What is shot peening? And how is it different from glass beading or sandblasting?
Below is a reply from Mike Eaton,
All springs have a finite life, that is one day the will fail. This failure can take more than one form. The most drastic is breaking. But the one we see the most is a gradual loss of hieght. Spring rate is not lost, just their ability to supoort weight which causes them to sag.
All good quality leaf springs are made for SAE 5160 which is a high alloy spring steel. Once formed and heat treated this steel retains it's memory. As the spring gradually sags it's memory also goes lower. If the spring is reset/re-curved while cold it's internal memory will bring it back to the low position. Even if no weight is applied to the spring it will sag back down.
The only way to correctly rearch a leaf spring is by annealing, reforming and then reheat treating it. We have written an on-line article describing how any why it is done,
http://eatonsprings.com/rearching.htm.
Due to the high volume of Mustang springs we make we must coat them to prevent rust. And thanks to the EPA we must use a paint which conforms to the rules and regulation. And the butt ugly black it is. Keeping in mind that spring steel begans to soften at 400 degrees F, to remove this one can use just about any method they would like providing the tempurature of the steel does not reach 400 degrees.
Because Ford Motor owns the right to the part number we can not stamp it onto the bottom of the new spring. So many do reuse the old bottom leaf and install it onto their new spring.
While shot peening removes the scale formed in the heat treating process, it's main function is to stress relieve the steel. We like to say that when the spring comes out of heat treat it's molecules are not happy. Shot peening makes for happy moecules and happy molecules result is a long lasting spring.
I hope this helps. Mike Eaton
