The wind power gearbox is one of the core components of wind power equipment. Each gear in the gearbox is a heavy-duty gear. The components are mainly composed of gears, bearings and other parts. They are relatively uncomplicated. However, because they are sealed inside the box, when a failure occurs, secondary disasters will quickly occur among each other. Therefore, the failure information of the first failed part is often easily covered up or completely destroyed, making it difficult to identify and judge the first broken part, and even the accurate cause cannot be given.
In this case, first of all, the factors considered in the judgment of the failed part should be inferred based on systematic analysis, and the real cause should be found through comprehensive analysis through multiple failure analyzes of the same type.
1. Several main forms of gearbox failure
Wind power gear failures can be divided into two categories: wheel body failure and gear tooth failure. Wheel body failure generally occurs rarely, so gear failure usually refers to gear tooth failure. The so-called gear tooth failure means that during the operation of the gear, due to some reasons, the gear teeth change in size, shape, material properties, etc. and cannot complete the work normally. The main failure forms of gear teeth include breakage, pitting, wear, etc.
When a failure box fails, system analysis is often required, and specific problems are analyzed in detail to eliminate secondary damage and determine the cause of the failure based on the failure characteristics of the primary failed part. Based on the cause, gearbox failure is mainly divided into the following types of failures:
1, broken tooth
The probability of tooth breakage in gear meshing is low, local tooth chipping is slightly more common, and there are very few cases where all teeth are sheared off at the same time. This situation is not mainly a design problem, but is often related to manufacturing. If it is a design problem, it will not pass the factory test at all.
The main possible reasons for the partial tooth chipping of the gear are that the core is too brittle, the impact resistance is poor, and the straightness of the axis between the meshing gear teeth is reduced, resulting in excessive local meshing stress on the gear teeth caused by deflection, resulting in local fatigue broken teeth. The other is hydrogen-induced cracks on the tooth surface caused by carburizing and quenching. These cracks may lead to tooth breakage.
Gear bald teeth (the phenomenon is that the root is broken and the cross section undergoes severe plastic deformation) is mainly caused by the soft core of the gear teeth and insufficient hardness. When the safety factor is high, even if the core is soft, it is more difficult to occur. Sometimes it appears in the form of local fatigue broken teeth. However, the gear's ability to withstand instantaneous large stress impacts is significantly reduced, and accumulation of plastic deformation of the gear teeth will occur, leading to final bald teeth.
There are two types of gear tooth fractures. One is fatigue fracture: when the gear is transmitting power, it is equivalent to a cantilever beam. The bending moment stress on the tooth root is the largest, and there is a very large stress concentration at the transition fillet of the tooth root. When transmitting load, the bending moment on the tooth root changes with the change of the meshing point. Long-term alternating load causes fatigue cracks to occur at the stress concentration of the tooth root. As the number of load repetitions increases, the cracks continue to expand, eventually causing the gear teeth to break.
The other is overload fracture: a short-term overload or excessive impact load causes the gear teeth to suddenly break.
In order to improve the breakage resistance of gear teeth, the following measures can be taken:
1) Select appropriate tooth width and module to ensure tooth strength;
2) using appropriate materials and heat treatment methods;
3) increases the tooth root transition fillet radius, reduces the tooth surface roughness, and strengthens the tooth root surface.
2, wear
Normal wear is very small. Meshing gear flanks have involute contact with almost no relative sliding; with lubrication, normal wear in accurately made gears is minimal. Extended, severe loss of lubrication can cause dry friction in meshing gears and bearings, but this is rarely seen and is primarily a management issue. Abnormal wear is a technical issue: binding greatly increases wear as sliding replaces rolling. For example, if the oil rapidly turns black but analysis shows no serious deterioration and only increased iron particles, consider abnormal binding, mainly in the bearings and less commonly in the gears. Abnormal wear often relates to failure to establish an effective oil film; increased wear debris and abnormal oil viscosity are also associated. Oil deterioration or increased water and other gear-corroding constituents can likewise increase gear wear.
During gear transmission, the gear teeth are subject to load, and there is relative sliding between the two teeth in contact at the same time, so wear occurs. If the tooth surface wear rate is in line with expectations, it is normal. Normally worn tooth surfaces are bright and do not affect the normal operation of the gear. When the tooth surface is severely worn, the involute profile is destroyed and the tooth side clearance increases, resulting in unstable transmission, impact and noise, and even tooth breakage.
Wear can occur for many reasons:
1) Relative sliding between the working tooth surfaces of gear teeth;
2) Metal particles, dust, pollutants, etc. enter the meshing area;
3) Poor lubrication.
