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In the hammermill we are dealing with velocities much greater than any we have considered so faro Tip velocities of the hammers start in about where the highest speed roll crusher operates (the 3500 f p m speed of the Edison roll crusher), and range from there up to 10,000 or 12,000 f p m. These high velocities are necessary because the hammermill does practically a11 of its crushing through transference of the kinetic energy in the free-swinging hammers to the material at the instant of impact, and this energy must be delivered without defecting the hammers very far from their noIoad radial position if the machine is to operate smoothly. The crushing blows may be delivered against free material (pure impact breaking) or against material supported on a breaker plate, in which case the action is a high velocity sledging. In either case the mass velocity force stored in the hammer does the work; the function of the motive power is to maintain, and restore, this energy-in which it is assisted by the kinetic energy in the rotor, supplemented in many machines by heavy flywheels mounted on one or both ends of the rotor shaft.
Most hammermills receive their feed on the down-running side of the rotor. In machines of this type the preliminary breaking is done while the material is supported against breaker plates whose inner surface is flush with the grate
circle at the lower end. In the grate zone the reduction is continued by interaction between hammers and grates, the process being a combination of "nut-cracking" action and impact of hammer-to-material and material-to-grate bars. In this type of mill a considerable portion of the total work is done in this zone; the clearance between hammer tips and grate surface is close; and, usually, the grates or the hammers, or both, are adjustable so that this close clearance may be maintained. One .particular make of mill, which does its work in the manner described, receives its feed directly over the top of the rotor, and is arranged for reversible operation-having two sets of grates symmetrically disposed on either side of the rotor. |
