Extension Springs

Definition: (x-ten-sha-on sp-ri-ng)
Extension springs, also known as a tension spring, are helically 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.

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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.

Applications:
Extension springs are used in trampolines, push and pull levers, rocking horses, screen doors, and anywhere else extended force is necessary. 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.

Design Considerations:
Designing an extension spring can be a complex affair if one does not understand the laws that govern them. To design an extension spring we must consider the basic design characteristics:

A: How much room do we have to place an extension spring in our product?
More importantly how much distance of travel will the extension spring need to have? 1. LIH (Length Inside Hook), (max) maximum (OD) outer diameter max. distance traveled, preloaded length, and type of hooks needed for the extension spring to fit into your black box.

B: 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. To see the extension spring formulas (click here)

1. 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. The key here is to figure out how much distance of travel the extension spring needs in order to function correctly, as well as give you the correct amount of force when the spring has traveled it's length. Lets assume you need 5 inches of travel, then the force your spring will generate is 50 lbs when extended 5 inches. Remember, the key here is for you to figure out how much force you need your spring to have at a certain amount of distance traveled.

C: What is preload and how do I get it?
Preload is stretching the extension spring a short distance from its free state, so one could have stored energy.
Example: Your extension spring will not give you any force until you stretch it. So, one must pull (pre-load) the spring to place the hooks of the extension spring on to your product. In the example of a trampoline one must pull the extension spring a short distance so you can have a tight rubber mat to jump on. Without preloading the extension spring, your rubber mat would not be nice and snug to jump on. There are two aspects here that go hand in hand one is rate and two is initial tension. The Spring Rate (lbs/in) plus the distance traveled will give you the final force of your spring.  But one has to take into consideration the initial tension. Initial tension is the tension between the coils sandwiched together. Every extension spring has initial tension. Lets say your spring has a rate of 10lbs/in plus your initial tension is 2 lbs.

This means that you'll get 2 lbs of initial tension force when you just barely pull the extension spring enough to see light between the coils. So the way to figure out your total force is initial tension + rate (lbs/in of extension) of spring.  This will give you the total amount of force your spring will give you for the amount of distance you will travel. 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.

D: I want many cycles of life (High Repeatability) How do I achieve this?
One must understand the logics of extension springs:

The Force Chart
For basic spring design consideration

More Force (MF)	Less Force (LF)
Small OD = MF	Large OD = LF
Less Coils = MF	More Coils = LF
Thicker Wire = MF	Thinner Wire = LF
More Travel = MF	Less Travel = LF
Outer diameter	Inner diameter

These are the basic design considerations. Please understand all the design considerations for they all work hand in hand.  Example: if my extension spring OD is too small, I may get a lot of load or force, but my extension spring will be to stressed and fatigued. This may cause it to take a set and not return to it's original length, or worse yet break all together.

Use these extension spring formulas to determine spring constant

R_f = GD⁴ / 8D³N
S = 2.55 PD / d³ (Correct for D/d)

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)

Torsional index resulting from initial tension in extension springs
Suggested Initial Tension Torsional Stress is the max to min on table below
 

Formula for Initial Tension
P = Sd³ / 2.55D = Load in lbs

Extension Spring Index:
To figure out my spring index take your outer diameter (OD) minus one (1) wire diameter (WD) = mean diameter (MD), MD divided by WD = Spring index. Example 0.500 OD  - .050 WD = .450 MD divided by .050 WD = 9 to 1 index, this is a good index. This means there are 9 wire sizes that divide into (1) one mean diameter. It is not recommended to go below a 5 to 1 index for a good extension spring design. It is important to be careful on your index because the smaller you go on index the tighter your spring displacement and the more strength your spring has. This of course will add more stress on your design. You do not want a lot of stress on your extension spring because stress and fatigue will cause setting of your extension spring. Example: You have an extension spring that measures .500 OD with a .100 WD, then take .500 OD minus - .100 WD = .400 MD divided by .100 WD = 4 to1 index or 4 wire size to (1) one mean diameter. A 4 to 1 index is to low of an index. A 4 to 1 index will only work if you need to generate a strong amount of force with a small amount of travel. But in the case where you need the same amount of force along a good amount of travel or (deflection) a  4 to 1 index will not work. In order to resolve the problem, you will need to open your index by making the outer diameter (OD) bigger. A  8 to 1 index (or higher) is a better index number to achieve a good amount of force along a greater distance of travel with less stress.

Number of coils in an Extension Spring:
Your number of coils is another topic of extension spring design that is greatly connected to stress and fatigue. 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. This is also coupled with the correct index to attain a low fatigue and stress level.  Once you achieve this balance, your extension spring will last many millions of cycle life (high repeatability). All this should be calculated to fit in the space of your application.

