The influence of auxiliary shaft on the dynamic balance measurement of wind blades

Fan users often say that it is very difficult to balance the fan. They don't know whether to do single-sided balancing or double-sided balancing. Some fans still have large vibrations after balancing. We analyzed the specific causes of fan balancing errors and came to a conclusion.

1. The balancing problem of the auxiliary mandrel.

Since the fan impeller is generally balanced with an auxiliary mandrel, it is necessary to consider whether the mandrel meets the balancing requirements. Since the mandrel is a rotor, it has its own imbalance. During the balancing process, the mandrel has been combined with the fan to become a rotor, so even if the whole is balanced on the balancing machine, it is only a surface balance. The residual imbalance of the mandrel corresponding to the correction plane of the rotor should be less than 10% of the allowable residual imbalance of the rotor. Usually, the mass of the mandrel is about 1/10 of the mass of the impeller. The imbalance of the mandrel itself may be equal to the impeller balancing requirements, so it will directly affect the final result of the impeller balancing. In addition, the key and keyway on the mandrel are also an issue that is easily overlooked during balancing. Some users may think that the mass of half a key or a whole key is light, and the radius is so small that it can be ignored. In fact, the key also has a considerable mass, generally tens of grams to hundreds of grams. The core shaft is several times smaller than the correction radius of the fan impeller. After division, there may still be tens of grams of unbalanced mass added to the fan.

2 The parallel offset between the core shaft center and the impeller center

is based on the basic principle of balance

U = M e = ur (1)

where U is the unbalance amount; M is the mass of the part; e is the distance between the center of rotation and the center of mass; u is the unbalanced mass; r is the correction radius.

If the center of the fan support position and the center of the core shaft axis are eccentric, according to formula (1), the eccentricity will produce unbalanced mass, and the generated error will be directly added to the fan.

In order to keep the eccentricity as small as possible, the reference of the processing core shaft should be kept completely consistent during the core shaft processing process. That is, the shaft neck support position on the core shaft and the impeller contact position must have a high degree of coaxiality. In order to prevent the wear of the core shaft surface from causing new errors, the core shaft must have hardness at the position where it contacts the rotor and the shaft neck. Only in this way can the mandrel produced meet the usual precision requirements. For example: the unbalance requirement of the impeller is 40g·mm/kg=40 μm, and the eccentricity of the mandrel or the concentricity between the center position of the mandrel and the journal position is 4 μm.

For example: the eccentricity of the mandrel = 6μm

requires a balancing requirement = 20g·mm/kg; the allowable residual unbalance = 20-6 = 14 g·mm/kg; for the balance of 2 planes: the unbalance of each plane = 7 g·mm/kg.

Even the best mandrel, even if the eccentricity is only 1μm, will produce an eccentricity of 5 to 10μm after a period of use. If the required balancing accuracy is smaller than this value, the corresponding adjustment can be made according to the eccentricity and angle of the mandrel during the balancing process.

In practice, it is also necessary to consider that the contact mating surfaces of the mandrel and the fan hole have tolerances, and the maximum eccentricity is half of the maximum matching tolerance.

For example: eccentricity e = 10 μm; tolerance s = 14 μm; maximum allowable eccentricity error e + s/2 = 10 + 14/2 = 17 μm.

3 Impeller inclination

If the impeller is installed on the actual shaft end face with inclination or the core shaft itself is bent, the couple imbalance that may be generated during rotation will be added to the impeller (see Figure 1). The formula for the couple imbalance caused by the slight inclination of the impeller is

The formula for the equilibrium of the couple on both sides of the plane is: Mu = urb

Figure 1: Unbalanced force generated by the impeller during rotation

The inclination of the impeller has a great influence on the balance. Usually, narrow fans only need to be balanced on one side. If the inclination reaches a certain degree, double-side balancing is required on the fan shaft. [page]

To illustrate this situation, an example is given to illustrate the situation where a pair of unbalanced couples are added to the impeller.

Assume that the mass of the impeller is m=700 kg, D=1000 mm, r=75mm, b=300 mm, L=1200mm, C=1500 mm.

Pi (correction plane end face runout) = 0.24mm, n=1000 r/min, G=6.3

Find: The unbalanced couple generated by the inclination of the impeller?

φ≈sinφ＝Pi /D=0.24/1000

Mu ≈1/4×700kg (1/3× 3002－5002―752) ×0.24/1000≈9003kg·mm2

Mu ＝urb

U (unbalanced couple)=9003/300≈30 kg·mmThe

allowable unbalance tolerance for the extended end impeller is

G=e ω

e =6.3/ (n/10)=0.063mm

Uper=me=700×0.063=44 kg·mmAllowable static unbalance Uper3＝L/(4 C)=44×1200/(4×1500)=8.8 kg·mmAllowable

unbalanced couple Uper1＝Uper2＝Uper (3L/8 b)=44×(3×1200/8×300)=66kg·mm＞ U (unbalanced force couple) is calculated to show that the unbalanced force couple caused by the impeller tilt is within the balance tolerance range. However, if the tilt angle, operating speed and part width change, the tolerance requirements will be exceeded. Because the value of 1/3 b2-R2-r2 is small, the value of Mu is small. Therefore, the unbalanced force couple caused by the tilt of a relatively wide impeller in disc parts is smaller than that of a relatively narrow impeller. If a relatively wide disc part is designed or manufactured, it is required that the unbalanced force couple cannot be ignored, and double-sided balancing is selected. On the other hand, for narrow disc parts, double-sided balancing can also be selected, but the end face runout of the part must be small enough. Moreover, narrow parts are more difficult to balance on the balancing machine because the two planes are relatively close, the separation of the correction planes will be poor, and the measured value will also be large. For fans that have been used for a long time, the inner holes of the shaft and impeller are worn due to time, and the fit is very loose and cannot be balanced. It is recommended to re-insert the sleeve between the shaft and the impeller, and control the radial runout and end runout of the new connection before balancing. You can also use the on-site balancing method and use an on-site balancing instrument to directly balance. This is also a good way to solve all errors. 4 Conclusion It is recommended that the production of fans should not only have the concept of balance, but also understand how to balance the fans. With the increase in fan speed, fan manufacturing is no longer a problem that can be solved by rough processing. From the above example, it can be seen that if the verticality of the fan is slightly bad, it will immediately produce an unbalanced couple and cause vibration, which cannot be solved by balancing methods. In addition, fan users are reminded that in addition to correctly calculating the balance tolerance of the fan, they should also pay attention to other factors that affect the balance of the fan, such as the core shaft and keys. (end)