Remarkable Safety Apparatus that Discovers Invisible Flaws
THE SPERRY DETECTOR CAR, when in service, moves over the track to be tested at a speed of five to nine miles an hour. When not engaged in detection, the car can do a speed of sixty miles an hour. The driving mechanism is a 220-
IT is no uncommon experience, when travelling to-
The car is searching for hidden defects in the rails, and its low speed is essential so that the electric current used for detection purposes may do its work, and reveal inside the rail-
If a rail cools out normally, in the open air, the outside skin during the cooling is obviously at a much lower temperature, by radiation, than the hot interior. If a curve be plotted showing the alteration in the physical properties of steel as its temperature is raised, it reveals that the steel becomes softer and more ductile as its temperature increases up to about 350 degrees Centigrade. It also shows that between 350 and 500 degrees the curve changes its direction, and a temporary increase of hardness takes place, until the 500-
A TRANSVERSE FISSURE. The hidden defect that has occurred in the head of a rail.
The hardness of steel is governed chiefly by the percentage of carbon it contains. The harder the steel, and the more rapid the rate of cooling, the greater are the stresses set up in the head of the rail while cooling through this zone. This is because the inside of the head is hotter than the outside. According to temperature, one part of the metal may be growing temporarily softer while the remainder is hardening.
The internal strain may thus become so great that tiny fissures, usually at right angles to the length of the rail, are formed in the centre of the head, where they are invisible.
Were these fissures to remain no greater in size than their original nucleus, there would be no danger. But the stresses set up in the rail by the passing of trains over it -
It was in America that fissure fractures first began to take place on an extensive scale. In countries where flat-
Analysis and microscopical examination of the rails concerned revealed the fact that no fault could be found with the steel itself. Attempts were made to ascertain whether fissured rails could be related to individual “heats” of steel; but, while some rails from any one heat were badly fissured, others were entirely free. Manufacturing methods came under review. The physical properties of different analyses were investigated; and endless statistics were compiled in an endeavour to get to the bottom of this metallurgical mystery. Meanwhile the fissure failures increased in frequency to an alarming extent until at length the trouble grew to the dimensions of a national menace. Indeed, up to the end of 1933 -
It was a British firm of consulting engineers -
Certain American steelworks have, therefore, adopted the oven method of cooling the rails as their standard practice. Others have instituted similar measures, such as packing the hot rails into brick-
THE RECORDING OPERATOR watches the track from the rear of the moving car. On the table before him is a paper strip, which moves at a speed proportional to that of the car, and on which is marked in ink a continuous record of the condition of both rails of the track. Twenty-
But although internal fissures are not unknown in British rails, the fissuring trouble here has up till now been more or less negligible. The reasons for this difference, as compared with what has happened on American railways, are several. In America the vastly increasing weight of locomotives and rolling-
The cause of the trouble has now been more or less definitely determined, so that fissuring can be reduced, if not entirely obviated, in future. There are, however, in American tracks, rolled during the last decade or two, tens of thousands of rails in the heads of which fissures are probably in course of development from their original nuclei. Each one of those fissures is a potential danger, and it is of the greatest importance that they shall all, if possible, be discovered and removed from the track in time.
THE RECORDING TABLE. The paper band on which the recording is done by seven pens moves at a rate equivalent to 1/16-
It was the late Dr. Elmer A. Sperry, a well-
After more than five years of research, during which other scientists were at work on this formidable problem, but without practical result. Dr. Sperry discovered the principle he sought. It was that of passing through a rail a low-
By 1928 the first primitive Sperry car was ready to give a demonstration on the railroads. It located a certain number of fissures, and when the rails concerned were broken at the point where they had been marked with paint, fissures were found to be present. With faith in the future of their discovery, the Sperry Company began to build their first fleet of detector cars, but Dr. Sperry himself died before construction was complete. The principle of detection was thus firmly established, though the method of working it out was as yet far from perfect. Sometimes a fissure would break a rail through shortly after one of the Sperry cars had been on one of its exploratory journeys over that particular track, and without leaving any paint-
The next step was to lay down a special test track of rails that were known to contain fissures. Twenty-
These tests led to the conclusion that each face of a transverse fissure constitutes a definite pole. If the polarity were alined in the same direction as that of the testing, fissures could easily be detected, even down to those of comparatively small size. But if the polarity were opposed to the direction of testing, then it was difficult, if not impossible, to detect any fissuring. New equipment was therefore devised to “pre-
A modern Sperry detector car is a self-
Behind this comes the galley, with cooking stove and refrigerator, and after that a lounge and dining-
THE ENGINE ROOM in which the current is generated for the work of detection. A current of 7,000 amps is delivered to the main detector brush carriage.
Towards the rear of the car is the large room housing the generating equipment for the detectors (which is quite distinct from the driving equipment), paint -
Current for the main detector brush carriage is delivered at 7,000 amperes by a 3-
At the rear end of the car is the recording compartment, containing the “brains” of the equipment. Here the recorder sits, with a clear view of the track as it slowly disappears from behind the car. Immediately before him is the recording table, over which an endless band of paper is moving at a speed so adjusted that each 1/16-
Seven parallel lines are seen on the paper, on certain of which momentary notches are seen. The right-
The searching units themselves are mounted in the centre of the rear end bogie truck of the car, on either side, being lowered on to the rail when detection is in progress, and lifted out of action when the car is travelling to and from its work. They are raised and lowered by pneumatic power. The searching unit consists of multi-
So sensitive is the searching unit that it will locate a fissure no greater in extent than five per cent of the cross-
THE CONFIRMATORY HAND TEST. When the recording chart shows a defect, the car is stopped and a confirmatory hand test is conducted. This second test indicates conclusively the exact nature of the flaw, its extent and location. If there is a serious defect, emergency orders are issued and the affected rail is replaced immediately.
Directly one of the defect lines on the roll has been notched by a searching unit, the operator is alert. First he looks at the rail to see, as it appears from below the car, if the paint-
Ready to the operator’s hand in the car are two dozen rubber stamps, in two racks, which are used for the marking of the roll, to number the fissures and record their size, to identify switches and crossings, to indicate mile posts and section posts, and for various other purposes. At the completion of the run, therefore, the diagram presents a continuous picture of the whole of the rails in the track over which the car has travelled. There are in all twenty-
In this remarkable vehicle nine different voltages are used, ranging from 1 to 700, for the detector equipment, lights, and drive. Eighteen different electrical machines, either generators or motors, are in constant use, and thirty-
In a typical month a Sperry car has covered 396 track miles, averaging fourteen miles a day, and as a result 195 rails have been removed from the track because of the presence of fissures.
No two days’ work are alike. One record obtained showed an average of no more than one fissure per 11.8 miles of track; another revealed sixty-
The question may be asked as to why the detection cannot be done at the steelworks or at the railway rail storage depot, before the rails are laid in the road. But the answer is that in their initial stage these sub-
THE SEARCHING UNIT seen in the lowered position in contact with the rail. To the left and right are the flexibly-
[From part 29, published 16 August 1935]
“Welding in Railway Service” on this website.