Special Springs product catalogue includes several different kinds of nitrogen springs. Each type of spring is characterized by the force range that it is able to exert and by its size. The tolerance associated with the spring height is about ±0.25 mm, a value which is guaranteed by Special Springs to its customers. The width of the tolerance zone of 0.5 mm, applied to the vertical dimension of the springs, is an objective that Special Springs is able to reach with the productive processes and the related technologies currently available. This generates a very low number of scraps of “out of tolerance” products, about a 0.5 %. Something has changed in this very successful scenario, when an important customer made a challenging request to the company: to guarantee a dimensional tolerance on the springs total height of ±0.10 mm, thus reducing the initial value of ±0.25 mm. At this stage it became necessary for Special Springs to understand the repercussions due to this reduction of the tolerance applied to the height on its own products, and in particular: how much would the “out of tolerance” scraps increase? Assuming that the percentage of scraps remains unchanged, which modifications should be required, thus generating the minimum possible costs? In order to find an answer to these two questions, Special Springs has instructed EnginSoft to investigate the tolerance analysis on a nitrogen spring. The tolerance analysis is based on a statistical approach in order to understand how the tolerances applied to the single nominal dimensions of the project affect, combining themselves, the final size, that may represent, as in this case, a very important product requirement

The model of the tolerance analysis has been built using the Cetol 6s software, developed by the American company Sigmetrix. The development of the model consists of the following phases: Definition of the contacts between the single parts of the assembly. The software allows to define in a flexible way any kind of kinematic joint, managing the single degrees of freedom which are constrained. Cetol 6s has furthermore advanced functionalities that allow to automatize this phase, thus dramatically reducing the development time of the model. In particular, when the assembly constraints are set in the 3D CAD, Cetol 6s is able to read and to import them inside the model. The single parts of a nitrogen spring that are involved in the model are four: body, ring, bush and rod. The rod rests on the bush in the area of the upper flange.; the bush is then constrained to the body by means of the ring that keeps it in the right place using the groove in which it is hosted. Definition of the scheme dimensioning of the single parts. This phase consists of the translation of the information included in the 2D drawings: the dimensions and the related tolerances (both dimensional and geometric) are included in the model and represent the quantitative information on the place variability and the relative orientation between plains, the axis, surfaces, etc. Furthermore, if datum notes (DRF – datum reference frame) are present in the 3D CAD model, dimensional and/or geometric tolerances, the software can one come identify and manage them automatically, Such information are imported in the model avoiding redundant action in the 3D CAD. Definition of the target size and analysis execution. Following the procedure described in the model, only some parts of the assembly are considered, that is those affecting the size to be calculated; the height, in this case. The method of constraints creation between the single parts of a set used by Cetol 6s allows to take into account in an automatic way only the geometric characteristics that somehow influence the size. The advantage is that of neglecting the total geometric complexity of the single parts, since it is not required by the objective under investigation: practically speaking this means a great saving of time in the model set-up phase.