Since the design requirements specify that the outer diameter of "check valve" is 68mm. Set the wall thickness of the thinnest part of the upper connecting sleeve as δ, The inner diameter of the mandrel is:
As the outer diameter of the "check valve" specified in this design requirement is 68mm. Wherein, the wall thickness at the thinnest part of the upper connecting sleeve is set as δ, and the inner diameter of the mandrel is: