spacing    between    the    rolls.    It    is    necessary to set the opening between the rolls slightly    closer    than    the    top    product    size required. With two coarse corrugated shells (fig. 6-7), the tip-to-tip setting produces    a    product    larger    than    two    smooth shells    set    at    the    same    distance. In   the   NCF,   the   hydrocone   crusher   is normally   used   as   the   secondary   unit.   These machines   have   a   conical   or   domed   crushing member    called    a    cone,    head,    or    sphere, which    moves    in    a    small    circle    around    a vertical axis inside a fixed bowl or mantle. The cone may be relatively stationary    at    the    top    and    move    at    the bottom    only,    or    be    mounted    so    that    the head    can    wobble    as    well    as    gyrate.    The crushing    head    is    free    to    turn    under    the thrust     from     the     material     being     crushed. The   fineness   of   the   product   is   adjusted   by raising    or    lowering    the    mantle. Effective feed and discharge arrangements are essential to the operation   of   any   cone   type   of   crusher.   For maximum    crushing    efficiency,    feed    must    be supplied    to    the    crushing    chamber    in    the optimum amount and distributed evenly around    the    entire    crushing    area. NOTE: Material received by the hydrocone   crusher   must   be   evenly   distributed nonsegregated     feed.     This     means     that     the material entering the crusher is evenly distributed around the entire crushing chamber,     with     fine     and     coarse     material well intermixed in the feed. Unevenly distributed or segregated feed results in poor manganese wear, reduced capacity, and high stresses within the crusher.     These     problems     can     cause     short bearing life and possible failure of major     components.     An     evenly     distributed, nonsegregated feed results in maximum capacity, a uniform product, and a smooth    running    machine. A   pressure   gauge   is   standard   on   all hydrocone    crushers.    Erratic    hydroset    control pressure    and    motor    amperage    (horse-power draw) readings are evidence of poor feeding conditions and uneven feed distribution    in    the    crushing    chamber.    The pressure gauge enables the operator to read    high,    average,    and    low    pressures    in the     hydroset     control     and     take     corrective measures    against    erratic    and    high-pressure conditions.    The    pressure    gauge    should    be checked regularly when the crusher is receiving    the    full    or    maximum    amount    of feed. NOTE:  When  excessive  fluctuations  occur in   the   pressure   in   the   hydroset   control   and horsepower   drew,   determine   and   eliminate   the cause     immediately.     Operating     the     crusher under these conditions can cause Figure 6-7.—Shell combinations for dual roll crushers. 6-6