624 MODERN STEAM PRACTICE . in modern practice solid arms are the rule , though some are made liollow in the casting as well as the naves . Much attention has been paid to wheel Castings in order to prevent them Cracking from the unequal expansion of the cast iron . A wheel properly propor - tioned… … Fig . 501 . — Cast - iron Wheel . A , Nave . b , Arm . c , Inside tyre . d , Outside tyre . e , Crank pin and boss . contracts rapidly , the great body of metal in the nave being still in a fluid state , which in cooling tears away from the rim ; it was there - fore found necessary to divide the…
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