OpenSpecEngineering

A quick reference for choosing a spring before you size it. The page compares the common spring types, lists the usual spring wires with their strength and temperature limits, gives the spring index range that coils well, and shows how Belleville discs stack. Designers and buyers use it to settle the type and wire first, then size the spring in the Spring Design calculator.

What the page covers

• Spring Types: compression, extension, torsion, Belleville (disc) washer, wave and constant-force springs, with the loading direction, typical uses and how much energy each stores for its space.
• Common Spring Wire Materials: music wire (ASTM A228), 302/304 stainless, chrome silicon, chrome vanadium, beryllium copper and Inconel X-750, with the tensile strength range and the maximum service temperature, from the Lee Spring guide [1].
• Spring Index: what happens below, inside and above the recommended range of C, following Shigley [3].
• Belleville Washer Stacking: parallel and series stacks and what each does to load and deflection, from the Solon bulletin [2].

Reading the tables

The spring index is C = D/d, where D is the mean coil diameter (outside diameter minus one wire diameter) and d is the wire diameter. Tensile strengths are in ksi (1 ksi = 6.895 MPa). The “~” marks an approximate range: thin wire sits at the top of it and thick wire at the bottom, so read the exact value for your diameter from the supplier’s wire table.

Limits and common mistakes

• The tables give no spring rate, stress or fatigue life; use the calculator for those.
• The maximum service temperature is where the wire starts to relax (lose load) noticeably. A spring that must hold its load near the limit needs a margin or a higher-rated wire.
• Don’t compute the index from the outside diameter. Using OD instead of the mean diameter overstates C by one.
• Friction between parallel Belleville discs makes the load higher on loading than on unloading, so a parallel pair is only roughly double one disc.

Worked example

Pick a wire for a compression return spring in an oven-door latch. The latch runs at up to 350°F, and the space allows a 12 mm mean coil diameter on 1.6 mm wire.

1. In Spring Types, the Compression row reads “Axial, compressive (pushes apart)”, which matches a return spring that pushes the latch closed.
2. In Common Spring Wire Materials, read the Max Service Temp column. Music wire stops at 250°F (121°C), so it is out. Stainless steel 302/304 gives ~130–325 ksi, 550°F (288°C) so choose it.
3. In Spring Index, work out C = 12 / 1.6 = 7.5. It falls in the 4–12 row: “Recommended range (about 6–12 preferred)”.

Check: 350°F leaves 200°F of margin under the 550°F limit,. The low end of the stainless range, 130 ksi, is about 896 MPa. Now enter the wire and coil sizes in the Spring Design calculator to get the rate and stress.