There are 15 types of failure modes of aero bearings and some measures to prevent slippage of aero engine bearings.
The failure of aviation bearings is a very serious problem, and the direct consequences of bearing failure are increased temperature, increased vibration or change in vibration state, increased power consumption of shafting, and change in the shape of the shaft trajectory.
1. Scratches and abrasions.
Due to the unclean filtration of lubricating oil, there are metal particles that have been worn down, or some impurities are mixed in during the installation process, and the size of these metal particles may be greater than the minimum oil film thickness, and due to the sliding phenomenon between the contact surfaces, the hard tiny granules that exist between the two contact surfaces will scratch the working surface. In addition, in the installation process, because the bearing parts are in contact with other hard objects with sharp angles, and the working area of the bearing parts is carved out of a certain depth, width, length of different grooves called scratches or scratches, if between the rolling several touch surfaces, fall into the foreign small particles of impurities or lubricating oil local damage, and there is a sliding phenomenon between the two surfaces, so that a group of small scratches appear on the working surface of the parts, called abrasions.
Preventive measures: (1) Strictly filter the lubricating oil. (2) Strictly prevent pollutants from entering people during the installation process. (3) In the process of cleaning and installation, it is strictly forbidden to use hard objects to carve the surface of the bearing.
2. Wear and tear. In a state of critical lubrication. When two objects in contact with each other move relative to each other, surface damage will occur, and if this damage continues, it will cause ablation, i.e., abrasion. Wear will cause the loss of dimensional accuracy of the part, and the worn surface will show speckle characteristics. This is shown in Figure 1-1. Due to the large amount of heat generated by friction, metal migration between the two contact surfaces will occur, causing the wear surface to exhibit phosphorus-like spalling, as shown in Figure 1-2.
Under the action of higher stress, or in the state of lack of oil, a large amount of frictional heat will be generated between the two friction objects, which is sometimes to make the contact area reach a melting state, so that there is instantaneous welding, and then it will be torn, and the consequence is to cause metal-to-metal migration of the contact surface, which is called adhesion wear.
Sometimes, due to some foreign impurities mixed into the contact surface, it can also cause severe wear and tear on the surface, which is called abrasive wear.
It is almost impossible to safely avoid the occurrence of wear and tear, and all effective measures can only be taken to reduce the occurrence of wear and tear, and the measures are as follows:
1) Improve the finishing technology of the structure and parts, and select the appropriate lubrication conditions.
2) Strict lubricating oil filtration measures to ensure the cleanliness of the working environment to reduce or avoid abrasive wear.
3. Slipping with light load.
In fact, it also belongs to a more serious wear, the rolling element is in contact with the raceway, and when the contact load is extremely light (or under the condition of excessive lubrication), due to the influence of inertial force, the linear velocity of the contact point of the two contacts is different, and the sliding phenomenon will occur between the two contacts, which is called light load slippage. Roller bearings are now slipping under light load, which will cause friction and wear, surface fatigue, raceway surface damage and other consequences of the raceway, and the early damage is characterized by surface rubbing and full-layer migration.
Measures to overcome light-load slippage of ultra-high-speed roller bearings:
1) In order to ensure that the rollers have a sufficiently high fatigue life.
2) The cage is made of high-strength lightweight materials to reduce its weight.
3) Improve the structure of the roller bearing to improve the smoothness of the lubricating oil in the bearing.
4) Adopt an elliptical raceway in the outer ring to increase the number of loaded rollers.
5) Preloaded hollow rollers are used to cause all rollers in the bearing to be evenly loaded without elliptical raceways.
6) Appropriately reduce the clearance to increase the number of rollers carried and increase the drag force.
4. Rust. Damage caused by chemical or electrochemical action between the metal and the surrounding environment medium is called rust or corrosion. When rust occurs, black or brown rust spots or rust pits are formed on the surface, and corrosion products appear, destroying the luster of the metal.
In bearings, there is also a phenomenon of tribocorrosion, which is characterized by the appearance of some pits on the contact surface. The formation mechanism is as follows: in the contact between the rolling element and the ferrule, under the action of positive pressure, the ferrule will produce a slight amount of plastic deformation and a slight relative motion between the two contact surfaces. The new metal surface formed by friction will be oxidized when subjected to a combination of chemical, mechanical and thermal effects, and the oxides formed will continue to accumulate and be pressed against the surface layer. Due to the high temperature formed by frictional heat, the lubricant will form aldehydes, ketones and carbonic acid, and these products will also react chemically with the material. Therefore, fretting corrosion will not only cause surface damage to the parts, but also reduce the contact fatigue strength of the parts.
Measures to prevent rust generation:
1) Pay attention to the storage of bearings, and try to avoid contact with corrosive media.
2) Wash carefully when installing the bearing, and wear gloves to avoid sweat stains from leaving on the bearing working surface.
