The main reason for excessive vibration of the mixer

Thread Content

There are generally three types of agitators: turbine, propeller, and paddle type； Its linear velocity is generally less than 20 m/s. These are all ordinary mixers, and their function is basically to stir mixed materials. In mechanical design, situations involving high temperature, high pressure, and high speed present significant challenges in terms of design, manufacturing, and assembly. Due to specific requirements, the core component of the company’s equipment – the mixer – uses a vertical mixing vessel; the mixing shaft is long, with a length of 2 meters. The mixer is equipped with two types of blades: one for propulsion and another for shearing. The motor used has a power of 22 Kw and operates at 1500 revolutions per minute. The mixers made initially had very unstable performance and poor reliability; some could last for several years without failing, while others had their bearings damaged after less than half a year, and some even experienced severe vibration right after being installed and tested. For this vibration issue, the reliability of the mixer was ensured through a redesigned structure, strict inspection of the part manufacturing quality, and improved assembly processes. Here is a brief summary of the three aspects: design, manufacturing, and assembly. After analysis, the main cause of the mixer’s vibration is likely to be (1) unreasonable design in terms of structure ； (2) The quality of part processing does not meet the requirements ； (3) The assembly process is incorrect. When the mixer is in operation and mixes the liquid inside the shear tank, it is subject to axial and radial forces. Moreover, as the liquid is added in stages, these two types of forces keep changing. As a result, the design of the vertical shaft and the selection of bearings have been altered from the original design to one in which the upper end is fixed while the lower end is free to move. For the bearings at the upper end, two angular contact ball bearings are used, installed back to back. Angular contact ball bearings can withstand both axial and radial forces, and they are suitable for high-speed operations. When installed back to back, the contact lines of the bearings extend along the axis of rotation, which increases the rigidity of their radial and axial support capabilities and enhances their resistance to deformation ； The lower bearing is a cylindrical roller bearing with separable inner and outer rings, designed primarily to bear radial forces. The inner ring moves to relieve the stresses generated by thermal deformation during operation. The areas on the stirring shaft that require tightening have been modified to use round nuts together with special retaining rings for those nuts ； Modify the assembly method at the impeller blades and shear blades to increase their contact area with the impeller shaft. Since the slender shaft rotates at high speeds, its rigidity must be high; therefore, a new selection was made regarding the material to be used. Imbalance is also one of the causes of vibration; for this reason, it is necessary to perform dynamic balancing on the lower thrust blades and the shear blades at the bottom. Generally, dynamic balancing should be carried out when the linear speed of the mixer is greater than 5 m/s. The machining quality of the components not fully meeting the requirements significantly affects the reliability of the equipment. In the case of mixing components, this is manifested in aspects such as coaxiality, cylindricity, perpendicularity, and surface roughness. For example, for two mixing shafts, if the deviations at the three bearing positions of one shaft are +0.02, +0.02, +0.02, while those of the other shaft are +0.02, +0.04, +0.06, then the vibration characteristics as well as the reliability of the mixer are better in the former case. Improper assembly is also a major cause of vibration, with bearing assembly being particularly important in this regard. Before installing the bearing, the shaft, housing holes, end caps, and other components that it will fit with must be thoroughly inspected first ； Used shafts and housing holes must undergo thorough precision inspection, and parts that do not meet the requirements should be repaired or replaced. Otherwise, assembly is not allowed. Excessive bearing clearance is also a major cause of vibration. The assembly of angular contact ball bearings has always been a relatively difficult task in bearing assembly. For angular contact ball bearings mounted in pairs, inner and outer steel sleeves of varying lengths are generally added between them, and an appropriate preload is applied based on the actual operating load. The operating clearance of angular contact ball bearings should ideally be zero or slightly negative. The dimensions of the inner and outer rings, as well as the level of preloading, have a significant impact on the operating conditions and lifespan of these bearings. To achieve the optimal operating clearance for angular contact ball bearings, it is first necessary to calculate the preload; generally, a smaller preload is suitable for high-speed applications, while a larger preload is appropriate for low-speed applications. Additionally, the preload should be slightly greater than or equal to the axial operating load. Then, for the adjustment of the operating clearance, actual measurements are taken, along with the grinding of the inner and outer rings and the application of preload; each pair of angular contact ball bearings has its own dedicated inner and outer rings. Eccentricity and wear caused by manufacturing and installation tolerances of couplings ； Missing matching connection nuts/bolts ； Worn coupling nut ； The fastening bolts are loose ； Changes in the stiffness of the shaft and bearings, etc. It is also one of the reasons for the mixer vibration. Great care must also be taken when transporting vertical shafts that are suspended in mid-air over long distances. The bumps encountered during such long journeys can often cause deformation of the shaft, loosening of bolts, or damage or loosening of bearings. Therefore, protective measures must be taken when transporting vertically suspended shafts over long distances; it is best to remove them for separate transportation or to place soft padding under them while they are suspended. To ensure the coaxiality of the shaft holes at the upper and lower end closures of the tank body, as well as the perpendicularity of the plane to the reference axis, mechanical machining should be carried out after the manufacturing processes such as welding and heat treatment of the tank body are completed. For the machining of such equipment, large machine tools are often required: such as horizontal machine tools, floor-standing machine tools, and boring machines. The manufacturing process of equipment should generally control the geometric and positional deviations of components during the cold forming and welding stages of the equipment. For example, during the rolling process of tube sections, it is necessary to control dimensions, roundness, the parallelism of the two end surfaces, and the perpendicularity of the end surfaces to the central axis, among other things. And after welding several tube sections together, the roundness of the cylinder body, the parallelism of its end faces, its perpendicularity, and the elevation of the cylinder body ； After the upper and lower end caps are formed by stamping, it is necessary to trim the straight edges of these end caps and prepare the welding grooves. During this process, the curvature height of the end caps, their overall height, their shape, as well as their perpendicularity to the central axis of the end face remain important parameters that need to be controlled. Generally, the central joint area (which contains a hole for the central axis) and the support joints used for installing mechanical seals and drive frames, which already have a certain amount of machining allowance, should be welded together after the initial processing of the end caps. After welding, further machining is carried out on the end faces of the end caps, the holes in the joints, and the planar support joints. Apart from the straight edges of the end caps, some amount of machining allowance should also be left for the remaining parts ； When welding the end caps and the cylinder body together, it is necessary to pay attention to controlling the flatness of the mating surfaces at the centers of the upper and lower end caps; a central light target and a laser theodolite are used to ensure the coaxiality between the axis holes at these mating surfaces and the central axis of the cylinder body ； Only by taking these measures can sufficient machining allowance be ensured on the final machined surface. The kettle body is generally also welded with jackets, reinforcement rings, guide plates, and internal accessories. Only after all these welding and assembly tasks are completed can the final machining take place. It should be noted that some devices require stress-relief heat treatment; in such cases, the final machining must be carried out after the heat treatment is completed.