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Grapple Head Damage to Sawlogs and its Impact on Processing

Grapple processors are being used extensively in harvesting Radiata pine in NZ. In order to harvest the increasing volumes of trees, harvesting technology that offers higher efficiency and reduces the safety risks to forestry workers will become the norm – grapple processors are in this category.

Grapple processors grasp a tree, cut it off at the stump, lower it to the ground and then delimb the stem as the processor head moves its way from one end of the stem to the other. In so doing the feed rollers can create surface damage to the stems; this damage taking many forms such as incisions and bruising to the surface and immediate subsurface of the logs.

The extent to which grapple processors damage logs and thereby reduce their value in wood processing operations is largely unquantified in NZ. The issue though is becoming increasingly important as more grapple processors are operating and with many different types of feed rollers.

The pictures below demonstrate the damage caused by grapple heads. On the left the log surface damage that is becoming more commonplace and on the right, the impact this can have on the boards manufactured from near the surface of the log.

Recently SWI contracted Keith Raymond, FFR, to undertake a preliminary study aimed at identifying the scale of grapple head damage and the impact this was having on SWI processing companies.

A survey of eight SWI manufacturing shareholders, who together process around 1.1 million cubic metres of logs per annum, showed that grapple head damage ranked moderately with regard to log quality problems but as there is no common specification for log damage. Current specifications are not written to include typical grapple roller damage and comparison of responses was difficult.

Grapple head damage results in:

  1. losses due to damage on opening board faces (typically around wane but sometimes across full faces),
  2. deep gouging reducing conversion to sawn lumber and
  3. increased sap stain incidence.

The questionnaire showed that:

  • sawlogs tend to have more damage than pruned logs and that larger knot sawlogs get more damaged than smaller knot-size saw logs
  • that more damage is assessed as 'high' on larger pruned logs (which may be due to branch size in the first whorl above the pruned zone)
  • that higher bark removal from grapple head harvesting resulted in logs drying out faster and for debris to attach to the cambium which ultimately dulls saws.

The cost of grapple head damage to the SWI processing companies was conservatively estimated at between $6 and $14 million dollars based on conversion losses ranging from 0.2 to 0.5%.

SWI will be undertaking further work in this area with a view to developing better and more meaningful specifications for grapple head damage so that growers and processors can speak a common language in dealing with the issue.

Suppliers can take proactive steps to minimise grapple head damage such as optimising landing layout, checking operating pressures and sharpness of feed rollers, matching roller types to the material being handled, improving operator awareness and technique to avoid damage, and updating equipment.