Feed force measurement – Simply know what nobody sees – Advantage through technology

High accuracy for hard wood and early wood decay

The feed force is the force required by the drilling needle to penetrate the wood. Besides the drilling resistance it is the second measured variable of the IML-RESI PowerDrill® and provides additional information about the condition of the wood inside. While the friction at the drilling needle shaft (shaft friction) can strongly influence the drilling resistance, it has only a minimal influence on the feed force. This means that the feed force together with the drilling resistance allows very realistic results to be achieved for the evaluation of the wood.

Display of the feed force as an additional measurement curve

The IML analysis software displays the feed curve together with the drilling resistance curve in a graph. Both curves are color-coded. For a precise evaluation of the measurement results, it is important to interpret both curves in relation to each other and not just one measurement curve in itself.

Without feed curve, shaft friction can hide wood defects

The friction of the shaft becomes greater the further the drilling needle presses into the wood. This influences the drilling resistance curve and causes the curve to rise steadily. The impact of the shaft friction cannot be separated from the actual drilling resistance. Consequently, the measurement curve of the drilling resistance curve remains rising even if the drilling needle penetrates a wood defect, although the drilling resistance decreases.

Measurement of the feed force provides high information value

In certain cases, only a comparison with the feed curve can show how strongly the measured drilling resistance curve is affected by the shaft friction and how large the damage in the wood actually is. This is because the feed curve drops significantly in the case of a wood defect and remains largely unaffected by shaft friction. In this way, especially the wood decay in the early stages becomes visible by the feed curve. Finally, the changes in the drilling resistance at that point in time are still so small that they can easily be covered by the shaft friction in the drilling resistance curve. The measurement of the feed force is also especially advantageous for hard wood species. This is because the shaft friction is higher with hard woods than with soft woods and therefore has a stronger influence.
The additional measurement of the feed force makes it possible, especially with hard woods, to detect wood decay much earlier and more precisely than the drilling resistance measurement alone.

Case studies

The following case studies are intended to illustrate the easy identification of wood decay through the synchronous recording of the drilling resistance and the feed force.

Example 1: Tropical hardwood with wood defect

The drilling curve (green) rises steadily with increasing drilling depth due to shaft friction. At a drilling depth of 35.5 cm the drilling curve falls off, indicating a wood defect. However, it is difficult to interpret the entire previous increase in the drilling curve. The feed curve (blue) drops significantly already at 31 cm, and from 35.5 cm it no longer shows any feed resistance. This indicates that timber extraction is well advanced. The drilling curve (green), on the other hand, remains clearly raised in its level due to the friction on the drilling needle shaft (at approx. 80% amplitute).

Example 2: Book with brittle cinder fungus

The drilling curve (green) does not drop significantly until 28.5 cm and indicates a larger wood defect. The feed curve (blue), on the other hand, falls off at 18 cm and indicates wood decay in the early stages up to a drilling depth of 28.5 cm. After that, the wood degradation increases significantly.

Example 3: Ash with brittle cinder fungus

The feed curve (blue) drops significantly from a drilling depth of 12.5 cm, while the drilling curve (green) remains at a similar amplitude level, which shows no further increase in the measurement curve between 12 – 14 cm.

Example 4: Robinia with heavy wood defect

The feed curve (blue) drops significantly further from 21.5 cm than the drilling curve (green), which does not drop below approx. 60% amplitude. In areas where there is almost no feed resistance (blue), there is most likely a hollow space or advanced wood decay in the trunk (e.g. brown rot). The difference between the amplitudes of the feed and drilling curves results from the shaft friction.

Example 5: Carob Tree (Alfarroba) with wood defect

From a drilling depth of 20.5 cm, the feed curve (blue) drops off significantly, while a slight drop in the drilling curve (green) is only visible at 22 cm and this is significantly less due to shaft friction. The deep feed curve (blue) indicates an advanced wood decay.


Practical experience on hard tree species has shown that the feed force measurement is only very slightly influenced by shaft friction. Especially for measurements on hard tree species in connection with an early stage of fungus-related wood decay, the feed force curve provides a large amount of information. Feed curve decreases cannot be covered by shaft friction effects as it can be the case with the drilling curve. An interpretation of the drilling resistance measurement result is considerably easier with the feed curve.

Most common areas of application

  • The advantage for a wood testing system (previously only a drilling resistance measuring device) of an additional measurement curve should be clear to everyone by now at the latest.
  • Tree inspection
  • Wood pole inspection
  • Construction wood inspection
  • Wooden playground equipment inspection
  • Truss Inspection

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