61 of the parking shift gear wheel at this moment the three convexities 71 of the dog clutch 7 mesh with the six teeth 61 of the parking shift gear wheel 6 separately and symmetrically as shown in fig3 thus transmission shaft 8 dog clutch 7 and the parking shift gear wheel 6 merge into one unit and prevent rotation of the transmission shaft 8 due to the engagement of groove 411 of stopper 41 and tooth 61 of the parking shift gear wheel 6 thus parking occurs when the parking gear is shifted as shown in fig4 the convexity 71 of the dog clutch 7 may potentially abut the tooth 61 of the parking shift gear wheel 6 thus the parking gear is not shifted because the dog clutch 7 is not able to engage the parking shift gear wheel 6 at this moment the dog clutch 7 pushes the parking shift gear wheel 6 toward the return spring 5 thus the return spring 5 is compressed as shown in fig5 and fig6 if the vehicle is moved slightly by external force or by gravity the transmission shaft 8 will slightly rotate the wheel and the convexity 71 of the dog clutch 7 will mesh with the teeth 61 of the parking shift gear wheel 6 as shown in fig3 at this moment.