54:2 Farm Mechanics. 



securing the least friction. On the other hand, if the bear- 

 ings are carrying heavy pressure at a slow rate of sliding, 

 as in the case of the axles of wagons, then the heavy thick 

 oil or grease will last longer, maintain less wear of the 

 bearings and ensure a smaller friction. But in any case 

 the rate of revolution of parts and the amount of friction 

 must be so related that sufficient heat is not developed to 

 burn the oil. Further than this it may sometimes happen 

 that the oil does not feed rapidly or completely enough to 

 all parts of the bearing to ensure perfect lubrication and 

 constant watchfulness is required on the part of every 

 operator of a machine to keep things in proper condition. 



683. Scrupulous Cleanliness of Bearings Next in impor- 

 tance to good lubrication of all slipping portions of a ma- 

 chine stands the maintenance of scrupulously clean bear- 

 ings, where the friction surfaces are free from both grit 

 or any gummy substance. Where sand or grit of any sort 

 has found its way to the bearings of machinery its grains 

 cut through the film of oil on both friction surfaces and 

 tend to lock the two together in the same manner that sand- 

 ing the rail under the driver of a locomotive does and thus 

 prevent slipping. 



Because of this tendency of dirt to get into bearings, 

 even under the best management, it is necessary to occa- 

 sionally overhaul the bearings of important machines and 

 carefully clean them, and very special attention should be 

 given to all those parts where the speed is high, because 

 not only is here where power is absorbed most rapidly by 

 needless friction but the wearing and injury to the ma- 

 chine is most rapid. 



684. Hot Boxes. The heating of boxes in machinery 

 may result from one or more of several causes: (1) insuf- 

 ficient lubrication, (2) dirt in the journal, (3) the box 

 may be screwed down too tight, (4) the belt may be too 

 tight, producing unnecessary friction, (5) the box may 



