Inner and Outer Diameter in Coil Compression Springs
When it comes to coil springs the term “diameter” can be tossed around often and it is an important word used in different parts of the coil spring dimensions. It is used to describe the width of the spring’s coils but it is important to pay attention to what kind of diameter is being referred to, especially when it comes to compression springs.
In compression springs, there are four main times where the word diameter is used in relation to spring specs. These are: wire diameter, outer diameter, inner diameter, and mean diameter. Each is abbreviated differently and refers to a specific and important part of the spring that will allow you to determine the size, shape and even rate of the coil compression spring. In this article we will address each of these types of diameters and explain the differences and importance of each.
Wire Diameter (WD)
Wire Diameter is abbreviated as WD. It is the width of the wire from which the coils of the metal spring are made of.
Inner Diameter (ID)
Inner Diameter is abbreviated as ID. It is the width of the “hole” at the center of the coils. Knowing this measurement will help you determine if the spring will fit over a shaft, mandrel, or tube, and if it will be adequately able to compress in such space.
Outer Diameter (OD)
Outer Diameter is abbreviated as OD. It is the width of your spring measured from outside the coil. Knowing this measurement is necessary in order to determine if your spring fits into the device it will be placed on. It is especially necessary to know this, if the spring will be inserted into a hole, tube or closely defined space.
Mean Diameter (MD)
The Mean Diameter of a compression spring is abbreviated as MD. It is right at the middle of the inner and outer diameters. It is measured from center of wire to center of wire. This is used when calculating the SPRING INDEX of a compression spring. You can learn more about Spring Index and the importance of it HERE
Measuring Wire Diameter Inner Diameter and Outer Diameter With a Caliper
One of the main reasons why we need to know and understand all the types of diameters is because these are necessary measurements for determining how small or big the compression spring will be. The wire diameter, outer diameter and inner diameter of your compression spring are all measurements that need to be taken with a digital caliper and each has a specific method for measuring it, so it is different depending on whether you are measuring the inner diameter (the inside space of the coils) or the outer diameter (the outside space of the coils).
For a video on How to Measure the Wire Diameter visit this youtube video: https://youtu.be/iX9wc75LEIU
For a video on How to Measure the Inner Diameter visit this youtube video: https://youtu.be/W10dM-cVkB8
For a video on How to Measure the Outer Diameter visit this youtube video: https://youtu.be/9b-DQR-L9tw
Determining Mean Diameter
The third type of diameter which is mean diameter can actually be calculated or determined by using the other diameter measurements: wire diameter, inner diameter and outer diameter in the following formulas:
Coil Spring Mean Diameter Formula
Mean Diameter = Outer Diameter – Wire Diameter Mean Diameter = Inner DIameter + Wire DiameterExample
You have a spring with a wire diameter of 0.035 inches, an outer diameter of 0.500 inches, and an inner diameter of 0.430 inches.
MD = OD – WD MD = 0.500 – 0.035 MD = 0.465 MD = ID + WD MD = 0.430 + 0.035 MD = 0.465Formula Diagram
Whether you are looking to design a custom spring or find a stock spring similar to the one you have, being able to measure each diameter in a compression spring correctly will allow you to accurately acquire the coil spring you need. If you need further assistance in selecting and purchasing the right compression spring and have a specific set of diameters you need it to have, feel free to contact us at: (951) 276-2777