SOME ASPECTS OF BEARING DIAGNOSIS IN PREVENTIVE MAINTENANCE

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1 SOME ASPECTS OF BEARING DIAGNOSIS IN PREVENTIVE MAINTENANCE Ilie Octavian POPP1 ABSTRACT: This paper presents some aspects of monitoring and diagnosis bearings in preventive maintenance and the importance of bearing repair as an alternative to replacing them. Knowing and understanding the value of bearing repair means knowing what repair can do, when to use it, and where to get it done. KEY WORDS: preventive maintenance, monitoring, diagnosis bearings, bearing repair 1 INTRODUCTION The growing investment machine tools and production systems requires their maximum availability. The complexity of such systems, which are highly automated and consist of many modules which are linked and have to work together without failure, increases the risk of breakdowns. Considering the trends of manufacturing developments, the following reasons can be pointed out to explain why monitoring technology is becoming more and more important in modern manufacturing systems: (1) Large-scale manufacturing systems should be operated with high reliability and availability because the downtime due to system failure has a significant influence on the manufacturing activity. To meet such a demand, individual unit processes should be securely operated with the aid of reliable and robust monitoring systems. Monitoring of large-scale systems is already beyond the capability of humans. (2) Increasing labor costs and shortage of skilled operators necessitate operation of the manufacturing system with minimum human intervention, which requires the introduction of advanced monitoring systems. (3) Ultra-precision manufacturing can only be achieved with the aid of advanced metrology and the technology of process monitoring. (4) Use of sophisticated machine tools requires the integration of monitoring systems to prevent machine failure. (5) Heavy-duty machining with high cutting and grinding speeds should be conducted with minimum human intervention from the safety point of view. Universitatea Lucian Blaga din Sibiu, Departamentul de Masini si Echipamente industriale, Emil Cioran 4, Sibiu, România ilie.popp@ulbsibiu.ro 1 52 (6) Environmental awareness in today s manufacturing requires the monitoring of emissions from processes (Popp, 2009). 1.2 Needs and roles of monitoring systems Considering the trends of manufacturing developments, the following reasons can be pointed out to explain why monitoring technology is becoming more and more important in modern manufacturing systems: Large-scale manufacturing systems should be operated with high reliability and availability because the downtime due to system failure has a significant influence on the manufacturing activity. To meet such a demand, individual unit processes should be securely operated with the aid of reliable and robust monitoring systems. Monitoring of large-scale systems is already beyond the capability of humans. Increasing labor costs and shortage of skilled operators necessitate operation of the manufacturing system with minimum human intervention, which requires the introduction of advanced monitoring systems. Ultra-precision manufacturing can only be achieved with the aid of advanced metrology and the technology of process monitoring. Use of sophisticated machine tools requires the integration of monitoring systems to prevent machine failure. Heavy-duty machining with high cutting and grinding speeds should be conducted with minimum human intervention from the safety point of view. Environmental awareness in today s manufacturing requires the monitoring of emissions from processes. The roles of the monitoring system can be summarized as shown in figure 1. First, it should be capable of detecting any unexpected malfunctions

2 which may occur in the unit processes. Second, information regarding the process parameters obtained with the monitoring system can be used for optimizing the process. The databank is necessary when the initial setup parameters should be determined (Popp, 2009). and more... Measurement units of measurement signals have specific responsibilities, local assessment of the measurements and conditional triggering of certain local events (warning, open / close relay power supply, etc.). And, last but not least, the transmission results in different formats to a centralized unit (control room). 3 PREDICTIVE MAINTENANCE (PDM) 3.1. Definition Fig. 1 Roles of the monitoring system 2 DIAGNOSIS SYSTEMS AND FAILURE DETECTING In current industrial practice there is an offer of a wide range of products, from portable measurement and control instruments, the continuous monitoring systems and complete software to centralize and evaluate the results. Monitoring tools cover a wide range of measurements: vibration measurements with FFT signal analysis and automatic evaluation of results; shock pulse measurements to determine the conditions of lubrication of bearings and early detection of wear; measurements of speed; temperature measurements; measurements of analog signals; determining the resonance frequencies of the machine; orbit calculation for sliding bearings. These equipment s detect operating faults since their infancy, thus offering the possibility of planning repairs or maintenance work, resulting in minimizing or eliminating downtime of the production process. A continuous monitoring system consists of sensors installed permanent camps dynamic equipment and connecting them to the monitoring unit. These sensors can be, for example: vibration transducer; shock pulse transducer; temperature sensor; speed sensor; The aim of predictive maintenance is first to predict when equipment failure might occur, and secondly, to prevent occurrence of the failure by