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Analysis of the Causes of Failure of TB Aircraft Landing Gear Wheel Bearings
Source: China Bearing Network Time: 2013-03-11
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First, the basic layout of the bearing and the force analysis TB aircraft wheel bearing is a tapered bearing produced by Timken (Timken); it consists of four parts: inner raceway, outer raceway, tapered roller and cage. Normally; inner raceways, outer raceways and rollers accept loads; and cages keep the rollers evenly spaced from one another; prevent bumps and conflicts from each other; and allow each roller to receive a flat load evenly and in turns. The inner raceway, the roller bar and the cage are collectively referred to as the raceway assembly. Generally it is separable from the outer raceway (the outer raceway is fixed on the differentiable hub); making the equipment bearing more convenient.
The bearing is made of low carbon steel; it is treated by external carburizing; it makes the bearing have suitable hardness; the induction function of fatigue resistance and tolerance; under normal operation; the maximum temperature range of the bearing is 120-150 ° C; 175 ° C; maximum period of touch stress is 2100 ~ 3100MPa; and the cage is generally made of low carbon steel.
Because of the several characteristics and characteristics of the tapered bearing; it can accept the meridional and axial induction loads. The larger the angle between the outer race and the middle of the bearing; the greater the ratio of acceptable axial thrust to warp thrust; The longer the touch line of the roller and the raceway, the stronger the load can be received. The aircraft is in a different working condition; the bearing is not the same:
1. The aircraft is in a stop condition; the bearing is primarily subjected to a stop load. The stopping load P of the aircraft's gravity attack is transmitted to the outer raceway through the bearing's roller; that is, the hub.P can be axially differentiated into an axial force N and straight to the outer raceway. The force F. As shown in the figure; the pressure of the external raceway caused by P is much greater than the force component of P on this wheel; a large pressure is applied to the raceway.
2. When the plane is taxiing on the ground; the first thing is to accept the straight load. Because the ground is not surely flat; the ups and downs of the plane are more than the gravity of the plane.
3. When landing; the first priority of the grounding of the wheel is to be subjected to a grand stop straight shock load; then the wheel accelerates at a high speed to reach the same speed as the aircraft. If the plane hits a heavy landing; the bearing is The damage caused by such an impact load in the case of non-operation is very large. If the aircraft is side-slip grounded; if the crosswind is large, the wheel is subject to a large lateral load; the wheel is subjected to the side. When the force is in conflict; because of the inertia effect; the aircraft has a tendency to skew toward one side. Therefore, the straight and lateral loads on the outer bearing are larger than the inner side.
Second, the common faults of bearings l. Rolling touches the fatigue caused by fatigue and rolling touch; it means that the bearing is ineffective at the working moment because of the alternating stress caused by the alternating stress on the touch surface or the subsurface of the touch surface. When the crack starts, the appearance is very damaged. Micro may not see it; until the final time of the process; cracks and shedding are manifested.
When the bearing receives the load; the rolling body and the raceway knead each other; the deformation and stress occur in the vicinity of the touched area; because the touch surface is very. The word 乖 乖 惶螅 惶螅 哟ビ 哟ビ σ σ σ σ . . . .. The touch stress in the rolling bearing is between 2000 and 4000 MPa. The touch stress determines the static bearing capacity of the bearing; the static bearing of the bearing can be the first to have the compressive stress distribution between the touch surfaces; the unique is the influence of the shear stress distribution. If the rolling body The touch surface is rolled over; the shear stress parallel to the roll direction will be changed according to the alternating stress. The amplitude reaches the maximum at a certain depth; it is called the maximum dynamic shear stress. The maximum dynamic shear stress is the attack on the touch fatigue crack. And the primary effect of the expansion; and the maximum dynamic shear stress plays a primary role in the plastic deformation under the touch surface. When the cylindrical roller passes the appearance of the raceway; the effect on the shear stress field of the sub-surface is shown graphically from right to left. The following figure.
When the roller rolls from azimuth 1 to azimuth 3; the stress field in azimuth 2 rotates by itself; this rotation is the primary factor to promote bearing damage. Because the shear stress changes from +Ï„2y to -Ï„2y; therefore its maximum shear The maximum positive value of the shear stress Ï„max becomes |2Ï„2y|; this value is far from the maximum shear stress Ï„max.
The above-mentioned shear stress theory is applied to the rolling bearing; the bearing is mounted on the axle; the inner ring is simply immovable; and the outer ring and the hub rotate together at the same high angular velocity. In general, the load direction on the shaft is not Changed; there is a little bit of force on the inner ring; the shear stress Ï„0 at this point; in each cycle (roller rolls over this point) is larger than the other points; so this is the earliest onset of fatigue damage However, with regard to the outer ring of the transformation; the point at which the maximum shear stress Ï„0 is accepted is modified; and the circumference of the outer ring is larger; therefore, the fatigue life of the touch is longer than that of the inner ring; the opposite is true; The outer ring is fixed with a rolling bearing; the outer ring has a shorter life than the inner ring.