5. Eccentric wear, load trajectory downward movement (climbing).
Eccentric wear is the wear phenomenon of the bearing in the working side, which is more obvious in the inner ring of the roller bearing. In the ball bearing bearing in the axial direction, the load trajectory on the outer ring roller channel is excessively biased to the other side of the channel center.
Overcoming methods: 1) The bearing installation must be in place so that the bearing is in a good load state.
2) Adjust the ball bearing clearance to prevent its value from being too large.
6. Pit pressure and collision injury.
Due to the action of static load or impact load, the working surface of the bearing part leaves a certain depth of permanent deformation, which is called pressure pit or impact. The preventive measures are as follows:
1) Strictly prevent parts from falling to the ground, and it is strictly forbidden to hit with hard objects, which will cause damage to the working surface.
2) When installing the bearing, keep the afterburner ring and the rolling element assembly in the correct position.
Edit 7, fatigue and fall.
Under the action of alternating contact stress, fatigue spalling occurs not only under the surface, but also on the surface.
Fatigue spalling that occurs under the surface, and the maximum shear stress occurs at a depth of about 0 from the surface under the action of alternating stress2 mm. First, the first microcrack is formed from the weakest point of the material, and then under the repeated action of alternating stress, the crack continues to expand and multiply. Finally, it leads to the peeling of the metal.
There are two causes of fatigue spalling that originate from the surface:
1) Due to the non-metallic inclusions, coarse carbides, micropores, wrinkles, pressure pits and surface metamorphic layers remaining on the surface of the raw materials in the process of forging, stamping, heat treatment, cold working and other factors of bearing parts, due to the action of contact stress, they are detached, separated from the parent body or used for plastic deformation to form spalling after the aggregation and expansion of micropores.
2) On the two rolling surfaces in contact with each other, due to the phenomenon of unevenness, if the bearing is in the boundary lubrication state when working, the two surface profile peaks interact with each other, so that those tiny uneven parts produce plastic deformation and fatigue. If this phenomenon continues, a large number of microscopic cracks will form, which will then expand and join together to form tiny spalling. This can cause surface damage. The further expansion of this damage will cause the micro-cracks to increase and cover the raceway surface, forming so-called pitting, which can then be further expanded and connected to cause fatigue peeling of the surface. It has been proved in practice that for the spindle bearing of high-speed boat engine, fatigue spalling is mostly spalling on the surface.
To overcome or reduce the premature occurrence of fatigue flaking, the following measures can be taken to:
1) Improve lubrication conditions and reduce the damage to the working surface caused by abrasive wear. Avoid direct metal-to-metal contact.
2) Adopt new materials or use high-purity vacuum smelting steel, and optimize the heat treatment process to improve the contact fatigue strength of parts.
3) Improve the surface quality and enhance its anti-wear and corrosion resistance.
4) Starting from manufacturing, increase the forging ratio, improve the fiber streamline direction, and improve the finishing process.
8. Current erosion.
When the local metal surface is heated and melted, the damage area is magnified, and the spherical particles of the metal can be seen after melting, and this damage is called electroerosion. The formation mechanism is that the two contact surfaces are used for some reason to generate static electricity, and at this time, a layer of oil film thickness is formed between the two sides, and the oil film plays an insulating role. When the electrostatic potential difference between the surfaces of two contacts is high enough to penetrate its insulation, a spark discharge occurs, resulting in current erosion.
Overcoming method: keep the parts in a good grounding state to avoid working ** due to the accumulation of charges to form a high potential and produce spark discharge.
9. The cage is deformed.
Under the action of external force, there is a large difference between the shape of the cage and the original shape, which is called cage deformation. The reason for this is that the cage is usually made of lightweight materials with low strength, and the lintel size of the isolating rolling elements is generally small. Therefore, under the action of external force, it is easy to make the cage plastically deformed.
The preventive measures are as follows:
1) Choose high-strength and lightweight materials.
2) Avoid hitting the cage with hard tools or gravity.
10. Cracks.
When the stress of the bearing part exceeds the strength limit of the material, the internal or surface of the bearing part will be fractured or partially fractured, and its macroscopic defect is called crack. It is characterized by sharp roots or edges. The causes of cracks are complex, and there are many influencing factors. Therefore, it is necessary to prescribe the right medicine from all aspects in order to prevent the occurrence of cracks.
11. The cage plating falls off or is convex and blistered.
Due to the improper treatment of the cage before the silver plating process, or there are some problems in the silver plating process, the bonding strength of the coating and the parent material is reduced, resulting in defects such as the coating falling off during work.
Preventive measures: improve the silver plating process and enhance the bonding strength of the plating layer and the parent metal.
12. Pole wear (cat's eye circle).
Its morphological characteristics are: concentric circles with a certain area range are formed at the two poles of the steel ball.
