People often ask how much a motor bearing's maximum vibration should not exceed. This question may actually be asking two things. First, what is the maximum vibration under which an engine bearing can operate; and the other, after installing the engine and testing vibration, what should be the maximum vibration.
For the first case, bearing selection can be checked and calculated considering environmental vibration as an impact load. In fact, most friends ask about the latter, that is, during the operation of the engine bearing, what should be the maximum vibration value of the engine and the bearing as a whole. Let's discuss this here.
There are many corresponding international and national standards for vibration limit values of engine bearings. Since we have standards, why do many people still have problems with the upper limit? The common question encountered is: are these limits for full load or no load? Are these limit values RMS or other values?
First, let's take a look at ISO2372 as shown in the following figure:
| Vibration Severity Range Limits（Speed） | Vibration severity ranges for machines belonging to: | ||||
| From ISO 2372 | |||||
| Input/sec（PK） | MM/sec（RMS） | Class I | Class II | Class III | Class IV |
| <20HP | 20-100 hp | >100 hp | >100 CV | ||
| 0.015 | 0.28 | A | A | A | A good) |
| 0.025 | 0.45 | ||||
| 0.039 | 0.71 | B | B | B | B(Allowed) |
| 0.062 | 1.12 | ||||
| 0.099 | 1.8 | W | W | W | C(Tolerable) |
| 0.154 | 2.8 | ||||
| 0.248 | 4.5 | D | D | D | D(Not allowed) |
| 0.392 | 7.1 | ||||
| 0.617 | 11.2 | ||||
| 0.993 | 18 | ||||
| 1.54 | 28 | ||||
| 2.48 | 45 | ||||
| 3.94 | 71 | ||||
It is easy to see from the table that the units of vibration intensity, including RMS velocity and peak displacement, are listed on the left.
So the question arises: should an engineer measure an engine under full load or no load conditions? From our understanding of engine bearings, the torque load of the engine itself, whether no load or full load, does not significantly affect the bearings.
Therefore, from the bearings' point of view, whether it is unloaded or fully loaded does not affect the result. However, this does not take into account the motor stator. Under real working conditions, the state of the motor stator and load can affect the overall vibration of the motor body. Note that this is not bearing vibration, but general vibration.
Therefore, when measuring an engine, there is a difference between no load and full load. This must be taken into account in actual measurements.
According to ISO10816, equipment vibration must be tested in all possible expected operational states, with the maximum value obtained in the tests serving as a representative of the vibration intensity.
That is, the maximum value is taken among all operational states of the equipment. These states include not only no charge and full charge, but more stringent considerations may also include other intermediate states.
The reason for this provision in the standard is to consider the maximum damage that vibration can cause to equipment and define the maximum vibration intensity allowed. (If engineers understand this principle, they will know why displacement is chosen for low-speed vibrations, velocity for medium-speed vibrations, and acceleration for high-speed vibrations.)
Naturally, the above-mentioned indications refer to the severity of equipment vibrations in general. For engines, there are some more detailed standards.
For example, GB/T 5171.1-2014 stipulates the vibration limit of the rated speed of small power converter motors under no-load conditions. The standard clearly defines that the measured object is unloaded vibration. Therefore, under this standard, only no-load vibration needs to be considered.
In practical scenarios, engine factory engineers may choose to measure engine vibration in reference to corresponding engine standards. When selecting, it's not just about reading the standards tables, the explanations that accompany them can be crucial. If a certain type of engine does not have the respective standard, it must, at a minimum, meet the stipulations for general equipment in ISO 2371.
Furthermore, in addition to the regulations of the above-mentioned standards, if an engine user provides a limit for engine vibration, the engine factory must measure it according to the customer's standards.
