Thermal Coil Spring Terminology

A note to begin with

As a specialized manufacturer in China that has consistently focused on the production of hot-coil spring equipment, we’ve deeply realized—through our interactions with customers—that our expertise in hot-coil spring manufacturing processes remains limited and often superficial. In particular, we’ve encountered situations where customers expect equipment manufacturers to explain issues related to hot-coil spring technology, leaving us feeling helpless. To address this gap, we’ve invited leading domestic experts in the field of hot-coil springs to periodically share insights on topics such as hot-coil spring terminology, production techniques, thermal processing methods, on-site management practices, and workstation tools. Moving forward, our company will continue to deepen our commitment to the hot-coil spring equipment industry, delivering increasingly professional, intelligent, and precision-focused equipment solutions that empower the global spring industry—and, ultimately, contribute to the high-quality growth of both domestic and international markets.

As a versatile basic component, springs serve a wide range of functions—including shock absorption, energy storage, motion control, and tension maintenance—while their applications span across virtually every sector of the national economy. From aerospace and aviation to power and nuclear equipment, high-speed railways, automobiles, and even everyday hardware products, springs can be found in countless areas of modern life.

Spring noun

Definition of a spring:

A spring is a mechanical component that utilizes the material properties and structural characteristics of its design to store and release energy through significant deformation during operation, making it widely used in the machinery industry and other sectors.

Definition of hot-coiled spring manufacturing:

The entire process of heating spring steel billets as a whole, winding and shaping them into springs on specialized equipment, followed by heat treatment, is known as hot-coiled spring manufacturing.

Distinguishing between cold-coiled and hot-coiled spring specifications:

Currently, when the cross-sectional size of spring materials in China exceeds φ20mm, the hot-coiling process is predominantly used for manufacturing. Meanwhile, well-established professional spring manufacturers have already achieved cold-coiling capabilities up to φ25mm.

The types of thermoformed helical compression springs, classified by their structural form, mainly include:

Cylindrical helical compression springs with either circular or rectangular cross-sections—including stepped or variable-section helical compression springs—are commonly used, though cylindrical helical compression springs with circular cross-sections account for the vast majority. Therefore, we will focus primarily on introducing hot-formed cylindrical helical compression springs that feature a circular cross-section.

Spring Terminology

In the processes of marketing, technical discussions, technology exchange, production manufacturing, preparation of technical documents and manufacturing procedures, as well as inspection and testing, we frequently encounter certain spring-related terms and associated requirements, which can be broadly summarized as follows:

Spring terminology is primarily categorized into: Basic terminology for spring characteristics, terms related to spring applications in mechanical and engineering fields, terminology for mechanism types, technical requirement terms, spring design and calculation terminology, manufacturing and processing-related terms, as well as testing and inspection terminology. We will provide a selective introduction:

Basic Terminology of Spring Characteristics ：

Compression spring: Axial-pressure springs typically refer to helical compression springs.

Spring Design and Calculation Terms ：

Spring stiffness: The load required to produce a unit deformation (1 mm) in the spring.

Elastic modulus: The ratio of stress to strain generated in the axial direction of the bar's cross-section.

Shear modulus: The ratio of shear stress to shear strain within the elastic limit.

Maximum and minimum stress: The maximum and minimum stresses generated by the spring within one vibration cycle.

Average stress: The half of the algebraic sum of the maximum and minimum stresses generated by the spring under cyclic loading.

Stress amplitude: The half of the algebraic difference between the maximum and minimum stresses generated in the spring under cyclic loading.

Spring safety factor: The ratio of stress causing spring failure (relaxation or fracture) to the intended operating stress.

Fatigue Strength: Springs can withstand the maximum stress level for an infinite number of cycles.

Creep and Relaxation: The phenomenon where a spring, under conditions of constant temperature and constant load, slowly undergoes plastic deformation over an extended period is called creep. Meanwhile, if the spring's deformation remains unchanged but its load-carrying capacity gradually diminishes after a certain time, this is referred to as relaxation.

