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Testing Machines

load, specimen, head, test, machine, applied, beam, weight and sliding

TESTING MACHINES. Testing ma chines are employed to determine the physical properties of metals, and other materials such as cement, used for engineering and structural purposes. They are used especially in the test ing of steel. The prime requisite of a material to be used in engineering operations is the property termed astrength,D which is its capa bility to withstand the action of forces that might be applied to it in various ways so as to produce tensile, compressive, bending, shear ing and twisting strains. Very often die force applied is a combination of two or more of these strains or stresses, and the metal is called to withstand also the effects of abrasion or wear. Physical tests are of two general classes — those made to determine the suitability of a grade of material for aparticular purpose, and those by which the effects of differences in chemical composition, and different methods of manufacture, on the properties of the material, are studied scientifically.

The most satisfactory method that may be employed for this purpose is to actually load a specimen of the material under test and gradually increase that load up to the breaking weight, and observe the effects thus produced. Such a method, however, is too slow and cumbersome, since the loads required for even the simplest commercial tests are very seldom less than 50,000 pounds, and loads ranging from 150,000 to 250,000 pounds are commonly used, and make the direct application of weight impracticable in most cases. Therefore, various machines have been devised by which any load, from the small est that will perceptibly affect the specimen, up to the breaking load, may be readily applied and its weight accurately determined.

Of these three are two general types — those by which the specimen is loaded by means of a hydraulic press, and those employing a screw generally combined with a train of gearing.

The most perfect testing machine, for straining, rupturing or crushing pieces of metal, without injury to the machine, while exactly measuring or weighing the strains, is the Emery testing machine, which has frictionless hydrau lic weighing heads, and is large and expansive of construction. It is used for government tests of important materials. Its disadvantages are due to the difficulty of keeping the heads filled with a suitable liquid, the difficulty of keeping the packing of the plungers in good order and the intermittent application of the load by the strokes of the pump.

In both types the load is applied in such a way that it acts through some form of weighing machine which enables the operator to deter mine instantly the weight of the load or the amount of the force being applied.

The general construction of the screw ma chine is shown by the accompanying illustration of the Riehle machine, which may be readily used to subject a suitable specimen to any one of the three classes of strains — direct tension, direct compression and the transverse or bend ing, so that the weight of the load can be ac curately determined.

Referring to the illustration, the operation of the machine may be described as follows: The sliding head (a) may be moved up or down by the two screws (bb) which are operated by power derived from any convenient source and transmitted by belts on the pulleys (cc). These pulleys are loose on their shafts, and being pro vided with a cross belt on one and an open belt on the other rotate in opposite directions. The lever (d) actuates friction clutches which con nect either of the pulleys with its shaft, so that the motion is communicated to the screws by gearing and thus raises or lowers the sliding head (a). By using the lever (e) and the hand wheel (f), the gears may be so combined as to give various speeds to the sliding head (a) as may be required in different classes of tests. The stationary head (g) is supported by col umns (hh) which rest upon the platform (k) of the scale. The weighing beam (1) of the scale carries a movable counterpoise (m) which works on rollers along the beam. A hand wheel (n) operates a screw or chain which lies along the top of the beam and enables the operator to move the poise out along the beam gradually as the load is increased and thus observe con stantly the weight of the load being applied. In some cases the poise may be arranged to be moved automatically by means of gearing op erated by electrical connections, the circuit be ing made or broken by the action of the beam as it rises or falls.

In making a tensile test, the specimen is held between two pairs of jaws, one in each of the heads (a) and (g), as shown at (t). The head (a) is then drawn down by the screws, and the pull on the specimen presses the stand ards of the head (g) down upon the platform of the scales and causes the weighing beam to rise. In a compression test, the specimen is placed upon the platform of the scale and the sliding head is drawn down upon it. In a transverse or•bending test, the specimen is placed on two supports carried by a heavy frame which is placed upon the platform of the scale, and the load is applied by means of a projection on the under side of the sliding head which presses upon the centre of the specimen when the head is drawn down.

Another machine of the screw type in exten sive use is that built by Tinius Olsen. It is similar in principle to the Richle, but it employs four screws to operate the sliding head.

These machines are not only used to test metal specimens, but may also be used to test wood, cement and other classes of building ma terial. In testing cement, small briquets of the material are made with a predetermined cross sectional area at the centre and are then sub jected to the tests as any other specimen.

In the testing of metals, the tensile test gives the simplest and most reliable data for deter mining the properties of the greatest interest in engineering operations.