A Study Of Active And Semi-active NVH Design For Forklift Trucks

Nowadays,more attentions have been paid to NVH problems in construction machineries due to more competitive environment and strict regulations.NVH studies of construction machineries have made a lot of achievements,whereas few on that of forklift trucks.At present,almost NVH problems are solved using well-developed models that use the typical "search problems-analyze problems-solve problems" path for problem solving.In such case,the achievement in optimization of a new product is usually limited and detrimental for the new product into the competitive market.Based on the above situation,a 3 tone forklift is taken as an example for the NVH active design and semi-active design,and three key technologies,namely modal avoidance,sound quality prediction modeling and sound absorption material matching,are studied and discussed.The specific work is as follows.Firstly,the concept and procedures of active and semi-active NVH design methods for forklift trucks are proposed,as well as comparison of the two design methods.Secondly,the modal avoidance strategy in active NVH design for forklift trucks is presented in details.The virtual prototype modeling and modal analysis of main parts and systems which include the frame,floor,engine cover,exhaust system and forklift mask system are carried out.Finally,the modal frequency avoidance table is obtained from the results of modal analysis,and the floor and frame are optimized based on the modal sensitivity analysis.Thirdly,the sound quality prediction model in active NVH design for forklift trucks is proposed.Five three-tone forklifts in three sound quality levels(good,moderate,and poor)are selected for the subjective and objective evaluation experiments.Based on the grey system theory,the prediction model of annoyance is established,and is assessed using the evaluation experiments.The results show that the prediction model based on grey system theory gives high accuracy.Fourthly,the matching strategy of the sound absorption material in active NVH design for forklift trucks is presented.The transfer path analysis of a forklift truck is carried out to obtain the largest contribution path to the measured target points and the corresponding main frequency band.The Kundt Tube test is conducted for a set of the sound absorption materials,in order to determine their acoustic performance.Then,the optimal acoustic material is selected according to the rule that the selected material best satisfies the engine noise reduction requirement.Finally,the experiments conducted on the forklift truck with the selected material gives significant noise reduction,indicating the reliability and feasibility of the proposed matching strategy.This thesis presents a good example for the active and semi-active NVH design procedures for forklift trucks.Three key techniques for the active and semi-active NVH design are modal avoidance,sound quality prediction modeling and sound absorption materials matching,which can be further extended to NVH designs for similar vehicles.