A spring impact hammer is a single-purpose device: store a known amount of energy in a compressed spring, release it, and let a defined striker hit the sample. IEC 60068-2-75 defines the geometries and the energy steps commonly used for IK code testing. The problem is that the number engraved on the body is the design value, not necessarily the delivered value after two years on a test bench.
Where the energy goes
Nominal energies in the standard series include 0.2, 0.35, 0.5, 0.7 and 1 J (with higher values used for other applications). The spring is compressed by a fixed travel, so the stored energy is proportional to the spring rate times the square of that travel. Anything that changes either term changes the impact.
- Spring fatigue or set: the free length shortens over thousands of cycles, so the same cocking travel produces less preload.
- Friction in the guide: dust, dried lubricant or a scored striker tube converts stored energy into heat before the head reaches the sample.
- Release mechanism wear: a rounded sear or a sticky trigger lets the striker drag as it releases.
- Striker condition: a chipped or deformed head changes the contact, even if the delivered energy is correct.
- Cocking travel: if the operator does not push fully to the stop, the energy is lower and varies shot to shot.
None of these produces an obvious symptom. The hammer still clicks, the sample still gets hit, and the report still says 0.5 J.
Verifying without sending it away every month
Formal calibration belongs in a lab with a pendulum or an energy reference device, and it should be on the equipment register like any other measuring instrument. Between calibrations, a simple repeatability check catches most drift: fire the hammer a fixed number of times into a reference surface, and compare the mark or the response of a reference sample against a baseline recorded when the hammer was known to be good.
| Check | Frequency | Action if it fails |
|---|---|---|
| Cocks to the stop, clean release | Every test session | Clean and re-lubricate the guide |
| Striker head condition | Every test session | Replace head |
| Reference-mark comparison | Monthly or every 500 strikes | Quarantine and send for calibration |
| Formal energy calibration | Annually, or per lab procedure | Adjust or replace spring assembly |
Keep the baseline sample. A hammer that produces a slightly shallower mark this month than it did six months ago is telling you something useful, even if the difference is within the tolerance you would accept on a single measurement.
Set-up errors that look like hammer errors
The hammer must be held perpendicular to the test surface and pressed firmly enough that the release cone sits flat before firing. On a curved or ribbed enclosure, a slight angle changes both the contact area and the effective energy delivered to the point of interest. Use the appropriate support fixture, and mark the impact points so the required number of strikes is distributed as the standard specifies rather than clustered in one convenient spot.
Sample temperature matters too. A polycarbonate cover tested at 5 °C behaves differently from one at 23 °C. Condition the samples and record the temperature; a brittle failure in a cold workshop is not a fair result for a product intended for indoor use.
What a defensible record looks like
Record the hammer serial number, nominal energy, calibration due date, number of strikes applied, sample temperature, support method, and the outcome at each impact point. When a customer challenges an IK rating, the question is rarely about the standard. It is whether the energy stamped on the report was the energy actually delivered on the day.
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