The reason for the wear of the two poles: in the three-point contact ball bearing that runs at high speed and bears a certain axial load, the steel ball will produce a large inertial dynamic load of centrifugal force and gyroscope moment, centrifugal force acts on the steel ball, always makes the steel ball press to the outer ring, resulting in the contact angle of the outer ring becoming smaller, and the contact angle of the inner ring increases, with the increase of speed, one of the half inner rings will be partially unloaded to complete unloading, at this time the bearing should be changed from three-point contact to two-point contact, if the bearing is not out of three-point contact at this time, due to the effect of gyroscopic torque, the steel ball will produce severe sliding on the groove of the unloaded inner half of the circle, causing abrasion or wear of the bearing parts.
This is typically manifested in the presence of many concentric circles ("cat's eyes") scratches on the steel ball.
Here's how to overcome it:
1) Increase the clearance and strictly control the diameter difference of the steel ball, so as to ensure that the bearing can be separated from the third point contact after operation.
2) When designing, select the appropriate two halves of the inner ring and design the gasket angle.
13. Discoloration by heating.
After a period of operation, a light brown deposit or gray-black (or purple) surface appears on the surface of the bearing, and the original metallic luster is lost.
The causes of discoloration are as follows:
1) Because the bearing surface is exposed to the hot air environment, due to the temperature rise, the oil film attached to the bearing surface produces the oxidation of the oil, forming a light brown oxide film, which is deposited and attached to the surface of the bearing.
2) If the bearing is in an abnormal working state, resulting in a sharp rise in the temperature of the working environment, the temperature at this time has greatly exceeded the tempering temperature of the bearing parts, resulting in serious burns, and the bearing surface has become gray-black.
Preventive measures: ensure that the oil circuit is unobstructed, and supply the required oil quantity of the bearing in time.
14. The size is enlarged or reduced.
After a certain period of bearing operation, the clearance decreases (reflected in the increase or decrease of the size of the inner and outer rings or the diameter of the rolling elements), and if the size changes greatly, the bearing is likely to hold the shaft.
Forming the cause of size expansion or shrinkage, the bearing parts retain a certain amount of residual austenite in the heat treatment process, with the change of time and temperature, it will gradually become a more stable martensitic, in the process of this transformation will appear volume expansion phenomenon, at this time the size of the bearing parts will expand. In addition, if the operating temperature of the bearing is higher than the tempering temperature of the bearing, the martensite decomposition process will occur, and the size shrinkage will occur. Sometimes stress relaxation in bearing parts is also a cause of dimensional changes.
Preventive measures: After quenching, the method of cold treatment is adopted to reduce the number of residual austenite; A tempering temperature higher than the operating temperature is used, and multiple additional tempering methods are used to eliminate residual austenite.
Edit 15, chatter.
The appearance of chatter marks can be divided into two situations:
1) Chatter marks generated by the engine when it is working.
In the local area of the raceway surface of the inner or outer ring, some axial parallel fringes that are closely arranged along the rolling direction appear, which are called chatter marks.
Formation mechanism: In high-speed radial cylindrical roller bearings, due to the non-roundness of the inner and outer ring raceways or the large difference between the roller diameters, the bearing rotation accuracy is reduced, which is the main reason for the formation of chatter.
Preventive measures: reduce the deformation of the inner and outer rings and reduce the difference between the diameters of the rolling elements, and improve the rotation accuracy of the bearing.
2) Vibration when the engine is stationary.
In the case of the engine at rest, due to the continuous non-rotating vibration, under the action of static load, it leads to the fretting wear traces formed by the roller pitch on the inner raceway of the cylindrical roller bearing, and this type of vibration mark does not affect the performance of the bearing, so it is also called false surface deformation, so it is allowed to exist.
In the non-rotating state, the bearing is subjected to significant vibration loads or overloads, which cause the balls or rollers to produce permanent shallow and smooth pressure pits on the raceway, and such deformations are generally not allowed.
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Preventive measures: prevent the vibration of transportation and strengthen anti-vibration measures.
The specific causes of aircraft engine bearing slippage.
Rolling bearings used in general machinery rarely slip due to their low rotational speed and the fact that there is always a load acting on them. But the main bearings of aviation gas turbine engines are prone to slippage because:
1) High rotational speed. Under the action of high speed, the roller has a tendency to break away from the inner ring raceway under the action of large centrifugal force;
2) The load is small. The rotor of the aero engine is made lighter, so that the radial load on the bearing is smaller, and when the aircraft is maneuvering, the load acting on the bearing will be smaller or even zero load in some cases, and the unbalanced force of the rotor will offset some of the radial load acting on the bearing in some cases, resulting in light load or zero load of the bearing.
