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Condition monitoring and fault diagnosis of machine tool equipment

Posted by: Chris Uli 2021-11-14 Comments Off on Condition monitoring and fault diagnosis of machine tool equipment

With the continuous progress of science and technology and the rapid development of modern production, the role of machinery and equipment as an important factor in determining the output, quality and cost of product production has become more and more obvious. During the use of equipment, different degrees of wear, fatigue, deformation or damage will inevitably occur. With the extension of time, their technical status will gradually deteriorate and the use performance will decline. As an important part of equipment management, equipment maintenance is an important guarantee for extending equipment life, ensuring normal production and preventing accidents.

1. Equipment failure rate curve

1.1 Equipment failure rate bathtub curve and characteristics

In daily life, people often habitually say the word “fault”. Under normal circumstances, people think that the so-called fault refers to the state where the equipment is forced to stop for some reason. However, even if the equipment is operating, if there are phenomena such as speed reduction, poor quality, energy loss, etc., it cannot be regarded as a normal state. These phenomena must be regarded as failures. It is worth emphasizing that failures are not just stagnant states.

Through the research and analysis of equipment failures, it is found that most of the mechanical equipment failure rate curves are shown in Figure 1. This failure curve is often called the bathtub curve. According to this failure curve, the change of equipment failure rate over time can be roughly divided into early failure period, occasional failure period and depletion failure period.

The early failure period is also called the running-in period for mechanical products. During this period, the initial failure rate was high, but over time, the failure rate dropped rapidly. The failures occurred during this period are mainly caused by defects in design and manufacturing, or caused by improper use. For example, the Shimada washing machine, the body cleaning equipment introduced by our company’s original factory, has continuous filter clogging alarms and other failures within two or three months after the completion of the installation and commissioning. This is the case. Entering the occasional failure period, the equipment failure rate is roughly in a stable state. During this period, failures occur randomly, with the lowest failure rate and stability. This is the normal working period or the best state period of the equipment. Most of the failures that occur here are caused by improper design, improper use and poor maintenance. The failure rate can be minimized by improving design quality, improving management, and strengthening maintenance. The use of our company’s equipment also fully reflects this situation. In the later stage of equipment use, the failure rate continues to rise due to wear, fatigue, aging, corrosion, etc. of equipment components. Therefore, it is believed that if major repairs are carried out at the beginning of the wear-out failure period, the failure rate can be economically and effectively reduced. The identification during this period is the most important.

1.2 Failure rate curve of modern equipment

With the development of science and technology, a large number of new technologies and new materials continue to emerge, especially the wide application of electronic technology and automation technology, and the equipment is developing in the direction of precision and automation. The structure of the equipment, the relationship between each work unit and the environment have become more and more complex, which brings new problems to equipment maintenance.

Through research, people have discovered that some equipment equipped with modern technology has a failure law that deviates from the bathtub curve. After recent years of investigation and research, in addition to the bathtub curve, the failure rate of the equipment has five situations, as shown in Figure 2.

Curve A shows a constant or slightly increasing failure rate, with a significant period of wear and tear. Curve B shows a slowly increasing failure rate, but there is no obvious wear-out period. Curve C shows that the failure rate of the new equipment has increased sharply from the low failure rate when it left the factory to a constant failure rate. Curve D shows that the failure of the equipment is a constant value, and the failure is often caused by accidental factors. The curve E shows that the equipment starts to have a high initial failure rate, and then drops sharply to a constant or very slowly increasing failure rate.

A statistical investigation of equipment failures found that 4% of the equipment followed the typical bathtub curve, 2% of the equipment followed the curve A, 5% of the equipment followed the curve B, 7% of the equipment followed the curve C, and 14% of the equipment followed the curve. D, no less than 68% of the equipment follows curve E. Generally speaking, in actual operation, the failure rate of the equipment should be a combination of one or several of the five curves shown in Figure 2 (the bathtub curve can be regarded as the combination of curves A, D and E). The rate may not be exactly the same as the failure rate of the equipment under investigation. However, the equipment failure rate depends on the complexity of the equipment. The more complex the equipment, the closer its failure curve is to curves D and E.

2. How to repair machinery and equipment

2.1 Main methods of equipment maintenance

(1) Plan maintenance method

As a preventive maintenance system, planned maintenance has been widely used in industrial enterprises since the 1950s, and has achieved good results in ensuring that the equipment is in good condition.
There are several ways to plan equipment maintenance:

  • (a) The standard maintenance law is the mandatory maintenance law. This method is often used for equipment that must strictly ensure safe operation. This method makes specific plans in advance for the maintenance date, category and content of the equipment. Regardless of the technical status of the equipment in operation, it is strictly implemented in accordance with the plan.
  • (b) Regular maintenance method. This method is based on the use of the equipment, with reference to the relevant maintenance cycle, to formulate the planned date and approximate maintenance workload of the equipment maintenance work. The exact work arrangement is specified in detail based on the preparation before each maintenance.
  • (c) Repair method after inspection. This law only prescribes the equipment inspection plan, and determines the maintenance date and content based on the inspection results and previous maintenance information.

