If your spring is going over a shaft or mandrel, you want to make sure that the inner diameter of your helical spring isn’t going to be too tight around the outer walls of the mandrel or shaft or even too small to fit over the shaft. The equation to calculate this helical spring dimension is to multiply the wire diameter by two and subtract it from the outer diameter as shown below.
Outer Diameter – 2(Wire Diameter) = Inner Diameter
OD – 2(WD) = ID
If your spring is going over a shaft please use this
"Spring Shaft Clearance Guide" as a reference.
Spring Shafts .100 to .250 allow for .010 to .015 clearance
Spring Shafts .251 to .500 allow for .020 to .025 clearance
Spring Shafts .501 to .750 allow for .025 to .030 clearance
Spring Shafts .751 to 1.000 allow for .031 to .040 clearance
Spring Shafts 1.001 to 1.500 allow for .040 .050 clearance
Spring Shafts 1.501 to 2.000 allow for .050 to .075 clearance