What Is Spring Index & How to Calculate It
The Spring Index is a derived property of helical coil springs that conveys vital design and manufacturing information about the spring by ratioing two of its most recognizable physical dimensions. For example, for custom spring projects, the spring index informs spring manufacturers about the feasibility of a given design. In contrast, it informs the design engineer about the fit and function of existing stock springs.
In this brief survey, we explore the definition of this property, how to calculate it, what it means for custom springs, and the performance and fit of stock springs. Once you understand the spring index and its importance, you will be better suited to shop stock springs for your design constraints. You will also have a much better understanding of the manufacturing effort needed to produce custom springs.
When shopping for springs, Century Spring’s dedicated customer support teams can help you navigate critical design criteria, including the spring index, to provide custom and stock spring solutions. We are the most trusted name in spring manufacturing.
What Is Spring Index?
The spring index is the ratio of the spring’s helical coil diameter to the spring’s wire diameter. In this way, it is a ratio of two defining features of springs: the Wire Diameter and the Mean Coil Diameter. The spring’s wire diameter represents the size of the wire that forms the helical coils of the spring. The mean coil diameter of a spring is the coil diameter as measured from the center of the helical coils to the middle of the wire diameter of the outer helical coils. It can be calculated as follows by taking either the Outer Diameter (OD) or Inner Diameter (ID) of the spring’s helical coils and then either adding or subtracting the spring’s wire diameter (d) from the values respectively as follows:
Knowledge of both the mean coil diameter and the wire diameter is required to calculate the spring index as defined by the simple formula below:
Where:
D is the Mean Coil Diameter
d is the Wire Diameter
With these two values in hand, you can calculate the spring index for any spring.
How to Calculate Spring Index
From the equation above, let’s calculate the spring index of an actual stock compression spring with an Outer diameter (OD) of 1.24 in. and a wire diameter (d) of 0.177 in.
As a side note, you can locate these dimensions directly from our spring catalogs when shopping for stock compression springs. However, when determining the spring index from an unknown spring in your possession, to ensure accuracy, carefully measure all dimensions. If you want instructions on measuring every critical dimension of your spring, check out our dedicated measurement guides for additional instructions on measuring critical dimensions for compression, extension, and torsion springs.
Back to our example, this means that the mean coil diameter (D) is taken to be:
Thus, the spring index is calculated as follows:
As we will detail further in the following sections, this spring index is right in the middle ideal range for manufacturability, which is to be expected for a stock spring. Generally, the spring index tells us that this spring will likely have a higher spring rate, a function of additional geometry to be detailed in the next section.
High vs. Low Spring Index: What Do They Mean?
The spring index generally tells you how tightly the helical spring coils are wound. Springs with low spring indices have a tighter wind than a comparable spring with a higher spring index.
Low Spring Index
Therefore, springs with low spring indices have comparatively larger stiffnesses and wire diameters, so they have relatively more resistance to applied loads. This follows that small spring index springs deform less for a comparable applied load.
Furthermore, springs with small spring indices have comparatively thick wire diameters and small ODs, meaning the coils are tightly wound. Since the wire diameter is on the denominator of the equation, springs with large wire diameters and small ODs must lower spring indices.
Low spring index springs are generally used in applications such as heavy machinery that require high load capacity and can tolerate minimal spring deflection.
Because of their compact shape, springs with spring indices less than 4.0 are challenging to manufacture, if not impossible.
High Spring Index
Conversely, springs with higher spring indices have comparatively lower spring rates and wire diameters, so they have relatively less resistance to applied loads. This follows that large index springs deform more for a comparable applied load, meaning they have more active spring coils.
Furthermore, springs with large index values have comparatively thin wire diameters and large OD. Since the OD is in the numerator of the equation, springs with large ODs and small wire diameters have large spring indices.
Large spring index springs are ideal for applications where large deflections must be accommodated, such as in actuated systems or mechanisms.
Springs with spring indices above 12 are expensive and impractical to manufacture because they have extremely large, flexible hoop shapes that approach the stability of a slinky. Additionally, the spring’s OD tolerance is a function of the spring index, meaning the larger the spring index, the larger the tolerance range on the spring’s dimensions.
Common Issues Caused by Incorrect Spring Index
It’s important to note that you do not shop for springs by the spring rate value alone. Helical spring selection most commonly begins with the spring rate and spring type.
An incorrect spring index means you do not have the correct spring for your application. Furthermore, the spring index serves as a “reality check” in custom spring design projects because it informs whether a spring design can be manufactured.
While a spring index less than 4.0 is difficult to manufacture, one less than 5.0 is not ideal and presents significant challenges to manufacturability that may make it impractical to produce. These challenges include a high scrap rate, longer tooling setups, and production runs. Additional care must be taken not to damage the tooling during the production of these springs.
Conversely, a high spring index larger than 20.0 is difficult or sometimes impossible to manufacture. However, even one greater than 13.0 presents similar manufacturing challenges, such as a high scrap rate, longer tooling and machine setups, and slow production runs. Springs produced to such dimensions also make it very difficult for the machine operator to hold specified tolerances on the OD and ID of the spring, the spring rate, and the spring's maximum load/deflection profile.
If you still have questions about spring design and selection, the team of spring design experts at Century Spring is available to help you create, refine, and optimize spring designs to fit specific tolerance, performance, and use case conditions. Let us partner with you today to show you how to select a spring with the ideal spring index for your application.
Century Spring: Your Partner in Precision Spring Design
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