4O FARM MACHINERY 



than with soft. Rolling friction is that met with in ball 

 and roller bearings, and is much less than sliding friction. 

 Roller bearings reduce- friction greatly. Ball bearings 

 may be used advantageously when end thrust is to be 

 overcome or where they can be used in pairs. They are 

 not suitable for carrying heavy loads. 



60. Lubrication. The object of lubrication is to reduce 

 friction to a minimum. A small quantity of oil is placed 

 in a box and a thin film adheres both to the surface of 

 the journal and also to the bearing, so in reality the 

 friction takes place between liquid surfaces. The lubri- 

 cant also fills the unevenness of the surfaces, so that there 

 is no interlocking of the particles that compose them. 

 Friction with a lubricant varies greatly with the quality 

 of lubricant and the temperature. 



6 1. Choice of lubricant. For heavy pressures the lubri- 

 cant should be thick so as to resist being squeezed out 

 under the load, while for light pressures thin oil should 

 be used so that its viscosity will not add to the friction. 

 Thus, for a wagon, heavy grease should be used, while 

 for a cream separator of high speed a thin oil is necessary. 

 Temperature must also be taken into account in choosing 

 a lubricant. 



Solid substances in a finely divided state, such as mica 

 and graphite, are used to reduce friction. The practice 

 seems to be a very good one. This is especially true with 

 graphite in bearings that can be oiled only occasionally, 

 as the bearings of a windmill. 



62. Bearings should be of sufficient size that the lubri- 

 cant will not be squeezed out from between the journal 

 and the bearing. In the design of machinery a certain 

 pressure limit must not be exceeded. -It is better to have 

 the journal and bearing made out of different materials, 

 as the friction in this case is less and there is a less ten- 



