Begin by finding the outside diameter (O.D.) and length or rate (strength) you require. You can reference the diagram shown on the right.
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Spring rate is the load (pounds) it takes to deflect (compress) the spring one theoretical inch, i.e., if the rate = 40 lbs./1 in., it would take 10 pounds to deflect it 1/4 inch, or 80 pounds for 2 inches, etc.
' If the length or rate is not known, but the installed working length (WL) is, then select a spring ' say 30% longer ' than the WL. You must know the load at the WL. Just subtract the WL from the spring's selected free length and multiply by its rate to obtain the load to compare with your required value.
' If the load required is not obtained, select a new candidate with either an increase or decrease in rate (strength) or free length.
' Be certain that the tabulated solid length (completely compressed) for your candidate spring indicates enough room for deflection and also that the deflection is not significantly greater than the tabulated 'Maximum Suggested' for stress reasons.
An Introduction to Extension Springs
All of Century Spring's stocked extension springs are of the constant diameter type with a variety of hook/loop styles. Each spring is wound with an initial tension force which offers a small deflection load for secure installation 'holding.' The initial tension is equal to the minimum force required to separate adjacent coils. Unless otherwise noted, Century extension springs are made with either machine hook, or full loop ends.
Note: If the spring needed for your application cannot be found in our inventory, we can fabricate it for you. Simply request a quote.
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Total Force = I.T. + Deflection x Rate
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If the required spring's O.D. and/or length are not important, review the 'Maximum Suggested Load' for springs in the size range you think you require. This value is the force created at the 'Maximum Suggested Deflection' and includes the initial tension. The suggested maximum deflection and loads listed are for a spring expected to give an average cycle life of around 50,000 cycles. As cycling increases, the maximum load should be decreased in order to support a long service life. For reference, the suggested max deflection is the distance a spring will travel over and above its free length. The use of a fish scale of an appropriate size can be a convenient aid in determining the load required.
If the spring needed for your application cannot be found in our inventory, we can fabricate it for you. Simply request a quote.
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Finding Spring Stress Rates
The suggested maximum-allowable stress value for an extension spring's body wire ranges between 30 and 45 percent of the material's minimum tensile strength (MTS). The suggested percentage values of the MTS to be used vary with the material type, and the MTS values for a given material vary with the wire diameter. Stresses in the spring hooks are normally higher than in the spring body because there is a bending stress in addition to the wire-torsion stress, specifically in the transition region between the last body coil and the hook. Therefore, an over-stressed extension spring can be expected to fail at the hook first. The suggested allowable hook stress in torsion is 30-45 percent (depending on the material) of its MTS, while that of bending is 75 percent of its MTS.
An estimate of the total stress of a common extension spring hook or loop can be determined with the information shown on the right.
There is an increment of stress which is due to the initial tension that adds to the stress complexity; this should be accounted for in many cases. The initial tension causes the spring body to be under constant stress, which roughly varies with the spring mean diameter to wire diameter ratio. Reference the figures below for a visual representation of stress due to initial tension as well as a chart showing the Wahl curvature stress correction.
Materials
Century Spring uses the highest grades of spring wire to fabricate extension springs. To create cost-effective warehousing of our stock spring inventory for our customers, we offer material certifications for custom springs only. For stock springs, we offer an optional material verification statement for a $25 fee. Certifications of conformance for geometric tolerances set by the Spring Manufacturers Institute (SMI) for our stock springs are available upon request.
'Spring Steel' is a term that includes: music wire, hard-drawn wire, and oil-tempered wire.
Stock extension springs are offered in Stainless steel 302 and 316 as specified.
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