To reduce tooth surface wear, you can:
1) improves tooth surface hardness;
2) reduces tooth surface roughness;
3) adopts a well-lubricated closed transmission.
3, tooth surface bite
If substantial flaking and scratching occur on the upper part of the tooth contact surface when gears in a gearbox mesh near the tooth tips (usually above the pitch circle), consider machining accuracy, the meshing clearance established during assembly, and component deformation. Locate machining and installation information in the technical documentation and combine it with measurement results to determine the actual cause. When local component deformation causes tooth-surface scuffing, the cause can be inferred from the symmetry and distribution pattern of tooth damage. Also examine whether tooth-surface hardness is sufficient and whether large gears contain soft spots or soft zones (ingot-type segregation may already exist in the cast blank and may escape quality inspection).
In high-speed and heavy-load gear transmission, the instantaneous high temperature generated due to high sliding speed will cause the oil film to rupture, causing the welding phenomenon between the tooth surfaces. The welding joint will be torn, and groove marks will be produced on the tooth surface along the sliding direction. This kind of gluing is called thermal gluing. In low-speed gear transmission, it is not easy to form an oil film and the friction heat is not large, but cold adhesion may also occur due to heavy load. This kind of gluing is called cold gluing.
Measures to reduce gluing:
1) Appropriately increase the gear surface hardness;
2) reduce gear surface roughness;
3) The two gears are made of different materials.
4, bearing stuck
Insufficient assembly clearance and small foreign particles can cause brief bearing jamming. Inadequate replenishment of lubricating oil can cause a marked rise in bearing operating temperature and abnormalities such as overheating damage. Oil starvation can cause thermal damage and plastic deformation in bearings, appearing as increased operating noise from the gearbox. In such cases, severe burn marks generally develop during the final stage of rapid failure, potentially obscuring the characteristic evidence needed to identify the failure. If noise increases, stop the machine promptly to inspect components for damage, determine the exact cause, assess the damage and clean the surrounding environment.
5, fatigue erosion
Fatigue erosion of planet gear, sun gear and internal ring gear. The phenomenon is that there are a lot of pits on the tooth surface and a lot of iron filings in the lubricating oil. This failure mode is often related to factors such as abnormal meshing clearance, degeneration of lubricant, more fine metal chips in the meshing gap, local meshing stress higher than the bearing capacity of the gear teeth (slight overload), improper heat treatment of gears, etc.
Bearing raceway erosion. Phenomenon: pockmarks and black oil. Pieces of raceways fall off. The main cause is overload. The support oil film is damaged by small foreign objects in the raceway, forming point-like local severe overloads, which is often prone to fatigue pitting corrosion behavior (denudation caused by thermal burns). In addition, fatigue pitting corrosion behavior can also occur when parts are improperly heat treated and some comprehensive properties are poor.
6, planet wheel cracked
Cracking in planetary gears is primarily a bore fatigue problem. Axial cracking in carburized gears is often associated with fatigue, which generally occurs in softer regions rather than in regions of high hardness. The gear bore fitted to the bearing outer ring is therefore often the origin of fatigue failure. Fatigue is frequently related to insufficient overall component stiffness. An excessively thin rim wall provides inadequate support. When the peak external alternating compressive force is too high, substantial elastic deformation occurs, momentarily reducing bearing clearance and causing brief binding. This produces short-duration, large-displacement sliding friction between the gear bore and the bearing outer ring, forming built-up deposits between them. These deposits generate high tangential tensile stress in the local area of the gear bore, creating a stress concentration. Once fatigue cracking initiates in axial microcracks, such as those at the bottom of plowed grooves, the stress concentration and normal alternating forces cause the cracks to propagate, ultimately leading to fatigue fracture of the gear.
The following are several illustrations of planet wheel bore fatigue:
2. Structural elements of gearbox
The main components in a gearbox are the meshing gears and the bearings that enable smooth rotation. Both gear-tooth meshing and contact between bearing balls and raceways involve Hertzian contact. In theory this is point or line contact; in practice it is elastic, nonuniform area contact. Relative component motion requires almost no relative sliding and only elastic deformation. Plastic deformation or severe wear on gear-tooth surfaces or bearing components indicates that they have departed from their normal loading conditions.
A speed-increasing gearbox necessarily contains gears and bearings, together with fixing and auxiliary components. The gears include planet gears, a sun gear and an internal ring gear. The outer ring gear remains stationary relative to the gearbox, while the planets both orbit and rotate. The central sun gear only rotates, but floats among several planets. Its straightness and the parallelism of the gear centerlines change within a certain range during operation. This creates the mechanical loading environment of the components. When failure occurs, first analyze the structure of the component system and its loading conditions.
