Radiator Fan Motor - 1MW Generators - Essay Example

Bearing Failure Radiator Fan Motor - 1MW Generators Report # Prepared by: John Brady (Affiliation) Background: The gas engine driven generators, with an individual capacity of 1 MW, installed to supply power to townships in remote areas of Africa serve a vital purpose in supplying electric power to the resident people. Given the location of these generators it is essential they operate on a continuous basis and down time due to breakdown is kept to the minimum. An essential component of the generators is the radiator that cools the circulating water required for maintaining engine temperatures. The radiator is of the Fin-Fan type with the water moving through the grid of horizontal finned tubes and air being blown over them by a large fan mounted below them. The fan is driven by a large electric motor mounted vertically. These motors have been giving frequent problems due to the failure of the bearing, mounted on the shaft, on the underside of the motor. Repeated failure of these bearings is a cause of concern since it leads to frequent power outages and the attendant maintenance problems. The premature failure of the bearing also leads to damage of the stator, rotor and other parts making this a problem with much larger consequences than just the collapse of a single bearing. This report is prepared and submitted with the object of analysing the cause of this frequent failure and to arrive at a permanent solution to this problem. Method of Analysis: The modus of analysing the problem is divided into three essential parts, these are: 1. A survey of literature available on the subject 2. Actual study and observation in the field 3. Collation and analysis of the data collected to determine the cause of failure and arrive at recommendations to overcome the problem Bearings are used to provide smooth and low friction movement between two moving parts; the movement may be rotary, as in this case, or lateral. Bearings have no wearable surfaces and are designed to fatigue after a time. With proper use this time is measured in years and not hours. The usual causes for premature failure of motor bearings are detectable from a close examination of the failed bearing since every different type of reason for failure will leave its own tell-tale signs on the bearing. Detailed examination of several failed bearings was done to determine: Whether all the bearings had failed due to similar cause, and What caused the bearings to fail The possible cause and effect of the bearing failure was analysed using the fishbone (Ishikawa) diagram method. Time Frame The study of the possible causes, collection of information analysis and preparation of report was completed over a period of 50 days. The approximate number of days spent on each activity and the scheduling of work are given in Appendix II attached to this report. Discussion The fishbone diagram A fishbone diagram was drawn to consider all possible causes that may lead to a premature bearing failure. The diagram is enclosed as Appendix-I to this paper. All aspects The four M's: Manpower, Material, Machine and Method The four S's: Suppliers, Surroundings, System and Skills The four P's: Policies, People, Procedures and Place Were considered with care and as a first step the four P's were rejected as not being relevant to this particular situation. The others were combined to form four major reasons Manpower and Skills Material and Suppliers Machine and Surroundings Method and System Every angle of how these could have led to the failure of the bearings was explored and these were eliminated one by one and the root cause of the bearing failure was isolated. The considered opinion of this writer is that the failure resulted from incorrect design and installation of the motors. A different approach was then adopted to determine the exact cause of the bearing failure by examining the engineering aspects of the design, alignment and installation of the motors that caused the bearings to fail. Technical evaluation The motor is vertically mounted with the fan mounted on the top. Both bearings are ball bearings with the following specifications: Top: SKF 6309 ZZ; dimensions - ID 45mm, OD 100mm, W 25mm Bottom: SKF 6307 ZZ; dimensions ID - 35 mm, OD 80MM, W 21mm It is the bottom bearing that is the one that fails. The bearings are sealed for life and do not need lubrication. Possible causes for bearing failure, description of the clues that lead us to surmise the reason for failure and observations recorded from an examination of the damaged bearings is recorded below. When we install a bearing in any rotating equipment it is the usual practice that the inner race of the bearing is interference fit with the rotating part, in this case the shaft of the motor, and rotates with the part. The outer race is fixed in position and remains stationary. The surfaces of the inner and outer rings and the rolling elements become dull as soon as the bearing goes into service, this is not a sign of wear, but an examination of the dull surfaces reveals the type of problem that caused the bearing to fail prematurely. Problems, possible causes and the observations of the pathway that shall link the failure to a particular type of problem are: 1. The radial load is rotating with the shaft: This is caused by an unbalanced rotating assembly or a bent shaft. The inner ring appearance. The load acts all of the time at the same place in the race way. Here the path pattern is at its widest, tapering off at the ends. If the load is only radial, the pattern will be in the centre of the race way and will extend around slightly less than half the race way circumference. The outer ring appearance. The path will extend around the entire race way. It will be uniform in width and if the load is only radial, it will be in the centre of the race way. Examination of the inner ring of all bearings reveals that the kind of problem did not cause these bearings to fail. The pattern is uniform on both races of the bearing. 2. The radial load is unidirectional. This is caused if the equipment is not operating at its best efficiency point, is misaligned, or if there is excessive pipe strain. The pattern is the same as we would expect in properly operating equipment, only more pronounced. The inner ring appearance. The path will be in the centre of the race way, uniform in width and visible around the entire circumference of the race way. The outer ring appearance. The pattern will be widest at the load point and tapering towards the ends. If the fit and clearances are normal the pattern will extend around to slightly less than one half of the raceway. It will be located in the centre of the race way, if the load is only radial. Both raceways shows a normal pattern, however the path is off centre and predominantly on the outer side of the bearing, showing the presence of a non-radial load on the bearing. However, we only eliminate the possibility that the fan was not operating at its optimum efficiency or that there was a problem with the alignment. 