Permanent deformation: The portion of the spring's free length change that cannot be recovered after unloading is referred to as permanent deformation.

Unfold length: The total length of the spring material when unfolded into a straight line.

The above terms are essentially derived from the content of GB/T 1805 standard, while the following introduction includes some practical techniques.

Spring free height: When measuring the total length of a spring in its unloaded state, please note the following points: For springs with a coil ratio C ≤ 6, you can measure them directly while standing upright—taking at least one measurement every 120 degrees to determine both the maximum and minimum dimensions. If the spring has a high coil ratio and exhibits poor stability when standing upright, place it on a flat surface and roll it during measurement, again recording the maximum and minimum dimensions.

Compression height: The compressed spring is fully pressed until all coils are tightly adjacent, reaching the total height of the fully compressed state. To measure the compression of the spring, the light-transmission method can be used: A light source is placed behind a powerful press, and the operator observes whether there is any visible light passing through the gaps between the spring coils from the front side. The maximum compressive load applied to the spring must not exceed 1.5 times the theoretically calculated compressive load.

Spring mean diameter: The average of the spring's inner diameter and outer diameter, used in spring design calculations.

Spring outer diameter: The outer diameter of the helical spring coil. It can be measured using general-purpose gauges or specialized inspection tools such as sleeve-type gauges, provided the sleeve length spans across at least 1.5 working coils.

Spring inner diameter: The inner diameter of the helical spring coil can be measured using general-purpose gauges, or it can be inspected with specialized tools such as gauge rods. The gauge rod must be manufactured with both go and no-go ends according to the dimensional tolerances specified in the drawing. Its length should extend across at least 1.5 active coils—or alternatively, it can be made 50 mm longer than the spring’s free height. To reduce weight, the structure can be designed as a hollow shape, which can be fabricated from seamless steel tubing.

Total laps: Count the total number of compression spring coils, including the ineffective coils at both ends.

Effective Rounds: The number of turns excluding the two non-effective ends.

Spin direction: Starting from one end of the spring, observe the direction in which the coils appear to vanish. If the vanishing direction is clockwise, the helix is right-handed; if the vanishing direction is counterclockwise, the helix is left-handed.

Supporting ring: The end coils in a helical compression spring that do not contribute to the spring's elasticity. Typically, support coils can range from 1.5 to 3.2. Japan's Mitsubishi Corporation has designed a large-opening spring with support coils measuring just 0.9.

Spring pitch: The axial distance between the centerlines of adjacent active coil cross-sections when the spring is in its free state. Recommended value: t < 0.5D (where D is the mean diameter of the spring).

Helix Angle: The angle formed between the central line of the helical spring material and the plane perpendicular to the central line of the helical spring. The recommended value is α = 5–9 degrees.

Variable pitch: The spiral spring has an uneven pitch to impart nonlinear characteristics to the spring.

Spacing: The axial gap between adjacent coils is measured along the axis direction.

Gap coefficient: The ratio of the spacing between effective turns to the material diameter.

Helical Ratio (Elasticity Index): The ratio of the mean diameter of a spring made from circular material to the material's diameter, or the ratio of the mean diameter of a spring made from non-circular material to its radial line width. Recommended value: C = 3 ~ 14.

Curvature coefficient: The correction factor for stress influence, corresponding to the winding ratio (elasticity index), is a physical quantity that measures how easily a spring can be formed. It is denoted by K.

Helical Spring Height-to-Diameter Ratio: The ratio of the free height of the helical spring to its mean coil diameter. Recommended value: b < 5.7.

Manufacturing and Engineering Terminology ：

Coil spring: The winding process of coiling wires or rods into a helical shape with curvature and torsion.

End-face grinding: The process of grinding the end faces of helical compression springs. When both ends of the spring require grinding, it is referred to as double-end grinding.

