What is a bearing
What is a bearing?
Bearing is an important component in contemporary mechanical equipment. Its main function is to support the mechanical rotating body, reduce the friction coefficient during its movement, and ensure its rotation accuracy.
Classification of bearings
Plain bearings are generally made of wear-resistant materials without inner and outer rings or rolling elements. It is often used at low speed, light load, lubricating oil filling and mechanical rotating parts with difficult maintenance.
The sliding contact surface of articulated bearing is spherical, which is mainly suitable for swing motion, tilt motion and rotation motion.
Rolling bearings are divided into radial bearings and thrust bearings according to the load direction or nominal contact angle they can bear. The radial contact bearing is a radial bearing with a nominal contact angle of 0, and the radial contact bearing is a radial bearing with a nominal contact angle greater than 0 to 45. The axial contact bearing is a thrust bearing with a nominal contact angle of 90, and the thrust angular contact bearing is a thrust bearing with a nominal contact angle greater than 45 but less than 90.
According to the shape of the rolling element, it can be divided into ball bearing and roller bearing. Roller bearings are divided into cylindrical roller bearings, needle roller bearings, tapered roller bearings and self-aligning roller bearings.
It can be divided into self-aligning bearing according to whether it can be self-aligning during operation — the raceway is spherical, which can adapt to the angular deviation and angular movement between the axis lines of the two raceways, and non self-aligning bearing (rigid bearing) — which can resist the angular deviation of the axis line between the raceways.
According to the number of columns of rolling element, it is divided into single row bearing, double row bearing and multi row bearing.
It can be divided into separable bearing and non separable bearing according to whether its components (ferrule) can be separated.
It can also be divided into various structural types according to its structural shape (such as whether there is a filling groove, whether there is an inner ring, an outer ring and a ferrule, the structure of the retaining edge, and even whether there is a cage).
According to its outer diameter, it is divided into micro bearing (< 26mm), small bearing (28-55mm), medium and small bearing (60-115), medium and large bearing (120-190mm), large bearing (200-430mm) and extra large bearing (> 440mm).
According to the application field, it is divided into motor bearing, rolling mill bearing, main bearing, etc.
According to the material, it is divided into ceramic bearing, plastic bearing, etc.
Deep groove ball bearing
Deep groove ball bearing is the most representative rolling bearing. Compared with other types of bearings with the same size, this kind of bearing has the advantages of small friction coefficient, high limit speed, simple structure, low manufacturing cost, high precision, no regular maintenance, large size range and many forms. It is the most widely used type of bearing. It mainly bears radial load and certain axial load. When it only bears radial load, the contact angle is zero.
After the deep groove ball bearing is installed on the shaft, the axial displacement in both directions of the shaft or housing can be limited within the axial clearance of the bearing, so it can be positioned axially in both directions. When the deep groove ball bearing has large radial clearance, it has the performance of angular contact bearing and can bear large axial load. Under the high-speed operation condition with large axial load, deep groove ball bearing has more advantages than thrust ball bearing. In addition, this kind of bearing also has a certain centering ability. When it is inclined 2 ′～ 10 ′ relative to the shell hole, it can still work normally, but it has a certain impact on the service life of the bearing.
Angular contact ball bearing
Generally, it is customarily called the six types of bearings represented by type 36 and 46 bearings, and the angular contact is generally 15 degrees, 25 degrees, 45 degrees, etc.
Self aligning ball bearing
The self-aligning ball bearing is a bearing equipped with round ball between the inner ring of two raceways and the outer ring with spherical raceway. The curvature center of the outer ring raceway surface is consistent with the bearing center, so it has the same centering function as the automatic centering ball bearing. In case of deflection of shaft and shell, it can be adjusted automatically without increasing the bearing burden. Self aligning roller bearing can bear radial load and axial load in two directions. Self aligning ball bearing has large radial load capacity and is suitable for heavy load and impact load. The inner diameter of the inner ring is a bearing with tapered hole, which can be installed directly. Or install it on the cylindrical shaft with tightening sleeve and disassembly cylinder. The cage is made of steel plate stamping cage and polyamide. The self-aligning ball bearing is suitable for bearing heavy load and impact load, precision instrument, low-noise motor, automobile, motorcycle, metallurgy, rolling mill, mining, petroleum, papermaking, cement, sugar pressing and other industries and general machinery.
Thrust ball bearing
Thrust ball bearings are divided into one-way and two-way. They can only bear axial load and can never bear any radial load. The thrust bearing is divided into two parts: tight ring and movable ring. The tightening ring is tight with the shaft sleeve, and the movable ring is supported on the bearing seat. The ring and rolling element are usually made of rolling bearing steel with high strength and good wear resistance. The surface hardness after quenching shall reach HRC60 ~ 65. The cage is mostly made of mild steel, or copper alloy cloth, bakelite or plastic.
