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# Spring Index

#### Definition of Spring Index

Spring index is the correlation between the mean diameter of a spring and the wire diameter of a spring. This proportion will determine the strength of the spring, the stress induced on the spring, and the manufacturability of the spring. On this page you will discover how to calculate your index mathematically using the spring index formula, how to use the index in spring design, how to measure a spring in order to determine index, and the most desirable range for a spring index to be in.

#### Formula For Calculating Spring Index

Index = Mean Diameter (D) / Wire Diameter (d)
Mean Diameter (D) = Outer Diameter (OD) - Wire Diameter (d)

#### Spring Index Calculation Example

• Outer Diameter: 1.000 inches
• Wire Diameter: 0.080 inches

• Mean Diameter = 1.000 in (OD) - 0.080 inches (d)
Mean Diameter = 0.920 inches
Index = 0.920 inches (D) / 0.080 inches (d)
Index = 11.5

In order to properly calculate the spring index formula you will need to know how to measure a spring. Please see the images below to see how to properly measure a compression, extension, and torsion spring. For a more detailed explanation on how to accurately measure your spring please visit our spring measurement page.

#### Using Spring Creator Calculator to Determine Index

To see how decreasing or increasing your index will effect the strength of your spring please use our Online Spring Calculators. Simply enter the dimensions you have measured using the diagrams above and hit calculate. The spring index will be in the "physical dimensions" section. Remember, to change your index you will need to manipulate the outer diameter of your spring and/or the wire diameter of your spring.

#### Manufacturability of Your Spring Based on Spring Index

The spring index on your spring is one of the first things a spring engineer will evaluate when deciding if your spring is manufacturable and what the cost will be to manufacture your spring. If you stick to the numbers below you will have lower production costs and more availability of spring makers to manufacture your spring.

• Spring Index: 0 - 3.9 : China Spring Products Corp Cannot Manufacture
• Spring Index = 4 - 5 : Your Spring Falls in the Difficult Category, higher cost
• Spring Index = 6-12 : Best Manufacturing Range Lower Cost
• Spring Index = 13 - 15: Your Spring is Just Above Good But Not Quite in The Difficult Range
• Spring Index = 15 - 25: Spring Falls in the Difficult Category, higher cost
• Spring Index = Above 25 : Very expensive and difficult to manufacture, tolerances are larger

#### Manufacturing a Large Spring Index

##### How Does My Index Affect The Strength Of My Spring?

The index of your spring is in direct correlation with how strong your spring will be. If you need to INCREASE the strength of your spring, you will need to reduce the index. There are two ways to reduce the index. The first way is to INCREASE the diameter of the wire. The second way is to DECREASE the outer diameter of the spring. If you need to DECREASE the strength of your spring, you will need to enlarge the index. There are two ways to enlarge the index. The first way is to DECREASE the diameter of the wire. The second way is to INCREASE the outer diameter of the spring. Please see the chart below for further information.