Abstract: In a method, when using a single-grip harvester head for felling and processing trees, of controlling the press power which, by means of a limbing knife assembly (6), is exerted on a stem (3) which is horizontally fed through th head with its butt end first, the diameter of the stem is measured and the press power is reduced in accordance with a control diagram as the diameter decreases during the feeding. The measured diameter values are registered, and each measured diameter value is compared with a predetermined diameter value which is selected from the diameter values of the stem (3) which are previously registered during the feeding of the stem. If a measured diameter value is found to equal or be greater than this predetermined diameter value, the press power is increased to a corrective power value, which is maintained until a later measured diameter value falls below a reference value.

Abstract: A single-grip harvester head comprises a frame (1, 2), which defines a tree feeding path, and two feeding roller arms (5), which are pivotable in relation to the frame (1, 2) about a pivot axis (A1) each defined by a pivot pin (6) and which support a rotatable feeding roller (7) each. Each feeding roller arm (5) is formed as an angular lever with two lever legs (5a, 5b), the feeding roller (7) being supported at the free end of one of the legs (5a) and the pivot pin (6) of the feeding roller arm (5) being positioned in the angular area between the legs. The two pivot axes (A1) are located in a common plane perpendicular to the tree feeding path. Each pivot pin (6) is articulated to the frame (1, 2) at one of its ends by means of a ball joint (13) and controlled at its other end, in such a way that the pivot axis (A1) is movable in said plane but fixed in a direction perpendicular to this plane.