there is another way that someone can make an extension spring, by equation :
For example read the following :
/* For Cartesian coordinate system, enter parametric
equation
/* in terms of t (which will vary from 0 to 1) for x, y and
z
/* For example: for a circle in x-y plane, centered at
origin
/* and radius = 4, the parametric equations will be:
/* x = 4 * cos ( t * 360 )
/* y = 4 * sin ( t * 360 )
/* z = 0
/*-------------------------------------------------------------------
x = 2 * cos ( t * 360 * 4 )
y = 2 * sin ( t * 360* 4 )
z = t * 3
add 3 last lines
the number 2 representing the radius off the spiral
the number 3 pitch
the number 4 number off coils
regards
Karavasilis V. Christos