Compression Springs

Wire Springs

A compression spring is designed to operate with a compression load, so the spring gets shorter as the load is applied to it.  A compression spring is an open-coil helical spring that offers resistance to a comprehensive force applied axially. They are usually coiled at a constant diameter, though they can be coiled in other needed forms such as conical springs, concave (barrel springs), convex (hourglass springs), or various combinations of these.

A Compression Spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. There are a large number of spring designs; in everyday usage the term often refers to coil springs.

Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication. Some non-ferrous metals are also used including phosphor bronze and titanium for parts requiring corrosion resistance and beryllium copper for springs carrying electrical current (because of its low electrical resistance).

When a coil spring is compressed or stretched slightly from rest, the force it exerts is approximately proportional to its change in length (this approximation breaks down for larger deflections). The rate or spring constant of a spring is the change in the force it exerts, divided by the change in deflection of the spring. That is, it is the gradient of the force versus deflection curve. An extension or compression spring has units of force divided by distance, for example lbf/in or N/m. Torsion springs have units of torque divided by angle, such as N·m/rad or ft·lbf/degree. The inverse of spring rate is compliance, that is: if a spring has a rate of 10 N/mm, it has a compliance of 0.1 mm/N. The stiffness (or rate) of springs in parallel is additive, as is the compliance of springs in series.

Depending on the design and required operating environment, any material can be used to construct a spring, so long as the material has the required combination of rigidity and elasticity: technically, a wooden bow is a form of spring.

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Free Length

Free length is overall spring length in the free or unloaded position. If loads are not critical, free length should be specified. When definite loads are required, free length should be a reference dimension that can be varied to meet load requirements. Pitch is the distance between centers of adjacent coils and is related to free length and number of coils.

Types of Ends

Types of ends available are: plain ends, plain ends ground, squared ends and squared ends - ground. To improve squareness and reduce buckling during operation, a bearing surface of at least 270° is required. Squared and ground springs are normally supplied with a bearing surface of 270° to 330°. Additional grinding results in thin sections. "Squared ends only" are preferred on springs with small wire diameters (less than 0.5 mm or 0.020"), a large index (greater than 12) or low spring rates. Squared ends cost less to manufacture than squared and ground ends.

Solid Height

Solid height is the length of a spring with all coils closed. For ground springs, solid height is the number of coils multiplied by wire diameter. For unground springs, solid height is the number of coils plus one, multiplied by wire diameter. If critical, solid height should be specified as a maximum dimension. After allowances are made for plating or other coatings, it is good practice to add one-half of the wire diameter to determine maximum solid height. With larger wire sizes and fewer coils, this allowance can be decreased. Solid height is often measured by applying a force equal to 110 to 150% of the calculated load at solid. If solid height is not critical, this dimension should be omitted.

Direction of Coiling

A helical compression spring can be either left or right-hand coiled. If the index finger of the right hand can be bent to simulate direction of coil, so that the fingernail and coil tip are approximately at the same angular position, the spring is right-hand wound (Figure S-3, below). If the index finger of the left hand simulates the coil direction, the spring is left-hand wound. If direction of coiling is not specified, springs may be coiled in either direction. Nested springs with small diametral clearances should be coiled in opposite directions.

Spring Rate

Spring rate for helical compression springs is defined as the change in load per unit deflection and is expressed as shown:

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