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WORLD PUMPS October 2008
www.worldpumps.com
Feature8
0262 1762/08 2008 Elsevier Ltd. All rights reserved
Fatigue failure
Fatigue failures inpumps part 3In this final part of a short series of articles on fatigue
failures in pumps, Rafael Ocampo uses real examplesto illustrate how inattention to detail during maintenanceand failure to recognize stress concentrations can leadto this common type of mechanical breakdown. A finalsumming up of the causes of fatigue failures and stepsto reduce their occurrence is also presented.
Following on from the second partin the series, this third articlecontinues the discussion of case
studies as real examples of the inuenceof stress raisers on the fatigue failure ofpumps and their motors.
Pump shaft failure
The rst case study considers the fatiguefracture of a pump shaft at the couplingend. The relevant factor in this case isthat the degree of misalignment was
found to be larger than the small valueallowed by the coupling type.
This case occurred in a vertical conden-sate extraction pump using an old designof over-running clutch coupling. Thecoupling hub is locked by a nut on thepump shaft end and the coupling core istted on the electric motor shaft. A set ofcylindrical rollers is installed in a cagebetween the core and the hub, transmit-ting the torque only in the correctrotation direction. In this particular designof pump, the axial hydraulic thrust and
the weight of the pump rotor aresupported by a thrust bearing locatedimmediately below the coupling and theinner ring of the bearing is locked inposition by the coupling hub and the nut(Figure 1). Because of the length of therollers, this coupling allows very littleangular misalignment between pumpand motor shafts.
As in every fatigue failure, the breakdownoccurred suddenly and nothing could bedone to prevent the damage to theinternal parts of the pump. The fracturewas located in a change of section in theshaft under the coupling hub ( Figures 1 and 2), and the appearance of thefracture surface suggested the presenceFigure 1. Sketch of the shaft ends with coupling and thrust bearing.
1 Electric motor shaft
2 Coupling core
3 Coupling rollers
4 Flat support plate
5 Coupling hub
6 Thrust roller bearing
7 Pump shaft
8 Washer
9 Slotted nut and cotter pin
1
2
3
5
6
7
498
Fracture surface
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of symmetric bending stress in rotationacting over a low stress concentrationfactor at a moderate cyclic load (Figure 3).
The load causing the failure is assumed tobe the bending stress caused by theactual angular misalignment being larger
than that allowed by the over-runningclutch coupling. The original motorsupport frame had been replaced and it ispossible that a slight dimensionaldifference caused misalignment. Mainte-nance engineers ought to have recognizedthe additional care required by this type ofcoupling during alignment and demandedmore accurate work from the maintenanceoperators and the use of more precisetools for the job. The change of section onthe shaft behind the thread is the fullyloaded stress raiser nearest to the load
application point, the rest of the pumpshaft being very smooth.
After the failure of the shaft, the couplinghub and the thrust bearing inner ring arefree and the whole pump rotor is also freeto move downwards, while still rotating athigh speed, until it collides with the pumprst-stage casing and bottom bush.Damage to the rst-stage casing, rst-stage casing wear ring and bottom bushare shown in Figure 4, while Figure 5shows the damage to the rst-stage
impeller front shroud and wear ring.
Motor shaft fracture
The second case study examines thefatigue fracture of an electric motor shaft.Again, the problem was found to stemfrom the actual misalignment beinglarger than that allowed by the couplingtype.
This case also occurred on a verticalcondensate extraction pump using an olddesign of over-running clutch coupling. Thepump involved was in fact a twin to thatdiscussed in the previous case study. Thistime the fracture was located not on thepump shaft but on the motor shaftimmediately behind the coupling core(Figure 6). As in the previous case study, theappearance of the fracture surfacesuggested the presence of symmetricbending stress in rotation acting over a lowstress concentration factor at a moderatecyclical load (Figure 7).
As with the twin pump, it is inferred that
the load causing the failure arises fromthe actual angular misalignmentexceeding that allowed by the over-running clutch coupling. As was the casewith the rst pump, the motor support
Figure 3. Appearance of pump shaft fatigue fracturesurface.
