Bearing speed and friction coefficient

      The speed of the bearing is mainly limited by the temperature rise caused by the frictional heat inside the bearing. When the speed exceeds a certain limit, the bearing cannot continue to rotate due to burns. The limit rotation speed of the bearing refers to a limit value that does not cause frictional heating that causes burns and can continuously rotate.
       Therefore, the ultimate speed of the bearing depends on the type, size and precision of the bearing as well as the lubrication method, the quality and quantity of the lubricant, the material and type of the cage, and the load conditions. The limit speeds of various types of bearings using grease lubrication and oil lubrication (oil bath lubrication) are respectively included in the bearing size table. The values ​​indicate the standard design of the bearing under normal load conditions (C/P>=13, Fa/Fr<= 0.25 or so) The limit value of the rotation speed when rotating downward. In addition, depending on the type and grade of the lubricant, the lubricant may be superior to other properties but not suitable for high-speed rotation.
       The corrected load condition of the limit speed C/P<13 (that is, the equivalent dynamic load P exceeds 8% of the basic dynamic load C), or the axial load in the combined combined load exceeds 25% of the radial load. The following formula corrects the limit speed. Na=f1*f2*n where na: corrected limit speed, rpm f1: correction factor f2 related to load conditions: correction factor n related to combined load: limit speed under normal load conditions, rpm (refer to bearing size Table) C: Basic dynamic load rating, N{kgf} P : Equivalent dynamic load, N{kgf} Fr: Radial load, N{kgf} Fa: Axial load, N{kgf} Limit of ball bearing with seal ring Speed ​​limit with contact seal (RS type) Ball bearings are limited by the line speed of the seal contact surface, allowing the line speed to depend on the rubber material of the seal.
       Precautions for high-speed rotation When the bearing rotates at a high speed, especially when the speed is close to or exceeds the limit speed stated in the dimension table, the following should be noted:
     (1) Using precision bearings
     (2) Analyze the internal clearance of the bearing (considering the reduction of the internal clearance of the bearing due to the temperature rise)
     (3) Analyze the type of material of the cage (for high-speed rotation, it is suitable to use a copper alloy or phenolic resin cutting cage. There is also a synthetic resin molding cage suitable for high-speed rotation)
    (4) Analyze the lubrication method (using the lubrication method suitable for high-speed rotation, such as circulating lubrication, jet lubrication, oil mist lubrication and oil-air lubrication). The friction coefficient of the bearing (reference). In order to facilitate comparison with the sliding bearing, the friction torque of the rolling bearing can be used as the bearing. The inner diameter is calculated by the following formula: M=uPd/2 where M: friction torque, mN.m{kgf.mm} u: friction coefficient, bearing load, N{kgf} d: bearing nominal inner diameter, mm friction coefficient u by bearing type The bearing load, speed, and lubrication method have a great influence.

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Causes and treatment of motor bearing overheating

     Causes and treatment of motor bearing overheating:

        1. Bearing damage. Should be replaced.

        2, rolling bearing grease is too little, too much or there are impurities such as iron filings. The capacity of the bearing grease should not exceed 70% of the total volume. Those with impurities should be replaced.

        3. The shaft and bearing are too tight or too loose. When it is too tight, it should be reground. If it is too loose, it should be set to the shaft.

        4. The bearing and the end cap are too tight or too loose. The bearing housing is machined when it is too tight, and the steel sleeve is placed inside the end cover when it is too loose.

        5. The motor ends or bearing caps are poorly assembled. Install or flatten the end cap or bearing cap stop and tighten the screws.

        6. The belt is too tight or the coupling is poorly assembled. Adjust the belt tension and correct the coupling.

        7, the sliding bearing lubricant is too little, there are impurities or oil ring stuck. Refuel, replace, repair or replace the oil ring.

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How to maximize the service life of the bearing?

Theoretically, the service life of rolling bearings in centrifugal pumps is 20,000 to 80,000 hours. However, the actual life depends on many factors - premature bearing failures can lead to costly equipment downtime and sometimes even more serious consequences. Bearing specialists from NKE (Austria) offer some simple but practical tips for optimizing bearing performance.


    Satisfactory bearing life begins with the correct bearing selection. From the outset, bearing designers have extended bearing life and equipment performance by choosing the right bearings for different applications. This process takes into account many factors such as load, hardness, bearing life prediction, operating environment, and so on.


