There are two types of loosening phenomena of threaded fasteners which sometimes cause serious accidents involving a variety of machines, structures and electronic devices: rotational loosening and non-rotational loosening of threaded fasteners. The former pattern frequently occurs due to shear loads repeatedly applied perpendicular to the bolt axis. This paper aims to clarify quantitatively how friction coefficients at contact surfaces and the geometry of threaded fasteners affect rotational loosening by conducting three-dimensional finite element analyses with the use of helical thread models. Numerical results showed that friction coefficient at nut bearing surface largely influences the loosening process, and that the introduction of fine screw threads and longer grip length are effective in preventing or mitigating the progress of thread loosening. Based on the foregoing results, a practical proposal is made for the prevention of thread loosening.
In April 2018, the International Maritime Organization (IMO) adopted its Initial GHG Strategy, which defined goals for the international shipping sector to achieve by 2050: reducing CO2 emissions per transport work by 70% and total GHG emissions by at least 50% compared to 2008. As for SOx emissions from ships, the 0.5% global cap on the sulfur content of any fuel oil on board ships took effect on 1 January 2020. Biofuel is described as carbon neutral and, in general, has low levels of sulfur. For these reasons, it is expected to be more commonly used as a marine fuel in the future. However, biofuel emits more NOx from diesel engines, because its oxygen content is higher than that of gas oil.
In this study, the authors conducted experiments to see how the use of emulsified biofuel can contribute to reducing NOx emissions. The experiments were conducted in a small high-speed four-cycle diesel engine using Fatty Acid Methyl Ester (FAME) made from palm oil, gas oil, FAME-based and gas oil-based emulsion fuels, both of which contain 30% of water. The results found that the NOx reduction rate of the FAME emulsion fuel is higher than that of the gas oil emulsion fuel, and that ignition delay times for the FAME emulsion fuel are shorter than those of the gas oil emulsion fuel.
One of the technologies to reduce PM emissions is an electrostatic precipitator. Electrical resistivity of the target particles is an important parameter in determining the performance of an electrostatic precipitator. If the resistivity of particles captured on the collection wall is low, re-scattering of the collected particles will occur. On the other hand, reverse ionization or the back-corona phenomenon occurs in the case of high resistivity, eventually resulting in sparkover. The resistivity of PM emitted from marine diesel engines is not fully understood. In this study, the authors developed a new instrument to measure particle resistivity, and used it to calculate the apparent electrical resistivity of PM exhausted from marine diesel engines. The results found that the apparent resistivity of PM exhausted from two-stroke engines was higher than that of PM from four-stroke engines. This is due to the high ratio of SOF contained in PM from two stroke engines. Therefore, when the ESP system is utilized to treat exhaust gas from two-stroke diesel engines, it is necessary to pay considerable attention to reverse ionization or the back-corona phenomenon.