Research on the Mass Balance of High-speed Cam Index

- Jan 26, 2021-

The cam index is a typical common mechanism. Because it can realize arbitrarily complex expected motion with a simple structure and has good motion rigidity, it has been widely used in various machinery for a long time.

 

When the cam index works at low speed, it can be treated as a rigid system. But when moving at high speed, the elastic deformation of the components caused by the inertial force will seriously affect the real movement of the mechanism, especially when the excitation frequency is close to the natural frequency of the system, the elastic deformation increases sharply, so when analyzing or designing a high-speed cam mechanism. It must be considered in accordance with the elastic system. The principles and methods of processing are different from those of rigid systems. At the same time, in order to ensure that the mechanism has the expected motion accuracy when running at high speed, and can maintain this accuracy for a long time, it is necessary to study not only its kinematics, dynamics, but also material, processing and tribological issues.

 

In addition, in recent decades, due to the widespread application of electronic computers and the development of adjacent disciplines, the research on high-speed cam theory has been continuously deepened. Therefore, it can be said that high-speed cam is a comprehensive problem with very rich content and involving multiple disciplines.

 

There are many factors that affect the dynamic characteristics of the high-speed cam divider, including inertial force (centrifugal force) caused by the eccentricity of the center of mass, damping of various parts, oil film oscillation of the bearing, temperature field, multi-support misalignment and other factors. As we all know, the higher the speed, the greater the centrifugal force. In modern times, many rotating machinery are in high-speed operation, such as engines, steam turbines, centrifuges, motor rotors and automobile wheels. Even if there is a small imbalance, a very large centrifugal force will be generated during high-speed rotation. Consider the following example: a cylindrical rotor weighing 10 tons and having a radius of 0.5 meters has an imbalance of 100 grams on its surface. When the working speed is 2000 r\min, a centrifugal force of 2000 (N) will eventually be generated according to the equation. If the speed is increased again, the centrifugal force will be greater. Therefore, even if there is an uneven mass distribution of several hundred grams on a rotor weighing several tons to several tens of tons, a large unbalanced force will be generated during high-speed rotation. The existence of unbalanced centrifugal force on the rotor will cause mechanical vibration of the machine rotor, bearing and installation foundation. In most cases, mechanical vibration is harmful. The main hazards are:

 

(1) The machine support is subjected to dynamic loads, which affects the normal operation of the support.

 

(2) The moving and static parts are worn out, the foundation is loose, or the machine's oil and gas pipes are cracked, and the automatic regulator fails, causing the machine to be frequently repaired or prematurely damaged.

(3) Disturb the surrounding mechanical equipment and instruments, so that the regulating device and protection system may malfunction and make the equipment and instruments unable to work normally.

 

(4) Noise is generated, which affects the physical and mental health of staff.

 

If the rotor speed is the same or close to the natural frequency of the rotor shaft system, the machine will resonate. When resonance occurs, the amplitude increases rapidly, and the unit will vibrate violently and cause damage. According to reports, in June 1972, when a 60MW steam turbine unit of the Power Plant in Japan was in trial operation, excessive vibration caused a runaway, which caused all the units to be scrapped. Based on experience and limited time, our factory only conducts research and discussion on the problem of eccentricity, and strives to find a way to balance mass to reduce or eliminate eccentricity, thereby improving its high-speed motion characteristics. How to establish the precise shape and dynamic analysis of high-speed cams under SolidWorks software is the focus of our research. For a long time, our engineers have conducted research on the following:

 

1) Comprehensive use of Matlab and SolidWorks software to generate accurate high-speed indexing cam 3D entities.

 

2) Use ComsmosWorks limited plug-in to analyze the inertial force, vibration, displacement, deformation and other dynamic characteristics of high-speed cams caused by eccentricity under high-speed conditions.

 

3) Using the trial weight method, drill holes on the cam at the eccentric position to remove part of the mass, reduce the amount of eccentricity, and improve the dynamic characteristics of the cam.

 

Finally, through a lot of measurements, our factory engineers figured out how to use the balance quality to improve the dynamic characteristics of the high-speed cam. It lays a theoretical foundation for practical applications in the future, improves production efficiency, reduces shipping time, and saves money and time costs for customers.