外文翻译--电梯传动轴的事故分析 英文版.pdf

Failure analysis of an elevator drive shaft A G ksenli I B Ery rek Faculty of Mechanical Engineering Istanbul Technical University G m ssuyu 34437 Taksim Istanbul Turkey a r t i c l ei n f o Article history Received 23 May 2008 Accepted 25 May 2008 Available online 11 June 2008 Keywords Drive shaft Elevator Failure analysis Notch effect Finite element method a b s t r a c t In this study failure analysis of an elevator drive shaft is analyzed in detail Failure occurred at the keyway of the shaft Microstructural mechanical and chemical properties of the shaft are determined After visual investigation of the fracture surface it is concluded that fracture occurred due to torsional bending fatigue Fatigue crack has initiated at the key way edge Considering elevator and driving systems forces and torques acting on the shaft are determined stresses occurring at the failure surface are calculated Stress analysis is also carried out by using fi nite element method FEM and the results are compared with the calculated values Endurance limit and fatigue safety factor is calculated fatigue cycle analysis of the shaft is estimated Reason for failure is investigated and concluded that frac ture occurred due to faulty design or manufacturing of the keyway low radius of curvature at keyway corner causing high notch effect In conclusion effect of change in radius of cur vature on stress distribution is explained by using FEM and precautions which have to be taken to prevent a similar failure is clarifi ed 2008 Elsevier Ltd All rights reserved 1 Introduction The elevator drive shaft investigated in this paper was in service for 30 years Elevator drive system is mounted at the bottom of the building Fig 1 It is reported that no maintenance was applied on the shaft during operation life The elevator is used in a building consisting of eight fl oor and 16 apartments The elevator has a four person 320 kg capacity Just before the accident two persons were leaving the elevator and because of sudden fracture of the shaft connection between pulley and electric engine was broken which lead the break system of the engine to fail Due to the weight of balance weight ele vator lifted upwards with a huge speed During the accident nobody was injured 2 Analysis of elevator drive system During analysis of elevator drive system it is concluded that torque which is produced by an electric motor is transmit ted by a worm gear to the shaft The electric engine which produces 6 5 HP is rotating 1500 rpm and reduction ratio of the worm gear is 28 6 The shaft rotates the pulley by a key The four main ropes of the elevator are placed on the pulley and by revolving in both direction of the drive shaft and pulley the elevator moves up and down Service speed of the elevator is 0 6 m s The shaft is supported in three points in form of journal bearings Fig 2 1350 6307 see front matter 2008 Elsevier Ltd All rights reserved doi 10 1016 j engfailanal 2008 05 014 Corresponding author E mail addresses goksenli itu edu tr A G ksenli eryurekb itu edu tr I B Ery rek Engineering Failure Analysis 16 2009 1011 1019 Contents lists available at ScienceDirect Engineering Failure Analysis journal homepage 3 Visual investigation of fracture surface After primary visual investigation it is revealed that fracture occurred at the keyway where the pulley is fasten at the shaft Fig 3 After analyzing fracture surface Fig 4 a typical torsional bending fatigue fracture surface is detected 1 Fatigue cracks initiated at the corners of the keyway and moved almost along the whole surface The small area of brittle fracture surface indicates a low applied stress Fatigue lines were detected only near to the brittle fracture region This might be due to the friction of two separated surfaces with each other causing the formed fatigue lines to disappear 4 Properties of material As no information with respect to the chemical composition of the shaft material was available the fi rst task in the failure analysis was the material identifi cation To determine shaft material chemical mechanical properties and microstructural analysis was carried out Fig 1 Elevator system inside the building Fig 2 Elevator drive system 1012A G ksenli I B Ery rek Engineering Failure Analysis 16 2009 1011 1019 4 1 Chemical analysis The chemical analysis of the shaft was carried out by atomic absorption spectroscopy and is reported in Table 1 4 2 Microstructure The microstructure of the shaft material was developed by etching after diamond polishing with 2 Nitral solution and was observed under the microscope Fig 5 A ferritic pearlitic and fi ne grain structure can be clearly seen 4 3 Mechanical properties Tensile and hardness tests were performed to determine the mechanical properties of the shaft which can be seen