Recently, an engineer raised a question regarding the selection of bearings for micro-motors: when conducting life verification calculations for bearings, the results often exceed one million hours. This makes life verification calculations for bearing selection seem meaningless.
Moreover, when it comes to micro-motors or fractional horsepower motors, what should we pay attention to when selecting bearings?
This is not an isolated case. Last year, when I was working on a motor application technical consulting project for a home appliance group, I encountered the same issue.
The conventional selection principles for ordinary small and medium-sized motors seem somewhat flawed in these smaller motors. If one has a sufficient understanding of motor bearing application technology, these problems are fundamentally consistent.
Given the widespread confusion among engineers, let’s sort out the key points for micro-motor bearing selection (this article only introduces rolling bearings).
I. General Principles
The basic principles for micro-motor bearing selection are essentially the same as those for ordinary motors. However, because the motor is very small and the bearing load is also very small, and the working environment state differs significantly from ordinary industrial motors, it might seem different.
Overall, micro-motor bearing selection still needs to satisfy the following basic principles:
First, the bearing size must meet the basic requirements of the equipment selection.
Second, the running life of the bearing should reach the expected duration.
Third, the selection should meet the noise requirements of the equipment.
We will now discuss the differences between micro-motors and conventional industrial motors in the following aspects:
II. Bearing Dimensions Must Meet Equipment Selection Requirements
Micro-motors are typically used in household appliances, robots, and other fields, sometimes as standard three-phase motors, single-phase motors, or servo motors. Since there are specific size requirements in equipment design, the external dimensions of the bearings are often constrained.
Moreover, due to the limitations of the bearing size and the equipment size design, considerations such as the system’s rigidity are also taken into account when discussing bearing dimensions.
For example, a common issue arises when the available space is extremely limited and the bearing volume is small, seemingly bearing little load; very thin-walled bearing housings are often chosen for support. When selecting bearing support, in addition to considering strength, the flexibility of the support deformation must also be taken into account to avoid impacting usage. (In micro-motors, these issues often manifest as bearing noise.)
III. Meeting Bearing Life Expectations
In conventional industrial motors, life expectancy calculations are often based on fatigue. Therefore, life expectancy calculations are typically based on verifying the basic rated fatigue life of the bearings.
However, in the field of micro-motors, both the motor shaft and the bearing are subject to small forces; thus, it’s hard for the actual bearing life to fall short of the theoretically calculated life. This doesn’t mean that the bearing can run indefinitely.
If one is well-versed in bearing application technology, they would know that besides the basic rated fatigue life of the bearing, other factors need to be considered during bearing life verification.
Firstly, the minimum bearing load. For rolling bearings, if the minimum load required for normal operation is not met, the bearing can fail prematurely due to excessive sliding friction. Therefore, verifying the minimum bearing load is essential.
Secondly, the bearing’s lubrication life. As mentioned before, in micro-motors, bearings hardly ever reach fatigue failure, but they can easily fail due to wear and tear caused by improper lubrication. Therefore, considering the lubrication of the bearing, including the choice of lubricant and the calculation of lubrication life, is crucial.
Extended reading: It’s worth noting that sometimes lubrication failure presents itself as bearing noise. However, due to the various factors that can cause noise, it’s often challenging to correlate the noise with the relevance of lubrication in real-world conditions.
Third, the bearing retainer and seals. Small bearings sometimes employ a nylon retainer, but such retainers have a specific temperature limit. If this limit is exceeded, the retainer’s lifespan can be shortened.
For specific calculations, customers can refer to “Motor Bearing Failure Diagnosis and Analysis” or “Motor Bearing Application Technology”. Besides the retainer, when using bearings with rubber seals, the temperature range requirement for the seals is also critical and must be taken into consideration.
IV. Meeting Equipment Noise Requirements
Micro-motors are often used in domestic appliances, small fans, small electronic devices, and robotics, among other fields. These devices are mostly used in living environments where there are stringent noise requirements, hence manufacturers pay particular attention to motor bearing noise.
There are many factors affecting the noise of micro-motor bearings. Generally speaking, in micro-motors, the chance of bearing seizure or burnout is much less likely than abnormal noise. Especially during factory inspections, a majority of products are deemed non-compliant due to noise exceeding the standard.
In essence, many bearing issues are manifested in the form of noise, which can be quite challenging to diagnose due to their variety. However, there are still discernible clues. Beyond the influence of installation techniques, paying attention to the following points in design selection can significantly reduce the occurrence of improper noise:
1. The dimensions and precision of bearing-related support components, including shaft and bearing housing tolerance selection, and shape position tolerance.
2. The choice of bearing lubrication. For household appliance bearings, choose a base temperature for lubrication that matches the actual operating temperature to avoid additional bearing noise caused by lubrication viscosity.
3. The acoustic characteristics of bearing-related parts, including the rigidity and strength characteristics of the bearing housing and casing, and whether these parameters contribute to the amplification and propagation of sound.
For instance, we once encountered a motor stator designed like a bell, one end connected to the bearing housing, bearing, and shaft, and the other end open. If the casing of this type of stator is thin, it becomes a sound amplifier, leading to noise propagation.
In addition to the three design factors mentioned above, there are many other aspects to consider in order to reduce bearing noise or noise manifested by bearings, such as bearing cleanliness, machining precision, whether the minimum bearing load is met, whether the bearing installation process is appropriate, whether the bearing storage and transportation are proper, and so on.
In summary, fundamentally, the selection of micro motor bearings is no different from the selection of bearings for standard industrial motors, only the focus and emphasis differ. We cannot simply apply the principles used in the selection of bearings for standard industrial motors directly.
Due to space limitations in this article, it is not possible to cover everything in detail. Feel free to raise your concerns and we can analyze specific issues individually.