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Bearing life factor

March 7, 2020

Bearing life factor

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The life of a rolling bearing refers to the cumulative number of revolutions, accumulated working hours, or mileage of a bearing before its components, such as rolling elements, inner and outer rings, or cages show the first signs of fatigue expansion after starting a bearing. Life is the most important design criterion and usage index of rolling bearings. How to determine the life of rolling bearings more accurately is always one of the important research directions in the bearing technology field, and it is also a long-term concern in the engineering community.
 
 
Chinese name
Bearing life factor
Foreign name
Coefficient of bearing life
Brief introduction
Life is an important indicator of bearings

classification

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Bearing failure can be divided into two types: dynamic failure and loss of accuracy.
Bearing
Stop failure is when the bearing stops rotating due to loss of working ability, usually due to factors such as seizure and fracture; loss of accuracy means that the bearing has lost the accuracy required by the original design due to size changes. Although it can continue to rotate, it is not Normal operation is prominently manifested by frictional torque , vibration, and temperature rise, which are usually caused by factors such as fatigue spalling, rust, wear, and gluing . Rolling bearings and space rolling bearings in service in the form of failure are mostly loss of accuracy.

form

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The common failure forms of rolling bearings are mainly in the following categories:
Bearing
(1) Fatigue spalling: Under the action of the contact load between the inner and outer rings of the rolling bearing and the rolling elements, the metal on the contact surface from the metal body appears to be spotted or flake. Pitting is often the initial form of fatigue spalling. It is a fatigue phenomenon caused by material fatigue. Pitting corrosion eventually expands into fatigue spalling. Fatigue spalling is related to bearing materials, heat treatment and manufacturing processes. At the same time, bearing selection, installation, coordination, lubrication, sealing, maintenance and other use conditions are also important causes of fatigue spalling.
(2) Surface plastic deformation: Surface plastic deformation is divided into general surface plastic deformation and local surface plastic deformation. The former is due to the contact between two rough surfaces and no dynamic lubrication film is formed between them. The latter is due to the local plastic deformation caused by original defects such as pitting and scratching on the surface of the material.
(3) Wear : Fatigue wear, adhesive wear, abrasive wear and corrosion wear are the basic forms of wear. The generation of wear is mainly related to factors such as poor sealing of the bearing, improper lubrication, falling off of the contact surface material particles and rust, etc. The wear can be reduced through measures such as improving assembly and processing technology, enhancing lubrication and avoiding intrusion of pollutants.
(4) Corrosion: Metal corrosion can be divided into chemical corrosion and electrochemical corrosion. The corrosion of bearings is mainly caused by the inside of the bearing or the lubricant contains corrosive substances such as alkali, acid, etc. The intrusion of corrosive substances caused by the failure of the sealing device and the bearing environment Caused by high humidity and improper bearing cleaning and storage.
(5) Burns: caused by improper slip, excessive preload, improper clearance selection, and poor channel surface contact.
(6) Cracks and defects: When the stress that the bearing component can withstand exceeds the fracture limit stress of the material, the internal or surface layer of the material undergoes overall and local fracture. Cracks and fractures are mainly caused by raw materials, heat treatment and processing technology. Non-destructive testing should be performed for cracks that are not visible to the naked eye. [1]

process

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Due to the special nature of the space environment, the bearings used in space are mostly solid-lubricated bearings. The failures caused by them are mainly in the following aspects: failures caused by processing, failures caused by installation, failures caused by lubricating films, and temperature Failure.
Bearing
Among them, the failures caused by the processing process are mainly related to the components' waviness, roughness, rolling body morphology, and manufacturing precision of the ferrules; the failures caused by the installation are mainly related to the fit tolerance , pre-tightening force, working gap and press- fitting ; The failure caused by the lubricating film is mainly due to the fretting wear, peeling wear and rolling body slip of the lubricating film; the failure due to temperature is mainly due to the change in the temperature difference of the working environment of the bearing and the extreme temperature. [1]