in Table 2 Considering mechanical chemical and microstructural analyze results shaft material is estimated as St52 0 Tensile yield elongation and hardness values are suitable for the catalogue values of St52 0 according DIN 1629 2 Fig 3 Failed shaft Fig 4 Fracture surface Table 1 Chemical composition of the shaft C0 22Al0 012P0 031 Si0 40Mn0 13S0 029 A G ksenli I B Ery rek Engineering Failure Analysis 16 2009 1011 10191013 5 Stress analysis By stress analysis minimum and maximum normal and shear stress values occurring at the fracture surface during oper ation is investigated At fi rst forces and torques acting on the shaft are determined By analyzing minimum stress value only the weight of the empty cabin 420 kg and balance weight 580 kg is considered In this case reaction forces causes a bend ing moment of 437 4 N m at the fracture surface resulting as normal stress value of 20 6 MPa Shear forces caused due to the loads of empty cabin ropes and balance weights forms a shear stress of 3 5 MPa By analyzing maximum stress value bal ance weight cabin weight with four persons inside each person is 80 kg and the total weight of the cabin is 740 kg torsion moment and impact ratio is considered In this case bending moment of 571 N m occurs at the fracture surface causing a normal stress value of 27 MPa Shear stress value due to shear force is 4 7 MPa The 6 5 HP electric engine rotates at 1500 rot min conversion ratio of the worm gear is 28 6 and drive system effi ciency due to worm gear mechanism is 0 7 Considering these parameters torsion moment is calculated as 887 7 N m Total shear stress is calculated under these circumstances as 25 7 MPa By visual examination it was determined that the transition from keyway ground to keyway side surface corner was approximately perpendicular and almost no radius of curvature RC was observed Fig 6 Theoretical RC cannot be zero it can only reach a value of 0 4 mm by fi ne milling cutter 3 Therefore by calculations RC is taken as 0 4 mm causing an enormous high notch effect The theoretical notch effect is analyzed in two states using rel evant tables 4 shear notch effect aS which is determined as 2 93 and bending notch effectaBis determined as 2 72 Fa tigue notch factor b considering geometry and material of the shaft can be calculated as 4 b 1 a 1 g 1 Fig 5 Microstructure of the shaft material 200 Table 2 Mechanical properties of the shaft Yield strength MPa Tensile strength MPa Rapture elongation Hardness BHN 33956918165 Fig 6 Failed shaft and fracture surface 1014A G ksenli I B Ery rek Engineering Failure Analysis 16 2009 1011 1019 wheregis notch sensitivity factor and its value is 0 85 4 Using Eq 1 shear fatigue notch effect bS is calculated as 2 64 and normal fatigue notch effect bN as 2 46 Impact ratio coeffi cient is taken as 1 2 Considering shear and normal stresses impact ratio q and fatigue notch effects equivalent stress rEQ is calculated using Shape Deformation Energy Hypothesis 5 as rEQ ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffi ffiffi ffiffi ffiffi ffiffi r q bN 2 3 s q bS 2 q 2 The summarized results can be seen in Table 3 5 1 Fatigue strength analysis Fatigue strength endurance limitrE of the shaft material was calculated as 6 rE 0 5 UTS 0 5 568 284 MPa 3 Considering size factor kD for 60 mm diameter as kD 0 77 and surface factor fi ne polished kS 0 95 6 the new endurance limitrIE is calculated as rIE rE kS kD 284 0 77 0 95 208 MPa 4 rIE 208 MPa From our calculations done before we know that average stress value rM is not zero To determine the effect ofrMon endurance limit we have to know or at least to estimaterM To calculaterM we have to consider the change of stress value occurring at the fracture surface in time But this is quite diffi cult By our calculations done before we calcu lated maximum and minimum stress values Minimum stress value occurred at empty cabin and stationary position veloc ity is zero and with no acceleration maximum stress value by acceleration of the elevator and four persons each person is assumed to be 80 kg inside the cabin But in real number of persons transported inside the cabin is not always the maxi mum transportation number of person four Therefore a Variable amplitude stress occurs at the fracture surface depend ing on the number of persons inside the cabin Fig 7 Each minimum peak demonstrates the stress value at the fracture surface at the time the elevator is not moving and is empty each maximum peak demonstrates the stress value occurring at the time of cruise of the elevator with person s in side the cabin and considering acceleration To calculate average stress value we have to transform Variable amplitude stress into Constant amplitude stress by assuming that always maximum stress occurs at the fracture surface always four persons are