The present invention relates to a circular saw for re-sawing or ripping lumber, timber and like wood, such as logs, blocks 5 cants and boards, into thinner pieces, wherein the machine includes to this end a plurality of saw blades which are mounted on a saw axle and dogged by the axle as it rotates, a drive motor, and power transmission means mounted between the motor and said axle, wherein the power transmission means 10 includes a belt, chain or like device which extends in a loop around two wheels of which one is intended to be driven by the motor and the other is intended to drive the saw axle.

Present-day requirements with regard to high production 15 rates, thin cuts, large depths of cut and smooth, even cut surfaces place high demands on the stability of both the sawing machine itself and the saw blades. The stiffness of a saw blade is dependent, among other things, on the thick¬ ness and radius of the blade. The natural resonance frequency 20 of a circular saw blade, and therewith its critical speed of rotation n _crit# is a function of saw geometry, which is defined as t/R ² , where t is the thickness of the blade and R is its outer radius. The saw blade should preferably work in the subcritical speed range, a suitable speed being n = 0.85

Z θ ⁿ crit •

In order to achieve the best possible yield, the saw cuts will also be as thin as possible. This requires the use of thin saw blades and it is necessary to give the blades the

30 smallest possible outer radius so as to obtain a high natural resonance index. However, it is not always possible to work with small saw blades, since there are occasions when the sawing machine is required to saw wood of larger sizes that require the use of a larger saw blade. Consequently, the mill

35 is normally forced to work with an excessively large and therewith thick saw blade during the major part of its production, resulting in thick cuts and therewith poor yield.

The yield loss increases by about 2 percentage units with each millimeter of increase in the width of a saw cut. A small blade not only reduces losses due to the width of the cut but also reduces the extent to which the wood needs to be oversized, because the wood is cut to size more accurately with smaller and thinner blades. In order to saw wood of different sizes to an optimum yield, it is thus necessary to use saw blades of mutually different diameters.

However, this means that it must also be possible to alter the speed at which the saw blades rotate, because a small blade will need to rotate at a greater speed than a large blade in order to achieve an optimal cutting speed. A change between saw blades of 750 mm in diameter and 550 mm in diameter requires a speed change of about 30% in order to maintain the same peripheral speed. Although this can be achieved with the aid of frequency-controlled electric motors or with the aid of hydraulic drive systems, such motors and systems are expensive in respect of powers in the order of 160-320 kW.

The main object of the present invention is to provide a circular saw with which wood can be sawn selectively with saw blades of two mutually different radii and which is con- structed to permit a switch to be readily made between two mutually different and predetermined blade speeds.

This is achieved in accordance with the invention with the aid of a circular saw of the kind defined in the first paragraph of this document which is characterized in that the transmission ratios between wheels and respective wheel axles are different; in that the wheels and their respective axles are mounted in a separate holder device; and in that said holder device can be fitted between the wheel axles while maintaining the centre distance between said axles, with a selected wheel connected to the drive motor and the other wheel connected to the saw axle so as to enable the saw

blades to rotate at one or the other of two predetermined speeds.

The saw thus enables a switch to be made between the two blade speeds very simply, therewith enabling the saw to be used for optimal sawing with two mutually different blade diameters.

The differences in said transmission ratios are conveniently achieved by using wheels of mutually different radii. With the intention of providing a very compact machine which can be readily insulated acoustically with the aid of a hood, the saw construction preferably includes a pillar stand which carries the drive motor and in which the saw axle is journal- led, wherein the blade-carrying part of the saw axle projects out from one side of the pillar stand and the other end of the saw axle and the shaft of the drive motor are provided with coupling devices on the opposite side of the pillar stand for connection with the axles of the wheels mounted in the separate holder device. The holder device is therewith suitably mounted on the pillar stand so that the device can be rotated through 180° after disconnecting the wheel axles from the drive motor and the saw axle respectively, and thereafter again connected to the drive motor and the saw axle for changing the rotational speed of the saw blades.

The holder device is preferably provided with an axle journal at half the centre distance between the wheel axles, this axle journal being guided in the pillar stand to permit the holder device to be rotated around said axle journal.

Other characteristic features of the invention will be apparent from the following Claims.

The invention will now be described in more detail with reference to an exemplifying embodiment thereof and also with reference to the accompanying drawings, in which

Fig. l is a perspective view of the main components of an

inventive saw;

Fig. 2 is a side view of the saw shown in Fig. 1; and

Fig. 3 is a perspective view of part of the saw shown in Fig. 1, with the saw blades removed.

The reference numeral 1 used in the drawings identifies an electric drive motor on whose drive shaft there is mounted a flanged coupling or clutch plate 2. The motor is supported by a pillar stand 3 in which a saw axle 4 is journalled. One end of the saw axle is provided with a flanged coupling or clutch plate 5 corresponding to the coupling plate 2 on the drive motor 1. The reference numeral 6 identifies a brake disc, while 7 identifies a pressure-medium activated brake yoke that coacts with the brake disc 6. In the case of the illustrated embodiment, the pillar stand 3 is intended to be mounted on a bottom stand 8.

