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Aviation History
1917
1917 - 0069.PDF
JANUARY 18. 191-. [/TIGHT interest to observe that the hysteresis low is greater lorgreater elongations. The initial length of an unstrctched rubber is taken as thehalf mean perimeter or 3 14 in. The bridge failed at a load -The landing gear bridgerings. with twelve rubber fitted 12 rubber rings z ins. x 2 ins. x T*j- in., similar to thosedescribed above in connection with Grade A rubber. Each ring was passed over a J-in. steel pin. The laboratory report of the test follows : Department of Mechanical Engineering, Massachusetts Institute of Technology, Testing Materials Laboratory, Dec. 21st, 1915. HVSTEKESIS TEST ON AEROPLANE SHOCK ABSORBER, AND TEST FOR ULTIMATE STRENGTH OF SAME. Specimen : Axle—Shelby tubing, 2 ins. in diameter by £ in. thick.Bridge and plate—Cold rolled steel, J in. thick. Rubbers—-Twelve 2 in. x Js in. commercial rubber rings.Pins—J in. steel. The Test:The elongations at the various loadings were obtained by taking the distance (average) between four pairs of Centrermseh marks. Three runs for hysteresis were made as follows : First Run. Second Run. Third Run.Load. Elongation. Load. Elongation. Load. Elongation. Utss. Ins. Lbs. Ins. Lbs. Ins. 2rOQO 2,300 3.500 4,000 3.500 3,000 a,5O© 2.OOO I, JOB 1,000 •22 •48 •78 •10 •40 •70 •08 •o7•98 •83 57 10 65 •21 1,000 2,000 3,000 4,000 3,000 2,000 1,000 500 •4* 1-25 1 -98 2-45 2-26 1-74 •70 •23 1,000 2,000 3,000 4,000 3,000 2,000 1,000 500 •52 1-37 2 -6o 2-45 1 91 •78 •28 At the beginning of the test, with an initial load of 500 lb.,tte* average distance between centre punch marks was 2 -27 ins. A fourth run was then made for ultimate strength. At aload of 9,750 lbs., with a corresponding elongation of 5 • 06 ins., the lower bridge failed by buckling under the axle, and onepi» at that point failed by shearing off on the inside of the plate. At the time of the failure there was a pronouncedodour of hot rubber, but none of the rubbers failed. (Signed) B. Q. JONES, H. A. HARMS.Discussion of Results. Fig. 6 is a plot of the three cycles of the hysteresis run.The hysteresis.loss drops from 47 per cent, to 28 per cent, as would be expected. The sub-permanent set is practicallyconstant at about 10 per cent, after the first cycle. For the hysteresis tests the load was run up each time to 41 per cent,erf the load which ultimately broke the bridge. At this point, 4,000 lb., the stress in the rubbers was only 267 lb. per sq. in.,which appears to account in a qualitative way for a lower Wsteresis loss than we had found for the individual rubberswfeen carried to a higher stress. In this connection it is of Fig. 5.—Detail of construction of landing gear bridgeused in test. of 9,750 lb., at which point the rubbers had stretched 161 percent, more than the original length and were stressed 650 lb. per sq. in. From previous tests these rubbers were found tobreak at about 900 lb. per sq. in. The modulus of elasticity computed from —* •• —2~ — 400 lb. per sq. in. This is somewhat greater than the highestvalue, 350, got from tests on individual rubbers. Here again the discrepancy comes from the fact that the rubbers in thetwo cases were not equally stressed. The buckling of the bridge before the rubbers were fullyelongated shows this landing gear in service might collapse on hard landing. Ordinarily a movement of 5 ins. for the shockabsorbing mechanism is small. Fig. 6.—Hysteresis loops for^twelve rubber rings inparallel as used in landingfgear bridge, Figs. 3, 4, and 5.
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