Analyze the cause of defects in bearing surface grinding

During the grinding process, the working surface of the bearing is ground by a grinding wheel that rotates at a high speed. Therefore, if the equipment is not operated and adjusted according to the operation instructions during grinding, various defects will occur on the working surface of the bearing, thus affecting The overall quality of the bearing. When the bearing is in precision grinding, the roughening requirements are very high, and the grinding marks appearing on the working surface can be observed by the naked eye.

    Performance showing cross spiral traces

    The reason for this kind of trace is mainly due to the poor straightness of the busbar of the grinding wheel and the unevenness. When grinding, the grinding wheel and the workpiece are only partially in contact. When the workpiece or the grinding wheel reciprocates several times, the performance of the workpiece will reappear. The spiral is visible to the naked eye. The pitch of these spirals is related to the workpiece table speed and the workpiece rotation speed, and is also related to the grinding wheel axis and the table rail.

    (1) The main reason for the formation of spirals

    1. The grinding wheel is poorly trimmed, the corners are not chamfered, and the coolant is not used for trimming;

    2. The guide rail guides too much lubricating oil, causing the workbench to float;

    3. The machine tool accuracy is not good;

    4. Grinding pressure is too large.

    (2) The cause of the formation of the spiral

    1. The V-shaped guide rail is not rigid, and the grinding wheel is offset when grinding, but the edge of the grinding wheel is in contact with the working surface;

    2. When the quarrel is repaired, the commutation speed of the worktable is unstable, and the precision is not high, so that the edge of the grinding wheel is slightly trimmed;

    3. The workpiece itself is poorly rigid;

    4. There are crushed and spalled sand on the grinding wheel and iron scraps from the workpiece grinding on the surface of the grinding wheel. For this purpose, the finished grinding wheel should be rinsed or brushed with cooling water;

    5. The grinding wheel is not well trimmed, and there are partial bulges.

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Analysis: Influencing factors of ball mill steel ball bearings

   A large number of tests at home and abroad have shown that the processing quality of cages, ferrules and steel balls has different degrees of influence on bearing vibration. The processing quality of steel balls has the most obvious influence on bearing vibration, followed by the processing quality of ferrules, the most important influence. The factors are the roundness, waviness, surface roughness, surface flaws of the steel ball and the ferrule.

    The most prominent problems of China's steel ball products are large dispersion of vibration values ​​and serious surface defects (single points, group points, pits, etc.), although the surface roughness, size, shape, and error are not lower than the level of the circle, but the set The rear bearing has high vibration value and even produces abnormal sound. The main problem is that the waviness is not controlled (no standard, no suitable test and analysis instrument), and the machine tool has poor vibration resistance. There are problems with the grinding wheel, grinding disc, coolant and process parameters. On the other hand, it is necessary to improve the management level and avoid any occasional quality problems such as bruises, scratches and burns.

    For the ferrule, the most serious vibration of the bearing is the channel waviness and surface roughness. For example, when the circularity of the inner and outer channels of small and medium-sized deep groove ball bearings is greater than 2μm, the bearing vibration will be significantly affected. When the internal and external channel waviness is greater than 0.7μm, the bearing vibration value increases with the increase of the waviness, and the channel is seriously injured. The vibration can be increased by more than 4dB, and even abnormal sounds can occur.

    Whether it is a steel ball or a ferrule, the waviness is generated by grinding. Although ultra-finishing can improve the waviness and reduce the roughness, the most fundamental measure is to reduce the waviness during the grinding process and avoid randomness. There are two main measures for bump injury:

