. Electric railway journal . e-pending upon the available material to be tested. Stand-ard commutator bars of a tapered section are used forthe test specimen of hard drawn copper bars. (SeeFig. 20 R). The method of making a test consists ofplacing the test piece on supports 1 in. apart, with dialgage under the middle to indicate the deflection. Theload is applied at the center of the bar until deflection is obtained. The stress is thenfigured from the following formula. Where S = S = Fiber stress in pounds per square = Load in pounds. L = Distance between supports in
. Electric railway journal . e-pending upon the available material to be tested. Stand-ard commutator bars of a tapered section are used forthe test specimen of hard drawn copper bars. (SeeFig. 20 R). The method of making a test consists ofplacing the test piece on supports 1 in. apart, with dialgage under the middle to indicate the deflection. Theload is applied at the center of the bar until deflection is obtained. The stress is thenfigured from the following formula. Where S = S = Fiber stress in pounds per square = Load in pounds. L = Distance between supports in inches. B = Width of piece in inches. D — Thickness of piece in inches. The results obtained from a sample section of a com-mutator bar showed a fiber stress of 21,350 lb. neces-sary to produce in. deflection. A Brinell hardness tester is shown in Fig. 3. Thehardness test can be made on almost any section andshape of test specimen not too large to be placed in thejaws of the testing machine. In case of gears and pin-. 0 20 40 60 80 100 HO 140 140 160 200 220 240 260hoiqnetiioition, Ampere. Turns per Inch of Length FIG. 17—MAGNETIZATION CURVE FOR CASTSTEEL SAMPLE Ions, the test is made on the finished product. test pieces are made up, they are I in. thick andtaper from 15 in. to 2 in. in diameter, as per sampleH, Fig. 20. How TO Obtain Brinell Hardness To carry out this test a 10-mm. hardened steel ball ispressed into the test specimen by a force of 500 or 3000kg. (depending upon the metals being tested), by hy-draulic pressure. The spherical impression of indenta-tion indicates the hardness of the material. The Bri-nell number is obtained by dividing the applied load bythe curved area of the impression, which is obtained bycalculation from the diameter of the impression. Some results which were obtained from actual testson metals are given below. Material Babbitt bearings . forged-steel pinions. Brinell HardnessNumber30512 324 Electric Railway Journal
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