采用英式中轴的自行车(如上海产的PA-17型凤凰牌车、PA-13型永久牌车等),它的左中轴碗螺纹是右旋的,而右中轴碗螺纹是左旋的。据认为,如此设计中轴碗螺纹旋向具有自动锁紧的作用。但在长期大量的使用过程中,常出现中轴碗(特别是右中轴碗)丝扣松动的现象,以致造成车体螺纹损坏的后果。本文对中轴碗受力进行了理论分析和实际观测,指出了中轴碗丝扣松动的原因在于其螺纹旋向设计得不合理,提出了左中轴碗螺纹应是左旋 The use of the British axis of the bike (such as the Shanghai-made PA-17 Fenghuang car, PA-13 permanent license car, etc.), the left axis of the bowl screw is right-handed, while the right axis of the bowl is left-handed of. It is believed that the design of the axial bowl screw thread rotation with automatic locking effect. However, in the course of a large amount of use for a long time, the looseness of the threaded shaft of the central shaft bowl (especially the right central shaft bowl) often results in the consequence of damage to the threaded body of the vehicle. In this paper, the theoretical analysis and actual observation of the axial shaft bowl stress were carried out. It is pointed out that the reason why the loose thread of the middle shaft bowl is loose is that the design of the thread rotation direction is unreasonable.