How to Source Springs and Wire Forms
Spring RFQ checklist: type, wire material and size, fit diameter, free length, loads at working heights, ends, stress relief, peening, finish and testing.
504 metal forming companies in the Noramark directory
Springs are specified by what they do, not only by what they look like. A compression spring with the right diameters and free length can still be wrong if its load at working height is off, and a spring maker cannot tell which value you care about unless you say. Most spring shops quote from a short specification: wire, diameters, coils, ends, and one or two load points with tolerances.
A good spring RFQ gives the type, material and wire size, the diameter that must fit (the outside diameter in a bore or the inside diameter over a rod), the free length, the load at one or two heights, the end type and the finish. Mark which values are free to change: spring makers adjust the coil count or free length to hit a load, and holding every dimension and every load at once over-constrains the spring. Wire forms and flat springs follow the same idea, with a drawing in place of the table.
RFQ checklist
17 things to settle on the drawing or in the request before you ask for spring quotes. Print it and tick it off against the print, or copy it into the RFQ.
Type and material
Spring type: compression, extension, torsion, wire form, or flat spring
Each type runs on different machines and is specified differently: extension springs by hooks and initial tension, torsion springs by leg angles and torque.
Wire material by spec: music wire (ASTM A228), oil tempered (A229), chrome silicon (A401), 302/304 stainless (A313), 17-7 PH, beryllium copper, or a nickel alloy
Material sets the allowable stress, temperature limit and corrosion resistance. Music wire is the default up to about 120 °C (250 °F); chrome silicon and stainless go hotter, nickel alloys hotter still.
Wire diameter, and whether the maker may adjust it
Wire diameter moves rate and stress more than any other variable, and standard sizes are cheaper and quicker to get.
Compression spring dimensions and loads
Outside or inside diameter with tolerance, and the bore or rod it works in or over
A compression spring grows in diameter as it compresses. With the bore or rod size, the maker can check clearance at the lowest working height.
Free length, with tolerance or marked as reference
Free length is the value makers usually adjust to hit a load, so hold it only if it matters to the assembly.
Load at one or two working heights, with tolerance (for example 50 N ±10% at 30 mm)
Load points are what the spring does. Makers hit them by adjusting coils or free length, so mark those dimensions as reference.
Spring rate, only if it matters more than a load point
Rate plus two loads plus free length plus coil count over-constrains the spring. Give rate or two loads, not all of them.
Maximum solid height, and the lowest height the spring reaches in service
A spring that goes solid in service can take a set or break. The maker checks stress at the lowest working height.
Total and active coils as reference, and direction of wind if it matters
Wind direction matters when springs nest inside each other or thread onto a part.
End type: closed and ground, closed not ground, or open
Ground ends sit square and add a grinding operation. Closed unground ends are cheaper, especially on light wire.
Extension, torsion and wire forms
Extension springs: hook type and orientation, initial tension, and load at an extended length
Hooks are where extension springs fail, and initial tension is coiled in. Both belong in the spec.
Torsion springs: leg lengths and angles, direction of wind, and torque at a stated deflection angle
A torsion spring must wind tighter under load. The wrong wind direction is a common and total failure.
Wire forms: a drawing with bend radii, leg lengths and angles, and the tolerances that matter
Wire forms are made on CNC wire formers or with custom tooling; the drawing decides which.
Processing, testing and delivery
Stress relief, shot peening and presetting (set removal)
Shot peening raises fatigue life on cycled springs, and presetting removes initial set so the spring keeps its free length.
Finish: oiled, zinc plated with a hydrogen embrittlement relief bake, e-coat, or passivated stainless
Electroplated high-strength spring steel must be baked promptly after plating to reduce the risk of delayed cracking.
Testing: load test sample size or 100%, cycle life test, and reports
Testing every spring and reporting loads costs time per part; sampling is the default.
Quantity and packaging: bulk, bagged, or tangle-free packaging
Open-ended springs tangle in bulk, and untangling them at assembly costs you time.
What to send with the RFQ
- A spring specification table or drawing: type, material, wire size, fit diameter, free length, loads and ends.
- The bore or rod it fits, and the working heights or deflections.
- Cycle life and operating temperature.
- A sample spring, if you are replacing one.
- Quantity and annual volume.
Lead times
| Prototype or first parts | Stock or near-stock springs: days. Custom springs in common wire: about 1-3 weeks for samples. |
|---|---|
| Production or repeat orders | About 3-6 weeks for production lots; longer for uncommon wire, plating or peening. |
Typical of U.S. job shops, from order or approved drawing to shipment; confirm with the shop for your part.
What drives the cost
- Material: stainless, chrome silicon, beryllium copper and nickel alloys cost several times music wire.
- Tight load tolerances, which need 100% testing and sorting.
- Ground ends, shot peening, presetting and plating.
- Complex wire forms that need custom tooling.
- Small quantities: setup dominates, and many spring makers have minimum order values.
Common mistakes
- Free length, coil count, rate and loads all given tight tolerances at once.
- An outside diameter with no bore size.
- Plating high-strength spring steel without an embrittlement relief bake.
- The wrong wind direction on a torsion spring.
- Ignoring solid height, so the spring bottoms out in service.
Questions to ask the shop
- Which values do you need fixed, and which can you adjust to hit the loads?
- Do you have this wire size in stock?
- Do you load test every spring or a sample?
- Do you shot peen and plate in-house, and how do you handle embrittlement relief?
- Can you supply samples before production?
Typical processes
Typical materials
Charts and calculators
Frequently asked questions
- What is spring rate?
- The load needed to deflect the spring one unit of length, in N/mm or lb/in. For a helical compression spring it rises with the fourth power of the wire diameter and falls with the cube of the mean coil diameter and with the number of active coils, which is why a small change in wire size moves it so much.
- Music wire or stainless steel for a spring?
- Music wire is stronger and cheaper and is the default for indoor, room-temperature springs; it rusts, so it is oiled or plated. 302/304 stainless resists corrosion and heat better at somewhat lower strength. Use 17-7 PH or 316 for harsher environments, and nickel alloys for high temperature.
- Why did my plated springs break?
- Probably hydrogen embrittlement. Electroplating puts hydrogen into high-strength steel, and springs can crack under load hours or days later. Specify a bake soon after plating (commonly several hours at about 190-220 °C / 375-425 °F, per ASTM B850 or the plating spec), or choose a finish such as mechanical zinc that does not charge the steel with hydrogen.
Build the RFQ: attach the print and this checklist
Attach the drawing (PDF) and the model (STEP), and put your answers to the checklist in the description. A Noramark sourcing specialist matches the request with U.S. shops that make springs and wire forms and contacts you within 1 business day.
