From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Explaining the use of the Wimperis Accelerometer for both railway and automobile applications.
Identifier | ExFiles\Box 113\2\ scan0023 | |
Date | 1st July 1912 | |
8 WIMPERIS ACCELEROMETER EQUILIBRISTAT USE For use as an equilibristat the instrument is so calibrated that each vertical division on the paper is equivalent to an excess, or deficiency as the case may be, of one inch of super-elevation. When using the instrument for this purpose it is so set on the floor of the coach that the pen moves across, instead of along, the length of the train. It is levelled when the coach is on a straight piece of track without super-elevation. The theoretical formula for super-elevation is— Super-elevation (in inches) = (V² . G) / (R . g) where V is the speed in feet per second, G is the gauge in inches, and R is the radius of the curve in feet. When the super-elevation is exactly equal to this amount the pen of the equilibristat moves along the zero line, but if the super-elevation is one inch in excess of this the needle points along the one inch excess line, and so on. The record given by the instrument can be examined by the chief engineer of a railway at leisure. It will show any places on the line where the speed and super-elevation are not in harmony; it will also show “slacks,” and, more important still, it will indicate the points where transition curves are needed, or where badly made transition curves have been laid. The purpose of a transition curve is to enable a train to enter or leave a sharp curve gradually instead of suddenly. The equilibristat chart shows a sudden steep rise or fall of the trace whenever such sudden changes occur. And the steeper the trace the greater the need for the permanent way to receive attention. A sudden rise of lateral acceleration is equivalent to a heavy blow, which is felt by passenger and stock alike. Mr. W. H.{Arthur M. Hanbury - Head Complaints} Shortt, in his paper on “Improvement of existing Railway Curves,” read before the Institution of Civil Engineers, remarks:—“ A rate of change of acceleration of one foot per second per second is the maximum that will pass unnoticed.” The more suddenly the equilibristat curve changes in height the worse is the blow. This renders the chart an effective indication of breaks of curvature due either to the absence, or to the ill design, of transition curves. Improvements of this kind are very necessary when a railway service is accelerated and the balance between the speed and the curvature of the line is thereby disturbed. The instrument when used for this work is sometimes also fitted with a convenient automatic mechanism for marking the paper every so many revolutions of the car wheels. This enables the train speed to be obtained and makes identification simple. If possible the transition curve should be designed to limit the rate of change of lateral acceleration to one foot per second in a second. Much more than this is detrimental to the comfort of passengers, to the durability of the stock, and may even lead to derailment. The charts in Figs. 3, 4, 5, 6 and 7, were taken to show the starting acceleration and braking of steam and electric (D.C. and A.C.) trains and the equilibristat diagram for a L. & S.W.R. express. ELLIOTT BROTHERS, Established 1800. CENTRAL BUILDINGS, WESTMINSTER, LONDON, S.W., and CENTURY WORKS, LEWISHAM, LONDON, S.E. 9 WIMPERIS ACCELEROMETER AUTOMOBILES RESISTANCE AND HORSE-POWER Indicating and Recording Accelerometers are also made with scales specially suitable for use on motor cars and motor wagons. The indicating instrument enables the tractive resistance of the vehicle to be measured. All that is necessary is to level the instrument on the floor of the vehicle by means of the adjusting screw until the needle reads zero when on a level road. (This is most easily done by selecting any road, level or not, and bringing the needle to zero; then turn the vehicle round so that its front wheels stand where its rear wheels previously stood and vice versa, note the reading of the needle and then turn the levelling screw until the needle returns halfway to the zero—when this has been done the needle is correctly set.) Then to measure tractive resistance at any speed, depress the clutch and take the reading of the accelerometer—the needle will point to the pounds per ton of resistance encountered. A knowledge of the tractive resistance, a knowledge of the tractive resistance, the speed and the weight of a car, enables the engine horse power (b.h.p.) to be at once obtained. Thus in the R.A.C. tests in July, 1912, at Brooklands, the h.p. of a given car was found by the accelerometer in this way. The tractive resistance = 165 lb. per ton, speed = 44'78 m.p.h, weight = 1'67 tons, Then b.h.p. = (165 x 44'78 x 1'67) / 375 = 33. This is an exceedingly easy and rapid way of measuring horse power. In all cases B.H.P. = (R x V x W) / 375 where R = tractive resistance in pounds per ton, V is the speed in m.p.h. and W the total weight in tons. AUTOMOBILE ACCELERATION It is often desired to obtain a permanent record of either the starting acceleration or the braking of a motor vehicle. For this the Recording accelerometer is desirable. The two alternative possible ways of measuring car acceleration are—(1) by deduction from a space time graph, (2) by the direct indications of an accelerometer. Of these two methods the former has the attraction that none of the measuring apparatus is carried on the moving vehicle and that it is a simple matter to measure the distance travelled at the end of equal time intervals; the slope of this space time graph at any point measures the velocity, so that a velocity time curve can then be constructed and the slope at any point of this velocity time curve measures the acceleration. This process is, however, both long and inaccurate, particularly at the gear change points. It is far more difficult to measure the acceleration of a motor car in this way than of any other moving vehicle, owing to the discontinuity of motion in the gear changing. The use of an accelerometer avoids these lengthy and troublesome calculations. It may appear at first sight that it is open to the objection that as the measuring instrument is carried on the moving vehicle it is subjected to a good deal of vibration, and possibly to zero-error should the floor of the car change appreciably in its upward or downward tilt relative to the road. Vibration, however, is actually useful in that it helps to overcome any statical friction there may be in the mechanism, and although difficulty due to change of tilt cannot be avoided by any attention to the mechanism of the accelerometer, nevertheless its amount has been the subject of careful study on a variety of cars, and been found to be less than can be measured in practice. This, therefore, is a suitable and easy means of measuring the acceleration on each gear. ELLIOTT BROTHERS, Established 1800. CENTRAL BUILDINGS, WESTMINSTER, LONDON, S.W., and CENTURY WORKS, LEWISHAM, LONDON, S.E. | ||