Hammermill and roll crusher maintenance and operation

Corrugations

Roll corrugations must be selected according to the materials to be processed and the finished product requirements. Coarser grooving provides longer life and higher capacities but will produce coarse or thicker finished products. Finer corrugations lead to finer products but will reduce capacity and wear out faster. Reviewing roll corrugations as grinding requirements change is an often-overlooked aspect of roll crusher grinding. 

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Roll speed differential

Roll speed differential is the difference in rotational speed between the two rolls in a pair and is one of the biggest reasons roll crushers produce a grind with a lower standard deviation than hammermills. The differential creates a shearing effect between the rolls that cuts the material rather than squishing it. Using a higher differential will reduce the amount of fines produced, improving the standard deviation. The differential ratio is limited by the size of the sheaves that can fit within the guard of the rollermill. Sheaves are the pulleys on the roll shafts that are connected via belts to spin the rolls at the designated speeds. If the pulleys are too large, they will contact either the other pulley on the paired roll shaft or the frame/guard of the machine. The most common roll speed differential in roll crusher grinding is 1.5:1. 

Roll parallel and roll tram

To maximize roll life and achieve consistent end-to-end performance, each pair of rolls must be parallel and tram, which means level with each other on a horizontal plane. If the rolls operate out of parallel, they will grind the material fine at one end and coarse at the other and will also wear unevenly. The same is true for rolls out of tram. Normally the tram adjustment will only be set when the operated rolls are changed or in some significant upset condition (such as lost elevator buckets and bolts appearing in the nip of the rolls). 

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When the rolls are not parallel or tram, material tends to "drift” in the nip of the rolls toward the more open end(s). This excess material may begin to “pile up” in the nip of the rolls and cause accelerated wear. Trying to make a fine grind when out of parallel or tram may cause the “close” end (or center of the rolls if out of tram) to make metal to metal contact. You can routinely check the roll parallel by using a feeler gauge when the machine is open or by taking samples from each end of the rolls to compare grind sizes. 

Balance between roll pairs

In double- or triple-pair machines, it is necessary to balance the work being done between the pairs of rolls to avoid premature wear on one of the roll pairs. When properly managed, each pair of rolls will wear at approximately the same rate. By using a “staggered” corrugation on the fast and slow rolls (with coarser grooving on the fast roll) all four (or six) rolls will be ready for recorrugation at the same time. Changing all rolls at once is usually the most efficient use of the time and effort required to make a roll change. 

Rolls must be operated in a way to avoid having the material pile up in the nip when the mill is running at full capacity. This is not normally a problem when the rolls are in good condition, as the corrugations will easily pull in the material and the motor load will limit the throughput. As the corrugations, in the top rolls especially, become dull, they will lose their effectiveness at pulling the grain in, and material may begin to pile up in the nip. This condition must be avoided since the material slipping in the roll nip will greatly accelerate roll wear. If the mill capacity decreases, or if the motor load does not go up when the feeder speed is increased, it is very likely that the material is not passing through the top pair of rolls. To maximize capacity and extend roll life, open the top pair of rolls slightly and/or reduce the feed rate to eliminate accumulation in the nip of the top rolls. If the finished product is too coarse, close the bottom rolls to obtain the desired finished particle size (Figure 5).

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