In general; the following elements simply lead to fatigue cracks originating from "appearance":
1 corrosive mottled; operation scratches; appearance noisy and external indentation can cause stress to gather and incur the rolling touch fatigue originating from the outside; crack,
2 smooth and poor. Generally caused by small surface discontinuities, such as falling off,
3 The oil film does not reach the critical thickness to constitute a large area of ​​the bearing damage.
Theoretically speaking, the load of the roller bearing is uniform on the raceway and the roller. However, the flaking originates from the raceway; it does not originate from the roller; this is very important for bearing inspection.
2. Rotating åŒåŒ rotating åŒåŒ refers to the relative motion between the compression cooperative inner ferrule and the shaft it cooperates with; this failure also has an episode. The rotation åŒåŒ occurs in the inner ferrule of the bearing; it makes the operation of the entire device similar a conflict transport device; the appearance of the inner wall of the bearing inner ring and the shaft is severely characterized by a scoring appearance characterized by a score; or even a short radial abrasion. The reason is that: on the one hand, the device makes the inner sleeve The upper shaft of the circle is loose; on the one hand, the plastic deformation caused by the abnormal unbalance of the medium load is used; and the middle line of the shoulder and the shaft may not be straight.
3. Metal adhesion metal adhesion refers to the accumulation or adhesion of material from any surface of two sliding touch bodies to another appearance; it is caused by the severe sliding of the bearing touch surface. It is characterized by two touches. The surface is welded; this failure is limited to the touch surface that satisfies the violent sliding motion; it is often aggravated. If left unchecked; the succession is the failure of the heat balance of the two fouling surfaces. The metal is partially discolored due to high temperature; the accumulation of metal is more severe.
The reason for the seizure is: the result is that the smooth grease is mutated or hardened by the contamination of dirt and foreign matter; the transition of the roll holding in the cage is severely hindered.
Smooth failure may be the primary cause of the adhesion of the cage data on the raceway and the roll. Second, the wear in the bearing has been carried out enough to cause the roller to be skewed or the device to be too loose; so that the transition intermediate line is not parallel to the axis. The sliding of the roller and the subsequent fouling will also occur. Adhesion is the sliding touch between the end face of the roller and the flange of the cage; and it has a large impact on the end of the raceway.
4. Wear failure of the bearing, although the rolling bearing is based on the rollover touch prototype; the conflict has a lot of impact. It not only causes the work noise to increase, but also causes other problems in the bearing operation; Shorter life.
The reason for the wear and tear of the bearing is that the smooth condition is not suitable; there may be dirt in the touch area; the hard particles are corrosive fluid; perhaps there is a high speed sliding touch on the touch surface. The impurity acts as an abrasive; Some unnecessary steel scraps; therefore, the abrasive material is added. Some solid and coarse granules are pressed between the roller and the raceway; during operation, the inner ferrule is transformed on the shaft. Water; acid and other corrosive liquids (including The wear of the smoothing agent is a wear-resistant failure; it is generally characterized by a reddish-brown covering layer and a small corrosion pit on the outer surface of the wide raceway. The wheel is close to the ground; simple adsorption of various impurities; many.
5. The deformed failure bearing causes the plastic surface to flex due to the partial overload of the overload; it is called plastic deformation failure, and the primary influence of the plastic deformation of the bearing on the bearing operation is essentially a few deformations. Therefore, the bearing is uniquely unable to accept overload; impact load and oscillation (unique when the bearing is not working). If the aircraft is in the process of landing; an eternal deformation of the excessive impact load; the dispersion of such indentations and depressions on the raceway must have a certain correspondence with the scattering of the ball. In the process of the differentiation of the bearing is the appearance that can be observed.
Softening is another plastic deformation phenomenon in the rolling bearing. Because the heat inside the bearing is greater than the heat released; then the unstable heat balance is caused; the operating temperature rises; the smoothing of the smoothing agent will be invalid; the bearing bite and Softening of steel; declining fatigue life. Severe thermal and soft damage of components; sometimes it can be identified by discoloration and obvious plastic deformation.
Third, strengthen the scientific protection of bearings (a) follow the prevention-oriented policy on the protection of bearings; the key is to adopt a prevention-based policy. Assume that the bearing fails during the life of the application; be sure to find out the cause of the failure And propose ways to eliminate and manipulate such failures; do preventive operations. Secondly; in the failure process of the bearing; often after a component fails; perhaps after a failure mode occurs; the bearing continues to operate abnormally and Initiating other failure modes; causing many parts to be damaged together; in the case of complex conditions, simply triggering more incidents. Therefore; pay attention to each failure mode. Once a failure of the bearing is found; Replace; to prevent the development of the incident.
