# Lifting height & Felling force

## The lifting height of the felling cut

The lifting height is the distance that the felling tool must push apart the felling cut before the tree falls on its own accord. The diagram shows the required lifting height for different tree diameters, including the saw cut. The values apply to Nordic tree species and trunk forms and should be considered as a guide, not as exact figures. Using the diagram you can, for example, see if you need several wedges in height to bring down the tree.

## Tree felling force

A felling tool must be able to provide the power needed to make the tree fall. The power, multiplied by the distance to the pivot of the hinge forms the effect known as felling force. The felling force that is required for the tree you plan to fell determines the felling tool you should select. The diagram will help you estimate the felling force and select the correct felling tool for complex felling work. The values apply to Nordic tree species and trunk forms and should be considered as a guide, not as exact figures.

The numbers indicate the maximum values for professional foresters, healthy trees and calm weather. The graphs assume that the hinge is of a suitable thickness and the depth of directional notch is about 20 % of the trunk diameter. You should also take into account a potential asymmetrical crown, curved trunk, the wind and the weight of any snow on the branches when selecting felling tools. The diagram has been produced by the Swedish forest researcher Tomas Gullberg at the University of Dalarna, Sweden.

## How to use the diagrams

1

Measure the tree’s diameter at 1.3 m height (DBH, diameter at breast height). Use callipers or measure the circumference using a logger’s measuring tape. The exact diameter is obtained by dividing the circumference by 3.14.

2

Base the DBH on the diagram’s x-axis. Go straight up until you hit the grey curve for the lean in question.

3

Go straight out to the left. Read the felling force in the y-axis.

4

Select a felling tool that provides a greater felling force than the reading.

Example: A tree with a 45 cm DBH that leans 1 m backwards, requires a felling force of 1400 daNm. A large breaking bar provides a little over 1200 daNm, which is insufficient. A large wedge produces about 1500 daNm which is just about enough. It would be appropriate to use two large wedges producing approximately 2600 daNm.

## Felling force and lifting height for different felling tools

1

### Hand force

Only useful on very small trees without any backward lean. Provides limited felling force. By using a 5 m pole and pressing higher up from the ground you can significantly increase leverage and the felling force. Maximum lifting height: Unlimited.

2

### Breaking bar

Comes in different handle lengths, giving different amounts of felling force. Avoid breaking bars with a long handle for very thick trees and large backward leans, as this may require a lifting height that is greater than you can handle. Maximum lifting height: Approximately 2 cm.

3

### Felling wedge

Provides very large felling force for thick trees, much less for narrow trees due to the shorter leverage. For demanding felling, you will need to knock in more wedges, first along the width and then in height. Maximum lifting height: Approximately 3 cm per wedge.

4

### Tree jack

A kind of mechanical jack that presses against the trunk. Provides a lot of felling force and is therefore a good choice for extensive felling work for medium-sized complex trees. Maximum lifting height: Unlimited.

5

### Winch and rope

The safest method for bringing a tree down in a specific direction. Works on both large and small trees. The felling force is affected both by the pull of the winch and the height of the fastening point. By attaching a winch 5 m up the trunk and pulling at 5000 N, you produce a felling force of 25 000 Nm. Maximum lifting height: Unlimited.

## Factors that affect the hinge

In advanced felling situations, the hinge may be subject to greater stress than at normal felling. When using felling wedges, the hinge is also subject to stress both along and across the grain. To maintain control over the felling direction, the hinge is designed to ensure that it is kept intact throughout the whole felling operation. You should therefore also consider the properties of various tree species. During winter, keep in mind that frozen timber is less flexible and breaks earlier. Decayed wood is also weaker than healthy wood.

Last edited

March 14, 2023