Small and medium-sized motors commonly employ a bearing end-cap structure. The assembly structure, installation accuracy, and lubrication system directly affect the system's reliability and operating efficiency. To compensate for the inconsistent axial expansion and contraction lengths of moving and stationary parts due to temperature changes, one end of the drive end and the other end of the non-drive end are always fixed while the other is floating. The following discussion primarily focuses on bearing selection, positioning, lubrication, and bearing structure systems, aiming to provide a complete and easy-to-maintain bearing system for motor configuration.
Regarding motor bearing selection and bearing systems
For motors with very small air gaps and high uniformity requirements, rolling bearings are preferable. Rolling bearings offer advantages such as small radial clearance, low friction loss, short axial dimension, and simple lubrication and maintenance; however, they have disadvantages including higher noise levels during operation and poorer performance under impact loads. For motors of H315 and above, ball bearings are used at the non-shaft extension ends. These bearings can withstand both radial and axial loads, keeping the rotor in the desired axial position and providing axial positioning. Spool bearings are generally used at the shaft extension ends to withstand larger radial loads. For smaller motors and all 2P small and medium-sized motors, ball bearings are used at both ends, but only one bearing can be used for axial positioning.
Rolling bearings must have protective devices for storing lubricating grease and preventing dust from entering.
Small-sized motors often use sealed bearings, which are characterized by the absence of an inner bearing cover and the use of an oil retainer ring heat-fitted onto the shaft, making assembly convenient. The bearing outer cover and end cover are integrated, which eliminates the need for an outer cover and screws for installing it, but the processing cost of the end cover is relatively high.
Sometimes, wave spring plates are added to the outside of the bearing at the transmission end of small motors to press down on the outer ring of the bearing, thereby reducing the axial working clearance of the bearing and reducing vibration and noise during motor operation. To further reduce noise, special low-noise bearings for motors can also be used.
Larger motors have oil-retaining grooves on both the inner and outer bearing covers to prevent dust from seeping in or grease from leaking out. If one end of the motor uses a cylindrical bearing assembly, both bearing covers can be tightly against the outer ring of the bearing. If both ends of the motor use ball bearings, an appropriate clearance should be left between the outer bearing cover and the outer ring of the bearing to compensate for errors in the axial dimensional chain of the motor and the thermal expansion and contraction of the shaft.
Large motors have an added bearing sleeve to the bearing system, which protects the bearings from contamination or damage when the motor is disassembled; however, such a structure is more expensive to manufacture.
With the increasing level of automation in production, motors are required to operate safely without maintenance. Bearings are the weakest link in motors. Nowadays, most small motors use enclosed rolling bearings, which do not require additional grease after lubrication. Bearings produced by reputable manufacturers can generally run continuously for more than two years. Medium-sized motors use bearing assembly structures that allow for lubrication without stopping the machine.
Normally, new grease is added to the inner bearing cover oil chamber, while old grease residue is forced into the outer bearing chamber and then collected in an easily removable container. This grease-adding device is much more effective than grease-adding from the outer bearing cover.
Regarding bearing grease
The grease used in rolling bearings consists of a lubricant and a thickener, and also contains a certain amount of peptizing agent and additives. The lubricant, primarily mineral oil such as spindle oil and cylinder oil, is the main lubricant. The thickener, in addition to its thickening function, also provides some lubrication; it is mainly composed of fatty acid salts (soap bases), such as calcium soap and sodium soap. Peptizing agents are used to improve the bonding between the soap base and the oil. Additives, such as graphite and molybdenum disulfide, can improve the properties of the grease.
(1) Dropping point. This is the temperature at which the first drop of grease begins to drip after being heated. It indicates the heat resistance of the grease. The maximum operating temperature of various greases should be 20-30°C lower than their dropping point.
(2) Penetration. The depth to which a cone-shaped weight of a certain amount of weight is dropped into the grease for 5 seconds at a certain temperature is the penetration of the grease, which indicates the viscosity of the grease. If the penetration is too small, it means that the grease is too hard and cannot completely fill the friction surface; if the penetration is too large, it means that the grease is too soft and is prone to leakage. When selecting grease, the operating conditions should be considered, such as environmental conditions, operating temperature, motor speed, etc. When the ambient humidity is high, a grease with strong water resistance should be selected. When the bearing speed is very high, a grease with a thinner consistency should be selected to avoid large frictional losses in the grease at high speeds, which would increase the bearing temperature and reduce the motor efficiency.
Common types of grease
(1) Calcium-sodium based grease. The thickener is a mixture of calcium soap and sodium soap. The dropping point is 120~130℃ and the penetration is 200~290 mm. The water resistance is weaker than that of calcium-based grease. It can be used in environments with water vapor or relatively humid conditions. The working temperature is 80~100℃. It is not suitable for very low temperatures.
(2) Composite calcium-based grease. It is composed of calcium soap, composite agent and lubricating oil. Dropping point is 180~220℃, penetration is 210~350 mm, it is moisture resistant, high temperature resistant and can be used at 150~200℃.
(3) Molybdenum disulfide composite calcium-based grease. It is made of composite calcium-based grease and molybdenum disulfide. It can withstand high temperature and moisture, and has strong pressure resistance. It is suitable for high-temperature load environments.
(4) Lithium-based grease. It uses lithium soap as a thickener, has a dropping point of 165~190℃, and a penetration of 202~380 mm. Its characteristics include cold resistance, heat resistance, water resistance, and good chemical stability. It can be used in environments with low temperatures and large temperature variations.
Bearing installation
Before installing bearings, they must be thoroughly cleaned. New bearings are coated with anti-rust oil and often contain small amounts of iron, dust, and other impurities; never rotate them by hand before cleaning to avoid scratching the rollers and raceway surfaces. Cleaning method: Immerse the bearing in No. 2 spindle oil at 90-120℃ for 5-10 minutes, gently agitate it, remove it, drain the anti-rust oil, then immerse it in gasoline several times to clean it. Remove it and dry it with compressed air. Improper cleaning is one of the main causes of bearing overheating and noise.
There are two methods for mounting bearings onto shafts: cold pressing and heat fitting. Cold pressing is only used for small-power motors; heat fitting is generally preferred. The heating chamber is filled with machine oil or transformer oil, and the oil temperature is maintained at 90-100℃. The cleaned bearing is placed in the oil and heated for about 20 minutes. After removal, it is heat-fitted onto the shaft journal. The heating temperature should not be too high, and the heating time should not be too long, to avoid tempering the bearing, which reduces its hardness and affects its service life.
When heat-fitting bearings, first install the bearing inner cover and clean the journal and shoulder areas, ensuring there are no burrs, paint residue, or dust. The heated bearing must be pushed firmly against the shoulder. If it doesn't fit properly, use a sleeve to hold the bearing inner ring in place and gently tap it into position with a hammer; directly striking the bearing is strictly prohibited. Heat fitting can be performed before or after the rotor is fitted into the stator.
After the bearing is installed, fill the gap between the inner and outer rings with grease, but only fill 1/3 to 2/3 of the gap inside the bearing cap. Do not overfill. Too much grease or unclean grease often causes the bearing to overheat and leak oil.
When removing the bearing, use a bearing puller to pull it off smoothly. The shoulder of the tool's clamp must press against the inner ring. Adjust the pull rod nut to apply even force around the bearing, and then turn the pull screw to pull it out.
