What are the contents of mechanical dynamics?

Mechanical dynamics is a science that studies the motion of machinery under the action of force and the force generated by machinery in motion, and designs and improves machinery from the perspective of the interaction between force and motion. The content of mechanical dynamics:

Mechanical dynamics is a subject that studies the motion of machinery under the action of force and the force produced by machinery in motion. The main contents of mechanical dynamics research can be summarized as follows.

I. * * Vibration analysis

With the high speed and heavy load of mechanical equipment and the light weight of structural materials, the natural frequency of modern machinery decreases, while the excitation frequency increases, which may make the running speed of machinery enter or approach the "* * * vibration zone" of machinery, causing strong * * * vibration. Therefore, for the supporting structures of high-speed mechanical equipment (such as high-speed belts, gears, high-speed shafts, etc.). ) Even the high-speed machinery itself must be checked for * * * vibration.

This check calculation is carried out in the design stage, which can avoid mechanical vibration accidents; When analyzing the fault, it will help to find the root cause of the fault and the troubleshooting methods.

Second, vibration analysis and dynamic load calculation

Modern mechanical design method is changing from traditional static design to dynamic design, and some special branches have appeared. For example, mechanical elastic dynamics is a specialized subject that considers the elasticity of mechanical parts to analyze the precise motion law and mechanical vibration load of machinery.

Third, the application of computer and modern testing technology.

Computer and modern testing technology have become the two wings of mechanical dynamics. The combination of them not only solves many problems that are difficult to be solved by traditional methods in vibration discipline, but also creates a series of effective practical technologies such as condition monitoring, fault diagnosis, modal analysis and dynamic simulation, which has become a very powerful modern means in production practice.

Various branches of mechanical dynamics have achieved fruitful results in the application of computers, such as MATLAB, AnAMS, CATIA, ANSYS and other large-scale simulation software have been widely used.

Fourth, vibration and isolation.

High speed and precision are important features of modern machinery and instruments. High speed is easy to cause vibration, but precision equipment often has extremely strict restrictions on its own and external vibration. Therefore, higher and higher requirements are put forward for mechanical vibration reduction and isolation technology. Therefore, the design, selection and configuration of vibration isolation equipment and the adoption of vibration reduction measures are also one of the tasks of mechanical dynamics.

Although mechanical dynamics has developed rapidly in recent years, there are still a lot of theoretical and technical problems to be explored, including the following aspects.

1, vibration theory problem

This kind of problem mainly refers to the nonlinear vibration theory problem. Nonlinear problems in engineering are often treated by simplified linearization or piecewise linearization on computers. This aspect needs further exploration.

At present, there is no unified and mature theoretical method for a large number of self-excited vibrations in engineering (such as conductor galloping, machine tool chatter, wheel vibration, cylinder and guide rail crawling, etc.). ), there are still many problems to be studied.

2. Virtual prototype technology

Dynamic simulation technology of mechanical system, also known as virtual prototype technology in mechanical engineering, is a computer aided engineering (CAE) technology that developed rapidly with the development of computer technology in 1980s. Using this technology can greatly simplify the development process of mechanical products, greatly shorten the development cycle of products, greatly reduce the development cost and cost of products, obviously improve the quality of products, improve the system and performance of products, and obtain optimized and innovative design products. Therefore, as soon as this technology appeared, people paid great attention to it, and various analysis softwares appeared one after another, such as MATLAB, ADAMS, ANSYS, CATIA, UG, Pro/E, SolidWorks and so on. For this work, there is still a considerable gap in China.

3. Study on vibration fatigue mechanism.

The fatigue failure of many mechanical parts is caused by vibration. How to combine vibration theory with vibration fatigue mechanism is still a hot topic.

4. Research on test technology theory and fault diagnosis theory.

The research on applicable, effective and cheap test and diagnosis equipment and technology is far from meeting the urgent needs of production.

5. Fluid-solid coupling vibration

When the fluid passes through the solid, it will excite vibration, and the vibration of the solid, such as conductor galloping, Karman vortex street vibration and bearing oil film oscillation, will in turn affect the flow field and flow pattern of the fluid, thus changing the vibration form.

6. Driving dynamics

For transportation machinery (such as automobiles, construction machinery, ships, etc. ), it is an extensive and important subject to introduce random vibration theory into its structural design, suspension design, seat design and vibration reduction design.

7. Micromechanical dynamics problems

Micro-machinery is not the reduction of macro-mechanical geometry in the traditional sense. When the characteristic size of the system reaches micron or nanometer level, many physical phenomena are very different from the macro world. For example, in micromechanics, the physical properties of the constituent materials themselves will change; The long-range effects of some microscale short-range forces and their surface effects will play a leading role in the microscopic field; On the microscopic scale, the friction problem of the system will be more prominent. The friction force is manifested by the interaction between molecules and atoms on the surface of the component, which is no longer generated by the positive pressure of the load. When the characteristic size of the system is reduced to a certain extent, the friction force can even be comparable to the driving force of the system. In the microscopic field, the influence of inertial force and electromagnetic force proportional to the higher power of the characteristic dimension L is relatively reduced, while the influence of viscous force, elastic force, surface tension and electrostatic force proportional to the lower power of the characteristic dimension is relatively increased. In addition, the deformation and damage mechanism of microscopic components is different from that of macroscopic components.

In view of the new characteristics of micromechanics research, the traditional theory and method of mechanical dynamics are no longer applicable. Micro-mechanical dynamics studies the constitutive relation of micro-component materials, deformation mode and damping mechanism of micro-components, elastic dynamic equations of micro-mechanisms, etc. The internal relations among molecular (or atomic) composition and structure of micro-component materials, elastic modulus and Poisson's ratio of materials, stiffness and damping of micro-components, and elastic dynamic characteristics of micro-mechanism are revealed, thus ensuring that the micro-electromechanical system can realize energy transfer, motion conversion, adjustment and control functions in a small space and achieve predetermined actions with specified accuracy. Therefore, the study of mechanical dynamics will achieve many innovative results, which not only has important scientific significance and academic value, but also has good application prospects.

The research methods of mechanical dynamics can be divided into two categories.

Dynamic analysis of (1) structure

For the forward problem of mechanical dynamics, dynamic analysis generally relies on a variety of dynamic analysis methods (such as modal analysis, modal synthesis, mechanical impedance analysis, state space analysis, modal perturbation method and finite element method) to establish the mathematical model of the structure or system, and then analyze the dynamic characteristics of the structure (such as dynamic simulation).

For the inverse problem of mechanical dynamics, the dynamic analysis usually begins with the dynamic experiment, and then the mathematical model of the structure or system is established according to certain criteria, and then the analysis is carried out by the method of parameter identification or system identification.

(2) Dynamic experiment

Structural dynamics experiments include modal experiments, mechanical environment experiments and simulation experiments. It is an indispensable link in the process of product design and production, which can not only directly evaluate the dynamic performance of products, but also provide necessary data for establishing a reliable mathematical model of dynamic analysis.