The six numbers a spring maker needs before anything can be quoted, and how to measure them from a part you already have.
See Compression Springs
A compression spring is defined by force at a working height, not by its free length. Before measuring anything, write down the force you need and the height at which you need it. Everything else exists to deliver that.
Wire diameter, outside diameter, free length, total coils, active coils and end type. Wire diameter is measured with a micrometer on a straight section of coil. Outside diameter is measured across the widest point of the body. Total coils are counted including the closed end coils; active coils exclude them. End type is squared, squared and ground, or open.
A spring with unground squared ends sits on roughly three quarters of a coil, which tilts it under load. Ground ends give a flat bearing face across a full coil. Specify ground ends when the spring seats against a flat surface and unground ends when it sits in a pocket or over a guide rod.
Spring rate is force divided by deflection and stays roughly constant until the coils start touching. Solid height is the wire diameter multiplied by the total number of coils, and your mechanism must never reach it. If your available travel forces the spring solid, you need a conical or tower design that nests into itself.
Carbon and 65Mn spring steel are the default for dry indoor use. Stainless is the answer for wet, washdown or outdoor positions. Music wire gives high strength in fine diameters. Note the environment on your enquiry, because it changes the material before it changes anything else.
Only partially. Wire diameter and coil geometry stay valid, but the free length will read short. Tell us it is a used part and we will allow for it.
Rate is the force per millimetre of travel. Load is the force at one specific height. A quote needs at least one of them, and preferably both.
Send the part, the load and the environment. A short email usually settles a specification faster than any guide can.
