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[Abstract] Through the pitting corrosion phenomenon of the gears of Shigoma reducer, the mechanism and process of pitting corrosion of gears are analyzed, the influencing factors of pitting corrosion of gears of reducer are analyzed, and the corresponding solving measures are worked out to reduce pitting corrosion of gears of reducer and prolong the service life of reducer. .
0 Preface
Shigoma reducer is the most common form of damage in use. Gear is an important part of mixer. Gear is the heart of reducer. Gear damage is one of the main reasons for the failure of reducer. Gear damage forms include pitting corrosion, fracture, wear, gluing and plastic flow of tooth surface. In the process of using reducer, pitting corrosion of gear is the most common form of damage. The serious pitting corrosion of reducer gear will cause great vibration of equipment, increase oil temperature, aggravate bearing wear, so that the reducer needs to be overhauled for 2-3 years. Through statistics on the maintenance of Shigoma reducer in 2016 by the Ministry of Business. It can be seen that pitting corrosion of tooth surface is the most common form of damage in the use of speed reducer of Shigoma mixer.
1 pitting mechanism of gears
1.1 pitting concept
When a pair of gears engage, the contact stress between the two tooth surfaces changes cyclically at the contact point (see Fig. 1). If the contact stress exceeds the contact fatigue limit of the tooth surface material, after a certain period of time, micro-fatigue cracks will appear in the inner surface of the tooth surface. With the propagation and propagation of the cracks, micro-fatigue cracks will appear. The metal surface of the tooth surface will produce flake spalling to form pits (see Fig. 2). This phenomenon is called pitting corrosion [1]. When pitting corrosion occurs, the bearing area of the tooth surface decreases rapidly and the contact stress increases sharply, which not only aggravates the fatigue damage of the tooth surface, but also destroys the correctness of the meshing of the tooth surface and causes considerable dynamic load, which eventually leads to large flakes of the tooth surface and scrap.
1.2 pitting mechanism and process
Pitting corrosion of tooth surface is a kind of surface fatigue wear, which is the result of cracking and separating micro-patches under the action of alternating stress on the tooth decay surface during gear meshing. According to Clarenchensky's theory of wear and fatigue, the basic mechanism of pitting corrosion is as follows: due to the existence of surface roughness and corrugation, the contact between two teeth surfaces is discontinuous; the wear is due to the local deformation and local stress in the actual contact area; the material of some parts of the frictional surface is destroyed mechanically. The fatigue failure of materials will depend on the alternating load on the contact zone. The pitting process consists of three stages of development. One is the interaction of the two sides; the other is the change of the surface properties of contact materials under the influence of friction; the third is the destruction of the surface and the peeling off of wear emblems. Surface interaction is the most important stage in these three stages of development, and the dual characteristics of interaction and the discontinuity of contact must be taken into account.
Causes and Countermeasures of pitting corrosion
2.1 Effect of Manufacturing and Assembly Accuracy of Reducer
One of the reasons for pitting corrosion in the early stage of gear reducer is that the assembly accuracy is low, resulting in the reduction of the contact area of gears and local overload, which makes the actual contact stress greatly exceed the allowable contact stress of gear material. This is because the two centerlines of the gear pair are not parallel or the cross deviation is too large, and the tooth orientation error is too large during gear processing. The inappropriate selection of gear material and heat treatment hardness results in low hardness of tooth surface, which can not meet the needs of operation, and is also an important cause of pitting corrosion.
2.2 Effect of Installation Accuracy
The non-standard alignment of the rear axle of the reducer and the motor causes the non-concentricity of the motor coupling and the input coupling of the reducer, resulting in uneven force on the input gear of the reducer, overload of the transmission gear on one side, pitting corrosion on the tooth surface, and failure of the reducer gear.
2.3 effect of load
There are two main sources of load for reducer of mixing equipment: one is that the load mainly comes from mixing and blade load, the medium density in the kettle is too high and the liquid level is too high, which will increase the mixing load and cause pitting corrosion of gears; the other is that the vibration of transmission shafting causes the load to increase, because the clearance between bearing and bearing hole becomes larger and the transmission shaft becomes larger. The bearing stiffness and other reasons lead to abnormal vibration of shafting and pitting corrosion of tooth surface.
Effect of 2.4 Lubricating Oil
Lubricating oil can reduce wear and tear in mechanical transmission, reduce vibration, lubricate and improve the life expectancy of equipment. Therefore, in the daily maintenance of gears, creating good lubrication conditions and adopting reasonable lubrication technology to reduce and reduce the pitting corrosion and wear of gear teeth surface are the topics that field technicians must pay attention to [3].
Effect of 2.4.1 Lubricating Oil Quality
The primary condition for pitting of gear teeth surface is micro-crack. Then during the working process, the crack is repeatedly loaded, and the crack expands continuously, leading to pitting corrosion. The crack originates from the surface produced by mechanical processing, or the uneven structure of material (including slag inclusion, pore and hard particles) and the excessive local shear stress. On the surface (below the surface), the crack source is repeatedly loaded, and the crack is gradually pulled apart, so that it extends to the surface and falls off. The viscosity of lubricating oil has a great influence on pitting corrosion. The thinner lubricating oil is easier to penetrate into cracks than the viscous lubricating oil in the working process, which results in crack propagation. The lubricating oil with high viscosity has strong cushioning and vibration absorption ability and strong load-bearing capacity. Lubricating oil with high viscosity can reduce shock, delay crack propagation and enhance pitting corrosion resistance of gears. If low viscosity lubricating oil is added, pitting corrosion of reducer gears will be easily caused. The phenomenon of lubricating oil emulsification caused by water entering the reducer box often occurs in production and use. It is found that the main source of water entering the reducer is the damage of oil seal at the input end of the reducer. After water enters the gearbox along the input shaft of the reducer, the lubricating oil emulsifies and deteriorates. It reduces the viscosity of lubricating oil, reduces the carrying capacity, and causes pitting corrosion of tooth surface.
Effects of 2.4.2 Additives
Adding a certain proportion of oil additives and extreme pressure additives to lubricating oil can reduce friction, increase the strength of boundary oil film, and improve the pitting resistance of gears. But there are corrosive substances in some additives, which will lead to metal tooth surface rust and cracks, and the presence of bubbles in additives will also cause pitting wear and tear off of tooth surface.
Effect of 2.4.3 Oil Temperature
Excessive oil temperature will reduce the viscosity of lubricating oil, which is not conducive to the formation of oil film between the gear teeth. The lubricating film on the gear meshing surface becomes thinner, and the load-bearing and anti-wear ability of the tooth surface decreases, resulting in pitting corrosion.
3. Measures to reduce pitting corrosion
(1) In the selection of reducer, appropriate gear material and heat treatment hardness are selected according to the load of equipment, and the installation of reducer is supervised by professional technicians to ensure the installation accuracy of gears and the contact accuracy of gears, so as to reduce pitting corrosion.
(2) When the stirring shaft is installed, the leveling of the stirring tank rack and the lead Brix and runout of the stirring shaft are strictly controlled. The lead Brix < L/1000 and the runout < 5mm reduce the runout and uneven load of the stirring system.
(3) Segmental installation of reducer and motor, alignment between reducer and motor, reducer and driven equipment according to GB50231 of General Code for Construction and Acceptance of Mechanical Equipment Installation Engineering, so that both ends jump no more than 0.1 mm, ensuring the installation accuracy of reducer.
(4) For the mixer reducer, reasonable control of gum spraying, control of material density in the autoclave, stabilization of liquid level in the autoclave, reduction of load and change rate of alternating load, thereby reducing pitting corrosion of gears.
(5) Reasonably select the material of bottom bearing and middle bearing, change the material of tin bronze and tetrafluoroethylene to tetrafluoroethylene+nylon, reduce the wear of bearing and stirring shaft and repair the gap between bearing and bearing seat in time to reduce the load increase of reducer caused by vibration, so as to prolong the service life of gear. Life.
(6) Choose the lubricant with proper viscosity, periodically check the quality of the oil, and replace it in time when problems arise. The connection between motor and reducer and the encrypted sealing glue of the oil seal cap of the input shaft of reducer prevent rainwater from entering.
(7) The mixer reducer is cooled by circulating water. The calcification in the circulating water will deposit in the inner cooling pipe, which will result in poor heat transfer efficiency of the inner cooling pipe and heat exchanger, and even blockage, which will cause excessive temperature of lubricating oil. Therefore, chemical cleaning of the inner cooling pipe of the reducer should be carried out regularly to ensure the heat removal effect of the inner cooling pipe and avoid temperature rise. High causes pitting corrosion of reducer gears.
(8) After choosing the suitable lubricating oil, the reducer can mix some additives of anti-wear and anti-pressure in proportion to enhance the lubrication effect of reducer gears and reduce pitting corrosion of gears.
4 Conclusion
There are many factors affecting pitting corrosion of Shigoma reducer gears, which are not only related to the manufacturing accuracy of the reducer itself, gear material and hardness ratio, but also closely related to the installation accuracy of the reducer and mixing system, load distribution, cooling and lubrication. Therefore, in the future, the equipment installation and maintenance of the reducer should be improved strictly according to the above measures, so as to reduce the pitting corrosion of the reducer gears and prolong the service life of the reducer.
