A Spring’s Outer Diameter and Inner Diameter
Both the outer diameter and inner diameter of a spring are basic and vital terms in spring design. Both terms assist in the design and purchase selection of the proper spring. If your compression spring is going inside a tube or enclosed space, you need to consider the outer diameter so you can ensure it will fit safely into such space. Outer diameter is also used to find inner diameter and mean diameter, two other terms common in spring design. If it is going around a tube or shaft then you will need to greatly consider inner diameter as well.
Outer diameter is the max width of a spring’s coil measured from the outside. It is a simple term because its name literally represents what it is, it is the diameter measured from outside to outside of the wire of the spring. If we looked at the coil spring from the top so that it looks like a donut, the outer diameter would be the diameter from outside to outside of the donut.
Inner diameter is another necessary term that is closely related to outer diameter and it is the width of the inside of a coil spring’s diameter, measured from the center of the helix. The inner diameter would be the diameter of the donut hole. There is a formula you can use to calculate inner diameter, and it is as follows:
Inner Diameter = Outer diameter - 2 wire diameters
(Wire diameter is the thickness of the wire the spring is made of)
Some coil springs are placed over a shaft or mandrel or have a tube going through them. In these cases, it is necessary to take into account the inner diameter of the spring as there needs to be just enough space for the spring to actually fulfill the required motion without getting stuck or causing abrasion to the tube it is surrounding.
Extension Spring Inner Diameter
Knowing the inner diameter of your extension spring might be required when you have an extension spring without hooks. Extension springs without hooks are used to eliminate the weak point of the hooks and therefore use a screw or bolt inside it. In these cases, the threads of the bolt and the wire diameter must be taken into account. Since the extension spring’s inner diameter must have good grip on the bolt on both cases, the inner diameter must be exactly the same as the outer diameter of the bolt, or even smaller. To calculate the inner diameter and compare it to the screw’s diameter, you must use the formula provided in the definition which shows you to subtract two wire diameters from the outer diameter.
Torsion Springs Inner Diameter
Most of the time, in torsion springs, a mandrel or shaft will go through this spring’s inner diameter. Therefore, you will have to make sure that the spring’s inner diameter will fit over the shaft or mandrel. It is important to note that a torsion spring’s inner diameter shrinks during deflection. Be aware of how much your spring is meant to travel to figure out how much the inner diameter will shrink under the deflection so that the spring won’t clamp on the shaft.
If your torsion spring’s inner diameter is too small, it will bite down on the shaft and won’t be able to travel any further. If the torque is still being applied after your spring’s inner diameter has already bit down on the shaft, you run the risk of damaging your torsion spring since you are pushing it beyond the limit. Our torsion spring calculator, Spring Creator, will calculate your torsional spring’s torque as well as the inner diameter after deflection.
As you can see, both inner and outer diameter are key factors that can be quite helpful when designing and buying a custom spring. If you need help or assistance designing or purchasing a custom spring, be sure to contact us today at: (951) 276-2777. We would be happy to help!