Abstract of Special Research Report (RR-2001)

National Institute of Occupational Safety and Health, Japan

The Three Dimensional Stress Analysis and Fatigue Strength Evaluation of Link Chains in Chain Hoists

RR-2001-1

Takashi HONDA

: The link chains are used in lifting machines, such as a chain hoist and a conveyor. They are very important components in those machines because the fatal accident may occur by the falling of a load when a link chain is fractured. Link chains used within the rated load are seldom fractured statically since the safety factors of them are defined as 4 or 5 in JIS B 8812. However they are usually fractured by fatigue. The fatigue fracture of a link chain occurs under small load further than the tensile strength of it, and it is difficult to detect the fatigue crack in the periodical inspection, such as visual observation and elongation measurement. Not only fatigue characteristics but also stress distributions have not been clear due to the complex shape of a link chain.
Therefore the fatigue of the link chain becomes large problem in loading facilities.
    In this paper, in order to clarify fatigue characteristics of link chain, the stress distribution in link chain considered as curved beam was calculated, and the result was compared with the analytical value of the three-dimensional finite element analysis (FEM) and the measurement values by strain gauges. In addition, fatigue tests of link chains were carried out in order to evaluate the fatigue strength, and then the fatigue fracture surfaces were observed by the scanning electron microscope (SEM).
    The main results obtained in this study are as follows :
  (1) The analytical results by FEM are agreed well with the surface stress measurement results by strain gauges.
  (2) The stress distribution in link chain considered as the curved beam is greatly different from FEM and measurement results by strain gages.
  (3) Fatigue cracks in all links used in fatigue tests are initiated in the point where the tensile stress analyzed by FEM becomes the maximum value.
  (4) As a result of fatigue tests, it is clear that the fatigue strength of link chain is almost the rated load.
  (5) The striations are observed in the fatigue fracture surfaces of link chains tested under limited loading conditions by the scanning electron microscope.

Experimental Study of Failure Characteristics of the Ground Causing the Overturning of Mobile Cranes

RR-2001-2

Satoshi TAMATE

: This study focuses on the instability of ground causing the overturning of mobile cranes. The mobile crane is a truck comprising machinery for hoisting equipment. The mobile crane is usually kept level during the hoisting operations by supports of four outriggers, which are located at each corner. When the outriggers happen to penetrate into the bearing ground, however, the crane becomes unstable with two types of mechanisms. One is static instability and the other is dynamic instability. Where the outriggers penetrate the ground gradually and the machinery rotates slowly, the overturning moment increases because the lever arm of the hook load increases due to rotation of the jib. Meanwhile, where the ground fails suddenly and the outriggers penetrate rapidly, the inertia moment due to angular velocity of the crane is an important factor in addition to the static instability.
This paper examines a series of experimental analysis to investigate the relationship between the instability of mobile crane and characteristics of the outrigger penetration due to failure of ground. Experimental simulation using a newly developed model crane is performed in a centrifuge at the National Institute of Industrial Safety (NIIS) to examine the realistic behaviors of both the overturning of mobile cranes and penetration of outriggers. It was found that mobile cranes become unstable due to rapid penetration of outriggers caused by brittle failure of the layered ground, which comprises a hard surface overlaying soft ground, comparing to slowly penetration due to ductile failure of the uniform soft ground. Back-analysis of experimental records was undertaken to ascertain the kinetic conditions for overturning, and it makes sure that mobile cranes satisfy the kinetic condition for overturning prior to reaching to the static limiting equilibrium where rapid penetration occurs. The relative instability index, which is the ratio of the critical settlement for overturning due to kinetic condition to the static critical settlement due to the static limiting equilibrium, indicates almost constant value where the ground condition is the same even though the condition of the crane (i.e., jib length and jib angle et al) is different. The result supposes that the stability of mobile cranes was classified by the ground condition.

Research of Earthquake Resistance of Tower Crane for Construction

RR-2001-3

Seiji TAKANASHI and Yutaka MAEDA

: A lot of tower cranes were severely damaged by the Hyogo-ken Nambu earthquake that had occurred on January 17, 1995. There were several major classifications of damage. The most extreme serious examples are the following three types. 1) Damage of mast part. 2) Damage of jib part. 3) Damage of base part. In this research, the damage of mast and the damage of the jib were examined. The purpose of this research is to propose measures to prevent such damage. However, the dynamic characteristics of the tower crane have not been examined enough. Therefore, at first, the free vibration test was done with the tower crane actually used on the construction site, and the dynamic vibration characteristics were examined. The lifting capacity of this tower crane is 200tm. The lifting capacity and the height of the crane were the same as the severely damaged tower crane. Next, the reduced-size model for the shaking table test was constructed based on those data. A reduced-size model (1/36 in length of the full scale) was utilized. The purposes of the shaking table tests were to confirm the damage situation of the crane and the effectiveness of the earthquake resistance measures, and to make an analytical model for a numeric simulation. The crane damaged at the mast was for the high rise building construction. This, crane is connected to the building by stays. The damaged part was a part where the stay had been installed. The shaking table test was able to confirm that. Moreover, it has been understood that the vibration characteristic of connected buildings strongly influences the vibration characteristic of the tower crane. When thinking about the earthquake resistance of the crane, it is necessary to think about both the characteristic in the building and characteristics of the crane. It was confirmed that improvement in earthquake resistance performance could be achieved by installing oil dampers into the stays. The validity of an analytical model for a numeric simulation could be confirmed by these experiment results. The crane damaged at the jib was for the low rise building construction. This crane can stand by itself and comparatively small. As for the crane of this type, the numerical analysis simulation did the examination. The crane of 20 tm, 40 tm, 60 tm and 100 tm were chosen as an analytical model. The following has been understood from an analytical result. In the tower crane of 60 tm, damage was little and negligible. It corresponds to the analytical result. It has been understood that the earthquake resistance performance is greatly different according to the up and down angles of the jib. To decrease the stress generated in the jib, the rotation restraint of the revolving frame was released. However, the stress has not become small. In static mechanics, this method is effective. However, it was not necessarily effective in dynamic mechanics. It was confirmed that the earthquake resistance performance improved by absorbing the rotation energy of the revolving frame. Moreover, the hook was used as TMD to decrease the stress of the jib. From the analytical results, it was understood that this method is not necessarily effective.

