When you’re sourcing custom springs, the technical terminology can feel overwhelming. This glossary breaks down the most common spring manufacturing terms and concepts that engineers and purchasing agents encounter during the design and procurement process.

Whether you’re writing an RFQ, reviewing a quote, or discussing spring specifications with a manufacturer, you’ll find clear, practical definitions here—not just the physics, but how each term affects cost, lead time, and performance.

Active Coils

The coils in a spring that actually compress, extend, or twist when the spring is loaded. Inactive coils (end coils) anchor the spring but don’t participate in the spring action. For a compression spring, the number of active coils directly determines the spring rate. A spring with 10 active coils will be stiffer than an identical spring with 8 active coils.

Closed and Ground Ends

An end condition for compression springs where the end coils are closed (wrapped tightly) and then ground flat on a grinding wheel. This creates a perfectly flat surface that allows the spring to stand perpendicular and distribute load evenly. Closed and ground ends cost 10-20% more than open ends and add lead time, but they’re necessary when the spring must bear precisely against a flat surface or when solid height is critical to the assembly.

Coil Diameter (Mean Coil Diameter / Outer Diameter)

The diameter of the coil measured from the center of the wire. The mean coil diameter (the diameter at the centerline of the wire) is used in spring rate calculations. The outer diameter is the maximum distance across the outside of the spring. These dimensions must be specified on a spring drawing, and they interact with wire diameter and coil count to determine the spring’s rate, solid height, and manufacturability.

Compression Spring

A spring designed to resist compression (pushing inward) and return to its original length when the load is removed. The most common type of spring in manufacturing, used everywhere from mouse clicks to automotive suspensions. Compression springs are defined by their free length, outer diameter, wire diameter, number of active coils, and material.

Design for Manufacturability (DFM)

The practice of designing a spring (or any component) to be easy and cost-effective to manufacture without sacrificing performance. DFM decisions include choosing standard wire diameters, appropriate spring index values, producible tolerances, and standard end conditions. Working with your manufacturer during the design phase to optimize DFM can significantly reduce cost and lead time without compromising the spring’s function.

End Condition

The configuration of the end coils on a spring. For compression springs, the four main options are open ends, closed ends, closed and ground ends, and closed and squared ends. Each end condition has different manufacturing requirements and costs. The choice depends on how the spring will be used—whether it needs to stand flat, be inserted into a hole, or compress against another component.

Extension Spring

A spring designed to resist being pulled apart (tension) and return to its original length when the load is released. Extension springs typically have hooks or loops at the ends to attach to other components. They’re used in applications like screen door hinges, garage door systems, and machinery guards. Extension springs require specification of their hook style and angle in addition to the standard spring parameters.

Free Length

The length of a spring when it is completely unloaded (no force applied). For a compression spring, this is measured from the flat surface of one end coil to the flat surface of the other. Free length is fundamental to spring design because it determines how much the spring can compress before it goes solid (all coils touching). Any change to free length affects both the spring rate and how the spring fits in an assembly.

Helix Angle

The angle formed between the coil of a spring and a plane perpendicular to the spring’s axis. A steeper helix angle (larger angle) means the coils are wound more loosely; a shallower angle means they’re wound more tightly. Helix angle affects spring rate and must be calculated based on the number of coils, mean diameter, and wire diameter. It’s rarely specified directly on a drawing; instead, designers specify pitch or coil count and let the manufacturer calculate helix angle.

Load

The force applied to a spring, typically measured in pounds-force (lbf) or Newtons. Spring specifications often include a load value at a specific deflection (e.g., “100 lbf at 1 inch compression”). This tells a manufacturer exactly how stiff the spring needs to be and helps them calculate the required wire diameter and coil count. Load and spring rate together define the spring’s behavior.

Music Wire

A high-carbon steel wire (typically ASTM A228) used to make springs. Music wire is very strong, has excellent fatigue resistance, and is the standard material for most industrial springs. It’s called music wire because it was originally used in piano strings. It’s available in standard wire sizes from most spring manufacturers and distributors, making it a cost-effective choice for most applications.

Number of Coils

The total count of complete revolutions in a spring coil. This includes both active coils (which contribute to the spring’s function) and inactive coils (end coils that anchor the spring). A compression spring might have 12 total coils, with 10 active and 2 inactive. The number of active coils is what matters for spring rate calculations, but the total coil count determines the spring’s overall length.

