Cutlery – Cutting tools – Saw
Patent
1986-09-24
1988-01-05
Watts, Douglas D.
Cutlery
Cutting tools
Saw
188 77R, B23D 5702, B23D 5900
Patent
active
047166541
DESCRIPTION:
BRIEF SUMMARY
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 gripped 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 kickback 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 operator'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 braking device is usually actuated by a so called kickback guard. The construction of such kickback guards are based upon the observation that the saw operator's front hand will fail to maintain its grip against the front handle of the power saw during 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 front hand if it will loose its grip due to a kickback. The hand thereby impacts the kickback guard with a force which will be transmitted to a brake means, for instance a brake band, which will 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 provide an automatic saw chain braking device which is actuated as a response to a detection of a predetermined angle of kickback, i.e. without having to affect the kickback guard. These automatic chain braking devices are, however, rather complicated and expensive, and they cannot be mounted on existing power saws.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a device of the aforesaid kind which does not require any co-operation of the saw operator's front hand, and which does not require any complicated automatic equipment, but which is of simple 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 will at least initially remain in its initial position when a kickback occurs, whereas the rear hand, which grips on the rear handle, will be wrenched relative to the saw operator's arm. This is due to physiological reasons according to which the hand can very easily be wrenched about the wrist from a physiological normal position, in which the hand and the arm are on a line with one another, upwards in approximately 30.degree. to a distinct stop in the wrist. Accordingly, the saw operator's rear hand will, 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 will initially be maintained due to a cramp contraction of muscles, but will then successively open, beginning from the little finger side of the hand. Only when the hand has been displaced by a wrenching about the wrist to said distinct stop, i.e. after a wrenching movement of approximately 30.degree., the saw operator's arm will be able to rotate upwards. However, at this time the power saw will often have swung so far upwards that the saw chain has already hit the saw operator's head. The upward movement of
REFERENCES:
patent: 4077125 (1978-03-01), Fuller
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