Abstract of Special Research Report (SRR-No.18)

National Institute of Occupational Safety and Health, Japan

Study on New Inspection Techniques for Lifting Apparatus --Focused on failure of crane wire ropes--


Masazumi TANAKA

: While recently the total number of occupational accidents is steadily decreasing in Japan, the number of crane accidents shows a trend of slight increase on the contrary, and the number of casualties to death due to the failure of lifting apparatus for these five years reached about 80.
    As the background of this situation, some factors have been pointed out, for example, like that taking care for safety during usage or inspection of them for cargo-working is insufficient, and besides, methods of strength assessment and inspection have not been established properly. And recently, the requires for revising such situations have increased.
    Among such situations, recently considerable numbers of crane accidents according to the fracture of wire ropes happened. And the prevention of possible similar accidents has become the most urgent problem for many persons concerned.
    In this specific research, therefore, main target was focused on this problem and the following subjects were carried out.
  (1) Case studies on some crane accidents caused by the fracture of wire ropes.
  (2) Precise experimental investigations on the properties of degradation of crane wire ropes.
  (3) Studies on countermeasures to prevent possible similar accidents.
    In Chapter 2 of this report, relating to the subject (1), the results of the precise investigations on the crane accidents due to the fracture of IWRC wire rope are introduced, and the direct and/or possible essential causes are studied, together with the other information on the degradation properties of this kind of wire ropes.
    In Chapter 3, as the main part of this specific research, following to the results of the study in Chapter 2, the results are introduced at first on an experimental investigation to certify whether or not the internal damage can occur under any allowed working condition, and mainly on the following thorough experimental investigations under wide range of testing conditions to clarify whether or not the internal damage can be a characteristic of IWRC wire ropes when they are used as travelling ropes. Moreover, a problem of the present inspection system is pointed out.
    Chapter 4 deals with the prediction of fatigue life of IWRC wire ropes, taking also the internal damage into accounts, as a part of the study on the subject (2). And Chapter 5 relates to the experimental investigation on essential properties of fretting fatigue of wire materials, as another part of the study. In Chapter 6, the subject (3) is investigated, mainly based on the results of the study on degradation property of wire ropes of this kind.
    Chapter 7 summarized the results obtained from this specific research.

Analysis of Fracture Accidents of Crane Wire Ropes and the Problem Included

Masazumi TANAKA

: Wire ropes are widely used for cranes, for example, on the lifting and derricking work, under the condition of heavy duty in many cases. And recently in Japan, considerable number of serious crane accidents happened, caused by the fracture of wire ropes.
    In this report, three crane accident cases which resulted in one or more casualties (death) were reviewed first, then the characteristics of rope failure were defined, and the problem of the present situation relating to the safety aspects were clarified from the accident analysis and other references.
    As to the first case, the derricking wire rope of a 60 ton crane suddenly broke when steel material were being hoisted in a building construction site. The result of the precise studies including spot investigation, analysis of the state of wire rope failure, the strength of the rope and the fractographic analysis of the feature of the fractured surfaces of wires showed that the direct cause of this accident was the remarkable decrease of rope strength due to internal wear and the following fatigue failure of wires. However, the essential causes seemed to be that the damage had proceeded preferentially at internal portion, and therefore, it was very difficult to detect it.
    As to the second case, similarly the derricking wire rope of a 50 ton crawler crane suddenly fractured when four H-section steels were hoisted hi the pier construction site. From the similar precise analysis as mentioned above, it was supposed that the main cause of this accident was also the remarkable decrease of rope strength to about one-tenth of that in the original normal state, and that although such heavy state of damage was due to the improper maintenance or management, the role of the internal failure was essentially important as it was in the first case.
    As to the third case, the main hoisting wire rope of 150 ton crawler crane broke when a 44 ton concrete block was being hoisted at a sea wall construction site. It was concluded that the cause was the drastic decrease of rope strength due to the severe abrasion on the surface and the remarkable corrosion especially in the internal portion of the rope, maybe with such damage caused by improper maintenance inspection.
    From the analysis of this kind of accidents including ones in other references, it was supposed that the IWRC (internal wire rope core) wire rope commonly had a characteristic of preferential internal failure when it was used as a travelling wire rope. As the crane users and even inspectors practically can not detectthe damage of this type correctly by means of the external observation which is common in Japan, the presentsituation would be serious from the safety point of view if the above mentioned supposition were true.
   This supposition will be confirmed in the next Chapter.

