Grips for Testing Lumber in Tension

This article describes grips designed for tension testing of full-size structural lumber without damaging the wood fiber. The grips are essentially U-shaped assemblies having heavy steel legs which are securely bolted to a heavy, inner steel block. The assemblies are mounted on the crossheads of the testing machine by means of a threaded stud and nut which are aligned on the axis of symmetry of the apparatus. Self alignment and rotational freedom of the assembly are obtained through a spherical seat between a bearing plate and the nut. The ends of lumber specimens are gripped within the assemblies by means of steel wedges which are, actuated by four pheumatic jacks interconnected through a gear sector and lever system. The gripping surfaces of the wedges are grooved transversely to the length of the specimen, the projection being slightly chamfered to provide a somewhat saw-tooth shaped profile. The wedges, which are 15 inches in length and can accommodate lumber of a full 8-inch width, have a slope of 1 to 11. Adjustable stops were installed to limit the total travel during load application, and hence the crushing action of the wedges during loading. Wedge-travel settings to transmit load effectively to failure of the lumber without unduly damaging the wood fiber were determined experimentally.

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