Tightness test is to assess the assembly bearing load
The purpose of the matching between the bearing and the shaft or bearing seat is to firmly fix the inner and outer rings on the shaft or bearing seat so that they will not slide harmful to each other.
If the mating surface slides, it will produce abnormal heating and wear, as well as early damage and vibration caused by powder entering the bearing due to wear, so that the bearing can not give full play to its function. In addition, the fit of the bearing can affect the radial clearance of the bearing. The radial clearance is not only related to the operation accuracy of the bearing, but also affects its service life.
Rolling bearing is a standard component, so when matching with relevant parts, its inner hole and outer diameter are reference hole and reference shaft respectively, which need not be marked in the matching. The main problem in determining the fit is the load state borne by the inner and outer rings of the bearing.
Generally speaking, tight fitting should be selected under the conditions of large size, large load, large vibration, high speed or high working temperature, while loose fitting should be adopted for frequent disassembly or swimming ferrule. Seals are extremely important for bearings. If there is a problem in the sealing part, it will have a great impact on the bearing. Seals with irregular shape, lack of meat, micro cracks and other defects are not allowed.
The bearing tightness test is to assess the assembly and working reliability of the sealed parts of the oil cylinder and its connections after assembly. Such as the sealing and connection of the cylinder port of the oil cylinder, the dynamic and static seals and sealing conditions on the oil cylinder piston (between the rod cavity and the non rod cavity), etc.
The pressure of bearing tightness test is very large, especially for the pressure test without rod cavity. If there is no tooling to bear, the pressure test by rod cavity and the great pressure acting on the piston rod through the piston can not be borne only by the connection of oil cylinder port. Influence of bearing load on accuracy
Therefore, this kind of pressure test is usually carried out after carefully assembling the oil cylinder before entering the commissioning stage of the final assembly machine.Influence of bearing load on accuracy
A. Improper assembly method
If the interference between bearing and shaft diameter or bearing seat hole is small, press in method is often used for assembly. The simple method is to use a copper rod and a hand hammer to knock the bearing race with interference fit symmetrically in a certain order to make the bearing press in smoothly. In addition, soft metal casing can also be driven by hand hammer or press. Influence of bearing load on accuracy
If the operation is improper, the seat ring will be deformed and cracked, or the hammer will hit the seat ring with non interference fit, which will cause indentation on the raceway and rolling element or indirect damage to the bearing.Influence of bearing load on accuracy
B. Improper temperature control during assembly
During assembly, if the interference between the bearing and the shaft diameter is large, the hot assembly method is generally adopted. Put the bearing into the oil barrel containing engine oil, and heat the outside of the oil barrel with hot water or flame. Influence of bearing load on accuracy
The process requires that the heated oil temperature be controlled at 80 ℃ ~ 90 ℃, generally not more than 100 ℃, and mostly not more than 120 ℃. After the bearing is heated, it is quickly taken out and sleeved on the journal. If the heating temperature is too high due to improper temperature control, the bearing will be tempered and the hardness will be reduced, and the bearing is easy to wear, peel off and even crack during operation.Influence of bearing load on accuracy
C. Improper adjustment of assembly time gap
The clearance of rolling bearing is divided into radial clearance and axial clearance. Its function is to ensure the normal operation and lubrication of rolling element and compensate thermal elongation.Influence of bearing load on accuracy
For bearings with adjustable clearance, because there is a positive proportional relationship between axial clearance and radial clearance, the required radial clearance can be obtained as long as the axial clearance is adjusted.Influence of bearing load on accuracy
However, they are generally used in pairs (i.e. installed at both ends or one end of the shaft). Therefore, it is only necessary to adjust the axial clearance of one bearing. The axial clearance is generally adjusted with gaskets, and some can also be adjusted with screws or thrust rings.Influence of bearing load on accuracy
Aalloy corrosion bearing load
Let’s briefly introduce the rolling sound of the bearing first. The size and sound quality of the rolling sound should be checked by the sound detector. The problem will be accurately judged with advanced tools. Even if the bearing has slight peeling and other damage, it will emit abnormal sound and irregular sound, which can be distinguished by the sound detector.
Then check the vibration of the bearing to see if the damage of the bearing is very sensitive. Whether peeling, indentation, corrosion, crack, wear, etc. will be reflected in the bearing vibration measurement. Therefore, the vibration can be measured by using a special bearing vibration measuring instrument (frequency analyzer, etc.), and the specific situation of abnormality can not be inferred by frequency division.
However, the values measured by each machine are different due to the service conditions of the bearing or the installation position of the sensor, so it is necessary to analyze and compare the measured values of each machine in advance to determine the judgment standard.
