Compression Springs

There are many factors to consider in spring design. The following is directed specifically toward compression springs although many elements apply to other springs as well.

Key Parameters and Reference Numbers

Spring Diameter: Helical compression springs can be described by three different diametric numbers:

outside diameter (“OD” ) is specified when the spring will function in a cavity

inside diameter (“ID”) is specified when the spring will function over a rod or shaft

mean diameter (“D”) is used in the stress and deflection calculations, and is equal to half the sum of the outside and inside diameters.

Wire Diameter: (“d”) is the diameter of the wire used to manufacture the spring and is a factor used to calculate spring index.

Spring Index: The ratio of the mean coil diameter to wire diameter (D/d). Springs with an index higher than 12 can tangle; springs with an index lower than 4 can be difficult to form. Therefore, for ease of manufacturing and packaging, the preferred spring index range is from 4 to 12.

Free Length: “L o ”; the overall length of an unloaded (“free”) spring.

Solid Height: “L s ”, the minimum length of a compression spring with all of its coils closed, when no further deflection is caused by additional load. If solid height is a critical application dimension, it should be specified as a maximum—generally figured as the solid height plus an allowance equal to half of the wire diameter.

Number of Coils: Active coils (“N a”) are the coils in a compression spring that are free to deflect under load. For squared-end springs, N a is equal to the total number of coils (“N t”) minus 2 (the turns at each end that are inactive, or “dead”, and in contact with the spring seat). The number of active coils in a plain-end spring is greater and depends on the seating method. The greater the number of active coils, the lower the spring rate.

Pitch: “p”, the distance between wire centers in adjacent active coils. Current recommended practice is to specify the number of active coils rather than pitch.

Spring Rate: The change in load per unit of deflection, generally expressed in pounds per inch. Spring rate is determined by the amount of force, in pounds, required to constrict a spring by one inch. Material size directly impacts spring rate. For example, increasing a wire diameter by 1 percent will result in a 4 percent stronger spring; decreasing diameter by 1 percent will result in a 4 percent weaker spring. Increasing the mean diameter by 1 percent will decrease the spring rate by 1 percent. Adding coils weakens the rate, while removing coils strengthens the rate.

Important Considerations for Spring Design

Type of End Treatments:

Plain (open) ends – a constant pitch is maintained for each coil through the end of the wire, without forming any dead coils.

Plain (open) ends ground – plain-end spring finished with a grinding operation to provide a flat plane.

Squared (closed) ends – the pitch of the end coils is reduced so that the end coils touch, forming one dead coil.

Squared (closed) ends ground – squared-end spring finished with a grinding operation to provide a flat plane.

Spring Engineers Tip: Plain ends are best suited for large-index, light-duty applications because they tend to cause the spring to bend, decreasing performance reliability. Using coil guides or shaped platens can help improve performance. Grinding operations provide flat planes, increased stability, and a more uniform load distribution into the spring. Squared ends are preferred on springs with wire diameters of .5 mm or less, an index greater than 12, or low spring rates. Squared and ground ends are preferred for applications requiring high-duty springs, close tolerances on rate or load, minimal solid height, and reduced buckling.

Hysteresis: The mechanical energy loss that occurs as a result of the spring ends’ tendency to rotate when compressed during cyclic loading and unloading. Spring Engineers Tip: For compression springs, hysteresis is generally low.

Coil Direction: Compression springs can be left- or right-hand coiled.

To determine the coil direction, hold the spring with its axis on a horizontal plane. If the direction of the coil, from top to bottom, can be simulated by bending the index finger of the left hand, the coil is left-hand wound; if it can be simulated by the index finger of the right hand, the coil is right-hand wound.

Spring Engineers Tip: If the spring will be mounted on a bolt, right-hand winding is preferable.

Squareness: Squareness, specified for springs in their unloaded position, refers to the angular deviation between the spring’s axis and a line normal to the end plains.

Spring Engineers Tip: For squared-and-ground springs, square-ness is generally specified within a 3-degree tolerance.

Parallelism: Parallelism describes the relationship between a spring’s ground ends as the maximum deviation in free length around the spring’s circumference.

Deflection: Movement of the spring ends when external loads are applied or removed.

Stress: Stresses are determined by spring dimensions and t he application’s load and deflection requirements. Compression springs are stressed in torsion. Maximum stress occurs at the inner surface of the wire; the load varies as the spring is deflected, producing a range of operating stresses that influence the life of the spring.

Spring Engineers Tip: For optimal longevity, higher stress ranges should be used with lower maximum stresses, and high maximum stresses should be used either with lower stress ranges or when the spring will be subjected only to static loading.

Load: Load, the force applied to a spring that causes deflection, can be determined by multiplying the spring rate by deflection. Loads can be classified according to their application requirements: static (the spring is expected to operate between specified loads a few times only), cyclic (the spring is expected to cycle between specified loads many times), or dynamic (the spring is subjected to a high rate of load application, causing periods of excessive stress).

Spring Engineers Tip : Loads should be specified at test heights between 15 and 85 percent of the full deflection range.

Buckling: Buckling occurs when a spring deforms in a non-axial direction. Once a spring buckles, it can no longer provide the intended force. Typically, buckling deformation accelerates rapidly and the spring fails.

Spring Engineers Tip : Compression springs with free heights greater than 4 times the spring diameter are prone to buckling, and benefit from guidance (either in a tube or over a rod).

Choice of Operating Stress – Static Conditions: Static applications require a spring to operate for a low number of cycles. Load-carrying abilities in static applications are limited by the tensile strength of the spring material, with a maximum stress that should fall between 35 and 50 percent of the minimum tensile strength. Exceeding the tensile-strength percentage can cause the spring to take a permanent set when deflected to solid.

Spring Engineers Tip : Presetting, or set removal, increases the load-carrying ability of springs in static applications by increasing the spring’s elastic limit. To preset, the spring is coiled longer than its required free length and then compressed to solid, causing the spring to set to its final desired length. This process increases the spring’s energy storage capacity to up to 75 percent of the material’s tensile strength, and is common for critical springs made from premium materials.

Choice of Operating Stress – Cyclic Conditions: Cyclic applications require a spring to operate repeatedly between specified loads. Load-carrying abilities in cyclic applications are limited by material fatigue strength. The optimum stress level is a balance between cost and reliability—reducing operating stresses will increase both reliability and cost.

Spring Engineers Tip : Maximum stress occurs at the wire surface, and any surface defects therefore reduce fatigue life. Shot peening can improve fatigue life by creating a residually-stressed surface area which resists fatigue cracks.

Spring Houston Specializing in Industrial Compression Springs 9740 Tanner Rd. • Houston, TX 77041 • Phone: 800.899.9488

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