The invention relates to the field of a guide bar of a chain saw for guiding a saw chain comprising a chassis member and a sheet part member, as well as of a method for manufacturing a guide bar .

Chain saws are handheld power tools and used for cutting wood and other material, e. g. in a forest, and a worker operating such a power tool is usually handling it during a whole working day. Such a chain saw comprises an engine and a related power source, normally a petrol tank or a battery providing electrical energy. The engine drives a saw chain which is tightened around a guide bar for a rotational movement along the peripheral edge of the guide bar. The standard material for a guide bar is met ^¬ al, usually iron. In case of a professionally used chain saw such a guide bar can have a length of more than 100 cm, what re ^¬ sults in a high weight of such a power tool. This causes stress on the worker and he gets tired soon.

In order to minimize the weight of a chain saw, different ap ^¬ proaches have been started. One starting-point for weight reduc ^¬ tion is to reduce the weight of the engine or the power source. E.g., only a small petrol tank is provided or the tank is filled only to such an extent as absolutely necessary. That, of course, reguires freguent refilling of the tank.

Even more relevant are approaches to reduce the weight of the guide bar which influences the fatigue behavior decisively due to the prying effect of the elongated extension of that compo ^¬ nent. For such kind of weight reduction it had been proposed in documents and by products on the market to substitute portions of the guide bar made solidly of iron by other material parts or parts designed lighter in weight. Such a specifically designed guide bar is proposed by US patent 4,961,263. This document discloses a chain saw with a guide bar having a main member made of solid material. For weight reduc ^¬ tion, a cutout extends through the main member of the guide bar. The opening in the main member defined by the cutout are closed by cover plates for maintaining the stiffness and are attached to the main member of the guide bar. A fill body is implemented in the cutout and is connected to the cover plates in a force- transmitting manner for obtaining torsion stiffness while weight reduction is realized. However, the design of this guide bar re ^¬ quire s high effort during manufacturing.

It is an object of the present invention to provide a chain saw with particularly low weight but high stiffness of its guide bar and manufactured without notable effort. Another object is to provide a method for manufacturing a steady guide bar for such a chain saw.

According to a first aspect of the invention, a guide bar for guiding a saw chain of a chain saw comprises a chassis member made of solid material, in particular metal. Instead of the chassis member being made of solid material, it may be also com ^¬ posed of at least two layers of solid material which are rigedly fixed together, in particular by spot-welding in case of metal- lie layers. The chassis member has two flat sides, which in par ^¬ ticular are arranged basically in parallel to each other, and an outer circumferential edge defining a guide groove for guiding the saw chain in its movement around the guide bar and along the circumferential edge. With such a design, the chassis member de- fines a frame part of the guide bar and comprises at least one through-hole or aperture formed at least approximately in an in ^¬ ner or a center area of the guide bar. Such through-hole or ap ^¬ erture is in particular a cutout hole. Depending on the desired degree of weight reduction and degree of accepted or justifiable reduction of stiffness, only one more or less large through-hole or aperture may be provided, or, instead, several smaller ones may be implemented. The chain saw further comprises a sheet part member which is arranged in and fit into the through-hole or ap ^¬ erture. For reduction of weight compared to a solid guide bar without such through-hole or cutout, the sheet part member is realized of smaller weight than the left out or cut out part of solid material. Naturally, in case of provision of more than one through-holes or apertures, provision of the respective number of sheet part members are favourable. Chassis member and the at least one sheet part member are connected to each other, thereby establishing a stiff structure for the guide bar, and, in that, compensating at least to a great extent the reduction of stiff ^¬ ness. At the transition zone or borderline between the chassis member and the sheet part member a groove is arranged at least on one, preferably on both, of the opposite flat sides of the guide bar which groove covers at least partially the sheet part member and the chassis member. In order to establish a rigid connection between the chassis member and the sheet part member, a fill material fills at least partially the groove.

With the substitution of a solid metal portion of the guide bar by the sheet part member it is desirable still to have a solid and stiff power tool, so that the sheet part member is prefera ^¬ bly basically of the same or even of higher stiffness than the left out or cut out part of solid material.

One specific embodiment provides for a through-hole or aperture and sheet part member which are essentially rectangular or oval. In the case of several smaller through-holes or apertures and respective sheet part members, these elements may be rectangular or oval as well, or they may be sguare or circular.

