Extension Spring Design Technical Information
Extension Spring Design
Definition of Extension Spring: Extension springs, also known as a tension spring, are helical wound coils, wrapped tightly together to create tension. Extension springs usually have hooks, loops, or end coils that are pulled out and formed from each end of the body. The function of an extension spring is to provide extended force when the spring is pulled apart from its original length.
Applications:
Extension springs are used in trampolines, push and pull levers, rocking horses, screen doors, and anywhere else extended force is necessary. You can find small extension springs, micro extension springs, large extension springs and heavy duty extension springs.
Extension springs can also be used as belts (Garter Springs, Snakes, and Drain Cleaners). Snakes are extension springs without hooks.
On snakes the coils are tightly wound together to give initial tension, and its long length allows it to make its way through the maze of the pipe it is unclogging.
The snake or extension spring is turned clockwise so that it acts like a screw threading itself into the clog.
Once the snake is imbedded into the mass it works at tearing it apart, thus unclogging the drain.
A Garter Spring is also a tension spring that is tightly wound.
However one end is usually cone shaped so as to permit it to thread into the other end of the garter spring making the entire spring into a spring belt or ring.
The belt or ring can now be used like a rubber belt or pulley, but with more give and flexibility.
They can also work their way around many pulleys like a serpentine belt and take on many shapes.
Example:
A trampoline uses many extension springs to create the bouncing effect.
Every time someone jumps on the trampoline, the extension springs are pulled apart and force is exerted.
This makes the extension spring want to go back to its original length, thus giving the inertia to fly into the air.
1. Force: How much force do I need to generate?
This question is more easily answered by thinking in terms of rate, which is pounds per inch (lbs/in) of extension or (distance traveled when pulled).
All extension springs have a rate of pounds per inch of extension .
Example: Lets say your spring is 10 inches long and has a rate of 10 lbs per inch / of extension.
This means if you pull the spring 1 inch in distance it will take you 10 lbs of force to do so.
The concept is simple, for every 1 inch of distance you pull or travel it will take you 10 lbs of force to do so.
Lets assume you need 5 inches of travel, then the force your spring will generate is 50 lbs when extended 5 inches.
2. What is pre-load and do I need it?
Pre-load is stretching the extension spring a short distance from its free state, so one could have stored energy.
Initial tension is the tension between the coils sandwiched together.
Every extension spring has initial tension.
Using the above example, if your spring is going to travel 5 inches of distance that will give you 50 lbs of force plus 2 lbs of initial tension giving you a total of 52 lbs of force.
3. Determine Spring Constant
Formula for Spring Constant
Rf = Gd^4 / 8D^3N
S = 2.55 PD / d^3 (Correct for D/d)
G = psi * 10^6
- G = Shear Modulus of Torsion
- d = Wire size
- D = Mean Diameter
- N = Number of active coils
- Rf = Rate of extension spring (in lbs./inch)
- S = Stress (lbs. /Sq. Inch)
- P = Load (lbs)
- M = Moment (inch-lbs.)
- D / d = Index correction (developed from Wahl Factor)
4. Determine Initial Tension
Formula for Initial Tension
P = Sd^3 / 2.55D = Load in lbs
The Spring Force Chart
Use this chart to adjust your spring force accordingly
| More Force (MF) | Less Force (LF) |
|---|---|
| Smaller OD = MF | Larger OD = LF |
| Less Coils = MF | More Coils = LF |
| Thicker Wire = MF | Thinner Wire = LF |
| More Travel = MF | Less Travel = LF |
Key Parameters:
1. Number of Coils:
Remember the chart above: Smaller OD = more force (MF), Larger OD = Less force (LF).
The same is true with the number of coils in your extension spring.
Less coils = A stronger extension spring.
More coils = weaker extension spring.
At the same time less coils = more stress and fatigue.
More coils = less stress and fatigue.
The perfect balance on your extension spring design is one that combines the right amount of coils for how much distance or travel you want to achieve along with the correct amount of force needed to do the job.
If you need help measuring the number of active coils you can watch our how to measure an extension spring video .
2. Hook Configuration.
Crossover Center Hooks
Extended Hooks
No hooks
Side hooks
Machine Hooks
General Step By Step For Designing An Extension Spring:
- 1. Determine if the extension spring has to fit over a shaft.
- 2. Determine if the spring is going to fit in a hole.
- 3. Does the extension spring need to fit into a fixed space.
- 4. What is the purpose of the extension spring.
- 5. What types of hook do you need?
- 6. Determine what type of material will be necessary.
- 7. Consult a spring engineer you trust.
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