By the Lee Spring Engineering Team

Quick Answer

Choose stock springs when a catalog part meets your load, size, and material requirements. They ship faster and cost less in low volumes. Choose custom springs when your application needs a specific spring rate, nonstandard geometry, special materials, or documented compliance, or when annual volumes are high enough that a purpose-designed spring lowers total cost.

Most companies buy both. The mistake is forcing one answer onto every application. Engineers tend to default to custom (“designed for the application”) and procurement tends to default to stock (“cheaper and faster”). The right answer depends on the six factors below, not on habit.

The 6-Point Framework

Work through these six questions in order. The first one alone settles many applications. The framework applies to every spring type, including compression, extension, torsion, wave springs, die springs, Belleville washers, and constant force springs. The fit parameters differ by type, but the decision logic does not.

Does a stock spring actually fit your envelope and load case?

If a stock spring exists in the right size for your assembly and delivers the required load or torque with margin, stock wins by default. The fit parameters depend on the spring type. A compression spring is defined by outer diameter, free length, spring rate, and solid height (its length when fully compressed). An extension spring is defined by free length, hook geometry, and rate. A torsion spring is defined by body diameter, leg lengths, and the torque it produces at a given deflection angle. Spring rate is the load, or torque, it takes to deflect the spring by one unit.

The odds of a match are better than most engineers assume. Lee Spring alone stocks more than 25,000 springs across compression, extension, torsion, and specialty types, ready for immediate shipment.

This test usually fails in three places: tight assemblies where solid height matters, applications needing a specific load at two different deflections (which pins the spring rate precisely), and designs where the spring must fit over a shaft or inside a bore with little clearance.

Rule of thumb: If you are modifying your design to accommodate the spring rather than the spring serving your design, you have crossed into custom territory.

How precise does performance need to be?

Stock springs are made to published commercial tolerances. At Lee Spring, stock compression and extension springs hold spring rate to ±10%. Standard compression springs hold solid height to +5% with no lower limit, hold outside diameter to tolerance bands that tighten as the diameter gets smaller (roughly ±0.002 in to ±0.055 in, or ±0.05 mm to ±1.40 mm, across the stock range), and hold squared-and-ground ends within 3° of square. Stock extension springs hold hook position inline within 22°. Stock torsion springs are held to published outside-diameter bands as well, with torque that changes in proportion to the deflection angle.

Tolerances also vary by spring type. Stock REDUX wave springs, for example, hold rate to ±25% rather than ±10%. The full tables are on Lee Spring's Engineering Notes and Tolerances page.

For a return spring on a latch, commercial tolerance is fine. For a pressure relief valve, a metering mechanism, or a medical device, it often is not.

Custom springs can be manufactured and tested to tighter tolerances, with load testing at specified heights, sorted rate groups, and certified test data. If your application requires performance documentation, you need a custom part with a controlled drawing. A catalog number is not a specification.

What is your annual volume?

Volume changes the economics completely. At low volumes, from prototypes through a few hundred pieces, stock springs are almost always cheaper because you avoid setup charges and tooling. This is why many engineers prototype with stock springs even when the production part will be custom.

At production volumes, from thousands to millions, custom springs frequently cost less per piece than stock equivalents. A custom spring is optimized to use exactly the wire, coils, and operations your application needs, and you are not paying a catalog distributor's margin.

Break-even is often lower than buyers expect. Ask your spring manufacturer to quote both paths at your projected annual volume before assuming stock is cheaper.

How fast do you need parts?

Stock springs ship in days. Custom springs typically take a few weeks depending on wire availability, complexity, and secondary operations, plus design and quoting time up front.

The lead-time question changes after the first order. Once a custom spring is tooled and proven, repeat orders run on normal production lead times, and blanket orders or vendor-managed inventory can put custom parts on your shelf as reliably as stock. Lead time is a strong argument for stock on new, urgent needs, and a weak argument against custom for recurring production demand.

Does your environment require special materials or finishes?

Stock catalogs concentrate on music wire and 302 stainless steel because those cover most applications. If your spring lives in salt spray, high temperature, aggressive chemicals, or a body-contact medical application, you may need 316 stainless, Inconel, Elgiloy, phosphor bronze, or a specific coating or passivation. The odds of finding your exact geometry in that material in a catalog drop fast.

Special environments are one of the most common and least controversial reasons to go custom.

Do you need traceability and compliance documentation?

If your quality system requires material certifications, lot traceability, Production Part Approval Process (PPAP) submissions, first article inspection reports, or compliance with standards such as ISO 13485 for medical devices or AS9100 for aerospace, a custom spring from a certified manufacturer gives you a controlled part number with full documentation.

Stock springs can sometimes be supplied with material certifications, but the documentation depth procurement teams need in regulated industries usually points to custom.

The Framework at a Glance

Factor	Points to Stock	Points to Custom
Fit	Catalog part meets envelope and load	Design constraints pin the geometry
Precision	Commercial tolerance acceptable	Tight rate or load tolerance, or test data required
Volume	Prototypes, low volume	Recurring production volume
Lead time	Needed this week	Recurring demand, plan-able
Environment	Standard materials work	Special alloys, coatings, temperatures
Compliance	No documentation needs	Certifications, traceability, PPAP required

Frequently Asked Questions

Are custom springs always more expensive than stock?

No. At production volumes, custom springs are often cheaper per piece because the design is optimized and distributor margins are removed. Setup costs dominate only at low volumes.

Can I prototype with a stock spring and switch to custom for production?

Yes, and this is a best practice. It is one reason Lee Spring maintains a 25,000-plus stock spring selection for immediate shipment. Validate the concept with a near-match stock spring, then have the production spring designed to your exact load case.

What tolerance can I expect from a stock spring?

Lee Spring stock compression and extension springs hold spring rate to ±10%, with published bands for outside diameter, solid height (+5% on standard compression), and end squareness. Torsion and wave springs have their own published tolerances (stock wave spring rate, for example, is ±25%). If you need tighter than the published rate tolerance, or a controlled tolerance on free length, that points to a custom part.

What information do I need to request a custom spring quote?

At minimum: the load case (force at deflection, or torque at angle), space envelope, environment, expected cycle life, annual volume, and any compliance requirements. A drawing helps, but a good spring manufacturer can create one from your application data.

How small can custom spring orders be?

Most manufacturers will run prototype quantities, though setup charges make small custom runs cost more per piece. Ask about prototype programs.

Not Sure Which Path Fits Your Application?

Lee Spring manufactures both. Browse 25,000+ stock springs ready to ship today, or send us your load case and our engineers will quote stock and custom options at your volume.

About Lee Spring

Lee Spring has manufactured springs for more than a century, offering 25,000+ stock springs ready for immediate shipment alongside full custom spring design and manufacturing. Our engineering team supports customers from prototype through production. Visit leespring.com.

Sources

• Lee Spring, Engineering Notes and Tolerances (leespring.com/engineering-notes-and-tolerances). Source for the ±10% stock spring rate tolerance, +5% solid height, outside-diameter bands, 3° end squareness, 22° hook position, ±25% wave spring rate, and the 25,000+ stock product count.

• Spring Manufacturers Institute (SMI), commercial spring tolerance guidance (Handbook of Spring Design). Corroborates the ±10% commercial spring rate tolerance as an industry norm.

• ISO 13485, Medical devices, Quality management systems (International Organization for Standardization).

• AS9100, Quality Management Systems for Aviation, Space and Defense (SAE / International Aerospace Quality Group).

• Production Part Approval Process (PPAP), Automotive Industry Action Group (AIAG).