Dynamic and static balance

1. Static balance
The static balance is calibrated and balanced on a correction surface of the rotor, and the residual unbalance after correction ensures that the rotor is within the specified range of the allowable unbalance when it is static. It is also called static balance and single-sided balance.

2. Dynamic balance

The dynamic balance is corrected and balanced on two or more correction surfaces of the rotor at the same time, and the residual unbalance after correction is to ensure that the rotor is within the specified range of the allowable unbalance during dynamic balance. Multi-faceted balance.

3. Selection and determination of rotor balance

How to choose the balance method of the rotor is a key issue. Its selection has such a principle:

As long as it meets the needs of the use after the rotor is balanced, if it can be statically balanced, do not do dynamic balancing, and if it can do dynamic balance, do not do static and dynamic balancing. The reason is very simple. Static balancing is easier to do than dynamic balancing, saving labor, effort and cost.

So how to determine the rotor balance type? It needs to be determined from the following factors and basis:

1. The geometric shape and structural dimensions of the rotor, especially the ratio of the diameter D of the rotor to the distance dimension b between the two correction surfaces of the rotor, and the supporting spacing of the rotor, etc.

2. The working speed of the rotor

Technical standards related to rotor balance technical requirements, such as GB3215, API610, GB9239 and ISO1940, etc.
3. Conditions for static balance of rotors In GB9239 balance standard, the conditions for static balance of rigid rotors are defined as:

If the supporting distance of the disc rotor is large enough and the axial runout of the disc part is small during rotation, the even unbalance (dynamic balance) can be ignored, then a correction surface can be used to correct the unbalance, that is, single-sided (static) balance. These conditions must be verified for the specific rotor. After balancing a large number of rotors of a certain type in one plane, the maximum residual couple unbalance can be found and divided by the support distance. If in the worst case this value is not greater than half of the allowable residual unbalance, single-sided (static) balancing is sufficient.

From this definition, it is not difficult to see that there are three main conditions for the rotor to be only balanced on one side (static):

(1) One is that the rotor geometry is disc-shaped;

(2) One is that the support spacing of the rotor when balancing on the balancing machine should be large;

(3) Another is that when the rotor rotates, the end face runout of its correction surface should be very small.

The above three conditions are explained as follows:

(1) What is a disc rotor is mainly determined by the ratio of the diameter D of the rotor to the distance dimension b between the two correction surfaces of the rotor. In the API610 standard, it is stipulated that when D/b < 6, the rotor can only be balanced on one side; when D/b ≥ 6, it can be specified as a condition for whether the rotor is a disc rotor, but it cannot be absolute, because what kind of rotor does the rotor do? Balancing also considers the working speed of the rotor.

(2) The support spacing should be large. There is no specific parameter regulation, but if the ratio to the rotor correction surface spacing b is greater than or equal to 5, it is considered that the support spacing is large enough.

(3) The axial runout of the rotor mainly refers to the runout of the end face of the correction surface when the rotor rotates, because any rotor is precisely machined for the balance test, and the behavioral tolerance between the rotor hole and the correction surface has been guaranteed after processing. The beating is small.

According to the above-mentioned conditions for single-sided (static) balance of the rotor, combined with the relevant technical standards for pumps (such as GB3215 and API610), the conditions for only statically balanced rotors are as follows:

(1) For the rotor of single-stage pump and two-stage pump, when the working speed is less than 1800 rpm, no matter D/b < 6 or D/b ≥ 6, only static balance can be done. However, if dynamic balance is required, it must be ensured that D/b<6, otherwise only static balance can be performed.

(2) For the rotor of single-stage pump and two-stage pump, when the working speed is ≥1800 rpm, if D/b≥6, only static balance can be done. However, the residual unbalance after balancing should be equal to or less than 1/2 of the allowable unbalance. If dynamic balancing is required, it depends on whether the balance of the two calibration surfaces can be separated on the balancing machine. If the separation cannot be separated, only static balance can be done.

(3) For some rotors such as open impellers, if the support at both ends cannot be achieved, only static balance can be done. Because the two ends cannot be supported, it is bound to be cantilevered, so it is very dangerous to do dynamic balancing on the balancing machine, and only one-sided (static) balancing can be performed on the balancing frame.

4. The conditions for the rotor to be dynamically balanced are stipulated in the GB9239 standard: if the rigid rotor cannot meet the conditions of the statically balanced disc rotor, two planes need to be balanced, that is, dynamic balance.

