Ultra-high-speed thermal behavior of a rolling element upon using oil–air mist lubrication K. Ramesh a,* , S.H. Yeo b , Z.W. Zhong b , Akinori Yui c a Gintic Institute of Manufacturing Technology, Nanyang Technological University, 71 Nanyang Drive, Singapore 638075, Singapore b Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore c Okamoto Machine Tool Works, Annaka, Japan Abstract Ultra-high grinding speed and super abrasives are the essential components of high-efficiency grinding. Despite having a high metal removal rate, this process is less spread throughout industry due to the high capital investment in the system. The spindle unit covers nearly 60% of the total outlay, on account of elaborate oil management for cooling and lubrication. This paper discusses the high-speed behavior of a rolling element upon using an oil–air mist system in an ultra-high-speed grinding spindle. A spindle unit was built with this system and experimented upon. The total heat generation due to friction and gyro-induced torque were evaluated. Droplet study was performed to establish the heat transfer components and the significance being reported. # 2002 Published by Elsevier Science B.V. Keywords: Oil–air mist lubrication system; Grinding spindle; Wheel speed; Grinding 1. Introduction The application of super abrasives in ultra-high-speed grinding technology paved the way for a high specific- material-removal rate [1,2]. The fragmentation of chip segments enables the deployment of a higher feed rate. However, the usage is fairly slight due to the heavy capital outlay. The ultra-high-speed spindle, which is a key compo- nent, covers nearly 60% of the total machine cost [3,4], and there is also an elaborate oil management in spindle design for cooling and lubrication [5]. The pressurized cooling oil in high-speed rolling element demands a cumbersome sealing arrangement. Heat generation, due to churning of the oil and frictional hydraulic power loss reduce the efficiency of the jet oil lubrication method. Therefore, an alternative lubrication and cooling method that simplifies the spindle construction with effective heat removal becomes essential. 2. Review of past development A high-speed spindle with a piezoelectric nozzle for oil–air lubrication was developed and reported, in this method the fluctuations of the oil–air supply being suppressed [6]. Holes were drilled into the outer race of the spindle bearing system and oil–mist is supplied to overcome the air turbulence around the rotating cage. The lubrication film thickness at different spindle speed has been measured and reported [3]. A draft plan to develop a high-speed grinding machine of 250 m/s outlines the advantage of the oil–air lubrication method [4]. It was established that effective lubrication is a key issue to enhance the performance of Si 3 N 4 angular-contact ball bearings [7]. Increase in spindle speed also increases the heat generation of a spindle bearing system [8]. Liquid droplet study con- firmed the effectiveness of convective heat transfer in cooling [9]. High performance cooling can be achieved in a mist- cooled heat exchanger [10]. A United States patent reports a spindle device having bearings lubricated with an oil jet [5]. A composite bearing system was adopted for the con- struction of an UHSG spindle and established the increase in natural frequency, damping ratio and decreased heat gen- eration [11]. In an UHSG spindle, contact stress fatigue was considered as the primary mechanism for the wear of the silicon nitride balls [12]. However, the contact stress pattern was controlled by the lubrication method. 3. Development of an ultra-high-speed grinding spindle Usually high-speed is related to smaller bearings and low power motors, but high rigidity is related to bigger bearings. Journal of Materials Processing Technology 127 (2002) 191–198 * Corresponding author. E-mail address: rkuppuswamy@gintic.gov.sg (K. Ramesh). 0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII:S0924-0136(02)00125-5