Mechanical damage
In severe cases, metal peeling and large-area random scratches occur on the contact surface; In general, bearing mechanical damage refers to different levels of groove marks on the alloy surface of bearing bush. Contact surface damage and ablation exist at the same time. The main reason for bearing mechanical damage is that the bearing surface is difficult to form oil film or the oil film is seriously damaged.
Bearing cavitation
Plastic deformation and cold work hardening occur in the outer layer, and the sliding bearing is subjected to repeated action of cylinder pressure (impact load). The deformation capacity is lost locally, and the grain is gradually formed and continuously expanded. Then, with the falling off of wear debris, the loaded outer layer forms a hole. Generally, when the bearing bush is pitted, the pit first appears, and then the pit gradually expands and causes the cracking of the alloy layer interface.
The crack extends along the parallel direction of the interface until it peels off. The main reason for the cavitation of sliding bearing is that the sudden change of the cross section of structural elements such as oil groove and oil hole causes the strong disorder of oil flow. Bubbles are formed in the vacuum area of oil flow disorder, and then cavitation occurs due to the collapse of bubbles due to the increase of pressure. Cavitation usually occurs in the high load area of the bearing, such as the lower bearing bush of the crankshaft main bearing.
fatigue pitting
Due to the overload of the engine, bearing fatigue pitting refers to. Make the bearing overheat and the bearing clearance is too large, resulting in fatigue damage, fatigue pitting or fatigue falling off in the middle of the bearing. This kind of damage is mostly caused by overload, excessive bearing clearance, or unclean lubricating oil and foreign matters mixed in it.
Therefore, when using, attention should be paid to prevent the bearing from overload and do not run at too low or too high speed; Adjust the engine to a stable idle state; Ensure the normal bearing clearance and prevent the engine speed from being too high or too low; Check and adjust the working condition of the cooling system to ensure that the working temperature of the engine is appropriate.Influence of bearing load on accuracy
Bearing alloy corrosion
The chemical impurities (acid oxides, etc.) in the lubricating oil oxidize the bearing alloy to form acidic substances. The corrosion area of the bearing alloy is generally impure lubricating oil. Cause the bearing alloy to fall off locally and form irregular small cracks or pits. The main causes of bearing alloy corrosion are improper selection of lubricating oil, poor corrosion resistance of bearing data, or rough engine operation and high temperature.Influence of bearing load on accuracy
Bearing melting
Local high temperature is formed, and there is a small convex metal surface in direct contact between the journal and the bearing friction pair. In case of lack of lubrication and poor cooling, the bearing alloy will be blackened or partially burned. This fault is often caused by too tight fit between journal and bearing; Lack of lubricating oil pressure is also easy to burn the bearing.Influence of bearing load on accuracy
Bearing load melting defect fracture
The causes of bearing fracture include defect and overload. When the external load exceeds the material strength limit, the part fracture is called overload fracture. During the operation of the bearing, defects such as overheating tissue and local burns will also occur, resulting in defect fracture.Influence of bearing load on accuracy
During the operation of the bearing, sometimes foreign hard particles or hard foreign matters or wear debris on the metal surface will be squeezed between the bearing surfaces, which will cause furrow like abrasion of the bearing.Influence of bearing load on accuracy
These hard foreign matters enter the interior of the bearing through the lubricating medium. With the continuous operation of the bearing, local friction generates heat, which is easy to cause local deformation of the friction surface and friction micro welding, or even local melting, and then bite.Influence of bearing load on accuracy
Generally speaking, the bearing itself does not produce noise. What we call noise is actually the sound effect of vibration between the bearing and the surrounding structure directly or indirectly.
