Abstract The opening and closing of platform screen doors is achieved by high-speed screw pair transmission. This transmission mechanism requires high transmission accuracy and a long service life. The high-speed screw pair transmission without off-center load mechanism is an improvement on the previous screw pair transmission mechanism, which can solve the problems of transmission, deviation, and service life caused by load, deformation, and installation errors in high-speed screw transmission. Keywords Platform screen door, transmission mechanism, high-speed screw, off-center load transmission Platform screen door systems in urban rail transit stations are a special type of equipment used in modern urban subways and light rail. The opening and closing of platform screen doors is achieved by high-speed screw pair transmission, which requires the transmission mechanism to have high transmission accuracy and a long service life. This paper discusses the improvement of the high-speed screw pair transmission without off-center load mechanism in the platform screen door system. 1 Working Principle and Existing Problems of the Original Mechanism Figure 1 is a schematic diagram of the original high-speed screw pair transmission mechanism of the platform screen door. The mechanism adopts a high-speed multi-head large-lead non-self-locking screw and nut transmission pair. The screw (2) is fitted with a wear-resistant nut (1). The screw is connected to the motor shaft through a coupling. The nut (1) and the door hanging plate (5) are connected through a clamp (3) and a hinge mechanism (4). The door leaf (6) is connected to the door hanging plate (5) by bolts. When the motor starts working, the screw (2) can rotate under the drive of the motor; the nut fitted on the outside of the screw and engaged by the screw thread moves linearly. Under the drive of the connecting fork and the hinge mechanism, the door hanging plate (5) and the door leaf (6) move to open and close the door. During the experiment, it was found that the mechanism requires a lot of manpower when manually opening and closing the door; and during the operation of the whole system, it was found that its movement was not very smooth, the transmission resistance was large, and there was a lot of movement noise. The following is an analysis of the reasons for this phenomenon in conjunction with the figure. [align=center] Figure 1 Schematic diagram of the high-speed screw pair transmission mechanism without off-center load for the shielded door[/align] Figure 2 is a schematic diagram of manually pushing the door. When a person applies force to the door leaf (3) as shown in the figure, a torque will inevitably be generated at point A, the axis of the lead screw (1). This torque forces the lead screw (1) and nut (2) to generate a large resistance at point B, thereby increasing the external force applied by the person. In other words, a person needs a lot of force to open or close the door leaf (3). At the same time, due to the large resistance between the lead screw (1) and nut (2), the wear between the lead screw and nut will inevitably be more severe. This will cause the entire system to move very unevenly and have a lot of noise during operation. Due to installation errors during the installation process, if the guide rail surface (7) of the door hanging plate (5) and the door leaf (6) is not parallel to the axis of the lead screw (2) during installation, and the nut (1) cannot adjust its axis to be parallel to the guide rail surface (7) on its own, the entire system will generate an off-center load during operation, resulting in large resistance, uneven movement, and a lot of noise. [align=center] Figure 2 Schematic diagram of manual door push chain[/align] 2 Improvement of the mechanism Based on the reasons analyzed above, the mechanism was improved. Figure 3 is a schematic diagram of the improved screw pair transmission mechanism without off-center load. The transmission mechanism adopts a high-speed multi-head large lead non-self-locking screw nut transmission pair. The stainless steel screw (13) is fitted with wear-resistant nuts (2) and (4). The brushless motor and the screw (13) are directly connected without reduction through a coupling. The door leaf (12) is connected in pairs on the door hanging plate (10) and (11). The door hanging plate (10) and (11) are connected through the trailer (15) and the guide rail (14). The trailer (15) bears the downward vertical weight and horizontal lateral pressure of the door leaf (12). The movement clearance of the door hanging plate (10) and (11) is adjusted by the trailer (15). Wear-resistant nuts (2) and (4) and a pair of door hanging plates (10) and (11) all adopt a non-eccentric load mechanism. The non-eccentric load mechanism adopts a fork-shaped hinge non-eccentric load mechanism. The rotatable pin (6) on the connecting fork (5) is inserted into the flange (1) and sleeve (3) connected