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 ot 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.

Description

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 US 3,163,052, US 5,021,033 and DE 36 25 277, with an outer surface consisting of 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.

If it is desired to displace the center of the ring further to the rear than the drive shaft in order to compensate especially great changes in chain length, it becomes necessary to transmit radial forces otherwise than as contact force between the cylindrical surfaces mentioned. If the diameter of the sprocket is less than half the inner diameter of the ring, the radial force can be transmitted without solid or roller bearings by means of a single intermediate roller to a small diameter pillar concentrically fastened to a flange on the ring. The diameters of the roller and the cylinder can be chosen to avoid any slipping.

Since all radial forces are transmitted through rolling contact between a few cylindrical surfaces the device according to the invention is very durable, and because of the small number of components, price and reliability will be favourable compared to devices where radial forces are transmitted through conventional roller or solid bearings. It will be easy to replace worn parts or the whole device. Since the sprocket is a standard item it may be possible to remove the ring only, and let the chain run directly on the sprocket if there is no need to compensate for elongation of the chain. It will also be possible to exchange one ring for a ring of different diameter to alter the ratio between tensions in the chain parts.

Detail description with reference to the figures Figure 1 shows a side view of the saw chain, the ring and the sprocket, figure 2 a section through the ring and the sprocket, figure 3 a section through of the ring and the sprocket where the sprocket is made with flanges, figure 4 a section through the ring and the sprocket where the ring is made with flanges. Figure 5 and 6 show sections through the ring and the sprocket where an intermediate roller is located between the sprocket and a pillar concentric with the ring. Figure 7 shows a section of the chain, the ring and the sprocket where the ring is so thin that the protrusions on the inside of the ring consist of drive link tangs reaching through the ring and emerging on the inside.

Figure 1 shows a saw chain comprising cutter links (10) , side links (11) and drive links (12) running around a ring

(13) which is supported by a drive sprocket (14) fastened to a drive shaft. The drive links (12) have tangs reaching into recesses (15) on the outside of the ring. On both sides of the recesses are substantially cylindrical surfaces (16,17) carrying the side links (11) and cutter links (10) of the chain and transmitting radial forces between the chain and the ring (13) . Tangential forces are transmitted from the ring (13) to the chain in the recesses (15) and as friction on the cylindrical surfaces (16,17). The deepest parts of the recesses (15) have openings (19) through which sawdust and lubricant brought by the chain can be ejected without clogging the recesses. The cylindrical surfaces (16,17) on the outside may be interrupted opposing the recesses (15) , since no chain links will be supported there.

The inside of the ring carries a plurality of protrusions (18) with a shape corresponding to the tangs of the drive links (12), and has cylindrical surfaces (20,21) on both sides of the protrusions. The number of protrusions (18) and recesses (15) can be equal or unequal. The inner sprocket

(14) is a traditional rim sprocket with a plurality of recesses (22) on its periphery, in which the protrusions

(18) of the ring are received. On both sides of the recesses (22) are cylindrical surfaces (23,24) in rolling contact with the cylindrical surfaces (20,21) on the inside of the ring. In their deepest parts the sprocket recesses (22) have openings (25) to the side or inwards where sawdust and lubricant can be ejected without clogging the recesses. Radial forces are transmitted from the sprocket to the ring as a contact force where the cylindrical surfaces touch, as long as the center of the ring is farther to the front than the center of the drive shaft.

The inside (26) of the sprocket is in known manner designed for mounting on the drive shaft with torque transmission through splines, claws or keyways.

Figure 2 shows in a cross-section through the contact point between the ring (13) and the sprocket (14) how the protrusions (18) on the inside of the ring are received in the recesses (22) of the sprocket.

Figure 3 shows in a cross-section how the sprocket can be made with flanges (27) for better lateral guiding of the ring (13) . Figure 4 shows how the ring (13) can be provided with flanges (28,29) for better lateral guiding, where the flange (29) on the terminating side of the drive shaft may be extended all the way to the center to prevent intrusion of sawdust. If desired, a spring may be applied to the flange (29) to hold the ring (13) in a well defined position when the chain saw is not running.

Figures 5 and 6 show in cross-sections through the contact point between the ring (13) and the sprocket (14) two embodiments where radial forces can be transmitted from the sprocket (14) to the ring (13) outwardly in the contact point as well as inwardly over a roller (30) to a pillar (31) affixed to the flange (29) of the ring. To keep the roller (30) aligned in the same plane as the centers of the ring (13) and the sprocket (14) without any slippage, the

roller is made with cylindrical surfaces (32,33) of different diameters, where the surface (32) in contact with the pillar (31) has smaller diameter than the surface (33) in contact with the sprocket. To assure the correct position of the roller (30) between the pillar (31) and the sprocket (14) when the radial force on the ring is directed outwards, the roller can be guided by simple plastic inserts, which do not carry any load.

Figure 7 shows a cross-section and figure 8 a perspective view of an embodiment of the invention, where the ring (35) is thinner compared to the ring in figures 2 to 6, and where the protrusions (34) from the inside of the ring consist of tangs of drive links (12) penetrating the ring through cutouts (36) and emerging on the inside. To get sufficient stiffness and lateral stability the ring is provided with flanges (37) extending radially inwards or outwards.