8. Gear Surface Treatment / Surface treatment when properly applied in accordance with usage conditions will improve the capabilities of gears.
Surface treatments are performed to make surface condition improvements on materials. The following are typical reasons for surface treatments, providing rust prevention and inhibiting abrasion by friction, to improve overall quality control.
- For corrosion resistance / For rust prevention
- For wear resistance
- For improvement in surface roughness
(Smooth surface) - For improvement in appearance
- Others
(Improvement in fatigue strength, etc.)
Electro-galvanizing
A typical plating method applied to prevent the rusting of steel. By applying a chromating process, the quality of the appearance also improves. Thickness of plating film is generally between 2 to 25 μm.
- Uni-chromate
Silver white in color with slight bluish tint. The rust preventative properties are less than electro-galvanizing but the price is lower.
- Chromate
It has a multi-color appearance consisting of red, yellow and green. It has more corrosion resistance than Uni-chromate.
- Black-chromate
The color is basically black, but it looks slightly red depending on product shapes. It has the most corrosion resistance of all 3 types of electro galvanizing.
Electroless Nickel Plating
A plating method produced without the use of electricity. It inhibits corrosion / wear resistance. This plating is suitable for products that have; uniform thickness of plating film (3-10 m), complicated shapes, or high-precision is required.
Black Oxide Treatment
A treatment from alkaline black oxidizing.
Applying a 140°C heated, strong alkaline processing liquid, the material is blacken by a chemical reaction within the steel. It is effective for rust proofing. On the surface it makes a ferrosoferric oxide film, a thickness that is less than 3 μm.
Raydent Treatment
Raydent is a registered brand of Raydent Industrial Co., Ltd. Expect excellent rust prevention performance from this treatment. The color is black and the processing is similar to plating, as it makes a Raydent film (1 to 2 μm) on the surface, which is extremely strong and will not separate.
Phoshate Treatment
A treatment from an Iron phosphate type coating. An Iron phosphate type film is a thin amorphous film used as a base coating for painted interior products.
- PALFOS M
A treatment with manganese phosphate that produces a plating thickness approximately 3 to 15 μm. Used as a rust prevention film and also applied to sliding components, as it is wear-resistant.
Solid Lubrication Treatment
A dry-coating spray, very useful as a solid lubrication treatment, where direct application of lubrication is not possible. Achieved by spraying on tooth areas, it allows the lubricant agent to adhere and dry. It also allows compounded molybdenum to form a disulfide metal texture, to discourage corrosion.
WPC Treatment
WPC is a treatment, effective in encouraging fatigue strength and reducing metal friction. This treatment helps improve fatigue strength, but does not improve bending strength. Applied by spraying the gears in very small amounts, 40 to 200 μm, at the speed of 100 m/sec or more. By using this application, the process instantaneously generates heat and melts the metallic crystal, creating a fine coating from rapid cooling, Usually involves very little dimensional change; expected change would be within 1 to 2 μm.
Related article – Work hardening of metal and surface treatments of gearing
Once metal material is plastically deformed, it becomes less deformable compared to before deformed. For example, after steel plate is bent by a press, you will need more power to bend the same place to the same direction again. This property is called “work hardening”.
The composition of the metal atoms causes work hardening. Before explaining causes of work hardening, let me show you the composition of the metal atom and the property of metal.
Metal atoms have special bond called “metallic bond”. With covalent bonds or coordinate bonds of non-metal material, valance electrons are fixed to atoms. With metaling bonds, however, valance electrons of each atom are shared between all atoms allowing valance electrons to move freely between atoms. The electron which moves freely is called “free electron”. This free electron gives metal luster, high conductive property, heat conductivity, malleability, and ductility.
Metal luster is brought by free electrons which reflect light. Free electrons conduct electricity and heat by moving in metal. Free electrons are also associated with plastic deformation (property of extending and deforming when external force is applied), or malleability and ductility of metal.
For example, rock sugar breaks into pieces if external force is applied. It occurs when bond of atoms of sugar (glucose) are partially cut by external force and crystals are broken apart. In contrast, metal bends or extends (plastically deformed) when external force is applied. This happens when the metal crystal is about to separate due to external force, the free electrons enters the part and the crystal structure of metal slides. In short, the free electrons act as glue so that crystals can move without breaking atomic connectivity. That is the mechanism of malleability and ductility of metal.
However, movement of free electrons or crystals is partly blocked due to sliding surfaces’ intersection. Imagine that tangled thread gets more densely entangled when tugged hard. Similarly, the free electrons and crystals cannot move any more when their movement is blocked. This is work hardening.
Work hardening is used as the method to harden metal other than heat treating, and sometimes applied to strengthen the tooth flank of the gear.
Shot peening is another method used for work hardening gears. This method works by spraying tiny steel balls on the finished gear surface like sandblasting to form a layer of slight plastic deformation. This method is used for small gears of automobile transmission.
Pitting occurs on the tooth surface due to metal fatigue when the gear of automobile transmission repeatedly transmits high load. At the beginning, the pitting appears as a mass of small dents on the tooth surface. As the pitting progresses, the gear surface may flake off or the gear tooth may break. Hardening the tooth surface of the gear effectively prevents pitting, and shot peening which causes work hardening on the surface layer, can be used as an effective measure against pitting.
相关链接 - 齿轮的表面处理