How to calculate the core shaft of a core coil spring

How to calculate the core shaft of a core coil spring
When there is a cored coil spring, due to the elastic deformation of the material to recover, the spring diameter, number of turns and pitch of the coiled spring will change after relaxation, resulting in elastic rebound, so the core shaft diameter D should be smaller than the spring inner diameter D1 There are many kinds of information about the calculation formula of the mandrel, but the calculation is more troublesome. Experience has proved that the following formula is more general.

Namely: D=(0.75~0.85)D1

Where D—the diameter of the mandrel;

D1—The inner diameter of the spring.