【正文】 eeded， such as in disk—drive spindle motors． Similarly， machine—tool spindles tolerate only minimal deflections to maintain precision cuts ． Consequently, bearings are manufactured with low NRR just for machinetool applications． Contamination is unavoidable in many industrial products， and shields and seals are monly used to protect bearings from dust and dirt． However， a perfect bearing seal is not possible because of the movement between inner and outer races． Consequently， lubrication migration and contamination are always problems． Once a bearing is contaminated, its lubricant deteriorates and operation bees noisier． If it overheats， the bearing can seize． At the very least， contamination causes wear as it works between balls and the raceway， being imbedded in the races and acting as an abrasive between metal surfaces． Fending off dirt with seals and shields illustrates some methods for controlling contamination． Noise is as an indicator of bearing quality． Various noise grades have been developed to classify bearing performance capabilities． Noise analysis is done with an Anderonmeter, which is used for quality control in bearing production and also when failed bearings are returned for analysis. A transducer is attached to the outer ring and the inner race is turned at 1,800rpm on an air spindle. Noise is measured in andirons, which represent ball displacement in μm/rad. With experience, inspectors can identify the smallest flaw from their sound. Dust, for example, makes an irregular crackling. Ball scratches make a consistent popping and are the most difficult to identify. Innerrace damage is normally a constant highpitched noise, while a damaged outer race makes an intermittent sound as it rotates. Bearing defects are further identified by their frequencies. Generally, defects are separated into low, medium, and high wavelengths. Defects are also referenced to the number of irregularities per revolution. Lowband noise is the effect of longwavelength irregularities that occur about to 10 times per revolution. These are caused by a variety of inconsistencies, such as pockets in the race. Detectable pockets are manufacturing flaws and result when the race is mounted too tightly in multiplejaw chucks. Mediumhand noise is characterized by irregularities that occur 10 to 60 times per revolution. It is caused by vibration in the grinding operation that produces balls and raceways. Highhand irregularities occur at 60 to 300 times per revolution and indicate closely spaced chatter marks or widely spaced, rough irregularities. Classifying bearings by their noise characteristics allows users to specify a noise grade in addition to the ABEC standards used by most manufacturers. ABEC defines physical tolerances such as bore, outer diameter, and runout. As the ABEC class number increase (from 3 to 9), tolerances are tightened. ABEC class, however, does not specify other bearing characteristics such as raceway quality, finish, or noise. Hence, a noise classification helps improve on the industry standard. 5. Conclusion The idea of partialLST to enhance performance of the parallel thrust bearing was evaluated experimentally. Good correlation was found with a theoretical model as long as the basic assumption of laminar ?ow in the ?uid ?lm is valid. At low loads with relatively large clearances, where turbulence may occur, the experimental clearance is larger than the prediction of the performance of both unidirectional and bidirectional partialLST bearings in terms of clearance and friction coe?cient was pared with that of a baseline untextured bearing over a load range in which the theoretical model is valid. A dramatic increase, of about three times, in the clearance of the partialLST bearings pared to that of the untextured bearing was obtained over the entire load range. Consequently the friction coe?cient of the partialLST bearings is much lower, representing more than 50% reduction in friction pared to the untextured bearing. The larger clearance and lower friction make the partialLST simple parallel thrust bearing concept much more reliable and e?cient especially in sealless pumps and similar applications where the process ?uid, which is often a poor lubricant, is the only available lubricant for the bearings. Acknowledgments The authors would like to thank Mr. J. Boylan of Morgan AMamp。 0:734. The load range extends from 160 to 460 N. The upper load was determined by the testrig limitation that did not permit higher loading. It is clear from ?gure 7 that the partialLST bearing operates at substantially larger clearances than the untextured bearing. At the maximum load of 460 N and speed of 1500 rpm the partialLST bearing has a clearance of 6 lm while the untextured bearing clearance is only lm. At 3000 rpm the clearances are and lm for the LST and untextured bearings, respectively. As can be seen from ?gure 7 this ratio of about 3 in favor of the partialLST bearing is maintained over the entire load range. Figure 8 presents the results for the bidirectionalbearing (see stator in ?gure 4(b)). In this case the LST parameters are Sp 188。 dimple depth , dimple diameter and dimple area density Sp= . These dimple dimensions were obtained with 4 pulses of 30 ns duration and 4 mJ each using a 5 kHz pulsating Nd:YAG laser. The textured portion of the unidirectional bearing was a= and that of the bidirectional bearing was a= . As can be seen from ?gure 2 both these a values should produce loadcarrying capacity vary close to the maximum theoretical test rig is shown schematically in ?gure 5. An electrical motor turns a spindle to which an upper holder of the rotor is attached. A second lower holder of the stator is ?xed to a housing, which rests on a journal bearing and an axial loading mechanism th