On the one hand, the planned maintenance method can restore the working ability of the equipment in a timely manner and prevent possible accidents; on the other hand, it can arrange the use and maintenance time of the equipment in advance, and prepare the people, money, and materials needed for the maintenance; at the same time, it can also The maintenance time can be shortened, and the maintenance quality can be improved.

(2) Equipment diagnosis technology and condition maintenance

In contrast to traditional inspections, equipment diagnosis technology has evolved from relying on the sensory judgment of personnel to the use of a large number of advanced scientific instruments and meters; from manual testing to display by electronic instruments during equipment operation; The use of scientific and convenient instruments and meters can also be undertaken by ordinary equipment operators; it has developed from detection under shutdown conditions to direct detection without shutdown; the purpose of detection has evolved from detection of strain (wear, corrosion, etc.) and prevention of failure to Predict the natural life of equipment. At present, this technology has been developed into a systematic equipment maintenance management method, which can monitor individual parts and main components of a complete set of equipment, and can also conduct a comprehensive monitoring of the entire set of equipment.

Equipment condition monitoring and maintenance is based on the information provided by the daily monitoring, regular inspection, condition monitoring and diagnosis of the equipment, through statistical analysis and processing, to determine the degree of deterioration of the equipment, and to plan appropriate maintenance before the failure occurs. Equipment condition maintenance is recognized as the most efficient method among equipment maintenance methods. The use of equipment status maintenance can grasp the hidden troubles in time and eliminate them in time, thereby improving the equipment integrity and utilization rate, improving the quality of equipment maintenance work and saving various costs, and improving the overall efficiency. Since 1998, the equipment department of our company has been committed to the improvement and development of equipment condition monitoring technology, and has achieved certain results, which will provide a strong guarantee for the comprehensive implementation of equipment fault detection and diagnosis in our company in the future.

(3) Post-maintenance method

In equipment maintenance work, there is another method called post-maintenance method, or fault maintenance method, which is a method of repairing if it is broken, and not repairing if it is not broken. This method is only suitable for a large number of general equipment. This kind of maintenance method is also very common in our company.
2.2 Comparative analysis of different equipment maintenance methods

The planned maintenance method considers that the equipment failure rate follows the bathtub curve. To effectively implement preventive maintenance, a maintenance plan must be formulated scientifically. If the plan is unreasonable, this method often results in a lot of waste. When performing preventive maintenance, it is necessary to accurately find the inflection point, so as to avoid excessive maintenance or the expansion of the maintenance scope, and obtain the best benefits. However, the life cycle of the equipment is closely related to the use environment, personnel quality, maintenance and other factors of the equipment. It is difficult to scientifically and reasonably determine the life of the equipment. Therefore, it is difficult for the equipment to implement planned maintenance effectively.

The traditional concept believes that the occurrence and development of equipment failures are directly related to the time of use, and regular scheduled disassembly and repair are commonly used. According to the modern equipment failure rate curve, it can be known that some equipment failures are related to the use time of the equipment, and some have nothing to do with the use time of the equipment. It depends on the actual situation. At the same time, many equipment failures have a certain incubation period, which can be detected by modern equipment diagnosis technology, and safe and economic decision-making maintenance can be carried out. Preventive maintenance cannot improve the reliability of the equipment.

As a new type of maintenance method, equipment condition maintenance has been applied more and more widely, and people have paid more and more attention to it. Our company has a certain status monitoring and fault diagnosis capabilities, and has carried out this work, the following brief introduction is as follows:

We have conducted monitoring of fixed measuring points for Russian internal milling. These measuring points are mainly selected on rotating machinery, such as the bearing seat of the motor, the bearing seat of the cutter head, and the supporting point of the spindle. The main reasons for choosing these measuring points are : Rotating machinery is the most widely used type of machinery among various types of machinery and equipment. Monitoring and diagnosis of rotating machinery is necessary, and it has received more and more widespread attention.

Rotating shaft assembly is also called rotor-bearing system or rotor system, which includes rotor (shaft, gear transmission, impeller, coupling, etc.), bearing (sliding, rolling), support (stator, frame, etc.) and sealing device The other parts are the main components of rotary machinery. The important thing is that the shaft assembly works under complex dynamic conditions, and its running state will determine the fate of the whole machine. From the perspective of diagnostic technology, the rotating shaft assembly is the core part of the rotating machinery and the main object of monitoring and diagnosis. Therefore, the research on the monitoring and diagnosis methods of the rotating shaft assembly has a certain typical and universal significance. Based on the analysis of various faults of rotating machinery, focus on the extraction and expression of the diagnosis information of the vibration signal of the rotating shaft assembly, the vibration monitoring method of the rotating shaft assembly (simple diagnosis method), the basic principle of fault diagnosis and the analysis of typical fault vibration characteristics (Precision diagnosis method).