The saw axle 4 extends out from the opposite side of the pillar stand 3 to an extent which enables a desired number of saw blades 9 to be fitted to the axle. The saw blades 9 are dogged by the axle 4 as it rotates, for instance by using a spline coupling or a profiled axle and providing the blades with holes whose shape corresponds to the shape of the axle. The saw blades 9 are loose in other respects, i.e. are seated freely on the axle so as to enable the blades to be moved readily in the longitudinal direction of the axle.

In this regard, the blades are held in their respective positions, determined by the saw-blade set-up concerned, with the aid of blade guides which each include a fixed part 10 and parts 11 which are pivotally connected to said fixed part. The pivotal parts define therebetween a slot in which a respective saw blade 9 is received and guided on both sides thereof. The reference numeral 34 identifies hose connections

through which an air-water coolant is delivered to the blade guides. When carrying out a blade change, the pivotal parts 11 of respective blade guides are lifted in the direction of the arrow A in Fig. 3 and the blades then removed from the axle 4.

Each of the fixed parts 10 of the blade guides is fixedly mounted on one of a plurality of mutually telescopic tubes 12-17. The positions of the tubes are set with the aid of servo-controlled hydraulic piston-cylinder devices 18 or with the aid of some other type of setting device, for instance screw jacks, each of which coacts through an associated rod 19 with the tube ends that project out on the other side of the pillar stand 3, through the medium of mounting element 20. The setting devices 18 thus enable the blade guides 10, 11, and therewith the positions of the blades 9, to be set very accurately in accordance with a desired saw-blade set¬ up. Stable attachment of the axle 4 is achieved by supporting its axle bearing or trunnion 21 with a support bearing 22 mounted on an arm 30 projecting out from the pillar stand 3; see Figs. 2 and 3.

The support bearing 22 can be moved axially in the direction of the arrow B shown in Fig. 3, so as to release engagement between the axle bearing 21 and the support bearing 22, therewith enabling the saw blades 9 to be readily removed from the axle and replaced with newly sharpened blades of the same size or of a different size, after having swung-up the blade guides 11. When changing between blades of different diameters, it is necessary also to change the rotational speed of the axle 4 so as to obtain an optimal cutting speed.

In the illustrated embodiment, the power transmission from the motor 1 to the axle 4 is effected with the aid of a flat belt 23 which extends in a loop around two wheels 24 and 25 whose axles are provided with mutually identical flanged coupling plates 26 and 27 respectively by means of which the

wheel 24 and the wheel 25 can be readily coupled with respective flanged coupling plates 2 and 5 on the motor 1 and the axle 4 respectively.

The cover surrounding the flat belt 23 has been omitted in the Fig. 2 illustration and the Figure shows a threaded tensioning device 31 by means of which the belt 23 can be tightened.

The wheels 24 and 25 of the illustrated embodiment have mutually different diameters, to enable the rotational speed of the axle 4 to be switched between two predetermined values. The wheels 24 and 25 are mounted with a constant centre distance in a separate holder device 28 which in the case of the illustrated embodiment includes a guide pin 32 mounted centrally between said wheels and coacting with a sleeve 33 mounted on the pillar stand 3. The rotational speed of the axle 4 is changed simply by releasing the mutually connected flanged coupling plates and slightly withdrawing the holding device 28 and rotating the same through 180°, so that the coupling plate 26 which was earlier connected with the coupling plate 2 on the motor 1 can now be engaged with the coupling plate 5 on the axle 4 and the other coupling plate 27 engaged with the coupling plate of said motor. The axle 4 will then rotate at the other of said two predeter¬ mined speeds. Neither the motor nor the saw axle is affected by this repositioning of the transmission means.

As a result of constructing the whole of the saw on the pillar stand 3, there is obtained a very compact construction wherein the saw blades 9 can be enclosed in a sound insulated protective casing 34. The casing 36 includes a feed opening 35 through which wood is fed to the blades, and a correspond¬ ing outfeed opening on the opposite side of the casing.

The aforedescribed circular saw construction enables saw blades to be readily exchanged and replaced in combination

with accurate guiding of the blades during a sawing opera¬ tion. The rotational speed of the blades can also be readily switched between two predetermined values, which are chosen to obtain optimum blade speeds. When the transmission provides a transmission ratio of about 1.15:1, there is obtained an upshift and a downshift in the speed of the saw axle that results in the cutting speed being generally equal to shifts between cutting blades of 0800 and 0600 or 0750 and 0 560, and so on.

Although the invention has been described in the aforegoing with reference to a preferred embodiment of a circular saw, it will be understood that the saw can be modified in several respects within the scope of the following Claims. For instance, the flat drive belt can be replaced with a ribbed or toothed belt or a chain which runs over toothed or ribbed wheels. The shift between the input and output axles of the wheels can also be achieved in a manner different to that shown and described.

Because the transmission means used in the inventive machine includes a unit that includes wheels which are mounted at a constant distance apart, the belt or like device can be made very tight so as to prevent tendencies of the belt to slip, since the belt tensioning forces are taken up by the unit and are not transmitted to the motor axle and the saw axle in the form of transversely acting forces.