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A lubricant is added between the two friction surfaces for relative motion to form a lubricating film, which separates the two friction surfaces, and the dry friction acts as friction between the lubricant molecules to control friction, reduce wear, cool down, and prevent rust. Buffering, vibration reduction, cleaning of dirt, transmission of power, sealing and leakage prevention can extend gear life. This method is called lubrication. When the gear is in operation, the tooth surface is subject to friction and wear due to relative sliding between the intermeshing tooth faces. For high-speed gear transmission, the friction and wear between the tooth surfaces is more serious, so lubrication is very necessary in the gear transmission. Lubrication is a key link in the gear train of metallurgical machinery, especially the gear transmission.
1 Lubrication state of the gear When the circumferential speed of the gear is v<12m/s, the gear teeth of the large gear are often immersed in the oil pool for oil lubrication. With the transmission of the gears, the lubricating oil is brought to the meshing tooth surface, and the oil can also be sucked onto the wall of the tank to dissipate or lubricate the bearing. In order to reduce the rotational resistance of the gear and control the temperature rise of the lubricating oil, the depth of the gear immersed in the oil generally does not exceed one tooth height; for the low speed stage of the multi-stage transmission, the maximum oil immersion depth should not exceed 1/3 of the large gear radius; For bevel gears, the oil immersion depth is 0.5 to 1 times the tooth width b. When the circumferential speed of the gear is v>12m/s, it should be lubricated with injection. The so-called injection lubrication is a lubrication method in which the lubricating oil is directly injected from the nozzle to the gear meshing portion with a certain pressure.
Lubrication of transmissions in mechanical equipment mainly includes lubrication of gears and bearings. The lubricating oil film established on the basis of comprehensive consideration of fluid dynamic effect, lubricating oil pressure-adhesive property and contact body deformation, and the lubricating state separated by the friction surface is called elastic hydrodynamic lubrication. According to the theory of elastic fluid dynamic lubrication, the tooth surface lubrication state is related to the contact stress of the tooth surface, the sliding speed and direction, and the lubrication performance. Since the gear transmission is characterized by short engagement time and simultaneous sliding and rolling, the sliding direction and size are sharply changed, the instantaneous pressure per unit area of ​​the contact surface is high, and the elastic deformation of the material is large. In this case, the film thickness ratio parameter λ for evaluating the effective lubrication of the gear is: λ = hmin / σ.
Where: hmin is the minimum oil film thickness when the gear is meshed, μm; σ(σ=(σ21+σ2)1/2, σ1, σ2 are the root mean squares of the two friction surface roughness, respectively, μm) is the synthesis of the surface of the two meshing gears Roughness, μm.
According to the test result of λ, the lubrication state of the gear is divided into three types: when λ < 0.4, the boundary is lubricated, the load is completely borne by the boundary film, and the tribological characteristics depend on the fixed surface property and the lubricant oil property.
When the gears are lubricated at the boundary, the surface of the gear teeth has more peak contact, which is prone to abrasion and adhesion (gluing). In order to prevent direct contact when the gears mesh, an oily agent and an extreme pressure agent should be added to the base oil to improve the performance of the lubricating oil to form an adsorption film and an extreme pressure lubricating film. When 0.4 < λ < 3, it is mixed lubrication, and as the value of λ increases, the proportion of the load on the oil film increases. When λ = 1, the boundary film bears a load of about 30% of the total load, and the tribological properties depend on the solid surface properties and the lubricant oiliness and viscosity. This lubrication state varies widely, some close to elastic hydrodynamic lubrication, and some close to boundary lubrication. When λ>3 ̄5, it is fluid lubrication, that is, complete oil film lubrication, the tooth surface is completely separated by the elastic fluid oil film, the friction of the tooth surface changes into the intramolecular friction inside the oil film, the friction coefficient is small, and the load is entirely borne by the oil film. The tribological properties depend on the viscosity of the lubricant, and the probability of damage such as tooth surface fatigue pitting, tooth surface gluing, and tooth surface wear is minimal, which is an ideal lubrication state.
2 Lubrication in the gear transmission and its impact 2.1 Lubrication against pitting corrosion and affecting the gear work, the contact stress on the working surface is a variable stress with time. Under the action of such high-variation contact stress for a long time, a small metal peeling may occur to form some fatigue shallow pits. This phenomenon is called tooth surface fatigue pitting. The degree of pitting and the time of occurrence depend mainly on the magnitude of the contact stress, the number of cycles of the load, the hardness of the material, the microscopic geometry of the surface, the lubrication state and the thickness of the lubricating film. Improve the hardness of the tooth surface, reduce the roughness value of the tooth surface, use the high viscosity oil and reasonable displacement, etc., can improve the ability of the tooth surface to resist fatigue pitting. High-viscosity lubricating oil has no effect on the penetration of cracks. At the same time, high viscosity is beneficial to the establishment of oil film and the increase of oil film thickness. The elasticity of oil can alleviate the impact and make the distribution of contact stress more uniform and reduce the maximum. The stress value enhances the pitting resistance of the tooth surface. Therefore, to improve the viscosity of the lubricating oil, the occurrence and expansion of surface fatigue pitting can be appropriately reduced.
