Spring Permanent Deformation Test: Standards, Methods, and Technical Requirements

The spring permanent deformation test, also known as the spring permanent set test, is a critical quality control procedure that measures the residual change in a spring's free length, angle, or geometry after it has been loaded to a specified deflection and released. This test verifies that a spring operates safely within its elastic limit and will maintain consistent load characteristics throughout its service life. Accredited third-party laboratories perform this testing in accordance with recognized standards frameworks such as GB/T, JIS, EN, and ISO/IEC 17025 laboratory quality systems.

Technical Overview

Permanent deformation, commonly referred to as permanent set, is the residual dimensional change that remains in a spring after an applied load is fully removed. In an ideal elastic spring, the free length returns to its original value upon unloading. However, when local stresses exceed the proportional limit or elastic limit of the spring material, plastic deformation occurs at the wire surface, and the spring does not fully recover. The spring permanent deformation test quantifies this irreversible change and is one of the most fundamental acceptance criteria in spring manufacturing and qualification.

The test is significant for several reasons. First, it validates that the working stress of the spring design leaves an adequate margin below the yield strength of the material. Second, it predicts load stability: a spring that exhibits set will deliver progressively lower force at a given installed height, which can degrade the performance of valves, suspensions, clutches, and precision mechanisms. Third, it supports manufacturing processes such as scragging (presetting), in which a controlled amount of permanent set is deliberately induced to stabilize the spring and improve its subsequent load-bearing capacity.

Permanent set testing is distinguished from stress relaxation testing. Permanent set is evaluated at ambient temperature and expresses dimensional change, whereas stress relaxation testing measures force decay at elevated temperature under constant deflection. Both phenomena are related to the same underlying plastic and creep mechanisms, but they are specified, measured, and reported differently.

Typical acceptance limits depend on the application and the governing standard or customer specification. For precision cold-coiled compression springs, a permanent set of no more than 0.3% to 0.5% of the free length after compression to solid height is a common requirement, while heavy-duty hot-coiled springs and suspension springs may be evaluated over multiple loading cycles with limits expressed in millimeters or as a percentage of free height.

Testing Samples

Cold-Coiled Helical Compression Springs

Cold-coiled compression springs manufactured from drawn wire such as carbon steel, chrome-silicon, chrome-vanadium, or stainless steel spring wire are the most frequently tested sample type. These springs are tested by compressing them to solid height or to a specified maximum deflection and measuring the resulting change in free length. Typical applications include valve springs, clutch springs, precision instrument springs, and general industrial springs produced to standards such as GB/T 1239.2 and JIS B 2704.

Hot-Coiled Heavy-Duty Springs

Large-section hot-coiled springs, such as railway bolster and side bearer springs, truck suspension springs, and heavy machinery springs, are tested to verify set behavior under high working loads. Because of their size, these springs require high-capacity compression testing machines with parallel platens and precise stroke control. Evaluation may involve repeated compression to solid height or to a specified load, with free height measured after each cycle.

Extension Springs

Extension springs are tested by applying a specified tensile load or deflection and measuring the permanent change in free length or in the distance between hook ends. Care must be taken to avoid bending or distortion of the end hooks during gripping, since hook deformation is evaluated separately from body set.

Torsion Springs

For torsion springs and double torsion springs, the permanent deformation test measures the residual change in the angular relationship between the legs after the spring is deflected to a specified angle. The result is expressed in degrees of angular set rather than in linear units.

Disc Springs, Wave Springs, and Flat Springs

Disc (Belleville) springs, wave springs, snap rings, stamped flat springs, spiral springs, and leaf springs can all be evaluated for permanent set using tailored fixtures. In many cases, these components are tested in stacked configurations or in the assembled condition to replicate service loading. Standards such as JIS B 2701 and design references including EN 13906 series documents provide guidance relevant to these spring geometries.

Material and Process Control Coupons

In addition to finished springs, manufacturers may submit preset or scragged production lots and sample springs at intermediate process stages to verify that heat treatment, shot peening, and presetting operations have been performed correctly and that the residual set meets the process window.

Testing Items

The spring permanent deformation test program typically includes the following measured and calculated items:

• Initial free length or free height measurement before loading
• Permanent set magnitude in millimeters (or angular degrees for torsion springs) after load removal
• Permanent set expressed as a percentage of original free length
• Load at a specified test height or specified deflection, before and after the set cycle
• Spring rate (stiffness) verification and rate change after set testing
• Solid height verification and comparison with drawing limits
• Load loss percentage at installed height due to set
• Set behavior over multiple loading cycles (first, second, and subsequent compressions)
• Elevated-temperature set or relaxation behavior where specified
• Visual inspection for surface defects, cracking, or permanent distortion of end coils and hooks

Testing Methods

Sample Conditioning and Preparation

Springs are conditioned at ambient laboratory temperature, typically 10 °C to 35 °C, before measurement. The free ends, bearing surfaces, and end coils are cleaned and inspected. The initial free length L0 is measured at defined contact points using a calibrated height gauge, dial indicator fixture, optical comparator, or vision measurement system. For springs with ground ends, the measurement plane and measuring force are standardized to ensure repeatability.

