How to Measure a Compression Spring
When measuring your compression spring, you must know on the following dimensions:
- Outer Diameter
- Inner Diameter
- Free Length
- Wire Diameter
- Total Coils
Please see the graphic on the right for help.
(You may calculate your inner diameter by subtracting two wire diameters from your outer diameter as shown in this formula:
O D - 2 * W D = I D )
How to Measure Your Spring
Springs are accurately measured with a dial caliper. The reason for this is because a caliper provides you with a more exact measurement. Wondering why this is so important? Well, take your spring's wire diameter as an example. In most cases, if the spring's wire diameter is off by 0.001" it'll affect your spring's rate and travel so one must have exact measurements when measuring a spring.
Another thing you must focus on is your end types. The end types available are the following:
Closed and Square The ends are closed and don't have pitch, therefore, not all the coils on your spring will be active only the coils that have pitch or space in-between the coils are active. In this case you may calculate your active coils by subtracting 2 coils from your spring's total coils: Total Coils - 2 = Active Coils.
Closed and Ground These ends are the same as closed and square ends but the ends go through a grinding process that grinds half a wire diameter off the first and last coils so that your spring will stand vertically. The calculation process for active coils is the same as the one used for closed and square ends. Total Coils - 2 = Active Coils.
Double Closed and Square ends These ends are closed but the first two coils and last two coils are touching. To figure out the active coils when it comes to this type of ends, you'll have to subtract 4 coils from your total coils: Total Coils - 4 = Active Coils.
Open Ends These ends are the opposite of closed and square ends because the ends don't close and don't have pitch. Therefore, all the coils of a spring with open ends are active.