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Structure parts and casting process of gear

Structure parts and casting process of gear

Issue Time:2021/07/24
Gear is a mechanical element with gears on the rim to continuously mesh and transmit motion and power.
Structure parts
Generally, there are gear teeth, tooth slots, end faces, normal surfaces, tooth top circles, tooth root circles, base circles and indexing circles.

Gear tooth
Referred to as tooth, it is each convex part used for meshing on the gear. These convex parts are generally arranged in a radial shape. The teeth on the paired gear contact each other, which can make the gear meshing and running continuously.

End face
It is the plane at both ends of the gear.

Normal surface
It refers to the plane perpendicular to the tooth line of the tooth.

Tooth top circles
Refers to the circle where the top of the tooth is located.

Root circle
Refers to the circle where the groove bottom is located.

Base circle
The generating line forming the involute is a pure rolling circle.

Indexing circles
It is the datum circle for calculating the geometric dimension of the gear in the end face.
Gear casting
Because most gears are made of cast steel. Here, I share some information about the production of gear castings and related heat treatment. The weight of gear castings usually ranges from a few kilograms to several tons.

Gear castings are usually made of high carbon cast steel, and some use alloy steel containing chromium, nickel and molybdenum to achieve high tensile strength. Generally, the physical requirements of large gears are lower than those of small gears.

As for the casting process, the floor forming process is usually applicable and can meet the normal requirements. As for cast steel gears, such as driven gears, gears and idlers, the floor forming process using quartz sand is a good choice. Why, because most parts of the gear need to be machined. Therefore, you do not need to use a higher casting process. In addition, the floor molding process using quartz sand is almost the only choice for medium and large steel castings.

With regard to heat treatment, of course, all steel castings must be standardized to eliminate internal pressure. Some parts of the gear casting can be welded. If the casting is welded by the foundry, the welding position must be annealed. If the hardness is very high after gear hobbing, you can anneal again to reduce the hardness and eliminate the internal hard spots. After machining and hobbing, the gear is quenched or hardened to improve the surface hardness of the gear teeth. For the pinion, you can carburize. For large driven gears, you can do surface quenching. The service life of gears without hardening treatment is very short, only a few weeks to a few months.

Because gear castings have higher requirements for materials, defects, processing and heat treatment. Moreover, the order volume of gear castings is relatively small. Therefore, many steel foundries are reluctant to manufacture.

Some gears are made by forging. The internal structure density of forged gear is better and the strength is higher. Forged gears can be used for more stringent working conditions. Cast gears have low strength, but are widely used in general working conditions. The cost of forged gears is high, while the cost of cast gears is relatively low. The buyer shall select the appropriate manufacturing process according to the cost and service conditions.
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