In such capillary passages as one between rubbing surfaces or in sealing spaces of oil hydraulic equipment, the roughness of the surfaces may become comparable to the oil film thickness. This paper reports an experiment on the effect of surface roughness on the capillary flow between parallel plates and compares the theoretical values obtained by the statistical treatment of roughness with the experimental results.
The test passages were made of two plates with almost the same roughness. The center line average heights R_A of the plates were 0.54, 1.65 and 3.45 microns. The smooth standard plates finished by lapping were also tested for comparison.
The tested range of mean film thickness ho was 15-60 microns, the pressure difference was 40-160 at., and the temperature was 30-90°C. The film thickness was adjusted by chemical plating, and the increase of film thickness according to the pressure increase was prevented by hydraulic balance.
From the experimental results, it is shown:
(1) The flow rate is decreased suddenly as the film thickness becomes smaller than a limit. (The limit is dependent on the surface roughness as expressed by, for example, R_A/h_o≥0.01. )
(2) The roughness of the plates gives a more prominent effect on the flow rate when the plates are machined perpendicular to the flow direction than when the plates are machined parallel to the flow direction.
The reduction o f flow rate calculated by the statistical treatment which is extended to three dimensional roughness coincides nearly with the experimental results in the case of the plates machined parallel to the flow, but does not coincide so in the case of plates machined perpendicular to the flow.

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