1920] Problems of Lubrication 65 



WEEKLY EVENING MEETING, 



Friday, February 27, 1920. 



Colonel E. H. Hills, O.M.G. F.R.S., Secretary and 

 Vice-President, in the Chair. 



W. B. Hardy, F.R.S. 



Problems of Lubrication. 



In lubrication a fluid or other body is used to decrease the friction 

 between opposed solid faces. The lubricant may act in one of two 

 ways. It may separate the faces by a layer thick enough to 

 substitute its own internal friction, modified by the mechanical 

 conditions in which it finds itself, for that of the solid faces ; or it 

 may be present as a film, too thin to develop its properties when in 

 mass, which reacts with the substance of the solid faces to confer 

 upon them new physical properties. In the latter case the solid faces 

 continue to influence each other, not directly, but through the 

 intermediation of the film of lubricant. There are indications that 

 these two types of lubrication — the one in which the solid faces inter- 

 vene only owing to their form, rate of movement, etc., and not by 

 their chemical constitution ; the other in which the chemical constitu- 

 tion is directly involved — are discontinuous states, in that the one 

 cannot be changed gradually into the other by simply thinning the 

 layer of lubricant. The change from the one to the other is 

 probably abrupt. 



It may by no means be asserted that resistance to relative motion 

 is always least when the solid faces are floated completely apart ; it 

 would indeed probably be truer to say of the best lubricants, that 

 friction is least when the "boundary conditions," to use Osborne 

 Reynolds's phrase, are fully operative. 



This address is concerned wholly with "boundary conditions,'' 

 and we get directly to the heart of the problem by certain simple 

 experiments. If a glass vessel, such as a bottle, be placed upon an 

 inclined pane of glass at a certain angle, it slips smoothly down. 

 The glass plate is an ordinary plate cleaned with a cloth. In the 

 usual sense of the word the surface of the plate is not lubricated, the 

 surface is " dry." The lower half of the plate is then wetted with 

 water, and the bottle is now found to slip on the unwetted part and 

 to be sharply pulled up by friction when it reaches the wetted part. 



Vol. XXIII. (No. 114) f 



