Peripheral Speeds

Several factors enter into the determination of the maximum practicable surface speed of crushing rolls. These are:

1. Size of the feed (maximum one-way dimension) ;
2. Crushing strength of the material;
3. Diameter of rolls;
4. Spring pressure and weight of the machine;
5. Coefficient of friction of the material.

The ways in which these factors influence roll speeds may be stated briefly as follows:

• (A) For any given 1'0)) diameter, the advisable maximum speed is an inverse function of the size of the feed.
• (B) For any given size of feed, the permissible speed is a direct function of the 1'011 diameter.
• (C) For any combination of feed size and for diameter, heavy-duty, high-spring-pressure rolls wiII stand higher speeds than wiII lighter rolls, provided that the angle of nip is not too great for the in creased speed.
• (D) In any given machine, soft, friable materials may, within the limitation suggested for (C), be handled at higher speeds than hard and tough materials.

A certain amount of momentary slip of the individual particles of material wiII occur in any set of rolls, regardless of diameter 01' size of feed. This slip is due primarily to the difference in velocity of the particles and the 1'011 faces at the moment of nip; the obvious tendency is for the slip to increase as the surface speed increases; also, it wiII increase as the angle of nip is increased.

Hence, to hold the slip within reasonable limits, the angle of nip must be decreased as the speed is increased.

Extra-heavy rolls may be run at somewhat higher speeds than indicated by the values given, unless the rock is extremely hard. Rolls of the class for which the chart was prepared may be run at higher than indicated speeds if the material is soft and friable. Light-duty rolls, on the other hand, should not be run at higher than indicated speeds on soft rock, and should be run somewhat slower on medium hard material. Light-duty rolls should not under any conditions be used for crushing hard rock or ore.

The chart (Fig. 4) may be used in another way, by adhering to the values given for speed vs. feed size, and selecting the class of rolls to suit the crushing characteristics of the material.