## 2 Mechanism Design and Kinetics  
There are many types of quick connector for hydraulic power transmission systems. The traditional poppet type and spherical type quick connectors make the hydraulic oil converge in the gap between the valve core and the coupling glove due to their irregular geometry and uneven contact surfaces. During the disconnection process of such coupling systems, this hydraulic oil flows out into the environment and causes pollution. In order to overcome this problem, environmentally friendly flat-face type coupling systems have been proposed, which have been widely used and recognized. In recent years, with the increase of operating pressures in hydraulic power transmission systems, the connecting characteristics of CFFCS are no longer sufficient to meet the requirements due to high coupling resistance. In order to make the coupling process more convenient, a NFFCS with an instantaneous pressure relief function is proposed in this research. The design of the NFFCS and its comparison with the CFFCS are presented below. Figure 1 shows the mechanism design of the CFFCS in the separated state. The plug connector on the left side and the socket connector on the right side of Figure 1 both have  
one-way self-sealing valves (1 and 4) that ensure the blockage of hydraulic fluid in the disconnected state. During the connection process, the pressure of the sealed high-pressure hydraulic fluid acts on the one-way self-sealing valve to generate a high amount of force (coupling resistance) that must be overcome to complete the connection process. Figure 2 shows the kinematics of the connection process of the plug and socket connectors of the CFFCS. Status A is the state where the plug and the socket have just made contact. During the course of pushing the socket to the left while the plug is fixed, Process 1 and Process 2 take place. Upon completion of these processes, the fluid path is fully opened. In order to facilitate their understanding, the theoretical models of the two processes are established, as shown in Figure 3, where only the influence of the mechanical spring element on the thrust is considered. Figure 1: Internal mechanism of the conventional flat-face coupling system (CFFCS). Figure 2: Kinematics of the CFFCS. It can be seen that during process 1, in order to move to the left, the pin (5) in the socket that is rigidly connected to the shell of the entire socket pushes the valve (1) of the plug to overcome the elastic resistance of the spring K2. At the same time, due to the reaction force given by the glove (2) in the plug, the glove (3) in the socket moves to the right against the elastic force of the spring K1. The theoretical mechanical model of this process is shown in Figure 2(a). d1 represents the distance between the glove (3) and valve (4) in the socket. When d1 reduces to zero, the entire system reaches status B. As shown in Figure 2(b), during Process 2, the valve (4) in the socket moves to the right along the guide (6). During this process, the springs K1, K2, and K3 work simultaneously. When valve (4) starts to move, the hydraulic fluid starts to flow through the entire coupling system. Finally, the fluid path is fully opened when the valve (4) makes contact with the guide (6) (i.e., d2 = 0). It can be seen that in the connection process of the EFFECTS, in addition to overcoming the resistance of the spring, it is also necessary to overcome the resistance due to the pressure of the hydraulic fluid acting on the single self-sealing valve (1). In actual operation, the resistance due to fluid pressure is usually very large and requires the use of additional tools to assist in establishing the connection. Therefore, the EFFECTS cannot meet the requirements for easy quick coupling under higher-pressure conditions. To solve this problem, we propose the NFFCS design that is presented below. Figure 4 shows the internal mechanism of the NFFCS. Compared to the EFFECTS, the main difference lies in the design of the plug connector.