ 2pf s x s cosð2pf s tÞ: (2) If the oscillation velocity amplitude is higher than the feed velocity the direction of the friction (1) will change within one period. In [4] it is shown that averaging the friction force over one oscillation period gives F  R ¼ mF N  2 p arcsin v f v s   |fflfflfflfflfflfflfflfflfflffl{zfflfflfflfflfflfflfflfflfflffl} g : (3) Eq. (3) can be interpreted as the normal friction force with unchanged friction coefficient and unchanged normal force modified by a friction reduction factor g. In this model, the friction reduction is solely depending on the ratio of the feed velocity to the vibration velocity controlled by the oscillation CIRP Annals - Manufacturing Technology xxx (2016) xxx–xxx A R T I C L E I N F O Keywords: Friction Bearing Control A B S T R A C T Sliding bearings have good damping properties and a high stiffness. However, stiction and poor sliding friction characteristics limit their use in applications, where high requirements are defined on the smoothness of motion. By inducing ultrasonic oscillations into the sliding contact it is possible to actively control the friction. The adjustable friction bearing uses this principle. This leads to a new bearing type that has the stiffness and damping properties of a sliding bearing, while at the same time, offering a linear friction characteristic without stiction. This paper presents basic principles for the design of ultrasonic bearings along with experimental results of a friction-adjustable machine table. ß 2016 CIRP. Fig. 1. Friction reduction: theoretical and experimental results using a steel–plastic combination with 50 Hz and x s within 30–80 mm. * Corresponding author. E-mail address: alexander.verl@isw.uni-stuttgart.de (A. Verl). G Model CIRP-1493; No. of Pages 4 Please cite this article in press as: Engel T, et al. Sliding bearing with adjustable friction properties. CIRP Annals - Manufacturing Technology (2016), http://dx.doi.org/10.1016/j.cirp.2016.04.084 Contents lists available at ScienceDirect CIRP Annals - Manufacturing Technology journal homepage: http://ees.elsevier.com/cirp/default.asp http://dx.doi.org/10.1016/j.cirp.2016.04.084 0007-8506/ß 2016 CIRP.