A novel method to design tolerance of aero-engine casing by integrating 3-D assembly tolerance with performance instability

Assembly quality of aero-engine casing plays a key role in the whole aero-engine, since it is directly related to the final function and dynamic performance. However, during the design phase, the tolerance analysis is usually conducted independently without any consideration of the effect on the dynamic characteristic. This paper aims to integrate manufacturing precision with dynamic performance instability together. First, the 3-D tolerance model of the aero-engine casing is constructed based on the Jacobian-Torsor theory. The target deviation from the tolerance model is defined as the input variable into the vibratory governing equation. Then, the effect of 3-D assembly deviation on the natural frequency is studied. The corresponding frequency distributions for different vibration modes are illustrated. Finally, the mapping relationship between assembly tolerance and fluctuation ratio of natural frequency is established through the 3-D fitted surface. Under the given constraint of performance stability, the optimized tolerance zone is obtained. This work provides a significant guidance for performance improvement and tolerance design in the aero-engine casing assembly.

A study of the compression stability of a coupler under initial asymmetric conditions

In practice, due to the influence of assembly deviation, clearance, vibration and other objective factors, the coupler will inevitably work under asymmetrical conditions in the buffing state. However, the existing theoretical studies on the compression stability of couplers did not pay enough attention to this condition, and most of the studies are based on the premise of ideal symmetry conditions. In this paper, the initial lateral deviation between the ends of a coupling coupler is taken as a typical asymmetrical condition, and the influence of the initial asymmetrical condition on the compression stability of the coupler is analysed by theoretical analysis and dynamic simulation. The results show that with the increase of the initial lateral deviation, the rotation angle of the coupler will also increase when it reaches the self-stabilizing equilibrium point. Therefore, the initial asymmetry will reduce the stability margin of the coupler, and then weaken the self-stabilizing ability and compression stability of the coupler. Improving the symmetry of the coupler is also one of the effective methods to guarantee the compression stability of the coupler and the running safety of the locomotive. When the static friction coefficient of the coupler tail can reach 0.3, the initial lateral deviation of the coupler should be limited to less than 7 mm, and the smaller the static friction coefficient, the higher the requirements for the initial lateral deviation.

Monomer model: an integrated characterization method of geometrical deviations for assembly accuracy analysis

Purpose Assembly accuracy is the guarantee of mechanical product performance, and the characterization of the part with geometrical deviations is the basis of assembly accuracy analysis. Design/methodology/approach The existed small displacement torsors (SDT) model cannot fully describe the part with multiple mating surfaces, which increases the difficulty of accuracy analysis. This paper proposed an integrated characterization method for accuracy analysis. By analyzing the internal coupling relationship of the different geometrical deviations in a single part, the Monomer Model was established. Findings The effectiveness of the Monomer Model is verified through an analysis of a simulated rotor assembly analysis, and the corresponding accuracy analysis method based on the model reasonably predicts the assembly deviation of the rotor. Originality/value The Monomer Model realizes the reverse calculation of assembly deformation for the first time, which can be used to identify the weak links that affect the assembly accuracy, thus support the accuracy improvement in the re-assembly stage.

The optimal control of assembly deviation for large thin-walled structures based on basic deviation patterns

The deviation vector at arbitrary location of large thin-walled structure caused by manufacturing process is different and has the characteristic of field distribution, which has great influence on the assemble quality. The deviation of each point on the part is not independent, and the final assembly deviation is difficult to be controlled. In this paper, the deviation field of large thin-walled structure is described by the linear combination of a series of basic deviation patterns. The deviation propagation model is established to quantify the contribution of basic deviation patterns between parts and assembly. A new two-step optimization method based on the adjustment of key control points of the part is proposed for the deviation control of large thin-walled structures. Firstly, the effective independent method is employed to obtain the optimal measurement points, which may characterize all basic deviation patterns of the part accurately. Then a new optimization model is developed to determine the key control points for special basic deviation pattern, which have little influence on the other basic deviation patterns. Based on the genetic optimization algorithm, the optimal key control points and the adjusted quantities for special basic deviation pattern are obtained, simultaneously. A case study on the assembly process of two cylindrical thin-walled parts with initial deviations measured by the Laser Scan Device is conducted. The basic deviation pattern with great influence on the deviation of assembly is determined firstly. The key control points and the corresponding adjusted quantities for this basic deviation pattern are calculated. The results indicate that the deviation of the assembled structure may be suppressed by the adjusted deformation of the key control points of parts. It is useful on the deviation control for the assembly process of large thin-walled structures.

