Custom springs and wire forms: types, rate, and design
SSG MFG engineering6 min readSprings & wire forms
How do you design a custom spring?
Design a custom spring by choosing the type (compression, extension, or torsion), then setting wire diameter, mean coil diameter, and active coils to reach the required rate and load. Rate for a helical compression or extension spring is k = Gd⁴ ÷ (8D³n). Keep the spring index D/d between 4 and 12, stay under the material's allowable stress, and specify loads at working heights.
What is the difference between compression, extension, and torsion springs?
| Type | How it loads | Typical ends or features | Common uses |
|---|---|---|---|
| Compression | Resists being pushed together | Closed and ground, closed not ground, open | Valves, switches, latches, return mechanisms |
| Extension | Resists being pulled apart; wound with initial tension | Machine loops, crossover loops, extended hooks | Levers, doors, counterbalances, return springs |
| Torsion | Resists rotation about its axis | Straight, bent, or hooked legs | Hinges, clips, ratchets, lids |
| Wire form | Depends on shape | Bends, loops, eyes, formed ends | Clips, retainers, hooks, handles, guards |
| Flat spring or clip | Bends like a beam | Formed from strip | Contacts, retaining clips, spring washers |
Compression springs are the most common and the easiest to make. Extension springs fail most often at the hook, where stress concentrates at the bend, so hook shape deserves as much attention as the body. Torsion springs should be loaded in the direction that winds the coils tighter, not the direction that opens them.
How do you calculate spring rate?
Rate is load per unit of deflection. For a helical compression or extension spring, k = Gd⁴ ÷ (8D³n), where G is the shear modulus of the wire, d is wire diameter, D is mean coil diameter (outside diameter minus wire diameter), and n is the number of active coils. Rate climbs with the fourth power of wire diameter, so small wire changes move it a lot.
| Material | G, psi | G, GPa |
|---|---|---|
| Music wire (ASTM A228) | 11.5 × 10⁶ | 79.3 |
| Oil tempered and chrome silicon | 11.2 × 10⁶ | 77.2 |
| Stainless 302/304 (ASTM A313) | 10.0 × 10⁶ | 69.0 |
| 17-7 PH stainless | 11.0 × 10⁶ | 75.8 |
| Phosphor bronze (ASTM B159) | 6.25 × 10⁶ | 43.1 |
Example: music wire 0.063 in in diameter, a 0.500 in mean diameter, and 8 active coils give k = (11.5 × 10⁶ × 0.063⁴) ÷ (8 × 0.500³ × 8) ≈ 22.6 lb/in (3.96 N/mm). The spring index is 0.500 ÷ 0.063 ≈ 7.9.
Shear stress is τ = 8FDK ÷ (πd³), where F is load and K is the Wahl correction factor, (4C − 1) ÷ (4C − 4) + 0.615 ÷ C, with C the spring index. Torsion springs use a different rate, about Ed⁴ ÷ (10.8DN) in lb·in per turn, where E is the tensile modulus and N is the number of active turns.
What is spring index and why does it matter?
Spring index is mean coil diameter divided by wire diameter, C = D/d. It sets how hard the spring is to coil and how evenly stress is distributed. The practical range is 4 to 12, and springs between about 6 and 9 are the easiest to hold to tolerance.
- Below 4: hard to coil, high stress on the inside of the coil, and tool marks.
- Above 12: diameter varies more, springs tangle in bulk, and long springs buckle.
- Slenderness: compression springs with a free length more than about 4× the mean diameter can buckle and need a guide rod or bore.
Which spring materials are most common?
| Material | Specification | Max service temp | Notes |
|---|---|---|---|
| Music wire | ASTM A228 | 250°F (120°C) | Highest strength in small diameters; lowest-cost high-stress choice; needs a coating outdoors |
| Hard drawn | ASTM A227 | 250°F (120°C) | Low cost for static loads |
| Oil tempered | ASTM A229 | 300°F (150°C) | Larger diameters, general purpose |
| Chrome vanadium | ASTM A231 | 425°F (220°C) | Shock loads and higher temperature |
| Chrome silicon | ASTM A401 | 475°F (245°C) | High stress and fatigue, such as valve springs |
| Stainless 302/304 | ASTM A313 | 550°F (290°C) | Corrosion resistance at lower strength than music wire |
| 17-7 PH stainless | ASTM A313 (631) | 650°F (340°C) | Higher strength and temperature than 302 |
| Phosphor bronze | ASTM B159 | 200°F (95°C) | Conductive and nonmagnetic |
What is fourslide forming and when is it used?
A fourslide, or multislide, machine feeds wire or flat strip from a coil and forms it with tools on four slides set 90° apart, often around a center post. It makes complex clips, contacts, retainers, and wire forms in one cycle at high speed. For small, multi-bend parts, its tooling costs less than a progressive die.
CNC wire benders cover the other end of the volume range. They form 2D and 3D shapes from a program with little or no dedicated tooling, which suits prototypes and low to medium volumes. Fourslide makes sense once volume justifies the tooling and setup; CNC bending makes sense while the design or demand is still moving.
How should you specify a spring on a drawing?
- Two loads at two working heights, or one load and a rate.
- The fit: maximum OD for a bore or minimum ID over a rod, not both.
- Wire material and specification, and end type.
- Free length and number of coils as reference dimensions.
- Maximum solid height, if the spring must not bottom out.
- Finish: passivation, zinc, black oxide, or shot peening for fatigue life.
- Wind direction and leg orientation for torsion springs.
Commercial load tolerance is about ±10%. Tighter load tolerance, such as ±5%, is possible with adjustment and 100% testing and sorting, at added cost.
How do you get a spring or wire form quoted?
Configure the spring in the SSG builder at /build, or upload a STEP file for a wire form, for an instant budgetary estimate and a DFM check on spring index, stress, and buckling. Include loads, fit, material, finish, and annual volume. An SSG rep then routes the RFQ to spring and fourslide principals matched to the wire size and volume, and returns firm quotes.