    First, reduce the vibration of the rolling surface grinding superfine, obtain good surface processing shape accuracy and surface texture quality to reduce vibration, the grinding super machine tool must have good vibration resistance, and the important structural parts such as the bed have vibration absorption, super The oil stone oscillating system of the precision machine tool has good anti-vibration performance; the grinding speed is improved. The foreign grinding 6202 outer raceway generally adopts 60,000 electric spindles, and the grinding speed is more than 60m/s. The domestic is generally much lower, mainly by the spindle and Main bearing performance limitations. In high-speed grinding, the grinding force is small, the grinding deterioration layer is thin, it is not easy to burn, and the machining accuracy and efficiency can be improved, which has great influence on low-noise ball bearings; the dynamic and static stiffness of the main shaft and its speed characteristics are low-noise ball bearings. Grinding vibration has a great influence. The higher the stiffness, the less sensitive the grinding speed is to the change of grinding force. The smaller the vibration of the grinding system is. The rigidity of the spindle bearing is improved. The random dynamic balance technology is used to improve the vibration resistance of the grinding spindle. Sex. The vibration speed of foreign grinding heads (such as Gamfior) is about one-tenth of that of domestic general spindles; it is important to improve the cutting performance and dressing quality of grinding wheel. The main problem of the current grinding wheel in China is that the uniformity of the structure is poor, which seriously affects the super-processing quality of the low-noise ball bearing; sufficient cooling to improve the filtration accuracy; improve the feed resolution of the precision feeding system, reduce the feed inertia; reasonable grinding and super-processing Process parameters and processing flow are factors that cannot be ignored. The grinding allowance is small, the shape tolerance is strict, the outer diameter of small and medium-sized ball bearings should not be super-finished, and the coarse grinding should not be separated to ensure good surface quality.

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Common bearing wear failures and troubleshooting methods

        In general, if the bearing is used correctly, it can be used until the fatigue life is reached. However, there will be accidents that are prematurely damaged and cannot be used. This early damage, as opposed to fatigue life, is a quality use limit called a fault or accident. Many of them are caused by inadvertent installation, use, and lubrication, and foreign objects intruding from the outside are insufficiently studied for the thermal effects of the shaft and the outer casing.
Regarding the damage state of the bearing, such as the ferrule of the roller bearing and the jam of the rib, it may be considered as a cause of insufficient lubricant, unsuitability, defects in the oil supply structure, intrusion of foreign matter, bearing installation error, shaft deflection If the song is too big, there will be coincidences of these reasons.

Therefore, it is difficult to know the true cause of the damage by only investigating the bearing damage. However, if you know the mechanical use of the bearing, the conditions of use, the structure around the bearing, and the situation before and after the accident, combined with the damage state of the bearing and several reasons, it can prevent similar accidents from happening again.

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27 reasons why the bearing is ringing

    After a long-term summary by the technicians, there are 27 reasons for the bearing to sound. There may be many that we have not encountered before, and will be slowly replenished later.
    1. The bearing is mixed with impurities such as sand or carbon particles to act as an abrasive.
    2. The bearing is mixed with moisture, acid or paint and other dirt, which acts as a corrosive effect;
    3. The clearance of the bearing is too small (the selection is not appropriate);
    4. The lubricating oil or grease type is incorrectly selected;
    5. Insufficient lubrication (oil level is too low, oil or grease leaks through the seal);
    6. The bearing is flattened by the seat hole (the roundness of the seat hole is not good, or the seat hole is not twisted straight);
    7. The horn of the bottom of the bearing housing is not flat (resulting in deformation of the seat hole or even cracks in the bearing housing);
    8. There are debris in the bearing housing hole (residues, dust particles, etc.);
    9. The seal is eccentric (wearing adjacent parts and rubbing);
    10. The bearing is subjected to additional loads (the bearing is axially tightened, or there are two fixed end bearings on one shaft);
    11. The thermal elongation of the shaft is too large (the bearing is subjected to static and indeterminate axial additional load);
    12. The clearance of the bearing is too small, and the rotation is too tight (the tightening sleeve is too tight);
    13. The bearing is noisy (caused by the end face of the roller or the slip of the steel ball);
    14. The bearing and the shaft are too loose (the diameter of the shaft is too small or the adapter sleeve is not tightened);
    15. The shoulder is too large (touching the seal of the bearing and rubbing);
    16. The shoulder of the seat hole is too large (the seal of the bearing is twisted);
    17. The gap of the labyrinth seal is too small (friction with the shaft);
    18. The teeth of the lock washer are bent (wearing the bearing and rubbing);
    19. The position of the oil ring is not suitable (the flange cover is encountered and friction occurs);
    20. There is a pressure pit on the steel ball or roller (caused by hammering the bearing during installation);
    21. The bearing is noisy (with external source interference);
    22. The bearing is discolored and deformed by heat (caused by the use of a spray gun to disassemble the bearing);
    23. The shaft is too thick to make the actual fit too tight (causing the bearing temperature to be too high or noise);
    24. The bearing is noisy (the bearing has fretting abrasion).
    25. The bearing housing hole diameter is too large, the actual fit is too loose (the bearing temperature is too high - the outer ring slips);
    26. The bearing housing hole becomes larger (the bearing hole of the non-ferrous metal is enlarged, or becomes larger due to thermal expansion);
    27. The diameter of the seat hole is too small (causing the bearing temperature to be too high).