performing maintenance. Monitoring for future failure allows maintenance to be planned before the failure occurs. Ideally, predictive maintenance allows the maintenance frequency to be as low as possible to prevent unplanned reactive maintenance, without incurring costs associated with doing too much preventative maintenance. Predicting failure can be done with one of many techniques. The chosen technique must be effective at predicting failure and also provide sufficient warning time for upcoming maintenance. Some techniques include vibration analysis, oil analysis, thermal imaging, and equipment observation. These are described in detail in condition based maintenance page. Choosing the correct technique for performing condition monitoring is an important consideration that is best done in consultation with equipment manufacturers and condition monitoring experts. When predictive maintenance is working effectively as a maintenance strategy, maintenance is only performed on machines when it is required. That is, just before failure is likely to occur. This brings several cost savings: minimizing the time the equipment is being maintained minimizing the production hours lost to maintenance, and minimizing the cost of spare parts and supplies. These cost savings come at a price, however. Some condition monitoring techniques are expensive and require specialist and experienced personnel for data analysis to be effective. Applications that are suitable for predictive maintenance include those that: have a critical operational function have failure modes that can be cost-effectively predicted with regular monitoring 53

3 Fig. 2 Location and nature of the bearing defect Unsuitable applications for predictive maintenance include those that: do not serve a critical function do not have a failure mode that can be costeffectively predicted 3.2 Advantages of predictive maintenance Compared with preventative maintenance, predictive maintenance: ensures that a piece of equipment requiring maintenance is only shut down right before imminent failure. This reduces the total time and cost spent maintaining equipment. 4 DIAGNOSING BEARINGS All equipment for assessing and analyzing the frequency made at this time in the world captures the vibration signal, processes and provides information on the technical state of camp, location and nature of defect. In modern industrial practice there are equipments and work procedures based on this principle. It is known that if a bearing get to vibrate, already bearing is compromised and does not provide reliability. It is shown a method of measuring and monitoring of the lubricant film thickness rolling elements and taxiways, which allow identification of the moment when, for one reason or another (as lubricant, additional tasks camps) film lubricant is thinning and surface roughness of the metal balls / rollers and roller paths, get in contact directly. By this method it can be determined what led to this situation, necessary corrections could be made in time, thus coming back to "wellness" page. Otherwise, the elements bearing wears, wear later will lead to the appearance of vibrations. Only in the due course, equipment to measure and analyze vibrations proves their usefulness, but already too late: the bearing is compromised. 54 This method it called Shock Pulse Method (SPM Barkov & co., 1997). The main advantage of the shock pulse method is just finding its expertise in shock. When a ball / roller hits the defective area path produces a shock wave in a metal machine table. These shock waves are starting with a short stroke that pays off quickly. In a recording time displayed on an oscilloscope, these waves are observed superimposed on the overall machine vibration. When their frequency corresponds to the frequency of crossing objects rolling or cage, is a testament to the defect occurred in the capital. SPM transducer shock wave excites the characteristic frequency 32 khz resonance, amplifying the signal. Circuit analysis of electrical and mechanical signal is calibrated to filter out other signals of vibration. Due to the sensitivity shock pulse method can be measured even overall lubrication of the bearing, the parameter dbc. Filtered transducer signal reflects the variation of pressure in the contact area between rolling objects and ways of rolling bearings. When the lubricant film is thick, the shock pulse is low, no significant peaks (green). Level increases when the lubricant film thickness decreases, but still not present significant amplitude components (yellow area). Wear causes strong pulses at irregular intervals (red zone- fig. 3). The shock pulse method can be extended by performing an FFT analysis (Fast Fourier Transformation) of the signal taken, leading to more thorough analysis possibilities. By identifying each bearing their own frequent (symptoms can now be identified these common SPM spectrum, thus diagnosing component even showing wear (running path inside / outside, roller / ball, cage). The advantages of implementing a maintenance system based on periodic measurements and analysis of shock pulse signal: - Saving of materials and spare parts; - Shortening the residence time in repair; - Increasing the availability of equipment. Basically, measurements are performed on all equipment camps (fig. 4). The instrument acquires a sample shock pulse signal, it breaks down, the Fast Fourier Transformation (FFT), resulting in SPM spectrum, and where frequencies are shown in the pulse occurs predominantly from running bearing. In terms of defects that can be detected can include lubrication failure / improper camp; early wear on the outer raceway; early wear on the inner raceway; early wear on the roller rolling object / ball; wear the cage early (Lipovszky, 1999).

