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Rough Surfaces Part 5

Tham khảo tài liệu 'rough surfaces part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Tribology produced by the passage of a ball or roller can lead to fatigue and this is a standard mode of failure of roller-bearings and gear teeth. It takes the form of pitting a pit being a small crater left in the surface as a result of a fragment of metal falling out. The presence of a lubricant does not prevent this for under elastohydrodynamic conditions the surface pressure distribution is essentially that found by Hertz for unlubricated contacts. Indeed one might argue that pitting is caused by lubrication certainly in the sense that without lubrication the surfaces would fail long before pitting could appear. However there is some reason to believe that lubricant is forced into surface cracks by the passage of the very high pressures and then acts as a wedge to help open up and extend the cracks. The first important appearance of quantitative surface roughness in tribology was in Dawson s 1962 work on pitting. Smooth surfaces are known to pit less readily. Dawson found from experiments with a disc machine using a small slide roll ratio that increasing the oil film thickness also reduced the tendency to pit and that the ratio of the surface roughness to the oil film thickness the D-ratio was the dominant parameter Fig. 9.6 . The correlation between the number of revolutions before pitting occurred and the -ratio holds over a 500-fold variation in D-ratio however for very low values of D the life was much longer than expected and the experiments were stopped without pitting occurring. Fig. 9.6 The life before pitting begins depends on the ratio of the initial surface roughness to the film thickness Dawson 1962 . It must be emphasized that the surface roughness measured was the initial value and that the roughness when pitting occurred was very much less. One can only speculate on the mechanism by which the discs remember their original finish. Note also that the no-pit limit appeared to be when the sum of the initial peak-to-valley roughnesses of the two .