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.

The size of machine suitable for compression tests on 2” X 2” sticks or for 2” X 2” beams with 26 to 36-inch span has a capacity of 30,000 pounds.

SPEED OF TESTING MACHINE

In instructions for making static tests the rate of application of the stress, i.e., the speed of the machine, is given because the strength of wood varies with the speed at which the fibres are strained.  The speed of the crosshead of the testing machine is practically never constant, due to mechanical defects of the apparatus and variations in the speed of the motor, but so long as it does not exceed 25 per cent the results will not be appreciably affected.  In fact, a change in speed of 50 per cent will not cause the strength of the wood to vary more than 2 per cent.[58]

[Footnote 58:  See Tiemann, Harry Donald:  The effect of the speed of testing upon the strength and the standardization of tests for speed.  Proc.  Am.  Soc. for Testing Materials, Vol.  VIII, Philadelphia, 1908.]

Following are the formulae used in determining the speed of the movable head of the machine in inches per minute (n): 

(1) For endwise compression n = Z l