Open Ends

An end condition for compression springs where the coil ends are left open (not closed). Open-end springs are the simplest and least expensive to manufacture, making them the default choice for cost-sensitive applications. The trade-off is that they don’t stand perfectly flat on a surface—they may rock slightly. For applications where flatness doesn’t matter, open ends offer significant cost savings.

Passivation

A chemical treatment applied to stainless steel springs to enhance corrosion resistance. During passivation, the steel is immersed in an acid bath that removes iron particles and oxides from the surface, leaving a chromium-rich oxide layer that protects against rust. Passivation is commonly specified for springs used in food equipment, medical devices, and outdoor applications. It adds cost and lead time but significantly improves corrosion performance.

Pitch

The axial distance between corresponding points on adjacent coils. In other words, if you measured from the top of one coil to the top of the next coil along the spring’s axis, that distance is the pitch. Pitch is rarely specified directly; instead, designers specify the number of coils, free length, and the spring is wound to fit. Understanding pitch helps when visualizing how tightly or loosely a spring is wound.

Rate (Spring Rate / Spring Constant)

The amount of force required to compress (or extend) a spring by one unit of length, typically measured in pounds-force per inch (lbf/in) or Newtons per millimeter (N/mm). A stiffer spring has a higher rate; a softer spring has a lower rate. Spring rate is calculated from wire diameter, coil diameter, number of active coils, and material. It’s one of the most critical specifications in spring design because it determines how much the spring will deflect under load.

Shot Peening

A surface treatment where small steel balls are fired at high velocity against a spring’s surface. This creates a layer of compressive stress that significantly improves fatigue resistance—the spring’s ability to withstand repeated compression and extension cycles without cracking. Shot peening is essential for springs in high-cycle applications like automotive suspension or industrial machinery. It’s a secondary operation that adds cost and lead time.

Solid Height

The length of a spring when it is compressed to the point where all coils are touching (no air gap between coils). At solid height, the spring cannot compress further without deforming permanently. Solid height is determined by wire diameter and total number of coils. Understanding solid height is critical for assembly design—you must ensure that the spring can compress enough to reach its required load without going solid.

Spring Index

The ratio of the mean coil diameter to the wire diameter. A spring index of 8, for example, means the mean coil diameter is 8 times the wire diameter. Spring index affects how easy a spring is to manufacture and how much stress the wire experiences. Springs with a low index (below 4) are difficult to wind and require specialized equipment. Springs with a high index (above 12) may be too loose and can be prone to buckling. Most manufacturers aim for a spring index between 5 and 12.

Stainless Steel (302 / 304 / 316)

A family of corrosion-resistant steel alloys commonly used for springs in applications requiring rust resistance. 302 stainless is the most common and cost-effective for springs. 304 and 316 offer slightly better corrosion resistance, particularly in harsh or salt-water environments. Stainless steel springs cost more than music wire but are necessary for food handling, medical devices, outdoor equipment, and any application exposed to moisture or chemicals.

Tolerance

The acceptable range of variation for a dimension or property of a spring. For example, a tolerance on outer diameter might be ±0.015 inches, meaning the spring can be anywhere within that range and still be acceptable. Tighter tolerances require more frequent inspection and slower production, adding significant cost. Looser tolerances are faster and cheaper to produce. The key is specifying only the tolerances actually required by the application.

Torsion Spring

A spring designed to resist twisting (rotational force) and return to its original angle when the torque is released. Torsion springs are wound in a helix like compression springs but are loaded by rotating the legs at the ends of the spring. They’re used in applications like door hinges, clothespins, and machinery brakes. Torsion springs require specification of leg length, leg angle, and direction of wind in addition to standard spring parameters.

Wire Diameter

The diameter of the wire used to wind the spring coil. Wire diameter is one of the most critical design parameters because it determines the spring’s stiffness, load capacity, and fatigue resistance. Wire is manufactured in standard sizes—0.020 inch, 0.025 inch, 0.032 inch, and so on—for common materials. Specifying a non-standard wire diameter complicates sourcing and adds cost, so best practice is to specify a standard size available from wire distributors.

Yield Strength

The amount of stress a spring material can withstand before it begins to deform permanently. When a spring is stressed beyond its yield strength, it won’t return to its original shape. Yield strength varies by material—music wire has a higher yield strength than soft steel, which is why it’s preferred for springs. Engineers must ensure that the stresses in their spring design stay well below the material’s yield strength, typically leaving a safety margin of 20-30%.

Need help translating these terms into a spring specification? Reach out to our engineering team for a design review or manufacturability consultation. We’ll make sure your spring spec is optimized for performance, cost, and lead time.