This blog is translated from German with DeepL.
Balance, balance quality and what you should know about it
Most of us may be familiar with “unbalance” from driving a car – the wheels “egg” and as a result the steering wheel vibrates at higher driving speeds. Where does this come from and how do you eliminate the unbalance?
High-speed rotating bodies run “out of round” due to manufacturing tolerances (fabrication and assembly) and possibly inhomogeneous material, generating unwanted vibrations. The center of mass and the axis of rotation are not congruent. The task of the balancing process is to reduce the unbalance to the required level. Completely vibration-free is theoretically possible, but in most cases it is not practical and also involves corresponding costs. So what is the required dimension and how do you get rid of the unbalance?
DIN ISO 1940 defines the balance quality G, a ratio of rotor mass, residual unbalance mass and its position on the rotor, and the speed of the rotor.
The standard specifies the balance quality to be achieved for various rotors. For fans, the balance quality G6.3mm/s is defined, for gas turbines or PC drives with G2.5mm/s. Piston engines have much higher balancing grades (G40mm/s). Gas turbine and PC drive have the same balance quality? Yes. Assuming that the gas turbine and the PC drive would run at the same speed ω, the quotient (u-r)/m should be the same for gas turbine and PC drive. The mass differences are very large, the geometric dimensions and the position of the unbalance mass as well. So it may be that the turbine has some 100g of unbalance mass u, while the PC drive may have just x-tenths of a gram of unbalance mass u. And yet the two rotors have the same balance quality. In general, the heavier the rotor, the greater the permissible unbalance mass may be. Therefore, the permissible residual unbalance Uzul is related to the rotor mass m.
To detect the unbalance, there are balancing scales and balancing machines. Balancing balances pick up the rotor with vertical axis and measure the unbalance on a rotor while it is NOT rotating. Balancing machines, on the other hand, have a horizontal or vertical axis position and measure the unbalance on the rotor while it is rotating. The rotor is usually driven by cardan or flat belt drive.
It is also important to know that there is static and dynamic balancing. Static balancing should not be confused with “non-rotating”. A static unbalance occurs when the axis of rotation has a parallel offset to the center of gravity axis. The unbalance is in one plane. Only a correction mass is required for balancing.
Dynamic balancing is required when the axis of rotation and the center of gravity axis are skewed. The unbalance is on two different planes and the rotor wobbles as a result. Two correction masses are required for balancing.
Rotors made of plastic may deform elastically during operation. This changes the position of the center of gravity. Such rotors are therefore best balanced at the rated operating speed. Balancing machines for this purpose must be specially designed and have an increased drive power.
Depending on the rotor type, design and material, mass is removed for balancing by drilling or grinding (e.g. brake disks) or balancing mass is applied (e.g. clipping lead weights to car wheels).
If the rotor does not have its own shaft and the balancing process is to be as efficient as possible, some manufacturers offer so-called interchangeable balancing mandrels. They have a standardized quick-clamping interface (e.g. HSK63) to the balancing machine and, on the other hand, take the rotor and secure it on the shaft. So while one rotor is in the balancing process, another balancing mandrel can already be mounted on the next rotor. This means that the balancing machine has little downtime.
Before purchasing a balancing machine, it is important to know the necessary or suitable balancing process for your component. It must be clear whether the design of the rotating body requires static or dynamic balancing. Likewise, a reasonable balancing speed must be determined in advance if possible. In addition, the manufacturer of balancing equipment must know the geometric dimensions of the rotating body and its weight. In practice, balancing machine manufacturers offer to balance a sample body from customers to confirm the choice of machine.