Chainsaw guide bars are during sawing subject to severe stresses due to vibrations, sawdust intrusion, insufficient lubrication etc. One situation with high risk of failure which has recently become very urgent for chainsaws mounted on forest harvester vehicles, is when a tree trunk sawn through leans backward, jamming the guide bar. In a corresponding situation with a hand-held chainsaw, this is often avoided by pressing wedges into the kerf behind the guide bar.

When a guide bar is jammed, the side plates of the bar are strongly pressed against the sides of the nose sprocket.

When the sprocket then continues rotating, the side plates and the sprocket will be so heated by friction that they will be deformed, softened or welded together, thereby damaging the guide bar.

The invention is a design of the sideplates of the guide bar, which greatly decreases the risk of such damage.

Description Investigation of the sequence of events when a guide bar is damaged following overheating at the nose sprocket has shown that when the central parts of the sprocket have been heated to a sufficiently high temperature, the sprocket suddenly buckles to a conical dish shape, the convex side being the hottest. This leads to even higher pressure against the hottest side, and lower pressure at the cooler side. On the cooler side the sprocket teeth will press harder against the sideplates, but they do not influence the deformation since they are cooled by the ambient air and since they cannot produce tangential stress. The conical deformation will thus grow even worse until the bearing or the rubbing surfaces are damaged, even if the jamming of the guide bar is

released.

It is earlier known to make the sideplate surfaces facing the sprocket with a shape to vary the clearance between the sprocket and the sideplate. The patent US 4,071,953 describes a guide bar where the clearance is greater near the center than at the sprocket teeth, in order to allow a limited conical deformation, but this arrangement leads to unstable and deleterious operating conditions for the sprocket roller bearing. The patent US 4,159,569 describes a guide bar where the clearance between sideplate and sprocket is smaller near the center than at the sprocket teeth, also in order to allow some conical deformation. That arrangement is beneficial for the bearing but leads to bad support for the saw chain on the sprocket teeth. Both these patent describe rotationally symmetric variations of the clearance.

According to the present invention the sideplates are deformed by coining or other suitable method to make the clearance between sideplates and sprocket greater at the rear half of the sprocket closer to the attachment end of the guide bar, and smaller at the front half of the sprocket. This makes the front half of the sprocket stably laterally supported all the way from the loaded front half of the bearing to those teeth carrying the chain. In case of a beginning conical deformation the rear half of the sprocket can deviate to either side with no worsening of the surface friction, the bearing condition or the support of the chain. This avoids the rapid damage during jamming which occurs with uniformly small clearance, as well as the disadvantages with rotationally symmetrical variation of the clearance mentioned above.

If the sideplates according to the invention are shaped by coining, the outside of the sideplates is preferably ground flat after coining to minimize the risk of jamming.

The invention is described in more detail with reference to the figures, where figure 1 shows a lengthwise section of a guide bar according to the invention, and figure 2 - 3 the surface of a sideplate facing the sprocket in two embodyments of the invention.

As shown in figure 1 the chain saw guide bar comprises at least in its front end two sideplates (11,12) reaching to the extreme front end of the guide bar, to the left in the figure, an one shorter middle plate (10) . Between the sideplates is fastened a bearing center (14) preferably by riveting. The bearing center is surrounded by a nose sprocket (13) which is supported against the bearing center by a plurality of rollers (15) . The sprocket (13) is at its periphery (20) provided with teeth to carry a saw chain. The figure shows a sprocket which is conically deformed by overheating, the front part of which is laterally guided by adjoining front parts of the sideplates, while its rear part, showh to the right in the figure, is freely deviating from the plane of symmetry into one of two recesses (16,17) formed in the sideplates (11,12) without touching the bottom of the recess. The outsides (18,19) of the sideplates are ground flat. The recesses need not be uniformly deep, but can be shallower near the bearing center (14) .

Figure 2 shows one embodyment of the front end of a sideplate (11) along the contour (21) of which a saw chain may run. At the extreme front end the chain is carried by teeth at the periphery (20) of the sprocket (13) . Dashed lines in the figure show where the middle plate (10) is to be mounted (22) and where the bearing center (14) is to be attached (23) . The recess (16) is located between the middle plate and the bearing center and is shaped like an arc encompssing at least 90 degrees and at most 200 degrees as seen from the midpoint of the bearing center. In this embodyment the recess (16) does not reach the contour (21) which ensures unbroken support for the saw chain along the contour.

Figure 3 shows an embodyment where the recess (17) reaches the contour (21) at least at one end (24) where it opens through the chain support surface. This minimizes the risk of plugging the recess by sawdust particles brought into the recess by the chain. To minimize the interruption of support for the chain, that end (24) of the recess should preferably be located in the region where the chain is carried by the sprocket teeth, farther to the front than the bearing center midpoint. In this embodyment the recess (17) encompasses at least 180 degrees and at most 240 degrees as seen from the midpoint of the bearing center.

Even if the recesses (16,17) are preferably formed by coining, other methods can also be used, such as milling or etching. The maximum depth of the recesses should be from 0.2 to 0.5 mm to allow some conical deformation of the sprocket while not unduly weakening the sideplates.