A particularly preferred embodiment is characterized by a design with chassis member and sheet part member that are connected by a fluid fill material that hardens over time, in particular the fluid fill material being a thermoplastic material. With such a design, the through-hole or aperture of the chassis member, po ^¬ tentially after having created at first the at least one through-hole or aperture, only needs to be filled with the sheet part member and these elements have to be interconnected by the fluid fill material.

The fluid fill material can be or comprise polyamide, in partic ^¬ ular glass fibre reinforced polyamide, e. g. PA6 or PA6 with glass fibres. Of course, other comparable materials may be con ^¬ sidered as well.

For a particularly relevant connection between chassis member and sheet part member, grooves may be arranged on both opposite sides of the guide bar and the rigid connection may be formed similar to a groove-and-tongue connection. Even more, if the connecting fill material is a thermoplastic joint, such kind of connection can be considered both between sheet part and thermo ^¬ plastic joint and between frame part and thermoplastic joint, which connections might be realized with opposite orientation to each other.

In a specific embodiment with arrangement of grooves on both guide bar sides, these grooves are interconnected via at least one opening in the intermediate wall between the grooves. This opening provides a connection or cohesion of the fill material the grooves at both guide bar sides are filled with.

For providing a favourable design, the inner or central edge of the chassis member abuts the outer or peripheral edge of the sheet part member, both inner or central edge and outer or pe ^¬ ripheral edge are tapered towards each other or face each other with reduced cross-section. With a particularly high reduction of the cross-section a very solid connection can be realized. In order to get a high stiffness of the completed guide bar, it might be favourable to design chassis member and sheet part mem ^¬ ber, in that the inner or central edge and the outer or periph ^¬ eral edge of the respective members fit closely together, so that there is no or very few fill material in the gap between said edges .

As already earlier mentioned, an interconnection of the fill ma ^¬ terial in the grooves on the two guide bar sides is favourable, since that will further improve stability and stiffness of the guide bar. To this end, the openings providing the interconnec ^¬ tion may be arranged in or at at least one of the inner or cen ^¬ tral edge of the chassis member and the outer or peripheral edge of the sheet part member. Instead of openings in any of these edges, also arrangement of indentations may be considered. The grooves on the two opposite guide bar sides result from tapering the inner edge of the chassis member and the outer edge of the sheet part member. The interconnection emerges from fill materi ^¬ al penetrating the openings or interdentations and thereby cou ^¬ pling the material within the two gooves .

According to a second aspect of the invention, a chain saw with particularly low weight but high stiffness of its guide bar and manufactured without notable effort may be also provided by a guide bar which comprises a chassis member made of solid materi- al, in particular metal, or comprising at least two layers of solid material which are rigidly fixed together, wherein the chassis member has two flat sides which in particular are ar ^¬ ranged in parallel to each other. An outer circumferential edge of this guide bar defines a guide groove for guiding the saw chain in its movement around the guide bar and along the circum ^¬ ferential edge. The chassis member defines a frame part of the guide bar and comprises at least one through-hole or aperture formed at least approximately in a center area of the guide bar, in particular a cutout hole. A preassembled sheet part member module is arranged in and fits into the through-hole or aperture and it is of smaller weight than the left out or cut out part of solid material. The through-hole or aperture and the preassem ^¬ bled sheet part member module may be essentially rectangular or oval. Chassis member and preassembled sheet part member module are interconnected, thereby establishing a stiff structure for the guide bar, and the sheet part member module comprises a sandwich laminate structure, in particular a metal fibre sand ^¬ wich laminate structure which provides, in connection with the chassis member, a particular stiff guide bar structure. With im- plementation and connection of a ready-to-use sheet part member also a low effort assembling is realized.

The guide bar according to the second aspect of the invention may be a specific embodiment of the guide bar according to the first aspect of the invention. Additionally or alternatively it may also comprise any of the specific individual features or el ^¬ ements of the above-described advanced embodiments.

