The Machine as a Safety Factor

In the case of the ASME codes for nuclear pressurised components, the questions of fatigue design and of flaw evaluation are dealt with separately in ASME Section III and Section XI Appendix A, respectively. The design S-A curve for machined butt welds typical of thick section pressurised components is set at a factor of two on stress range or twenty on cyclic life, whichever is more conservative, below the mean of S-N data developed on smooth cylindrical specimens in air. (A somewhat similar design curve obtained by a different method from experimental S-A data for machined butt welds is given in British Standard 5500.) These safety factors are intended to encompass any adverse influence of minor weld defects, size effects, data scatter and environment. As far as environmental effects are... 

More equations can be found in the literature [B.9, B.13b, B.48, B.55, B.71]. If the specific force, the average density of the product, and the desired throughput capacity of the machine are known, with the sheet thickness or the briquette size a roller size can be calculated and a machine can be selected based on its maximum specific force capability. The drive power is determined from the torque requirement during testing and safety factors are added to all machine and process parameters. As always, after installation it is normally necessary to readjust and optimize all conditions in-line. 

As a result of the Eleventh Circuit Court of Appeals decision (AFL-CIO versus OSHA), OSHA s permissible exposure level for tetrachloroethylene, which was lowered to 25 ppm in 1989, was returned to 100 ppm (OSHA 1993). Based on human exposure data, Stewart etal. (1981) concluded that a TLV of 100 ppm contained no safety factor for individuals more susceptible to the subjective and neurological symptoms of tetrachloroethylene. Based on human data, the ACGIH (ACGIH 1995) TLV-TWA is 25 ppm. The geometric mean exposure of dry cleaning machine operators was 22 ppm (Ludwig et al. 1983), a value close to the ACGIH TLV-TWA. 

The quality of moulded theimosets is highty dependent on a precisely repeatable cycle time. This is true also of thermoplastics, but is not so critical as in thermosets. The one phase of the (tycle not controlled is part removal from the mould this is at present done tty the machine operator. Whilst he does this, the safety gate protects him and the restart of the next (tycle is under his control and is delayed until he completes removal of the part and closes the safety gate. The human variability thus introduced (a change firom (tycle to cycle of onfy a few seconds) can affect product repeatability. This is one factor leading to the replacement of the human operator Ity robots for part removal. 

Another factor in airflow requirements during the casting of any flammable solvent (s) in the presence of heat is safety. There are published regulations as to the volume of airflow that must be maintained to remain below the lower explosion limit (LEL). Each solvent system should be studied before using a specific casting machine to determine whether the machine will be operating in a safe zone with respect to the LEL. Information about these calculations can be found in the published literature. Further discussion of this topic will be included in Section 4.3 and in Appendix 2. 

The interlinking of cybernetics and the science of safety results from the fact that technical installations are operated and monitored by human beings who are involved within the effective range of that same installation. They are using the machine but are likewise exposed to its dangers. Human behavior as well as the behavior of the machine, on the other hand, are dependent on the conditions of their environment. The environment, in turn, is often influenced by them in various ways, e.g., by the production of waste, sewage, noise, and alien substances into the air. Man, on the other hand, is able to influence these environmental factors. Each and every technical installation is thus embedded in a man-machine-environment-system characterized by mutual interaction (Figure 3.12). 

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