Bidirectional thrust angular contact ball bearing
Double row tapered roller bearing
The contact angle of thrust angular contact ball bearing is generally 60 °. The commonly used thrust angular contact ball bearing is generally a two-way thrust angular contact ball bearing, which is mainly used for the spindle of precision machine tools. It is generally used together with double row cylindrical roller bearing, and can bear two-way axial load. It has the advantages of high precision, good rigidity, low temperature rise, high speed, convenient assembly and disassembly, etc.
Thrust roller bearing
It includes thrust cylindrical roller bearing, thrust tapered roller bearing, thrust needle roller bearing and thrust self-aligning roller bearing.
Needle roller bearings are equipped with thin and long rollers (the roller length is 3 ~ 10 times the diameter, and the diameter is generally no more than 5mm). Therefore, the radial structure is compact. Its inner diameter size and load capacity are the same as those of other types of bearings, and its outer diameter is the smallest. It is especially suitable for the supporting structure with limited radial installation size. Needle roller bearings are used in different occasions, The bearing without inner ring or needle roller and cage assembly can be selected. At this time, the journal surface and housing hole surface matched with the bearing are directly used as the inner and outer rolling surfaces of the bearing. In order to ensure that the load capacity and operating performance are the same as those of the bearing with ferrule, the hardness of the raceway surface of the shaft or housing hole, The machining accuracy and surface quality shall be consistent with the bearing ring. Purpose combined needle roller bearing is a bearing unit composed of centripetal needle roller bearing and thrust bearing components. It has compact structure, small volume and high rotation accuracy. It can bear a certain axial load while bearing a very high radial load. Moreover, the product has various structural forms, wide adaptability and easy installation. Combined needle roller bearings are widely used in machine tools, metallurgical machinery, textile machinery, printing machinery and other mechanical equipment, and can make the mechanical system design very compact and dexterous.
Outer spherical ball bearing
The outer diameter surface of the outer ring of the outer spherical ball bearing is spherical, which can play the role of centering.
Spherical Roller Bearings
Self aligning roller bearings have two rows of symmetrical spherical rollers, which mainly bear radial load and axial load in either direction, but can not bear pure axial load. The outer race raceway of this kind of bearing is spherical, so its centering performance is good and can compensate the coaxiality error. When the shaft is forced to bend or the installation is not concentric, the bearing can still be used normally. The centering performance varies with the bearing size series. Generally, the allowable centering angle is 1 ~ 2.5 degrees. This type of bearing has a large load capacity, In addition to the radial load, the bearing can also bear the axial load of two-way action and has good impact resistance. Generally speaking, the allowable working speed of self-aligning roller bearing is low. It is suitable for working under heavy load or vibration load.
Flange bearing outer wheel is provided with flange flange. The utility model is characterized in that the structure of the main engine can be simplified, the size of the main engine can be reduced, and the bearing can be positioned more easily.
Bearing with seat
A component of a radial bearing combined with a seat. There is a base plate for mounting screws on the support surface parallel to the axis of the bearing.
A rolling bearing composed of the above two or more bearing structural forms in a set of bearings at the same time. Such as needle roller and thrust cylindrical roller combined bearing, needle roller and thrust ball combined bearing, needle roller and angular contact ball combined bearing, etc.
Linear bearings are divided into metal linear bearings and plastic linear bearings.
Metal linear bearing is a low-cost linear motion system, which is used for infinite stroke and cylindrical shaft. Because the bearing ball is in point contact with the shaft, the service load is small. The steel ball rotates with minimal friction resistance, so as to obtain high-precision and stable motion.
Plastic linear bearing is a linear motion system with self-lubricating characteristics. Its biggest difference from metal linear bearing is that metal linear bearing is rolling friction and there is point contact between bearing and cylindrical shaft, so it is suitable for low load and high-speed motion; The plastic linear bearing is sliding friction, and there is surface contact between the bearing and the cylindrical shaft, so it is suitable for medium and low speed movement under high load.
Material of bearings
Characteristics of bearing steel:
1. Contact fatigue strength
Under the action of periodic load, the contact surface of the bearing is prone to fatigue damage, that is, cracking and peeling, which is an important damage situation of the bearing. Therefore, in order to improve the service life of bearing, bearing steel must have high contact fatigue strength.
2. Wear resistance
During the bearing task, not only rolling friction but also sliding friction will occur between the ferrule, rolling element and cage, so that the bearing parts will be worn all the time. In order to increase the wear of bearing parts, maintain the stability of bearing accuracy and extend the service life, bearing steel should have good wear resistance.
Hardness is one of the important qualities of bearing quality, which has an indirect impact on contact fatigue strength, wear resistance and elastic limit. The hardness of bearing steel in service shall reach HRC61 ~ 65, which enables the bearing to obtain high contact fatigue strength and wear resistance.
4. Rust resistance
In order to avoid corrosion and rust of bearing parts and finished products during processing, storage and application, it is required that bearing steel should have good anti rust performance.
