Garter Springs

Definition:
Garter Springs are helical extension or compression springs whose ends are connected in order to form each spring into a circle which exert radial forces. This means that the garter spring is an endless spring, which provides a clamping force around an object.

Applications:
Garter springs are primarily used to maintain controlled pressure of a radial lip seal on a shaft and to compensate for lip wear, changes in volume or stiffness of the elastomer caused by the fluid, and the effect of temperature changes and time. Other uses include small motor belts, electrical connectors, and piston-ring expanders. Garter springs can be found in automobiles wherever you can find oil, such as the transmission.

Design Considerations:
Garter spring ends may be fastened together by interlocking loops, stacking one coil of the female end into the rib end, or soldering (for belt applications) or by screwing one end into the other. Regardless of the connecting joint used, its strength must be such that the joint will not separate when the spring is extended to its maximum diameter.

In order to design a garter spring you must know the following specifications.

1. Material: The most commonly used material for garter springs is Stainless Steel (type 302) or spring temper (AMS 5688). Astm A228 Music Wire can be used as well as many other types of material.
2. Assembled Length: The length of your garter spring depends solely on the application you plan to use. The length can also be adjusted by trimming the non-tapered end to your needed length and assemble by screwing the tapered end into the non-tapered end as shown below.
3. (a) OD: The outer diameter of the overall garter spring must be considered. Measure the lip diameter or cavity diameter where the garter spring will be used to determine the correct OD for your application. Once this is known then you must measure the radius on the lip or cavity radius in your product to determine the correct garter spring outer diameter.
   
   (b) ID: The inner diameter of the non tapered end must also be considered. The non tapered female end ID must fit over the male tapered tip. This allows one end to screw into the other correctly. Thus creating a ring or belt.
   The outer diameter is measured as follows:
   The inner diameter is measured as follows:
4. How much force do I need to generate? This question is more easily answered by thinking in terms of rate, which is lbs/per inch of extension or (pull). Each garter spring has a rate of pounds (lbs) per inch of extension. In the case of the garter spring which in essence is an extension spring, the garter spring will pull apart radially expanding its circle. Unlike the extension spring which is pulled linear, the garter spring expands it diameter when pulled apart.
   
   To determine the necessary force to work in your product we first need to know how many inches of total travel the spring will expand. Once we know that, we will understand how much travel is needed to accomplish our goal. Total travel and the rate of your spring will tell you how much force your garter spring will generate.
   
   Example: Your garter spring needs to have a 10.000 inch outer diameter and you need to open up the garter spring to an 11.000 diameter thus giving you 1.000 inch of radial travel. At this 11.000 inch diameter you require 20 lbs of force for your garter spring to function correctly in your product.
   
   The following formula will help you determine how much rate your spring should have: 20 lbs divided by Pie (3.14) = 6.369 lbs You need a 6.369 lbs/per inch extension spring to do the job. The 20 lbs is your load at an 11.000 diameter or 1 inch of radial expansion but the 6.369 lb rate is the master because for every 1 inch you expand radially you must X's (times) it by pie (3.14) to know how much force you will generate at one inch of radially expanded distance. If any preload is needed use the same formula above to determine the preloaded force.
5. I want many cycles of life (high repeatability). How do I achieve this?
   To achieve this we must first understand the logistics of Garter springs. Take a look at the force chart below to understand.
   
   
   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
   All equal = More stress	All equal = Less stress
   & Fatigue	& Fatigue

   
   
   These are the basic spring design considerations. Please understand all the design considerations because they all work hand in hand.
   
   Example: if my garter spring OD is too small, I may get a lot of load or force, but my garter spring will be too stressed and fatigued. This may cause it to take a set and not return to its original diameter or worse yet break all together. This is where my garter spring index plays a huge part.
    

Garter 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 4 to 1 index for a good garter 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 garter spring because stress and fatigue will cause setting of your garter spring.

Example: You have an garter 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 6 to 1 index (or higher) is a better index number to achieve a good mount of force along a greater distance of travel with less stress.

Number of coils in an Garter Spring:
Your number of coils is another topic of garter 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 garter spring. Less coils = A stronger garter spring, more coils = weaker garter spring. At the same time less coils = more stress and fatigue, more coils = less stress and fatigue. The perfect balance on your garter spring design is one that combines the right amount of coils for how much distance or radial travel you want to achieve, along with the correct amount of force needed to do the job. The total number of coils coupled with the correct index allows you to attain a low fatigue and stress level. Once you achieve this balance, your garter spring will last many millions of cycles of life (high repeatability). All this should be calculated to fit within the space of your application.

Garter Spring Material:
Garter springs can be constructed from many types of materials. Some examples of materials are MW (Music Wire), SS (Stainless Steel), PB (Phosphor Bronze), etc. The most important aspect of choosing the right material for your extension spring is identifying your garter spring's environment then coupling it with the correct type of material to do the job correctly.

Example: If your garter spring will be in fluid 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 garter 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 have been 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 garter 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 garter spring you will be better prepared for spring design. Remember, a great garter spring is one that will function properly in the confined parameter of your product with low stress and high cycles of life.