Sourcing Springs and Coils From China: The Spec Sheet Is Not the Part

Springs are one of those parts that look dead simple on a drawing and then ruin your week in production. I have watched a buyer approve a "music wire compression spring, 10mm OD, 25mm free length" and then receive reels that sagged 8 percent after a month in the field. The spring was real. The spec sheet was real. The problem is that a spring is a force-versus-deflection curve, not a list of dimensions, and most Chinese catalogs only sell you the dimensions.

Here is how I handle this now, after getting burned enough times.

Wire material decides everything downstream

The material is the first line item and the one people skip. Music wire, which the Chinese suppliers usually call ASTM A228 equivalent or "piano wire," is your default for small springs under about 3mm wire diameter. It gives high tensile strength and good fatigue life at a low cost. Where buyers go wrong is assuming every order is music wire. A lot of shops will quietly switch to a cheaper drawn wire with a lower carbon content, and the spring still measures correctly at rest, but it loses load fast once it cycles.

Stainless 302 and 304 are common when you need corrosion resistance, but understand the trade: 302 stainless springs are typically around 15 to 20 percent lower in allowable stress than music wire for the same diameter, so a straight material swap with no redesign means a spring that either has to be bigger or will not hold the load. If your application sits outdoors or near moisture, 302 is often the right call and you accept a slightly larger coil.

For valve and high-temperature work you will see 17-7 PH and Inconel. Those are expensive and you should be asking why you need them, not just ordering them.

What to actually put on the drawing

A useful spring order has all of this, not part of it:

Material grade and wire diameter with the tolerance, for example 1.20 mm ±0.02
Outside diameter and inside diameter, since the rod or bore the spring sits on is what matters
Free length
Solid height, which is the fully compressed length and decides whether the spring binds
Load at a stated deflection, usually two reference points such as load at 30 percent and 60 percent deflection
Spring rate, expressed as N/mm
End condition: closed, closed and ground, or open. Closed-and-ground is what you want for a flat seat unless cost forces otherwise
Direction of wind, right hand or left hand
Finish: plain, zinc plated, powder coated, or passivated for stainless

If the supplier only quotes you OD, length, and wire gauge, you are going to get whatever their machine happened to produce that day. The load points and rate are the contract.

The tolerance game on rate and load

Industry-standard rate tolerance is usually plus or minus 10 percent, and load tolerance at a reference point is often plus or minus 10 to 15 percent. That sounds fine until you stack it. If your assembly needs a spring to deliver a window of, say, 20 to 28 newtons and the supplier is allowed to drift 15 percent either way from a nominal 24, you can fall outside your window at both ends. When the spring is critical to function, tighten the tolerance in the purchase order and expect to pay a bit more and accept a slightly higher rejection rate. Ask specifically whether they test 100 percent of the batch or just sample, because sampled load testing means some units are out.

One practical tip: ask for the measured load data from the last production run before you place the bulk order. A supplier who can produce that sheet is usually running real QC. A supplier who "can send a certificate" but has no data is a risk.

Fatigue and cycling

If your spring cycles, fatigue is your enemy and stress range is the number to control. Springs that never fully relax, meaning you keep some preload on them, last vastly longer than springs that go from loaded to zero every cycle. Set a minimum preload and a maximum working stress well under the material's allowable for the wire size, and ask the factory what stress they design to. If they cannot answer that question in engineering terms, move on.

Shot peening is an option for high-cycle springs and does meaningfully improve fatigue life. It is also easy to claim and hard to verify, so if you are paying for it, ask for the process parameters or a sample test.

Plating, hydrogen, and the failure nobody connects

Electroplated springs, especially zinc or zinc-nickel on high-strength wire, can suffer hydrogen embrittlement. The spring passes incoming inspection and then cracks days later under load. If your spring is plated and made of high-tensile wire, insist on a post-plating bake and ask for it in writing. For stainless, passivation rather than plating avoids the whole problem and is usually what you want.

Why a small order and a big order are different animals

The first sample springs may be hand-wound on a manual coiler by a skilled operator and will look great. The production run goes on an automatic CNC coiler and the tooling setup shifts the rate. Always approve a production-representative sample made on the automatic machine, not a hand sample, and hold onto it as your golden reference. When the next batch arrives with a different rate, that reference is what settles the argument.

Springs are a commodity part with a non-commodity engineering core. Pay for the curve, not the dimensions, and you will stop discovering the difference in your field returns.