3. The radial load is multidirectional. Caused by cavitation, too tight interference fit, preloading, or cooling of a bearing outside diameter are all common causes of this problem. On both the inner ring and the outer ring the path is all around the race way, widest where the load was the greatest. Cavitation is not expected in a fan and an examination of the failed bearings eliminates possibility of the failure having been caused by improper fitting and assembly of the bearings. 4. An oval compression of the outer ring. This is caused by an out of round housing. The inner ring appearance. The path extends around the entire ring and is uniform in width. The outer ring appearance. Two wider paths where the ring was distorted to the oval shape. No evidence was found that pointed in this direction. The housings are well machined and the fit good. 5. The axial load is unidirectional. This is the condition when there is an axial load on the bearing. In our case an axial loading is to be expected because of the downward thrust caused by the weight of the rotor and the fan blade and also the thrust caused by the reaction against the air being blown up by the fan. On both the inner and outer rings. The pattern will extend around both raceways and is displaced axially from the centre towards in the direction of the thrust. Observation of the failed bearings clearly points to this having been the main cause of the premature failure of the bearings. Other factors that could have caused the failure of the bearing and which need to be eliminated in order to completely isolate the cause of failure are observed from different view-points. The observations and conclusions in our case are given below. We must look out for evidence of rubbing and for evidence of corrosion and damage. Solid particles can enter a bearing's raceways and leave the following signs: Scores or small indentations in the precision races and rolling elements. Interference with the transfer of heat within the tight tolerances, causing discoloration, thermal expansion, seizing etc. The particles could come from Varnish and "coke" that forms where the lubricant overheated, parts of the ball cage that have broken loose due to a lack of lubrication, pieces from a failed grease or lip seal, contaminated lubricant and a host of other sources. Scoring or discoloration of the bearings must be carefully and expertly examined to evaluate those that caused the bearing to fail and those that might have taken place as a result of the failure. No scoring or discolouration of the bearings is observed which can lead us to conclude that there was a problem with the lubricant or overly tight fitting of the bearings. Lack of lubrication can eventually cause the bearing to seize, such cases are comparatively easy to see and no observation leads to in this direction. We need to look for smearing of the metal. When two non lubricated surfaces slide against each other, under load, the material transfers from one surface to the other in the form of a smear. In addition metal melts and then re-hardens causing localized stress that can produce cracks. Sometimes when the load is too light for the speed the centrifugal force may throw the balls out. The outer race will smear on the outside diameter if it slides during operation due to an improper "slip fit". This slipping can also cause "fretting corrosion" as the protective oxide film is worn away from the metal surface. We also look for evidence of static vibration. One will see indents in the raceway that could be either shiny or rusted in the bottom. The frequency of the vibration has no affect, but greater energy causes greater damage. Roller bearings are more susceptible to this type of damage because the balls, in a ball bearing, can roll in many directions. The equipment was located too close to another piece of equipment that was vibrating. This can be a big problem during storage. The shaft was not locked during shipment. In addition to vibration, equally spaced indents can be caused by: The bearing was installed by pressing on the wrong race. The bearing was driven too far up a tapered shaft. Damage can also be caused by electric current. It will show up on both the races and the rolling element. The bottom of the depression will be dark in colour. Since there is nothing in a bearing to wear out, flaking or spalling is a sign of normal fatigue. Overloading however, can cause premature fatigue. The following causes of bearing overloading need to be investigated carefully: The bearing housing is out of round The shaft is oversized The bearing was driven up too far on a tapered shaft Misalignment between the fan and its driver The rotating assembly is out of balance The shaft is bent The pump is operating too far off of its best efficiency point Pipe strain Water hammer in the lines Cavitation The bearing had a quality problem to start with Shaft thermal expansion The bearing housing is being cooled, causing the outer race to shrink, increasing the load Excessive axial thrust Pulley driven design Conclusions: A careful examination of all the bearings and using comparison charts provided by some bearing companies it is possible to arrive at the exact cause of the bearing failure most of the times. In cases where two or more elements cause the bearing failure it becomes difficult to isolate individual causes and requires a very detailed study and sharing of experience with the site staff. The cause of bearing failure in the generators is isolated to be the faulty design of the motors for the purpose they are being used. These motors are good by themselves, but were never designed or built for use in a vertical orientation. The major difference between a motor used in horizontal and vertical operations is the choice of bearings at both ends. While ball bearings suffice in horizontal applications where there is very little axial load, they are utterly useless in a vertical application where the weight of the rotor and the fan has to be supported - 35Kg in this case. Recommendations: There appears to be no way to change the orientation of the motors and the radiators without undertaking major modification work. This is not possible at the present locations without causing major outages of power and quality of the work carried out may also become a factor. The motors are of good design and build and can be used in the present location but with modification. Ideally both bearings must be changed to a thrust - roller bearings design. Temporary solution can be found by changing just the bottom bearing (the one that is failing) to a thrust bearing which can take the load of the stator. Fishbone Diagram Appendix II Time Taken for the Study The total time taken is divided into roughly the following pattern: Scheduling of the Study Read More