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Another common concern is, what happens if you don't apply preload to a motor bearing? Let's clarify something before we dive in: when we talk about "preload" here, we're specifically referring to axial preload, not radial preload.
We've discussed the basic configurations of motor bearing systems in other articles. Now, let's explore which motor bearings require axial preload based on their fundamental setups.
For small horizontal motors, especially those not subject to significant external axial or radial loads, a common setup involves two deep groove ball bearings.
These two deep groove ball bearing configurations typically come in either a fixed end + floating end setup or a crossed locating arrangement.
In a fixed end + floating end configuration, applying preload causes the floating end bearing to bear a certain axial load. This load is then transmitted through the shaft system to the fixed end, making that bearing also subject to some axial load. Under this axial force, the axial clearance in both bearings is compressed to zero.
When the bearing operates with this preload, any potential collision between the rolling elements and the raceways due to residual clearance is suppressed. This directly leads to a reduction in motor bearing noise.
What happens if you don't apply preload in this setup? The scenario described above won't occur, and consequently, the motor bearing noise will be higher. However, this increased noise typically doesn't significantly impact the bearing itself. For industrial motors or applications where noise isn't a critical concern, not applying preload might not cause any harm. So, is a bit more noise always a problem for your setup? Not necessarily!
For small motors with a crossed locating structure using two deep groove ball bearings, the shaft's thermal expansion during stable operation can naturally create some axial preload on the bearings at both ends. This acts similarly to the manual preload, helping to reduce noise. However, since thermal expansion is influenced by factors like temperature and shaft dimensions, it can be difficult to control precisely. Because of this, it's often more reliable to manually apply axial preload.
In both the fixed end + floating end and crossed locating structures, applying axial preload not only reduces bearing noise but also helps prevent false brinelling.
Small motors that feature a pulley on the shaft end might experience larger radial loads. In such cases, you sometimes see a cylindrical roller bearing + deep groove ball bearing configuration.
If you apply axial preload to this type of motor bearing system, it will help reduce axial clearance and lower the noise of the deep groove ball bearing. However, for the cylindrical roller bearing, the effect is almost negligible. Why? Most cylindrical roller bearings used in motors are NU or N series, which are not designed to carry axial loads and allow for excellent axial movement internally. So, even if axial preload is applied to them, the most it will do is cause a small axial displacement, having almost no impact on the bearing's internal operation.
Medium-sized horizontal motors commonly utilize cylindrical roller bearing structures. Sometimes, spherical roller bearings are also employed.
For cylindrical roller bearings, the most prevalent setup in motors is the two cylindrical roller bearings + one deep groove ball bearing configuration. If preload is applied to this shaft system, its effect is similar to the single cylindrical roller bearing + deep groove ball bearing setup discussed earlier. However, in this bearing system, the primary load-carrying bearings are the cylindrical roller bearings. Therefore, applying preload to the deep groove ball bearing (which mainly serves a locating function) will have a less noticeable noise reduction effect. Moreover, medium-sized motors typically operate in conditions where a certain amount of normal bearing noise is acceptable. Consequently, applying axial preload in these configurations often provides minimal benefit.
If a spherical roller bearing structure is used, applying axial preload can lead to uneven loading on one row of rollers within the double-row bearing. If the preload isn't very large, it might not significantly affect bearing noise. However, if the preload is substantial, it could even cause issues like overheating due to uneven load distribution within the bearing. For these reasons, applying axial preload is generally not necessary in configurations using spherical roller bearings.
When a motor is designed to withstand a specific amount of axial force, the shaft system or its locating end will be clamped by this force. In such situations, applying an opposing axial force would weaken this effect, while applying an axial force in the same direction would enhance it.
Therefore, for motors that inherently handle axial loads, the decision to apply axial preload, along with its magnitude and direction, must be determined based on the specific operating conditions. There's no one-size-fits-all solution here.
Similarly, for vertical motors, their loading conditions are inherently equivalent to those motors subjected to axial forces. Thus, the need for and specifics of axial preload also need to be determined based on the particular operating conditions.
So, while preload can be a great tool for noise reduction and preventing issues like false brinelling, it's not a universal solution, is it? Each motor configuration demands a thoughtful approach!
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