Excitation caused by the change of the number of loaded rolling elements. When a radial load is loaded on a bearing, the number of loaded rolling elements will change slightly during operation, that is: 2-3-2-3 This causes an offset in the direction of the load. The resulting vibration is inevitable, but can be reduced by axial preloading, which is applied to all rolling elements (not applicable to cylindrical roller bearings in bearings).Influence of bearing load on accuracy
When the waviness of components is closely matched between the bearing ring and the bearing seat or transmission shaft, the bearing ring may be deformed by matching with the shape of adjacent components. If deformation occurs, vibration may occur during operation. Influence of bearing load on accuracy
Therefore, it is important to machine the bearing pedestal and drive shaft to the required tolerances.Influence of bearing load on accuracy
Local damage due to incorrect operation or installation, a small part of the bearing raceway and rolling element may be damaged. During operation, rolling over damaged bearing parts will produce specific vibration frequency. Vibration frequency analysis can identify damaged bearing components. Influence of bearing load on accuracy
Vibration behavior in applications in many applications, the stiffness of sliding bearings is the same as that of surrounding structures. Because of this feature, as long as the bearing (including preload and clearance) and its configuration in the application are correctly selected, it is possible to reduce the vibration in the application.Influence of bearing load on accuracy
Before the bearing is delivered for routine inspection, its oil seal package can be removed, and it can be immersed in washing gasoline or acid free kerosene containing 3% engine oil. Turn its outer ring and carefully wash the junction of cage, rolling element and raceway.Influence of bearing load on accuracy
It is required to thoroughly clean the original oil traces left in any hidden parts. After the bearing is cleaned, the attached cleaning oil shall be drenched and dried on the workbench lined with clean neutral lining paper, or dried slowly in the far-infrared oven. The bearing ring, cage or any rolling element shall not be rotated during the whole process from washing oil drying to air drying or drying.Influence of bearing load on accuracy
The bearings that have been dried or dried shall be dripped with lubricating oil as required and delivered for inspection immediately. At this time, there is no oil film protection on the outer surface of the bearing, and it is not allowed to directly touch the skin of hands during the whole inspection process. Influence of bearing load on accuracy
The operator shall wear clean gloves or apply antirust agent on his hands, but the antirust agent on his hands must be avoided from mixing into the rolling surface of the bearing, otherwise the measurement results will be deviated.Influence of bearing load on accuracy
After the inspection, the bearing shall be immediately delivered for use or treated with anti rust inventory. The whole inspection process shall also be completed quickly to avoid corrosion caused by too long. Note that using pure gasoline without engine oil for cleaning will also cause corrosion.Influence of bearing load on accuracy
Before inspection, the bearing must be placed on a metal plate in the inspection site together with the gauge block, standard parts and measuring instruments used for inspection, so as to homogenize the temperature and room temperature of these objects quickly.Influence of bearing load on accuracy
Bearing temperature is too high: during the operation of the mechanism, the part where the bearing is installed is allowed to have a certain temperature. When touching the shell of the mechanism with your hand, it should be normal not to feel hot. Otherwise, it indicates that the bearing temperature is too high.Influence of bearing load on accuracy
The lubricating oil viscosity is too high or the lubricating oil quality does not meet the requirements; The mechanism assembly is too tight (insufficient clearance); The bearing assembly is too tight; The bearing race rotates on the shaft or in the shell; Excessive load; Broken bearing cage or rolling element, etc.Influence of bearing load on accuracy
Bearing noise: the rolling bearing is allowed to make slight running noise during operation. If the noise is too loud or there is abnormal noise or impact sound, it indicates that the bearing is faulty.Influence of bearing load on accuracy
The causes of rolling bearing noise are complex: 1) the mating surfaces of the inner and outer rings of the bearing are worn. Due to this kind of wear, the matching relationship between the bearing and the shell, the bearing and the shaft is damaged, resulting in the axis deviation from the correct position and abnormal noise when the shaft moves at high speed. Influence of bearing load on accuracy
When the bearing is tired, the metal on its surface will peel off, which will also increase the radial clearance of the bearing and produce abnormal noise. Insufficient bearing lubrication, dry friction and bearing breakage will produce abnormal sound. Influence of bearing load on accuracy
After the bearing is worn and loose, the cage is loose and damaged, which will also produce abnormal noise and damage to the bearing. During the disassembly and inspection of rolling bearing, the fault and damage cause of bearing can be judged according to the damage of bearing.Influence of bearing load on accuracy
Metal spalling on the raceway surface: the bearing rolling body and the inner and outer raceway surfaces are subjected to periodic pulsating load, resulting in periodic contact stress. When the number of stress cycles reaches a certain value, fatigue spalling will occur on the working surface of rolling element or inner and outer race raceway.Influence of bearing load on accuracy
If the load of the bearing is too large, it will aggravate this fatigue. In addition, improper bearing installation and shaft bending will also produce raceway spalling. The fatigue spalling of bearing raceway will reduce the operation accuracy of shaft and cause vibration and noise of mechanism.Influence of bearing load on accuracy
Bearing burn: the burned bearing has a return fire color on its raceway and rolling element. The causes of burns are generally insufficient lubrication, unqualified or deteriorated lubricating oil quality, and too tight bearing assembly.Influence of bearing load on accuracy
Plastic deformation: uneven pits appear on the contact surface between the raceway and the roller of the bearing, indicating that the bearing has plastic deformation. The reason is that under the action of large static load or impact load, the local stress on the working surface of the bearing exceeds the yield limit of the material, which generally occurs on the bearing rotating at low speed.Influence of bearing load on accuracy
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