In this blog post, we’ll examine the roles of motors and rotating shafts—key components of mechanical devices—and explore the principles behind how unbalanced mass causes vibration and damage in these devices, as well as the importance of balancing to mitigate these issues.
What is the relationship between machines and motors?
We are now in the 21st century—the era of machines and automation. It is difficult to find a place without machinery anywhere you go, and recently, machines have even come to take over tasks that humans could perform; it is hard to imagine the 21st century without them.
What do you think is the most important component of the machinery that is indispensable to modern life? I would say, without a doubt, it is the motor that drives the movement of the machinery. A motor is a component that receives electrical energy from a power source, converts it into kinetic energy, and drives the various parts of the machine. If such a motor fails and cannot operate properly, the entire machine will be unable to fulfill its intended purpose.
Why is the rotating shaft affected by mass imbalance?
In the structure of a motor, the component that transmits the kinetic energy converted from electrical energy to the mechanical device is called the output shaft (Shaft). The shaft rotates at high speeds to transfer energy to the mechanical device. One of the most common problems arising from these rotating shafts and rotating bodies is vibration and deformation caused by mass imbalance. Unbalance is a phenomenon in which centrifugal force is generated around the rotating shaft due to an uneven mass distribution in the rotating component; it is known as a primary cause of vibration in rotating machinery.
Mass unbalance arises from the uneven mass distribution of rotating elements connected to the shaft. Let’s imagine that the rotating element connected to the shaft is a disk of uniform thickness. The geometric center and the center of mass of the disk connected to the shaft may not coincide due to subtle imbalances in mass distribution, manufacturing tolerances, material inconsistencies, or wear that occurs during operation. Consequently, when the rotating component spins, centrifugal forces resulting from mass imbalance are generated; these forces act repeatedly on the shaft, bearings, and other supporting structures, causing vibration. In this case, the greater the degree of mass imbalance—that is, the farther the distance between the rotating shaft and the center of mass—the greater the force generated by the imbalance. If this force exceeds the limit that the shaft or supporting structure can withstand, it can lead to shaft deformation or component damage. Imbalance thus acts as a major cause of machinery failure and is, in fact, one of the primary causes of vibration in rotating machinery.
How can unbalanced mass be reduced?
To reduce the vibration and deformation caused by this unbalanced mass, we perform a balancing process. Balancing is a series of operations that adjusts the mass distribution of rotating components to reduce residual imbalance to an acceptable level and minimize noise and vibration, thereby ensuring that high-speed rotating machinery can operate reliably over a long period. Balancing is also necessary to improve product quality, extend the service life of bearings and consumable components, minimize power loss, and reduce operator fatigue. Mass is added or removed at various points on the rotating component, which is then rotated to measure vibration and imbalance. Subsequently, mass is added at appropriate locations or removed from the opposite side to minimize the vibration and imbalance of the rotating component that occur during actual operation. The ISO 21940 series of international standards also defines the basic balancing process as checking the mass distribution of a rotating component and, if necessary, adding or removing correction masses to ensure that residual imbalance falls within acceptable limits.
The image on the right shows the blades of a common electric fan. The sticker attached to the upper left corner of this fan is an example of a mass added through the balancing process. You will easily notice that even among products of the same model from the same manufacturer, the locations of these stickers vary.
Why is unbalanced mass the arch-enemy of mechanical devices?
As described above, unbalanced mass is a phenomenon that has a significant impact on mechanical devices, particularly those containing components that rotate around a shaft. Furthermore, unbalanced mass can occur even in rotating components machined through the most precise processes, due to various machining tolerances, material inconsistencies, assembly conditions, and wear during use. Therefore, it is practically difficult to completely eliminate imbalance in rotating components, nor is it necessary to eliminate all imbalance. However, it is clear that balancing—which minimizes unbalanced mass and keeps it within acceptable limits to enhance machine reliability and maintain stable performance for as long as possible—is an indispensable process for the sound operation of machinery.
