windup of "compression" spring

[subsearobot]

Hi all
I have a weird (to me) application for a "compression" spring.
If axial torsion is applied, does the length of the spring decrease or increase, depending of the direction of torque? Or does only the coil diameter change? (for my application, imagine a die-spring)

Are you familiar with equations that would predict the magnitude?

Ultimately, I am interested in understanding how the spring in an impact driver is designed; I have a similar application.

https://www.youtube.com/watch?v=f0gSJa3L_7c&t=145s

<--- skip to 2:20 & 3:11

thanks!!

 

[btrueblood]

Torsion springs will shorten in length and diameter as they are "wound up" (loaded in the direction of winding). In garage door springs, this is typically countered by fixing the ends, which adds an axial extension load to the spring as it winds up, and helps prevent buckling. Get a copy of Wahl's Mechanical Springs, or other spring design guides, torsion springs are covered there.

 

[desertfox]

Hi superseabot

Watching the video it appears to me that the compression spring doesn’t get wound up, it rotates freely with the drill and acts as a compression spring would normally. I think if you apply torsion to a compression spring it would buckle and it’s length would change in either direction depending whether you are unwinding or winding the coils.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[3DDave]

The shaft in the impact wrench has a spiral cut as mentioned in the video. When the striker is stopped the shaft acts as a wedge to raise the striker and compress the spring. When the striker raises far enough to clear the impact hammer it is rotated until it gets to the far side of the hammer where the spring shoves the striker down and it rotates along the spiral to slam into the hammer again.

 

[subsearobot]

3DDave, nice. i was thinking that he misunderstood the mechanism of the spiral cut shaft. your explanation makes more sense.
thanks!