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Bearing use and cooperation

Core Tip: Use of Bearings Precautions for Use Rolling bearings make precision parts, so they need to be cautious when using them. They use high-performance bearings, if not used properly.
Bearing use
Precautions for use
Rolling bearings make precision parts, so they need to be cautious when using them. They use high-performance bearings. If they are used improperly, they can't achieve the expected performance. Therefore, the following items should be used when using bearings:
▲ Keep the bearings and their surroundings clean. Even if the smiling dust that is invisible to the naked eye enters the bearing, it will increase the wear, vibration and noise of the bearing.
▲, use the installation carefully, do not allow strong stamping, do not allow the hammer to directly hit the bearing, not allowed to pass the pressure through the rolling elements.
▲ Use appropriate and accurate installation tools, use special tools as much as possible, and try to avoid using cloth and short fibers.
▲, to prevent the corrosion of the bearing, directly take the bearing by hand, to wash the sweat on the hand, and apply high-quality mineral oil before operation, especially in the rainy season and summer to pay attention to rust.
Bearing cooperation
▲, the choice of cooperation
The inner and outer diameters of the rolling bearing are manufactured to standard tolerances. The tightness of the bearing inner ring to the shaft and the outer ring to the seat hole can only be achieved by controlling the tolerance of the journal and the tolerance of the seat hole. The inner ring of the bearing and the shaft are matched by a base hole, and the outer ring of the bearing and the seat hole are made by a shaft.
The correct choice of fit, you must know the actual load conditions, operating temperature and other requirements of the bearing, but it is actually very difficult. Therefore, most of the cases are based on the use of lint selection.
▲, load size
The amount of over-win between the ferrule and the shaft or casing depends on the size of the load. The heavier load uses a larger over-win, and the lighter load uses a smaller over-win.

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Technical requirements for sliding bearing scraping

Core Tips: 1. Basic Requirements It is necessary to make the journal and the plain bearing evenly and finely contact, and also have a certain matching clearance. 2. Contact angle It is the central angle of the contact surface between the journal and the sliding bearing.
1. Basic requirements
It is necessary to make the journal and the sliding bearing evenly and finely contact, and also have a certain matching clearance.
2, contact angle
It is the central angle of the contact surface between the journal and the sliding bearing.
The contact angle should not be too large or too small. If the contact angle is too small, the pressure of the sliding bearing will increase. In severe cases, the sliding bearing will have a large deformation, accelerate the wear and shorten the service life; if the contact angle is too large, the formation of the oil film will be affected, and good liquid lubrication will not be obtained.
Experimental studies have shown that the limit of the contact angle of the plain bearing is 120°. When the sliding bearing wears to this contact angle, liquid lubrication is destroyed. Therefore, under the premise of not affecting the pressure condition of the sliding bearing, the smaller the contact angle, the better.
From the theoretical analysis of the frictional force distance, when the contact angle is 60°, the friction torque is the smallest. Therefore, it is recommended that for a sliding bearing with a rotational speed higher than 500r/min, the contact angle is 60°, and the sliding bearing with a rotational speed lower than 500r/min is used. The contact angle can be 90° or 60°.
3, the point of contact
The actual contact between the journal and the plain bearing surface can be expressed in terms of the actual number of contact points per unit area. The more the contact points, the finer and more uniform, the better the scratch-off of the sliding bearing, and the worse, the sliding bearing is not good.
Generally speaking, the more fine the contact points, the more difficult it is to scrape. The contact points should be determined in the production according to the performance and working conditions of the sliding bearings. The information listed in the table below is for reference:
Sliding bearing speed
(r/min) contact point
(number of contacts per 25 x 25 mm area)
100 or less 3~5
100~500 10~15
500~1000 15~20
1000~2000 20~25
2000 or more 25 or more
For Class I and Class II precision machines, the above table data can be used. Class III precision machines can be halved according to the above table data.