to the wear-resistant nuts (2) and (4). The connecting fork is connected to the left hanging plate (10) and the right hanging plate (11) by a rotatable connecting shaft (7) and a hinge (8) with a movable joint through a pin (9). [align=center] Figure 3 Schematic diagram of the improved high-speed screw drive non-eccentric load mechanism for the shielding door[/align] The following analyzes the case where the degree of freedom of the nut is restricted: assume that the plane of the front view at the top of Figure 3 is the X-Y plane. In the absence of any constraints, the nut (4) has 6 degrees of freedom, namely rotation about the X, Y, and Z axes and linear motion in the X, Y, and Z axis directions. First, let's analyze the X-axis situation: Assuming the door panel (10) is the constraint body, the rotation of the lead screw (13) causes the nut (4) to move linearly, but it cannot rotate around the X-axis; and due to the restriction of the pin (6) on the connecting fork (5), the nut (4) cannot move linearly in the X-axis direction relative to the door panel (10). Therefore, the off-center loading mechanism restricts the nut (4) to two degrees of freedom in the X-axis direction. On the Y-axis, due to the action of the rotatable connecting shaft (7) on the connecting fork (5), the nut (4) can rotate around the Y-axis; and due to the action of the hinge (8), the nut (4) can make a slight linear translational motion in the Y-axis direction relative to the door panel. On the Z-axis, due to the action of the pin (6) on the connecting fork (5), the nut (4) can rotate around the Z-axis; and due to the action of the hinge (8), the nut (4) can make a slight linear translational motion relative to the door hanger in the Z-axis direction. The off-center loading mechanism does not restrict the nut (4) in the four degrees of freedom in the Y and Z-axis directions, which is exactly what we want. This is because the rotation of the lead screw (13) drives the nut (4) to make linear motion, and through the off-center loading mechanism and the door hanger, it drives the door leaf to move. Therefore, it is necessary to require that the nut (4) cannot rotate and cannot move along the X-axis relative to the door hanger. That is to say, it is necessary to restrict the nut (4) in two degrees of freedom in the X-axis, but not restrict the other four degrees of freedom, because there are bound to be errors in the manufacturing and installation of the lead screw and guide rail. Once the lead screw (13) and guide rail (14) deviate slightly from their theoretically parallel positions, since the nut (4) is not restricted in its four degrees of freedom in the Y and Z directions, the nut (4) can adjust its axis by means of the live joint on the non-eccentric load mechanism, avoiding additional eccentric load between the nut (4) and the lead screw (13) and increasing resistance. This setting reduces wear, increases service life, lowers noise, and improves transmission accuracy, thus enabling the entire system to operate smoothly. 3. Experimental Results After conducting a 1 million-cycle life test on the improved door operator, it was found that the entire system operated smoothly, had high transmission accuracy, and low noise. When manually opening and closing the door, the mechanism does not require much manual effort to easily open or close it. This indicates that the improved mechanism meets the usage requirements. A simplified diagram of the nut tooth profile change before and after the test is shown in Figure 4. Regarding other components: no wear was measured on the door guide rail; although there was wear, it could continue to be used. After 1 million trouble-free operating cycles, all components of the door operator can still be used, and all performance remains unchanged. [align=center]Figure 4[/align] 4 Conclusion The improved transmission non-offset load mechanism, through a high-speed multi-start non-self-locking screw and nut pair, not only converts the rotational motion of the motor into the linear motion of the door leaf, but also meets the requirements of smooth movement, low noise, and long service life. The direct connection between the motor and the screw without reduction reduces motion noise, improves the transmission efficiency and reliability of the door operator, and reduces comprehensive errors caused by manufacturing, installation, deformation, and load, thus simplifying the door operator mechanism. The nut and door hanging device adopt a non-offset load force condition, improving the smoothness of movement, extending service life, and reducing the overall error caused by manufacturing, installation, deformation, and load. The non-offset load mechanism limits motion noise. Experiments have proven that the improved high-speed nut of the shielded door moves in 2 out of 6 degrees of freedom, thus eliminating the need for a screw-driven non-offset load mechanism that meets the usage requirements.