An assembly deviation calculation method based on surface deviation modeling for circumferential grinding plane

Performance of mechanical product is highly influenced by assembly deviation. Due to manufacturing errors, the real part surface is machined with morphology deviations, which would cause mating surface deviating from ideal position in assembly behavior, consequently leading to assembly deviation. Meanwhile, the random variation of relative position and orientation between two non-ideal parts also affects the assembly deviation. To efficiently obtain the maximum assembly deviation considering the comprehensive influence of two factors above for circumferential grinding plane, an assembly deviation calculation method based on surface deviation modeling is proposed in this paper. In this method, morphology deviations models of part surfaces are firstly established from the deviation function. The randomness of two factors are represented by a multivariate group with randomness containing deviation function coefficients and three deflected parameters. Then based on surface deviation modeling method, differential evolution algorithm is applied to search the maximum assembly deviation, which involves the construction of fitness function by implementing optimized progressive contact method and iterative operations of mutation, crossover and selection. Finally, the effectiveness of this method is illustrated by an assembly in the end.

A generic integrated approach of assembly tolerance analysis based on skin model shapes

Nowadays, assembly tolerance analysis has become a challenging problem to predict the accuracy of a final assembly and examine whether specified tolerances satisfy assembly functional requirements (AFRs) for ensuring product assembly performance. Skin model shapes can be addressed to represent part geometric tolerances with manufacturing defects, thereby providing high fidelity surfaces that can replace nominal or ideal surfaces and significantly improve the accuracy and reliability of assembly tolerance analysis. However, their application in easy-to-use assembly simulation is limited by the level of detail required for manufacturing defect simulation and the complicated calculation process for integrating these defects into the tolerance analysis. Therefore, to overcome these issues in predicting assembly deviations in the early design stage, we propose a generic integrated approach of assembly tolerance analysis based on skin model shapes. First, two methods are introduced for modelling and generating skin model shapes according to different mate types of assembly key features. Second, a calculation method of assembly deviation propagation is developed by the integration of skin model shapes and stream-of-variation theory with accuracy and efficiency guarantees. Besides, a slightly modified relative contact positioning method is presented, based on different surface and progressive contact method, to obtain deterministic contact points and contact positioning errors between key mating joint surfaces. And then, the deviation values of AFRs are calculated, considering the inevitable manufacturing and assembly process errors. Finally, a typical mechanical assembly on assembly tolerance analysis is used as a case study to demonstrate the effectiveness of the proposed approach.

Critical joint identification for efficient sequencing

Identifying the optimal sequence of joining is an exhaustive combinatorial optimization problem. On each assembly, there is a specific number of weld points that determine the geometrical deviation of the assembly after joining. The number and sequence of such weld points play a crucial role both for sequencing and assembly planning. While there are studies on identifying the complete sequence of welding, identifying such joints are not addressed. In this paper, based on the principles of machine intelligence, black-box models of the assembly sequences are built using the support vector machines (SVM). To identify the number of the critical weld points, principle component analysis is performed on a proposed data set, evaluated using the SVM models. The approach has been applied to three assemblies of different sizes, and has successfully identified the corresponding critical weld points. It has been shown that a small fraction of the weld points of the assembly can reduce more than 60% of the variability in the assembly deviation after joining.