Sleeve vs Antifriction Bearings - Siemens Industry, Inc.

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6.3 Inch25 O CAmbient40 O c40 O C Brg HousingFig. 11 – Thermal network A/F BrgB. Sleeve Bearing: Re-Lubrication & Lack of LubricationProper lubrication is important for any bearing type.However, sleeve bearings are more tolerant of lubricantabuse than anti-friction bearings. If the lubrication is nonoptimal,than the sleeve bearing will give better warningthan anti-friction bearings would, especially if the bearingsare equipped with bearing temperature detectors.Sleeve bearings are lubricated with high-grade turbine oil.It is important to use the proper viscosity oil. If a differentviscosity is used than what the bearings were designedfor, than several things may occur: inferior rotor-dynamicperformance, change in bearing oil film thickness, orincrease in bearing heating.Unless otherwise stated, it is assumed that this oil ismineral based. Synthetic oils are becoming morecommon, but their compatibility with paint and sealingsystems must be verified. Non-compatible oils will bechemically aggressive to the paint and sealing systemscausing leaks and foreign material getting into the bearing.Catastrophic failure may result!The oil must be maintained at the proper level. Properlevel is indicated by having the oil level centered on thebull’s eye or to the appropriate mark on a sight glass/gage.Additionally, constant level oilers can be used to assureproper oil level on sleeve bearing motors.The variation allowance in the oil level should be kept ata minimum. If the level is too high it can leak out, or slowthe oil ring down to the point that it is incapable ofdelivering the required oil to the shaft journal/bearing. If itis more than ¼ in low it can cause failure. Refer to OilRing subsection for additional details.C. Notification of Imminent Failure2.95 Inch60 O C ShaftTemperature50 O C BearingSleeve bearings are more likely to give you a warningimminent failure than anti-friction type bearings. Manytimes sleeve bearings will begin to wipe under highvibration conditions and while this is happening RTD willshow some higher temperature levels and then return tonormal, but when you open up the machine you will seeheavy wear patterns and bearing wiping. Also whenSleeve bearings begin to fail babbit material will begin tocontaminate the oil causing it to turn black. Thisdiscoloration can also be seen when shaft currents exist.Anti-friction bearings on the other hand can fail ratherquickly with little to no warning. Many cases have beennoted where the operator walked past a motor when it wasrunning very well and moments later the bearings failedcatastrophically causing the rotor to hit the stator resultingin total destruction of the motor.One scenario is quite possible. When for any reason thebearing begins to run hotter than the bearing housing theclearance in the bearing (which can be as low as one ortwo mils on anti-friction bearings) can quickly be used up. .Fig. 11 shows the typical temperatures that can be seenon open motors with ambient air on both sides of thebearing. A 6.3-inch OD bearing that has a 10 degree Crise above the bearing housing temperature will expand1.12 mils per inch for every 100 deg C. For this 6.3-inchbearing that would be .71 mils for every 10 O C. Thediametrical clearance between the bearing and thehousing ranges from 0-2 mils. The bearing internalclearance ranges from 1-2 mils. This would result in a totalaverage clearance of 2.5 mils. If the differentialtemperature between the housing and the bearing in thisexample were 35 O C the clearance would be used up.Above that the bearing would be damaged. The bearingwould then be subjected to extreme compressive stressesas the bearing starts pushing against itself, the shaft, andthe bearing housing. Initially the tighter clearances mayactually result with the motor running with less vibration.But once this process begins, a thermal runaway conditionwill follow. Eventually, the outer race will expand to thepoint of locking in the bearing housing. The inner raceexpands and begins to spin on the shaft and the balls getimbedded in the inner and outer race. At this point thebearing has catastrophically failed. If the motor is not shutdown very quickly the rotor can quickly hit the statorcausing extensive and very expensive damage.Another condition that may exist is axial loading betweenbearings. When the rotor assembly and shaft heats up andexpands axially, the bearing, which is intended to float inthe housing, could possibly, due to excessive heat,expand radially to a point where it could seize in thehousing not permitting axial growth. This would result inexcessive axial loading of both bearings. This failure modewill look like excessive thrust loading.D. Bearing ReplacementIt is not possible to replace anti-friction bearings withoutremoving the bearing housings and uncoupling the drivenequipment. Then a significant amount of access room isstill required for the necessary equipment to remove theold bearings. In general the motor must be removed forbearing replacement.Sleeve bearings, on the other hand, can be replaced withthe lower bearing housings intact and driven equipmentPage 10 of 13
