Spring Design


Nomenclature
Gshear modulus of the spring material
dwire diameter
DOoutside spring diameter
h1spring operating height 1
h2spring operating height 2
P1spring force at height 1
P2spring force at height 2


Good spring designs are dependent on many factors including dimensional and load requirments. Considerations should be given to the following:
- Environment: Will the spring be operating in a cold or hot environment? Will it be in contact with corrosive media? If so, what kind?
- Life expectancy: How long must the spring survive or work under the specified conditions without breaking or setting?
- Load: Is a load required and if so, over what range of deflection must be maintained? What is the frequency and velocity of the load application?
- Space: Where will the spring be functioning? During what application and in how much space? Is there sufficient room for the spring to operate fully without causing high stress that can increase the risk of failure?

Springs are usually heat treated. Generally, there are two ranges for heat treating parts:
- Low temperature: Low temperature (350°F to 950°F) heat treatment can be applied to parts after forming to reduce residual stresses, stabilize parts dimensionally, or increase or restore the set point.
- High temperature: High temperature heat treatment is used to strengthen annealed material after forming.

Burrs can be minimized during the wire forming process. However, sharp edges are a natural result of the cut-off operation. Sharp edges are generally harmless and removal of such adds more costs without adding any value to the spring performance. Give a second thought to adding the note "remove all burrs and break all sharp edges" to your drawing.

An increase in wire size by 1% will result in a 4% stronger spring. The converse is also true. If you increase the mean coil diameter by 1%, the change in spring rate will decrease by 3%. If the material size changes by 1%, there will be a 3% change in torsional stress.

Adding a coil weakens the rate. Removing a coil strengthens the rate. For example, if you have a compression spring that has 10 active coils and a spring rate of 90 lbs/in, and you want to get a rate of 100 lbs, you need 9 active coils.

(Eq1)    
number of active coils =
single coil rate
required spring rate

In any spring, some portion of the end coils will probably be inactive. The number of inactive coils varies depending on the spring end configuration and mating component geometry. The following equations give approximate active coil coints, assuming that the springs are compressed between parallel plates:

For closed ends (ground or unground)NaNt − 2
For open ground endsNaNt − 1
For open unground endsNaNt

In practice, the number of inactive coils varies slightly as a spring is compressed. If the spring output at two operating heights is known, the number of active coils over the operating height range can be calculated using the following equation for any end configuration:

(Eq2)    
Na =
Gd4(h1h2)
8(DOd)3(P2P1)

Do the following in order to determine if a spring is right-hand-wound (RHW) or left-hand-wound (LHW): If you look down the axis of a spring, if the free end of the wire is the same orientation as you look at your hand and curl your index finger, then it is right-hand-wound. Same for if it matches your left hand then it is left-hand-wound.

Spring Index is the ratio of mean coil diameter to wire diameter or radial dimension of the cross section. The preferred index range is 4 to 12. Springs with high indexes tangle and may require individual packaging, especially if the ends are not squared. Springs with indexes lower than 4 are difficult to form.

Most extension springs are wound with initial tension. This is an internal force that holds the coils tightly together. The measure of the initial tension is the load necessary to overcome the internal force and just start coil separation. Unlike a compression spring, which has zero load at zero deflection, an extension spring can have a preload at zero deflection.

This built-in load, called initial tension, can be varied within limits, decreasing as the spring index increases. If the designer needs an extension spring with no initial tension, the spring should be designed with space between the coils.


Corrosion Resistance

Many coatings are available that can provide adequate corrosion resistance for wire types that would not themselves resist corrosion. These include powder coating, phosphating with an oil dip or spray, and plating in some cases. Typically, a coated spring produced from a traditional spring material will involve less cost than producing a spring from stainless steel. If stainless steel wire is required, type 302 stainless steel is generally the first choice. This wire can yield very corrosion-resistant springs for most environments. When the application calls for high operating temperatures as well, 17-7 PH wire can also be considered.





Related
▪ L - Spring Clip Fastener
▪ L - Spring Materials
▪ L - Stress in Helical Springs
▪ L - Types of Springs
▪ L - Spring Constant