How Do You Measure an Extension Spring for a Perfect Fit?

You need to replace a broken extension spring, but you're not sure how to measure it. Ordering the wrong size means more delays, wasted money, and a product that still doesn't work.

To order the correct extension spring, you must accurately measure three critical dimensions: the Outer Diameter (OD) of the coils, the Wire Diameter (դ), and the Length Inside the Hooks (LIH). These three measurements are the universal language for defining the size of any extension spring.

Over my 14 years in this industry, the most common reason for a returned order is an incorrect measurement. A customer will often measure the total length from tip to tip, but that's not the industry standard and can lead to errors. Getting these three core sizes right from the start is the most important step in getting a replacement spring that fits and functions perfectly on the first try. Let's walk through exactly how to take these measurements like a pro.

Why Are There Two Different Diameter Measurements?

You measured the width of your spring, but the order form asks for two different diameters. This is confusing, and you're worried about choosing the wrong one and getting a part that fails.

The two critical diameters are Outer Diameter and Wire Diameter. Այն Outer Diameter (OD)[^1] is the total width of the spring's body, which tells you if it will fit. The Wire Diameter is the thickness of the wire itself, which determines the spring's strength.

I often explain this to clients by comparing a spring to a donut. The Outer Diameter is the measurement across the entire donut, which tells you if it will fit on the plate. The Wire Diameter is like measuring how thick the doughy part is. That thickness is what gives the donut its substance, just like the wire's thickness gives the spring its strength. A tiny change in the wire diameter can make a massive difference in the pulling force. That's why we always ask for both measurements; one is for fit, and the other is for function.

The Two Most Important Diameters

One measurement is for size, the other is for strength.

• Outer Diameter (OD): This is the measurement across the outside edges of the spring's coils. It is the most common way to specify the coil size because it relates directly to the space the spring will occupy in your assembly.
• Լարերի տրամագիծը (դ): This is the thickness of the actual wire used to make the spring. It is the single most important factor in determining the spring's force and stiffness. For accurate measurements, you should always use a tool like digital calipers.

What's the Right Way to Measure an Extension Spring's Length?

You carefully measured your spring from one end to the other. But the replacement you ordered was too short, and now your machine is still down, waiting for the right part.

The correct way to measure the length is "Length Inside Hooks" (LIH). This is the distance from the inner surface of one hook to the inner surface of the other. This measurement defines the spring's length when it is installed and relaxed.

This is probably the biggest source of confusion when ordering springs by size. Many people naturally measure the "Overall Length," from the very tip of one hook to the tip of the other. The problem is that the thickness of the hook wire is included in that measurement, and it can vary. A manufacturer will almost always assume you mean Length Inside Hooks, as this is the standard that dictates function. I had a client in the agricultural industry who kept ordering springs that were too short. It turned out his team was measuring the overall length. Once we corrected that one measurement, the parts fit perfectly.

Measuring Length for Function, Not Form

The functional length is what truly matters for performance.

• Length Inside Hooks (LIH)[^3]: This is the industry standard. It measures the functional length of the spring between the points where it connects to your product.
• Body Length: This is the length of only the coiled portion of the spring. While useful for design calculations, it is not the primary measurement needed for ordering.
• Overall Length (OAL): This is the total length from the absolute ends of the hooks. It should generally be avoided as it can be an inconsistent and misleading measurement.

How Do These Sizes Affect the Spring's Performance?

You have the correct dimensions for your spring. But the original part failed, so how do you know that a replacement of the same size won't just break again?

The size dimensions directly control the spring's force and stress. A thicker wire or a smaller coil diameter will create a stronger, stiffer spring. If a spring is failing, you may need to increase the wire diameter to make it more durable.

When a customer comes to us to replace a failed spring, I always ask them how it failed. Did a hook break? Did the coil stretch out and not return to its original length? Knowing this helps us recommend a better design. Simply ordering an identical replacement might not solve the underlying problem. Օրինակ, if the spring is being over-stretched, a longer spring with the same force might be needed. Or if it's breaking from fatigue, a spring made from a slightly thicker wire will have lower internal stress and last much longer. The size isn't just about fit; it's the blueprint for the spring's performance and lifespan.

From Size to Strength

The dimensions are a recipe for the spring's behavior.

• Stiffness (Գարնանային դրույքաչափ): This is primarily controlled by the wire diameter and the outer diameter. A thicker wire makes the spring stiffer, while a larger coil diameter makes it less stiff.
• Stress and Durability: The stress on the wire is also determined by these dimensions. A spring with a very small diameter coil made from a very thick wire is under extremely high stress and may fail quickly. A good design balances these factors for a long life.

Եզրափակում

To order an extension spring by size, focus on three key numbers: Outer Diameter, Լարերի տրամագիծը, and Length Inside Hooks. These precise measurements ensure you get a spring that fits and performs correctly.

[^1]: Understanding OD is crucial for ensuring the spring fits properly in your assembly.
[^2]: Learn how wire thickness impacts spring strength and performance, essential for your project.
[^3]: Discover why LIH is the standard measurement for ordering springs and ensuring fit.
[^4]: Find out why OAL can lead to incorrect sizing and how to avoid costly mistakes.