Creep and Tempering: Low-temperature tempering of the spring held at a specified length.

Pressurized Tempering: Low-temperature tempering of the spring held under a specified load.

Shape-Change Heat Treatment: A deformation heat treatment involving plastic deformation, applied to steel products in the metastable austenitic state before undergoing martensitic and/or bainitic phase transformations.

Isothermal Quenching: After austenitizing, cool down to M at a sufficiently rapid rate. _S A heat treatment process conducted at temperatures above the specified point to maintain warmth, thereby preventing the formation of ferrite or pearlite and ensuring that the austenite fully or partially transforms into bainite. Finally, the material is cooled to room temperature without a prescribed cooling rate. _S The transformation temperature is the point at which austenite begins to transform into martensite during cooling.

Low-temperature annealing: Low-temperature heat treatment is used to relieve internal stresses and enhance various material properties, such as the elastic limit, yield strength, or fatigue resistance, as well as to stabilize shapes.

Stress-relief annealing: The heat treatment process involves heating to a suitable temperature and holding it, followed by cooling at an appropriate rate. It is used to relieve internal stresses (generated during spring forming) without significantly altering the material's microstructure.

Quenching hardening: After austenitizing, the steel product undergoes cooling designed to transform most of the austenite into martensite, with possibly some retained as bainite, thereby achieving hardening.

Tempering: Heat treatment applied to steel products after quenching or other thermal processes, involving one or more heating steps to a specific temperature (＜A _C1 Then, maintain the temperature and allow it to cool at an appropriate rate to achieve the desired properties. Typically, tempering reduces hardness, though in some cases, it may even increase hardness—A _C1 It is the temperature at which austenite begins to transform during the heating process.

Annealing: The heat treatment process involves heating the metal to a specific, appropriate temperature and holding it at that temperature to achieve uniformity, followed by cooling under controlled conditions. After cooling to room temperature, the metal’s microstructure becomes closer to equilibrium.

Shot peening: It involves high-velocity spraying of nearly spherical, hard particles onto material or mechanical components to induce residual compressive stress and achieve work-hardened cold deformation, thereby enhancing their fatigue life and resistance to stress corrosion.

Hot Shot Peening: Shot peening of springs is performed within a specific temperature range. Heat shot peening leverages the age-hardening effect of steel, typically resulting in higher residual stresses—especially in high-hardness steel springs.

Establish a handling procedure: The process of applying a load or torque exceeding the spring's maximum working capacity before use, causing a certain degree of permanent deformation to enhance its resistance to stress relaxation and improve durability.

Room-temperature setting treatment: Standing treatment conducted at room temperature.

Heat Setting (Heated Setting Treatment): A stress-relief treatment performed at a temperature no higher than the low-temperature (stress-relief) annealing temperature.

Test and Inspection Terminology ：

Armand's Test Print: A specialized test specimen used to measure shot peening intensity.

Shot peening intensity: It depends on the kinetic energy applied per unit area of the workpiece within a given time, as assessed by the Arman test specimen at the saturation point.

Alman Arc Height: The arc height value of the Arman test specimen measured over the specified span.

Coverage Rate: The ratio of the area where indentations are formed by shot peening impact to the total area of the test surface.

Surface defects: Defects on the spring surface that occur during manufacturing or use, such as flaky roughness caused by overheating during the heating process, as well as mechanical damage, surface folds, rust, and corrosive residues arising from the spring-forming process.

Decarbonization: During the manufacturing process, the phenomenon occurs where the carbon content in the surface layer of the spring decreases during hot working and heat treatment. After decarburization, the spring's fatigue limit will be reduced.

Surface hardness: Hardness measured on the spring's surface.

Test Load: Used to determine the spring characteristics, applying a static load to the spring.

Length Test: Test the spring's length under the specified load.

Residual shear strain: After removing the spring load, the residual shear strain in the spring remains.

