DRIVE SPROCKET FOR CHAIN SAWS

Abstract: Drive sprocket for chain saws, comprising an outer ring (11) transmitting force on its outside (13) to a chain and with a large internal gear surface (15) on its inside meshing with a smaller external gear surface (16) on the drive shaft or on an inner ring (17) supported by the drive shaft, and one or more distance elements (19, 20, 24, 27) maintaining the center of the outer ring (11) at a certain distance from the center of the drive shaft, allowing the center of the outer ring to move along a curved path around the drive shaft to compensate for changes in chain length and to automatically ensure a tension force in the chain.

Description: DRIVE SPROCKET FOR CHAIN SAWS Background Excessive wear and hazards are unwanted results when the average tension in a saw chain occasionally becomes so low that the chain is no longer safely guided in contact with the guidebar. This will lead to sharp tension peaks in the chain and impacts against the guidebar, or the chain can leave the groove in the guidebar or the drive sprocket. To avoid this, chain saws have long been made with guidebars which after mounting of the saw chain can be moved forward relative to the drive sprocket to create tension in the chain, and with means to lock the guidebars in this position. Wear and heating during use will lengthen the chain, however, and even a small elongation will lower the tension considerably. The cutting forces in severe use will also lengthen the chain elastically, resulting in lower tension in the upper outgoing part of the chain. It has long been known from the patents US 1,016,664 and US 2,316,997 to make the guidebar slidingly movable in use, acted upon by a spring to ensure a constant sum of the tension in the outgoing and ingoing parts of the chain. Disadvantages are that friction and wear will be as large in idling as in cutting, and that impacts against the nose part of a movable guidebar may push it back and lower the tension in the chain. The invention concerns a drive sprocket to be mounted on the power head of chain saws, designed to make the tension in the chain proportional to the sum of friction and cutting forces, making it high while cutting when high tension is needed, and making it low while idling to avoid unnecessary wear. Description of the invention The invention is described with reference to the figures, where figure 1 shows an axial section through a drive sprocket, figure 2 a perspective view of a partly disassembled drive sprocket, figures 3 and 4 sections through alternative forms of drive sprockets and figure 5 shows in principle the relations between the forces acting on a part of the saw chain. A drive sprocket according to the invention comprises an outer ring (11) having on its outside (13) indentations (12) corresponding to the drive links of the saw chain. The indentations may be only slightly wider than the thickness of the drivelinks, with the side links of the chain being carried by the outside (13) of the outer ring, or they could be substantially as wide as the outer ring, with the side links being carried by the ridges (14) between the indentations (12) . The inside of the outer ring (11) is shaped as a large internal gear surface (15) meshing with a smaller external gear surface (16) on a drive shaft or on the outside of an inner ring (17) supported on the drive shaft with moment transferring grooves or splines on the inside. The smaller gear surface (16) comprises fewer teeth than the large gear surface (15) and to allow meshing the gear surfaces (15,16) have different centers. The appropriate distance between the centers for correct meshing is ensured by one or more distance elements (19,20) journalled in the outer ring (11) through outer bearings (22) concentric with the outer ring, and journalled on the drive shaft or the inner ring (17) through inner bearings (21) concentric with the drive shaft. The outer and inner bearings (21,22) can be solid bearings or bearings with rolling elements. The drive shaft is powered by an engine, and according to the invention the guidebar around which the saw chain runs in service is preferrably clamped at a certain distance from the drive shaft, and this distance can be changed within certain limits when the clamping is released. Figures 1-2 show an embodiment of the invention where there are two distance elements (19,20) and the difference in diameter between the large gear surface (15) and the smaller gear surface (16) is large enough for the distance elements to be rigidly connected in the space between the gear surfaces. The distance elements (19,20) are preferrably made with raised radial ridges (23) serving as a visual indication of the angular position of the distance elements when the guide bar and saw chain are being fitted, and as counterpart of a safety catch on the engine to limit the angular movements of the distance elements. Figure 3 shows an axial section through another embodiment of the invention with one distance element (24) , the bearings of which lie enclosed in a groove (25,26) in each gear surface. To facilitate assembly in this case, at least two of the components must be split. Figure 4 shows an embodiment where a distance element (27) is journalled on a neck (28) at the end of the drive shaft (29) and on a peg (30) extending inwardly from a raised cover (31) on the outer ring (11) . Figure 5 explains in principle the equilibrium of the forces acting on the saw chain and the outer ring (11) , where only those forces which are balanced by external forces are shown. The lesser force U is acting on the outgoing part (41) of the chain, and the higher force U+W+F is acting on the incoming part (42) of the chain, where W is the cutting force and F is the friction, summed for all links of the chain. Moment equilibrium around the meshing point between the gear surfaces (15,16) requires that U=A(F+W)/S where A is the distance from the the meshing point to the incoming part (42) of the chain and S is the diameter of the large gear surface (15) . The tension force U in the outgoing chain part (41) is thus always positive and substantially independent of the mounting, wear and heat expansion of the saw chain. Changes in the length of the chain due to wear, heat expansion and elastic strain will cause the distance elements (19,20) to rotate a limited angle around the center of the smaller gear surface (16) , moving the center of the large gear surface (15) along a curved path, the diameter of which equals the difference between the diameters of the gear surfaces (15,16). The distance from the outer ring (11) to the guide bar will then automatically assume such a value that the force U in the outgoing part (41) of the chain is determined by the above relation. The diameter M of the smaller gear surface (16) does not influence the forces, but the largest length change of the chain that can be accomodated is related to the difference between diameters S-M. For calculation of the elastic strain in the chain account must be taken of the internal inertial force T=V 2 Q/2 which is balanced by centrifugal forces in curved parts of the chain, where V is the chain velocity and Q the chain weight per unit length. This inertial force T does not affect the motion of the chain. The elastic elongation of the chain is thus dependent on the value of 2U+2T+F+W. For a chain saw of traditional design with constant distance between the guidebar and the drive sprocket this elongation will be constant, and the tension force U which is crucial for the safe running of the chain must then decrease in service as the inertial force T, the friction F and the cutting force W all increase. Gradually the force U will also decrease more as the chain is elongated by wear and heat expansion. For a chain saw with a slideable guide bar acted on by a constant spring force, this force will equal 2U+F+W, and even in this case the tension force U will decrease in service, although less than if the. guide bar was fixed. In a chain saw according to the invention, the tension force U will increase in service, however, when the requirements for safe running of the chain are greatest. When idling the tension force U is smaller causing less wear. This arrangement thus produces higher safety for the running chain and less wear than any prior art. At stand-still the chain is without tension, which simplifies removal and fitting of new chain. The drive sprocket according to the invention has been described above as embodiments fitting a saw chain with a single row of drive links running in a groove along the edge of the guide bar, but can be adapted equally well to saw chains straddling the guide bar edge and to chains transferring power from a drive shaft to another sprocket without cutting, as for powering an auxiliary tool.