Specifically, in any of the embodiments according to the first or second aspect of the invention, the sheet part member or the preassembled sheet part member module is composed of two outer thin sheets, in particular metal plates, arranged in parallel to each other and an inner metal fibre structure provided between the thin metal plates. Such an arrangement reguires notably few material while achieving the intended stiffness of the sheet part member or preassembled sheet part member module. Prefera ^¬ bly, the outer thin sheets are fixed with each other at their peripheral edges, which edges are arranged offset to the interi ^¬ or of the outer thin sheets and in a direction opposite to each other. In particular, the offset of the edges of both outer thin sheets is realized egually, so that a centered tapering periph ^¬ eral edge of the sheet part member will result, with the effect of enabling similar or identical grooves to be filled with fill material when the sheet part member has been assembled into the chassis member. Further, according to the invention, a method for manufacturing a guide bar, in particular a guide bar made of solid material or comprising at least two layers of solid material, of a chain saw, provides the following steps

- cutting out at least one weight reduction hole in the center or an inner area of the guide bar, thus creating a chassis member defining a frame part of the guide bar,

- inserting a lightweight sheet part member into the weight reduction hole, and

- joining together the light weight sheet part member and the chassis member by filling fluid material, in particular thermoplastic material, into a gap or a groove at the tran ^¬ sition zone or borderline between chassis member and sheet part member, which fluid material hardens over time.

The filling of the fluid material may be executed with a casting or injection moulding process. The guide bar may be of a type as described in any of the above-mentioned embodiments. In particu ^¬ lar, the sheet part member is of lightweight structure and it is formed to fit in to the weight reduction hole.

The cutout process can be operated from both guide bar sides un ^¬ til the interior is removed. Preferably, an inner circular edge or a medial strip of smaller width than the width of the chassis member is retained. With such circular edge or medial strip a more stable connection to the sheet part member to be implement ^¬ ed will be obtained. After having finalized the cutout process, the cross-sectional view of the chassis member at the area of the upper or lower chain guide may look like a tuning fork. To go even further with the inventive method, the sheet part member comprises an outer peripheral edge of reduced width which will be arranged in alignment with the inner circular edge or medial strip of the chassis member. The grooves that are reached at both sides of the guide bar, which is constituted by the chassis member and inserted sheet part member, can be filled therafter with the fill material, in particular thermoplastic material to provide a thermoplastic joint.

A particularly preferred method is characterized by grooves that are arranged at both guide bar sides, which grooves are inter ^¬ connected via at least one opening and they are at least par ^¬ tially filled during the manufacturing process with fill material. The fill material is a fluid material which flows through the opening to pass from the groove at one guide bar side to the groove at the opposite side. If thermoplastic material is used as fill material, filling of the grooves may be provided during the casting or injection moulding process and the supply of the material may be arranged from one side only.

The invention will be described more fully hereinafter with reference to the accompanying drawings, in which

Fig. 1 is a schematic diagram of a motor driven chain saw with a guide bar with a part of a housing cut out;

Fig. 2 is an enlarged section view of the guide bar as shown in Fig. 1; Fig. 3 is a detail view of an end section of the guide bar as shown in Fig 1 with a split of a thermo plastic joint;

Fig. 4 is a detail view of a segment of the guide bar as shown in Fig. 3, but without thermoplastic joint;

Fig. 5 is the view of Fig. 4, but with the thermoplastic joint partially filled; Fig. 6a is an enlarged section view of detail VI as indi ^¬ cated in Fig. 2; and

Figs. 6b to 6d are alternative designs to the embodiment shown in

Fig. 6a.

A motor-driven chain saw 1 includes a housing 3 having a drive motor (not shown) for driving a saw chain (not shown) . The housing 1 comprises a rear handle 5 with a throttle lever 7 and a forward handle 9 with a protector 11 mounted forward of the han ^¬ dle 9. A guide bar 13 is detachably mounted to the forward part of the motor housing 3, which guide bar 13 is provided for guid ^¬ ing and tightening the saw chain. At the rearward end of the guide bar 13, a slot 15 as well as two attachment holes 17 are arranged to permit a one-sided at ^¬ tachment of the guide bar 13 to the housing 3 with the aid of bolts. The saw chain is redirected at the forward free end of the guide bar 13. For this purpose, a sprocket 19 is provided which is arranged by use of attachment rivets 21. The saw chain is guided in a peripheral guiding groove arranged at the circum ^¬ ference of the guide bar 13.