4 Fig. 3 Lubricant film thickness measurement Specialized software associated test equipment must contain a rich database symptoms being able to automatically diagnose the probable causes of failure of bearing. At the end of a round of measurements, it releases a report of measurements, including assessment of shock pulse signal level standards in force, diagnosing and locating possible causes of wear of the bearing and the recommendations on what action should be taken when appropriate Bearing problems caused by inadequate lubrication have many forms, including roller end scoring from metal-to-metal contact and excessive wear on rollers and raceways. Excessive heat generation can cause cone large rib face scoring or "welding" damage or deformation. In extreme cases, inadequate lubrication causes rollers to skew or slide sideways, leading to bearing lockup. There are signs that indicate when bearing lubrication is inadequate or in excess. If these signs are detected soon enough, conditions can be corrected before catastrophic damage occurs. With inadequate lubrication, gauges and operating temperatures will show the bearing is running hot, and it will become noisy from metal-to-metal contact. With excess lubrication, the lubricant can break down because of increased operating temperatures, as indicated by a dark brownish-black color. With proper lubricant application and a regular maintenance schedule, bearing damage from lubrication problems can be avoided. Here are some tips to ensure proper bearing lubrication. A suggest a predictable pattern in achieving sustained culture change, which is a function of effort and time it s showed in fig. 5. Fig. 4 Measurements of the machine bearings 5 THE IMPORTANCE OF LUBRICATION Lubrication is more than simply an additive to a machine. Instead, it is an integral component of that machine and as such should be closely monitored to ensure it maintains its quality. Essential to all the moving parts, especially bearings, lubrication serves to carry away heat and protect bearing surfaces from corrosion and wear. Too little lubrication causes additional friction and heat, which can cause premature bearing damage. Excess lubrication, at high speeds, can generate additional heat from churning, which causes the lubricant to break down thermally, leading to eventual bearing damage. Fig. 5 Culture of bearing lubrication (Kennedy, 2005) 6 CASE STUDIES IN DATA ANALYSIS THROUGH PREDICTIVE MAINTENANCE The case studies representing vibration analysis will present examples and comparisons of bearing faults, unbalanced conditions and impact testing for natural frequencies. 55

5 6.1 Vibration Data Figure 6 represents a bearing problem with an outer race defect. Notice the side bands around the outer race fault frequency? This is caused by modulation in the time waveform. If you look at the time waveform below the spectrum, you can see the amplitude modulation that is present. Generally, this type of modulation will represent the operating frequency of the rotating element of the machine in question. Other information that is present on the data presented is the bearing type SKF 22222CC. This information is entered into the software to allow the software to label the fault frequencies within the spectrum. The software has a database that has more than 10,000 bearing fault frequencies calculated. This helps cut down on valuable time and makes the analyst s job much easier. Vibration analysis can be a powerful diagnostic tool. Identifying bearing failures, gear problems, and imbalance and misalignment conditions are not the only positives this technology provides. Determining operating parameters such as potential resonance conditions pays big dividends when designing and determining operating speeds. should be conducted. In figure 7, an example of the results of an impact test is provided. Fig.7 The results of an impact test [7] The data in figure 7 represents a valid impact test. Trying to operate a rotating machine within 20 percent of 7,313.1 CPM will cause poor operating conditions and an unreliable machine throughout the life of the machine Oil Analysis Fig. 6 Vibration Data from a Bad Bearing [7] The pressure to continue to speed up or change operating conditions will only increase with the highly competitive manufacturing facilities that we work in on a daily basis. The problem with changing speeds of a rotating machine is that at some point we venture into the natural frequencies of the structure or rotating element. When the operating speed of the rotating machine and the natural frequency become too close, a condition called resonance is excited. This will increase the amplitude of the