On the Propeties of Internal Damage of Crane Wire Ropes

Masazumi TANAKA

: As it was introduced in the previous chapter, the presice investigations on the crane wire rope fracture accidents showed that the remarkable internal damages formed prior to the accident were found in many cases in IWRC (independent wire rope core) wire ropes.
    If this kind of damages were formed even under the usual working conditions, the present inspection system would be insufficient, because the usual inspection mainly depends on the external observation by naked eyes and therefore the internal damages usually could not be detected correctly.
    However, it is not clear whether such supposition is true or not, because the working conditions or history of working loads could not be confirmed for all cases of accidents, and under such situation it would be difficult to decide the target of studying countermeasures for preventing the accidents of this kind.
    In this study, therefore, to improve such ambiguous situation, a thorough experimental investigation mainly consist of S-bending fatigue tests and the following pres ice analysis of damaging condition of the ropes was carried out to certify at first whether the remarkable internal damage as mentioned above could be formed even under the allowed loading conditions, and then to confirm the relations between the fatigue damaging conditions and the various testing conditions referring to the real working conditions.
    The main results obtained are as follows.
  (1) Two kinds of IWRC wire ropes showed the remarkable internal wire breaking damage in many cases preferential to the external ones under the S-type bending fatigue, not only under the allowed D/d (ratio of sheave diameter to rope diameter) and the rated loading condition as the basic ones for this study, but under other mechanical conditions with the rope tension and load frequency widely changed.
  (2) They also showed the similar internal wire breaking damage under the S-type bending fatigue in many cases excluding limited conditions, under the environmental conditions of insufficient rope grease, clean water dropping and salt water dropping.
  (3) Under the U-bending condition, essential damages for both wire ropes were also the internal ones, although the external wire breakings were more than those in case of S-bending.
  (4) From such results, it may be concluded that the IWRC wire rope, when it is used as a travelling wire rope, has a characteristic that the internal damages are apt to occur prior to the external ones.
  (5) Nevertheless the IWRC wire ropes with such characteristics are widely used for cranes, the present inspection method practically relies on the external observation by naked-eyes which can not essentially detect the internal damages. This situation seems to be fairly uneasy ones, and it is necessary to take any countermeasure to improve it in a hurry.
  (6) It wouldn't be an appropriate method to prevent the internal damages by means of the change of working conditions when the IWRC wire ropes were used for cranes. Hereafter, it would be important, for example, to develop the simple and convenient NDT (Non Destructive Testing) methods capable of detecting damages in the ropes even if those were the internal ones, and/or to study a method to make the discarding standards more strict, and so on.

Analysis of Progression Properties of Wire Breaking Damage

Masazumi TANAKA

: To decide the reasonable inspection period and to improve the quality of safety management for crane wire ropes, it is inevitable to predict reliable fatigue life under well known working conditions and moreover to grasp the progression properties of wire breaking damage. However, so far the fatigue life of wire ropes has been assessed hi many cases practically by the number of load repetitions until the number of visible wire breaking detectable by external observation reaches a specific number. And the case of the fatigue life assessment which takes the internal damages especially interested in this specific study into consideration or the study on the progression properties of all wire breaking patterns including internal wire breaking have not been found.
    In this study, therefore, S-bending fatigue tests was carried out and the quantitative expression of increasing behavior of wire breaking numbers for one kind of the IWRC wire ropes (IWRC 6xFi(29)) previously studied in Chapter 3.
    The main results obtained are as follows.
  (1) In both cases of D /d = 16 and 12.8 (D: sheave diameter, d: rope diameter), the preference of internal wire breaking in progression property through all life does not change, and therefore, the ratio of internal wire breaking is larger in the early stage than in the later stage.
  (2) The dependence of the maximum number of visible wire breaking C v max on the number of the repetition of S-bending N is approximated by following formula.
        C v max= A·N m
here, A and m are constants, being 8.81 × 10-21 and 5.61 respectively for D / d = 16, and 1.66 × 10-17 and 5.61 respectively for D / d = 12.8. The influence of D / d is larger for the coefficient A.
  (3) For D / d = 16, the dependence of the maximum of total external wire breaking numbers of strands C 0 max and the the maximum of total wire breaking number of the strands C t max on N is approximated by the following formula.
        C 0 max= 3.68 × 10-15 · N4.25
        C t max= 1.03 × 10-13 · N3.88
  (4) For D / d = 16, C t maxis given by the following expression, as a function of the maximum of the visible wire breaking number C v max.
        C t max= 7.3 C v max0.69
    This kind of formula axe thought to be very useful, when studying the damaging state of wire rope including the internal ones.

