Abstract of Special Research Report (SRR-No.15)
National Institute of Occupational Safety and Health, Japan
Prevention of Failure of Overhead Travelling Cranes Due to Degradation of components
Introduction
SRR-No.15-1 |
Yoshio KITSUNAI |
: The fatigue life of overhead travelling cranes in general has been designed based on the strength of the stress range at 2 x 106 cycles for the components. Recently the operation rate of cranes has remarkably increased year by year due to economical request. For example, according to a report in a steelworks, the recent operation rate per year of overhead travelling cranes reaches about 10 times as compared to 20 years ago. As a result, some of cranes constructed in the 1960's have already reached the design life. So that the fatigue cracks are often found at highly stressed regions such as weld toe of gusset plate and of rib plate. Once the fatigue cracks are initiated, they continue to grow slowly in the early stage of the fatigue crack growth life. As long as crack growth occurs, the potential for more serious distress exists. Therefore the cracks must be repaired at an early stage before the cracks grow up to the critical crack size. Rational methods of maintenance as well as of design are required to achieve an optimum structure throughout its service life and to secure safety for workers. |
Survey of Degradation for Overhead Travelling Cranes
SRR-No.15-2 |
Yoshio KITSUNAI and Yutaka MAEDA |
: In recent years the operation rate of overhead travelling cranes has increased due to economical request, so that the damages mainly associated with fatigue have sometimes been reported near welded area such as a toe of termination of fillet weld which has been subjected to high stresses. Hence survey of damages in connection with fatigue and degradation of the material was performed using existing literatures and based on field search. This survey involves information on damage causes, locations, service term, capacity or hoisting load etc. The results of the survey may give us useful information on detection of damage and determination of inspection term for cranes. |
Monitoring of Service Loading for Overhead Travelling Crane
SRR-No.15-3 |
Yutaka MAEDA, Yoshio KITSUNAI and Etsuji YOSHIHISA |
: Cranes have been experienced loads or stresses of variable amplitude in random. In case of application of damage tolerance concepts for cranes, a reliable prediction must be made of number of load cycles that will propagate a crack from a certain starting size to the permissible size. The prediction of fatigue crack growth rate and growth time of a crane requires the input of relevant crack propagation data and stress history. In order to produce reliable fatigue life or crack growth data for cranes, stress spectra acting on the main girder in an overhead travelling crane was examined under service condition. The main girder of the crane used was composed of conventional pipe structure. The principal specifications of the crane were: the maximum hoisting load was 49 kN, the span was 16.3m, and the crab trolley wheel base was 3.25 m. The monitoring of the stresses was carried out using strain gauges glued on the selected components of the girder. The data obtained were recorded into a histogram recorder and a data recorder. Rain-flow method was used as a stress counting technique. The stresses acting on the girder are a function of the hoisting load, movement of the crane and frequency of operations. A 49 kN weight which corresponds to the safe working load was used during the stress measurement. A typical movement of the crane employed was as follows; the test weight was lifted by a crab trolley at an end of the girder, the trolley was traversed from the end to the opposite side of the girder, further the main girder with the crab trolley was traveled approximately 20 m on the runway girder, then turn back to the starting position tracing the original root, and the weight was lowered. A series of crane operation above mentioned was cycled 20 times during the stress measurement to make histogram of stress range acting on the selected components in the girder. As a result, the majority of the stresses monitored occupied by relatively low stress ranges below 20 MPa, which have a little influence on the fatigue damage of the girder. However, the girder was subjected relatively high stresses when the lifting or lowering was stopped abruptly by braking. Moreover, the stress of the component located in the middle of the girder reached approximately 30 MPa as the crab trolley traversed from one side to the opposite side on the main girder. The stress spectrum monitored for the main girder under service conditions was found to be expressed by the Weibull distribution. |
Estimation of Fatigue Crack Grouwth Behavior of HT80 Steel Weldments under Variable-Amplitude Loading
SRR-No.15-4 |
Etsuji YOSHIHISA and Yoshio KITSUNAI |
: Many welded joints exist in structural members of overhead travelling cranes. Since actual service loads for the cranes are generally random, these welded joints are under complex loadings. In the evaluation of fatigue strength of these joints, it is essential to clarify the effect of such complexity in load, which is added to the effect of welding-induced residual stress. Recently, damage tolerant design method is adopted in designing certain kinds of machines. In these machines, prompt relevant action based on the knowledge of fatigue crack growth is necessary on cracks detected by inspection in service and in fabrication. As it is considered that this design method will be adopted for many kinds of machines which include cranes, the estimation of fatigue crack growth behaviors will be a matter of great importance.
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Fatigue Strength and Life Prediction for Gusset Welded Joints
SRR-No.15-5 |
Yoshio KITSUNAI, Yutaka MAEDA and Etsuji YOSHIHISA |
: Weld toe in the gusset plate attached with chord members in overhead travelling crane is often an origin of fatigue cracks. Reliable data on fatigue strength and fatigue crack growth behavior of gusset plate are required for assessment of the integrity or determining the inspection period of cranes. The present study is focused on the following issues to assure the safety of the bridge girders in overhead travelling cranes, (1) determining the fatigue strength of gusset plates which tend to be an origin of cracking, and (2) predicting the life of fatigue crack growth for the gusset plates. |
Fatigue Strength of Repaired Weldments
SRR-No.15-6 |
Yoshio KITSUNAI, Yutaka MAEDA and Etsuji YOSHIHISA |
: Overhead travelling crane and their relative components are often experienced fatigue cracking. When crack is found in a structural member of a crane, welding is often employed to repair the crack because of its compatible with easy handling. The repair and strengthening of the component in the cranes occasionally oblige us to work at small place or high location under insufficient lighting, so that welding defects are sometimes introduced during the operation. Moreover, inadequate procedure for the repair weld acts to decrease markedly fatigue life of the repaired components. Therefore, a minimum requirement for the quality of the welding is necessary. However the influence of repair weld method on the fatigue strength of structural members was not fully understood. More research is needed to evaluate the effectiveness of the repair weld. |
Concluding Remarks
SRR-No.15-7 |
Yoshio KITSUNAI |
: In this study, survey of failure of cranes, monitoring of stress spectrum acting on components in a girder of a crane, fatigue crack initiation and propagation lives of welded joints, estimation of fatigue crack growth life, and weld repair method were examined to prevent failure of overhead travelling cranes due to damages associated with fatigue and degradation of materials. The results are summarized as follows:
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