Length inside Hooks (LIH)  
 Body length  (BL)

Extension spring length inside hook (LIH) is the length of your extension spring measured from the inside of one hook to the inside of the following hook as an overall length from end to end.  In this case it's the inside of one end hook across the spring length to the inside of the other end hook.  Body Length is your stacked coil height. To determine your body length the following formula is required. Example: Lets say your extension spring has a 10 coils and your wire diameter (WD) is .050 then you would multiply .050 wd x 10 coils = .500 + one wire size = .550 body length (BL). Why one more wire size? Because your spring has 10 coils but  the 1st coil you count is really on the second wire.  A coil on any spring isn't a complete coil until you follow it around a complete 360 degrees (a full circle or turn), thus the need to start counting your coils on the second wire. An easy way to figure out body length is to count all the wire sizes in the spring. If you have 10 coils you will have 11 wire sizes, thus giving you 11 wire sizes of body length or solid height. Please be very clear on this so you may understand how to obtain the correct Body Length (BL) or Solid Height (SH). Many engineers fail to understand this simple concept and thus end up with incorrect prints.

Extension Spring Hook Configurations:
Most extension springs have hooks which transmit the force to your product. From a economic manufacturing point of view, you as a designer or engineer should avoid having extended or long length hooks especially with small springs. This ensures that your end extension spring design will be economical as well as functional. Many designers/engineer never take this into account, causing them to end up with complicated hook configurations that are overkill. Please do not limit yourself either. You should consult the advice of a professional spring design engineer to asses your design criteria while your still in the prototype stage. They can point out great ways to assemble your extension spring to your product with very simple hooks. A lot of times the extension spring designs are very complex and cannot be manufactured. Remember a great spring is all in the design, including all the above factors we took into account to get here. Sometimes less is more. There are many different types of hooks that can be added to your extension spring. Extension springs can have machine hooks coming right off the body, side hooks, loops, cross-over hooks and double loops, as well as many more end configurations. Just about any kind of end configuration can be manufactured into your extension spring.



Mounting an Extension Spring:
One has to take into consideration how you will mount your extension spring. You may use posts for connecting the hooks onto, you can use fasteners like shoulder bolts, you can use holes in sheet metal for the extension spring hooks to go into, you can taper the end of an extension spring so as to thread the spring onto a bolt or fastener. Remember the Heli-Coil, that's a spring that acts like a thread for your fastener. You can do the same with an extension spring by determining the right wire diameter and couple it with your fasteners pitch and outer diameter. Your fastener or bolt will thread right into the extension springs inner diameter. There are many possibilities with extension springs, your main objective should be to achieve proper function in the defined parameters of your product with low stress & fatigue along with affordability and repeatable results.

Extension Spring Material:
Extension springs can be constructed from many types of materials. Some examples of metal springs are music wire springs, SS (Stainless Steel), PB (Phosphor Bronze), etc. The most important aspect of choosing the right material for your extension spring is identifying your extension spring's environment then coupling it with the correct type of material to do the job correctly.
Example: If your extension spring will be in salt water, then your choice would be 302-SS. If your spring environment is indoors or enclosed in a housing then Music Wire will be sufficient. If your extension spring is subject to high temperatures, above 800 degrees, then your obvious choice is Inconel X 750. If you are looking for conductivity then use Phosphor Bronze with gold electroplating. These are just a few examples that I have covered. For more information go to the Properties of Common Spring Materials page.

Plating and Coating:
When coating a spring the following types of platings are available: Zinc Plating, (white, blue, gold, & black are available) offers corrosion resistance, Nickel Plating (gives a very bright chrome looking finish or you can have black finish) offers good corrosion resistance, Black Oxide, Shot peering to reduce stress and fatigue, powder coating offers corrosion resistance and gives you the options of choosing just about any color under the sun. Electro Gold plating offers your springs conductivity if used in electrical applications. Galvanized wire is great economical choice for a spring, it offers the user a pre-galvanized wire that is pre-galvanized so the spring maker purchases this wire with the galvanization already on it, they do not have to send it out for a secondary process after the spring is made, thus saving the buyer a substantial amount. A word of advice about material selection: Take the time to ask a spring engineer his or her opinion of your particular spring environment. They can professionally asses the extension spring environment and recommend the correct material for the job.

Word for Thought:
Once you have understood these basic principals for creating and engineering your unique extension spring you will be better prepared for spring design. Remember, a great extension spring is one that will function properly in the confined parameter of your product with low stress and high cycles of life.

If you are looking for a spring rate calculation or determining your extension spring break detection please use our extension spring calculator.