A die spring may meet its initial load specification and still lose return force, develop wear, or crack after repeated use. For buyers, the useful question is whether a fatigue test represents the tool being purchased for. A cycle count without working heights, fixture details, and acceptance criteria gives an incomplete basis for comparing suppliers.
This guide explains what to agree before die spring fatigue testing, what evidence to request, and how to turn the result into a purchasing decision. The equipment images and video show the test setup. They do not establish a rated life or load for an unspecified spring.
Define the purchasing decision the test should support
Start by stating the purpose: qualify a new specification, compare candidate springs, or investigate a field failure. These purposes need different evidence. Qualification requires an agreed operating condition and acceptance plan. A comparison requires equivalent conditions. Failure analysis also needs information about the original installation and the damaged sample.
Static inspection and cyclic testing complement each other. Static inspection establishes initial dimensions and force at specified heights. Cyclic testing follows changes after repeated compression. A passing initial load check does not by itself demonstrate performance throughout service.
Review the die spring range against the tool envelope first. If space or load requirements call for a special design, include the operating conditions in the request for custom springs. This gives the supplier a clearer basis for a technical response.
Specify the complete operating envelope
Separate preload from working travel
Use three heights with an agreed meaning: L0 is free length, Li is installed height at the beginning of the working stroke, and Lw is the minimum working height. In the same units:
- Preload deflection: L0 − Li.
- Working travel: Li − Lw.
- Maximum total deflection: L0 − Lw, equal to preload deflection plus working travel.
If total deflection is expressed as a percentage, state the reference length: (L0 − Lw) ÷ L0. Machine stroke may exclude preload or include clearance elsewhere in the mechanism. Confirm the spring's actual height range rather than copying machine travel into the spring specification.
Check permitted travel against the particular spring series and manufacturer data, including clearance from solid height. There is no universal operating percentage for every die spring. Color alone cannot establish a load class, standard, or interchangeability. For dimensions, specify wire diameter for round wire and section width and thickness for rectangular wire.
Include guidance, frequency, and environment
Describe the guide post, bore, or mandrel; support surfaces; and any possible side load. State cycling frequency, dwell periods, inspection intervals, lubrication, temperature, and exposure to dust or corrosive media. An increased test frequency may change heating or dynamic behavior, so an accelerated result needs a justified relationship to service conditions.
For exposed applications, include the proposed surface treatment or aftertreatment in the plan. Otherwise, buyers may compare results from samples whose finish or environment differs from the component they intend to order.
Agree acceptance criteria before cycling begins
The buyer and supplier should agree the target cycles, sample quantity, inspection points, measurement methods, and stop criteria before testing. Identify each sample and its batch. If several springs share a fixture, identify their positions as well. This makes an unusual result traceable.
Set permitted height and load changes from the application requirements, product information, and the agreed verification plan. Avoid adopting an arbitrary pass limit from an unrelated spring. Where a result approaches the acceptance boundary, consider measurement repeatability before declaring a change significant.
| Evidence to request | What the record should establish | Why it matters |
|---|---|---|
| Initial and staged free length | Measurements after consistent unloading, rest, and temperature conditions | Separates lasting height change from inconsistent measurement conditions |
| Load at agreed heights | Initial and repeat readings using the same method and verified instrument | Shows whether required force is retained |
| Appearance and contact condition | Cracks, wear location, corrosion, coating damage, and end contact | Connects a change to physical evidence |
| Fixture and cycling log | Guidance, completed cycles, frequency, interruptions, and adjustments | Defines the conditions under which the result is valid |
Height loss, load loss, and cracking are separate observations. Permanent set is not automatically the same failure mode as a fatigue crack. A report should state which criterion was reached, when it was reached, and what evidence supports the diagnosis.
Follow the agreed stopping procedure for cracking, breakage, jamming, or fixture instability. If the target is completed without an observed failure, report that outcome within the tested conditions and duration. Do not convert it into an unsupported guarantee of a longer service life.
Use fixture evidence to interpret the result
Photos should show platen alignment, end support, and the relationship between the springs and their guides. Missing guidance, uneven contact, or side friction can dominate a test. In a multi-spring setup, one outlier may indicate a position problem that deserves investigation before the entire batch is rejected.
If a sample is moved, a guide is replaced, or a setting changes, record the reason and inspection point. Treat the later observations as data from the revised condition. Combining all readings without noting the adjustment can conceal the cause of an apparent improvement.
For one-sided wear, review guidance, parallelism, and contact first. For free-length loss, verify total deflection and measurement conditions. For end cracking, retain close-up images and review contact or handling damage. Increasing spring force before checking the installation can leave the original cause unresolved.
Turn the report into a clear RFQ and next action
A useful RFQ includes the drawing or sample, outside and inside diameters, free length, wire or section dimensions, installed height, minimum working height, and required loads. Add cycle frequency, guidance details, environment, the proposed finish, and the failure criterion that matters to the tool.
For replacement work, provide installation photos and identify whether the old sample has already lost height or suffered wear. Its measured dimensions may differ from the original design. A supplier should understand those limitations before using the sample as the sole dimensional reference.
The final report should connect sample identity, test conditions, staged measurements, photographs, and any interruptions to the decision. Possible actions include continuing verification of the current design, correcting guidance, or reviewing the spring specification. Record the reasoning so purchasing and engineering teams can refer to the same evidence.
If a design or process changes after testing, review whether the earlier result still applies. Changes in material, geometry, heat treatment, finish, or installation may require further verification. The spring winding process page provides manufacturing context for a supplier discussion; it does not replace the test record.
Frequently asked questions from buyers
Can I compare suppliers using cycle count alone?
No. Compare working heights, preload, frequency, guidance, environment, sample quantity, and acceptance criteria. Different conditions can produce different outcomes even when the reported cycle counts appear similar.
Does completing the target cycles prove the spring's maximum life?
It establishes performance only within the recorded test conditions and completed duration. If testing stopped at the target without failure, there is no measured failure life beyond that point.
Can springs of the same color be substituted directly?
Confirm dimensions, load at working heights, permitted travel, and the applicable product series or standard. Color systems vary and cannot establish interchangeability on their own.
What should a report say if the fixture was adjusted?
State what changed, why, and at which inspection point. Keep the observations before and after the adjustment identifiable, so the result is not presented as one uninterrupted test under unchanged conditions.
Can an old spring sample support an RFQ without a drawing?
Yes, as a starting point. Include installation dimensions, required loads, photos, and the sample's damage history. Agree the original design requirements before treating worn or deformed dimensions as a replacement specification.
Discuss the application with Dingli
Effective die spring fatigue testing links the tool's operating envelope to repeatable measurements and agreed acceptance criteria. Buyers gain more useful evidence when they request working heights, fixture details, load retention, and observed failure modes alongside the cycle count.
Cixi Dili Spring Co., Ltd. has worked in spring R&D, production, and sales since 1995 and can discuss custom requirements from samples or drawings. Send your drawing, installed and working heights, load requirements, and available test or failure records to jackchen@dinglispring.com, or contact us on WhatsApp +86 13586942004, to review the application.