5. Processing performance
In the process of consumption, bearing parts have to go through many cold and hot processing procedures. In order to meet the requirements of small quantity, high efficiency and high quality, bearing steel should have good processing properties. For example, cold and hot forming performance, cutting performance, hardenability, etc.
In addition to the above basic requirements, the bearing steel shall also meet the requirements of appropriate chemical composition, average external structure, less non-metallic impurities, compliance with the specification for external appearance defects, and the external decarburization layer shall not exceed the regular concentration.
Function of bearing
In terms of its function, it should be support, that is, it is literally used to support the bearing, but this is only a part of its function. The essence of support is to be able to bear radial load. It can also be understood that it is used to fix the shaft. Bearing quick, easy and excellent automatic selection is included. It is to fix the shaft so that it can only rotate and control its axial and radial movement. The motor can’t work without bearings. Because the shaft may move in any direction, and the motor requires that the shaft can only rotate. Theoretically speaking, it is impossible to realize the function of transmission. Moreover, the bearing will also affect the transmission. In order to reduce this impact, good lubrication must be achieved on the bearings of high-speed shaft. Some bearings already have lubrication, which is called pre lubricated bearings, while most bearings must have lubricating oil. During high-speed operation, friction will not only increase energy consumption, What’s more terrible is that it is easy to damage the bearing. Changing sliding friction into rolling friction is one-sided, because there is something called sliding bearing.
The lubrication purpose of rolling bearing is to reduce the internal friction and wear of bearing and prevent burning and sticking; Extend its service life; Discharge friction heat and cooling to prevent bearing overheating and lubricating oil aging; It also has the effect of preventing foreign matters from invading the interior of the bearing, or preventing rust and corrosion.
The lubrication methods of bearings are divided into grease lubrication and oil lubrication. In order to make the bearing function well, first of all, choose the lubrication method suitable for use conditions and purposes. If only lubrication is considered, the lubricity of oil lubrication is dominant. However, grease lubrication has the advantage of simplifying the structure around the bearing. Compare the advantages and disadvantages of grease lubrication and oil lubrication. Pay special attention to the amount of lubrication, whether oil lubrication or grease lubrication. Too little lubrication will not fully affect the service life of the bearing. Too much will produce large resistance and affect the speed.
The sealing of bearings can be divided into self sealing and external sealing. The so-called bearing with seal is to make the bearing itself into a device with sealing performance. Such as bearing with dust cover, sealing ring, etc. The seal occupies little space, is convenient for installation and disassembly, and the cost is relatively low. The so-called bearing external sealing performance device is to manufacture a sealing device with various properties inside the installation end cover. Bearing external seal is divided into non-contact seal and contact seal. Among them, non-contact seal is suitable for high-speed and high-temperature occasions, with different structural forms such as gap type, labyrinth type and gasket type. Contact seal is suitable for medium and low speed working conditions. The commonly used structural forms are felt seal, leather cup seal and so on.
According to the requirements of bearing working conditions and working environment for sealing degree, various sealing forms are often used comprehensively in engineering design to achieve better sealing effect. The following main factors shall be considered for the selection of bearing external seal:
- Bearing lubricant and type (grease and oil);
- The working environment of the bearing and the size of the occupied space;
- The support structure of the shaft has the advantages of allowing angular deviation;
- Circumferential speed of sealing surface;
- Operating temperature of bearing;
- Manufacturing costs.
Installation and maintenance of bearings
Key points for attention
From the perspective of use, the following points should be paid attention to to to ensure that the bearing can work reliably:
- 1. Improve lubrication quality, control oil pressure, temperature and flow, and strengthen oil filtration.
- 2. Fuel oil and lubricating oil meeting the requirements shall be used.
- 3. It is disadvantageous to control the temperature state of diesel generator set under the condition of supercooling and overheating. In cold weather, the diesel engine shall be preheated before starting, and the crankshaft shall be turned by hand to make the oil enter the friction surface.
- 4. The surface quality and geometry of bearing and journal shall be strictly guaranteed.
- 5. The bearing clearance shall be appropriate. If the generator set is too large, it will produce impact. If it is too small, it will have poor lubrication and may burn the tile.
How to ensure the reliable operation of bearings
Generally speaking, from the perspective of use, pay attention to the following points:
- 1. The bearing clearance shall be appropriate. If it is too large, there will be impact. If it is too small, the lubrication will be poor and the bearing may burn;
- 2. The surface quality and geometry of bearing and journal shall be strictly guaranteed;
- 3. Improve lubrication quality, control oil pressure, temperature and flow, and strengthen oil filtration;
- 4. Fuel oil and lubricating oil meeting the requirements shall be used.
In order to give full play to the bearing and maintain its due performance for a long time, regular maintenance (regular inspection) must be done. Through proper regular inspection, it is very important to find faults early and prevent accidents before they happen, which is very important to improve productivity and economy.
