Cutlery – Cutting tools – Saw
Patent
1993-07-15
1995-03-14
Jones, Eugenia
Cutlery
Cutting tools
Saw
144 34R, 83819, B27B 1714
Patent
active
053967054
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
Tree harvesting machines with saw chain and guidebar of types corresponding to those used with hand-held chain saws are known from among others the patents U.S. Pat. No. 3,991,799, 4,116,250, 4,161,200, 4,219,059 and 4,552,191. In these the guidebar is attached to a mounting base rotatable around the driving shaft from the motor to the drive sprocket for the saw chain. The guidebar and saw chain are advanced towards the tree to be felled by a hydraulic cylinder acting on the mounting base or a shaft connected to it.
In tree harvesting machines the saw chain is highly stressed, runs at high speeds and wears rapidly. This leads to expensive downtime for changing or tensioning of the saw chain, and to the possibility that the saw chain becomes so slack that it will jump off the guidebar.
It is important for the safety that the saw chain is tensioned with appropriate force, since too low force might allow the links of the chain to tilt and leave the guiding groove of the guidebar, and too high force will impair the lubrication of the saw chain and wear the rivets too soon.
In prior art, the tensioning force in the saw chain is normally adjusted by loosening the bolts holding the guidebar, followed by moving the guidebar lengthwise by a screw mechanism, and finally retightening said bolts. This does not ensure proper control of the tensioning force since there is no means for measuring the force, and since the force is affected by wear and temperature rise of the saw chain in use.
This adjustment procedure requires that the machine is stopped, and that the operator or another skilled person performs the adjustment with appropriate tools. The loss of production time and the need to leave the operator's cab make it likely that the adjustment is seldom done except when the saw chain or the guidebar has to be changed, which is too infrequent.
SUMMARY OF THE DISCLOSURE
The invention is a mounting base for a guidebar with an automatic tensioning device which holds the saw chain tensioned with a predetermined force without any action from the operator, and which allows a quick change of the saw chain without need for loosening of any bolts. This permits more operating time and improved lifetime of the saw chain.
Automatic tensioning of the saw chain is known in prior art from the patents U.S. Pat. No. 3,194,284 and 3,636,995, where a mechanical spring in the mounting base urges the guidebar outwards, U.S. Pat. No. 2,316,997 where a spring in the machine urges the nose part of the guidebar outwards by a lever connection, U.S. Pat. No. 2,532,981 and 4,486,963 where said nose part is urged outwards by a spring within the guidebar, and U.S. Pat. No. 3,279,508 where the whole guidebar is urged outwards by a spring in the guidebar. In practice, none of these arrangements has been a success, since a guidebar urged outwards by a mechanical spring with a force suited to the saw chain can also be urged inwards by some undesired external force acting on the nose part of the guidebar, such as when felling closely standing trees, with the result that the tension in the saw chain is lost so that it jumps off the groove in the guidebar. This can also happen if the tree to be felled leans backward and the non-cutting part of the saw chain is jammed. A method is shown in U.S. Pat. No. 4,486,963 to make the force required for inward displacement of the guidebar is higher than the force urging the guidebar outwards, but since this method depends on dry friction, it does not function properly in practice due to the effect of generous lubrication and strong vibrations in the guidebar.
The invention describes a mounting base designed to urge the guidebar outwards with a limited and predetermined force, still leaving the guidebar totally unaffected by external forces against the nose part of the guidebar.
A mounting base according to the invention comprises a rigid plate, rotatably journalled around the drive shaft. In the plate is a radial slot, along which a guide piece can slide, said guide piece being attached to the guid
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Jones Eugenia
Sandvik AB
Schrock Allan M.
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