VIBRATION DAMPENING SAW CHAIN

Abstract: A saw chain length including articulated alternating centre drive links (14) and side links. Cutting links (16,22) provide certain of the side links and a safety side link (20,24) precedes each cutting link. The safety side link and its opposing tie strap link have shaved heel portions (42) and thereby are raised off the guide bar when normally tensioned around the guide bar. The safety link (20,24) is configured whereby engagement thereof by material being cut during a cutting operation induces pivoting of the safety link against the opposing tension force, which tension force thereby absorbs the impact prior to engagement of the heel portion with the guide bar edge.

Description: The invention relates to a saw chain in the form of a continuous loop designed to be received on a chain saw guide bar. A chain saw comprises a loop of saw chain consisting of interconnected links, some of which carry cutting teeth, a guide bar that guides and supports the loop of saw chain for rapid rotation around the guide bar, and a power head which drives the saw chain by way of a drive sprocket and which also provides handles that the sawyer grips for directing the cutting action of the chain saw. Wood is non-homogeneous and its resistance to cutting is inconsistent and therefore rough. The chain saw is necessarily a very powerful tool and one that inherently demands concern for both safe operation and cutting performance. A primary concern of recent years arises from the occurrence of "kickback". Kickback occurs when a cutting tooth "hooks" into a limb or log (generally while passing around the outer end or nose of the guide bar) in such a manner as to induce rearing of the chain and bar and possible injury to the sawyer. Such kickback action has been a target on which design improvements have been concentrated, and it has been substantially reduced by providing guard-like protrusions that precede the cutting teeth. A secondary consideration of chain saw design relates to the incidence of vibration. The very nature of a rotating cutting chain when engaging and cutting a tree or log, whereby small wood chips are rapidly ripped from the log, creates vibration. This vibration, over time, is detrimental to the sawyers health and causes what is termed in the industry as "white fingers disease". This is a numbness that develops in the sawyers hands after extended use of a chain saw. The invention of U.S. Patent 4 122 741 has as its objective a reduction in the vibration that occurs in the operation of a chain saw. It was determined that vibration was largely caused when a cutting edge engaged a hard spot in the wood, for example, a knot. The cutting link was pivoted about its heel and because the cutting edge was forward of the heel, it pivoted deeper into the wood causing the chain to jerk or vibrate. This action was alleviated by changing the geometry of the cutting link whereby the heel was elevated off the bar so that a cutting impact caused the cutting edge to pivot around the front rivet, that is, out of the wood. Not disclosed or suggested in U.S. Patent 4 122 741 is a safety link which is often added to the sequence of chain links to decrease the likelihood of kickback at the nose of the guide bar. A saw chain incorporating such a safety link is disclosed in U.S. Patent 3 910 148. On the straight reach of the bar a guard portion of the safety link projects toward the wood but is normally short of the cutting edge and does not cut. Its presence in the chain link sequence during a typical cutting operation nevertheless induces vibration that is not explained by the theory of U.S. Patent 4 122 741. Furthermore, the vibration is present even where the guard portion is located over the rear rivet of the link, that is, where rearing of the link would not increase the outward projection of the guard portion. Whereas the increased vibration is a problem for the sawyer, nevertheless the safety link is necessary for solving a problem of higher priority, that is, reducing the likelihood of kickback. Consequently the sawyer has heretofore simply lived with the vibration caused by the safety links. U.S. Patent 3 910 148 discloses a saw chain formed into a continuous loop and designed to be entrained on the edge of a guide bar of a chain saw, the saw chain comprising alternating centre links and pairs of side links, and front and rear rivets pivotally interconnecting the centre links and the side link pairs, some of the side link pairs comprising a cutting link and an opposed tie strap link, each of the side link pairs which includes a cutting link having a preceding side link pair comprising a safety link and an opposed tie strap link, the links of the preceding side link pair each having a toe portion and a heel portion positioned under the front and rear rivets respectively, each of the toe heel portions having bar engaging surfaces, and each safety link including an upwardly projected guard portion. In accordance with the invention, such a saw chain is characterised in that the bar engaging