Dynamic and static balance

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I. Static balance: Static balance is achieved by performing balancing on one correction surface of the rotor; the residual unbalance after this process ensures that the rotor is within the permitted limits for unbalance in a static state. Static balance is also known as one-sided balance. II. Dynamic balancing: Dynamic balancing involves performing the balancing process on two or more correction surfaces of the rotor simultaneously. The remaining unbalance after this process is reduced to ensure that the rotor remains within the specified limits for allowable unbalance during operation. This type of balancing is also known as double-sided or multi-sided balancing. III. Selection and determination of rotor balancing. How to choose the method of rotor balancing is a key issue. There is a principle governing its selection: provided that it meets the requirements for the rotor’s intended use after balancing, if static balancing can be performed, then dynamic balancing should not be done; and if dynamic balancing can be carried out, then both static and dynamic balancing should be avoided. The reason is simple: static balance is easier to achieve than dynamic balance, saving effort, labor, and costs. So how is the rotor balancing type determined? It is necessary to determine this based on the following factors and criteria: 1. The geometry and structural dimensions of the rotor; in particular, the ratio between the rotor diameter D and the distance b between its two end faces, as well as the spacing between the rotor supports, etc. 2. The operating speed of the rotor is governed by the technical standards related to rotor balancing requirements, such as GB3215, API610, GB9239, and ISO1940, etc. 3. Conditions for static balance of the rotor: In the GB9239 balancing standards, the conditions for static balance of a rigid rotor are defined as follows: if the spacing between the supports of the disc-shaped rotor is large enough and the axial runout of the disc portion during rotation is very small, so that couple imbalance (dynamic balance) can be ignored, then the imbalance can be corrected using a single correction surface, achieving single-sided (static) balance. It is necessary to verify whether these conditions are met for a given rotor. After balancing a large number of rotors of a certain type on a plane, the maximum remaining couple unbalance can be determined and divided by the support distance. If, under the worst-case conditions, this value is no greater than half of the allowable residual unbalance, then single-plane (static) balancing is sufficient. It is easy to see from this definition that the conditions under which the rotor can achieve unidirectional (static) balance are mainly threefold: (1) one of them is that the rotor has a disc-shaped geometry ； (2) One is that the support spacing when the rotor is balanced on a balancing machine should be large ； (3) Another requirement is that when the rotor rotates, the end-face runout of its correction surface must be very small. The above three conditions are explained as follows: (1) What constitutes a disc-type rotor is primarily determined by the ratio of the rotor’s diameter D to the distance b between its two end faces. According to the API610 standard, when D/b < 6, it is sufficient to perform single-sided balancing of the rotor ； When D/b ≥ 6, this can be used as a criterion to determine whether a rotor is a disk-type rotor; however, it should not be taken as an absolute rule, as the type of balancing required for the rotor also depends on its operating speed. (2) There are no specific parameters stipulated for the support spacing; however, a ratio of the support spacing to the distance b between the rotor alignment surfaces of ≥5 is considered to indicate that the support spacing is sufficiently large. (3) The axial runout of the rotor refers primarily to the end-face runout of the reference surface as the rotor rotates; since any rotor undergoes balancing tests after being finely machined, the tolerances between the rotor’s holes and the reference surface are ensured, resulting in very low end-face runout. Based on the conditions for single-sided (static) balancing of the rotor mentioned above, and in conjunction with the technical standards related to pumps (such as GB3215 and API610), the requirements for rotors that are to be balanced only statically are as follows: (1) For single-stage and two-stage pump rotors, when the operating speed is < 1800 revolutions per minute, static balancing alone is sufficient, regardless of whether D/b < 6 or D/b ≥ 6. However, if dynamic balancing is required, it is necessary to ensure that D/b < 6; otherwise, only static balancing can be performed. (2) For the rotors of single-stage and two-stage pumps, when the operating speed is ≥ 1,800 rpm, if D/b ≥ 6, only static balancing is required. However, the remaining unbalance after balancing must be equal to or less than 1/2 of the allowable unbalance. If dynamic balancing is required, it is necessary to check whether the balance of the two correction surfaces can be achieved separately on a balancing machine; if not, only static balancing can be performed. (3) For some rotors such as open-type impellers, if it is not possible to provide support at both ends, performing only static balancing is sufficient. Since the ends cannot provide support, a cantilever structure is necessary; as a result, performing dynamic balancing on a balancing machine is very dangerous, and single-sided (static) balancing can only be carried out on a balancing stand. 4. The conditions for dynamic balancing of rotors are stipulated in the GB9239 standard: For any rigid rotor that fails to meet the conditions for static balancing applicable to disc-shaped rotors, balancing must be performed on two planes; this is known as dynamic balancing. The conditions for rotors requiring only static balancing are as follows (the G0.4 level of static balance accuracy represents the highest precision; generally, the dynamic balance accuracy for pump impellers is specified as G6.3 or G2.5): (1) For rotors of single-stage and two-stage pumps, when the operating speed is ≥ 1,800 rpm and D/b < 6, dynamic balancing must be performed. (2) For multi-stage pumps and combined rotors (3 stages or more), dynamic balancing of the combined rotor shall be performed regardless of the operating speed. IV. Dynamic Balancing Test The dynamic balancing test is the process of conducting dynamic balance checks and corrections on the rotor to meet the required standards for use. When parts are components that move in a rotational motion, such as various drive shafts, spindles, fan impellers, water pump impellers, cutting tools, and the rotors of electric motors and steam turbines, they are collectively referred to as rotating bodies. Ideally, when a rotating body is rotating and when it is not rotating, the pressure exerted on the bearings remains the same; such a rotating body is a balanced rotating body. However, for various