Tolerance design of multistage radial flow submersible pumps

The functional quality of a product assembly is heavily dependent upon one or two critical dimensions of the assembly. For example, the quality of a ball bearing is based on the clearance between the balls and the inner or outer racings. These critical dimensions result from the cumulative effect of two or more functional dimensions. A tolerance analysis has to be conducted for identifying the functional dimensions that are affecting a critical dimension. Once the functional dimensions are identified, then the tolerances for these dimensions are to be allocated from the critical dimension tolerance. Thus the tolerance design of a product consists of tolerance analysis and tolerance allocation. Dimensional tolerancing are designed to ensure that products produced will meet the designed requirements, e.g. functionality, minimum cost and maximum interchangeability. In the present study, tolerance design is carried out for a multistage radial flow submersible pump assembly. One of the critical dimensions of the pump assembly is the axial play between the impeller and the volute casing. The impeller has to rotate inside the casing without rubbing the wall. An axial play of 2+/-0.5mm is required to be maintained for preventing the rubbing action. As the number of functional dimensions controlling this axial play is more than fifteen, an assigned tolerance of +/-0.5mm on the critical dimension is very difficult to achieve. The method adopted for achieving the required axial play is discussed in details in this paper.