What Are The Classifications And Commonly Used Materials of Shafts?

A shaft is one of the essential components in machinery, used to transmit motion and power for various rotating transmission parts. Typically, shafts support rotating parts together with bearings and frames, and facilitate motion and power transmission through couplings or clutches. In shaft design, considerations must include the design requirements of bearings, couplings, frames, and transmission parts that make up the shaft system.

Classification of Shafts:

Shafts can be classified based on different load conditions, shapes, and application scenarios.

a) According to the load conditions:

• A shaft that only withstands bending moments without transmitting torque is called a stationary shaft (heart shaft). Stationary shafts can be further classified into fixed stationary shafts (which do not rotate during operation) and rotating stationary shafts (which rotate during operation). Stationary shafts are primarily used to support various mechanical components.

• A shaft that only transmits torque without bearing bending moments is called a transmission shaft. Transmission shafts primarily transmit power by bearing torque.

• A shaft that both transmits torque and bears bending moments is called a rotating shaft. Various transmission parts primarily transmit power through rotating shafts.

b) According to structural shape:

• Shafts can be classified as solid shafts, stepped shafts, hollow shafts, etc. Hollow shafts are usually designed for applications where the shaft diameter is large and weight reduction is required due to their complex manufacturing process.

c) According to geometric axis shape:

• Shafts can be classified as straight shafts and crankshafts, among others.

Common Materials for Shafts:

There are many types of materials used for shaft forgings. When designing, materials are primarily selected based on requirements for shaft strength, rigidity, wear resistance, and the heat treatment method used to achieve these requirements. Manufacturing process considerations are also taken into account. Since shafts are typically subjected to alternating stress during operation, the most common failure mode of shafts is fracture due to alternating stress. Therefore, shaft materials should have a certain toughness and good fatigue resistance, which are basic requirements for shaft materials.

Common materials for shafts include medium-carbon quality carbon structural steel. For shafts with lower loads or less critical importance, ordinary carbon structural steel can also be used. Medium-carbon alloy steel can be used for shafts under higher stress, limited dimensions and weight, and with specific requirements. Alloy steels are sensitive to stress concentration, so the structural shape of alloy steel shafts should minimize stress concentration sources, and low surface roughness values are required.

Cast iron has poor toughness, so it should be used as little as possible for shaft materials. However, for structurally complex and less critical shafts, ductile iron or high-strength cast iron can be used as shaft materials. Although materials with high strength limits also have slightly higher elastic moduli, the differences in elastic moduli among various steel materials are not significant. Therefore, selecting materials with high strength limits solely to improve shaft rigidity is inappropriate. Shafts are generally manufactured from rolled round steel or forgings through machining. For smaller diameter shafts, rolled round steel can be used. For larger diameter or critical shafts, forgings are commonly used for manufacturing.