Performance guarantees

Discrepancies between the calculated performance of machinery and apparatus and the corresponding measured or actual values are due either to errors in design and construction¿referred to as consistent errors¿or to uncontrollable variations and irregularities in the materials and manufacturing processes. The latter are distinct from the former in that they are governed by the vagaries of chance. They are responsible for the difficulty in giving guaranteed values for technical performance and thence for the introduction of tolerances on these guarantees. Tolerances have always been, and still are, a vexed question between manufacturers and purchasers¿partly because the purchaser naturally prefers a definite guarantee and partly because tolerances are often associated with commercial astuteness, ignorance in design or incompetence in manufacture. This view may, but should not, be justified. For apparatus in mass production the probability distribution can be determined by direct analysis of the performance for each product. This direct method is not applicable to most engineering machinery since the quantity production of identical units is too small. The paper discusses the various factors involved in the determination of performance guarantees and develops practical methods for assessing these considerations and deriving therefrom appropriate values both of guarantee and, if necessary, of tolerance. Modern methods of production are assumed in which the principles of quality control are applied to the manufacture of component items in both material and labour.