CRUSHING AND PULVERIZING MACHINERY. 443 



however, give a product practically equal in fineness to the 30- or 32- 

 mesh screens used in clinker-grinding. With these screens the mill 

 will turn out 2|- to 3 tons of raw material (equivalent to 8 to 10 barrels) 

 per hour, taking slightly less power than when running on clinker. 



The repair costs of a Griffin mill were stated, at a plant which has 

 always used these mills extensively, to vary with the material crushed 

 in about the following ratio: 



Repair costs on clinker : repairs on raw mix : repairs on coal. 

 11 : li : 1 



Kent mill. The Kent mill is a comparatively untried machine, 

 but deserves mention here because of the favorable results reported 

 for it by Professor Newberry, and G. H. Fraser (see p. 468). 



The Kent mill is shown in Fig. 95, the casing being broken out, 

 and one fixed check-ring B being partly broken away to show the feed 

 chutes A, the free revolving ring C, the three crushing-rolls G, and the 

 bottom discharge outlet F. 



Referring to the interior view, A is the feed-chute, which enters 

 the casing at opposite sides above one of the three rolls G and feeds 

 into the angle between this roll and the ring C. The three rolls G (of 

 which one is driven) are within and support the ring C, being drawn 

 yieldingly against its concave inner face at three points by stiff springs 

 acting against the bearing yokes carrying the shafts of the rolls. These 

 yokes slide in lugs on the casing and pull outward according to the 

 adjustment of the springs by their screws. The convex faces of the 

 rolls fitting the concave inner face of the ring hold it in position side- 

 ways, so that the ring always tracks on the rolls, but it is also checked 

 against too much side play by fixed check-rings B fastened on the inside 

 of the casing at a slight distance from the edges of the free ring, so as 

 to leave a free space D between in which the free ring can play and 

 through which the fine material may escape to the discharge chamber E f 

 which surrounds the ring C and at its lower part meets the discharge 

 outlet F. The rings B are cut away for a space above the outlet F. 

 The fixed rings can be easily replaced if ever worn out. 



In operation, the free ring is cushioned by the rolls and held cen- 

 trally, both axially and laterally, thereby, but can yield to pass a hard 

 substance or to cushion unequal thrusts, and can play sideways between 

 the fixed rings B to equalize variations of charge between any roll and 

 the ring. The driven roll drives the ring by contact with its inner 

 face, the other rolls being passive and free to revolve by contact with 

 the ring. The rolls and ring run on each other at like surface speeds. 

 The charge streams in between the ring and one roll, passes the latter, 



