Experimental investigation of laser surface textured parallel thrust bearings I. Etsion*, G. Halperin, V. Brizmer and Y. Kligerman Dept. of Mechanical Engineering Technion, Haifa 32000, Israel Received 14 September 2003; accepted 29 December 2003 Performance enhancements by laser surface texturing (LST) of parallel-thrust bearings is experimentally investigated. Test results are compared with a theoretical model and good correlation is found over the relevant operating conditions. A compari- son of the performance of unidirectional and bi-directional partial-LST bearings with that of a baseline, untextured bearing is presented showing the benefits of LST in terms of increased clearance and reduced friction. KEY WORDS: fluid film bearings, slider bearings, surface texturing 1. Introduction The classical theory of hydrodynamic lubrication yields linear (Couette) velocity distribution with zero pressure gradients between smooth parallel surfaces under steady-state sliding. This results in an unstable hydrodynamic film that would collapse under any external force acting normal to the surfaces. However, experience shows that stable lubricating films can develop between parallel sliding surfaces, generally because of some mechanism that relaxes one or more of the assumptions of the classical theory. A stable fluid film with sufficient load-carrying capacity in parallel sliding surfaces can be obtained, for example, with macro or micro surface structure of different types. These include waviness [1] and protrud- ing micro-asperities [2–4]. A good literature review on the subject can be found in Ref. [5]. More recently, laser surface texturing (LST) [6–8], as well as inlet roughening by longitudinal or transverse grooves [9] were suggested to provide load capacity in parallel sliding. The inlet roughness concept of Tonder [9] is based on ‘‘effective clearance’’ reduction in the sliding direction and in this respect it is identical to the par- tial-LST concept described in ref. [10] for generating hydrostatic effect in high-pressure mechanical seals. Very recently Wang et al. [11] demonstrated experi- mentally a doubling of the load-carrying capacity for the surface- texture design by reactive ion etching of SiC parallel-thrust bearings sliding in water. These simple parallel thrust bearings are usually found in seal-less pumps where the pumped fluid is used as the lubricant for the bearings. Due to the parallel sliding their performance is poorer than more sophisticated tapered or stepped bearings. Brizmer et al. [12] demon- strated the potential of laser surface texturing in the form of regular micro-dimples for providing load-car- rying capacity with parallel-thrust bearings. A model of a textured parallel slider was developed and the effect of surface texturing on load-carrying capacity was analyzed. The optimum parameters of the dimples were found in order to obtain maximum load-carrying capacity. A micro-dimple ‘‘collective effect’’ was identi- fied that is capable of generating substantial load-car- rying capacity, approaching that of optimum conventional thrust bearings. The purpose of the pres- ent paper is to investigate experimentally the validity of the model described in Ref. [12] by testing practical thrust bearings and comparing the performance of LST bearings with that of the theoretical predictions and with the performance of standard non-textured bearings. 2. Background A cross section of the basic model that was analyzed in Ref. [12] is shown in figure 1. A slider having a width B is partially textured over a portion B p ¼ aB of its width. The textured surface consists of multiple dimples with a diameter 2r  p , depth h  p and area density S p . As a result of the hydrodynamic pressure generated by the dimples the sliding surfaces will be separated by a clearance h  0 depending on the sliding velocity U, the fluid viscosity l and the external load W  . It was found in Ref. [12] that an optimum ratio exists for the param- eter h p ¼ h  p =h  0 that provides maximum dimensionless load-carrying capacity W ¼ W  h  0 2 =lULB 2 where L is *To whom correspondence should be addressed. E-mail: etsion@ tx.technion.ac.il 1023-8883/04/0800–0295/0 Ó 2004 Plenum Publishing Corporation Tribology Letters, Vol. 17, No. 2, August 2004 (Ó 2004) 295