Why is it necessary to perform dynamic balancing on the rotor, and how is dynamic balancing carried out?

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Why is dynamic balancing necessary? During the manufacturing and assembly processes of the rotor, the finally assembled rotor can never achieve perfect mechanical axisymmetry (a condition known as axial eccentricity); as a result, there is always a certain amount of imbalance present. This amount of imbalance is usually referred to as the initial imbalance. 　　There are various reasons that can cause a rotor to have an initial degree of imbalance, but if such imbalance is present, it will generate a centrifugal force as the rotor rotates. This centrifugal force increases gradually as the rotational speed rises. Centrifugal force is transmitted to the machine through the bearings, causing vibration throughout the machine, generating noise, accelerating bearing wear, reducing the machine’s lifespan, and even leading to control failures and serious accidents. 　　Dynamic unbalance is the most common type of unbalance. It is a combination of static imbalance and couple imbalance. 　　After dynamic balancing correction, an unbalanced rotor not only has its couple imbalance eliminated but also its static imbalance removed; at this point, the rotor’s principal axis of inertia and its axis of rotation coincide perfectly, resulting in a balanced rotor. But ideals are plump while reality is gaunt; it is impossible to balance an unbalanced rotor so that its imbalance amount becomes zero. This is due to the limitations of the precision of the dynamic balancing equipment and the rotor itself. Thus, the concept of balance accuracy arises – that is, the most reasonable order of magnitude that can be achieved under the existing conditions, thereby meeting both the requirements of production and daily life as well as those related to economic efficiency. Under what circumstances is dynamic balance testing necessary? In the previous article, we outlined the situations in which dynamic balance is required for pumps; that can be used as a final guideline, as it is based on practical experience. 　　Theoretically, in practical applications, dynamic balance testing is required for any component on the rotor, as well as for rotors that have not had their components replaced after maintenance. 　　During the assembly process, the gaps between various components meet the installation standards; however, for the entire rotor, the cumulative error may exceed the dynamic balance accuracy required for that rotor. Especially for some rotor components equipped with bearing housings, they should be dynamically balanced before final assembly. Requirements for the precision of a dynamic balancing machine: The minimum achievable residual unbalance is the smallest amount of residual unbalance that can be achieved when using the balancing machine to balance a rotor; it serves as a performance indicator of the balancing machine’s maximum balancing capacity. 　　A rigidly supported balancing machine can be expressed directly by the minimum residual unbalance on the balancing surface, with the unit being grams per millimeter; sometimes grams per centimeter is also used. 　　The minimum achievable residual unbalance is influenced by factors such as the type of balancing machine, the measurement method, the transmission mechanism, the type of bearings, the planar separation ratio of the correction surfaces, and the sensitivity of the balancing machine. 　　To achieve a high level of rotor balance accuracy (i.e., a very small residual unbalance), it is necessary to eliminate as many factors as possible that affect this accuracy. Among these influencing factors, the transmission method of the balancing machine and the imbalance of the transmission components have the greatest impact. The journal accuracy of the rotor must also be strictly controlled, as well as in the case of rotors equipped with impellers, rotors whose center of gravity is not located at the center of the rotor, and rotors that have bearing housings and packing for sealing purposes. Balancing processes and methods: For an unbalanced rotor, its degree of imbalance is measured and corrections are applied to eliminate that imbalance; this is the process of rotor balancing, also known as a balancing test. It is an important process in rotor machining. 1 Selection of the correction surface 　　The process of eliminating the imbalance of the rotor and bringing it into a balanced state is called balance correction. Balance correction is carried out in a plane perpendicular to the rotor axis, which is called the correction plane. The method of correcting balance within a single correction plane is called single-sided balance or static balance, while the method that requires correction in two or more correction planes is called double-sided balance or multi-sided balance, also known as dynamic balance. 　　For rotors with a large initial unbalance and excessive vibration during rotation, single-plane balancing must be performed first to eliminate static unbalance prior to dynamic balancing verification. Sometimes, due to an improper selection of the alignment plane (i.e., the center of gravity is not within the selected alignment plane), correcting the static balance can actually increase the couple imbalance. Therefore, it is best to perform the alignment within the plane where the center of gravity lies, in order to reduce couple imbalance. When deduplication is not permitted on the plane where the centroid lies, it should generally be performed on two planes located on either side of that plane. 　　For rigid rotors, there are generally static imbalance and couple imbalance. To achieve balance, the imbalance can be corrected in any two arbitrarily chosen correction planes perpendicular to the axis, which is known as double-sided balancing. Correction methods generally involve adding weight (such as using counterweights) or removing weight (such as drilling holes). The position of the correction plane is generally determined by the structure of the rotor. 　　To reduce the time and effort required for balancing operations, it is necessary to minimize the amount of correction. To this end, the distance between the two correction surfaces as well as the correction radius should be increased as much as possible under feasible conditions, in order to achieve good balancing results. Click to view -- Summary of Chemical Engineering Skill Training Courses for 2023 2 Verification Methods The imbalance of a rotor occurs when its principal axis of inertia does not coincide with the axis of rotation; balance correction involves altering the mass distribution of the rotor so that its principal axis of inertia aligns with the axis of rotation, thereby achieving balance. Common correction methods include adjusting the correction weight, adding weight or removing weight, etc. 　　To increase stiffness, screw connections, riveting, or welding can be used; these methods enable the rotor to achieve better balance and higher precision, while also being convenient and safe. 　　De-duplication often employs methods such as drilling, grinding, chiseling, and milling. For the rotor components and impellers of pumps and compressors, grinding is used to remove excess weight, or counterweights are added to the couplings. 　　The choice of correction method depends on the rotor structure, process requirements, and the geometry of the correction surface, among other factors. Generally, the locations for adding or removing weight are determined during the design of the rotor. 3 Correction error　During the balancing process, in addition to the measurement errors of the balancer, there are also errors arising from inaccurate balancing corrections (including the magnitude and position of the correction). Such errors are referred to as correction errors. It can be divided into correcting angle error, correcting amplitude error, correcting radius error, and correcting plane position error, etc. 　　In actual calibration, the above four types of errors usually occur together, and they should be taken into account comprehensively during analysis. In addition, the ratio between the initial imbalance and the remaining imbalance, as well as the impact of the imbalance reduction rate of the balancing machine, should also be considered.