Static Compression Permanent Set Test

The spring is placed between parallel, flat platens on a spring compression testing machine or universal testing machine. It is compressed at a controlled rate to solid height or to the test height specified by the drawing or standard, held at that position for a specified dwell time (commonly 10 seconds or as required by the specification), and then fully released. After a defined recovery period, the free length L1 is remeasured. The permanent set is calculated as L0 minus L1, and the set percentage is calculated as (L0 minus L1) divided by L0, multiplied by 100. The result is compared with the allowable limit in the applicable standard, such as GB/T 1239.2, JIS B 2704, or the customer technical specification.

Repeated Cycle Set Test

For suspension and safety-critical springs, the compression-to-solid or compression-to-maximum-load procedure may be repeated for a specified number of cycles, for example three to five consecutive compressions. The free height is recorded after each cycle. This method reveals whether the spring stabilizes after the initial cycles or continues to accumulate set, which is an important indicator of inadequate presetting or insufficient yield margin.

Load-Hold and Elevated Temperature Set Test

Where the spring operates under sustained load or at elevated temperature, a load-hold test may be performed in which the spring is held at a specified deflection for an extended duration at ambient or elevated temperature. The residual dimensional change and the load decay at the test height are measured after release and cooling. This method bridges permanent set testing and stress relaxation testing and is common for engine valve springs, exhaust-system springs, and thermal actuator springs.

Torsional and Extension Set Test Methods

Torsion springs are fixtured on a mandrel with the legs located against calibrated stops, deflected to the specified angle, held, released, and remeasured for angular set. Extension springs are loaded in tension using suitable hook or thread fixtures to the specified load or deflection. In both cases, the measured permanent deformation is compared against the drawing tolerance or standard limit.

Measurement Uncertainty and Equipment Requirements

Reliable set measurement requires compression platens with verified parallelism, load cells calibrated traceably to national standards, and length measurement resolution appropriate to the set limit, typically one-tenth or better of the allowable set value. Accredited third-party laboratories operating under ISO/IEC 17025 quality systems maintain calibrated spring testers, force-deflection testers (spring raters), and metrology equipment, and report measured values together with test conditions, dwell times, loading rates, and the applicable acceptance criteria.

Applications

Spring permanent deformation testing supports quality assurance and product qualification across a broad range of industries:

• Automotive engineering: valve springs, clutch springs, suspension coil springs, brake return springs, and seat and door hardware springs
• Railway engineering: bolster springs, side bearer springs, and coupling springs subject to heavy static and dynamic loads
• Industrial machinery: die springs, hydraulic valve springs, safety valve springs, and pneumatic cylinder springs
• Precision instruments and electronics: contact springs, switch springs, relay springs, and battery contacts where stable force over long service life is essential
• Petroleum, chemical, and power generation equipment: spring hangers, pressure relief valve springs, and turbine springs exposed to sustained stress and temperature
• Construction and hardware: garage door springs, fastener springs, and snap rings
• Process validation: verification of scragging, presetting, heat treatment, and shot peening operations in spring production

FAQ

What is permanent set in a spring?

Permanent set is the residual change in a spring's free length, free angle, or overall geometry that remains after the applied load is removed. It occurs when localized stresses during loading exceed the elastic limit of the spring material, producing a small amount of plastic deformation. Permanent set reduces the load a spring delivers at its working height and is a key indicator of design margin and manufacturing quality.

Which standards govern spring permanent deformation testing?

Testing is commonly performed to national and international standards such as GB/T 1239.2 for cold-coiled compression springs, GB/T 23935 for hot-coiled cylindrical helical springs, JIS B 2704 for helical compression and extension springs, and the EN 13906 series for cylindrical helical spring design and evaluation, as well as customer technical specifications. Testing laboratories should operate under an ISO/IEC 17025 quality framework to ensure calibrated equipment and traceable results.

What is a typical acceptance criterion for permanent set?

Acceptance limits vary by application. For precision cold-coiled compression springs, the permanent set after a single compression to solid height is frequently limited to 0.3% to 0.5% of the free length or to a small specified value in millimeters. Suspension and heavy-duty springs are often assessed over multiple compressions with limits defined in the product drawing. The governing specification always takes precedence over generic values.

How is the permanent set value calculated?

The initial free length L0 is measured before loading. The spring is then compressed, extended, or torqued to the specified position, held for the required dwell time, and released. After recovery, the final free length L1 is measured. The permanent set equals L0 minus L1, and the set percentage equals the set value divided by L0 and multiplied by 100.

What is the difference between permanent set and stress relaxation?

Permanent set is a dimensional change measured at ambient temperature after the load is removed, whereas stress relaxation is the loss of load measured while the spring is held at constant deflection, usually at elevated temperature. Both mechanisms stem from plastic flow and creep in the material, and both reduce the force delivered by the spring in service, but they are tested and reported as separate quantities.

Why do manufacturers deliberately preset or scragg springs?

Scragging is a controlled production process in which springs are compressed beyond the elastic limit, typically to solid height, to induce beneficial residual compressive stresses in the wire surface. A correctly scragged spring will show negligible additional set during subsequent proof testing and service, and it gains improved load capacity and fatigue performance. Permanent deformation testing is used both to qualify the scragging process and to confirm final product stability.

What sample size and test conditions are used?

Sample quantities are defined by the applicable standard or sampling plan, often ranging from a few pieces for type testing to statistical lots for production surveillance. Tests are conducted at room temperature with controlled loading rates, parallel platens, and specified dwell and recovery times. All test parameters, including the number of cycles and the measurement method, must be documented in the test report to ensure reproducibility.