There are various types of titanium gear damage, including fracture, surface fatigue (pitting, spalling), plastic flow, galling, gluing, corrosive wear, scorching, etc. In order to prevent gear failure, people will first consider the problem from a mechanical point of view, such as selecting excellent gear materials, reasonable gear design parameters, advanced heat treatment technology and high-precision machining technology, which are all essential and effective. .
It cannot be ignored that reasonable lubrication also has obvious effects on avoiding, mitigating and delaying gear failure. Therefore, gear lubricant is regarded as one of the components of gear mechanism.
1. Peeling
The spalling mechanism is the same as that of pitting corrosion. The stress caused by improper heat treatment or the larger metal chips caused by subsurface cracks is removed from the hard tooth surface or case hardened tooth surface to form spalling. It is caused by material defects, overloading or other usage problems. Pitting corrosion and spalling are often referred to as metal surface fatigue, and the viscosity, type and additives of lubricating oil have an effect on this wear pattern.
Viscosity has the greatest influence on the fatigue life of titanium gears. Most agree that the higher the viscosity, the longer it takes to develop fatigue. This is because the oil film is interposed between the tooth surfaces in the rolling contact, so the pressure distribution becomes smooth; the micro-convexities on the rolling surface are covered by the presence of the oil film; the external load changes have a buffering effect and other reasons.
Different types of lubricating oils have a considerable impact on the generation of rolling fatigue. For example, naphthenic oil of the same viscosity takes longer to produce fatigue than paraffin-based oil. In addition, the molecular structure is related to the service life, that is, the oil with the same viscosity, the more rings in the molecule, the longer the service life. The polarity and activity of lubricant molecules increase, which generally shortens the rolling fatigue life. For example, if the OH (hydroxyl group) in the same alcohol is replaced by the polar COOH (carboxyl group), the fatigue life will be reduced by 1/4; the ester ratio Carboxylic acids and alcohols have long lifetimes. These are all reasons for the different effects of chemical surface activity on the generation and development of cracks.
The effects of additives are complex, with inconsistent results under different conditions. The results of the study with a rolling four-ball machine show that the addition of dibutyl phosphite and chlorinated paraffin reduces the life, while the addition of elemental sulfur increases the life.
Different types and amounts of additives have a considerable impact on the service life. Sulfurized terpenes and zinc dialkyldithiophosphates can significantly improve the fatigue life when adding about 2%. However, when the amount of addition increases, the effect of prolonging the life gradually becomes smaller; and when a certain amount of addition is reached, the life is reduced instead.
2. Plastic flow
This is the deformation of the tooth surface due to the surface stress exceeding the elastic limit of the gear material due to the heavy load. This is usually the case in softer materials, where the surface material may be squeezed along the tooth end face and tooth crest, eventually forming a burr on the tooth flank. Pitch line wrinkled protrusions or tooth root depressions also fall into this category.
If this kind of damage is caused by strong vibration or shock load, the high-viscosity lubricant has the effect of buffering the load, but this problem cannot be solved by changing the lubricant alone.
3. scratches
This is a type of abrasive wear. When hard particles larger in size than the thickness of the oil film separating the tooth surfaces enter the gear meshing area, the tooth surfaces are scratched in the sliding direction. These particles may be dust, sand, casting Scale, gear or bearing material, or any other wear debris, enter the lubrication system in various ways.
The above impurities can enter in the air through a loosely sealed cover or an open inspection hole, or it may be caused by impurities mixed in without careful cleaning of the box or rotating parts; wear debris may also be the product of gear wear, through experimental analysis The types of these particulate materials can be indicated.
Increasing the viscosity of the lubricant can increase the thickness of the oil film, which can reduce galling, but cannot cure it. The best solution is precision filtration of emulsifiers and improved maintenance conditions to remove abrasive particles. Once these issues are resolved, the damage to the gear surfaces will also stop.
4. gluing
Gluing is the damage caused by the melting of the metal caused by the breakdown of the oil film. If the degree of gluing is light, there are signs of tearing in the sliding direction of the oil, and if it is serious, the tooth surface is damaged and cannot be used continuously. The parts that are easy to cause gluing are the beginning and end of the meshing surface, that is, the tooth root of the pinion and the tooth top of the large gear, or the tooth top of the pinion and the tooth root of the large gear during deceleration. This is because the slip velocities are high in these places and there is generally a large force at the onset of engagement. There are many factors such as load, slip speed, friction coefficient, material, manufacturing error, stress concentration and lubricating oil, etc., which are very complicated for the conditions to produce gluing.
Heat weakens the oil film more easily than mechanical force. A considerable part of the gear load is transmitted by boundary friction, so the load and speed increase, the heat generation of the contact part also increases, and the risk of gluing is high. The heating of the contact part is proportional to pυT (p is the maximum contact stress, υ is the relative slip velocity of the tooth surface, and T is the distance from the node to the mesh). The moment the tooth surface meshes can reach a relatively high temperature, and the oil film rupture at the contact point is caused by this high temperature.
Viscosity has a great influence on gluing. The high viscosity of the oil is difficult to cause scratches, because the higher the viscosity, the easier the oil film is to form, and the thicker the oil film is, so that the proportion of the load in the field of fluid lubrication is larger. This is why oil with high viscosity has good anti-glue properties.
The type of lubricating oil and additives have a great influence on gluing, and extreme pressure oil has much better anti-gluing resistance than non-extreme pressure oil. Extreme pressure additives also vary according to their types and the amount added to the oil. Generally, those added in large amounts have good adhesion resistance. Oils with different extreme pressure resistance levels have different adhesion resistance. Oils with a high level of extreme pressure resistance have good adhesion resistance. The more extreme pressure agent added, the better the anti-scratch performance of gear oil.
The amount of oil and the method of lubrication also have a great influence on gluing. When the oil supply in the gear meshing area is insufficient, the gear heats up and the anti-glue performance decreases. As the amount of oil increases, the anti-glue load increases, and circulating oil injection is better than oil bath lubrication.
5. Corrosion and wear
Corrosion and corrosion wear can occur on lightly pitted or rusted tooth surfaces or on exposed unpainted metal surfaces. Corrosion can be caused by condensed water in the oil or water escaping from heat exchangers; it can also be caused by acids or corrosion additives in the lubricating oil.
Certain lubricating oil additives can prevent rust on the gear surface, thereby preventing corrosion. Others prevent the oil from oxidizing and forming acids. If the cause of corrosion is known to be external, it can be corrected.
6. Burnt
If the teeth of the gear are subjected to intense heat, the hardness of the material will decrease. The softened material will be removed quickly because it cannot withstand the corresponding load.
This phenomenon is characterized by the discoloration of the gear surface due to high temperature, which must be corrected if the heat source is external, and the lubricant type and lubrication method should be reconsidered if it is a friction problem.