machine by as much as 20 times and can cause it to shake itself apart. To determine the frequencies to stay away from when modifying or designing a machine, a test such as an impact test 56 In figure 8 below, the ISO contaminant level is 22/21/17. This far exceeds the standard set for all operating equipment in most industrial applications. For each numerical increase in ISO contaminant level, the amount of contaminants in the oil doubles. If the standard is a 16/14/11, then the increase in contaminants in the oil for a 22/21/17 is 64 times dirtier than the standard. Oil analysis is a reliable predictive maintenance tool and is very effective in detecting contaminants in oil that are a result of ingressed dirt or internal wear debris generated by the effect of machine degradation and wear. An increase in contaminant levels accelerates the wear-out process of all components in industrial machine applications. Contaminants in oils can be prevented. Good filtration on the return side of hydraulic power units will help in taking out dirt and other ingressed particles. Usually 3-micron filtration with a 200 beta ratio is the standard set for most machinery. Eliminating leaks and insuring that clean oil is used to refill any oil that may need replacing is another best practice that can help maintain oil cleanliness.

6 Has the bearing been subjected to sudden changes in lubrication or temperature? Early detection of a problem through routine checks can spare companies' unnecessary downtime and expense and help to capitalize on the capabilities and benefits of bearing repair. Regardless of the manufacturer, type of bearing, or application, proper treatment may preserve a bearing and restore it to like-new condition for continued use. Bearings with little or no damage often can be reconditioned and recycled easily and at low cost (Breaz, s.a., 2005). Fig. 8 Oil Analysis Data [7] 7 BEARINGS REPAIR: AN ALTERNATIVE TO REPLACEMENT Bearing repair is not a new concept, nor has it changed a great deal over the years and that's good news. Repairing damaged bearings is a precise science that has been fine-tuned over time through careful and gradual enhancements to provide superior results. Just as new designs and technologies improve bearings, growing expertise and technology of bearing repair continues to increase the reliability and performance of reconditioned bearings, keeping it an economical alternative to purchasing new bearings. Knowing and understanding the value of bearing repair means knowing what repair can do, when to use it, and where to get it done. 7.1 When to repair Although it offers many benefits, reconditioning is not always the best option for a damaged bearing. The challenge of properly utilizing bearing repair services is determining if and when a bearing needs repair, and deciding which options is the best economical and long-term decision. Visual inspection serves as the first step in deciding if a bearing needs repair. Careful review of additional criteria assists in determining the need for repair, such as: Is the bearing nearing or has it exceeded its suggested life expectancy? Have operating temperatures exceeded 200 F? Has the bearing been exposed to excessive vibration? 7.2. Remanufacturing process Once a bearing is damaged, it should undergo a rigorous process to determine if it is a candidate for repair. Bearings to be repaired should be thoroughly cleaned. Next, and the most critical step, is a detailed inspection on 100% of the bearing. Inspection begins with a careful review for major damages such as fractures, significant spalling, and bluing due to excessive heat. Any evidence of these usually results in scrapping the bearing. If the bearing passes the first stage of inspection, it should undergo further review. This step serves to reveal a mode of damage, assessment of damage, and the scope of repair. The degree of bearing repair typically fits into three types. Type I reconditioning is used for minor trouble spots and includes polishing, resetting of internal clearances, and spot grinding of any small surface defects to prevent further damage. Type II repair is for bearings with more extensive damage. This level involves major procedures such as regrinding of races, new spacers, and polishing of the bore and cup OD. The regrinding of raceways will also necessitate the manufacture of oversize rollers to maintain bearing geometry and clearance. Type III remanufacturing takes Type II repair one step further by replacing one or more of the major components of the bearing that are not repairable. These levels of repair have traditionally been suited for bearings with a minimum bore size of 200 mm. However, reclamation service can be done for bearings as small as 75 mm Expertise in bearing repair Technology and material advancements contribute to an increased potential for bearing remanufacturing and reconditioning programs. However, experience in recognizing and treating various types of damage is the only way to guarantee an accurate inspection and proper repair. It is important to have any bearing repair performed by properly trained and experienced personnel, 57

