There are a variety of vibration issues caused by machine-generated vibration, such as vibration and noise pollution generated by factories, deterioration of the work environment and livability around machines, and adverse effects on vibration-sensitive equipment. In this issue, we will discussanti-vibrationan effective countermeasure against such machine vibration.
Basic Concept of Vibration Isolation
The problem caused by machine vibration is that the vibration spreads and is transmitted from the machine ⇒ the floor ⇒ the surrounding area, and can occur here and there. To prevent this from happening, don't you think the best way is to cut the vibration at the foot of the machine, which is the source of the vibration, rather than taking measures in the middle of the vibration propagation path?
Vibration isolation is a vibration reduction technique that reduces the amount of vibration transmitted to the floor by placing a soft elastic material between the machine and the floor. Figure 1 shows an image of vibration transmission reduction by vibration isolation.
Figure-1 Image of vibration propagation reduction by anti-vibration
What is a soft elastic body?
We often use such “vibration countermeasures” in our daily lives, such as placing a washing machine on a rubber mat to prevent vibration from being transmitted to the room below, or placing audio speakers on rubber blocks to improve sound quality, etc. We can imagine that “placing something soft on the floor reduces the vibration that is transmitted.
So, what kind of “soft elastic material” is actually used for machine vibration isolation?
Typical examples include anti-vibration rubber, coil springs, and air springs. The characteristics of each are summarized in Table 1. Although the materials of these springs differ, they are all elastic and have the characteristics of springs. The characteristics of a spring are the ability to expand or contract in proportion to the applied force (F=kx).
As shown in Table 1, the softer the elastic material used for vibration isolation, the more effective it is. The softness is indicated by theIndividual Vibration Frequency.
表-1 機械防振に用いる弾性体（防振材）の種類と特徴
Natural frequency for softness
In Table-1, the natural frequency expresses the softness of an elastic body. To be precise, it is the natural frequency of a spring-mass system (model like Figure-2) in which a machine (mass) is placed on top of an elastic body. What is natural frequency?
Imagine that you press down a little on the weight in Figure-2 and release your hand the next instant. Then the weight will vibrate with a certain motion, right? The frequency of the vibration at that time (how many times it reciprocates per second) is called the natural frequency,
- Low frequency = soft (vibrates slowly)
- High frequency = stiff (vibrates in fast motion)
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This natural frequency is very important and is closely related to theVibration propagation rate, which expresses the effectiveness of vibration isolation.
Figure-2 Mechanical model of a machine (mass) on an elastic body
Relationship between vibration transmission coefficient (vibration isolation effect) and natural frequency
As shown in Figure-3, the excitation force f of a machine is input to the floor as f' through an elastic body. The ratio of f to f' is the vibration transmission coefficient and represents the effect of vibration isolation.
Figure-3 What is vibration transmission coefficient?
This vibration transmission coefficient varies with frequency, and its characteristics are determined by the natural frequency of the vibration isolation system.
As an example, Figure-4 shows the theoretical curves of vibration transfer coefficient for a natural frequency of 12 Hz and 3 Hz. In the graph, the horizontal axis is the frequency and the vertical axis is the value of the vibration transmission coefficient. The range where the vibration transmission coefficient is less than 1 is the area where the effect of vibration isolation can be demonstrated.
The following can be found for the vibration transmission coefficient
- The lower the natural frequency, the greater the effect, starting in the lower frequency range.
- Conversely, there is a region near the natural frequency where it becomes easier to transmit vibration (amplification).
Relationship between vibration transmission coefficient (vibration isolation effect) and natural frequency
Summary & Key Points
The above describes the basic concept of vibration isolation and that the softer the elastic body (lower natural frequency), the higher its effectiveness.
So, is it OK to put as soft an elastic material as possible under the machine when taking anti-vibration measures?
In fact, it is not that simple. If the machine is made too soft, vibration of the machine itself may exceed the allowable displacement, resulting in operation problems, or the natural frequency of the vibration isolation system may not be designed properly, resulting in a larger vibration transmission.
To prevent this from happening, it is important to design a vibration isolation system that takes into account all of these factors: the machine's excitation force characteristics, the characteristics of the vibration being generated and to what extent it should be reduced, and the allowable displacement of the machine itself. Please leave it to Yacmo, the vibration isolation professionals. If you have a machine vibration problem, please feel free to contact us!
Please see more examples of wind sway countermeasures on the Construction Results page. Please take a look at them as well!
