DRIVE SPROCKET DEVICE

Abstract: Drive sprocket device for chain saws, where the saw chain runs over two free and separate but cooperating drive rings rolling around a sprocket part and guided by the sprocket and elements at its ends to keep the distance between the drive rings slightly larger than the thickness of the drive links of the saw chain.

Description: DRIVE SPROCKET DEVICE Background In patent application SE 9302842-1 was shown a device for automatic tensioning of the saw chain of a chain saw, where the chain runs around a loose outer drive ring rolling on a drive sprocket of traditional rim type The drive ring had a row of orifices, through which the drive links of the saw chain enter recesses in the periphery of the drive sprocket. Axial displacement between the drive ring and the drive sprocket are prevented by flanges. To get a large enough contact surface between the drive links and the recesses, the drive ring must be made thin, which makes the bridges between orifices vulnerable and difficult to manufacture. Even with a thin drive ring, there is a tendency that the contact surface against the leading edge of the drive links becomes too small, which makes the links climb the drive sprocket in a forward direction if the rotational velocity is non-uniform, as is often the case with high speed piston engines. If the bridges are damaged the drive ring will split and may cause chain damage- or other malfunction. The present invention concerns an improved drive sprocket device, with two separate but cooperating drive rings. Description The invention is described with reference to figures 1 - 2, showing cross sections through two embodiments, with the motor end of the chain saw upwards, where the chain is supported by the drive rings, and the cutting nose end downwards, where the chain has left the drive rings to run around the guide bar of the chain saw. Figure 1 shows an embodiment for use mainly with saws with horizontal shaft. A clutch drum (11) is fastened to a first end of a tubular sprocket part (12) with ridges on the outside, as is earlier known as a spur sprocket. Against the ridges two rings CONFIRMATION COPY (14,15) are freely rolling. The ridges may have uniform height throughout their length, or preferably have a higher center (25) with lower suppbrting portions (18) for the rings, which allows smoother rolling motion. The inner diameter of the rings is larger than the diameter of the supporting portions (18) . At the second end of the tubular sprocket part (12) is a washer (13) , preferably held to the end of the drive shaft by an elastic retaining ring. The width of the rings is such that they can roll freely on the supporting portions (18) without jamming between the drive links (16) , the drum (11) and the washer (13) , but still supporting the sidelinks (17) of the chain in a radial direction. If desirable to get enough stiffness for the rings, they may have flanges adjacent to the drum (11) and the washer (13) . For chain saws operating with vertical shaft, figure 2 shows an improvement, where the supporting portions (18) have been joined by fixed rings (20) fastened to the sprocket part (12) while supported by still lower portions of the ridges, the outside of the fixed rings serving as continuous cylindrical rolling surfaces (19) for the drive rings (22,23) . Axial displacement of the drive rings can be further restrained by inwardly directed flanges (24) guided by grooves (21) adjacent to the fixed rings, to keep the drive rings parallel to each other and ensure a spacing between rings to fit the drive links (16) even when the chain has not started moving and is not tensioned. The invention is not restricted to the embodiments shown in the figures. The drive rings need not be equal to each other, and may have combinations of outward and inward flanges. Fixed rings may be used without adjacent grooves and flanges on the drive rings. The drive ring edges facing the drive links can preferably be slightly rounded or tapered to facilitate entry of the drive links. If no clutch drum (11) is needed, another washer may serve to restrain the drive rings from being displaced towards the first end of the tubular sprocket part (12)

Claims: CLAIMS 1. Drive sprocket device for chain saws, where the saw chain runs over two loose and separate but cooperating drive rings (14,15,22,23) rolling around a sprocket part (12) with ridges, and the drive links (16) of the saw chain penetrate between the drive rings to enter recesses between the ridges of the sprocket part (12) , characterized by the drive rings having equal diameter, and being guided by the sprocket part and elements at its ends (11,13) to keep the distance between the drive rings slightly larger than the thickness of the drive links. 2. Drive sprocket device for chain saws according to claim 1, characterized by at least one drive ring having an inwardly directed flange (24) guided by a groove (21) in the sprocket part (12) to keep the drive rings from coming too close to each other. 3. Drive sprocket device according to claim 1 or 2, characterized by the drive rings being supported by supporting portions (18) of the ridges of the sprocket part (12) . 4. Drive sprocket device according to claim 1 or 2, characterized by the drive rings being supported by cylindrical surfaces (19) of the sprocket part (12) .

Inventors: LEINI ARVO, LEINI, ARVO
Applicants: SANDVIK AB
Application year: 1995
Main classification: B27B17
IPC codes: B27B17