Residual Stress: The internal stress remaining in a material or component after external forces and thermal effects have been removed.

High-temperature loading test: A test in which the spring is clamped to a constant length and held at a constant high temperature for a specified period of time.

Creep Test: Under constant temperature and constant force, the change in length of a spring or the test specimen is measured over a specific period of time. There are two types of creep tests, classified as tensile creep tests or compressive creep tests depending on the type of stress applied. Sometimes, these tests are conducted at room temperature.

Relaxation Test: Under constant temperature and constant force, the change in length of a spring or the test specimen is measured over a specific period of time. There are two types of creep tests, classified as tensile creep tests or compressive creep tests depending on the type of stress applied. Sometimes, these tests are conducted at room temperature.

Fatigue Testing: Under cyclic or alternating stress applied to the spring or the test specimen, determine its fatigue life or endurance limit. Depending on the type of stress, fatigue tests can be categorized into torsional, axial load, rotating bending, and plane bending fatigue tests.

Environmental Testing: An experiment testing the changes in surface conditions of materials or springs when exposed to environments of light, heat, wind, and rain.

Salt Spray Test: After the salt spray test conducted in a sodium chloride solution at the specified concentration and temperature, the material was tested for its corrosion resistance against rusting and blistering.

Eddy Current Flaw Detection Test: Non-destructive testing that applies an alternating magnetic field with coils (via AC current) to conductive springs or materials, and detects defects based on eddy current variations caused by imperfections.

Penetrant Testing: Non-destructive testing for detecting metal surface defects utilizes the capillary action of liquids, allowing penetrant fluids to seep into open defects on the surface of solid materials. These defects are then revealed by applying a developer, which brings the penetrated penetrant back to the surface, clearly indicating the presence of flaws. This test can be performed using pigments that are visible under either visible or ultraviolet light.

Magnetic Particle Inspection Test: Coat the magnetized steel sample with a magnetic suspension containing ultra-fine magnetic powder, and utilize the phenomenon where defects disrupt the magnetic flux lines in the magnetic field, causing the magnetic powder to adhere specifically to the defective surface—this method enables non-destructive testing to identify surface crack defects. This technology can be performed using pigments visible under either visible or ultraviolet light.

Existing Standards for Hot-Coiled Springs

Currently, there are international standards, national standards, industry standards, and enterprise standards for hot-coiled springs. At present, we generally follow the national standard GB/T 23934-2015, "Technical Specifications for Hot-Coiled Cylindrical Helical Compression Springs."

China Spring Standard:

GB/T23934 — 2015 Technical Specifications for Hot-Coiled Cylindrical Helical Compression Springs

GB/T23935 — 2015 Cylindrical Helical Spring Design and Calculation

TB/T2211--2010 Technical Specifications for Supplying Hot-Coiled Helical Compression Springs for Locomotives and Rolling Stock

International Spring Standards ：

International standard:

ISO11891--2012 Technical Specifications for Hot-Coiled Helical Compression Springs

ISO683-14 / EN10089 - 2003 Quenching and Tempering Hot-Rolled Spring Steel – Delivery Technical Specifications

U.S. Standard: ASTM--A125 　Hot-Coiled Spring — Technical Specifications

EU Standards: EN13906 Circular Wire and Bar Helical Compression Springs: Calculation and Design

EN13298 Railway Applications – Suspension Systems – Steel Spiral Suspension Springs

German standard: DIN2096 　Spiral Compression Springs Made from Round Metal Wire and Bars: Quality Requirements for Hot-Coiled Compression Springs

UK Standard: BS1726 　Hot-Coiled Spring — Technical Specifications

Japanese Industrial Standards: JISB2704 　Spring Design and Testing Methods

International Railway Union Standard: UIC822 　Supply Specifications for Compression Springs Used in Locomotives and Rolling Stock

Russian Federation National Standard: T0CT1452--2011 Technical Specifications for Tire Cylindrical Rails and Railway Lifting Equipment