Technical Paper

Vibration reveals "signs of friction"
– Tribology as an entry point to equipment maintenance –
Part 1: The relationship between tribology and sound/vibration

Tribology is the science of friction, wear, and lubrication. Friction inevitably occurs when machine parts move. And this friction often causes vibration. Here, we will explain the reasons and actual phenomena in an easy-to-understand way.

Vibrations caused by changes in friction

　Friction is taught as a "force that stops movement" (=resistance force), but in reality, friction is not always constant; its strength changes depending on speed, the force applied, the surface condition, and whether or not lubrication is present. This "change in friction" triggers vibrations and sounds when combined with the inherent "susceptibility to vibration (natural vibration)" of an object or part.
　The most typical example is stick-slip (Figure 1). This occurs when the friction when stationary (static friction) is high and the friction after starting to move (kinetic friction) is low. When a force is applied little by little, it suddenly starts moving at a certain moment and then becomes easy to stop again, so the cycle of stopping → sliding → stopping repeats, resulting in a jerky vibration at a low frequency.

Next, vibrations can increase even if the vehicle continues to slide without stopping. When the strength of friction changes with speed, and it appears that "resistance decreases as speed increases" within a certain range, friction can actually amplify vibrations rather than weaken them. This is called friction-induced vibration. Furthermore, high-pitched noises like the "squeak" of brakes are often caused by friction linking vibration modes of parts together, making them prone to sudden vibrations at specific frequencies. In this case, it is not necessarily accompanied by repeated stopping and sliding. Friction noise is a state where these high-frequency vibrations are transmitted through the air and heard as sound, and the cause cannot be determined by a single factor.
　Also, if small vibrations continue for a long time, the contact surface can rub against itself minutely, roughening the surface, accumulating wear particles, and making the friction even more unstable. This is called fretting, and it is a problem around bolted parts and bearings. Countermeasures include using lubricants to reduce the difference in friction, coating the surface to stabilize sliding, changing the shape and rigidity of parts to make them less prone to resonance, and absorbing vibrations with rubber or vibration-damping materials.

Vibration caused by changes in shape due to wear

　When parts wear down and their shape and surface change due to abrasion, it can cause machines to rattle or make noise. New parts have uniform shapes and make stable contact, but with continued use, scratches appear on the surface, corners become rounded, and slight steps develop. Then, each time the parts rotate or slide, small impacts are repeatedly made as they overcome these steps, becoming the "seeds" of vibration. Furthermore, as the "gap" widens due to wear, parts are more likely to collide and then separate, often resulting in vibrations accompanied by collisions (rattling vibrations). This concept can also explain why bicycle brakes and chains rattle and make more noise as they get old.
　In bearings that support rotating shafts, if small indentations or peeling occur on the rolling surface, each time the balls or rollers pass over them, a "clunking" impact occurs, which is perceived as a high-pitched sound by the ear and appears as a high-frequency vibration when measured. Even with gears, wear on the tooth surfaces can cause the meshing to gradually shift, disrupting the transmission of force and potentially increasing vibration at certain rotational speeds. Furthermore, the accumulation of wear particles (worn-down particles) worsens lubrication, leading to unstable friction and a vicious cycle of increased vibration.

Vibrations caused by lubrication

　Vibration caused by lubrication is a phenomenon in which vibration occurs even in the presence of oil or grease, or where vibration increases or decreases due to changes in the lubrication state. The role of lubrication is to reduce friction and wear by preventing metals from rubbing directly against each other and allowing them to slide on a thin film of oil. However, the oil film is not always maintained at the same thickness; as shown in Figure 2, its thickness changes depending on the rotational speed (relative speed), load, temperature, oil viscosity, and supply amount. If this change is large, the contact fluctuates between a state where the parts are "floating on the oil film" and a state where they are "slightly touching," causing the frictional force to fluctuate and resulting in vibration.

For example, if the rotation is slow, the oil is not drawn in easily, resulting in a thin oil film, which makes it easier for the unevenness of the metal surface to come into contact with the oil. Conversely, if the rotation is fast, the oil film thickens and becomes smoother, but under certain conditions, the oil may be stirred up, increasing resistance and potentially making the rotation unstable. Unstable oil film formation can also lead to vibrations. If the oil temperature rises and the viscosity decreases, the oil film thins, which can cause a sudden increase in vibrations and noises.
　Furthermore, while lubricating oil acts like a "cushion," if it is too soft, the parts may become more prone to wobbling. For example, in sliding (journal) bearings, the oil film acts like a spring or damper, supporting the rotating body. If the oil film is not stiff enough, vibrations such as oil whirl, where the center of rotation wobbles, may occur. Conversely, if the oil is too stiff or there is too much of it, the stirring resistance can generate heat, leading to a decrease in viscosity and foaming, which can worsen vibrations.

The effects of vibration on wear and lubrication

　Vibration significantly alters friction, wear, and lubrication. First, regarding friction, even slight vibrations in parts disrupt contact, causing repeated sticking and sliding, leading to fluctuations in frictional force, abnormal noise, and even greater vibration. Changes in contact due to vibration can also cause localized heat generation and increased friction. Next, regarding wear, prolonged slight reciprocating slippage can easily lead to fretting wear, where the surface is worn down and produces powder. This powder acts like an abrasive, accelerating wear in a vicious cycle. In terms of lubrication, vibrations cause fluctuations in the oil film thickness, leading to breakage and increased metal-to-metal contact. Mixing air with the oil or uneven distribution of oil also destabilizes lubrication, increasing friction and wear. For example, minute vibrations in bolted connections can cause fretting damage and changes in interface conditions, increasing the risk of loosening and performance degradation. In bearings, the oil film cannot be maintained, leading to scratches and pitting, creating a chain reaction of increased vibration. When vibrations occur, observe changes in sound, temperature rise, and oil contamination to identify the cause. Vibration can push oil out or create bubbles within the oil, so the oil film can break down even if the amount of oil supplied is sufficient. The solution is to reduce the source of vibration by centering and balancing the parts, and to choose oil with the appropriate viscosity. Taking action early will prevent malfunctions.

summary

　Tribology holds the key to understanding the causes of sound and vibration. It may sound a little complicated, but in short, it's about friction, wear, and lubrication. Vibration is not a "name of a malfunction," but rather the result of a change in contact conditions occurring somewhere in the machine (Figure 3). Therefore, knowledge of tribology is essential for investigating the causes of noise and vibration and for taking countermeasures.