rotating bodies in engineering, due to multiple factors such as material inhomogeneity or blank defects, errors occurring during machining and assembly, and even asymmetrical geometric shapes present in the design, the centrifugal inertial forces generated by each tiny particle on the rotating body fail to cancel each other out during rotation. These centrifugal inertial forces are transmitted via the bearings to the machinery and its foundation, causing vibrations, noise generation, accelerated bearing wear, and shortened machine lifespan. In severe cases, this can lead to catastrophic accidents. To this end, the rotor must be balanced to achieve the permitted balance accuracy level, or the amplitude of the resulting mechanical vibrations must be reduced to an acceptable range. V. Balancing Methods 1. Process balancing method The testing system used in the process balancing method is less susceptible to interference, offers high balancing accuracy and efficiency; it is particularly suitable for performing individual balancing of rotating mechanical components in the production process. It plays a significant role in the field of dynamic balancing, and this method is widely used in steam turbines and aero engines. However, the process balancing method still has the following problems: (1) The speed at balance is different from the operating speed, resulting in a decrease in balancing accuracy. For example, there are many rotors that are flexible rotors with a second-order critical speed. Given the limited rotational speed of balancing machines, if these rotors are balanced using conventional methods, it is impossible to effectively prevent the imbalance caused by deformation of the rotors at high speeds.  (2) Balancing machines (especially high-speed vertical balancing machines) are expensive.    (3) The rotor that has been balanced on a dynamic balancer may not maintain its balance accuracy after being installed in the machine. Since the support conditions during dynamic balancing differ from those under which the rotor operates in actual working conditions, and since the fit between the rotor and the balancing equipment also differs from the fit between the rotor and its own shaft, even a rotor that has been precisely balanced on a dynamic balancer prior to shipment may experience a decrease in balance accuracy after processes such as transportation and reassembly. Consequently, when operating at its working speed, it may still generate unacceptable levels of vibration.    (4) Some rotors, due to size and weight limitations, are difficult or even impossible to balance on a balancing machine. For example, in the case of extra-large rotors such as those used in large generators and turbines, the lack of suitable balancing equipment often makes it impossible to perform balancing ； For large high-temperature steam turbine rotors, elastic thermal warping often occurs, and it disappears automatically after the machine is shut down. Such rotors require thermal dynamic balancing, and it is clearly impossible to achieve this balance using a balancer.    (5) The rotor must be removed in order to perform dynamic balancing, which results in long downtime, slow balancing speeds, and significant economic losses. 2. On-site dynamic balancing method for the entire machine To overcome the drawbacks of the aforementioned process balancing method, the on-site dynamic balancing method for the entire machine was proposed. The balancing operation of the assembled rotating machinery in its installed condition on-site is called overall on-site balancing. This method uses a machine as the balance machine base; by utilizing sensors to measure vibration data from relevant parts of the rotor, data processing is carried out to determine the imbalance amounts and their locations on various balance planes of the rotor. The imbalance is then eliminated through either removing mass or adding mass, thereby achieving high-precision balancing. 　　Since on-site dynamic balancing of the entire machine is performed directly on the machine itself, there is no need for a dynamic balancer; only a low-cost testing system is required, making it quite economical. Furthermore, since the rotor is balanced under actual operating conditions, there is no need for additional processes such as assembly, allowing the entire machine to achieve a high level of balance accuracy while in operation. As an important branch of on-site dynamic balancing technology for complete machines, online dynamic balancing technology is also experiencing rapid development and holds great promise. Since the process balance method is one of the earliest classical dynamic balancing methods. The on-site dynamic balancing technology for complete machines was proposed to address the problems existing in process balancing technology. VI. Balance accuracy – Precision class: Gg.mm/kg. Examples of rotor types: G4000 – Crankshaft drives for low-speed marine diesel engines with a single number of cylinders and rigid installation; G1600 – Crankshaft drives for large two-stroke engines with rigid installation; G630 – Crankshaft drives for marine diesel engines with rigid installation ； Crankshaft drive components G250250 for rigidly mounted large four-stroke engines; crankshaft drive components G100100 for rigidly mounted high-speed four-cylinder diesel engines; crankshaft drive components for six-cylinder and multi-cylinder diesel engines. Complete (gasoline, diesel) engines for cars, trucks, and locomotives. G4040 automobile wheels, wheel rims, complete wheels ； Driving components for engines used in cars, trucks, and locomotives. G1616 crusher, parts for agricultural machinery ； Individual parts for (gasoline, diesel) engines used in cars, trucks, and locomotives. G6.36.3 Gas and steam turbine, including the rigid turbine engine rotor of marine (commercial ship) main turbines ； Turboscharger ； Machine tool drive components ； Rotors for medium and large motors with special requirements ； Small motor rotor ； Turbopump. G2.52.5 Gear for the main turbine of sea vessels (merchant ships) ； Centrifuges, pump impellers ； fan ； Rotor components of aircraft gas turbines ； flywheel ； General parts of machine tools ； Ordinary motor rotor ； Individual parts of engines with special requirements. G11 tape recorder and record player drive components ； Grinding machine drive component ； Small armatures with special requirements. Spindle, grinding wheel, armature, and gyroscope of the G0.40.4 precision grinder. Considering both the technological advancements and economic feasibility, the International Organization for Standardization (ISO) established the globally recognized ISO 1940 balance grades in 1940. This standard categorizes rotor balance levels into 11 grades, with each grade differing from the previous one by a factor of 2.5. Balancing machines are designed to meet requirements ranging from the most stringent G0.4 to the least stringent G4000. The unit is grams per millimeter per kilogram (gmm/kg), and it represents the eccentric distance of the imbalance from the rotor’s axis.