Research On The Theory And Application Of Virtual Tolerance

In mechanical manufacturing industry,one of the most important quality evaluation indices for a machine is the precision of the product.Meanwhile,precision and error are closely related.Larger error levels cause a lower precision.For the error generated in the process of processing and assembling,error compensation method is often applied to assure the assembly accuracy required.But there are no concepts for the expression in a visual way when related to the amount and range for the error compensation.It is not conducive to comprehensive analysis of errors and error compensation.ISO/TC213 has discussed the issue of describing error compensation,but no appropriate expressions are proposed until the emergence of virtual tolerance.In September2016,ISO attached great importance to virtual tolerance and arranged specially a symposium?Chinese input on potential new work on virtual tolerances?at the41 st annual meeting of the international standard organization.At the meeting,other colleagues in the tolerance field from all over the world,discussed and rated highly with this new concept.The organizing committee believes that there is a theoretical basis for the existence of virtual tolerance and its use value,and they hope to further improve its theory and expand its scope of application.In this context,this paper makes a thorough study of virtual tolerance.Virtual tolerance,defined as the tolerance with upper deviation smaller than lower deviation,is hard to be understood and appreciated for its opposition to the current definition of tolerance.This paper started from a concrete example of gearbox calculation for the purpose of introducing the concept,and followed with comparing the differences in the process of assembling between an elastic part(a spring)and a rigid part(a pad)to analyze the essence of the virtual tolerance.A quantitative change to qualitative change process from tolerance to virtual tolerance is demonstrated by the change from the positive to negative of the tolerance value.The change of expression form didn't lead to the change of the basic category,both are inside the range of tolerance.The concept ofPositive and Negative Numbers and Sets in mathematics,the Principle of Dimensional Homogeneity in physics and Dialectics of Nature in philosophy provided strong theoretical basis for the rationality of virtual tolerance's existence.In practical application,virtual tolerance requirements for the dimensions of seals with compressibility and resilience is put forward to quantitatively express the dimension range that meets the requirement of resilience,which resulted in great convenience in actual production.For the assembly chain with compensation rings,allocating virtual tolerance directly to the compensation ring can fulfill the synchronous computation of error and error compensation.Comparing the results obtained by Extremum Method,Traditional Probability Method and Probability Method based on virtual tolerance,it is illustrated that the third one is more reasonable in calculating error compensation.Besides,the range of fake waste products can be determined accurately and quickly.Direct calculations can be performed by the basic formula of dimension chain when introduced virtual tolerance to the assembly dimension chain,which avoids the complicated analysis and judgment process and simplifies the calculation.Anisoplan-Anisoamount Fitting Method is proposed when applied the concept of virtual tolerance to the Fitting Method,which greatly reduces the amount of repair and reduces the manufacturing cost.The necessity of the existence of virtual tolerance is proved through the application of virtual tolerance in specific examples.