Spring Materials & Their Affect on Rate

As a spring designer, I must have a fair understanding of a number of disciplines, with spring materials being a major player.  But, instead of the intimate details of grain structure and chemical composition, I am more concerned with the practical result when a force is applied to a given material.

Also, I work with many clients who understand their applications thoroughly, but rely on me to be their source for the material selection for their application. This is especially true since I still see blueprints from the 1940s and materials have greatly improved since that era. The introduction of stringent quality practices have made for more consistent, high-quality materials grades. But no quality plan can accurately predict the chemical mix that results when a given number of chemical compounds, with their own variations in properties, get processed together in that heated mass that gets drawn down to spring steel.

The most important of these properties in spring work is the modulus of elasticity in torsion-symbolized as “G”. The rate formula (also known as gradient or slope) is as shown.

R = Gd4/8Na03

The other factors are:

d = round material size

Na = active coils

D = mean spring body diameter

Both the body diameter and active coils are parameters that offer the spring maker some chance for adjustment of spring rate.  But, the wire size will be purchased within a tolerance and the spring maker may have to compensate for that material variation.  Also, the modulus is not a characteristic that is certified of measured-it is assumed.  For carbon steels, G is 11,500,000 PSI. But, in reality, G can be anywhere from 11,200,000 PSI for large wire to 11,900,000 PSI for smaller wires. This means the G has a direct affect on the spring rate and the spring maker has no control mechanism. They must vary other parameters in the formula to compensate for rate. The one positive in that scenario is that G is not raised to an exponent such as material size or body diameter.  Now to my point. I will sometimes be asked what the difference in spring rate will be from one wire type to another. For example, a common request is to alter material from carbon steel such as music wire, to stainless steel. The G of stainless materials is less than that of carbon steels. This means something else will have to give, usually coils. A comparable stainless steel 302 spring will require less coils to boost the rate back up to the same value of the music wire spring. This creates more pitch, which creates higher stress-and that can be a total game changer. In other words, two springs with the same identical geometry (diameter, coils and wire size), but made from different materials, can have different rates. This is strictly due to G, a parameter that cannot be altered.

So be sure and consult a spring engineer if material changes are made that leave one material realm and switch to another. Although spring geometry can be altered to compensate for rate changes, those very changes may negatively impact the design, and other materials that are more exotic may be needed to match spring performance.

Too, I have had conversations with customers who do not understand the difference between the modulus and the hardness/tensile strength.  They are two different characteristics, but easy to understand. The modulus determines the rate/force that can be expected from the spring when it travels. The hardness determines how far the spring can travel before it takes a permanent set.  So in the case of carbon steels, they all have different tensile values, but the modulus is the same for all.  So they will all create the same force at a given height, but will take set at different heights from one material type to another.

Spring Essentials …

Modulus of Elasticity in Torsion for various materials and alloys

Carbon steels (Hard Drawn, Oil Tempered, Music)= 11,500,000 PSI

Stainless 300 series (302,304,316) = 10,000,000 PSI

Stainless 17-7PH = 11,000,000 PSI

Alloys (Chrome Vanadium, Chrome Silicon)= 11,500,000 PSI

lnconel X-750 = 11,500,000 PSI

Beryllium Copper= 7,000,000 PSI

Monel 400 & 500 = 9,600,000 PSI

 

By: Randy DeFord, Engineering Manager Mid-West Spring & Stamping