Additives in lubricating oils, especially extreme pressure additives, often cause corrosion and accelerate pitting corrosion if not used properly. In order to prevent pitting, the composition and amount of additives, gear materials, contact stress, load properties, gear working speed and ambient temperature should be taken care of and should not be used casually.
Lubrication methods and oil supply also have a great impact on pitting. From the perspective of preventing fatigue pitting, the oil supply should not be excessive. If the oil supply is too much, some oil will be squeezed between the tooth surfaces of the mesh, resulting in local high pressure and increased tooth surface contact stress. At the same time, the amount of oil penetrating into the crack under high pressure is also increased, which promotes the occurrence and development of fatigue pitting. But in order to prevent adhesion, there should be sufficient amount of water. Therefore, the amount of oil supply should be considered comprehensively to achieve the best lubrication effect.
2.2 Lubrication against the surface of the tooth surface (adhesive) and affect the gluing is due to the complete loss of the lubrication film of the tooth surface, so that the metal of the meshing tooth surface directly contacts under a certain pressure, resulting in adhesion, while the relative tooth surface Movement causes the metal to tear off the tooth surface causing adhesive wear. Lubrication prevents the build-up of adhesion.
For mineral oils without oil and extreme pressure agents, the higher the viscosity of the oil, the thicker the oil film formed, the better the adhesion, the easier it is to prevent the direct contact of the tooth surface, the stronger the resistance to gluing; the addition of mineral oil Oily agents and composite mineral oils can be physically and chemically adsorbed to form a stronger oil film than mineral oil to prevent adhesive wear; for extreme pressure gear oils containing extreme pressure agents, chemical reactions with the gear surface. The inorganic covering film is formed to lose the chance of adhesive wear.
2.3 Effect of Lubrication on Abrasive Wear and Influence When sand, dust, iron filings, metal powder, etc. enter the meshing part of the gear teeth, the abrasive wear of the tooth surface will be caused. The lubricating oil does not improve the wear of the abrasive particles, and the oil can only wash away the foreign impurities from the tooth surface. If the wear particles are suspended in the oil, it will act as an abrasive and the wear of the abrasive particles will continue to develop.
In order to prevent abrasive wear, it is important to always pay attention to the cleaning and replacement of the lubricant. Oil bath lubrication uses a lower viscosity lubricant to facilitate the precipitation of abrasive particles. The lubricating oil is filtered and used, and the lubricating oil is sampled and tested regularly, and purified regularly.
3 Metallurgical mechanical gear transmission requirements and selection of lubricating oil performance 3.1 Requirements for lubricating oil performance Metallurgical enterprise equipment is complex, the operating environment is harsh, the overall working characteristics are: high transmission power, high tooth surface contact stress; low circumferential speed; The impact load is large, and it often bears a large overload load; the working environment temperature changes greatly; the dust is more. Therefore, it is necessary to improve the lubrication effect, adapt to the working conditions of the working environment, and ensure good transmission performance to extend the service life of the equipment. The requirements for lubricating oil performance of metallurgical mechanical gear transmissions are: (1) Adhesiveness is appropriate. The viscosity of the lubricating oil is mainly based on the pitch line speed and the force of the gear.
(2) Good anti-wear performance. Regarding the anti-wear properties of gears, it can be generally divided into 4 grades: first grade, pure base oil, no anti-wear additive, suitable for medium speed medium load gear transmission; secondary, base oil with oil additive, suitable for Medium-speed medium-duty gear transmission; three-stage, base oil with medium extreme pressure additive, suitable for medium-speed and high-load gear transmission; four-stage, base oil with all-in-pressure additive for high speed, high load and strong impact Gear transmission for loads.
(3) Regular inspection and timely replacement to keep the lubricating oil clean.
Lubricating oils for long-term recycling require anti-oxidant lubricants with deep refining and anti-oxidant; those with fire hazard require olefin-resistant or non-combustible lubricants such as phospholipid oil, mixed phosphoric acid oil or water-based emulsified oil.
3.2 Selection of Lubricating Oils The type and viscosity of lubrication can be appropriately selected by empirical formula and graph method, which is generally determined by trial and error in practice. Table 1 selects the type of lubricant according to the type of gear and the load conditions, and is for reference only.
4 Conclusion With the continuous advancement and development of science and technology, the requirements for the quality of metallurgical machinery and equipment control are also constantly improving. It can be expected that the lubrication of gear transmissions in metallurgical machinery will play an increasingly important role in the use of metallurgical equipment.
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