transported According to this assumption change of stress value versus time can be seen in Fig 8 In this case average stress value is 108 MPa Considering average stress value before calculated endurance limit is modifi ed By the analysis criteria of Goodman 6 is regarded Fig 9 and modifi ed endurance limitrII E is calculated as rII E r I E 1 rM UTS 208 1 108 569 170 5 Considering modifi ed endurance limitrII E and equivalent stress rEQ fatigue safety factor t can be calculated as t rIE rEQ 170 162 1 05 6 5 2 Fatigue life analysis From Eq 6 can be seen that fatigue safety factor is quite very low t 1 05 For detailed fatigue life analysis life cycle is estimated by following assumptions The total elevator displacement inside the building is by eight fl oors approximately 21 m The building consists of 16 apartments In half of the apartments there are two and other half there are three persons living Each person is using the elevator twice a day The elevator has been used 340 days in a year and shaft is in operation for 30 years The diameter of the pulley is 400 mm and by each rotation the elevator is moving 1 257 m Considering these explanations total life of the shaft was approximately 6 8 106cycles For further fatigue analysis stress cycle S N curve of the shaft is estimated To draw the S N curve of the shaft accord ing to Juvinall and Shigley 7 8 stress value rF where fatigue failure cycle at 103cycles occur can be calculated as rF m UTS 7 Table 3 Stress force and torque values occurring at the fracture surface during operation Normal stress MPa Shear stress MPa Equivalent stress MPa Shear forceTorqueTotal Minimum50 69 2 9 254 Maximum79 715 167 382 4162 A G ksenli I B Ery rek Engineering Failure Analysis 16 2009 1011 10191015 where m 0 9 for bending The stress value at Nf 103cycles according Eq 7 is 512 MPa The second point at the graph is the endurance limit value which is 106cycles for steel According to these explanations estimated S N diagram can be seen in Fig 10 By low stress values and high cycle fatigue failures can occur between 106and 107cycles 9 Therefore estimating life cycle value of 6 8 106cycles for fatigue failure supports our thesis Fig 7 Stress time graphic acting on the fracture surface depending on the number of persons inside the cabin Variable amplitude load Fig 8 Stress time graphic acting on the fracture surface Constant amplitude load Fig 9 Determining modifi ed endurance limit considering average stress value of 108 MPa according to Goodman 1016A G ksenli I B Ery rek Engineering Failure Analysis 16 2009 1011 1019 6 Finite element analysis To examine stress distribution at the keyway and fracture surface fi nite element method FEM was applied By the anal ysis ANSYS program was used A precise geometrical model of the shaft was built up Since the shaft is too long to be ana lyzed completely as a three dimensional object only the keyway and fracture region was modeled in detail Fig 11 High stress regions especially at the corner of the keyway can be clearly seen The dominant effect of the raise in stress values is the low radius of curvature causing a high notch effect The aim of stress analysis using FEM was to verify our stress calculations done before From Fig 11 can be seen that stress values at the fracture surface were close to the calculated ones 7 Discussion By increasing radius of curvature RC value stresses occurring at the keyway corner could be decreased effectively To determine the effect of RC on stress distribution fi nite element analysis is carried out By this examination RC value was increased stepwise for visual analysis of decrease in stress values which can be seen in Fig 12 Dramatic decrease of stress values at keyway corner can be clearly seen For further investigation the effect of change in RC on stress and fatigue safety factor is analyzed in detail which can be seen in Figs 13 and 14 By increasing radius of curvature even from 0 5 mm to 2 mm would decrease stress value from 163 to 104 MPa and an increase in fatigue safety factor from 1 05 to 1 78 Figs 13 and 14 demonstrate that an increase of radius of curvature would Fig 10 Estimated stress cycle diagram Fig 11 Finite element mesh of the shaft A G ksenli I B Ery rek Engineering Failure Analysis 16 2009 1011 10191017 probably prevent the failure of the elevator drive shaft In conclusion it is determined that fracture of the shaft occurred due to faulty design or manufacturing of the keyway low radius of curvature causing a high notch effect Fig 12 Effect of radius of curvature RC on stress distribution using FEM Fig 13 Effect of RC on stress distribution Fig 14 Effect of RC on fatigue safety factor 1018A G ksenli I B Ery rek Engineering Failure Analysis 16 2009 1011 1019 8 Summary Failure analysis of an elevator drive shaft is investigated