Torque control method is widely used when tightening bolt-nut connections due to its easy operations. However, it inevitably involves the essential problem of generating a fair amount of bolt preload scatter, even when applying the same amount of torque. The primary reason for this is the scatter of friction coefficients of thread and nut loaded surfaces. By using tightening equipment which can accurately evaluate the scatter of friction coefficients, this study has shown that the use of plain washer is effective to lower the scatter of friction coefficient of the nut loaded surface, thus lowering the bolt preload scatter as much as 10% of the target value.

In recent years, power trains that consist of propeller propulsion and motor drive systems have been widely used for battery electric boats, which are gaining popularity as a type of small boats, as well as for diesel electric ships that are renowned for their good maneuverability. The AC motor drive system used for these boats has a response time of just a few milliseconds — much quicker than that of the internal combustion engine. In addition, it is technically possible for the system to accurately monitor motor control signals (current, voltage, torque and rotational speed) in real time. In our previous report, we measured (torque signals, rotational speed signals and bubble behavior around the propeller) in the motor propulsion system under constant torque control in a circulating water channel through such means as analyzing high-speed video images. By analyzing rotational speed signals, we confirmed ahead of other researchers around the world, that it is possible to detect generated bubbles, measure its quantity, monitor the resulting thrust reduction and accurately gauge its magnitude. This report, has verified the results of our experiments theoretically by using the propeller characteristic equation. It has also demonstrated the possibility of improving propulsion efficiency enabled by suppressing the generation of bubbles and stabilizing the motor power output.

In this study, we aimed to improve the propulsion efficiency of the motor drive system by suppressing the load fluctuation caused by the generation of air bubbles around the propeller. Experiments were conducted to verify the detection of air bubbles around the propeller using status monitor signals from the motor drive and the correlation between the monitor signals and the thrust in the AC motor drive system, when driven by the constant rotational speed control method. Experimental data such as the motor power fluctuation were compared, and verified by comparison with previously reported experimental results. This established that the correlation between the monitor signal from the motor drive and the bubble generation and quantity around the propeller, as well as the thrust, can be expressed using the propeller characteristic equation and the motor characteristic equation for the motor drive system when driven by the constant rotational speed control method. We showed that the monitor signal of the motor drive system can be considered a sensing signal, and used to stabilize the motor power and thrust fluctuations in the motor drive system when driven by either the constant torque control or the rotational speed control method.

  Torque control method is widely used when tightening bolt-nut connections owing to its easy operation. However, it usually involves a fair amount of scatter in bolt preloads. In contrast, elastic angle control method can provide bolt preloads with higher accuracy, though its tightening operation requires longer working hours and higher costs. In this study, a new tightening principle is proposed by combining the two methods, which achieves greater accuracy and ease of operation simultaneously. The proposed method can precisely estimate the critical value of “nut factor” in the course of tightening operations, thereby modifying the objective torque into the proper one enabling the attainment of the target bolt preload. Tightening equipment, which was manufactured for trial based on the proposed principle, successfully tightened the target bolts with scatter staying within 5% of the objective bolt preload.