Spring relaxation rate testing quantifies the loss of spring force that occurs when a spring is held under sustained deflection or load, providing critical evidence of long-term performance and dimensional stability. Performed in accordance with recognized standards such as GB/T 23935, GB/T 1239, EN 13906, ASTM A125 and JIS B 2704, this test is essential for validating valve springs, clutch springs, disc springs, spring washers and other safety-critical components. This article explains the testing principle, sample requirements, test methods, calculation formulas and typical applications of spring relaxation rate testing.
Technical Overview
Spring relaxation refers to the time-dependent decay of spring force when a spring is maintained at a fixed deflection (constant strain) or under a sustained load. As a small portion of the elastic strain is gradually converted into micro-plastic deformation, the load that the spring delivers at a given length decreases even though its external geometry may appear essentially unchanged. The relaxation rate expresses this load loss as a percentage of the initial load and serves as a key acceptance index for springs that must provide stable clamping, sealing or actuation force throughout their design life.
Relaxation is closely related to, but distinct from, creep. Creep describes the progressive increase of deformation under constant stress, whereas relaxation describes the progressive decay of stress under constant strain. Both phenomena are driven by similar microstructural mechanisms, including dislocation movement, grain-boundary sliding and diffusion-assisted plastic flow, and both are strongly accelerated by elevated temperature and high working stress. Because most preloaded springs in service are installed at a fixed length, relaxation testing provides the most direct simulation of real operating conditions.
The relaxation rate R after a holding time t is generally calculated as: R = (F0 − Ft) / F0 × 100%, where F0 is the initial load measured at the specified test height before the holding period, and Ft is the load measured at the same height after the holding period. When the test is performed under constant load, relaxation is instead evaluated through the change of spring height, expressed as ΔH / H0 × 100%, or through the permanent set remaining after release.
Testing is typically carried out in accredited third-party laboratories in accordance with product and method standards such as GB/T 23935, the GB/T 1239 series for cold-coiled and hot-coiled helical springs, EN 13906 for cylindrical helical springs, EN 16983 for disc springs, ASTM A125 for heat-treated helical steel springs, and JIS B 2704, or according to the specific technical requirements stated on the customer drawing.
Testing Samples
Helical Compression Springs
Helical compression springs are the most frequently tested sample type. They are usually cold-coiled from oil-tempered carbon steel wire, chromium-silicon or chromium-vanadium alloy wire, or precipitation-hardened and austenitic stainless steel wires. Typical examples include engine valve springs, fuel injector return springs, clutch springs and die springs.
Extension Springs, Torsion Springs and Retaining Rings
Extension springs and torsion springs can also be evaluated for relaxation using purpose-designed fixtures that hold the specimen at a specified extension length or twist angle. Spiral retaining rings and snap rings are similarly checked for force retention after installation at a fixed working diameter.
Disc Springs and Spring Washers
Disc springs (Belleville springs), wave springs and toothed or split spring washers are tested individually or in stacked columns. For these components, relaxation is often assessed by measuring the residual force of the stack after compression to a fixed height, following the requirements of the applicable product standards.
Special and Custom Springs
Stamped flat springs, conical springs, constant-force springs, battery contact springs and other custom elastic elements can be tested with dedicated fixtures designed to reproduce the installed condition specified by the customer.
- Samples should be submitted in the as-delivered condition, complete with any shot peening, presetting (scragging) or heat treatment already applied in production.
- Specimens must be free of corrosion, nicks, handling damage and visible defects that could bias the result.
- A minimum of three identical specimens is normally recommended so that scatter can be assessed statistically.
- Free height, outside diameter, wire diameter and material certificate should be recorded before testing.
- Samples are typically conditioned at laboratory ambient temperature (23 ± 5 °C) for a defined stabilization period before the initial load measurement.
Testing Items
- Room-temperature relaxation rate at a specified deflection and holding time
- Elevated-temperature relaxation rate under simulated service thermal conditions
- Load loss (F0 − Ft) after a defined holding duration, such as 24 h, 48 h, 100 h, 500 h or 1000 h
- Permanent set and free height change after release of the test deflection
- Relaxation curve, describing load decay as a function of holding time
- Spring load-deflection characteristic verification before and after the relaxation exposure
Testing Methods
Constant Deflection (Constant Strain) Method
This is the most common approach. The spring is first compressed to the specified test height and the initial load F0 is measured with the spring deflection fully seated. The spring is then held rigidly at this constant length for the specified duration, either at ambient temperature or inside a temperature chamber. After the holding period, the spring is released, allowed to recover for a defined rest interval, and the load Ft is measured again at exactly the same test height. The relaxation rate is then calculated as R = (F0 − Ft) / F0 × 100%.
