CHAINSAW GUIDE BAR

Abstract: Chainsaw guide bar with a nose sprocket between two side plates (15), a longitudinal groove (11) on each longitudinal edge and a curved groove (14) around the nose through which the sprocket teeth protrude, where the thickness of the central part (19) of the sprocket from the bearing to and including the lower part of the teeth is considerably larger than the width of the longitudinal grooves (11).

Description: CHAINSAW GUIDE BAR Background Guide bars for chainsaws are commonly made with a toothed sprocket between two side plates to reduce chain friction as the chain turns around the nose ofthe guide bar. Because of the high chain velocity and insufficient lubrication in an environment with vibrations, sawdust and resin, the nose sprocket is prone to damage and in many cases limiting for the guide bar durability. Among the types of sprocket damage three types are especially relevant. Because of high chain velocity and high chain tension on vehicle-born chainsaws. the sprocket bearing has a much higher temperature in service than on hand-held chainsaws, and traditional bearing designs may be insufficient. Apart from the risk of bearing failure, the high tension force combined with the high bearing temperature may cause thermal buckling of the sprocket to a conical or disc shape. Such buckling will cause rubbing friction against the side plates, causing more heat and more buckling until the sprocket disintegrates or welds to the side plates. A third problem is that the side plates are often squeezed together when a tree trunk has been cut through by a vehicle-born chainsaw. This is most noticeable at the guid bar nose and will often cause the sprocket to stop and the chain to break. The invention concerns a guide bar with less risk of overheating ofthe sprocket bearing, with less risk of heat buckling ofthe sprocket even if the bearing is heated, and with less risk of chain breakage if the guide bar is squeezed between parts ofthe tree trunk. Description A guide bar according to the invention is shown seen from a point slightly off the plane ofthe bar in figure 1. The guiode bar har on each longitudinal edge two slide rails (12) and between them a longitudinal chain groove (1 1) with a thickness slightly exceeding the thickness of the drive links ofthe saw chain, and with a depth large enough to keep the drive links from scraping the groove bottom. When the saw chain is moving, the drive link will be adequately guided by the groove, while any forces in the plane ofthe CONFIRMATION PY guide bar will act on the contact surface between the side links ofthe chain and the slide rails (12), at least as long as the longitudinal edges are slightly convex. At the nose ofthe guide bar, the chain is carried in the plane ofthe guide bar by contact forces between the drive links and a toothed sprocket, the teeth (13) of which run in a curved groove (14) between two side plates (15). The sprocket is carried by a roller bearing with a bearing center riveted between the side plates. With vehicle-born chainsaws. the part ofthe chain travelling around the nose is rarely used for cutting, and has less need for accurate guiding than when it travels along the longitudinal sides. According to the invention the thickness ofthe nose sprocket including the part adjoining the bearing as well as the lower part ofthe teeth (13), as well as the width of the curved groove (14) can be chosen to get the optimal durability ofthe sprocket, exceeding the thickness ofthe drive links and the width ofthe longitudinal groove (11) preferably by 10 - 50 % . This choice reduces the friction between the drive links and the side plates (15) and reduces heat creation in the nose region. Through the lateral mobility ofthe chain, the wear ofthe sprocket teeth (13) will be spread over a larger width and less harmful. Another advantage is that the roller of the roller bearing can be made longer than with a sprocket of traditional thickness, thereby reducing stress, heating and wear. This has been previously suggested in patent US 3,263,715, where the sprocket was made from an inner thicker part and an outer thinner part as thin as the drive links. This two-part design will reduce stress and wear ofthe bearing, but will not reduce friction between drive links and side plates, and will increase the stresses from radial wedging in the outer sprocket part. A third advantage ofthe invention, compared to traditional thin sprockets as well as two- part sprockets according to US 3,263,715 is that the resistance to heat buckling is greatly increased, making it unlikely that incipient buckling due to bearing heating will lead to friction against the side plates and buckling failure. A fourth advantage ofthe invention is that the transition zone (16) between the curved groove (14) and the longitudinal grooves (1 1) will be gradually wider than the longitudinal grooves. When a tree trunk is just sawn through, its weight shifts from the stump to the feller head ofthe tree harvester vehicle, and there is a great risk that the guide bar is jammed and squeezed between the trunk and the stump. In the region between the guide bar body and the nose sprocket bearing the side plates are easily deformed since there is no support between them. The increased space for the chain in the transition zone (16) makes it less likely that the chain will be obstucted by such deformation. The groove in the transition zone is preferably made with a gradually decreasing width towards the longitudinal groove, to make sure that the drive links enter the longitudinal groove (11) without impacts. Figure 2 shows a cross section of one embodiment of the invention where the nose sprocket is made as one integral part with constant thickness in the central part (19) from the bearing race (17) to the lower part of the teeth, considerably thicker than the width of the longitudinal groove (1 1). The outer part ofthe teeth (13) have a thicknesss intermediate between the thickness ofthe central part (19) and the width of the longitudinal groove (1 1). In this case the side plates (15) are profiled with the edge regions forming the curved groove ( 14) offset inwards to make the groove narrower than the thickness ofthe central part (19) ofthe sprocket. The groove width and the bearing thickness can then be optimized independently. Unequal thickness may, however, make it difficult to exchange a worn or damaged sprocket, but could preferably be chosen for such guide bars where the whole nose is replaceable as a unit, including sprocket, bearing and nose side plates. For guide bars with non-replaceable noses it is preferable to make the side plates flat and the sprocket with equal thickness all the way to the side plate edge. The sprocket teeth serve only to carry the drive links to lift the saw chain a certain height above the side plates. The teeth should not contact the side links, since the contact forces could cause friction between sprocket and side plates. The teeth should thus be either so short that they terminate below the side link contour as described in patent CA 863 686, or with thin tapered tips which can enter between the side links as described in patent CA 981 156. The invention can with advantage be applied to guide bars of several known types. On a laminated three layer guide bar with non-replaceable nose, the front end ofthe side plates is gradually offset outwards to make room for the thicker sprocket. If desired, the outside of the side plates can be ground flat. On a solid guide bar with non-replaceable nose the curved groove and the slit for the sprocket are milled to a greater width than the longitudinal groove, with a gradual transition. A replaceable nose is made with a gradual offset outwards ofthe central part, and if needed to fit the sprocket, a partial offset inwards of those edge portions forming the curved chain groove. The offsetting lines should be curved rather than straight, to give maximum stiffness and less risk of clogging by sawdust.