Fretting Fatigue Behavior of Wire Rope Steel and SNCM439 Steel

Yoshio KITSUNAI, S.Genesh Sundara Raman and Muthuswamy Kamaraj

: It is known that one of the most important factors in failure cause of wire rope is fretting fatigue, which occurs as a result of relative cyclic slip at the interface between two surfaces in intimate contact. However, study of fretting fatigue in wire rope is limited because fretting phenomenon is very complex and reasonable fretting fatigue test is difficult. In this study, to assess the important variables that can affect fretting fatigue of wire rope, fretting fatigue behavior of a cold drawn eutectoid steel rod, which has been used to make wire rope, was examined by clamping the contact pads with the same material. Fretting fatigue strength is affected various factors. The effects of specimen configuration, fretting test apparatus, contact pressure and pad span on fretting fatigue strength were examined using two types of specimens and contact pads made of SNCM439 steel.
    The fretting fatigue specimen for the wire rod is a total length of 280 mm with button head ends an a 90 mm long gage length with the cross sectional area of 33.24 mm2. Two types of specimen geometry were machined from the rods of 16 mm and 25 mm dia. of SNCM439 steel. One is the same configuration as the wire rod specimen. Another is rectangular configuration with the cross section of 5 x 10 mm and gage length 70 mm. Bridge type contact pads having the span lengths of 20, 30 and 50 mm of the same material as the fatigue specimens were prepared. Two types of fretting fatigue test apparatus are used. One is a screw type apparatus of which contact pressure was applied by threaded screw attached to the proof ring. Another is an oil pressure type apparatus.
    The plain and fretting fatigue tests were performed under a sinusoidal wave at frequency of 10 Hz using a 98 kN capacity closed loop servohydraulic fatigue testing machine. The fretting fatigue tests were carried out under contact pressure of 60, 96.5 and 100 MPa, and a stress ratio of 0.1.
    The slip amplitude between the specimen and the contact pad was determined using a contact type or lazier type displacement sensor. The frictional force between the specimen and the contact pad was measured using the strain gages bonded to the central part of the contact pad holder or the contact pad. The fractured and fretted damaged surfaces of the specimens were examined by an optical microscopy and a scanning electron microscopy to determine fracture morphologies, crack initiation and propagation modes of fretting cracks.
     Based on the results of this experimental program, the following conclusions were drawn:
  (1) The fretting fatigue limit of wire rod and SNCM439 steel decreases about 55-70% and 34-57%, respectively, as compared with the plain fatigue limit.
  (2) The fretting fatigue life of both wire rod and SNCM439 steel specimens decreases with increasing contact pressure due to increasing frictional force.
  (3) The fretting fatigue strength of different types of specimens shows approximately same fatigue limit at a given contact pressure.
  (4) Under a given contact pressure, the fretting fatigue life tends to decrease with increasing pad span because of increasing relative slip amplitude, regardless of specimen configuration.
  (5) The fretting fatigue life or strength is controlled primarily by both relative slip amplitude and contact pressure.
  (6) The fretting fatigue life measured by the hydraulic type clamping apparatus tended to decrease, as compared to that obtained by the screw type apparatus.
  (7) Fractographic examination of the tested specimens revealed the fretting cracks were initiated at a shallow angle to the contact surface and the direction of crack growth changes to the loading axis. Fretting damage surface showed the wear tracks with heavy plastic deformation.

