An Experiment in Oil Migration on Metal Surfaces

I've often read that oil can spread around inside a watch and that proper watch oils are designed to "stay in place". I was a little skeptical that oil could move around all by itself until I tried an experiment involving a comparison between Moebius 9010 (a Swiss oil designed for general use in wristwatches) and "Mobil 1" 5W-30 (an automotive engine oil). Here are the results:

The areas of interest are indicated by purple arrows. At left, a test to see if an excessively-oiled metal-to-metal bearing would leak oil out onto the surface surrounding the bearing hole. At right, a test to see if drops of oil deposited onto a metal surface would migrate (move or spread). The barrel at right was mounted vertically to show any effect due to gravity.

This is a shot of the comparative test set-up after 36 hrs. The Moebius oil drop at right has stayed in place with a very slight spread delineated by the purple lines. The Mobil oil at left shows some unexpected behavior. The main part of the oil drop has sagged downward due to gravity, a lower surface tension, and a somewhat larger initial drop size than that of the Moebius oil. However, a thin film has formed and continued to grow, apparently following the circular machining marks on the metal surface. It appears that gravity has aided the migration of the lower part of the film. There remains a line of demarcation between the film and the edge of the original drop.

This is an external shot, after 17 hours, of the barrel arbor bearing hole, the inside of which was oiled excessively with the Mobil 1 automotive oil. The purpose of this test was to see if any oil is able to exit by overcoming the capillary force induced by the bearing clearance gap. It is seen that no spreading has occurred so far, although the oil "ring" itself is clearly visible. The small blob of Mobil 1 at left was added later to test spreading of a drop on a horizontal, smoother surface.

I have drawn no firm conclusions yet and am still in research mode. Therefore, conclusions will be published later as additions to this post.

Best regards,

xpatUSA

More Oil Migration

It was so fascinating to see those stress patterns that I just had to play a little more. This time, the goal was to simulate the gap between a cap and a pivot jewel such as found on a balance cock. The top of a pivot jewel is slightly curved and the bottom of a cap is flat. When oiling balance pivots, the goal is to provide sufficient oil to form a ring of oil around the end of the pivot between the jewels.

Another blob of motor oil was placed between the crystals not far from the middle and two clothes pins provided the necessary fringe pattern to simulate the juxtaposition of a flat surface and a curved surface. Sure enough, the blob headed for the thinnest gap - albeit very slowly, The pics below show the position after 6 hours elapsed time (that radial shadow line is a deep scratch outside on one of the crystals):



It appears that there comes a point where the blob stops moving, since it did not go all the way to the center. This point is likely when the molecular attraction force between the oil and the surface of the glass is equal to the motion force that is due to the decreasing gap (or gap force gradient). When the gap decreases the oil spreads out, thereby increasing the attraction force between the oil and the surface. Also when the fringe lines are wider, the gap gradient is smaller.

These experiments have shown quite conclusively that oil tends to move to the narrowest gap and stay there. So fears of random oil migration if a watch is left in drawer are groundless. By the same token, running a watch occasionally to "spread the oil around" is a waste of time IMHO. Also, since the oil moves as a blob, insufficient oiling will leave dry zones - not a good thing.

Sorry about the terminology, I never really studied capillary action or tribology for that matter!

Best regards,

xpatUSA