Claims: CLAIMS 1. Chainsaw guide bar comprising a nose sprocket the innermost part of which is a roller bearing race (17) between two side plates (15), a longitudinal groove (1 1) on each longitudinal edge and a curved groove (14) around the guide bar nose, through which the sprocket teeth (13) protrude, characterized by the central part (19) of the sprocket,from the bearing race (17) at least to and including the lower part ofthe teeth having a thickness considerably larger than the width ofthe longitudinal grooves (1 1). 2. Chainsaw guide bar according to claim 1, characterized by the thickness ofthe central part (19) having a thickness 10 - 50 % larger than the width ofthe longitudinal grooves (1 1). 3. Chainsaw guide bar according to claim 1 or 2, characterized by the central part (19) of the sprocket having uniform thickness. 4. Chainsaw guide bar according to claim 1 or 2, characterized by the width ofthe curved groove (14) being at least 10 % larger than the width ofthe longitudinal grooves (1 1), with a gradual stepless decrease of the width in a transition zone towards the longitudinal groove. 5. Chainsaw guide bar according to claim 1 or 2, characterized by the thickness ofthe lower part of the sprocket teeth (13) and the width ofthe curved groove (14) being smaller than the thickness ofthe central part (19) ofthe sprocket, but larger than the width of the longitudinal grooves (11).

Inventors: LEINI ARVO, LEINI, ARVO
Applicants: LEINI ARVO, SANDVIK AB, LEINI, ARVO, SANDVIK AKTIEBOLAG
Application year: 1997
Main classification: B27B17
IPC codes: B27B17