Claims: CLAIMS 1. Drive sprocket for a chain, in particular a saw chain, comprising an outer ring (11) having a first center and on its outside indentations (12) for transmitting forces to the chain, and designed to be powered from a drive shaft having a second center, characterized in the center of the outer ring (11) never coinciding with the center of the drive shaft but held at a constant distance from the center of the drive shaft by distance elements (19,20,24,27), and in the center of the outer ring (11) being able to assume different positions along a curved path under influence of forces in the chain, and in the outer ring (11) having on its inside a large internal gear surface (15) meshing with a smaller external gear surface (16) which is concentric with the drive shaft. 2. Drive sprocket according to claim 1, characterized in the distance elements having outer bearings (22) concentric with the outer ring (11) and inner bearings (21) concentric with the drive shaft. 3. Drive sprocket according to claim 2, characterized in comprising an inner ring (17) supported by the drive shaft and carrying on its outside the smaller external gear surface (16) . . Drive sprocket according to claim 3, characterized in the inner ring (17) having on its inside grooves (18) for power transmission. 5. Drive sprocket according to claim 2, characterized in having two distance elements (19,20) connected in the space between the gear surfaces (15,16) . 6. Drive sprocket according to claim 2, characterized in the outer and inner bearings (22,21) being roller or ball bearings. 7. Drive sprocket according to claim 2, characterized in the distance elements being made from a synthetic material with low friction.

Inventors: SUNDSTROEM ERIK, SUNDSTROEM, ERIK
Applicants: SANDVIK AB
Application year: 1992
Main classification: B27B17
IPC codes: B27B17, F16H7