NOSE SPROCKET FOR CHAIN SAW GUIDE BARS

Abstract: Nose sprocket for chain saws to carry the drive links of a saw chain on sprocket teeth and gullets being provided between the teeth, where the gullet bottom is shaped as part of an ellipse or ellipse-like curve, which compared to circular gullet bottoms achieve less risk of fatigue cracks, at least the same space for sawdust and a possibility to strengthen the sprocket bearing.

Description: Nose sprocket for chain saw guide bars. 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 ofthe 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 ofthe chain. To make a thin sprocket work with teeth so that it functions, it is necessary that the saw chain is carried at a well defined radius from the center ofthe sprocket bearing. This is accomplished if the ends ofthe chain drive links at its end portions are carried by the flanks ofthe sprocket teeth. When sawing timber with large chain velocity and coarse sawdust, it is important that there is space for sawdust between the underside ofthe drive links and the bottom ofthe gullet between the sprocket teeth, and that the drive link does not hit the sprocket in such a way that the pointed bottom part ofthe drive link is deformed, since its shape is vital for the distribution of lubricating oil along the path ofthe saw chain along the edge ofthe saw guide bar. The gullet bottom has been shaped in different ways. In the patent US 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 US 3,279,508 the gullet bottom is shaped as a circular arc, tangent to straight portions ofthe flanks, but this does not leave ample space for sawdust under the drive link. In the patents US 3,263,715, US 3,498,346 and US 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 diameter 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 larger depth. The same problem has long been known from drive sprockets of chain saws, and was there resolved by letting the sawdust leave the gullets in an axial direction but this is not possible in a nose sprocket. The present invention is a shape ofthe sprocket gullet, which better than previous designs will satisfy demands for ample space under the drive link and a limited depth. 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 ofthe bearing and less risk of buckling ofthe nose sprocket due to thermal stresses. Description The invention is described with reference to the figures, where figure 2 shows a gullet between teeth of a sprocket according to the invention. compared to previously known shapes, and figure 1 shows curvatures and other details ofthe gullet shape. 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) ofthe 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 large outer race radius is desirable to achieve a low bearing load over the rolling elements, but will reduce the cross section so much that the benefit of lower stress concentration is lost. A increased gullet radius will, if the gullet bottom is a circular arc and the length ofthe flank regions (11) are preserved, reduce the cross section so much that the benefit of stress concentration is lost unless the outer bearing radius is reduced.. According to the invention, the circular arc shape is substituted by another curve, where the curvature is largest (14) at the bottom (12) and smallest (15) at the end (16) where the curve meets the straight flanks (11). The curve is preferably a portion of an ellipse or ellipse-like oval curve, and could preferably be made with a slight undercut (17). In figure 2 a gullet bottom shape (21) according to the invention is compared with corresponding curves from previously known nose sprockets, where one curve (22) shows a gullet with very small bottom radius according to US 3,124,177, and a second curve (23) shows a gullet with circular arc bottom without undercut according to US 3,279,508, and a third curve (24) shows a gullet with circular arc bottom with undercut according to US 3,589,415. Figure 2 shows that a gullet according to the invention (21) has much less depth than a gullet with small radius (22) or a gullet with circular arc shape and undercut (24), and at least not greater depth than a gullet with circular arc without undercut (23). A gullet according to the invention thus allows at least equal or greater cross section compared to other gullets, and no risk of deforming the drive link lower point (18). Figure 2 also shows that the space for sawdust in a gullet according to the invention (21) is considerably larger than in a circular gullet without undercut (23), and that the amount of undercut (17) near the flanks is less than for a circular gullet with undercut (24), thus ensuring that no clear strength loss ofthe tooth occurs. Stress concentration at a gullet according to the invention is determined by the radius (14) at the deepest part (12), since the cross section at the undercut (17) and at the transition to the flanks (16) is considerably greater than at the deepest part As this radius (14) is greater than for any ofthe other gullets, a gullet according to the invention causes less stress concentration. This means less fatigue risk, if the bearing outer race is constant, or a possibility to enlarge the outer bearing radius with longer bearing life but without increased fatigue risk. The gullet is, according to the invention, preferably made with a slight undercut (17) which gives more space for sawdust, and which makes it somewhat simpler to punch or mill the tooth shape ofthe nose sprocket compared to a curved gullet without undercut.

Claims: Claims 1. Nose sprocket for chain saws, to carry the drive links of a saw chain on flanks of teeth as the saw chain travels around the nose of a saw guide bar, characterized by the bottom ofthe gullet between the teeth being shaped with varying radius of curvature, said radius being largest (14) at the deepest part (12) ofthe gullet and smallest (15) where the gullet bottom meets those portions ofthe flanks (11) carrying the chain. 2. Nose sprocket according to claim 1, characterized by the bottom ofthe gullet being shaped as part of an ellipse or ellipse-like curve. 3. Nose sprocket according to claim 1 or 2, characterized by the gullet bottom being undercut (17) relative to an imaginary extension of the flanks.

Inventors: SUNDSTROEM ERIK, SUNDSTROEM, ERIK
Applicants: SANDVIK AB, SUNDSTROEM ERIK, SANDVIK AKTIEBOLAG, SUNDSTROEM, ERIK
Application year: 1996
Main classification: B27B17
IPC codes: B27B17, F16H55