Arrangement for achieving a reciprocal motion in particular for a chain saw

An arrangement that is turnable around an axis of rotation (15′), arranged to achieve a reciprocal oscillatory motion (“S”) of a guide bar (5) in a chain saw arrangement (1), which chain saw arrangement (1) is supported by a harvester unit (100) for cutting of timber (103), which oscillatory motion (“S”) is performed in a plane (“P”) of the guide bar, the arrangement comprising:

• a bearing arrangement (10);
• two shaft parts, the two shaft parts being arranged to interact axially and are, via the bearing arrangement (10), connected and rotatably arranged, about said axis of rotation (15′), in relation to each other,
• the two shaft parts comprising a stator part and a rotor part arranged as i) an outer stator part and an inner rotor part or ii) an inner stator part and an outer rotor part;
• a pump unit (8) that, using an imparted hydraulic pressure, turns the rotor part in relation to the stator part and thereby turns the guide bar (5);
• wherein the stator part is arranged to engage with the rotor part,
• wherein the stator part comprises two outer discs (11, 12) and an intermediate disc (13),
• wherein the rotor part comprises a disc (14) axially arranged between the two outer discs (11, 12) and arranged to cooperate with the intermediate disc (13),
• wherein the two out discs (11, 12), the intermediate disc (13), and the disc (14) of the rotor part are arranged in parallel and centered about the said axis of rotation (15′),
• wherein the two out discs (11, 12), the intermediate disc (13), and the disc (14) of the rotor part are arranged so that the intermediate disc (13) and the disc (14) of the rotor part together form at least one cavity (13″) between the two outer discs (11, 12),
• wherein the pump unit (8) is arranged to impart the hydraulic pressure in said cavity (13″) so that the disc (14) of the rotor is thereby turned in relation to the intermediate disc (13), such that hydraulic fluid is then allowed to penetrate between common sliding or bearing surfaces (110-116, 131, 141) between the stator part and the rotor part.