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Compression Springs Compression Springs

Enter your Outside Diameter and Free Length
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Wire Diameter
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Solid Height
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Extension Springs Extension Springs

Enter your Outside Diameter and Length Inside Hook
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Length Inside Hook
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Wire Diameter
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Max. Load
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Initial Tension
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Torsion Springs Torsion Springs

Enter your Outside Diameter and Total Coils
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Inside Diameter
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Free Position
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Wire Diameter
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Body Length
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Max. Torque
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Max. Deflection
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Rate
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Leg Length
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Material Type
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Conical Springs Conical Springs

Enter you Small Outside Diameter, Large Outside Diameter and Free Length
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Small Outside Diameter
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Large Outside Diameter
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Free Length
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Rate
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Total Coils
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Solid Height
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Wire Diameter
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Material Type
*Required Field

Spring Index

What is it?

Definition of Spring Index: Spring index is the proportion between the wire diameter and coil diameter (how tight the coils are). It can help determine key characteristics of a coil spring and affects its manufacturing complexity.

 

How to Calculate Spring Index

Step 1.) Calculate Mean Diameter

D = Douter – d

Step 2.) Calculate Index

I = D ÷ d

Example

D = 1.5" (Douter) – 0.15" (d)
D = 1.35 inches

I = 1.35" (D) ÷ 0.15" (d)
I = 9

Key

  • I = Index
  • D = Mean Diameter
  • Douter = Outer Diameter
  • d = Wire Diameter
Spring Index ExamplesSpring Index Examples

How it affects strength(force)

 

The index tells you how tight a spring's coils are and it can directly affect the strength of such. A spring with very tight coils tends to have a stronger spring rate. That is why the lower the index, the more strength/stress the spring tends to have. Therefore, the higher the index the weaker the spring tends to be.

 

Decreasing the outer diameter or increasing the wire diameter will tighten your spring's index and make your spring stronger. However, if you either increase the outer diameter or decrease the wire diameter, your spring's index will grow.

 

Spring Index and Force Relation

More Force (Tighter Index) Less Force (Larger Index)
Smaller OD Larger OD
Thicker Wire Thinner Wire
Less Coils More Coils
Tight Index Spring ManufacturingTight Index Spring Manufacturing
Large Index Spring ManufacturingLarge Index Spring Manufacturing

How it affects manufacturing and cost

 

Most spring engineers can quickly determine if the spring you inquire about is manufacturable by simply looking at the index. Since very tight coils (low index) require special tooling and techniques, they can either cost more due to their manufacturing complexity or be out of the spring supplier's manufacturing capabilities. The same applies for very high indexes where the spring can be quite flimsy and hard to manipulate during the compression spring manufacturing process.

 

A good spring index range is between 5 and 15. When a spring has an index that falls between this range, it is usually within most manufacturer's possibilities and does not tend to be as tedious to work with. To better understand the relation between spring index and the manufacturing costs, please take a look at the chart below.

 

Spring Index vs Manufacturing Complexity

Spring Index Manufacturing Complexity
0 - 3.9 Not manufacturable by Acxess Spring.
6 - 12 Ideal Manufacturing Range. Lower cost.
13 - 15 Ok, but not great. Might start to increase cost.
15 - 25 Spring Falls into the “Difficult” category. Higher cost.
Above 25 Hard to manufacture. Greater tolerances needed. Expensive.

We are always ready to help you understand what you need from a spring and how to get the best price for it. Call us now at (951) 276-2777.