Figure 2. Location of fatigue fracture in a change ofsection on the pump shaft.
Figure 4. Damage to rst-stage casing, wear ring and bottom bush.
Figure 5. Damage to rst-stage impeller front shroud and wear ring.
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frame had been replaced. The stressconcentration factor this time is thecorner of the keyway, where the crackclearly starts (Figure 6).
The damage in this case was limited tothe motor shaft. No damage was
sustained by the pump because thepump rotor remained in its workingposition supported by the thrust bearing.
Helpful hints
The frequent occurrence of fatiguefailures in industry is a clear indication ofhow difficult it can be to design anapparently simple spare part successfully,and how easy it can be to produce anincorrect design that will fail in servicewithout any obvious warning signs.
A few theoretical hints will help to dealwith fatigue failures in everyday life.
Fatigue failures of parts, even those madefrom plastic materials, do not usually
involve any external manifestation of plasticdeformation and the breakage occurssuddenly, without previous indication thata failure is going to occur.
Under rotating bending loads, the fracturegenerally occurs on a surface that is
perpendicular to the rotation axis,regardless of whether the load is high orlow. The appearance of the fracture surfacein different cases will present features thatcan give an indication of the type andmagnitude of the load and also about thepresence of a stress raiser.
The reduction of fatigue strength incorrosive media is very considerable, so thecareful selection of the material is of thehighest importance as well as preventingcrevices and other factors favouring
corrosion. The form of machine elements is of special
importance. Abrupt changes of formshould be avoided; stress concentrationpoints of any type on the surface shouldbe reduced to the minimum; forces shouldbe directed away from the stressconcentration points, introducingadditional smaller concentration pointsfunctioning as stress relieving points forlarger stress raisers.
Many decisions taken while designing,
manufacturing or repairing pump partscould lead to fatigue failures if the possibleintroduction of stress raisers is not verycarefully checked and avoided. Although itis not always carried out, a very carefulnal inspection of parts manufactured onsite will reveal harmful details.
Practical experiences such as thosediscussed in the case studies presented inthis paper also yield helpful indicationsfor dealing with fatigue.
Fatigue failures can occur under cyclicstresses in rotating pump parts, in rotatingpump parts without apparent cyclic loadand even in stationary pump partswithout apparent cyclic load. Small andapparently unimportant pump parts arealso subjected to this type of failure.
Manufacturing spare parts on site canpresent a serious risk of producing partsprone to fatigue failure if the basic fatiguetheory is unknown or neglected.
Repair jobs and spare parts manufacturedon site should be carefully inspected byskilled engineers in order to detect andremove stress raisers inadvertentlyintroduced by inexperienced personnel.
Repair and/or maintenance malpracticescan introduce weak points in different
pump parts leading to later fatiguefailures. Maintenance personnel shouldtake into account the care required bydifferent types of equipment, especiallywhen replacing important parts.Misalignment should be recognized as asource of harmful stresses on pump andmotor rotating parts, and the type ofcoupling installed and its particularfeatures taken into account.
Final word
When repairing a pump or any othermachine, the aim is to return it as close aspossible to its original design condition.However, if you do not know that conditionexactly and you do not have the requiredknowledge regarding machine design, youcan hardly carry out a reliable and durablerepair job.
Through many years of industrial experi-ence, the author has witnessed countlesscases proving that lack of knowledge andtechniques concerning the design of
machine elements on the part of eldengineers frequently leads to malpracticesin repair and maintenance that are sourcesof fatigue failures. The same is true for lackof design knowledge applied to manufac-turing spare parts. Only the knowledgeand application of basic fatigue theory, theaccumulation of practical experience andthe thorough inspection of everycomponent of the equipment will help toreduce the prevalence of this type offailure in industry.
Figure 6. Fracture location on the motor shaftimmediately behind the coupling core.
Figure 7. The fatigue fracture surface of the motor shaft.
ContactRafael Ocampo
Tel: +53 43 51991 5
E-mail: [email protected]
mailto:[email protected]:[email protected]:[email protected]