The rotor conditions for static balance only are as follows (balance static G0.4 level is the highest precision, in general, the dynamic balance static state of the pump impeller selects G6.3 level or G2.5):

(1) For the rotor of single-stage pump and two-stage pump, when the working speed is ≥ 1800 rpm, as long as D/b < 6, it should be dynamically balanced.

(2) For multi-stage pumps and combined rotors (level 3 or above), no matter how much the working speed is, the dynamic balance of the combined rotor should be done.

4. Dynamic balance test

The dynamic balance test is a process of dynamic balance detection and correction of the rotor to meet the requirements of use.

When the parts are rotating parts, such as various drive shafts, main shafts, fans, water pump impellers, tools, motors and rotors of steam turbines, they are collectively referred to as revolving bodies. In an ideal situation, when the rotating body rotates and does not rotate, the pressure on the bearing is the same, and such a rotating body is a balanced rotating body. However, due to various factors such as uneven material or blank defects, errors in processing and assembly, and even asymmetric geometric shapes in design, various revolving bodies in engineering make the revolving body rotate. The centrifugal inertial force generated by the tiny particles cannot cancel each other out. The centrifugal inertial force acts on the machine and its foundation through the bearing, causing vibration, noise, accelerated bearing wear, shortened mechanical life, and can cause destructive accidents in severe cases.

To this end, the rotor must be balanced so that it reaches the allowable level of balancing accuracy, or the resulting mechanical vibration amplitude is reduced within the allowable range.

5. Balance method

1. Process balance method

The test system of the process balance method suffers less interference, has high balance accuracy and high efficiency, and is especially suitable for single-body balance of rotating mechanical parts in the production process. It currently plays a very important role in the field of dynamic balance. Engines generally use this balancing method. However, the process balance method still has the following problems:

(1) The rotational speed during balancing is inconsistent with the working rotational speed, resulting in a decrease in balancing accuracy. For example, many rotors are disturbing rotors with second-order critical speed. Due to the limited speed of the balancing machine itself, if these rotors are balanced by process, the imbalance caused by the deformation of the rotors at high speed cannot be effectively prevented.

(2) The balancing machine (especially the high-speed vertical balancing machine) is expensive.

(3) The well-balanced rotor on the dynamic balancing machine is difficult to guarantee the balance accuracy after installation. Because the supporting conditions during dynamic balancing are different from the supporting conditions of the rotor under actual working conditions, and the coordination between the rotor and the balancing device is also different from the matching conditions between the rotor and its own rotating shaft, even if it has been achieved on the dynamic balancing machine before leaving the factory For the rotor with high precision balance, after transportation, reassembly and other processes, the balance accuracy will inevitably decrease before use, and when it is running at the working speed, it may still generate unacceptable vibration.

(4) Some rotors are difficult or even impossible to balance on the balancing machine due to the limitation of size and weight. For example, large rotors such as large generators and turbines often cannot be balanced because there is no corresponding large balance device; for large high temperature steam turbine rotors, elastic thermal warping is generally prone to occur, which will disappear automatically after shutdown. This type of rotor needs to be thermally balanced, which is obviously impossible to balance with a balancing machine.

(5) The rotor must be removed for dynamic balancing, which results in long downtime, slow balancing, and large economic losses.

2. On-site dynamic balancing method of the whole machine

In order to overcome the shortcomings of the above-mentioned process balance method, the on-site dynamic balance method of the whole machine is proposed. The balancing operation of the assembled rotating machinery in the field installation state is called overall field balancing. In this method, the machine is used as a dynamic balancing base, and the vibration information of the relevant parts of the rotor measured by the sensor is used for data processing to determine the unbalance and its orientation on each balance correction surface of the rotor, and to remove weights or weights. Eliminate the unbalance, so as to achieve the purpose of high-precision balance.​​

Because the on-site dynamic balancing of the whole machine is directly connected to the whole machine, there is no need for a dynamic balancing machine, but only a set of low-cost testing systems, so it is more economical. In addition, since the rotor is balanced under the actual working conditions, no reassembly and other processes are required, and the whole machine can obtain high balance accuracy under the working state.

As an important branch of the on-site dynamic balancing technology of the whole machine, the online dynamic balancing technology is also in vigorous development and has great prospects. Because the process balance method is the earliest classical dynamic balance method. The on-site dynamic balancing technology of the whole machine is proposed to solve the problems existing in the process balancing technology.

6. Balance accuracy