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How Do We Design Shafts in Mechanical Design?

by: May 08,2024 35 Views 0 Comments Posted in Engineering Technical

A shaft is a cylindrical object that goes through the middle of a bearing, wheel, or gear, but a few are square. A shaft is a mechanical part that supports rotating parts and rotates with them to transmit motion, torque, or bending moment. It is usually a metal round rod, and each section can have different diameters. The parts that rotate in the machine are installed on the shaft.

1. Classification of axes

There are many types of shafts, which can be divided into straight shafts, crankshafts and flexible shafts according to the shape of the axis. Crankshafts are often used in reciprocating machinery (such as internal combustion engines, air compressors, etc.), and flexible shafts can flexibly transmit rotational motion to the required position and are often used in medical equipment. In this article, we only discuss straight shafts.

Straight shafts can be divided into three categories according to their load-bearing conditions: rotating shafts, spindles and transmission shafts.

Rotating shaft: It bears both bending moment and torque when working. It is the most common shaft in machinery, such as the shafts in various reducers;

Spindle: Used to support rotating parts and only bear bending moment without transmitting torque. Some spindles rotate , such as the shafts of railway vehicles, etc., and some spindles do not rotate, such as the shafts supporting pulleys, etc.;

Transmission shaft: It is mainly used to transmit torque without bearing bending moment, such as the long optical shaft in the crane moving mechanism, the drive shaft of the car, etc. The material of the shaft is mainly carbon steel or alloy steel, but it can also be made of ductile iron or alloy cast iron. The working capacity of the shaft generally depends on the strength and stiffness, and also depends on the vibration stability when the rotation speed is high;

2. Shaft material

Common materials for shafts are carbon steel and alloy steel. Carbon steel is cheaper than alloy steel, less sensitive to stress concentration, and has better mechanical properties, so it is widely used.

The blank of the steel shaft is generally made of rolled round steel or forgings. The internal structure of forgings is relatively uniform and has good strength, so important shafts should be forged blanks.

3. Shaft structural design

The structural design of the shaft is to determine the reasonable shape and all structural dimensions of the shaft, which is an important step in the design of the shaft. It consists of the type, size and position of the parts installed on the shaft, the fixing method of the parts, the nature, direction, size and distribution of the load, the type and size of the bearing, the blank of the shaft, manufacturing and assembly process, installation and transportation, and the alignment of the shaft. deformation and other factors. The designer can design according to the specific requirements of the shaft. If necessary, several plans can be compared to select the best design plan. The following are general shaft structure design principles:

cross-sectional shapes with equal strength dimensions or large cross-sectional coefficients ;

Easy to accurately position, stabilize, assemble, disassemble and adjust parts on the shaft;

Adopt various structural measures to reduce stress concentration and improve strength;

Easy to manufacture and ensure accuracy;

4. Axial positioning and fixation of parts on the shaft

Commonly used methods for axial positioning and fixing of parts on the shaft include shaft shoulders, shaft collars, locking rings, sleeves, round nuts and stop washers, elastic retaining rings, shaft end retaining rings and conical surfaces.

5. Shaft maintenance

5.1 Correction of shaft bending deformation

When the deformation of the shaft is too large, it can be corrected by cold pressing or local flame heating. The supporting position during correction should be correct, and special attention should be paid to avoid stress concentration at the corners of the stepped shaft due to correction.

5.2 Repair of journal wear

Usually, grinding processing is used to eliminate the geometric error of the shaft, and then metal spraying or brush plating can be performed. In severe cases, it can be repaired by overlay welding or inserting. When inserting, the sleeve and the shaft have an interference fit.

5.3 Repair of splines, keyways and threads

the splines can be milled based on the worn splines. After the keyway is damaged, the keyway can be appropriately enlarged or the old keyway can be welded and plugged and equipped with a new key. When the threads on the shaft are damaged, overlay welding should be performed and the threads should be re-turned.

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