7 because unnecessary repairs can lead to additional damage and limited bearing life. Common reconditioning mistakes include: Improper roller polishing that creates flat spots Excessive deep spot grinding Mixing of preset components. In addition to expertise, proper equipment is required not only to fix the problem, but also to ensure all damage has been reviewed and addressed. Magnifying glasses and proper lighting and measuring equipment are essential to perform thorough inspections and repairs. CONCLUSIONS The benefits of predictive technologies are evident in the examples of data provided in this paper. Simple measurement of vibration will indicate if the vibration is normal, still acceptable or unacceptable, but we will provide information on which the defect and where it is located. The problem is more complicated for complex machinery where vibration evaluating a camp, we will not know if they come from working, the clutch, the gear etc. The advantages of implementing a maintenance system based on periodic measurements and analysis of vibration signal: - Saving of materials and spare parts; - Shortening the residence time in repair; - Increasing the availability of equipment for production. Bearing design takes into account the use and application of the bearing and establishes an appropriate prediction for service and fatigue life. No matter who the manufacturer is, bearings often deviate from these expectations due to factors such as contamination, inadequate lubrication, and misalignment. When a bearing is damaged, the entire machine of which it is a part suffers. Conventional practice suggests that a damaged bearing should be scrapped and replaced. The growing popularity of bearing repair has helped companies understand its value by providing an efficient and cost-effective way to resolve the problem. Bearings often can be returned to original specification for less time and money than purchasing new ones. A quality repair and reconditioning program can result in significant savings compared to discarding and replacing bearings. Depending on the scope of the work, bearing repair can save as much as 50% to 90% of the cost of purchasing a new bearing. 58 Beyond the cost, repairs often save time compared to ordering replacement bearings, reducing costly downtime. Another value-added service of bearing repair is using damage analysis as a tool to identify difficult conditions and prevent future problems. The data provided illustrates how vibration analysis can detect gear meshing defects in power transmission units, bearing defects in rotating equipment, imbalance conditions in fans and the detection of natural frequencies of structures supporting rotating equipment. Each problem represented and diagnosed has proved to save thousands of dollars in repair cost and downtime. Oil analysis data provided has shown to identify contaminants in oil that can cause parts to wear out prematurely and reduce equipment life. Identifying wear particles in machine oil can detect bearing problems and gear problems that could otherwise go undetected. REFERENCES Popp, I., Barsan, I. (2009) Considerations regarding the monitoring system for preventive maintenance, Annals of the Oradea University,Vol.8, No.18, ISSN , pp Popp I., Barsan, I. (2009) Considerations regarding the monitoring and diagnostic of manufacturing systems for preventive maintenance, PROCEEDINGS of the ICMaS, Vol. 4, ISSN , University POLITEHNICA of Bucharest. Barkov A.V., Barkova N. A., and A. Yu. Azovtsev, (1997) Condition Monitoring and Diagnostics of Rotating Machines Using Vibration, VAST, Inc., St. Petersburg, Russia. Lipovszky, G., Solyomvari, K., Varga, G. (1999)Vibration Testing of Machines and their Maintenance, Akademiai Kiado, Budapest. Kennedy, S., (2005) Learn about condition monitoring beyond oil analysis, temperature and vibration in Sheila Kennedy's monthly Technology Toolbox column, "New tools for PdM" Breaz, R.E., Bologa, O.C., Tera, M., Deac, C., (2011) Low-cost motion control solution for industrial manufacturing systems, 9th IEEE International Conference on Industrial Informatics (INDIN), pp , ISSN , Lisabona. [7] * * * A collection of maintenance papers on the Internet.