High‑Frequency Mechanical Impact (HFMI) inherently produces vibration because the process relies on repeated local impacts at the weld toe. This is a known drawback. The practical questions are: how large is the vibration exposure in real work, and how can we minimize the risk?
Real‑World vs. Declared Vibration Values
A key limitation is that declared vibration values for hand‑held tools—measured in laboratories according to EN ISO 28927—do not fully describe actual hand–arm vibration (HAV) exposure during real work. Laboratory type‑tests are conducted under standardized conditions (ideal posture, controlled feed force, new components, consistent loading). In contrast, field use involves higher grip forces, variable working positions, and harder or uneven materials—all of which affect the vibration transmitted to the operator.
Across field measurements on percussive pneumatic tools (e.g., chipping hammers, riveting hammers, impact scalers), real‑world vibration magnitudes often measure ~1.5–2.5× higher than declared values. This “factor‑of‑two” gap is widely recognized in occupational health practice and by tool manufacturers. Also for Weld-Hits HFMI equipment, this factor is valid, and real vibration values vary from 3–7 m/s² depending on the pressure setting.
Weld‑Hit HFMI: Vibration in Context
Hand–arm vibration is a key factor in metal fabrication and construction. Weld‑Hit’s typical real‑world HFMI levels of 3–7 m/s² place the process in a middle range: clearly lower than many traditional percussive tools, and comparable to common grinders, drills, and impact drivers used daily. This matters for perception: while HFMI is “impact‑based,” its HAV profile is not extreme in the broader tool landscape and can be managed with established exposure‑control routines.
Exposure Points System
Many organizations use EU:s exposure point system that maps A(8) exposure to points:
- Exposure Action Value (EAV) – 2.5 m/s² A(8) = 100 points
- Exposure Limit Value (ELV) – 5.0 m/s² A(8) = 400 points
As a rule of thumb (for a given tool setting), exposure points scale with time and roughly with the square of vibration magnitude. Staying well below 100 points keeps you in the green zone; between 100–400 points requires documented controls; ≥ 400 points must be avoided.
Example Use Case — Production Scenario
A customer needs to treat 60 short welds (~7 cm each) during a shift. With typical HFMI parameters, this results in ~7 exposure points, leaving a large margin to the action level of 100 points (see points system below). Work can also be shared among several workers to reduce individual exposure further.
The marked area in the figure below corresponds to the zone if one worker performs all the treatments. Higher working pressure gives higher vibration values but shorter treatment time. Making the choice of parameters quite neutral, but process control is also essential and important for ergonomics and the working environment.
Process recommendations for control and ergonomics:
- Air pressure: Start at 3.0–3.5 bar. This provides good process control, reduces required operator force, and limits peak vibration—an ideal baseline for training and consistent results.
- Technique: Keep a relaxed grip, maintain a stable stance, and brace the tool effectively; avoid unnecessary force.
- Maintenance: Ensure that the tool tips are in good condition; keep the pneumatic supply clean and stable.
Measurement note:
For this case, a field vibration sensor was used to record total exposure. The same type of sensor can be purchased and used for periodic on‑site verification of operators’ cumulative HAV exposure.
Recommended Usage of Weld‑Hit HFMI Equipment
A good working environment starts with knowing where HFMI adds value, typically transverse fillet welds relative to the main stress, at attachments, brackets, and stiffeners. Treatment is applied at one weld toe against the main member, with a typical local length up to ~200 mm (bridges and very large structures may require longer treatments). For most structures, 2–10% of total weld length benefits from HFMI.
Weld‑Hit recommends viewing HFMI as a process integrated with welding. Perform it continuously during welding (same position and setup), rather than batching all HFMI at the end of a shift. This minimizes re‑clamping, improves quality consistency, and supports higher overall efficiency.
Two planning examples
- Manual welding
Assume an arc‑on factor of ~35% and 10% HFMI treatment length. A typical shift yields ~6 m of HFMI treatment—about 12 minutes trigger time—corresponding to ~8 points. Well inside the green zone. - After a welding robot
Assume a robot arc‑on factor of ~70% and 10% treatment length. You may reach ~16 m of HFMI per shift—about 32 minutes trigger time—corresponding to ~24 points and still clearly in the green zone.
These are conservative “worst‑case” scenarios from a vibration standpoint—and remain far from the 100‑point action level.
Recommended Operating Envelope
We recommend staying in the dark green zone with < 65 points per operator and shift. The table below summarizes typical relationships between air pressure, working speed, vibration level, and allowable time/length. Max 65 points corresponds to 40 meters, which should be more than enough for all cases when it is integrated with welding and applied only to critical areas of the structures.
Reference: EU guidance on worker protection from mechanical vibration