    Correct storage


    In principle, all bearings should be kept in their original packaging before installation. They need to be left in a clean, moisture-free, relatively constant temperature environment. Rolling bearings should be kept away from dust, water and corrosive chemicals. Shaking and vibration can permanently damage the mechanical properties of the bearing, so vibration must be avoided during handling and storage.


    Basically, all bearings must be stored horizontally, as some heavier bearings may be deformed by their own weight after standing for a long time.


    Pre-coated greased (or sealed) bearings require special care because the density of the grease changes after prolonged storage. Thus, when the bearing is used for the first time, there will be a certain degree of rotational noise. Therefore, the shelf life of such bearings should be controlled on a first-in, first-out basis.


    Clean


    Cleaning is very important for rolling bearings. The surface roughness of the rotating surface of the bearing ring and the rolling elements is typically 1/10 μm. Such a smooth surface is very sensitive to damage caused by contaminants. The lubricating layer between the rotating faces is usually 0.2 to 1 μm, and particulate impurities larger than the particle size of the lubricant may be excessively crushed by the rolling elements to generate partial pressure in the bearing steel, which may eventually cause permanent material fatigue. In addition to this, dust particles in the external environment can be up to 10 μm in size, which can also cause damage to the bearings. Therefore, a clean, dust-free environment is critical to the storage and installation of the bearings.

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Analysis of the Causes of Overheated Bearing Temperature in Cryogenic Low Pressure Expander

The expander used in the cryogenic device belongs to a velocity turboexpander, which transfers energy in response to changes in the velocity of natural gas. The high-pressure natural gas is close to isentropic expansion in the expander, causing a drop. At the same time, the output work, consume energy, increase the enthalpy drop, reduce the natural gas outlet temperature, and achieve the purpose of refrigeration. The expansion impeller belongs to the radial axial flow reverse radial impeller, the impeller of the coaxial brake centrifugal turbocharger is semi-open, and the expander and supercharger are single. The expander is the main equipment of the cryogenic station refrigeration system, which plays an important role in cooling and recovering energy. The design flow rates of the high and low pressure expanders are 18623/17388 Nm3/h, the rotational speed is 39000/47000r/min, the inlet temperature of the high and low pressure expanders is -69.15/21.55 °C, and the outlet temperature is -105.32/-39.55 °C. Through analysis, the main factors affecting the bearing temperature of the expander are lubricating oil, water cooler, sealing gas, accessory quality, maintenance quality, etc. The oil film thickness of the lubricating oil is the main reason for restricting the bearing temperature, and the bearing temperature is the influence. The key to long cycle and efficient operation of the expander.

    1 Problems with the expansion unit

    In May 2005, the expansion unit showed high bearing temperature and chain shutdown phenomenon for a week (data shown in Table 1), which caused the unit to be in a lower load operation state. The refrigeration negative temperature was around -50 °C, which was far from the designed cooling temperature.

    2 Expander bearing lubrication mechanism

    The lubrication and cooling of the shaft and bearing of the expander are mainly based on lubricating oil. The main function of the lubricating oil is to provide lubrication and cooling for the shaft, bearing and seal. A dynamic pressure oil film of a certain thickness is formed between the journal and the bearing to form an oil film contact, the oil film holds the shaft, the bearing and the shaft are not in direct contact, and the shaft and the bearing are in fluid friction, and the friction factor is small. At the same time, the metal chips and impurities falling off between the friction pairs are taken away by the lubricating oil, which reduces the abrasive wear, removes the heat generated by the friction, and prevents the shaft and the bearing from being too hot to burn the tile.

    3 Low-pressure expander structure and lubricant flow

    The structure of the low-pressure expander is mainly composed of an expander casing, a rotor shaft, an impeller, a bearing, a key, a back bolt, a heat insulation plate, a back pressure seal, a sealing rubber ring, a temperature probe, a vibration probe and the like. The auxiliary system includes a supercharger, a sealed gas system, a cold blowing system, an accumulator, a balancing valve, and the like.

    Lubrication of bearings and shafts of low pressure expanders

    Lubricating oil of L-TSA68 is lubricated. The viscosity index of lubricating oil is 55.08~82.28mm2/s, and the flash point (opening) index is ≥185°C. The lubricating oil process is lubrication of the oil tank-rough filter-oil pump-lubricating water cooler-double filter-line valve-pre-machine filter-shaft and bearing of the expander.

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