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Factors affecting bearing life and control

Core Tips: 1. Material factors affecting bearing life The early failure modes of rolling bearings mainly include cracking, plastic deformation, wear, corrosion and fatigue. Under normal conditions, the main contact is
1. Material factors affecting bearing life

The early failure modes of rolling bearings mainly include cracking, plastic deformation, wear, corrosion and fatigue. Under normal conditions, they are mainly contact fatigue. In addition to the service conditions, the failure of bearing parts is mainly limited by the hardness, strength, toughness, wear resistance, corrosion resistance and internal stress state of the steel. The main intrinsic factors that affect these performance and status are as follows.

1.1, martensite in hardened steel

When the original structure of high carbon chromium steel is granular pearlite, the carbon content of quenching martensite under quenching and low temperature tempering obviously affects the mechanical properties of steel. The strength and toughness are about 0.5%, the contact fatigue life is about 0.55%, and the crush resistance is about 0.42%. When the carbon content of the quenched martensite of GCr15 steel is 0.5% to 0.56%, the anti-failure ability is the strongest. Comprehensive mechanical properties.

It should be noted that the martensite obtained in this case is cryptocrystalline martensite, and the measured carbon content is the average carbon content. In fact, the carbon content in the martensite is not uniform in the micro-region, and the carbon concentration around the carbide is higher than that away from the original ferrite portion of the carbide, so that the temperature at which they begin to undergo martensite transformation is different. Thereby, the growth of the martensite grains and the display of the microscopic morphology are suppressed to become cryptocrystalline martensite. It can avoid the microcracks that are prone to occur in the quenching of high carbon steel, and its substructure is dislocation-type lath martensite with high strength and toughness. Therefore, only when the medium carbon cryptocrystalline martensite is obtained when the high carbon steel is quenched, the bearing parts can obtain the substrate with the best failure resistance.

1.2. Retained austenite in hardened steel

High carbon chromium steel may contain 8% to 20% Ar (residual austenite) after normal quenching. The Ar in the bearing parts has its advantages and disadvantages. In order to eliminate the disadvantages, the Ar content should be appropriate. Since the amount of Ar is mainly related to the austenitizing condition of quenching heating, how much it affects the carbon content of quenched martensite and the amount of undissolved carbide, it is difficult to correctly reflect the influence of Ar amount on mechanical properties. For this reason, the austenitic conditions were fixed and the austenite thermal stabilization treatment process was used to obtain different amounts of Ar. The influence of Ar content on the hardness and contact fatigue life of GCr15 steel after quenching and low temperature tempering was studied. With the increase of austenite content, the hardness and contact fatigue life increase, and then decrease with the peak value, but the peak Ar content is different, the hardness peak appears at about 17% Ar, and the contact fatigue life The peak appears around 9%. When the test load is reduced, the influence of the increase in the amount of Ar on the contact fatigue life is reduced. This is because when the amount of Ar is small, the effect on the strength reduction is small, and the effect of toughening is more obvious. The reason is that when the load is small, a small amount of deformation of Ar occurs, which reduces the stress peak and strengthens the deformed Ar processing strengthening and the stress strain induced martensite transformation. However, if the load is large, the large plastic deformation of Ar and the base will locally generate stress concentration and rupture, thereby reducing the life. It should be noted that the beneficial effect of Ar must be under the stable state of Ar. If it is spontaneously transformed into martensite, the toughness of the steel will be drastically reduced and embrittled.

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