TECHNICAL FIELD

The present invention relates to a device for braking the saw chain of a portable power saw, the saw having a front handle to be gripped by one hand of the saw operator, and a rear handle to be gri ped by the operator's other hand, below referred to as the rear hand, the device having a brake means capable of braking the saw chain by means of an actuating member arranged to be operated by the saw operator

BACKGROUND ART

When using a portable power saw there is a risk of so called ickback accidents, i .e. the saw sword with its saw chain is violently swung upwards so that it hits the saw operator.

This type of accident often seriously injures the saw opera¬ tor' s head, arms or upper body.

In order to reduce the risk of kickback accidents, the power saw is often provided with a chain braking device which is actuated as a result of a kickback. The chain bra ing de¬ vice is usually actuated by a so called kickback guard. The construction of such kickback guards are based upon the obser vation that the saw operator's front hand wi ll fai l to main- tain its grip against the front handle of the power saw du¬ ring a kickback. Therefore, the kickback guard consists of a safety handle or safety "glove" being pivotably mounted in front of the the front handle and intended to catch the fron hand if it wi ll loose its grip due to a kickback. The hand thereby impacts the kickback guard with a force which wi ll be transmitted to a brake means, for instance a brake band, which wi ll brake the coupling drum and thereby brake the saw chain. In some power saws the motion of the kickback guard is arranged also to release the drive motor of the saw from the coupling drum.

However, it has turned out that the saw operator's front hand does not always hit the kickback guard with sufficient force in order to actuate the chain braking device.

In order to solve this problem it has been suggested to pro¬ vide an automatic saw chain braking device which is actuated as a response to a detection of a predeter ined angle of kickback, i .e. without having to affect the ickback guard. These automatic chain braking devices are, however, rather complicated and expensive, and they cannot be mounted on e is ing power saws.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a device of the aforesaid k nd w ich does not require any co-opera¬ ion of the saw operator' s front hand, and which does not requ re any complicated automatic equipment, but which is of si ple construction and capable of being mounted on existing power saws. This object is achieved by the braking device constructed in accordance with the present invention and having the characteristic features set forth in Claim 1.

Further developments of the invention are set forth in the depending claims.

The present invention is based upon the observation that the rear arm of the saw operator wi ll at least initially remain in its initial position when a kickback occurs, whereas the rear hand, which grips on the rear handle, wi ll be wrenched relative to the saw operator's arm. This is due to physio¬ logical reasons accord ng to which the hand can very easi ly be wrenched about the wrist from a physiological normal po¬ sition, in which the hand and the arm are on a line with one another, upwards in approximately 30° to a distinct stop in the wrist. Accordingly, the saw operator's rear hand wi ll, if a kickback occurs, initially be wrenched - together with

the power saw - relative to the saw operator's arm. The saw operator's hand grip on the rear handle wi ll initially be maintained due to a cramp contraction of muscles, but wi ll then successively open, beginning from the little ^' finger side of the hand. Only when the hand has been displaced by a wrench ng about the w ist to said distinct stop, i .e. after a wrenching movement of approximately 30°, the saw operator' arm wi ll be able to rotate upwards. However, at this time the power saw wi ll often have swung so far upwards that the saw chain has already hit the saw operator's head. The up¬ ward movement of the power saw is, therefore, substantially a rotary motion with the centre of rotation be ng located approximately at the centre at the rear handle, i .e. in or in the vicinity of the rear hand.

Accordingly, the ai of the invention is to use the physio¬ logical coercion wrenching movement of the rear hand in or¬ der to actuate the saw chain brakin»g device ^' . This aim is achieved by locating the actuating member in such a position relative to the rear handle that it can be operated by the operator' s rear hand as a result of a wrenching movement of the rear hand relative to the rear arm of the saw operator if a kickback of the power saw wi ll occur.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention wi ll be described in more detai l with referenc to a preferred embodiment thereof i llustrated in the accom- paying drawings; in which

Figure 1 is a perspective view of a normal working position when using a conventional portable power saw, whereas Figure 2 i llustrates the conventional power saw during a kickback. Figures 3a and 3b i llustrate the saw operator' s rear hand in a normal working position, and during a kickback, respective ly. Figure 4 is a perspective view of a conventional portab le power saw being provided with a braking device in accorda

ce with the invention. Figure 5 i llustrates schematically a side view of a braking device in accordance with the in¬ vention and operatively connected with a brake means for the saw chain.

