Cutlery – Cutting tools – Saw
Patent
1998-07-06
1999-11-02
Rachuba, M.
Cutlery
Cutting tools
Saw
30384, 474156, 474157, B23D 5702
Patent
active
059746734
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
To carry a saw chain around the nose of a chain saw guide bar, a thin sprocket with teeth has been used for a long time. These carry the saw chain without friction against the edge of the guide bar nose. This causes less wear and needs less power. It also makes it possible in certain situations to cut with the upper, outward moving part of the chain.
To make a thin sprocket work with teeth so that it functions properly, it is necessary that the saw chain is carried at a well defined radius from the center of the sprocket bearing. This is accomplished if the ends of the chain drive link at its end portions are carried by the flanks of the sprocket teeth. When sawing timber with large chain velocity and coarse sawdust, it is important that there is space for sawdust between the underside of the drive links and the bottom of the gullet between the sprocket teeth, and that the drive link does not hit the sprocket in such a way that the pointed bottom part of the drive link is deformed, since its shape is vital for the distribution of lubricating oil along the path of the saw chain along the edge of the saw guide bar.
The gullet bottom has been shaped in different ways. In the patent U.S. Pat. No. 3,124,177 the gullet is V-shaped with straight flanks, with the disadvantage of concentrated stresses at the gullet bottom due to wedging action when the drive link is pushed into the gullet. In the patent U.S. Pat. No. 3,279,508 the gullet bottom is shaped as a circular arc, tangent to straight portions of the flanks, but this does not leave ample space for sawdust under the drive link. In the patents U.S. Pat. Nos. 3,263,715, 3,498,346 and 3,589,415 the gullet bottom was made as a circular arc, undercutting the straight flank portions so that the teeth were somewhat undercut. The latter shape leaves enough room for sawdust and has been much used.
Chain saws for mineral or coal use low chain velocity and small sawdust, and have less need for free space under the drive links.
There is a demand for nose sprockets with smaller diameters and fewer teeth, but it has until now been difficult to combine sufficient space for sawdust with sufficient material thickness between the gullet bottom and the sprocket bearing, as spacious, undercut gullets have a larger depth. The same problem has long been known from drive sprockets of chain saws, and was to resolved by letting the sawdust leave the gullets in an axial direction but this is not possible in a nose sprocket.
BRIEF SUMMARY OF THE DISCLOSURE
The present invention is a nose sprocket having an improved gullet shape. The nose sprocket of the present invention includes a gullet having a bottom portion with a varying radius of curvature. The varying radius of curvature is largest at a deepest part of the bottom portion of the gullet and smallest where the bottom portion meets teeth which carry the central drive links of a saw chain. For a certain material thickness determined by the need to avoid fatigue cracking in the gullet bottom, it is thus possible to either reduce the sprocket diameter to get a lighter and safer guide bar, or increase the bearing diameter to get better lifetime of the bearing and less risk of buckling of the nose sprocket due to thermal stresses.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of an embodiment of a nose sprocket according to the present invention; and
FIG. 2 is a side view of a gullet in the nose sprocket shown in FIG. 1 compared to prior gullet shapes.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The bending moment which acts on the nose sprocket when a drive link is pressed into the gullet is largely determined by the angle between the tooth flanks (11) which in turn depends on the drive link shape. A large cross section with reduced risk of deformation or fatigue cracks is achieved, i.e. by making a large distance between the deepest part (12) of the gullet bottom and the outer race (13) of the bearing large, and by reducing stress concentration by making large radii at these points (12, 13). A larg
REFERENCES:
patent: 3124177 (1964-03-01), Ekrud
patent: 3263715 (1966-08-01), Dobbertin
patent: 3279508 (1966-10-01), Ehlen et al.
patent: 3498346 (1970-03-01), Gasner et al.
patent: 3589415 (1971-06-01), Locati
patent: 4387615 (1983-06-01), Lemery
patent: 4492030 (1985-01-01), Beerens
patent: 4754549 (1988-07-01), Fischer et al.
patent: 4970789 (1990-11-01), Bell
Rachuba M.
Sandvik AB
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