Shaft parts are one of the most commonly encountered components in mechanical systems. They serve as a support for transmission elements, transmit torque, and bear loads. Typically, shafts are rotating components where the length is greater than the diameter. They usually consist of outer cylindrical surfaces, conical surfaces, inner holes, threads, and corresponding end faces. The shape varies depending on the design, and shafts can be classified into several types, such as straight shafts, stepped shafts, hollow shafts, crankshafts, and more.

The ratio of the shaft’s length to its diameter determines whether it is considered a short or long shaft. If this ratio is less than 5, it is called a short shaft; if it exceeds 20, it is referred to as an elongated shaft. Most shafts fall between these two extremes.

Shafts are supported by bearings, and the portions that interface with the bearings are known as journals. These journals act as the reference points for the entire shaft. Due to their critical role, they require high levels of precision and surface quality. The machining requirements depend on the function and operating conditions of the shaft. Key aspects include:

1. Dimensional Accuracy: Journals used for supporting the shaft must have high dimensional accuracy (IT5–IT7). For journals that mount transmission components, the required accuracy is slightly lower (IT6–IT9).
2. Shape Accuracy: This refers to the roundness and cylindricity of journal surfaces, conical sections, and Morse tapers. These features must meet specific tolerances, especially for high-precision applications.
3. Azimuth Accuracy: The coaxiality between the journal of a gear and the bearing journal must be maintained to ensure smooth transmission and reduce noise. Typical radial runout for standard shafts is 0.01–0.03 mm, while high-precision shafts (such as main spindles) may require up to 0.001–0.005 mm.
4. Surface Finish: The outer diameter of a shaft typically has a surface roughness of Ra 2.5–0.63 μm. Bearings that work with the shaft require even finer finishes, often Ra 0.63–0.16 μm.

Blanks and Material Selection: The choice of blank material for a shaft depends on the application, production volume, available equipment, and design requirements. For simple shafts with small diameter differences, bar stock is commonly used. However, for stepped shafts or those with significant diameter variations, forgings are preferred. This not only saves material but also reduces machining time and improves mechanical properties.

Understanding the machining technology of shaft parts is essential for ensuring the performance and longevity of mechanical systems. Proper attention to dimensional accuracy, surface finish, and alignment will significantly impact the overall efficiency and reliability of the machine.

Related Bearing Knowledge:

• Tapered Roller Bearings and Pump Shaft Bearing Systems
• Common Classification Methods for Rolling Bearings
• Bearing Clearance and Its Importance
• Analysis of Sliding Bearing Anti-Friction Layer Plating Technology (Part 1)

This article was originally published on China Bearing Network.

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