| Centrifugal compressors are widely used in many industrial fields,and play key roles in the national basic industries.The quality of the design,manufacturing and assembly of the shaft and impellers are essential for the performance of the compressor.In order to transmit specified torque,the main shaft and the impellers in the rotor system should be tightly assembled.In such assembly,the interference fit,which is also called as shrink fit assembly,are usually used due to its high reliability.However,the main shaft may be curved to some extent after the assembly,i.e.,a shaft bending deformation may occur.Unfortunately,such bending deformation may become more serious and the frequency of the occurrence may also increase when the compressor size becomes larger.Once the shaft bending deformation exceeds the specified value,it will result in some knotty problems,such as vibration,noise,and impact abrasion,and so on.Therefore,for the manufacturer,it is very important to reveal the mechanism of the shaft bending deformation induced by shrink fit and to provide some methods to the reduce the shaft bending deformation.In general,the assembly of the main shaft is carried out in a large factory building,and there is no special environmental isolation device to maintain the homogeneity of the environmental temperature.Moreover,in order to speed up the cooling rate,fans are often used to accelerate the air convection,and then the temperature field along the circumference of the main shaft is inhomogeneous.Based on finite element analysis,we found that the inhomogeneity of the temperature field around the shaft and impeller can lead to a considerable shaft bending deformation.Specially,the more inhomogeneous the cooling is,the greater the bending deformation will be.In addition,it is also found the contact pressure between the interference surfaces can promote the recovery of the bending deformation,while the existence of the friction force on the interface will hinder the recovery.Because both the contact pressure and the friction force are related on the interference,the interference and the friction coefficient have a coupling effect on the bending deformation.When the friction coefficient is equal to 0,the bending deformation is very small,but decreases monotonically with the increase of the interference.If the friction coefficient is not equal to 0,the bending deformation will increase first and then decrease with the interference.By simulating the assembly of two impellers,it is found that when the two impellers are cooled at the same cooling modes,the bending deformation produced by the two impellers will superimpose,and the assembly time interval will not have considerable influence on the bending deformation,but cooling modes of the two impellers will have a significant influence on the shaft bending deformation.In the present thesis,the influences of shape errors with respect to the impeller hub or the shaft on the bending deformation are also studied.It is found that the shape error has a considerable influence on not only the stress distribution and friction torque between the interference surfaces,but also the bending deformation of the main shaft.At the same time,the shape error and the inhomogeneous cooling conditions have a coupling effect on the bending deformation.Finally,in order to reduce the bending deformation caused by the interference fit,two methods are proposed.One is to shorten the contact length between the hub of the impeller and the shaft,and another one is to design a circumferential chamfer at the bottom of the impeller disc.The finite element simulations show that the two above methods can bot effectively decrease the bending deformation caused by inhomogeneous cooling during the interference fit,while it has slight influence on the bending deformation induced by the shape error. |
PDF Full Text Request