Locating Maximum Stresses in Tooth Assemblies of Inserted-Tooth Saws

Breakage of the parts of inserted-tooth assemblies of inserted-tooth log saws has been attributed to the interaction of the assembly parts during the cutting action and ultimately to metal failure due to fatigue. An investigation was made to determine to what extent the design of the saw-tooth assembly was a factor in this problem. The location and magnitude of the maximum stress was the primary purpose of the investigation. A stress-sensitive brittle lacquer technique and a technique using photoelastic plastic were tested. The technique involving the use of solid plastic sheets (with reflective backing) attached to the assembly parts was the most satisfactory. In the photoelastic plastic trials using a Style B tooth in a saw segment that was mounted in a special fixture in a universal testing machine, a stress of about 30,000 psi occurred on the back of the tooth under a load of about 750 pounds. In another test with the Style B assembly and saw segment to investigate stresses within the tooth assembly, due to the action of inserting the tooth and holder in the tooth socket, a maximum compressive stress of 105,250 psi was found at a point adjacent to the hole in the holder which receives the leverage pin of the tooth inserting wrench. The compressive stress developed from the act of inserting the tooth and holder was about 59.7 percent of the strength of the steel. Additional trials to obtain stress data resulting from the action was a Style 3 and the portion involved included three adjacent tooth assemblies, the center assembly being the one used for the inserting procedure. Three different holders were used An original holder classified as a tight fit; an original holder classified as a loose fit; a new holder manufactured by a different saw manufacturing firm. The maximum stress values in these three holders were found to be as follows: Tight holder, 84,400 psi; loose holder, 21,100 psi; and new holder 147,600 psi. A test for contact bearing points for the holder in the socket showed only the end points were involved, indicating a flexing action of the holder. This condition together with the high concentration of stresses conceivable could contribute to metal fatigue. The need for more research was indicated.

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