Factors affecting life

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In addition to structural design, there are mainly four aspects of material, manufacturing, use and lubrication technology.
Bearing
(1) Material impact: In terms of bearing material technology, the life of the bearing is ensured mainly by means of material selection, material guarantee and heat treatment. Rolling bearings are generally made of high-carbon chromium bearing steel, and the chemical composition is almost unchanged. However, different smelting methods have different purity materials and have a significant impact on life. Under the same contact stress conditions, the contact fatigue life of Zhen IV4 ceramic bearings is better than that of bearing steel bearings; at high speeds, light loads and small impact loads , Ceramic ball bearings can be preferred. It can be seen that the impact of materials on bearing fatigue life is very significant.
2) The effect of surface roughness: Fatigue cracks usually originate on the surface, so the surface has a great effect on the life of the part. The smoother the surface, the longer the fatigue crack initiation time. The surface treatment technology of rolling bearing rolling elements and rings can change the hardness of the surface layer of the rolling elements, the residual stress distribution and the overall strength of the material, thereby improving the bearing life.
(3) Effect of temperature: Whether it is a fatigue model based on subsurface stress or a model starting from a defect on the surface, the influence of the heating in the contact area of ​​the bearing on the fatigue life is not considered. In fact, after continuous operation of the bearing, it will inevitably be accompanied by a certain temperature rise. The magnitude of this temperature rise should be related to the thickness of the lubricant film between the contact pairs, the surface characteristics of the contact parts, and the load and speed . At the same time, after the temperature rises, there must be a temperature distribution in the bearing ring and rolling body, and thermal deformation due to the influence of thermal expansion will affect the accuracy of the bearing. Therefore, the impact of temperature rise and heat on fatigue life cannot be ignored.
(4) the impact of lubrication technology: in bearing lubrication technology, mainly through the selection of lubricants and lubrication methods to improve the life of the bearing. Lubrication technology has become one of the most critical factors in improving bearing life. Especially for sealed bearings, grease life has become another "synonym" for bearing life, that is, grease life is bearing life.
(5) Influence of running speed: Bearing fatigue life is related to instant contact time. The instantaneous contact time refers to the time required for the rolling element to roll over the ring and the ring raceway to contact the ellipse width under the maximum load. As the speed increases, the instantaneous contact time increases, and the fatigue life of the bearing decreases. The slower the running speed of a bearing, the longer its life in revolutions. On the other hand, the length of the instantaneous contact time will also affect the surface residual stress, which indirectly affects the fatigue life.
(6) Effect of load: The effects of rolling element parameters and curvature coefficients on the fatigue life of deep groove ball bearings. Studies have shown that the magnitude of the load has a very large effect on bearing fatigue. The fatigue life of rolling bearings depends largely on the maximum rolling body load. Therefore, the increase in load results in a significant increase in the maximum rolling element load and a reduction in fatigue life. [2]

Classification of calculation methods

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Different bearing fatigue life calculation methods produce different calculation results. Two calculation methods are given below: LP theoretical algorithm and ISO international standard theory.
Bearing
On algorithms. The ISO international standard algorithm is a simplification of the LP theoretical algorithm. The bearing support ring is made rigid, while the actual ring is a non-rigid body and the ball bearing has greater contact deformation under the same load. The SO standard ignores centrifugal force and gyro torque. . Under the radial load, the ISO standard is based on the assumption that the internal clearance of the bearing is zero and that the contact load between the rolling elements and the channel remains uniformly distributed. Therefore, the relative error of the ball bearing calculation results is relatively large. Both P theory and traditional ISO national standard theory ignore the fatigue life of rolling elements. [3]

Measures to improve life

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1. Increasing the number of rolling elements: As the number of rolling elements increases, the radial load on each rolling element decreases, and the contact load of each rolling element decreases, and the bearing fatigue life increases.
2. Increasing the diameter of rolling elements: Increasing the diameter of rolling elements is equivalent to the increase of the bearing structure and the bearing capacity of the bearing. Therefore, the bearing life increases with the increase of the diameter of the rolling elements.
3.Reasonably set the contact parameters of rolling elements and raceways: For deep groove ball bearings, the inner groove curvature coefficient Z of the bearing is less than 0.52, and the outer groove curvature coefficient Z is less than 0.53. Also note
with
Match between. In addition, it is also possible to reduce the pitch diameter of the bearing to minimize the pre-
  tightening force under the premise of ensuring that the bearing does not slip . At high speeds, the rolling body uses ceramic materials instead of steel materials.