Guidebars for chain saws have long been made with toothed nose sprockets as shown in the patents US 3,124,177, US 3,263,715, US 3,762,047. In these cases the sprockets have a bearing in the plane of the sprocket and a non-rotating bearing center. It is also known from the patents US 2,783,794 and US 4,506,444 to make toothed sprockets with double bearings and a rotating bearing center, which makes the sprocket run more stably and allows connection of various auxiliary equipment.

Sprockets according to prior art for guidebars of forestry chainsaws have all used roller bearings to .reduce the need for lubrication. Some ancient designs such as US 6,350 have used double solid bearings, but were only used in stationary wood-working machines where ample lubrication could be arranged. The nose sprockets and their bearings are subject to more wear and damage than other parts of the guidebar, and it is common that a worn and damaged sprocket has to be replaced. Roller bearings make removal and installation of nose sprockets difficult and expensive.

The present invention concerns a guidebar for chainsaws, provided with a nose sprocket with double solid bearings of a new type, designed to simplify replacement and installation of the sprocket.

Description The invention is described with reference to the figures, where figure 1 shows a lateral view of a guidebar according to the invention, provided with a hollow shaft for connection of auxiliaries in a manner described in US 4,506,444, figure 2 the nose portion of a guidebar without possibility for connecting auxiliaries, figure 3 an edge view of a guidebar, figure 4 a section of the guidebar shown in figure 1, figure 5 a section of the guidebar shown in

figure 2, figure 6 a section like figure 5 but with the addition of a lubricant reservoir in the sprocket center, figure 7 an edge view of a guidebar in the process of installing a nose sprocket and figure 8 a bearing cup.

A guidebar as shown in figures 1,2 and 3 comprises a guidebar body (12), made with a groove (13) along its edges, in which a saw chain (14) is guided. The rear end of the guidebar body has slots and holes of known configuration for attachement to the motor part of the chainsaw. In the front end a rotatable nose sprocket (20) is provided. The guidebar body (12) may also be provided with holes for affixation of auxiliaries.

The nose sprocket (20) is rigidly united with a shaft extending on both sides. The shaft can be solid (22) or hollow (21) . If the ends of a hollow shaft (21) are accessible as shown in figure 4 and its inner surface is non-cylindrical, preferably triangular with rounded corners, it is suitable for powering an auxiliary tool, such as a grinding wheel, a pump or a capstan. A hollow shaft can also be used as a lubricant reservoir as shown in figure 8.

The ends of the shaft (21,22) are journalled in thin bearing cups (15,17) of a material which suffers little friction and little wear in cooperation with the material of the shaft (21,22). Preferably, the shaft (21,22) is made integrally with the sprocket (20) from a stiff polymer such as polyacetal or polyimid, with admixture of friction reducing substances such as PTFE, graphite or molybdenum disulfide, and the bearing cups (15,17) are made from metal such as bronze, brass or steel. It is also possible to make the sprocket (20) and the shaft (21,22) from metal, and the bearing cups (15,17) from a polymer or from polymer coated metal.

The bearing cup (15,17) is made with a collar on the side adjoining the sprocket (20) as shown in figure 8, and it may

have a continuous bottom (17) if it is to be used with a sprocket with a solid shaft (22) as in figure 5, or if it is to be used with a hollow shaft (21) to store lubricant as shown in figure 6. The cup (15) may have a circular opening in the bottom as shown in figures 1 and 4, if it is to be used with a hollow shaft for powering auxiliary tools.

The bearing cups (15,17) are located in circular openings in the side plates (18) extending from the guidebar body (12) . To prevent the bearing cups from following the rotation of the shaft, which would destroy both the cups and the side plates, the bearing cups (15,17) are made with one or more nibs (19) fitting into corresponding notches (23) in the periphery of the circular openings.

Installation of a nose sprocket in the guidebar is made as shown in figure 7 by prying the extending side plates (18) slightly apart with help of a screwdriver, whereupon the nose sprocket (20) with shaft (21,22) and bearing cups (15,17) is pushed in between the side plates. When the bearing cups (15,17) are aligned with the openings in the side plates (18) , the side plates will spring together elastically to lock the bearing cups (15,17). The length of the extending portion of the side plates is chosen sufficiently large to permit a total elastic spring-back.

Removal is done in an analogous way by prying the extending side plates (18) so far apart that the nose sprocket (20) with the bearing cups (15,17) can be slid out forwards from the guidebar.

The collar of the bearing cup (15,17) is as is shown in the figures supported between the inside of the side plates (18) and the sprocket (20) . The thickness of the collar will then determine the spaceing between the sprocket (20) and the sideplates (18) . If a narrower spaceing is desired, the collar can be recessed in the side plates (18) or the sprocket (20) or both.