Attrition can be brought about by tension, compression and shear stresses, compression
being most important when related to animal feeds. Shear and tension tests may
give much information but are often more difficult to perform (Luyten et al., 1992). In
practice, therefore, more emphasis has been put on compression devices. The actual
assessment, however, when pellets are subjected to attrition by means of different
devices, is derived from properties of the more fundamentally desired quantity ‘energy’
needed for fracture or abrasion of pellets. Actual routine assessments are often expressed
just as ‘kg force’ after simulation of impact devices (mainly fragmentation) or as
‘percent fines returned’ after simulation of transport (mainly abrasion).
Several types of empirical devices for the routine evaluation of pellet quality have
been developed (Melcion and Delort-Lava], 1981). In general, these routine devices test
the hardness of pellets or their durability or resistance to attrition stresses (Pfost, 1963).
Some important devices are discussed in the following paragraphs with the often used
quantities ‘hardness’ and ‘durability’ as pellet quality parameters.