As shown especially in the section view of Fig. 2, the guide bar 13 is composed of a chassis member 23 constituting a frame part of the guide bar 13 and being made of solid iron as well as of a sandwich laminate sheet member 25 arranged in a central area of the guide bar chassis member 23 in such a way that both sides of the guide bar 13 show flat surfaces.

The reason for the above-described composition of the guide bar 13 is the intention to reduce the weight of the guide bar 13 and thereby for the entire chain saw. On the other hand, weight re ^¬ duction measures shall not end up in a reduction of stiffness, in particular torsion stiffness, of the guide bar 13. In order to cope with such balancing act, the chassis member 23 of solid iron comprises in its central area an almost rectangular cutout 27 extending over almost the entire length of the guide bar 13 to provide space for the integration of the sandwich laminate sheet member 25 which is of smaller weight but of comparable stiffness than a similarly shaped solid metal sheet. The cutout 27 and the filling by light weight sheet member 25 can be inter ^¬ rupted by connecting transverse struts or more than one cutout can be arranged.

As can be seen from Fig. 2, the cutout 27 is realized by a re ^¬ moval of an area extending nearly over the entire width of the guide bar 13. Only a comparably small frame portion persists, just defining the peripheral guiding groove 29. In fact, the cross-sectional view of Fig. 2 shows a Y- or tuning fork shaped profile of the frame portion which means, the cutout 27 does not leave a flat inner face or edge but a circumferential medial strip 31 is arranged which is smaller in its width than the thickness of the chassis member 23.

As can be also seen from the cross-sectional view of Fig. 2, the sandwich laminate sheet member 25 is arranged by completely in ^¬ filling the cutout area, i. e. there is no space between the outer edge of the sheet member 25 and the inner edge, defined by the circumferential medial strip 31, of the chassis member. Such arrangement enhances stiffness of the completed guide bar 13.

The sandwich laminate sheet member 25 is composed of two thin outer iron sheets 33 which are arranged in parallel with such a distance that their outer faces define a width of the completed sandwich laminate sheet member 25 which is the same as the width of the chassis member 23. The thin outer iron sheets 33 are fixed with each other at their peripheral edges 35, which edges

35 are arranged offset to the interior of the thin outer iron sheets 33 and in a direction opposite to each other, thus defin- ing an inner hollow space of the sandwich laminate sheet member 25 which hollow space is filled with air and with countless mi ^¬ croscopic steel fibres 37 that bind the inner surfaces together. With the offset-arrangement of the peripheral edges 35 fixed with each other, the framed outer edge of the sandwich laminate sheet member 25 is tapered compared to the interior of the sheet member 25 and has a thickness of nearly the same of the circum ^¬ ferential medial strip 31. After insertion of the sandwich laminate sheet member 25 into the cutout 27, abutting medial strip 31 and framed outer edge 35 define grooves 39 on both sides of the guide bar 13.

Chassis member 23 and sandwich laminate sheet member 25 are fixed together with the aid of a thermoplastic joint 41 filling the grooves 39 on both guide bar sides, thereby establishing a smooth surface of these sides.

Fig. 3 shows a detail view of an end section of the guide bar 13. The theroplastic joint 41 is shown as only partly filling the groove 39 for a better understanding of the composition.

The enlarged views of Figs. 4 and 5 allow further detailed in ^¬ spection of the installation of the sandwich laminate sheet member 25 in the chassis member 23. While Fig. 4 shows a portion of the groove 39 without thermoplastic joint 41, the exposure of Fig. 5 reproduces the view of Fig. 4 but with the groove 39 partly filled. As will be noticed from these enlarged views, the medial strip 31 comprises several indentations 43 which could be arranged in an eguidistant way and provide an opening between the grooves 39 on both guide bar sides. These indentations 43 allow a close contact between the thermoplastic joints 41 at both guide bar sides, thus further improving the stability of the composition. With Fig. 6a a further enlarged sectional view of the joint be ^¬ tween chassis member 23 and sandwich laminate sheet member 25 is shown, in particular illustrating the tapered framed outer or peripheral edge 35 abutting the circumferential medial strip 31 of the Y- or tuning fork shaped profile of the frame portion.

