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.