Install of bearings
Whether the bearing is installed correctly affects the accuracy, service life and performance. Therefore, the design and assembly department should fully study the installation of bearings. It is hoped that the installation will be carried out according to the operating standards. Items of operating standards are usually as follows:
- (1) Clean the bearing and its related parts
- (2) Check the size and finish machining of related parts
- (3) Installation
- (4) Inspection after bearing installation
- (5) Supply lubricant
It is hoped that the bearing package will be opened just before installation. General grease lubrication, do not clean, directly fill with grease. For lubrication with lubricating oil, it is not necessary to clean in general. However, bearings for instrument or high-speed use shall be cleaned with clean oil to remove the antirust agent coated on the bearings. The bearing with rust inhibitor removed is easy to rust, so it can not be placed regardless. Moreover, bearings sealed with grease can be used directly without cleaning.
The installation method of bearing varies according to the bearing structure, fit and conditions. Generally, the inner ring needs interference fit because it is mostly shaft rotation. Cylindrical bore bearings are usually pressed in with a press or hot installed. In the case of taper hole, it can be directly installed on the taper shaft or installed with sleeve.
When it is installed on the shell, there is usually more clearance fit, and the outer ring has interference. It is usually pressed in by a press, or there is a cold shrinkage fit method for installation after cooling. When dry ice is used as coolant and cold shrinkage is used for installation, the moisture in the air will condense on the surface of the bearing. Therefore, appropriate anti rust measures are required.
Maintenance of bearings
The bearing is disassembled for regular maintenance and replaced. After disassembly, if the bearing continues to be used or the condition of the bearing needs to be checked, the disassembly shall be carried out with the same care as during installation. Pay attention not to damage all parts of the bearing, especially the disassembly of the interference fit bearing, which is difficult to operate.
It is also very important to design and make disassembly tools according to needs. During disassembly, the disassembly method and sequence shall be studied according to the drawings, and the matching conditions of the bearing shall be investigated to ensure the safety of the disassembly operation.
For the removal of the outer ring and the outer ring with interference fit, several outer ring extrusion screws are set on the circumference of the shell in advance, and the screws are tightened equally while being removed. These screw holes are usually covered with blind plugs, tapered roller bearings and other separate bearings. Several cuts are set on the cover shoulder of the shell. Use cushion blocks, disassemble with a press, or knock gently.
The disassembly of the inner ring can be pulled out with a press. At this time, pay attention to make the inner ring bear its pulling force. In addition, most of the pulling fixtures shown are also used. No matter what kind of fixture, it must be firmly clamped on the side of the inner ring. Therefore, it is necessary to consider the size of the shaft shoulder, or study the processing of the upper groove at the shoulder in order to use the drawing fixture.
The inner ring of large bearing is disassembled by oil pressure method. Oil pressure is applied through the oil hole arranged in the bearing to make it easy to pull out. For the bearing with large width, the oil pressure method is used together with the drawing fixture for disassembly.
The inner ring of cylindrical roller bearing can be disassembled by induction heating method. It is a method of drawing after heating part in a short time to expand the inner ring. Induction heating is also used where a large number of such bearing inner rings need to be installed.
When removing the bearing for inspection, first make appearance records by photography and other methods. In addition, confirm the amount of remaining lubricant and sample the lubricant before cleaning the bearing.
- a. The cleaning of bearings is divided into rough cleaning and fine cleaning, and a metal grid can be placed at the bottom of the container.
- b. During rough cleaning, remove grease or adhesion with a brush in the oil. At this time, if the bearing is rotated in oil, pay attention to the damage to the rolling surface due to foreign matters.
- c. During fine cleaning, rotate the bearing slowly in oil and carry out it carefully.
The cleaning agent usually used is neutral water-free diesel oil or kerosene, and warm alkali liquor is sometimes used as required. No matter what kind of cleaning agent you use, always filter it and keep it clean.
After cleaning, apply antirust oil or antirust grease on the bearing immediately.
Inspection and judgment of bearings
In order to judge whether the removed bearing can be reused, its dimensional accuracy, rotation accuracy, internal clearance, mating surface, raceway surface, cage and sealing ring shall be emphatically checked. Since large bearings cannot be rotated by hand, pay attention to the appearance of rolling elements, raceway surfaces, cages, edge retaining surfaces, etc. the higher the importance of bearings, the more careful the inspection is.
Causes of heating of rolling bearing and its elimination
- Low bearing accuracy: select the bearing with specified accuracy grade.
- Bending of main shaft or non concentricity of box hole: repair the main shaft or box.
- Poor lubrication: select the specified brand of lubricating materials and clean them properly.
- Low assembly quality: improve assembly quality.
- Bearing inner shell race: replace the bearing and related wear parts.
