机械外文翻译实验研究激光加工表面微观造型平行的推力轴承Word文件下载.docx

1、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 ispresented showing the benets of LST in terms of

2、 increased clearance and reduced friction.KEY WORDS: uid lm bearings, slider bearings, surface texturing1. IntroductionThe classical theory of hydrodynamic lubrication yields linear （Couette） velocity distribution with zero pressure gradients between smooth parallel surfaces under steady-state slidi

3、ng. This results in an unstable hydrodynamic lm that would collapse under any external force acting normal to the surfaces. However, experience shows that stable lubricating lms can develop between parallel sliding surfaces, generallybecause of some mechanism that relaxes one or more of the assumpti

4、ons of the classical theory.A stable uid lm with sucient load-carrying capacity in parallel sliding surfaces can be obtained, for example, with macro or micro surface structure of dierent types. These include waviness 1 and protruding microasperities 24. A good literature review on the subject can b

5、e found in Ref. 5. More recently, laser surface texturing （LST） 68, 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 eective clearance reduction in the slidingdirection

6、 and in this respect it is identical to the par- tial-LST concept described in ref. 10 for generating hydrostatic eect in high-pressure mechanical seals.Very recently Wang et al. 11 demonstrated experimentally a doubling of the load-carrying capacity for the surface- texture design by reactive ion e

7、tching of SiC parallel-thrust bearings sliding in water. These simple parallel thrust bearings are usually found in seal-less pumps where the pumped uid is used as the lubricant for the bearings. Due to the parallel sliding their performance is poorer than more sophisticated tapered or stepped beari

8、ngs. Brizmer et al. 12 demon-strated the potential of laser surface texturing in the form of regular micro-dimples for providing load-carrying capacity with parallel-thrust bearings. A model of a textured parallel slider was developed and the eect of surface texturing on load-carrying capacitywas an

9、alyzed. The optimum parameters of the dimples were found in order to obtain maximum load-carrying capacity. A micro-dimple collective eect was identi-ed that is capable of generating substantial load-carrying capacity, approaching that of optimumconventional thrust bearings. The purpose of the prese

10、nt 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-texturedbearings2. BackgroundA cross section o

11、f 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 Bp =B of its width. The textured surface consists of multiple dimples with a diameter，depthand area density Sp. As a result of the hydrodynamic pressure generated by th

12、e dimples the sliding surfaces will be separated by a clearancedepending on the sliding velocity U, the uid viscosity l and the external loadIt was found in Ref. 12 that an optimum ratio exists for the parameter that provides maximum dimensionless load-carrying capacity where L isthe bearing length,

13、 and this optimum value is hp=1.25. It was further found in Ref. 12 that an optimum value exists for the textured portion a depending onthe bearing aspect ratio L/B. This behavior is shown in gure 2 for a bearing with L/B = 0.75 at various values of the area density Sp. As can be seen in the range o

14、f Sp values from 0.18 to 0.72 the optimum a value varies from 0.7 to 0.55, respectively. It can also be seen from gure 2 that for a 0.85 no optimum value exists for Sp and the maximum load W increases with increasing Sp. Hence, the largest area density that can be practically obtained with the laser

15、 texturing is desired. It is also interesting to note from gure 2 the advantage of partial-LST （a 1） over the full LST （a = 1） for bearing applications. At Sp= 0.5, for example, the load W at a = 0.6 is about three times higher than its value at a = 1. A full account of this behavior is given in Ref

16、. 12.3. ExperimentalThe tested bearings consist of sintered SiC disks 10 mm thick, having 85 mm outer diameter and 40 mm inner diameter. Each bearing （see gure 3） comprises a at rotor （a） and a six-pad stator （b）. The bearings were provided with an original surface nishby lapping to a roughness aver

17、age Ra= 0.03 lm. Each pad has an aspect ratio of 0.75 when its width is measured along the mean diameter of the stator. The photographs of two partial-LST stators are shown in gure 4 where the textured areas appear as brighter matt surfaces. The rst stator indicated （a） is a unidirectional bearing w

18、ith the partial-LST adjacent to the leading edge of each pad, similar to the model shown in gure 1. The second stator （b） is a bi-directional version of a partial-LST bearing having two equal textured portions, a/2, on each of the pad ends. The laser texturing parameters were the following; dimple d

19、epth, dimple diameter and dimple area density Sp= 0.60.03. 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= 0.73 and that of the bi-directional bearing was a= 0.63. As c

20、an be seen from gure 2 both these a values should produce load-carrying capacity vary close to the maximum theoretical value.The test rig is shown schematically in gure 5. Anelectrical motor turns a spindle to which an upper holder of the rotor is attached. A second lower holder of the stator is xed

21、 to a housing, which rests on a journal bearing and an axial loading mechanism that can freely move in the axial direction. An arm that presses against a load cell and thereby permits friction torque measurements prevents the free rotation of this housing. Axial loading is provided by means of dead

22、weights on a lever and is measured with a second load cell. A proximity probe that is attached to the lower holder of the stator allows on-line measurements of the clearance change between rotor and stator as the hydrodynamic eects cause axial movement of the housing to which the stator holder is xe

23、d. Tap water is supplied by gravity from a large tank to the center of the bearing and the leakage from the bearing is collected and re-circulated. A thermocouple adjacent tothe outer diameter of the bearing allows monitoring of the water temperature as the water exit the bearing. A PC is used to co

24、llect and process data on-line. Hence,the instantaneous clearance, friction coecient, bearing speed and exit water temperature can be monitored constantly. The test protocol includes identifying a reference “zero” point for the clearance measurements by rst loading and then unloading a stationary be

25、aring over the full load range. Then the lowest axial load is applied, the water supply valve is opened and the motor turned on. Axial loading is increased by steps of 40 N and each load step is maintained for 5 min following the stabilization of the friction coecient ata steady-state value. The bea

26、ring speed and water temperature are monitored throughout the test for any irregularities. The test ends when a maximum axial load of 460 N is reached or if the friction coecient exceeds a value of 0.35. At the end of the last load step the motor and water supply are turned o and the reference for t

27、he clearance measurements is rechecked. Tests are performed at two speeds of 1500and 3000 rpm corresponding to average sliding velocities of 4.9 and 9.8 m/s, respectively and each test is repeated at least three times.4. Results and discussionAs a rst step the validity of the theoretical model in Re

28、f. 12 was examined by comparing the theoretical and experimental results of bearing clearance versus bearing load for a unidirectional partial-LST bearing. The results are shown in gure 6 for the two speeds of 1500 and 3000 rpm where the solid and dashed lines correspond to the model and experiment,

29、 respectively. As can be seen, the agreement between the model and the experiment is good, with dierences of less than 10%, as long as the load is above 150 N. At lower loads the measured experimental clearances are much larger than the model predictions, particularly at the higher speed of 3000 rpm

30、 where at 120 N the measured clearance is 20 lm, which is about 60% higher than the predicted value. It turns out that the combination of such large clearances and relatively low viscosity of the water may result in turbulent uid lm. Hence, the assumption of laminar ow on which the solution of the R

31、eynolds equation in Ref. 12 is based may be violated making the model invalid especially at the higher speed and lowest load. In order to be consistent with the model of Ref. 12 it was decided to limit further comparisons to loads above 150 N.It should be noted here that the rst attempts to test the baseline untextured bearing with the original surface nish of Ra= 0.03 lm on both the stator and rotor failed due to extremely high friction even at the lower loads. On the other hand the partial-LST bearing ran smoothly throughout the load range. It was found that the post-LST lapp