surface of each heel portion is spaced below the rear rivet a distance less than the distance of the bar engaging surface of the toe portion below the front rivet, whereby tension applied to the saw chain about the guide bar that normally maintains the rivets in alignment raises the heel portions of the links of the preceding side link pairs off the guide bar edge. The invention is based on a re-evaluation of the cutting chain, not simply in consideration of how a cutting link functions to cut wood chips, but how the combination of the safety links and cutting links, in a safety design cutting chain, act in concert to generate vibration. In such evaluation, it was concluded that the presence of the guard portion or protrusion preceding the depth gauge and cutting edge of a cutting link, engages and absorbs impact by its proximate location in the kerf, that is, it engages irregularities or chips present in the kerf. In U.S. Patent 4 122 741, the jerking vibration is thought to be induced by the cutting edge of a cutting link digging in deeper as the link is pivoted. This jerking action is conveyed through the chain links and felt by the sawyer as a form of vibration. It is avoided by changing the geometry of the link to cause the cutting edge to pivot away from, rather than into, the kerf. In contradistinction to this, it is thought that safety link induced vibration is caused by the rapid, brief, non-cutting engagement of the protruding guard portion with an irregularity or chips in the kerf. This causes the safety link to snap back and in effect to hammer the guide bar. The present invention is thus directed to alleviating the vibration contributed by the hammering action of the safety link by designing the safety link, in combination with the chain link guard portion, preferably at the rear of the link, whereby the tension of the chain in operation acts as a shock absorber. The snapping back action described is simply absorbed by the chain tension and not by the guide bar. Vibration in a safety link chain has been found to be significantly reduced in accordance with the invention. The invention is further described below by way of illustration, with reference to the accompanying drawings, wherein: Fig. 1 is a side view of a length of a saw or cutting chain embodying the invention, as supported on a guide bar in free running condition; and Fig. 2 is a view similar to that of Fig. 1 but showing the chain, and particularly a safety or bumper link thereof, in reaction to engagement with a log or tree in a cutting action. In the drawings, a broken line 10 represents the peripheral edge of a guide bar in which a groove is formed, the bottom of the groove being represented by a broken line 12. A length of a saw chain in accordance with the present invention includes centre drive links 14 having drive tangs 15 that ride in the groove of the guide bar. These drive tangs 15 entrain the chain on the grooved edge of the guide bar and are engaged by the teeth of a drive sprocket located at the rear end of the guide bar. Interconnecting the centre drive links 14 are side links that, in general, ride on the bar edge represented by the line 10. These side links include, in sequence back to front, that is, left to right in the drawings, first, second and third, and fourth pairs of side links. The first pair of side links consists of a left hand cutting link 16 having a cutting edge 36 preceded by a depth gauge 34, and a tie strap 18 opposite the cutting link. The second pair of side links consists of a right hand safety or bumper link 20 with a protrusion or guard portion 32 and a tie strap opposite the bumper link and hidden thereby. The third pair of side links consists of a right hand cutting link 22, also with a cutting edge and depth gauge, and a hidden tie strap opposite thereto. The fourth pair of side links consists of a left hand safety or bumper link 24 with guard portion 32 and a tie strap 26 opposite thereto. The pairs of side links are pivotally connected to the drive links 14 by rivets 28. The links just described represent a full sequence of the links in the saw chain, which sequence is repeated with a drive link 14 preceding side links 24,26 which in turn is preceded by side links 16,18 etc. The general relationship just described of the safety link protrusion or guard portion 32, the depth gauge 34 and the cutting edge 26 is not new with the present invention. Prior safety links have taken many forms, commonly with a guard portion projecting outwardly, or upwardly from the bar as depicted, to the approximate height of the depth gauge 34, with a leading front edge that is configured so as to avoid hooking, that is, it is slanted upwardly and rearwardly. In accordance with the present invention, the bumper guard or safety link guard portion 32 projects rearwardly and upwardly over the vicinity of its rear rivet 28r, as distinguished from front rivet 28f. A general specification for the guard portion is that the outermost projection of the guard portion 32 (outermost being upwardly as viewed in the