- Too much axial force: clean and adjust the clearance of sealing ring between 0.2 ~ 0.3mm, correct the diameter of impeller balance hole and verify the static balance value.
- Bearing damage: replace the bearing.
Safekeeping of bearings
The bearings are coated with an appropriate amount of antirust oil and packaged with antirust paper. As long as the packaging is not damaged, the quality of the bearings will be guaranteed. However, for long-term storage, it is better to store it on a shelf 30cm above the ground when the humidity is lower than 65% and the temperature is about 20 ℃. In addition, the storage place should avoid direct sunlight or contact with cold walls.
Quality inspection of bearings
Chinese standard for bearings
1. National standard for vibration acceleration (commonly known as Z standard)
The standard was formulated earlier. The quality grade of the bearing is determined by measuring the vibration acceleration value when the bearing rotates. It is divided into three quality grades: Z1, Z2 and Z3, from low to high. At present, domestic bearing manufacturers are still using the vibration acceleration value to measure the quality of the bearing, which only simply reflects the fatigue life of the bearing.
2. Vibration speed standard (commonly known as V standard)
Since the original vibration acceleration standard has not been abolished, the standard appears as the standard issued by the Ministry of machinery industry. It is formulated with reference to the European standard and in combination with the actual situation and needs of our country to detect the vibration speed of the bearing to divide the quality grade of the bearing (equivalent to the national standard). It is divided into five quality levels: V, V1, V2, V3 and V4. The quality grades of various ball bearings are V, V1, V2, V3 and V4 from low to high; The quality grades of roller bearings (cylindrical and conical) are V, V1, V2 and V3 from low to high.
It reflects the quality of the bearing by detecting the vibration B velocity of the bearing in different frequency bands (low frequency, medium frequency and high frequency). It can be generally analyzed whether the bearing has geometric dimension problems (such as steel ring ellipse), quality problems of raceway / rolling element and quality problems of cage, which is significantly better than investigating the bearing quality by vibration acceleration. At present, bearings exported to Europe, China’s military and aerospace industry all carry out bearing quality inspection according to this standard. At the same time, it provides a feasible means to detect the quality of bearings imported from Europe and distinguish fake imported bearings.
There are two parallel standards in bearing quality inspection, and bearings with high quality grade of “Z standard” may not have good quality performance when tested with “V standard”, and there is no corresponding relationship between the two. This should be paid special attention in the quality inspection of bearings.
Quality discrimination of bearing
We usually identify the quality of equipment bearings from the following aspects:
1. Whether the outer packing of the bearing is clear
Double row tapered roller bearing
Generally, brands produced by regular manufacturers have their own special designers to design external packaging, and arrange factories with qualified production conditions for production. Therefore, the packaging of products should be very clear and unambiguous from lines to color blocks.
2. Whether the steel seal of the bearing is clear
Each bearing product will be printed with its brand words and label on the bearing product body. Although the font is very small, the products produced by regular manufacturers use steel printing technology, and press the font before overheating treatment. Therefore, although the font is small, it is deeply concave and very clear. In general, the font of counterfeit products is not only fuzzy, but also floating on the surface due to the rough printing technology. Some of them can even be easily erased by hand or the manual marks are serious.
3. Whether the bearing has noise
Hold the inner sleeve of the bearing with the left hand, and move the outer sleeve back and forth with the right hand to make it rotate. Listen to whether there is any noise during the operation of the bearing. Due to the backward production conditions of most counterfeit products and complete manual workshop operation, impurities such as dust and sand will inevitably be mixed in the bearing during the production process, so there will be noise or poor operation when the bearing rotates. This is the key to judge whether the products come from the brand products of regular manufacturers with strict production standards and machine operation.
4. Surface of bearing
Whether there is muddy oil on the surface requires special attention when purchasing imported bearings. As there is still a certain gap between domestic antirust technology and foreign advanced manufacturing countries, it is easy to leave thick oil traces when antirust the bearing body, which feels sticky when touching with hands, while almost no traces of antirust oil can be seen on the bearings originally imported from abroad. According to industry insiders, a particularly careful person can smell a special smell on imported bearings, which is the smell of antirust oil.
5. Whether the chamfer is uniform
The so-called bearing chamfer is the junction of horizontal and vertical surfaces. Due to the limitation of production technology, counterfeit bearing products are not handled satisfactorily in these corners, which we can easily distinguish.
6. Packing of bearings
Bearing packaging is divided into inner packaging and outer packaging
After the bearing has been manufactured and passed the inspection, it shall be cleaned and antirust treated, and then put into the inner packaging, so as to achieve the purpose of waterproof, moisture-proof, dust-proof, impact proof, maintaining the quality and accuracy of the bearing and facilitating use and sales.
The inner packaging of bearing is divided into three categories according to the rust proof period:
- ① Short anti rust period packaging: the anti rust period is 3 ~ 6 months. It is suitable for bearings that are delivered in large quantities to the same subscriber and put into use in a short time. By mutual agreement, simple packaging is adopted on the principle of convenience.
