I am interested in designing a spring which moves with the model so that it can be put into a larger assembly and appear realistic.  The tutorial below is the best I've found.  It is really good, but I need to be able to change the pitch and diameter fo the spring towards one end, otherwise it won't look realistic to more esoteric users.  Has anyone found anything that allows gradual changes of pitch and diameter of the helix that can still be used dynamically?  I think I could do this with equations, but I have not been able to get anything equation based to animate properly since a rebuild is required for each calculation.

http://www.solidworks-apac.com/2009/12/21/how-to-animate-a-spring/