AU5950900A - Chain saw guide bar with replaceable nose - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): KAPMAN AB Invention Title: CHAIN SAW GUIDE BAR WITH REPLACEABLE NOSE The following statement is a full description of this invention, including the best method of performing it known to me/us: CHAIN SAW GUIDE BAR WITH REPLACEABLE NOSE Background Guide bars for chain saws have often been made with replaceable noses, either to make it possible to repair a guide bar where only the nose sprocket or its bearing have failed, or to facilitate installation of a nose sprocket on guide bars the body of which is made from solid plates. Several different contours have been suggested for the joint between the guide bar body and the replaceable nose, mainly for the purpose of getting a rigid connection. Some of those are described in patents U S 3,762,047 and U S 3,955,279.

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Some other contours have been suggested to fill other needs, such as a softer deformable joint where the nose may be reused after straightening if it had been accidentally bent out of the plane of the guide bar without damage to the sprocket, as shown in U S 4,722,141 or U S 3,765,088.

ooo* •go* In case of such accidental bending, however, there are still two problems not addressed by prior inventors. Because of the localised bending of the plates of the replaceable nose S and the very strong stress concentration at the corners of the contour, it is very likely that cracks develop at the bending or straightening of the nose. There is also a great risk that deformation of the nose plates will be such that it will grip the guide bar body tightly and be difficult to remove.

The present invention concerns a replaceable sprocket nose for a chain saw guide bar, which compared to prior art can be bent and restraightened with less risk of cracking, and which is easy to remove from the guide bar even if deformed.

Description Replaceable noses are commonly made to include two outer plates, one center plate, one roller bearing and one sprocket. The plates and the center of the roller bearing are held together by welds or rivets. The rearward parts of the plates have extensions which together form a protrusion which fits into a recess in the front end of the guide bar body, 2 to align the nose with the guide bar. Usually, the nose protrusion is retained in the recess by one or more rivets or other fastening elements.

In prior art, the extensions of at least the outer plates at their base form sharp corners with the transverse contours of the plates. This makes a very sudden transition in bending stiffness, and since most of the protrusion is supported by the guide bar recess, all deformation out of the plane must occur in a very restricted area at those comers. Even relatively small deformations caused by dropping the saw to the ground may then cause •local cracking, which may then fracture at straightening or at a later deformation.

oooo According to U S 4,722,141 the deformation can be spread over a slightly larger area by making the extension of the center plate short and rounded to make a gradual stiffness transition. In many cases this is insufficient, and may even cause new problems as the ••go o deformation of the outer plates will be saddle-shaped and more difficult to straighten, even if there are no cracks.

According to DE GM 7143776 the deformation can be spread over a larger area by making cutouts in the outer plate extensions, but this will reduce the effective width and strength of the extensions.

According to the invention, there are no sharp corners at the base of the extensions, but at least the outer plates are made with rounded cutouts in the transverse contours adjoining the extensions, which allow the deformation to take place as pure tension over a certain length of the plates in front of the extensions in addition to the length corresponding to the center plate extension.

It was described in U S 2,693,206 that a cutout at the center line of the free end of the protrusion can be used to facilitate removal of a worn-out nose by by application of force to an inserted screwdriver or similar tool. This will be functional if the protrusion is not deformed, but applying a compressive force on a deformed protrusion will tend to make the deformation more severe, and for a deformed protrusion it may also be necessary to vary the direction and point of application. If according to the invention there are cutouts at both sides of the extensions at their base, application of a force can be made in two places, and will cause stretching of the protrusion rather than compression, both of which will make it much easier to remove a deformed nose.

Preferably, the cutouts should form a through hole, which may require a corresponding cutout in the center plate. The shape of the cutouts could be made to fit any of the service tools supplied with the chain saw. It is obvious that the invention could be applied to any type of replaceable nose, not only those with sprockets, but also noses with smooth or 10 grooved rollers, noses with wear-resistant alloy layers or noses with power take-off sprockets.

