The need to redesign an ammonia valve line gave Henry Valve Co. of Illinois the opportunity to make the conversion from gray iron to Ductile Iron as shown. Size (thinner valve walls) and weight reductions (45% reduction) were achieved without sacrificing performance.

PRINCIPLE NO. 7-

DESIGN WITH ADEQUATE SAFETY

Both automobile steering knuckles and plow-shares are made in Ductile Iron. It is obvious these castings have different safety requirements. Safety requirements vary within wide limits, with the above two examples representing extremes.

The designer is in command of all controls necessary to achieve whatever degree of safety is needed. These controls, however, need to be applied judiciously, because increasing part safety invariably increases cost. The controls available are:

1. Approximating the ultimate load carrying capacity of the material to various predetermined degrees. Under static loads the maximum permissible stress equals the yield strength. Usual design stresses vary from 50 to 75% of the yield strength (0.2% offset proof stress). Parts exposed to frequently varying loads should be designed on the basis of the endurance limit. Design stresses of 50 to 100% of the endurance limit are customary, corresponding to a very high, and low margin of safety, respectively.
2. Estimating the effect of potential emergency overloads involves the determination of the most likely failure mechanism. Emergency operating conditions can seldom be foreseen. Neverless, the occurrence of such conditions should be considered. Excessive static or dynamic loads may cause failure either through deformation of the