The Mechanical Properties of Wood

Excerpt: ... used only for tension and cleavage tests, consists of a steel cage above the movable crosshead and rests directly upon the weighing platform. The top of the cage contains a square hole into which one end of the test specimen may be clamped, the crosshead containing a similar clamp for the other end, in making tension tests. For testing long beams a special form of machine with an extended platform is used. (See Fig. 29 .) The weighing platform rests upon knife edges carried by primary levers of the weighing apparatus, the fulcrum being on the bed of the machine, and any pressure upon it is directly transmitted through a series of levers to the weighing beam. This beam is adjusted by means of a poise running on a screw. In operation the beam is kept floating by means of another poise moved back and forth by a screw which is operated by a hand wheel or automatically. The larger units of stress are read from the graduations along the side of the beam, while the intermediate smaller weights are observed on the dial on the rear end of the beam. The machine is driven by power from a shaft or a motor and is so geared that various speeds are obtainable. One man can operate it. In making tests the operation of the straining screws is always downward so as to bring pressure to bear upon the weighing platform. For tests in tension and cleavage the specimen is placed between the top of the stationary cage and the movable head and subjected to a pull. For tests in transverse bending, compression, and cleavage the specimen is placed between the movable head and the platform, and a direct compression force applied. Testing machines are usually calibrated to a portion of their capacity before leaving the factory. The delicacy of the weighing levers is verified by determining the number of pounds necessary to move the beam between the stops while a load of 1,000 pounds rests on the platform. The usual requirement is that ten pounds should accomplish this movement....