DESCRIPTION OF A - PREFERRED EMBODIMENT

Figure 1 i llustrates a normal working position for a conven¬ tional portable power saw 1 having a sword 2 and a saw chain 3. A housing 4 for the saw is provided with a front handle 5 which is to be gripped by one hand 6 of the saw operator, and a rear handle 7 to be gripped by the other, rear hand 8 of the saw operator. For a better understanding of the phy¬ siological positions of the hands 6 and 8, these have been shown without safety gloves, although such of course should be used by the saw operator.

The power saw is further proyided with a conventional kick¬ back guard in the shape of a safety or parry handle 9 being pivotably mounted in the housing 4 in front ofthe front handle 5. The actuating function of the safety handle 9 is schematically shown in Figure 5. The safety handle 9 is connected to one end of a brake band 11 extending enti rely or partly ci rcular about a coupling drum 10 that drives the saw chain 3. In its repose position, shown in Figure 5, the brake band has been brought out of contact with the coupling drum 10. When subjected to a forward movement, for instance due to a kickback the safety handle 9 wi ll be displaced to the position 9 ' shown in a dashed line in Figure 5, this dis- placement causing the brake band 11 to be brought into con¬ tact with the coupling drum 10 such that the motion of the saw chain 3 wi ll be stopped.

Figure 2 i llustrates how the front hand 6, during a kickback, fai ls to maintain its grip on the front handle 5. The hand 6 is thereby caught by the safety handle 9 which is thus affected by the hand 6 by an impact force which brakes the

saw chain in the manner described above.

The rear hand 8 of the saw operator wi ll grip on the rear handle 7 only by a cramp contraction of muscles during the kickback, but wi ll wrench from a normal position, shown in Figures 1 and 3a> to a stop position that is determined by the physiological properties of the hand, shown in Figures 2 and 3b. This wrench extends over an angle of approxima¬ tely 30°.

The above-mentioned wrench of the saw operator' s rear hand is used, according to the present invention, in order to operate an actuating member which is capable of actuating a brake means for the saw chain. The actuating member is shown in Figures 4 and 5 in the form of an L-shaped metal tube 12, one shank 13 of which having its free end 14 being angularly bent and rigidly connected to the safety handle 9 by means of two screws 'or ri ets 15 and 16. The other shank 17 of the L-shaped tube 12 is orientated with its lengthwise di rection extending substantially perpendicular to the plane of the saw sword 2, and extending a few mi llimetres above the back of the rear hand 8. It has turned out that the wrench of the hand 8 that wi ll occur during a kickback, in the manner described above, is sufficient to affect the shan 17 with such an upwardly di rected force that the end 14 of the L-shaped tube 12 wi ll transmit a forwardly directed force to the safety handle 9, thereby displacing the same forwardly to the position shown in dashed lines in Figure 5. The wrench of the hand 8 wi ll thereby cause a braking of the saw chain 3 which, in conventional power saws, instead is achieved by a catching of the front hand against the safety handle 9.

It wi ll be apparent that the actuating member 12 may have a shape different to the L-shape i llustrated. Accordingly, any other means which is capable of detecting a wrench of th rear hand may be used for actuating the chain braking device

in accordance ith the invention. In the embodiment shown the actua ing member 12 is operated by the back of the hand, but it may alternatively be operated by the front part of the hand or by the palm.

In the embodiment shown ^' in Figure 5 the safety handle 9 acts directly on the brake band 11. It is also conceivable, in an alternative embodiment, to connect the handle 9 or the actuating member 12 to the brake band 11 via a biased latch which is held by a spring and which is to be released by the handle 11 or by the member 12 as a result of a kickback, such that the brake band 11 will be brought into contact with the coupling drum 10. In this case the latch must, of course, again be biased before the power saw can be used again. Such latches are known per se in conjunction with saw chain bra¬ king devices for power saws, and are, therefore, not illustra ^¬ ted here.

In an alternative embodiment, not shown, the actuating mem- ber 12 is not secured to the safety handle, but is instead mounted on the housing 4 in a manner to enable it to be dis¬ placed by the wrenching movement of the rear hand. The actu¬ ating member is then preferably pivotably mounted on the housing 4 and operatively connected to a pneumatic, hydrau- lie or electronic means capable of being operated as a re¬ sult of a pivoting movement of the actuating member caused by the wrenc ng of the rear hand of the operator. Such means are known per se to those skilled in the art and are, therefore, not shown or described in detai l.