How to Count Spring Coils: Counting Active & Inactive Coils
The total number of spring coils in your helical coil spring, including the active spring coils that operate during spring deformation, gives you the information you need to verify that your spring is suited for your application. For example, the spring stiffness (or spring rate) is a direct function of the number of spring coils and helps you determine whether your spring is functioning within its specified displacement limits.
Counting the number of spring coils on your spring is an easy exercise if you understand the distinction between the coil spring’s total number of spring coils, which consists of the spring’s active spring coils and inactive spring coils. After you understand the distinction between these types of spring coils, you can easily count the total number of spring coils in your spring by following a simple procedure.
In short, verifying the number of spring coils, both active and inactive, on your spring gives you the knowledge you need to ensure spring longevity and proper function in your application.
Why Counting Spring Coils is Important
The number of helical spring coils directly determines several important spring properties. Understanding the spring rate is the foundation for understanding how spring coils impact spring performance.
The spring rate (k) is a direct function of the number of active spring coils (N):
Where:
- G: Material Shear Modulus, a material property that describes the deformation response of the spring under shear stresses
- d: Spring Wire Diameter
- D: Mean Coil Diameter, calculated as the spring’s Outer Diameter (OD) minus the spring’s Wire Diameter
- N: Number of Active Spring Coils
This relationship shows that the spring rate is inversely proportional to the number of active coils. Holding the other spring parameters the same, a spring with more coils will be less stiff than a spring with fewer coils. Therefore, a particular spring with fewer coils (thus fewer active coils) will be stiffer and experience less deformation per unit load.
A lower-stiffness spring will experience much more displacement per unit of force on the spring. For example, a helical coil compression spring with relatively high stiffness will compress less distance, meaning that when all other spring design properties are held constant, a helical coil spring with fewer helical coils can withstand larger loads.
Alternatively, using our spring catalog design information, you can calculate the number of active spring coils (N) directly by rewriting the above equation as follows:
Understanding Active vs. Inactive Coils
In simplest terms, active spring coils are displaced under the design load and store energy during deformation. Any spring coils that do not experience displacement under loads are inactive spring coils, such as those that comprise the spring ends. Inactive spring coils provide stability to the spring as the interface between the spring and mating machine components. The inactive and active spring coils comprise the total number of spring coils. Active coils = total coils - 2
A spring must have the manufacturer-specified number of total coils to function properly in the application. For example, a large compression spring in a car's suspension system will not be able to safely absorb the impact of the car wheel hitting a pothole if it is missing an inactive coil where it mounts to the suspension.
Step-by-Step Guide: How to Count Spring Coils
Step 1: Identify the Total Number of Coils
A complete 360-degree rotation of spring wire defines a single spring coil. Any coil that does not traverse the full 360 degrees is a partial spring coil. This simple principle allows you to count the total number of coils in your helical coil spring.
To count the total number of coils in your spring:
- Start at one end of the spring at a fixed reference point.
- Visually track the coil around 360 degrees until you arrive at the next, representing one single spring coil.
- Continue counting coils, tracking the coil wire around the spring towards the opposite end of the spring.
- When you reach the end of the spring, add up any partial coil counts as follows: 180 degrees represents ½ coil count, while 90 degrees represents a ¼ coil count.
This procedure can count the total number of coils in any type of helical coil spring, regardless of the end type.
Step 2: Determine the Active Coils
To count the number of active spring coils, follow the same general procedure as the previous step, except skip the ends of the spring and start counting where the first full helical coil begins. Count carefully from here towards the other end of the spring, terminating your count at the last full helical coil before the spring’s end wire configuration begins.
Alternatively, you can also determine the number of active spring coils by taking the total number of coils in your spring (as determined in the previous step) and subtracting the inactive spring coils from this value.
The number of inactive spring coils is a function of the spring end type for helical coil compression springs. For example, a closed-end compression spring has flat ends formed from the wire ends so the spring can stand vertically on a flat surface. An open-end compression spring does not have this end feature formed from the wires; rather, the ends are cut in the middle of their helical coil progression.
Step 3: Verify Coil Count with Manufacturer Specifications
After obtaining the total spring coil count, you can browse the manufacturer specifications by the total coil count and other spring properties to verify the spring specification. You can use this to obtain all the information you need.
Our expansive helical coil spring catalog includes thousands of stock springs across every type. You can filter by outer diameter, wire diameter, spring rate, and the total number of spring coils.
Helical Coil Compression Springs
Helical Coil Extension Springs
Helical Coil Torsion Springs
If the number of spring coils you counted does not match a standard stock spring in our catalog, it might have a broken coil, or it might be a custom spring design. In such cases, Contact Us so we can help you verify your spring is undamaged and will function as intended in your application.
Common Mistakes When Counting Spring Coils
The most common spring coil counting mistakes occur when counting compression springs and directly result from not correctly understanding the inactive spring coils at the end of your spring.
For example, most compression springs have closed ends, with one inactive spring coil at each closed end. However, compression springs with double closed-end configurations have two inactive spring coils at each end of the spring. Finally, an open-end on a compression spring end has no inactive spring coils, but an open-end with ground coils at the end (called “open-end ground” springs) has ½ inactive spring coils per end.
Compression Spring End Type | No. Inactive Spring Coils (per end) |
Closed End | 1 |
Double Closed End | 2 |
Open End | 0 |
Open End, Ground | 0.5 |
Helical coil extension and torsion springs also have different end types, which will require a careful assessment of whether the coils at the ends are active or inactive. Check both ends of your spring because some springs have different configurations at each end.
For more detailed information on spring end types, check out our dedicated guide to spring end types.
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