# Compression springs and extension springs

I am an artist trying to figure out which extension spring to buy to hold back three compression springs. I am using 3 compression springs with the following attributes:
Rate lbs/in = 1.77
(these are the specs in the catalog)
They are stacked in a row end on end. How do I figure out the extenion spring that would be needed to hold back three of these compression springs in cocked position? I tried a extension spring with the following attributes, but it could only hold 2 of the compression springs, not three: load=3.69lbs
Rate lbs/in = 6.05
Tension lbs = .36
I don't know what any of the measurements mean, help!

 ebaines Posts: 10,033, Reputation: 5529 Expert #2 Feb 12, 2008, 02:05 PM
It's a bit difficult to follow you. I assume that the compression spring will be compressed from their rest position, and the extension spring will be arranged to somehow keep the compression springs from springing back to rest position, correct? What we would need to know is how far are you planning to compress the compression spring set, and then we can tell you how much the extension spring must be extended to counteract them. For example, if you want the compression springs to be compressed by 1.1 inches, this requires a force of 1/3 of 1.95 pounds = 0.65 pounds. This load can be generated by the extension spring if it is stretched 0.65/6.05 = 0.11 inches.
 ebaines Posts: 10,033, Reputation: 5529 Expert #3 Feb 13, 2008, 07:00 AM
Quote:
 Originally Posted by mylardatter sorry I didint know if I should repost to ask you questions or just mail you... SO, my totally confusing problem: Yes, the springs are being compressed from rest and the trigger is holding them in compressed posion so they don't spring back to resting. The springs are each about 3 inches long so all togehter are about 9 inches. When completely compressed they are all together 2 1/2 inches. The trigger is upright and is held upright by the spring, but when it is pulled (like a gun trigger) it releases the compressed springs. The extension spring I have currently is about an inch long, and estends about another 1/2 inch when pulled, so it is not extending a long way. When it is at resting at one inch it is not strong enough to hold all three springs back. It can however hold 2 (so my guess is that there might be some kind of ratio that will work to figure out what will hold 3) First, when I stack all three springs, am I tripleing the load? Or does it multiply? How does that work? Second, is there an equation that tells you how much force it takes to hold down a spring? Ie keep it compressed? Would the anser to that equation be the rate of lbs/inch I should look for in an extension spring? Or is it more complicated than that? I will draw a picture if you need me to! Thanks so much!
I have taken the liberty to copy the personal message you sent me here, as you should always post follow-up questions to the forum as opposed to using personal messages.

First, 3 springs in a row together act as spring that is 1/3 as stiff as a single spring. The formula for calculating spring loads is F=kx, where k is the spring coefficient in pounds/inch and x is the spring displacement from rest in inches. This formula assumes a perfect linear relatoinship between force and displacement - in real life this may not be the case, especially if the springs are compressed to the point where the coils begin to interfere with each other, or if a spring is stretched so far as to take on a permanent stretch.

From the data you have provided, the spring coefficient for the 3 springs stacked together and in compression is 1/3 of 1.77 pound/inch, or 0.59 pounds/inch. Since you want to compress the stack by 6.5 inches from a rest length of 9 inches to a compressed length of 2.5 inches, that requires 3.84 pounds of force. The extension spring has a spring coefficient of 6.05 pounds/inch, so it must be stretched 0.64 inches in order to balance the force of the compression spring.

One other item - you don't mention how the extension spring and compression springs are connected to each other - if through the trigger then is the trigger acting like a lever, with a center pivot and the springs attached at either end? If this is the case then the relative lengths of the two lever arms will affect the answer. Hope this helps.

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