coupled. This can be a significant feature if riggingrequirements make it extremely difficult to remove themotor.Anti-friction bearings failures tend to be catastrophic innature, oftentimes providing little prior warning. In somecases the bearings fail to the point of having the rotor dropdown into the stator, essentially destroying the entiremotor. Even if the entire motor is not damaged, the shaft(and thus rotor assembly) must oftentimes be replacedafter a catastrophic bearing failure, as the inner race tendsto first spin and later weld itself to the shaft.When sleeve bearings fail, they tend to be lesscatastrophic, and generally provide information as to thedegrading state of the bearings. When the bearings failcatastrophically, the bearing itself tends to be sacrificial,as the bearing babbit is much softer than the shaft steel.Generally it is possible to clean up the shaft with minorregrinding of the bearing journal surface.V. PERFORMENCEA. Efficiency vs. Bearing TypeIt is sometime asked whether anti-friction bearings canachieve better efficiency than sleeve bearing machines. Ingeneral the answer is that the difference is insignificant..A/F bearings do normally generate fewer losses butnormally the losses are a small percentage of the totallosses having little effect on the total efficiency. Regardingthis question, the following example for a 500 frame 2 pole300 HP and 1250 HP motor may help:Total. Brg. Efficiency .Loss Loss A/F SleeveW/O in kW Brg. BrgHP Brg. A-F / Sleeve.. Loss .300 13.5 .52/ .75 94.09 94.001250 46.7 .52/ .75 95.17 95.15As can be seen from the above, the difference is small.B. Service life vs. Theoretical lifeThe calculated life expectancy of any bearing is based onfour assumptions.1) Good lubrication in proper quantity will always beavailable to the bearing.2) The bearing will be mounted without damage.3) Dimensions of parts related to the bearing will becorrect.4) There are no defects inherent in the bearing.In general, sleeve bearings have an extremely long life.Anti-friction bearings that are oil mist or sump lubricatedalso do and will reach their expected L10 fife. Greaselubricated bearing rarely reaches their expected L10 life.C. Rotor-Dynamic PerformanceBearing selection can have a significant impact on rotordynamic performance. Table III shows the results of acase study of a similar motor with different bearings. Therotor under study was for a 3600 RPM 1250 Horsepoweropen motor. The rotors were similar, with the obviousexception of bearings (and the required subsequentchanges in the shaft).Notice that as the stiffness decreases, so does theresonant frequency. However, highly damped resonancesare not considered to be critical speeds. Per API 684,amplification factor is defined as a measure of a rotorbearingsystems vibration sensitivity when operated in thevicinity of one of its lateral critical speed [3]. Furthermore,this same standard defines critical speed as a shaftrotational speed that corresponds to a non-criticallydamped (amplification factor >2.5) rotor system resonancefrequency. This is an important definition as a highlydamped resonance should not be misinterpreted as acritical speed. The justification in this lies in the fact thateven if a motor were to operate right at this speed, theincrease in vibratory response would be so low that nodamage to the motor or bearings would occur. This isevident when reviewing the calculated rotor dynamicperformance of the 150 Partial arc sleeve bearing to the6315 anti-friction bearing, in particular, with respect to therotor response to unbalance as illustrated in Fig. 11:Fig. 11 compared the predicted increases in vibration dueto an unbalance of 4W/N, which is equal to the residualunbalance limit for each end of the rotor in API 541motors. The peak to the right corresponds to the 6315AFB bearing, and the peak to the left, the 150 degreepartial arc bearing. Note the significantly different scalesfor the two curves (6315 corresponds to the scale on theright, and the 150 degree partial arc, the scale to the left).Furthermore, note that the vibration of the 6315 rotorwould be 34 times higher at its resonant speed comparedto operating speed. Clearly, vibration severity of thismagnitude cannot be tolerated for any period of time.150 Sleeve BearingVibration mils Pk-Pk0.30.250.20.150.10.05010001500Rotor Response to Unbalance (4W/N)2000250030003500400045005000Rotor Speed - RPMFig. 11 Rotor response to unbalance550060006500700032.521.510.506315 AFB Vibrationmils Pk-PkLikewise, the 150-degree partial arc-bearing rotor wouldbe 1.2 times higher at its resonant speed compared tooperating speed. The rotor can operate at this speedindefinitely without any harm to the rotor or bearing.Page 11 of 13
Page 1 and 2: Copyright © 2001 IEEE. Reprinted f
Page 3 and 4: cool the oil such the bearing speed
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Page 9: AirSealVent ChamberA. Anti-Friction
Page 13: VI.COMPARISONObviously, both types

Sleeve vs Antifriction Bearings - Siemens Industry, Inc.

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