There are two common types of bearings used in motors : rolling bearings and sliding bearings. Different bearing structures have different requirements during assembly, maintenance, and disassembly inspection. These are things that any motor manufacturer and repair/maintenance unit should be familiar with. Ms. Can has compiled this information and is sharing it with you.
Assembly of motors using rolling bearings
Assembly is the reverse of disassembly. Before assembly, components must be inspected. Ideally, the coaxiality and other dimensional tolerances of the frame (the stator already has windings at the assembly site), end covers, bearing sleeves, inner and outer bearing covers, and rotor should be checked to ensure they conform to the drawings before assembly. For explosion-proof motors, the explosion-proof surfaces should also be checked for unacceptable defects. Before installing bearings and inserting the rotor, the stator cavity should be thoroughly cleaned. The assembly site should be clean and dust-free. The junction box should be installed before inserting the rotor. 204-1 anti-rust oil should be applied to the threaded parts of the fastening bolts and machined surfaces. Bearings are a critical component during assembly; special attention should be paid to the assembly of rolling bearings.
(1) Before assembling the bearing, the components inside the bearing are installed first, such as the inner fan, balance ring, curved ring, bearing inner cover, etc.
(2) Bearings should be strictly inspected. If there is no inspection equipment on site, experienced workers should carefully select them (they should rotate smoothly and make a quiet and harmonious sound...).
(3) The bearing should be heated before being installed on the shaft. The heating temperature is about 100°C. It can be boiled in oil for 5 to 10 minutes, and then immediately installed on the shaft. After cooling, it can be cleaned. For small motors, it can also be installed cold without heating.
(4) Regardless of whether it is cold installation or hot installation, the bearing shall be driven into the shaft with a sleeve.
(5) The markings on the bearing outer sleeve should face outwards.
(6) When using grease lubrication, apply an appropriate amount of grease.
(7) There are various types of grease, including sodium-based, calcium-based, calcium-sodium-based, complex calcium-based, lithium-based, and molybdenum disulfide grease. Currently, lithium-based greases of grades 1 to 3 are widely used. Customers using electric motors can choose according to their own habits and experience, and do not necessarily have to completely follow the grade selected by the manufacturer. However, when replacing grease, the original grease must be thoroughly cleaned.
(8) After applying the grease, the motor should be installed immediately to prevent the bearing parts from being contaminated.
Precautions for disassembling and assembling motors using sliding bearings
(1) Precautions during disassembly, assembly and cleaning
● Open the bearing cover and flip out the bearing shell. Thoroughly clean the anti-rust grease from the shaft, bearing shell, and all components. Wipe them completely clean with a lint-free cloth soaked in gasoline. Check the bearing shell alloy surface for damage and any hard particles embedded in the bearing shell alloy.
● The clearance between the bearing and the shaft is pre-selected at the factory and does not need to be scraped or repaired if there is no damage during disassembly and inspection.
● Scratches on the upper bearing surface are usually related to misalignment of the axis. Scratches on the thrust surfaces at both ends of the bearing indicate that the magnetic center and the mechanical center are significantly misaligned, and should be adjusted during reassembly.
● During disassembly and inspection, carefully examine the marks formed by the bright spots produced by the scratches on the running surface of the bearing. Ideally, these marks should be evenly distributed along the axial direction across the entire bottom of the lower bearing with uniform width. If the bright spots formed by the scratches are uneven in width, it may be due to bearing assembly problems or shaft bending. Scratches in the same direction on the bearing may be caused by hard particles entering the running surface of the bearing, or by a rough journal surface. During disassembly and inspection, these should be repaired (removing hard particles and polishing the rough surface of the journal with an oilstone) or the bearing should be replaced.
● Inspect the bearing surface for pits caused by electro-corrosion due to shaft current. If this is found, check the bearing insulation for damage.
● Check if the floating seal inside the bearing can move up and down. If it is stuck, it is usually caused by burrs, which can be removed. Otherwise, check its size and shape again, and replace it if necessary.
(2) Precautions when replacing bearing shells
●Inspect the journal to check for any protrusions or scratches. If any are found, remove them with an oilstone.
●Inspect the junction between the oil inlet groove and the inner circumference of the new bearing shell for a smooth transition. If not, scraping is required.
●Insert the lower bearing shell, allowing the shaft to rest on it. Manually rotate it three times, then lift the shaft and flip the lower bearing shell out to inspect for the marks of friction. Under normal circumstances, the marks should be evenly distributed along the axial direction without any skewing, and the area in contact with the journal should contain approximately 75% of a bright band. If this area is not achieved, gently scrape off any protruding points, reinstall the lower bearing shell, and repeat the above process until the shape and area of the friction marks meet the requirements.
Do not damage the bearing insulation when flipping the bearing.
(3) Add lubricating oil
The need for lubricating oil replacement largely depends on the operating conditions of the motor—load, speed, running time, operating temperature, etc. After the motor is put into operation, regular inspections are conducted, and the decision to replace the lubricating oil is primarily based on the inspection results. Generally, the oil change interval for self-lubricating bearings is about 4-6 months; for compound lubrication bearings, it is about 2-3 years.
(4) Monitoring of bearing temperature
Bearings are typically equipped with platinum resistance thermometers (Pt100) to monitor bearing temperature. Sometimes, even if the bearing temperature does not exceed the limit, significant fluctuations are abnormal. If the temperature suddenly rises considerably, the machine should be stopped for inspection. The bearing temperature setpoint should be: alarm 85℃; cut-off 90℃.
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