Decomposing Detonation Propagation Properties of Ozone / Oxygen mixtures

RR-2001-4

Takaaki MIZUTANI and Hidenori MATSUI

: Ozone is the strong oxidizing agent which has been used for water treatment, pulp bleaching and others. Recently, along with development of ozone generator, high concentration of ozone up to 20 vol.% in oxygen has been able to be obtained. It is well known that ozone is rapidly decomposed into oxygen with detonation by some ignition sources. But the decomposing detonation properties of high concentration ozone in a tube are not sufficiently clear. It is important to clarify the decomposing property for safe handling of ozone.
In this study, the decomposing detonation of ozone/oxygen mixtures of up to 20 vol.% of ozone in oxygen under high pressure of up to 1.0 MPa in a tube were experimentally investigated. The mixtures were ignited by a driver detonation of the stoichiometric oxy-hydrogen mixture. Detonation properties such as wave velocity and pressure were measured with piezo electric transducers mounted along the tube. Slow and constant detonation propagation profiles were obtained. We also investigated the quenching ability of a wire gauze as well as the concentration limit for detonation propagation. Under these experimental conditions, direct initiation of detonation by the driver detonation of the stoichiometric oxy-hydrogen mixture was easily achieved at much lower concentrations than the limit of deflagration. The observed detonation properties, such as wave velocity and pressure, fairly agreed with C-J calculated values. The detonation velocity (900 to 1,200 m/s) and the pressure ratio to initial pressures (5 to 9.5) were not affected by the initial pressure of the mixtures. Near the detonation limit, typical spinning detonations with oscillatory pressure waves were observed.

New Evaluation Method for Antistatic Efficiency of Floors and Floor Coverings

RR-2001-5

Atsushi OHSAWA

: This paper presents a new evaluation method for antistatic efficiency used in floors and floor coverings. Flooring is one of essential elements for controlling static charge at work area, because the tribocharging between footwear and floor during walking causes the rise of human body potential. The human body charging often introduces electrostatic discharges. Moreover they cause unexpected electrostatic hazards, such as fires and explosions in flammable atmosphere, failure operations in electronic facilities and damage of electronic devices in many areas of industry. To prevent such hazards and failures due to the human body charging we use antistatic (dissipative) or conductive floor connected to ground as well as suitable footwear and clothes.
In this paper we characterize the electrostatic properties of floors numerically using an equivalent circuit model. In general the electrostatic properties for preventing hazards are determined by resistance path to ground (leakage resistance) and decay time of static charge. To obtain them the steady state and transient solutions of surface potential distribution are solved by using the equivalent circuit model. The modelling was in good agreement with the experimental results of several floor coverings. Note that the method can apply to not only floors, but also worksurfaces.
The surface potential distributions on square floors were numerically calculated when voltage was applied at the center of top surface and the bottom surface was bounded by a grounded conductor. A dimensionless equation for the surface potential was obtained. The equation showed that the surface potential is determined by the value of χ = ρ_sL²/(ρ_vδ) where ρ_s : surface resistivity, ρ_v: volume resistivity, δ: thickness and L: side length of the floor. Physical meaning of χ is the ratio of surface resistance, R_s = ρ_s, to total volume resistance of floor, R_v = ρ_vδ/L ². Calculated leakage resistance and charge relaxation, which are the functions of material physical quantities and dimensions of floors, will be useful for designing and installing floors in industry. As a result we found that the condition for effective antistatic efficiencies is 10² <= χ < 10⁷ . Consequently we can conclude that the condition is applicable to the evaluation method of the antistatatic floors and floor coverings.

Statistical Analysis of Industrial Accidents by Risk Curve

RR-2001-6

Shigeo HANAYASU, Masao KAJIYAMA and Kazuyoshi SEKINE

: The objective of statistical analyses for industrial accident occurrences is to provide various basic information for the decision making on the policy of loss prevention or risk management. For this purpose, characterizing and evaluating of the potential risk of accident occurrence quantitatively that correspond to the results of statistical analyses is required. This paper proposes a new statistical technique which give a quantitative index that enable to characterize the potential risks of industrial accident occurrences in many engineering systems. An approach taken in this study is based on the concept of the risk curve, in which the relationship between exceedance frequencies and their corresponding magnitude of damage consequence of hazardous events is described in a log-log scale. The basic understanding and interpretations of risks for accident occurrence have been examined with the risk definition determined by Kalpan and Garrick, and the risk curve has been clarified to be of a useful method for characterizing the risk of hazardous events. Furthermore, based on the collected various actual accident data, the risk curve can be essentially expressed with a mathematical form uniquely determined by a one parameter distribution. It was concluded that the distribution parameter that characterizes the risk curve can be adopted as an safety index for a possible methodology that statistically assesses risks of accident occurrence of an operating system.

JNIOSH