drawings) is located rearward of a vertical line, as viewed, centered between the front and rear rivets. It is also desirable that the right safety link 20 precede the left cutting link 16 so that interference does not occur therebetween. The desirability of this configuration for the safety links 20 and 24 is for the purpose of accommodating shock absorbing pivoting of the safety links as will now be explained. It will be observed from Fig. 1 that the tie strap 18, opposite cutting link 16, and the cutting link 22 have front and rear bar engaging portions 38,40 (sometimes referred to as the toe and heel portions of the link) that ridge, in a non-cutting mode, directly on the edge 10 of the guide bar. Although not shown, it will be understood that the cutting link 16 and the tie strap opposite cutting link 22 similarly have toe and heel portions 38,40. This is true of every other like pair of side links. The intermediate side links, comprising the safety link 20 and the opposing hidden tie strap, and the safety link 24 and its opposing tie strap 26 are provided with toe portions 38 but with heel portions 42 which are raised off the bar edge in the non-cutting mode so as to create gaps 44. Although the gaps 44 can vary, it has been found that for a 0.95 cm (3/8 inch) pitch chain, wrapped around a guide bar and tensioned in accordance with chain saw manufacturer's instructions, this gap should be in the range of about .025 cm. (.010 inch) to .064 cm. (.025 inch) and preferably about .051 cm. (.020 inch), that is, the heel portion 42 is .051 cm. (.020 inch) shorter than the heel portions 38 and 40 in its extent below the rivet or, more precisely, below the rivet hole opening. A rivet hole opening 46 is illustrated for side link 26 and represents the rivet holes provided in the side links for all of the rivets 28, including rivets 28r and 28f. Fig. 2 illustrates the reaction of the safety link 20 or 24 when it engages material to be cut. Arrows indicate the force 48 applied against the protrusion or guard portion 32. The result is that the link pairs comprising safety link 20 and the opposite tie strap, and safety link 24 and tie link 26, rather than transmitting the force 48 as a jarring impact against the bar edge represented by line 10, rotate around the pivot point under the front rivet 28f, as permitted by the spacing between the heel portion 42 and the bar edge, that is, the gap 44. The gap 44 is essentially closed, depending on the angle and degree of force 48, but because of the chain tension, this pivoting of the links is strongly resisted, for example, in a manner similar to a strong spring holding the links away from the bar. The force 48 is thus largely absorbed by the resisted pivoting of the links and to a substantial degree, vibration is alleviated. The invention is not limited to the specific embodiment herein described and depicted, but can be variously embodied within the scope of the appended claims.

Claims: A saw chain formed into a continuous loop and designed to be entrained on the edge of a guide bar of a chain saw, the saw chain comprising alternating centre links (14) and pairs of side links, and front and rear rivets (28) pivotally interconnecting the centre links and the side link pairs, some of the side link pairs comprising a cutting link (16,22) and an opposed tie strap link, each of the side link pairs which includes a cutting link having a preceding side link pair comprising a safety link (20,24) and an opposed tie strap link, the links of the preceding side link pair each having a toe portion (38) and a heel portion (42) positioned under the front and rear rivets (28f,28r) respectively, each of the toe and heel portions (38,42) having bar engaging surfaces, and each safety link (20,24) including an upwardly projected guard portion (32), characterised in that the bar engaging surface of each heel portion (42) is spaced below the rear rivet (28r) a distance less than the distance of the bar engaging surface of the toe portion (38) below the front rivet (28f), whereby tension applied to the saw chain about the guide bar that normally maintains the rivets (28) in alignment raises the heel portions (42) of the links of the preceding side link pairs off the guide bar edge. A saw chain as claimed in claim 1 wherein the outermost point of the guard portion (32) is rearward of a vertical line centered between the front and rear rivets (28f,28r). A saw chain as claimed in claim 2 wherein the guard portion (32) is projected upwardly and rearwardly at a position substantially over the rear rivet (28r). A saw chain as claimed in claim 1, 2 or 3 wherein the differential in the spacing of the bar engaging surfaces of the toe and heel portions (38,42) is between .025 and .064 cm. (.010 and .025 inch). A saw chain as claimed in claim 4 wherein the differential in the spacing of the bar engaging surfaces of the toe and heel portions (38,42) is about .051 cm. (.020 inch).

Inventors: MACGAVIN, DONALD J.
Applicants: BLOUNT, INC.
Application year: 1987
Main classification: B27B33
IPC codes: B27B33