Balancing of Spindles
Air-guided spindles are predominantly used when maximum precision and/or frequencies of more than 100,000 rpm are required. To make this possible, the rotor must be very well balanced. AeroLas has built up its very own know-how for the balancing of rotors, consisting of hardware and software with bespoke algorithms, which are not available to this technical level anywhere else on the market. This ensures that the spindles reliably meet customer requirements for precision and running smoothness.
The rotor is balanced either directly in the finished spindle, or in a balancing stand which is stored just like the spindle itself. Imbalance is measured using either acceleration sensors or high-precision distance sensors and is individually adjusted for each spindle type. This is necessary because AeroLas manufacturers different spindle types with the shaft located on the inside or outside (Figure 1).
A self-developed module is used to calculate the imbalance from the measured acceleration or deflection of the shaft. This is very flexible to use and can be adjusted for any measurements.
Figure 1: Examples of spindles with the shaft on the inside (internal rotor, right) or outside (external rotor, top)
Balancing at rated speed
In the literature, one usually finds a distinction between static and dynamic unbalance. A static unbalance occurs when the centre of gravity is not on the rotary axis, but rather parallel to the centre of gravity axis. In this case, it is possible to balance the rotor without the need to rotate it. Typically, rotors also have a dynamic unbalance, i.e. the rotary axis does not correspond to one of the axes of inertia. In this case, it is only possible to determine the unbalance on the rotating part.
The unbalance increases quadratically with the speed. This is why the sensitivity in the measurement of the unbalance increases with the frequency. At AeroLas, the rotor is balanced at proxy with the frequency later used in the application. For this purpose, a bespoke balancing module developed in-house is used to determine the balancing matrix for the respective spindle at the nominal speed. With their help, the unbalance can then be quantitatively determined and compensated for.
The example in Figure 2 shows the result of balancing a grinding spindle operated at 3,000 rpm. The spindle is an external rotor, therefore the unbalance can only be measured indirectly with contactless distance sensors via the deflection of the shaft. By85*4
balancing, the concentricity at the front of the spindle could be reduced from 20 µm to less than 3 µm at the nominal speed. There is also a sharp increase in the imbalance as the speed increases.
Figure 2: Balancing a spindle in two planes at a speed of 3,000 rpm. The spindle is attached on one side. The imbalance is determined indirectly via the deflection of the spindle.
Balancing in 3 Planes
Usually, a rotor is only balanced in two planes. At very high speeds, however, this may not be enough to achieve the desired precision. The reason being that a bending mode of the rotor could be triggered at a high rotary speed. This increases the concentricity of the tool.
Figure 3 shows a spindle which runs at 180,000 rpm. For this spindle, the concentricity of the tool could be significantly improved by additionally balancing the shaft in a third plane (Figure 4).
The shaft is first balanced in two planes (planes 1 and 2) to largely eliminate the two rigid body movements – the translatory and tilting mode. Next, the third plane is balanced to minimise the bending of the shaft. The deflection and resulting bending of the shaft is measured directly on the tool using a capacitive distance sensor (Figure 5). The graph shows a reduction in concentricity of the tool to less than 1 µm (p-p) by balancing in the third plane.
Figure 4: Spindle shaft at 180,000 rpm, shown with 3 balancing planes. Relevant vibration modes of the shaft.
Figure 3: Spindle for operation at 180,000 rpm. Radial and axial guidance of the shaft and complete spindle.
Figure 5: Measurement of the concentricity of the tool. Concentricity error before and after balancing in the third plane.