00. 0 Figure 1 shows an exterior view of a replaceable nose (10) fitted to the front end of a guide bar The nose has an protrusion (13) serving to align the nose to the guide bar and fasten it by means of bolts or rivets (16):On each side of the protrusion there is a S0: 0transverse contour (14) adjoining the end of the guide bar. Close to the protrusion (13) *"each transverse contour (14) has a cutout The general shape of the cutout may be round or oblong to fit some specific service tool of the chain saw, but the cutout should .g be rounded at least where it meets the protrusion.

Figure 2 shows a sectional view between the center plate (22) and an outer plate (21).

The outer plates extend the full width and far enough in front to form a laterally guiding groove for a saw chain carried radially by a nose sprocket The sprocket is supported by a bearing comprising rollers (29) and a bearing center (28) attached to the outer plates.

Each outer plate is provided with an extension (24) forming part of the nose protrusion At the base where the extension meets the transverse contour (30) the latter is made with a rounded cutout (23) preferably merging tangentially to the extension and forming an outward corner with the transverse contour. The longitudinal edges of the extensions may be parallel or converging towards guide bar.

The center plate (22) is considerably smaller than the outer plates, and is provided with a short extension (31) in the direction of the protrusion It is attached to the outer plates, preferably by welds or rivets The center plate has cutouts with generally the same shape as the cutouts (23) of the outer plates The short extension (31) is preferably rounded or triangular. The front contour of the center plate should be a concave circular arc, to keep a generally constant distance from the sprocket teeth, in order to minimize the risk of debris being compressed and caught between the arc and the teeth.

If the replaceable nose is accidentally bent out of the plane of the guide bar, the deformation will be less concentrated than with a prior art nose, and the bending will be spread over a distance including the cutouts (23) and the short center plate extension (31).

To remove the deformed nose, the fasteners (16) in the protrusion are removed, and a 9 suitable elongated tool such as a screwdriver is inserted through a cutout and juggled to o* push the protrusion away from the guide bar. If the deformation still makes it stick, the tool can be alternated between the cutouts.

9 The replaceable nose according to the invention is a safety feature, and to prevent the use o: of less safe noses where operators and operation methods are adapted to the safety, the guide bar may as shown in figure 3 be made to only accept noses according to the invention. The guide bar (32) has a transverse contour (34) corresponding to the contour of the outer plates. A central part (33) of the guide bar has a reduced thickness corresponding to that of the center plate The part with reduced thickness may have edges which are parallel or diverging towards the nose, and a pocket (36) large enough to accept the short center plate extension On at least one side of the pocket the guide bar is provided with a low lug (35) which will fill part of a cutout (15) of the nose. This makes it possible to apply the nose according to the invention to a new guide bar type where all safety features are utilized, as well as to older guide bars, but noses according to prior art can only be applied to corresponding prior art guide bars.

The invention has been described with reference to replaceable noses with sprockets, which are the most commonly used, but can equally well be used for replaceable noses with smooth chain-carrying rollers or with solid edges, both of which are known from prior art. Likewise, the nose can be made from a thick plate by deep milling instead of joining three thinner plates.

It is to be understood that, if any prior art publication is referred to herein, such reference does not constitute an admission that the publication forms a part of the common general knowledge in the art, in Australia or any other country.

For the purposes of this specification it will be clearly understood that the word "comprising" means "including but **.not limited to", and that the word "comprises" has a corresponding meaning.

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Claims (5)

1. Replaceable nose for a chain saw guide bar, attachable to the guide bar body by a rearward protrusion (13) from a transversal contour the protrusion having parallel or rearwards converging edges, characterized by cutouts (15) in the transversal contour where it meets the protrusion.

2. Replaceable nose according to claim 1, characterized by smooth rounded transitions from the cutouts to the protrusion.

3. Replaceable nose according to claim 2, made from two outer plates (24) and one center plate (22) welded together, the outer plates (24) forming parts of the protrusion 10 having holes (16) for attaching it to the guide bar body with fastening elements, characterized by a center plate (22) having a considerably shorter extension (31) *.00 rearwards relative to the transversal contour.

4. Replaceable nose according to claim 3, characterized by the center plate having a circular arc front contour.

5. The combination of a replaceable nose according to any of the preceding claims, and a guide bar having at least one low lug (35) at its front transversal contour, partly filling at least one of the cutouts (15) of the nose. Dated this 20th day of September 2000 KAPMAN AB By their Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia