As I think of subjects to write about for this column, I try to stay with typical issues that come up on a weekly basis. Last issue, I wrote about converting loads to deflections. This is a crucial skill for any spring designer.
Another subject that comes up frequently is material changes. It’s fairly common for customers to alter a design by changing material types to improve temperature handling capabilities or corrosion problems. However, many alloys and exotic materials have properties that alter the physical dimensions of the spring. This can have an impact on the application and sometimes require changes to mating parts.
The one crucial parameter that takes center stage is the modulus of elasticity in torsion, which determines the spring rate for helical designs such as compression and extension springs. If the modulus changes, then so does the active material- this translates to the coils required to produce the needed rate.
Carbon steels and alloys such as Chrome Vanadium and Chrome Silicon all have the same modulus, at 11,500,000 PSI. Moduli tends to be lower with larger material sizes, but it is considered a constant for the sake of convenience. Designers do not have the option of picking a modulus as they can tensile ranges, so a fixed modulus is used.
Special alloys such as stainless steels and Inconel are a different case. The stainless steels of the variety in the 300 series (302, 316, 321, etc., aka 18-8) are drawn materials whose modulus tends to be at the value of 10,000,000 PSI. This lower modulus means that active material will be less in order to produce the same spring rate. Less active materials means more pitch and higher stress. Higher stress can make the difference between a design that is functional and one that isn’t. This means that just switching materials from carbon to stainless is not as simple as it sounds. Changing material types from high-tensile carbon steel to stainless that cannot handle the stress and has associated set and load loss, is not an option. A redesign will be required to compensate for the higher stresses. Many times this means the customer will need to alter the dimensions of the application to accommodate the new spring dimensions.
Stainless steel 17-7 (aka 631) is a bit different. This stainless type has a modulus of 11,000,000 PSI, which makes it very close to carbon steels. This means a slight change will be needed and may not translate to a marked dimensional change in spring. However, this stainless requires heat treatment after forming to obtain the tensile properties, which is an added cost.
The opposite case exists with an exotic such as spring temper Inconel X-750, which is used for extreme temperatures up to 1100°F. The modulus (depending on your source) comes in around 11,700,000 PSl. This means very little change will occur in the active material length. But this material will require heat treatment at 1200°F for 4.0 hours-a marked increase over simple stress relief of 450°F for 30 minutes for music wire. This change will also translate to higher part cost.
So be certain to contact your favorite designer if you need to change material types. If you are not changing types outside a given class (such as carbon steels, which include hard drawn, oil tempered and music wire) then no rate changes will result- just the set characteristics.
But any time a substantial material type change will be required, it would be best to get feedback on what changes will result.
By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping