Bearing work in view and fault handling (2)
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When bearings operate under high-speed and high-temperature conditions, it's important to use oil lubrication. If grease is used, it can be replaced with oil for better performance. Circulating oil not only provides smooth operation but also helps dissipate heat effectively.
Viscosity plays a key role in the performance of lubricating oil. It directly affects how well the oil flows and the thickness of the oil film formed between moving parts. For bearing applications, the ideal viscosity at operating temperature usually ranges from 12 to 15 cSt.
The higher the speed, the lower the viscosity should be, while heavier loads require higher viscosity. Common types of lubricating oils include machine oil, high-speed machine oil, turbine oil, compacting oil, transformer oil, and cylinder oil.
There are several methods of oil lubrication:
a. **Oil Bath Lubrication** – This is the most common method, suitable for low and medium-speed bearings. Part of the bearing is immersed in an oil reservoir, and as the bearing rotates, it picks up oil and redistributes it before returning to the reservoir.
b. **Drip Lubrication** – This method ensures a controlled supply of oil to the bearing. The oil is usually applied at a rate of one drop every 3–8 seconds. Too much oil can cause excessive heating.
c. **Circulating Oil Lubrication** – In this system, oil is pumped to the bearing, filtered, cooled, and then reused. This method is ideal for high-speed bearings due to its cooling effect.
d. **Spray Lubrication** – Oil is mixed with air and sprayed as a fine mist into the bearing. This method not only cools the bearing but also prevents contamination. It’s commonly used for high-speed and high-temperature applications.
e. **High-Pressure Oil Injection** – A pump delivers high-pressure oil through nozzles into the bearing. This is necessary when conventional methods fail at high speeds. The nozzle is typically placed between the inner ring and the cage.
In special applications, solid lubricants are used. These can be added to grease or embedded into materials like engineering plastics or sintered metals. Solid lubricants such as molybdenum disulfide (3–5%) help reduce wear, improve heat resistance, and provide performance in extreme environments like high vacuum, corrosion, radiation, or very low temperatures.
Regular maintenance of lubricants is essential. Over time, grease may degrade due to mechanical stress, aging, or contamination. Bearings must be replenished regularly based on their size, speed, and operating conditions. When the temperature exceeds 70°C or rises by 15°C, the interval for re-lubrication should be halved.
Most double-shielded bearings come pre-filled with grease at the factory. For example, HRB bearings use standard grease that suits normal operating conditions. In many cases, the grease lasts longer than the bearing itself, so additional lubrication is unnecessary unless specified.
For oil lubrication, the replacement cycle depends on usage conditions. In clean environments with temperatures below 50°C, oil can last up to a year. However, if the temperature reaches 100°C, it should be changed every three months or less.
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