Initial tension is an important term related to extension springs. It refers to the tension that is store between an extension spring coils, which is what keep them together. The initial tension is released once the spring is extended to the point where you can barely see some light pass through the coils. This value directly affects the load required to pull an extension spring, that is why it is so important to keep it in mind when designing or buying a tension spring.

Rate is the constant force that a spring exerts per one unit (1 inch or 1 millimeter) of distance traveled. Therefore, it is expressed in pounds per inch or newtons per millimeter. Because the spring already has initial tension before deflection, this value is additional to the load determined by the spring rate. To further explain this, let's illustrate it differently.

Let’s say we compare calculating the force that your extension spring exerts, to the amount of money you make at a job. If this were a regular compression spring, then rate would be the only thing you consider, and you would take into account only rate to find out the amount of force needed. If you have a regular paying job, you get paid certain amount of dollars per hour, so you would take into account only the hours you worked to know how much you get paid. Now, because this is an extension spring, you have to factor in the initial tension, just as if your job paid commission on top of your regular pay.

For example:

Let's say you get paid $5 dollars an hour and you work for 1 hour you would have to get paid $5 usd but if you made $10 dollars in commission you would actually have to get paid $15 usd.

Now, if your spring rate says that your spring requires 5lbs of load to extend one inch of travel, then, you have to add to that the initial tension that it demands. For example, let’s say the initial tension is 10lbs then you would actually need 15lbs for it to extend enough for it to have one inch of travel.

initial tension plus rate

Knowing what the initial tension of your spring is, is very important. At Acxess Spring, we make it easy for you to get detailed information on your extension spring and accurate calculations. When you input the dimensions of the tension spring you want, Spring Creator ( automatically calculates initial tension so you can accurately design the right extension spring for the job. Should you need additional assistance purchasing the perfect custom spring, contact us directly at: (951)276-2777