Background _a Chainsaws are traditionally made with guide bars, where the

5 saw chain runs with the drivelink tangs in a groove along ^ the perimeter of the guide bar and is carried at the front end of the guide bar by a toothed sprocket inserted in the guide bar. The guide bars are either solid made from a single steel plate with a milled or ground groove, or 10 laminated made from three thinner plates joined by spot welding, with the groove is created by making the middle plate smaller than the side plates. The guide bars are usually hardened along the edges to improve the wear resistance where the chain slides along the guide bar, and 15 unhardened or only slightly hardened between the edges.

It has been shown, however, that guide bars of the known types when used in vehicle-born tree harvester machines are easily damaged if the grip of the machine around the tree

20 trunk is not firm enough. A common type of damage is when the guide bar is bent near the end attached to the machine, and it is often difficult or impossible to straighten such a guide bar, because of cracks in the hardened edge and fractures near the spot welds, where hard brittle regions

25 border on soft heat affected zones.

In patent US 5,052,109 is taught that soft annealing of a zone across the width of the guide bar near the end attached to the machine will concentrate any bending to this zone, 30 where the hardness both at the weld spots and at the previously hardened edges is low enough to avoid fracture, and that such a guide bar is easy to straighten after it has

, been bent. Disadvantages with this method are that the force a guide bar can withstand without bending is lower, and that _t 35 the edges may become wrinkled during the concentrated bending and difficult to get smooth when the rest of the bar is straightened.

In patent US 4,965,934 is taught how the weld spots of a laminated guide bar can be made ductile without annealing or with a low temperature annealing that does not diminish the hardness imparted to the side plates during previous hardening. Disadvantages with this method are that some risk of cracks at the edges remains and that the lower middle plate hardness lowers the stiffness of the bar.

The present invention concerns a guide bar where the risk of cracks at the edges is eliminated without lowering of the bending resistance, and where any bending will not be so concentrated that the edges could get wrinkled.

Description of the invention

The invention is described with reference to the figure, which shows a side view of a guide bar comprising one attachment end (11) with hole for fastening bolts and oil supply, one central part (12) with very slightly curved edges (13) , one front end (14) and one toothed sprocket (15) inserted with its bearing into the front end. With vehicle born tree harvesters the lubrication of the saw chain (10) is usually so well controlled through internal oil channels in the guide bar that there is no appreciable wear on the slightly curved edges (13) of the central part (12) , which in consequence does not need as high hardness as is traditionally specified. The wear on the edges is concentrated to those regions where the saw chain (10) impacts the edges after traveling a shorter or longer path without support from the edges, since high peak forces occur and lubrication can not easily be arranged there. Such regions are where the chain part coming from the drive sprocket first impacts the guide bar edge (16) at the attachment end (11) , and where the saw chain after having traversed the front end (14) while elevated and supported by the sprocket (15) settles again on the edge (17) at the front end. Within these limited regions (16,17) the edge should be considerably harder than the slightly curved edges

make the guide bar reversible, the edge should preferrably be made with the higher hardness also in the symmetrically located regions (16a,17a).

At the front end (14) the bending moment is small since any forces are at a short distance, and at the attachment end (11) no bending is possible since it is clamped between rigid blocks. Thus the edge in the regions (16,17) can be harder and more wear resistant than the rest of the edge without risk of edge cracking.

When no cracks are initiated at the edge, the rest of the guide bar can be made harder than usual, lowering the risk of concentrated bending and edge wrinkling considerably. Suitable hardness values are for the regions with highest hardness (16,17) 60-64 and for the rest of the guide bar 48- 53 according to the Rockwell C scale. Traditional hardness values for guide bars where the whole length of the edges are4 hardened is 59-61 for the edges and 42-47 for other parts of the guide bar.

Before regional edge hardening, the entire guide bar is preferrably hardened and tempered to the hardness 48-53 HRC. For laminated bars this is done after spot welding, thus eliminating any heat affected zones near the weld spots. Alternatively, the guide bar can be made from two pre- hardened side plates and a boron alloy middle plate as taught in US 4,965,934.

The edge hardening of the hard regions (16,17) can be done with a gas flame or with inductive heating in known ways. According to the invention, the length of each hardened region (16,17) should not exceed one third of the length of the entire guide bar.