Chain saw bar

Cutlery – Cutting tools – Saw

Patent

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Details

30387, B27B 1702

Patent

active

045570540

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a bar for a chain saw of the type having a peripheral groove in which the tongues of the chain are guided, and faces on either side of the groove to support the chain as it travels around the periphery of the bar.
Various constructions of chain saws bars are known wherein the nose portion of the bar incorporates a roller or sprocket rotatably mounted to guide the chain as it passes from the top edge to the bottom edge of the bar. The provision of the roller or sprocket reduces the wear on the chain and wear on the nose portion of the bar. In these known constructions the bearing supporting the roller or sprocket is usually of an axial length equal to the width of the groove in the edge face of the bar. This restriction on the bearing size limits the effective life of the bearing.
Because of the restriction on the axial length of the bearing, the diameter of the bearing should be reasonably large to provide a bearing that has a satisfactory working life. It will be appreciated that the diameter of the bearing influences the diameter of the sprocket or roller, and as the bearing diameter increase so does the nominal diameter of the roller or sprocket and so the radius of the path of the chain around the nose of the bar is also increased.
It is a recognised fact that the severity of kick-back during operation of a chain saw is increased with increases in the radius of the path of the chain around the nose. Accordingly from the point of view of safety, the nominal diameter of the roller or sprocket should be kept small. Thus the sprocket or roller diameter should be increased to improve bearing life, and should be reduced to improve safety by reducing the severity of kick-back. Accordingly in the known designs, a compromise must be made between bearing life and safety.
It is the object of the present invention to provide a bar for a chain saw incorporating a structure at the nose of the bar that will permit a bearing of the larger size to be incorporated, thus have a longer effective life, without the need for a corresponding increase in the sprocket or roller nominal diameter.
According to the invention there is provided a chain saw bar having top and bottom longitudinal edge faces along which the chain travels and a central longitudinal groove in said edge faces to receive drive tongues of the chain, and a bar nose structure at the foreward end of the bar to guide the chain from the top to the bottom edge face of the bar, said nose structure comprising an extension of the bar of less thickness than the bar and having one side face substantially co-extensive with a side wall of the groove in the bar, a roller supported for rotation by a bearing structure mounted on said extension, said roller providing an arcuate surface to guide the chain from the top to the bottom edge face of the bar and defining with the extension a groove to receive the chain drive tongues.
Conveniently the extension is of a thickness equal to the distance from said wall of the groove to the side face of the bar adjacent said wall. In this way the face of the extension, which defines with the roller the groove at the nose of the bar, is co-extensive with the internal face of the groove in the edge of the bar. Preferably the roller may incorporate a sprocket tooth formation, that is located in the groove formed by the roller and the extension, to co-operate with the chain in the conventional manner as the chain passes around the nose of the bar. The sprocket teeth may be formed on a member that is attached to the roller to rotate therewith, or may be formed integral with the roller.
As the sprocket is attached to or integral with the roller, the total thickness is increased and the diameter of the sprocket may be smaller than possible in a conventional sprocket constuction.
The nose structure of the bar enables the roller to be of a greater axial length than in previously known constructions, and therefore the bearing may also be of greater axial length than the width of the bar edge groove, which was the limitin

REFERENCES:
patent: 2929092 (1960-03-01), Carpenter
patent: 3995370 (1976-12-01), Allard et al.

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