Lubricating ball bearings

Ball and roller bearings are lubricated with oil or grease, either of which might be mineral or synthetic, and it is extremely difficult to decide just how the lubricant behaves. Many bearings are “sealed for life” like those on motor-car wheels. I have a drilling machine with sealed-for-life bearings that I have used regularly for 50 years. The bearings show no signs of wear or deterioration. Just how the grease manages to find its way around the tracks and the balls is difficult to either imagine or observe and the sheer durability of the grease is remarkable.

 

Other ball races can only function if they are force lubricated and then the way in which the lubricant gets into the spaces is not in question.

 

It becomes a design to be finalised by making and testing but whilst dimensions can be chosen and realised in hardware it is not so easy to choose a lubricant. It is not immediately obvious what properties are required for the lubricant. We know from the previous work that we need a high viscosity to achieve high pressures in the gap but we also know that we want low viscosity to let the lubricant flow quickly into the spaces between the balls. Density is not very important. But the ability to wet the balls and the tracks is important.

 

Whatever means of introducing the lubricant is used we ultimately have a set of balls that roll along a track covered with lubricant. We must look for some help to think about the flow pattern. I thought that I might get some help from simple experiments. The following is the outcome of those experiments.

 

Figure 16-33 shows a table tennis ball being rolled through a thick film of steam oil. Steam oil is very viscous and it obviously wets the celluloid of the ball.

 

Even this light ball cuts through the film of oil and leaves a wake, like banks on either side of a country road, that immediately starts to close under the action of gravity and possibly the intermolecular forces and certainly surface tension but is resisted by viscosity.

The oil clings to the ball and stays on to take part in the motion ahead of and under the ball. The process of displacing the oil in front of the ball can be seen in figure 16-34 as a brown collar of oil and the profile of the oil clinging to the ball can be seen to change as it is carried over the ball. Figure 16-35 shows that the oil that is pushed to the sides is “stretched” between the ball and the surface and tries to form the sort of web that is shown in fig 7-5 of Chapter 7. The web of oil breaks to form a trail of oil over the ball and a trail along the plate. I know that, in a real ball bearing the oil would be in a track that is hollow, but it seems to me that the oil will still form this web and break to let some oil be carried round the ball into the path of the next track. Centrifugal action will tend to bring this oil back to the crown of the ball.  I have put this together in figure 16-36 to give a mechanism for lubrication ball bearings. There I have suggested that the oil that is carried round by a ball joins with oil that is already on the track in a process that must involve surface tension and then the oil is squeezed and pushed sideways to form the parallel wake that appears in figure16-35. Some of the oil is carried on the ball to the next point of contact. I have shown the two balls in figure 16-36 with a considerable gap between them. I think that the raised profile of the parallel wakes changes quickly in the way that is shown in figure 16-37 and there must be some minimum gap that lets these wakes spread ready for the next ball to arrive. It seems like a viable mechanism to me and I think that the way that the lubricant is carried round with the ball may be the key to an understanding these bearings and especially sealed-for-life bearings. It might certainly be the case for bearings lubricated with grease.

I thought about grease and wondered whether it would be worth using the same ball with a slurry of icing sugar in water. Figure 16-37 shows that the same web as in 16-35 forms in the icing sugar only now it is a semi solid and we can see the process more clearly Figure 16-38 shows the front of the ball and the icing sugar that has come over the ball amalgamating with the wave forming under the ball

 

If this ball were to be one of a pair with the second following closely behind the first there would be nowhere for trails of oil or icing sugar to change shape to lubricate the second ball. A balance must be struck between the number of balls to share the load bearing and the need for spacing between the balls to give successful lubrication.

 

The behaviour of lubricant is clearly complex but it does seem to be the case that, more or less by chance it works very well. It raises the question of the ideal combination of properties for a lubricant.