Correlative Study Between Bolt Preloading Sequence And Accuracy Retention Of Machine Tool

Poor accuracy retention problems occur prevalently when operating domestic high grade Computerized Numerical Control (CNC) machine tools, which has become a bottleneck to restrict their competence in the market. Preloading anchor bolts installation method is used in most of the installation for heavy duty machine tools. The capability to retain machine tools’ geometric precision is the key for their overall performance and service accuracy after they are installed. Both the elastic interaction between the anchor bolts during preloading and the creep relaxation of the anchor bolts after preloading can lead to the changes in machine tools’ geometric sizes and reduce their accuracy retention. Regarding machine tools’ poor accuracy retention problem, the study focused on the elastic interaction between the anchor bolts and their creep relaxation after preloading. The main research contents are summarized in the following aspects:By simplifying multi-bolt joints as the ’spring-node’ model and introducing the elastic interaction stiffness, a generalized model for multi-bolt elastic interaction was built up for arbitrary bolt joints topology. Combined with the power-law model based on time-hardening, a generalized model for the changes of preloading capacity in multi-bolts creep relaxation was established as well.The impacts of elastic interaction by a variety of preloading levels, bolts distances and joint structures were computed and analyzed via the generalized model of elastic interaction and assisted with numerical simulation method. The results showed that bolt distances had a significant influence on elastic interaction; the preloading levels are independent of interaction stiffness, rigidity stiffness and elastic transfer range.Based on construction process of anchor bolt preloading, numerical simulation and experiments on bolt preloading process were conducted to verify the elastic interaction model, the influence pattern of elastic interaction on bolt’s residual preloading capacity was analyzed as well.Regarding the features of anchor bolt’s preloading distribution and embeddedness, a new anchor bolt preloading strategy was established based on minimum assembly deformation errors and equalized residual preloading capacity. Multi preloading sequences for the machine tools’ anchor bolts were designed, the impacts of guide rail mounting surface and residual preloading capacity by different preloading anchor bolts sequences were quantitatively analyzed in the paper. The results revealed that a reasonable planning on the order of preloading machine tools’anchor bolts could be implemented by the strategy.Considering the joint effects of elastic interaction and room temperature creep deformation on preloading capacity reduction, a model was built up to analyze the variation of the bolt’s creep relaxation and a simulation model on machine tools’ anchor bolt creep relaxation was proposed. The influence pattern of long term creep relaxation on machine tools’geometric accuracy recession was analyzed quantitatively in this paper as well. After 9000 hours of creep simulation, it showed that the anchor bolt preloading creep relaxation accounted for 14%, the relative recession of the straightness and the depth of parallelism on guide rail mounting surface of the machine bed was 3.2 μm and 2.4 μm respectively. This result revealed that the developed model provided a significant guidance and great engineering value for the optimization of anchor bolt preloading on the basic parts of heavy duty machine tools and their capability to retain the geometric precision.