Cutlery – Cutting tools – Saw
Patent
1996-04-18
1997-12-23
Watts, Douglas D.
Cutlery
Cutting tools
Saw
74438, 474156, B27B 1708
Patent
active
056996195
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
In chain saws power is transmitted from a rotating shaft to a saw chain running along a guide bar. The rotating shaft is on handheld saws usually a tubular shaft extending from a centrifugal clutch drum, on vehicle born saws usually the solid shaft of a hydraulic motor. In known saws the guide bar is rigidly mounted in relation to the rotating shaft when actually sawing. With this arrangement the outgoing chain part will lose some tension since the ingoing chain part is extended by elastic strain, and the whole chain by heat expansion and wear. The contact force between the outgoing chain part and the guide bar will then decrease, with a risk that the chain can leave the chain groove and cause an accident if the tension reaches zero. It is desirable to ensure that the tension is always high enough in the outgoing chain part in spite of cutting forces, impacts or chain extension, and low enough to avoid breaking when a heated chain cools.
The problem has been long known from various chain or belt transmissions and has been solved either by adjustable idler wheels, or by pivoting drives where the output sprocket is not concentric with the pivot axis. If the radius of the output sprocket is larger than the distance between the pivot axis and the sprocket center, the ratio between tensions in outgoing and incoming chain parts will always be a positive number and the tension in the outgoing part can never reach zero. This arrangement was marketed as the "Sespa" gear around 1950.
Corresponding arrangements adapted to the special demands of chain saws have been described in patents SE 469 515 and DE 41 37 409. Chain saws differ from other applications by an extremely difficult environment with severe vibrations and large amounts of moist or oily sawdust, and both cited patents describe variations of the "Sespa" concept where the space inside the drive has been filled out and sealed to keep out sawdust.
Another problem with the drives described in the cited patents is the transmission of radial forces, since the space for roller bearings is very restricted. The patents also describe the possibilities of filling the space between gears with low-friction material acting as a solid bearing, or letting the gear teeth carry the radial load, which in practice will greatly shorten their lifetime.
The present invention concerns an improved drive sprocket device for use on chain saws, where forces are transmitted between a drive shaft and a saw chain without use of roller bearings, solid bearings, gear teeth or other vulnerable elements, utilizing only elements which have been proven insensitive to sawdust.
SUMMARY OF THE INVENTION
According to the invention the drive sprocket device comprises one outer ring and one inner sprocket. The inner sprocket is mounted on the drive shaft and is basically a traditional rim sprocket as described in patents U.S. Pat. No. 3,163,052, U.S. Pat. No. 5,021,033 and DE 36 25 277, with an outer surface comprising two sustantially cylindrical portions and between them rows of recesses shaped to receive the tangs of drive links of the chain.
The outer ring is made with an outer surface similar to the outer surface of the sprocket, with two cylindrical portions and between the a row of recesses shaped to receive the tangs of the drive links. The inside of the ring is made as two cylindrical portions and between the a row of protrusions of a shape corresponding to the tangs of the drive links. In a special embodiment of the invention, the protrusions are actual tangs of the chain reaching through the recesses on the outside and emerging on the inside.
In normal use, the center of the ring should be located in front of the drive shaft, which allows the radial force and a large part of the tangential force to be transmitted as contact force between the smooth cylindrical surfaces on the inside of the ring and the outside of the sprocket. No bearing is needed in such a case.
To reduce the transverse force on the protrusions, the ring or the sprocket can be made with guiding flanges.
REFERENCES:
patent: 3163052 (1964-12-01), Oehrli et al.
patent: 3669162 (1972-06-01), Irgens
patent: 3857179 (1974-12-01), Haupt et al.
patent: 5021033 (1991-06-01), Sundstrom
Sandvik AB
Watts Douglas D.
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