weight through a given space. The diagrams obtained from the dynamometer consisted of a series of waving lines of varying elevation and with different average ordinates. The undulations were produced by changes of speeds probably caused by the inequalities of belt lacings, etc. Fig. 19. The general appearance of these diagrams is shown in Fig. 19. The dynamometer was calibrated three times: first, by attaching a brake to the same shaft, and comparing the diagrams with the brake readings; secondly, by direct pull with a spring balance against the springs of the dynamometer and thus obtaining the ordinate for a given belt-pull; thirdly, on the same principle as the first, but a spring balance was used, to measure the brake weights, instead of scales. The object of these calibrations was to obtain the ordinate corresponding to any given belt-pull. The following results were obtained: CALIBRATION OF DYNAMOMETER. Comparison with Prony Brake. 1st Trial-Brake pulling against load of 52 pounds on Fairbanks Scale. The diameter of the brake pulley, including belt, was 23 inches; the dyna mometer pulley, including belts, was 17 inches. CALIBRATION OF DYNAMOMETER. 2d. Method, by direct pull against springs of dynamometer. This method was employed a number of times, and gave uniform results, the variations from the results of this trial and the first and third, as previously given, are believed to be errors incident to the use of the brake. The mean of these three results corresponds very closely to this last, and, where plotted, gives a straight line, whose equation is Y = 0.046 +0.20, Y being expressed in inches and X in pounds. The diagram of the curves is shown, and in interpreting the results was used instead of the equation. (See Figure 20.) The engines employed in the investigation to be described were of several types and of various sizes, styles and proportions. The first was a small Straight Line Engine built in the Sibley College workshops, but modified from time to time for purposes of experiment, in such manner that it represented the ordinary type of directly connected engine with throttle regulation. It was tried both with its usual balanced and with an unbalanced valve. Another engine was a traction-engine built by the Lansing Iron Works with locomotive style of valve motion; and others, by the same company, were of the automatic type, and compound and condensing engines, both the latter having balanced valves. FRICTION OF STRAIGHT LINE ENGINE. The first engine tested was the 6 x 12 inch Straight Line Engine built by Prof. J. E. Sweet, while connected with Sibley College. This engine had been modified for experimental purposes in many ways, but still retained the principal characteristics of the Straight Line Engine. The valve gear is arranged for a fixed cut-off, at any part of the stroke less than five-eighths, and the valve can be changed, by removing the pressure plate and fastening on a back, from a balanced valve to an ordinary slide valve. For these tests, the power was obtained from the water-wheel and main driving shaft, in the Sibley College shops, the speed of which was not always uniform and was beyond the control of the investigators. The power was measured by passing it through the transmitting dynamometer. The speed was measured by a hand speed indicator, and also by an attached tachometer, which had been carefully calibrated. The tachometer results could only be used to correct errors, as readings to single revolutions could not be made. In making this trial, and all others, the engine was first heated up by steam, the steam-chest cover and cylinder heads were removed to prevent pump action, then, as quickly as possible, the dynamometer cards cards were taken. These diagrams were generally taken with the engine complete, and then successive cards with part after part removed. The engine was turned by power applied to its main driving-wheel. The speed of the engine varied from 200 to 244 revolutions, and the results were corrected in accordance with the law, known to be true for that engine, that the friction varied directly as the speed. This correction, however, did not seriously change the results. Twenty-nine successful dynamometer cards were obtained, each of which may be considered as the average of several observations. The practical condition of working of a plain slide valve, with steam on, could not be obtained in these trials, because the cylinder was open to the air, and the friction obtained is no doubt too high for that particular case. Table I. presents a record of the distribution of friction found in this engine and all essential data from later comparisons with other results from tests of the various engines afterward employed. Table II. exhibits the method of variation of friction, and Tables III. and IV. its computed amounts. Condition of Engine. Warmed up by Steam. Cylinder head off. Steam-chest cover and pressure-plate off. All cocks open. Engine Complete. 1.122 0 Piston and piston-rod dropped. 1.159 0 1.189 0 Pressure-plate and steam-chest Balanced valve converted into slide valve. Steam pressure on back of valve. |