- ② General rust proof packaging: the rust proof period is one year, which is suitable for general-purpose bearings.
- ③ Long rust proof packaging: the rust proof period is two years. It is suitable for special and precision bearings.
The packaging materials in the bearing include polyethylene plastic cylinder (box), kraft paper, plain and wrinkled polyethylene composite paper, carton, polyethylene or polyethylene plastic film, nylon fastening belt or plastic woven fastening belt, waterproof high-strength plastic belt, linen bag, etc. The above materials shall be qualified in the corrosion resistance test.
How to judge whether the bearing is refurbished
Bearings are widely used and are very common parts. In order to make more money, many unscrupulous businesses charge them with old ones and renovate the recycled old bearings. The service performance is not as good as that of good bearings. Today, let’s share the differences between refurbished bearings and good bearings, and how to identify whether the bearings are old bearings or refurbished bearings when buying bearings, so that you don’t waste money.
Difference between refurbished bearing and good bearing
- 1. The dimension of the refurbished bearing changes. The outer diameter is small and the inner diameter is large, which can be measured with a ruler.
- 2. Watch the bearing surface for accuracy and smoothness. The refurbished bearing is rough.
- 3. Rotate the bearing by hand to renovate the uneven rotation of the bearing.
- 4. Watch the clearance between the cage and the rolling element, and the bearing clearance is greater than the normal clearance.
- 5. Look at the bead refurbished bearing. The size of the bead is different, and there will be wear and deformation.
- 6. Look at the slideway. There is irregular wear and deformation on the refurbished bearing slideway.
A good way to identify refurbished bearings
- 1. Bearing accuracy: the surface of the refurbished bearing is rough, the chamfer size is uneven, and some still have black skin and pitting.
- 2. Some inferior bearing materials are ordinary steel, carbon steel, Maanshan Steel and iron pipes, and most of these materials are white, and the color of special bearing steel is green. However, just looking at the color can not completely determine that hardness needs to be done. Ordinary steel has low hardness.
- 3. Some small manufacturers have simple dipping technology, uneven heating of bearings and slight deformation.
- 4. Rotate the bearing to see if the bearing rotates evenly and the sound is continuous without noise.
- 5. Looking at the cage, the regular manufacturer’s bearing cage is regular and free of burrs, and the rivet pier head of the riveted bearing cage is the same size, while the irregular manufacturer’s processing technology and equipment are simple, so the cage is rough, some riveted structure cages are unevenly divided or offset, and the rivet pier head is different in size or offset.
- 6. Packaging: the packaging of regular manufacturers is consistent, and the font on the packaging is clear. The packaging of some counterfeit bearings is inconsistent or simple and informal
- 7. For the end face font, both the steel font and the laser font of the original bearing are very clear and uniform. The laser font has a certain depth, and the counterfeit bearing font has broken marks. The laser font will disappear after being gently wiped with gauze, while the regular laser font will leave fuzzy font marks after being wiped with gauze.
- 8. According to the certificate of conformity, the bearings of regular manufacturers have certificates of conformity, and the size of the certificates is the same. Most of the certificates are oil proof paper. The font is clear. Some manufacturers have query information on the certificate, and some counterfeit certificates have fuzzy fonts. The paper is ordinary paper, and the size of the certificate is different. It is recommended to keep a formal manufacturer’s certificate for comparison and reference.
- 9. Antirust oil, imported bearings are not without antirust agent, but there is no trace. The bearing surface is clear, while some counterfeit imported bearings coated with domestic antirust agent feel sticky in hand, which will taste like antirust oil.
- 10. Bearing typing specifications, such as bearing model, trademark and year code. Some fake bearing fonts are engraved, even if some fonts are blurred by laser.
- 11. Personally.
Parameters of bearing
The number of revolutions or hours that a bearing experiences before pitting under a certain load is called bearing life.
The service life of rolling bearing is defined by the number of revolutions (or the number of working hours at a certain speed): the bearing within this service life shall have preliminary fatigue damage (peeling or defect) on any bearing ring or rolling element. However, no matter in laboratory test or in actual use, it can be clearly seen that the appearance of the same bearing under the same working conditions is very different from the actual service life. In addition, there are several different definitions of bearing “life”, one of which is the so-called “working life”, which means that the actual life of a bearing before damage is caused by wear and damage, which is usually not caused by fatigue, but by wear, corrosion, seal damage and other factors.
In order to determine the standard of bearing life, the bearing life and reliability are linked.
Due to the difference of manufacturing accuracy and material uniformity, even if the same batch of bearings with the same material and size are used under the same working conditions, their service life is also different. If the statistical life is 1 unit, the longest relative life is 4 units, the shortest is 0.1-0.2 units, and the ratio of the longest to the shortest life is 20-40 times. 90% of the bearings do not produce pitting corrosion, and the number of revolutions or hours experienced is called the rated life of the bearing.
