Rubbing and Rolling Friction.

Progress, Volume III, Issue 4, 1 February 1908, Page 122

Rubbing and Rolling Friction.

It may be taken as an axiom that so long as rolling friction only, as defined above, is present in a bearing there will be no appreciable heating ; but the moment rubbing friction between metal and metal occurs warming up begins. This may be due to three causes : (1) Insufficient supply or poor quality of oil ; (2) the presence of a foreign substance, such as grit in the bearing ; or (3) badly prepared surfaces. In the first case rubbing friction will occur all round with disastrous results in a very short time ; the shaft or pin expands with the heat, making matters worse, and finally becomes such a tight fit that the engine pulls up or something breaks. In the second case, rubbing friction is only set up in the small area affected by the grit ; but the heat generated there soon spreads, causing the bearing to close up as before, and the result will be bad scoring of the brass, particularly in the place where the grit is present. Another point to be remembered in regard to hot bearings is that above a certain temperature the oil chars, losing its lubricating properties. When this stage is reached, under ordinary systems of lubrication no amount of oil will cool a bearing, and the engine must be shut down.