3. Appearance structure and force analysis
Helical and spur teeth each have distinct characteristics. Helical teeth produce relatively little meshing impact, distribute operating loads more evenly and smoothly, and generate less meshing noise. However, this structure produces a circumferential shear-force component that must be counteracted by thrust bearings or other components, increasing structural complexity. Spur teeth are noisier during meshing. Compared with helical teeth at the same meshing clearance, they experience greater instantaneous contact impact between teeth and thus produce more noise, but almost no circumferential force component. Each has advantages and disadvantages, so neither is substantially superior overall.
When failure occurs, the force exerted on the gear itself and the surrounding environment, as well as traces of damage to the surrounding environment, etc. should be considered.
There must be fatigue problems for high-frequency reciprocating and alternating force-bearing parts. Gearbox design mainly considers the anti-contact fatigue factor of gear teeth. Therefore, contact fatigue is often a key direction considered by designers. However, gear tooth surface contact fatigue is sometimes related to the accuracy of hot and cold processing. The appearance processing can be inspected through a variety of means, while the internal heat treatment quality can only be found through tooth shape samples, first inspection, random inspection, etc., which is relatively difficult and prone to missed inspections.
When the tooth surface carburization forms angular block carbides distributed along the crystal, poor contact fatigue resistance is caused by quenching soft spots, etc., which may not necessarily exist everywhere on the gear surface, and sometimes exist locally, resulting in missed inspections. This problem and risk can be solved by increasing inspection and inspection frequency.
Load analysis should consider more than steady-state meshing forces. If the safety factor is too low, the design may meet steady-state requirements but have limited resistance to abnormal transient impacts or additional loads, increasing failure probability. System impacts relate to external shock loads, flexible cushioning design and rotational inertia. Gearbox impacts from emergency braking or tangential wind must be considered. Impact forces can sometimes be several times steady-state forces. Although brief damage may not immediately cause failure, its accumulation substantially contributes to failures such as gear cracking and broken teeth.
4. Improper material selection and heat treatment
Material selection is generally based on the parameters in the material manual and general industry experience. There are relatively few problems. People are generally very careful when using new materials. They dare not apply them in large quantities after a large number of experimental verifications. Generally, there will be no problems. However, sometimes the wrong materials are used. This is generally related to lax control of raw materials entering the factory or confusion in storage and transportation, and is a quality management issue.
Abnormal tissue conditions caused by improper heat treatment of parts, and problems caused by insufficient inspection or identification errors are relatively common. Mainly reflected in the fact that the factory only checks the hardness, and the metallography only checks the tooth shape. The judgment and identification of part of the metallographic structure are often ignored. The tensile impact only checks the tooth root, and other positions are not paid much attention to. This often results in the inability to fully control the heat treatment.
Repeated failures have shown that bore fatigue or tooth fracture can readily occur when the heat treatment quality is acceptable but the designed rim thickness is too small, or when slow quench cooling produces large amounts of non-martensitic structures, leaving the gear relatively soft through its wall thickness and creating excessive upper bainite with poor mechanical properties. In comparison, large angular blocky carbides and hardness gradients at the tooth surface are relatively easy to detect and are generally revealed by tooth-profile specimens. However, indicators such as tooth-surface carbon and hydrogen content often receive insufficient attention, which can sometimes lead to hydrogen-induced delayed cracking of carburized tooth surfaces. Penetrant inspection should be performed after carburizing, quenching, and tooth grinding to detect microscopic cracks on the tooth surfaces. Undetected cracks may cause tooth fracture and even serious secondary damage, making it impossible to identify the true cause of the accident accurately.
(The carburizing process will also see hydrogenation at the same time, and hydrogen-induced cracking will occur due to the weakening of grain boundaries caused by incomplete dehydrogenation in the later stage)
5. Operation management
This issue is relatively difficult to control because many people are involved and their professional specialties vary considerably. Some technical procedures used in operational management should be developed jointly by design personnel (experts) and on-site maintenance personnel (experts). As issues are identified through operational management by the various parties, items should be promptly updated, added, or removed to create operating and maintenance procedures better suited to the actual conditions. Operating records and log management should also be used to compile process data. Together with the system's own automatic monitoring data, these records greatly assist incident investigations, helping to identify fault types quickly and providing substantial support for early assessment of failure causes.
6. General common sense of troubleshooting when finding faults
First, make quick decisions to prevent secondary disasters from continuing to occur;
Secondly, record all possible phenomena of the accident, which can be in the form of written records, images, audio, etc.;
Thirdly, protect the damaged section from contact with hot, humid and oxidizing environment;
Finally, actively search for information such as gearbox documents and operating data for future reference.
Source: Internet