Rated dynamic load
In order to compare the bearing capacity against pitting corrosion, it is specified that when the rated life of the bearing is one million revolutions (106), the maximum load that can be borne is the basic rated dynamic load, expressed in C.
That is, under the action of rated dynamic load C, the reliability of this bearing working for one million revolutions (106) without pitting failure is 90%. The greater C, the higher the bearing capacity.
For basic dynamic load rating:
- 1. Radial bearing refers to pure radial load;
- 2. Thrust ball bearing refers to pure axial load;
- 3. Radial thrust bearing refers to the radial component that produces pure radial displacement.
Causes of bearing noise
1. Excitation caused by the change of the number of loaded rolling elements
When a radial load is loaded on a bearing, the number of rolling elements bearing the load will change slightly during operation, resulting in the deviation of the load direction.
2. Local damage
A small number of bearing raceways and rolling elements may be damaged due to incorrect operation or installation. During operation, damaged bearing components will produce specific vibration frequencies. Vibration frequency analysis can identify damaged bearing components. This principle has been applied to condition monitoring equipment to monitor bearing damage.
3. Accuracy of relevant parts
When the inner and outer rings of the bearing are closely matched with the bearing seat or transmission shaft, the inner and outer rings of the bearing may be deformed due to improper matching with the shape of adjacent components. If deformation occurs, vibration and noise may occur during operation.
If the equipment operates in a polluted environment, impurities may enter the bearing rolling elements, resulting in vibration sound. In this case, a disturbing noise can usually be heard.
The causes of rolling bearing noise are complex. One is the wear of the mating surfaces of the inner and outer rings of the bearing. Due to this wear, the matching relationship between bearing and shell, bearing and shaft is damaged, resulting in the axis deviation from the correct position. The shaft makes abnormal noise when moving at high speed.
In addition, insufficient bearing lubrication, dry friction and bearing breakage will produce abnormal sound. Bearing wear, cage looseness and damage will also produce abnormal noise.
Different bearings have different noise levels?
- 1. The noise of ball bearings is lower than that of roller bearings, and the (friction) noise of bearings with less sliding is lower than that of bearings with more sliding; More balls, thicker outer ring and less noise;
- 2. The noise of bearings using solid cages is relatively lower than that of bearings using stamping cages;
- 3. The noise of bearings with plastic cages is lower than that of bearings with the above two cages;
- 4. The noise of bearings with high precision, especially those with higher rolling element precision, is relatively lower than that of bearings with low precision;
- 5. The noise of small bearings is relatively lower than that of large bearings.
Of course, this is true in theory. There are many factors that affect the noise in practical application.
30 reasons for bearing noise
- 1. Impurities in grease;
- 2. The cage is broken;
- 3. The bearing raceway is rusted;
- 4. Bearing noise (interference from external vibration source);
- 5. Unqualified ferrule raceway (manufacturer’s problem);
- 6. The bearing clearance is too small or too large (manufacturer’s problem);
- 7. The diameter of the seat hole is too small (resulting in excessive bearing temperature);
- 8. Eccentric sealing ring (contact with adjacent parts and friction);
- 9. There are sundries in the bearing seat hole (residual chips, dust particles, etc.);
- 10. Bearing noise (caused by roller end face or steel ball slipping);
- 11. The shaft shoulder is too large (touching the seal of the bearing and causing friction);
- 12. The clearance of labyrinth seal ring is too small (friction with shaft);
- 13. The teeth of the lock washer are bent (touching the bearing and rubbing);
- 14. The shoulder of the seat hole is too large (it distorts the seal on the bearing);
- 15. The position of the oil slinger is inappropriate (touching the flange cover and rubbing);
- 16. Impurities such as sand or carbon particles are mixed into the bearing to act as an abrasive;
- 17. Wear of steel ball and raceway (unqualified grinding or bumping of products);
- 18. The thermal elongation of the shaft is too large (the shaft bears static indeterminate axial additional load);
- 19. The bearing is flattened by the seat hole (the roundness of the seat hole is not good, or the seat hole is not straight);
- 20. Dirt such as water, acid or paint is mixed into the bearing to corrode;
- 21. The fit between the bearing and the shaft is too loose (the diameter of the shaft is too small or the tightening sleeve is not tightened);
- 22. The clearance of the bearing is too small and too tight during rotation (the tightening sleeve is too tight);
- 23. There are pressure pits on the steel ball or roller (caused by knocking the bearing with a hammer during installation);
- 24. The bearing is discolored and deformed by heating (caused by removing the bearing with a spray gun);
- 25. The shaft is too thick, which makes the actual fit too tight (resulting in excessive bearing temperature or noise);
- 26. Insufficient lubrication (oil level is too low and improper storage leads to oil or grease leakage through the seal);
- 27. The sizing block on the bottom surface of the bearing seat is uneven (resulting in seat hole deformation and even crack of the bearing seat);
- 28. The diameter of the bearing seat hole is too large and the actual fit is too loose (the bearing temperature is too high and the outer ring slips);
- 29. The bearing seat hole becomes larger (the bearing seat hole of non-ferrous metal is supported larger, or becomes larger due to thermal expansion);
- 30. The shaft bears additional load (the shaft bears axial tightness, or there are two fixed end bearings on one shaft).