- Step 1: Measure and record the free height and characteristic dimensions of the specimen.
- Step 2: Apply a seating preload cycle, if required by the specification, to stabilize the end coils.
- Step 3: Compress to the specified test height and record the initial load F0.
- Step 4: Maintain the constant deflection for the specified holding time at the specified temperature.
- Step 5: Release the spring and observe the prescribed recovery period.
- Step 6: Recompress to the identical test height, record the final load Ft, and calculate the relaxation rate.
Constant Load (Constant Stress) Method
In this variant, a fixed load, usually applied by a lever system, dead weight or servo-controlled actuator, is maintained on the spring throughout the test. Relaxation is manifested as a progressive reduction of spring height, and the result is expressed as the height change ΔH relative to the initial height H0, or as the residual permanent set after unloading. This method is particularly useful for disc spring stacks and springs operating under weight or pressure loading.
Elevated-Temperature Relaxation Testing
For springs serving in thermally loaded environments, the test is conducted inside an environmental chamber that maintains the specified temperature, commonly between 100 °C and 400 °C depending on the application, with a typical uniformity requirement of a few degrees across the specimen. The spring is allowed to soak at temperature before the initial load is recorded, so that thermal equilibrium is achieved. The holding time is defined by the product standard or customer specification, and the load is remeasured either in the hot condition, using a load cell integrated with the chamber, or after controlled cooling to ambient temperature.
Equipment and Measurement Accuracy
Testing is performed on dedicated spring relaxation testing machines or universal testing systems equipped with precision load cells, rigid compression platens and, where required, temperature chambers. Load measurement systems of accuracy class 0.5 or 1 are generally used, displacement measurement resolution should be adequate to return to the test height within a tight tolerance, and all instruments must be traceably calibrated. Test reports should identify the specimen, material grade, test height, temperature, holding time, initial and final loads, calculated relaxation rate, and the governing standard or drawing clause.
Applications
Spring relaxation rate testing is applied across industries in which long-term force stability directly affects safety, sealing or control accuracy:
- Automotive engineering: valve springs and valve spring retainers, clutch and brake springs, fuel injection return springs, seat belt and latch springs, battery contact springs in electric vehicles.
- Railway engineering: disc spring assemblies for bolted joints on rails, switch machines and pantograph systems, where sustained clamping force is essential.
- Electrical power equipment: contact pressure springs in circuit breakers, switchgear and relays, which must maintain contact force for decades of service.
- Precision instruments and hardware: springs in timing devices, measuring instruments, locks and couplings that require minimal force drift.
- High-temperature industries: springs in boilers, turbines, valves and exhaust systems, verified by elevated-temperature relaxation testing.
- Quality control and product certification: relaxation data are used to validate material selection, shot peening and presetting processes, heat treatment quality and design margins during type testing or incoming inspection.
FAQ
What is the difference between spring relaxation and spring creep?
Creep is the gradual increase of deformation when a constant load or stress is applied, while relaxation is the gradual loss of force when the spring is held at a constant deflection or strain. Both reflect time-dependent plastic deformation of the material, but they describe different boundary conditions. Most installed springs operate at fixed length, so relaxation rate is the more relevant acceptance parameter.
What relaxation rate is typically considered acceptable?
Acceptance limits depend on the application and the governing standard or drawing. Precision components such as engine valve springs are often limited to load losses of a few percent after extended holding at operating temperature, while general engineering springs may tolerate up to about 10 percent. The specific limit must always be agreed against the product specification.
How long does a relaxation test take?
The holding time is defined by the specification and commonly ranges from 24 hours to 1000 hours. Short-term screening tests of 24 h to 100 h are used for routine quality control, while long-duration tests are used for qualification of safety-critical springs and for generating relaxation curves used in design.
Can the test be performed at elevated temperature?
Yes. Elevated-temperature relaxation testing uses an environmental chamber to hold the spring at the specified temperature, typically between 100 °C and 400 °C, throughout the test. This is essential for valve springs, turbine-related springs and other thermally loaded components, because relaxation accelerates significantly with temperature.
How should specimens be prepared before testing?
Specimens should represent the final production condition, including all heat treatment, shot peening and presetting operations. They must be visually inspected, dimensionally characterized, and conditioned at laboratory ambient temperature before the initial load measurement. Damaged or corroded samples should be excluded, and a sufficient number of replicate specimens should be tested to evaluate scatter.
Which standards govern spring relaxation rate testing?
Commonly referenced documents include GB/T 23935, the GB/T 1239 series, EN 13906, EN 16983, ASTM A125 and JIS B 2704, together with customer drawings and technical specifications. Where a relaxation requirement is not explicitly standardized, the test height, temperature, holding time and acceptance limit are agreed between the purchaser and the accredited third-party laboratory performing the test.