In mechanical design, strength calculation is a key link to ensure that mechanical parts and structures operate safely and reliably under predetermined working conditions. It not only relates to the service life of the machinery but also directly affects the operation safety and economy of the equipment. Therefore, in the process of mechanical design, reasonable strength calculations must be made for the parts, and appropriate materials and structural forms should be selected accordingly.



One, Basic Concept of Strength Calculation



Strength refers to the ability of materials or structures to resist failure under load conditions. During the working process, mechanical parts often bear various loads such as tension, compression, bending, shearing, and torsion. Therefore, the task of strength calculation is to judge whether the part meets the strength requirements based on the load conditions of the part and the material properties.



Strength calculation usually includes two major categories: static strength calculation and fatigue strength calculation. The former is suitable for situations with stable and slowly changing loads; the latter is used to analyze the life and reliability of parts under alternating loads.



Two, Common Strength Calculation Theories



1. Maximum Normal Stress Theory (First Strength Theory)

It is believed that when the maximum principal stress of the material reaches the ultimate stress of the material, failure will occur. It is suitable for brittle materials such as cast iron.



2. Maximum Shear Stress Theory (Third Strength Theory)

It is believed that the main factor causing material failure is the maximum shear stress, which is suitable for the design calculation of plastic materials.



3. The Theory of Shape Change Energy (Fourth Strength Theory)

It is believed that material failure is due to the shape change energy reaching the limit value, and it is widely used in the safety evaluation of plastic materials in modern engineering.



4. Allowable Stress Method

In practical engineering design, the allowable stress method is usually used for strength checking. That is, calculate the maximum stress in the part and compare it with the allowable stress of the material. If the calculated stress is less than or equal to the allowable stress, it meets the strength requirements.



Three, Strength Calculation Examples of Typical Parts



Taking shaft-type parts as an example, their strength calculation usually includes the calculation of combined bending-torsional strength. First, calculate the bending moment and torque according to the loading diagram, then calculate the equivalent stress according to the third or fourth strength theory, and finally compare it with the allowable bending stress of the material.



For parts such as gears, bolts, and springs, there are also corresponding standard calculation methods and empirical formulas, such as the calculation of bending fatigue strength of gear root, and the calculation of preload and tensile strength of bolts.



Four, Application of Modern Calculation Methods



With the development of computer technology, finite element analysis (FEA) has become an important tool for mechanical strength calculation. It can simulate the actual loading conditions by establishing a three-dimensional model of the part, accurately calculate the stress distribution at each point, and thus more scientifically evaluate the structural strength. This method is particularly effective for complex structures or nonlinear problems.



Five, Conclusion



Strength calculation in mechanical design is a systematic and rigorous process that requires comprehensive consideration of mechanical principles, material properties, loading conditions, and manufacturing processes. Only by conducting accurate strength calculations and reasonable structural optimization at the design stage can we ensure that the mechanical equipment operates safely and reliably with stable performance. Therefore, mastering scientific strength calculation methods is an essential basic skill for every mechanical engineer.