variable pitch helix

[unknown1]

I'm trying to come up with the equations for a variable pitch helix we're
using for a closed end spring.
I think I have everything I need but I'm having a hard time coming up with
the pitch for the middle coils.
The end coils have a pitch of the wire dia then I have a transition from
the closed ends that is variable pitched and about one wire dia long. Then
I have the center coils at a specific pitch. Problem is I'm not sure how I
need to calculate the center coil pitch.
Any ideas?
__________
Stanton Mapson, Product Engineer
Parker Hannifin Corp. - Quick Coupling Division
252 N. 134th Street (68520)
P.O. Box 81247
Lincoln, NE 68501 USA
Phone: 402.486.9104
Fax: 402.486.9102

[fburke]

Hi Stanton,

Don't bother calculating the middle pitch, use some relations to do it for
you.
Imagine you have a datum "BOTTOM" and a datum "TOP"
They are a distance "HEIGHT" from each other.
The first third of your coils go up to "DTM1" which is "HEIGHT/3" from
"BOTTOM"
The last third of your coils go down to "DTM2" which is "HEIGHT/3" from
"TOP"
Then just align your helix lengths to the datums and make your pitch
HEIGHT/(# of coils)
Make three separate springs to simplify your life.
If you build your spring like this, you can also stretch it to different
lengths by changing the HEIGHT (real number) parameter.


Also remember only to use helixes when you absolutely HAVE to.
They bog down large assembly navigation terribly.
Whenever I can, I use a stack of doughnuts instead of a helix, and noboby
knows the difference.
(My doughnut stacks have changeable heights too... mmm doughnuts.) 🙂

Best regards,

Frederick W. Burke III
Mechanical Design Engineer
ISE Corporation
7345 Mission Gorge Road, Suite K
San Diego, California 92120
(619) 287-8785 x-151