However, as shown in Fig. 6b, alternatively to such Y- or tuning fork shaped profile the frame portion may also be shaped similar to an "H", thereby adapting the outer contour of the tapered framed outer or peripheral edge 35 of the sandwich laminate sheet member 25, thus enabling a more eguable distribution of the thermoplastic joint which is not only arranged on both guide bar sides but also fills the space between the abutting sheet member 25 and chassis member 23. With such a design, the stability of the connection joint could be improved but the manufac- turing process of the H-profile of the frame portion of the chassis member 23 could be a bit more complex.

Figs. 6c and 6d illustrate further alternative embodiments which differ from the embodiments of Figs. 6a, 6b in a substitution of the inner portion of the sandwich laminate sheet member 25 filled with microscopic steel fibres by an inner portion being or comprising a foam plastic core 37a. With such a core sub ^¬ stance, the outer iron sheets 33 of the sandwich laminate sheet member 25 can be designed in an open manner, i. e. they do not need to be fixed with each other at their peripheral edges 35, but they may be completely untapered as illustrated in Fig. 6c or the peripheral edges 35 may be bent only with their uttermost periphery according to Fig. 6d, the latter one being favourable due to the possibility to provide a smoother transition from thermoplastic joint 41 to outer surface of the sandwich laminate sheet member 25. Both embodiments according to Figs. 6c, 6d may advantageously be connected with a Y- or tuning fork shaped frame portion of the chassis member 23, but of course other so ^¬ lutions for the structure of the joint are possible. The manufacturing the guide bar 13 is as follows. A main body 45 of the guide bar 13 is manufactured using a solid iron sheet of the width of the completed embodiment . By way of milling or cut ^¬ ting-out, in particular laser-cutting, a cutout is arranged in an area of the solid iron sheet which defines the central area of the completed guide bar 13. The cutout 27 is realized by abandonning removal of the medial strip 31 as described above. After the cutout process a chassis member 23 of the main body 45 is created.

A light weight sheet member 25 is manufactured by creating a sandwich laminate structure according to the sheet member embod ^¬ iment described above. The light weight sheet member 25 is in ^¬ serted into the cutout 27 of the chassis member 23 of the main body 45 by aligning the medial strip 31 with the tapered periph ^¬ eral edge 35 of the sheet member 25, thus defining approximate rectangular grooves 39 on both guide bar sides.

Thereafter, the arrangement of chassis member 23 and light weight sheet member 25 is put into an injection moulding tool and by way of injection moulding the grooves 39 are filled with thermoplastic joint 41. Since the sprue of the injection mould ^¬ ing tool is arranged only at one guide bar side, the thermo ^¬ plastic material is first filling the groove 39 arranged at the same side, but flowing through the indentations 43 provided in the medial strip 31 and filling completely also the groove 39 arranged at the opposite side.

A specific end portion 47 of the guide bar 13 housing the sprocket 19 but which end portion 47 shall not be further de ^¬ scribed here, is finally arranged at the main body 45 of the guide bar 13.

The guide bar 13 as describe above and as shown in the figures is made of solid metal. A different design of the guide bar, not shown in the figures, provides a guide bar which is composed of three layers of solid metal sheets which are welded together, in particular by spot-welding. In this specific embodiment the two outer layers are designed in that they also define the two side walls of the peripheral or guide groove 29 for the saw chain while the inner layer defines the width of the guide groove as well as the width of the circumferential medial strip 31 of the cutout 27. With such a construction, the two outer layers may be manufactured with identical cutouts, while the inner layer re- ceives a smaller cutout in order to establish provision of the circumferential medial strip 31. Both arrangements, however, ei ^¬ ther a solid metal guide bar or a guide bar of multilayer construction are comparable with respect to their stiffness.

List of reference numerals

1 chain saw

3 housing

5 rear handle

7 lever

9 forward handle

11 protector

13 guide bar

15 slot

17 attachment holes

19 sprocket

21 rivets

23 chassis member

25 sandwich laminate sheet member

27 cutout

29 peripheral groove

31 medial strip

33 thin sheets

35 peripheral edges

37 steel fibres

37a foam plastic core

39 grooves

41 thermoplastic joint

43 indentations

45 guide bar main body

47 guide bar end portion