The most common compression spring is a straight cylindrical spring. There are also conical, barrel, and hourglass compression springs. The pitch or spacing between the coils can be constant or variable. See the figure below for compression spring configurations. See Helical Compression Spring Terminology.
Helical Compression Spring Technolog
Notes refer to diagram at Leftside of page. Per Spring Manufacturers Institute Handbook of Spring Desig
Mean coil diameter = (O.D. + I.D.)/2 or O.D. - d
Free Length = The overall length of a spring in the unloaded position.
Pitch = The distance from center to center of the wire in adjacent active coils. Recommended practice is to specify the number of active coils rather than pitch.
Spring Index = D/d = ratio of mean coil diameter (D) to wire diameter (d). The preferred index range is 4 to 12. Springs with high indexes tangle and may require special packaging. Springs with indexes lower than 4 are difficult to form.
Squareness = Angular deviation between the axis of a compression spring and a normal to the plane of the ends. Squareness can be measured by standing a sample spring on end on a horizontal flat plate and bringing the spring against a straightedge at right angles to the plate. The spring is rotated to produce a maximum out-of-square dimension es. Normally squared and ground springs are square within 3º when measured in the free position.
Parallelism refers to the relationship of the ground ends, and is determined by placing a spring on a flat plate and measuring the maximum difference in free length around the spring circumference.
Bearing Surface: To improve squareness and reduce buckling during operation, a bearing surface of at least 270º is required. Squared and ground springs are normally supplied with a bearing surface of 270º to 330º. Additional grinding results in thin sections.
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