(b) Applying scientific management methods; formulating the optimal protection plan for repair and management is a systematic project; in this process, we must implement prevention-oriented policies and reliability-based repair thinking. Dynamic process; in this process; combine our rich repair experience with repair theory; rise to a higher level; draw up the optimal repair plan. This will not only make each of our stand-alone Repair quality progress; and benefit the progress of the protection quality of the entire fleet.
For example, the problem of the front wheel bearing of the 8913 aircraft. Because of the scale of the wheel, the kind of å²¢ å²¢ æ°² æ°² æ°² æ°² 诘å›å©Šè‰¿ 诘å›å©Šè‰¿ 讨 讨 讨 讨 讨 讨 åˆ åˆ åˆ åˆ åˆ åˆ åˆ åˆ åˆ åˆ åˆ åˆ åˆç½´ä¿šé¾‹ 罴俚龋 罴俚龋 沟 è‹« è‹« è‹« è‹« è‹« è‹« è‹« è‹« è‹« è‹« . . . .
The smooth condition of the bearing in the long-term stop operation is poor; the working environment of the bearing is abnormally bad. The aircraft of our hospital is mainly used for flying education practice; the number of landings is high; the frequency of impact load is high; the airport environment is poor. The characteristics of the aircraft bearing traces are analyzed; there have been multiple failure modes in the past. Each failure mode has been attacked on other bearings; it is often attacked in a single way; rust and reddish brown materials indicate excessive bearing Wear; Fe2O3 fine particles; there are signs of fatigue and shedding. Because of the large gap in the future; the stick is severely skewed; the sliding conflict occurs; a lot of heat is attacked; and the metal is carried and adhered; the outer race has a stick spread. The skewed indentation is marked; the bearing receives a large impact load in the future due to the skew of the roller; the plastic deformation occurs. According to this situation, a reasonable repair plan is drawn up.
1 punctuality to make smooth grease; time distance is the calendar time for half a year; the minimum limit ensures the smoothness and sanitation of all aircraft axles in the fleet; also check the bearing situation. Theoretical proof; this is very useful The lowest cost method.
2 Strengthen the protection and protection of the aircraft at the time of parking. The protection of the bearing at the time of parking is an important project of the working aircraft. When operating the aircraft; drag the aircraft; make the bearing smooth.
3 A more comprehensive view of the bearing timing; a consistent view of the specification; once the bearing is found to be ineffective; timely replacement.
The main points of view of the common faults. The number of aircraft life is now in the old age; fatigue failure is very significant; very simple to find the outer raceway showed fatigue and shedding. From the previous analysis we can know; the outer raceway fatigue life The number is larger than the inner raceway; if the outer raceway is also exhausted; it is clear that the fatigue life of the bearing has been fully reached; the bearing and the hub should be replaced in time. For example; strengthen the view of the outer ring seat. Although it does not belong to the bearing; However, it is made of aluminum alloy with the wheel hub; it is affected by the acid substance oxidized by the smooth grease; the corrosion is much more severe than other places. Because of the landing; the effect of accelerating the impact load; the simple break; the looseness of the bearing outer raceway The middle line is out of tune; it has a great influence on the bearing operation. For example, because the frequency of flying and falling is high; strengthening the view of deformation failure is also a key point; the unique aircraft attack is heavy and future.
(c) Precautions in theoretical work.
In addition to the inspection and protection of the bearings in the theoretical operation, there are some unique things to note:
1 pair of bearings should be completely hygienic before smoothing; do not leave residue. Because the bearing is close to the ground; the bearing is very simple with foreign impurities; there are also worn products and acid substances smoothed by oxidation of grease; and moisture must be To remove the cleanliness, because the bearings are removed in a volatile solvent; the moisture will quickly condense on the outside; it will be difficult to volatilize after being greased; and it will cause corrosion to the steel; therefore, the water should be blown off before smoothing.
2 to prevent pollutants. Bearings should be viewed and protected on a clean workbench; do not be on the ground; prevent foreign matter from entering. Also prevent the bearings from being beaten.
3 correct smooth. Use the aircraft manufacturer's regular trademark and grade of smooth grease; the amount of smooth grease should account for 1/3 1/2 of the shell space.
4 correct equipment bearing; do not force the equipment. Do not overtighten the equipment; but do not be too loose; prevent the axial clearance is too large.
5 Make the protection of the aircraft's shock absorption system; ensure that the aircraft has excellent shock absorption function. The aircraft is landing; the impact load and oscillation will be affected during the running process. It is very important to ensure proper shock absorption conditions.
5 correct use of the aircraft; foster students' safety awareness; prevent aircraft from falling.
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