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This post was last edited by foam_ZPTK on 2019-7-9 23:46. 1. Selection of the correction plane: The process of eliminating the imbalance in the rotor so that it is in a balanced state is called balance correction. This correction is carried out in a plane perpendicular to the rotor’s axis, and this plane is referred to as the correction plane. The method of correcting the balance within a single correction plane is known as eliminating the imbalance of the rotor; the process of bringing it into a balanced state is called balance correction. This balance correction is carried out in a plane perpendicular to the rotor’s axis, and this plane is referred to as the correction plane. · For rotors with a thin disc shape, the couple imbalance is very small, so in practice only one-sided balancing is performed. For example, flywheels, grinding wheels, fan blades, clutch discs, and rotors whose maximum outer diameter is 5 times or more their net length, etc. · For rotors with a large initial imbalance and excessive vibration during rotation, one-sided balancing must be performed before dynamic balancing to eliminate the static imbalance. Correction* should preferably be carried out in the plane where the center of gravity lies, to reduce couple imbalance. If overlapping is not allowed in the plane where the center of gravity lies, it should generally be done in two planes on either side of that plane. · For rigid rotors, there are generally static imbalance and couple imbalance. Its imbalance can be corrected in any two arbitrarily selected correction planes perpendicular to the axis, namely so-called dual-plane balancing. Correction methods generally involve weighting or deduplication. 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. · For rotors such as crankshafts, since the angular positions for unbalance correction are limited, using two correction surfaces is not sufficient to achieve balance; therefore, a three-or five-surface approach is required. For rotors whose actual operating speed is close to or exceeds the critical speed, they are already flexible under operation; therefore, the deflection caused by rotation must be taken into account during balancing. When the actual operating speed is close to the critical speed, balancing can be achieved using two or more correction planes at different speeds; when the rotor speed far exceeds the first-order critical speed and reaches the second-order critical speed, a balancing method involving four or more correction planes is necessary. 2. Number of correction planes The number of correction planes, as well as the choice of axial positions, is determined based on the principles of the mode shape method; there are the N-method and the N+2-method, that is, the number of correction planes is determined according to the order N of the mode shape to be balanced. The main principle is to use N+2 planes for balancing lower-order modes and N planes for balancing higher-order modes. Regarding the selection of the plane for correcting the axial position, the following two points need to be considered: · It should enable the counterweight to produce a greater balancing effect under the corresponding mode of vibration; · The feasibility and convenience of adding weight to that plane. However, in practical balancing, the selected correction plane must not only balance the first and second orders but also the third order; it is quite difficult to meet these two conditions. Therefore, the correction planes are generally selected as evenly distributed as possible within the effective length of the rotor; this approach can approximately satisfy the above two conditions and is also beneficial for reducing higher-order unbalances. For the influence coefficient method, the determination of the number of correction planes and their axial positions should be based on the mode shape method; otherwise, it may result in an excessively large calculated correction mass that is impractical to implement, or it may significantly disrupt the balance of higher-order mode shapes. 3. Balanced speed The goal of balancing is to ensure that the vibration of the rotor remains within acceptable limits within a certain speed range. For flexible rotors whose operating speed is at least greater than the first-order critical speed, it is necessary not only to ensure that the vibrations at the operating speed meet the required standards, but also to ensure a smooth passage through each critical speed during start-up and shutdown. 4. Vibration measurement point at equilibrium For the mode shape method, in theory, one vibration measurement point is sufficient. For the influence coefficient method, the system of equations has a solution if the number of correction planes is equal to the product of the number of measurement points and the number of equilibrium speeds. In practice, to reduce measurement errors, multiple vibration measurement points are often taken, and methods such as least squares are used for analysis. The determination of the number of measurement points includes: the selection of the axial position of the measurement points and the selection of their direction. The principles for selecting the axial position are: · High original vibration level; · Proximity to the correction plane, with sensitivity to the weighting applied to that plane. Other measurement points should be discarded as much as possible; on the one hand, this is to reduce the number of measurement points, and on the other hand, because the accuracy of the influence coefficients for these points is poor, including them in the equations significantly reduces the accuracy of the correction calculations. The determination of the measurement direction: theoretically, the vibrations in the XYZ directions of the bearing can all be used as a basis for balance calculations. However, those that have a good linear relationship with the unbalanced mass are vertical vibrations and horizontal vibrations; axial vibrations, on the other hand, are less suitable for this purpose. Moreover, horizontal and axial vibrations often contain them.