On the Countermeasures for Preventing Wire Rope Fracture Accidents

Masazumi TANAKA

:Through many analysis carried out by the author on the wire rope fracture accidents in cranes, remarkable internal damages were found to exist in IWRC wire ropes. And from the results of such analysis and the thorough certification tests on these kinds of wire ropes, it was concluded that the internal damage was one of the general characteristic for these kinds of wire ropes when they were used as travelling ropes. On the basis of the conclusion, an important problem in occupational safety was pointed out that the existing inspection methods in our country mainly by means of naked eye's observation could not detect correctly such kinds of internal damages.
    In this chapter, the results of the studies by concerning persons (Ministry of Labor, Japan Crane Association, wire rope makers and crane users) and the performance certification tests by the author on a new commercial wire rope tester were summarized and discussed to resolve this serious problem.
    Countermeasures introduced are as follows,
  (1) Direct observation of the inside of ropes, opening them using tools called clamping jaws.
  (2) Making the discarding standard for wire breakings more strict(safe).
  (3) Shortening of the working term.
  (4) Elimination of the factors accelerating the internal wire breakings.
  (5) Proper treatment of the rope by sufficient supply of rope grease.
  (6) NDT (non destructive testing) methods with higher detectability of internal damages.
  (7) Measurement of the rope elongation.
  (8) Measurement of diameter or circumference of ropes.
    Finally, two concrete methods or countermeasures for the internal damages of IWRC wire ropes were selected and recommended as realistic ones.
    The first one is "the simple inspection manual for crane wire ropes" proposed and under sale at low price from The Japan Crane Association. This is a combined method of measure (2) with other ones, hi which the simple naked eye's observation method is adapted first, with more strict discarding level for various kinds of damages than usual ones (for example, provided in Cranes Structure Code), and the rope with higher damage level should be discarded, or one can use it moreover if he could certify the safety of the rope by means of more reliable inspection methods.
    The second one is an electro-magnetic damage inspection method using new detecting apparatus developed recently by a rope maker hi Japan. According to the study on its detecting ability of internal wire breaking, it has realistically satisfactory performance for checking the safety of IWRC wire ropes.
    Of course, these are not ideal ones, and more suitable or reliable inspection methods are strongly expected among the persons concerned.

Concluding Remarks

Yutaka MAEDA

: This specific research started with the occasion of the Southern Hyogo-prefecture earthquake in 1995, which caused a great deal of damage to many cranes. The dynamic effect of earthquake on jib cranes and tower cranes is not prescribed in the standard for proof against earthquake of crane.
    Considering the damage of cranes by this earthquake related in the chapter 2, the tower crane used in the construction site was selected as the main object of this specific research. Results of the studies are summarized as follows;
  (1) Survey on damage of cranes due to the Southern Hyogo-prefecture Earthquake
    Jib cranes and tower cranes suffered more damage compared with bridge cranes (container cranes) and overhead travelling cranes. Damage of portal jib cranes was particularly concentrated around roller path, and about a half of this damage was due to buckling of the columns. The tower cranes fixed with stays were severely damaged, while the tower cranes without stay suffered relatively minor damage. Main damage cause of overhead travelling cranes was falling down and derailment due to large plastic deformation of runway girders. Container cranes and unloaders installed along waterfront of reclaimed lands were severely damaged due to movement of the foundation associated with soil liquefaction.
  (2) The vibration characteristics of a tower crane used in the construction site
    To obtain the dynamic characteristics, an actual tower crane was excited by sudden release of the lifted load or by stopping the rotation. The natural frequencies of the tower crane standing by itself were 0.27 Hz in the lateral direction of jib axis and 1.4 Hz for the tortional vibration of the mast, and the first mode damping factor was 1.2%. The natural frequencies of tower crane standing with stays were 0.48 Hz in case the stays were connected without play, and 0.34 Hz with play, and the damping factors were 1.2% and 3.4%, respectively. This showed that the damping of a crane was greatly influenced by the play between stay and crane.
  (3) Vibration test by the model of a construction tower crane
    Vibration tests using tower crane model were performed using a shaking table and free vibration analyses were performed by using a rigid-body pendulum model. Natural frequencies of the tower crane were similar in both the results of experiments and analyses. It was assumed that the tower crane had a similar natural frequency to the dominant one of the earthquake from the experiments and analyses.
  (4) Buckling strength of stays for construction tower cranes
    Compression tests were performed to the actual sized stays composed of steel H-beams and jacks.
The experiment showed that jacks of the stays were broken when the slenderness ratios of beams were less than 100, and the steel H-beams were buckled when these ratios were more than 100.
    Therefore, it was assumed that the buckling strength of the stays was equal to the compression strength of the jacks when the slenderness ratio was less than 100, and was equal to the Euler's buckling load when this ratio was over 100.

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