These two factors will make the drag force generated by friction become very small, or even zero, so that it will inevitably cause bearing slippage and slip damage. In the case of ball bearings, they are generally not prone to slippage because they are also subject to axial loads.
Some measures to prevent slippage of aero engine bearings.
Measures to increase the drag force are adopted to prevent the bearing from slipping.
Reduce the bearing clearance, so that the rollers can still maintain contact with the inner ring raceway under the action of centrifugal force. For example, the roller bearings (intermediate bearings) of the CFM56 engine supporting high-pressure turbines use small or even negative clearance to reduce slippage; Another example is the compressor front roller bearing of the WJ6 engine after a long period of test run and serious slip damage, the clearance of the bearing is reduced from 0070~0.095mm is reduced to 0045~0.065mm, eliminating slippage.
However, the use of measures to reduce clearance can cause other serious problems and should be treated with caution, especially when it comes to bearings at the hot end of the engine.
In the early aviation gas turbine engine, in order to solve the balance problem of the cage, the cage of the main bearing was mostly positioned in the outer ring, but this design was easy to cause the bearing to slip and rub damage. This is because when the cage is positioned on the outer ring, the oil film between the outer ring and the cage hinders the normal movement of the roller cage under the action of viscosity, causing slippage.
If the cage is positioned in the inner ring, the oil film existing between the inner ring and the cage will give the roller cage assembly a drag force to make it move normally under the action of viscosity. In this way, the cage that was originally positioned in the outer ring is changed to be positioned on the inner ring, not only reduces the resistance, but also increases the drag force, obviously can reduce the slip damage, of course, this also has to improve the machining accuracy of the cage to improve its balance.
When editing the assembly, an additional radial or axial load is applied to the bearing, that is, a "preload" is applied to the bearing, so that when the bearing works, there is always a load between the inner and outer ring raceways and the rollers, and there is no light load or zero load, so as to increase the drag force. The methods of applying preload to the bearing are: using non-round bearings, axial springs to preload the bearings, using hollow rollers and adjusting the direction of fuel injection to the bearings.
The outer circle of the outer ring of the bearing is made non-round, while the housing hole of the casing-mounted bearing is still round. Commonly used non-circular bearings include elliptical bearing outer ring and three-petal bearing outer ring. For example, when the elliptical bearing is pressed into the bearing housing hole, the long axis of the ellipse (the protruding part of the outer ring) is to act on the roller at the place a pre-loaded load, such as the long axis is in the horizontal position, so that several rollers in the horizontal position are always in contact with the inner and outer rings and act on a certain load, therefore, in addition to the action of gravity load on the lower several rollers, the roller rod at 90 ° on its left and right is also subjected to a certain amount of preload, so that the number of rollers bearing the load increases to about 60% of the total number of rollers, It can generate a certain drag force and overcome bearing slippage.
"Slippage" is used to indicate whether the rolling bearing is slipping and how severe the slippage is.
When the actual speed of the cage is equal to the theoretical speed of the cage, the bearing does not slip, and its slippage degree is zero; As long as the actual speed of the cage is lower than the theoretical speed, the bearing will slip, and when the actual speed of the cage is zero, the slippage degree is 100%.
Generally, the slip degree of the bearing is obtained by measuring the actual speed of the cage. When developing the RB211 engine, Rolls-Royce used radioisotopes to detect bearing slippage. A very small radioactive source made of cobalt or iridium (CO 60, IR 192) filament is fixed on the cage, and the speed of the working cage is measured by using the principle of inverse square law (that is, the radioactive intensity transmitted to a certain point is inversely proportional to the square of the distance from the radioactive source to the point), so as to find out whether the bearing slips and calculate the degree of slippage. The rotational speed of the cage can also be measured by using the principle of cutting magnetic field lines.
Prevents the axial load acting on the ball bearing from changing direction.
As mentioned above, the thrust bearing of the RB211 fan rotor is an intermediary bearing, and the ball bearing has been damaged less than half a year after it was put into use (April 1972) because the axial force direction of the fan rotor will change during operation.
Therefore, the above-mentioned measures of changing the positioning of the outer ring of the cage to the positioning of the inner ring have solved the problem to a certain extent, but because the axial load reversal problem has not been solved, for a long time, in various modifications of RB211 (524B, 524C, 524D4 and 524D4Upgrade, etc.), there are still occasional incidents of slip damage to the bearing. For this reason, in the last modification of the RB211, the RB211524G H (put into service in 1989), the diameter of the unloading chamber sealing ring was changed from 55620mm increased to 73152mm, which eliminates the problem of axial load reversal of the rotor during work.
The slippage problem of this bearing was basically solved. The Trent engine, which is derived from the RB211 524G H, not only adopts a large-diameter balance chamber sealing ring, but also adds a spring and a small bearing at the rear end of the low-pressure turbine shaft to preload the rotor, which fundamentally solves the problem of bearing slippage.
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