Dimensional chains in which under the terms of production it is economically expedient to appoint wider admissions to the making links of dimensional chains, allowing thus at some small part of products an exit of the sizes of the closing link out of tolerance zone limits, have to pay off by a probability-theoretic method. The quantity of such rejected products is defined by coefficient of risk of tD.
The dimensional chain consists of the closing link and components. The size which turns out during the processing or assembly of a dimensional chain the last is called closing. The making links of a dimensional chain share on increasing and reducing. Such link of a dimensional chain at which increase and constancy of the sizes of other components of links, the size of the closing link increases is called as the increasing link. Such link of a dimensional chain at which increase and constancy of the sizes of other components of links, the size of the closing link decreases is called as the reducing link.
In dimensional chains in which 100% interchangeability has to be provided, admissions pay off on a maximum minimum method. The calculation procedure on this method is rather simple, however thus too strict requirements are imposed to the accuracy of the making links (and consequently costs of production increase, however complete interchangeability is carried out.
Dimensional chain call set of the geometrical sizes located on the closed contour, the surfaces defining mutually situation (or axes) one or several details and directly participating in the solution of an objective.
The method of a maximum minimum imposes strict requirements to the accuracy of the making links it is connected with the assumption that limit values of errors of the making links are realized and they are combined in the worst way. From here small admissions.
In a real situation most often economically expediently using a teoretikoveroyatnostny method to appoint wider admissions to the making links, allowing thus at some small part of products an exit of the sizes of the closing link out of tolerance zone limits.
The direct task – synthesis of accuracy of a dimensional chain – has no unambiguous decision since the set admission of the closing link and coordinate of its middle can be received at various combinations of characteristics of the making links. In formulas (– (we have in each equation of unknown so much, how many the making links in the considered dimensional chain. Therefore solutions of a direct task in many respects is defined by training of the designer and his experience. He has to appoint coordinates of fields of admissions for constructive reasons so that the equation was carried out (.