The following is a detailed analysis of the monitoring and diagnosis process of Russian internal milling machine tools. This work starts with determining the measuring point and establishing the database; the second step uses the Zhentong 904 data collector to detect the vibration at the measuring point and obtain the measured value; the third step is to transmit the measured value back to the computer for data processing and analysis , And compare it with the standard value to get the current operating conditions of the machine tool and the use and damage of each component; the fourth step is to pass the analysis result to the user, and prepare the spare parts according to the damage degree of the parts indicated in the report. The figure on the right is the work flow of the data acquisition and computer software structure system diagram.

From the monitoring of Russian internal milling, the mechanical part has been operating normally so far, and the vibration value of each measuring point of the machine tool is within the normal use range. The test results at this stage are analyzed. The analysis process is shown in Figure 1~Figure 4. Shown. Figures 1 to 4 are all measuring point trend analysis graphs. In these figures, “×” represents the actual measured value of all measuring points, and the red dotted line represents the future development trend of the measuring points analyzed by the software.

The machine tool can be used indefinitely according to the trend of high frequency and displacement of the measuring point; the machine tool can be used for about 120 days according to the trend of acceleration; the machine tool can be used for about 280 days according to the trend of speed.

However, it is not possible to fully adopt this technology in equipment maintenance work. This is because: (1) With the development of science and technology, there are already many equipment that can be repaired with equipment diagnostic technology, but as a new technology, it is still under continuous development and improvement, not all of them Machines and equipment can be repaired with equipment diagnosis technology. (2) From an economic point of view, if all equipment adopts the state-of-the-art maintenance method, a large economic investment is required. If these diagnostic equipment are only used within the enterprise, the utilization rate is low, and part of the maintenance resources may be idle at ordinary times. This greatly increases the maintenance cost, which is economically inappropriate for the enterprise.

3. Choice of machine equipment maintenance strategy

Every unit has equipment of different numbers and models. Some equipment is complicated, some equipment is simple. For large and medium-sized enterprises, the quantity and variety of equipment are large. With the advancement of the production technology of enterprises, more and more types of equipment are owned, and the technicality is getting stronger and stronger. Different equipment uses different environments and different failure rate curves. How to choose equipment maintenance strategy and carry out equipment maintenance?

(1) According to the importance of equipment, adopt corresponding maintenance strategy.

Generally speaking, the larger the enterprise, the greater the number of devices. Large and medium-sized enterprises have a very large number of equipment. Different equipment plays different roles in the production and operation of the enterprise, and the maintenance methods used are also different. When formulating a maintenance strategy for machinery and equipment, the importance of the equipment should be classified first. There are many classification methods, of which ABC classification is a commonly used and better method. In a unit, the number of Class A equipment is small, and it plays a large role, accounting for about 10% of the total. This kind of equipment has a complicated process and is a key equipment in the production process of an enterprise. Once these equipment fails, it will cause major economic losses and have a major impact on the enterprise. Therefore, this type of equipment is required to have very high reliability during operation. This type of equipment should be the focus of equipment maintenance. For these key equipment, appropriate maintenance methods should be selected from the perspective of maintenance economy. It is best to use conditional maintenance for this type of equipment, change planned maintenance to targeted maintenance, and implement conditional maintenance. This can greatly reduce maintenance workload and maintenance costs, and achieve the effect of less input and more output. If there is no corresponding diagnostic equipment, or the cost of the state maintenance method is too high, the result of the technical and economic analysis is inappropriate, and the enterprise cannot afford it, the standard maintenance method should be adopted, or the regular repair method should be adopted.

(2) According to the statistical results of equipment failure information, select the appropriate maintenance method

When repairing Class B and Class C equipment of an enterprise, an appropriate repair method should be selected according to the equipment failure information. For these devices, the fault information is the basis for determining the maintenance method of the device. Through the analysis of the failure information, the type of equipment failure can be judged and the maintenance method can be selected. Equipment failure information mainly includes statistics on the number of equipment failures (frequency of shutdowns) and the consequences of failures (length of downtime, economic losses, etc.).

Collect statistics on the frequency (number of times) and downtime of equipment in one year (or half a year, quarter, month). Draw a chart of equipment downtime and downtime frequency. Each unit has a lot of equipment. Just list the equipment with the longest downtime and the most downtime, which accounts for more than 80% of the total. For equipment with long downtime and high frequency, planned maintenance and regular maintenance are adopted to completely eliminate failures. For equipment that has a low frequency of shutdown, but requires a longer downtime once it is shut down, the process and causes of the failure and shutdown should be analyzed, and the status should be monitored and repaired. The maintenance strategy adopted for equipment with high frequency of shutdown and short downtime is preventive maintenance, but the skills and fault judgment level of the operators must be improved, and the general maintenance can be completed by them. For equipment with short downtime and low frequency, the method is not to be broken and not repaired, that is, to repair after the fact.

The machining industry has a large number of machinery and equipment, which is the basis for safe production. The requirements for their operation quality are very high, and the annual maintenance costs are also very large. In order to better use equipment maintenance methods, we must classify equipment and adopt corresponding maintenance methods, and in the future equipment maintenance work, condition monitoring and fault diagnosis will gradually expand the scope, which will become Class A equipment The mainstream of maintenance methods.

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