Lubrication

I suppose that the first bearings that were invented were probably runners for land sledges and ice sledges. Applied lubrication would have been impossible but some woods have natural lubricants in them and are more durable than others. The arrival of wheels must have shown the need for lubrication to be applied to the hubs and the earliest lubricants were animal grease and organic oils produced by rendering such greases. There are very few of these organically derived lubricants and none, with perhaps the exception of oil from whales, was as good as the lubricants derived from mineral oils when petroleum became readily available and refined. The evolution of internal combustion engines depended on the success of these oils. More recently lubricants with desirable combinations of viscosity, film strength, resistance to high temperatures and low absorption of water have been synthesised by chemical methods and, using these lubricants, modern gas turbine engines for aeroplanes can be run for total times of 100,000 hours (=11 years) without being dismantled provided that the synthetic lubricant used in the ball bearings is monitored and changed at prescribed intervals.

 

So we have a range of liquid lubricants that can be used to go between the sliding or rolling surfaces of our bearings and give us reliable performance over a very long time.

 

As I worked through that chapter the physics of plain bearings showed that usefully high pressures would only be developed in thin films of lubricant and it seemed to me that I should go back and make a few simple practical checks. I have an accurately machined block of tungsten carbide with two flat annular faces. It is shown in figure 16-3. When it is resting on that face it exerts an average pressure of  or about 1.2 psi. This is a very modest pressure by engineering standards. I found a thick piece of duraluminium plate in the as-rolled condition and polished it. I placed it on a table that was not quite level and put some synthetic oil on the plate to form a film. I then rested the block on this film. It gently slid down the slope but it was also in electrical contact with the aluminium plate. That contact resistance was low enough to carry current to light a car bulb from a battery.

 

So the film was thick enough to float the block but thin enough for imperfections in the surfaces to be in electrical contact. I wondered just how flat the plate was and rubbed a diamond hone over it and this showed a random pattern of closely spaced high spots that must be almost undetectable by ordinary measurement.

 

I left the block on the plate for half an hour reasoning that the pressure in the oil film would push the oil out until the block and the plate came into contact unless any strength that the film might have kept them apart. After the half hour the block was not easily moved but, when it did move, it continued to move easily. My impression was that there was solid friction initially but, almost instantly, the oil separated the two and permitted viscous sliding.

 

When I separated the bock and the plate both were uniformly covered with a very thin film of oil. I rubbed my finger over the film and could scarcely see the oil on my finger pad. The film was very thin.

 

I tried rolling a steel, bearing ball of 7/16 inch diameter weighing 5.5 gram across a fairly thick film of oil and the ball cut a path through the film but carried oil away to dry patches to continue the trail. I made several trails in all directions and after a while the oil had parted at these trails to form islands of oil. It was not wetting the duraluminium. I changed to glass with the same result. I changed to a mineral oil intended for lubricating my lathe. This did wet the duraluminium and floated the tungsten carbide block. When a ball was rolled over a film of this oil it produced the same trails but these gradually closed. The oil wetted the duraluminium. I changed to a glass mirror with the same result.

 

After doing these tests I felt that for almost no effort I had learnt a great deal that is useful to an engineer. Anyone can repeat them.

 

Clearly bearings that are lubricated can only be successful if the lubricant is always in place when the bearing is operating. It follows that there must be a steady supply of lubricant and some system to deliver it to the sliding or rolling surfaces. This is a part of the design  but it must not be supposed that we can lubricate every mechanism that we can devise. We cannot always find ways to introduce the lubricant especially in reciprocating bearings like those of engine cross heads and in oscillating bearings like those in vehicle suspensions. Indeed vehicle suspensions have only become long lasting with the introduction of bonded rubber bushes that distort and do not slide.

 

By implication there are two groups of bearings, one is the plain bearing where surfaces slide one relative to the other and rolling bearings typified by ball bearings where balls roll between two lubricated tracks. Each group of designs subdivides again many times to adapt to all the possible engineering applications and we must see how they work. I will start with plain bearings and then deal with rolling bearings.

 

It turns out that in order to understand these plain bearings we must start with flow between parallel flat plates.