Historical development of bearings
In the early form of linear motion bearing, a row of wooden rods were placed under a row of prying plates. Modern linear motion bearings use the same working principle, but sometimes use balls instead of rollers. The simplest rotating bearing is the shaft sleeve bearing, which is just a bushing clamped between the wheel and the axle. This design was then replaced by rolling bearings, that is, many cylindrical rollers were used to replace the original bushing, and each rolling element was like a separate wheel.
An early example of a ball bearing was found on an ancient Roman ship built in 40 BC found in Lake nano, Italy: a wooden ball bearing was used to support a rotating table. It is said that Leonardo da Vinci once described a ball bearing around 1500. Among various immature factors of ball bearing, a very important point is that there will be collision between balls, resulting in additional friction. But this can be prevented by putting the balls into small cages. In the 17th century, Galileo first described the ball bearing of “cage ball”. At the end of the 17th century, British C. wallow designed and manufactured ball bearings and tried them on mail cars, and British P. worth obtained the patent of ball bearings. The earliest practical rolling bearing with cage was invented by clock craftsman John Harrison in 1760 to make H3 chronometer. At the end of the 18th century, H.R. hertz of Germany published a paper on the contact stress of ball bearings. On the basis of Hertz’s achievements, R. Stribeck of Germany and a. pamgren of Sweden have carried out a large number of tests, which have contributed to the development of rolling bearing design theory and fatigue life calculation. Subsequently, N.P. Petrov of Russia applied Newton’s law of viscosity to calculate bearing friction. The first patent on the ball channel was obtained by Philip Vaughn of camassen in 1794.
In 1883, Friedrich Fisher put forward the idea of using suitable production machines to grind steel balls with the same size and accurate roundness, which laid the foundation of the bearing industry. O. Reynolds of England made a mathematical analysis of Thor’s discovery and derived Reynolds equation, which laid the foundation of hydrodynamic lubrication theory.
Bearing industry overview
According to the data of the National Bureau of statistics, in 2011, there were 1416 enterprises in China’s bearing manufacturing industry (with an annual sales revenue of more than 20 million yuan), realizing a total industrial output value of 193.211 billion yuan, a year-on-year increase of 27.59%; The sales revenue was 191.097 billion yuan, a year-on-year increase of 30.30%; The total profit was 12.523 billion yuan, an increase of 26.54% over the previous year. It is estimated that by 2015, China’s bearing output is expected to exceed 28 billion sets, and its main business income is expected to reach 210 billion yuan, becoming the world’s largest bearing production and sales base.
At present, China’s bearing industry is mainly faced with three prominent problems: low production concentration, low R & D and innovation ability and low manufacturing technology level.
First, the industrial production concentration is low. Among the world’s bearing sales of about US $30 billion, the world’s eight major multinational corporations account for 75% ~ 80%. Germany’s two major companies account for 90% of its national total, Japan’s five companies account for 90% of its national total, and the United States one company accounts for 56% of its national total. The sales of the 10 largest bearing enterprises in China, such as bearing shafts, account for only 24.7% of the whole industry, and the production concentration of the top 30 is only 37.4%.
Second, the ability of R & D and innovation is low. The basic theoretical research of the whole industry is weak, the participation in the formulation of international standards is weak, there are few original technologies and few patented products.
At present, our design and manufacturing technology is basically imitation, and our product development ability is low. Although the matching rate of domestic main engines reaches 80%, the supporting and maintenance bearings of important main engines such as high-speed railway passenger cars, medium and high-grade cars, computers, air conditioners and high-level rolling mills are basically imported.
Third, the level of manufacturing technology is low. The manufacturing process and process equipment technology of China’s bearing industry develop slowly, the NC rate of turning is low, and the automation level of grinding is low. There are only more than 200 automatic production lines in China. Advanced heat treatment processes and equipment that are crucial to bearing life and reliability, such as controlled atmosphere protection heating, double refinement, bainite quenching, etc., have low coverage, and many technical problems have not been solved. The R&D of new types of bearing steel, the improvement of steel quality, and the R & D of lubrication, cooling, cleaning and abrasive tools can not meet the requirements of improving the level and quality of bearing products. Therefore, the process capability index is low, the consistency is poor, the product processing size dispersion is large, and the internal quality of the product is unstable, which affects the accuracy, performance, service life and reliability of the bearing.
Source: China Bearing Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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