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446                  METALLURGY OF CAST  IUON.
wheel is turned before being" put into service, and it certainly must be soft in order that it can be turned. These hard, gritty wheels will fail in the thermal test, or by severe brake application. Regarding the depth <>f chill, it should not exceed 3.4 inch in the throat, or 15-16 inch in the center of the tread. The minimum should not be less than ]„( inch in the throat, or .".-» inch in the center of the tread. Assuming that we have the
maximum   depth  of  chill—.....*5~*6   inch      we   get   the
blending of the white iron through the entire tread, and begin to crowd the dang'er line and gain nothing, as the highly chilled wheel will shell out and become capable of sliding more readily than a medium chilled whiH'L In breaking up three hundred defective wheels that were removed on account of shelled spots, 95 per cent, showed a high chill.
** The design of a pattern is one of the essential factors in I IK; man 11 facture of the cast wheel, other than the thickness of flange, shape of hub, and tread. The designing of the pattern should be left to the discretion of the manufacturer. A large percentage of wheels that fail in the brackets can be ascribed to a poorly designed pattern; too light brackets will crack because they cool more rapidly than the plate of the wlu'f-l, which would cause a strain on them; too heavy a bracket will throw the strain on the plates, causing tin- f'l.tfi's to crack. For those who are not familiar v, nil tin- drop u*st used in testing wheels, Pig. 93 •/i',« ^ ,tii inspiration of the 'Ban* drop, and Fig. 94 the M l II. »!/»p. It \vill be noted that, the hammer of !•)• P ni drop striki-s tin* single plait- of tin- wheel {*<•»• <»!!'! A ««n Fu'a, M..*K Tin* hammer of the M. I'. P». ilr*,p -trila-s tin.- hub of the wlii.-el (si-c letter A on i;ig. is claimed for it, 1 the steel-tiredrcf.    Ilirlinn* whrii thr pomm-»'r-r.r-,