Steam Locomotive Construction and Maintenance/Chapter X
CHAPTER X
INSPECTION AND TESTING
The testing work in connection with locomotives may be divided into two main categories—
(a) The testing of the materials of which the engine is constructed.
(b) Testing and trials of finished locomotives.
Testing and Inspection of Materials. This is done at the works of the various manufacturers of the plates, bars, axles, tyres, etc., who provide facilities in the shape of testing machinery, in which specimens cut from the material are broken and their physical behaviour noted. The railway companies, if the materials are for use in this country, or the consulting engineers, if the engines are being built for abroad, have their representatives or inspectors at the works of the manufacturers, to pass the materials for acceptance if the tests prove satisfactory, or reject them if they do not come up to the specified requirements.
Tensile Tests. The most important physical test is known as the tensile test. A piece of the plate is machined to definite rectangular or circular form and dimensions, and placed in the jaws of the testing machine. The specimen is then pulled by the direct action of a hydraulic cylinder, the pull being counterbalanced and measured by a long steelyard on which there is a movable weight. After a certain time the piece begins to stretch, and the first sign of this is measured on the piece by a pair of compasses which have been set to a definite and specified length varying from 2 to 10 ins. according to the test specimen. This length is marked on the specimen before the test is begun. The point at which the first signs of permanent stretch are observed is termed the yield point, and the stretching denotes that the limit of perfect elasticity of the material has been passed. The position of the weight on the steelyard at this moment is recorded, after which the pulling is continued until the specimen breaks, the position of the weight being then noted again. The steelyard is marked directly in tons, so that the position of the weight shows without further calculation the total number of tons pull on the specimen. The number of tons divided by the known original area of the section of the specimen in square inches, gives the yield point or the ultimate breaking strength of the material in tons per square inch.
Just before the specimen breaks it pulls out somewhat after the manner of a piece of toffee. The extension of length over that of the specimen, as it was originally, and also the contraction of area where it has broken, are also recorded, and give a measure of the ductility of the material.
Table I shows the ultimate tensile strength and minimum permissible elongation for the most important materials used in locomotive construction.
| Name and use of Material. | Ultimate tensile strength tons per sq. in. |
Percentage elongation above original length not less than |
| Mild steel boiler plates | 26 to 32 | 22% in 8 ins. |
| Mild steel boiler rivets | 24 to 28 | 27% in 8 diameters |
| Mild steel frame plates | 28 to 32 | 20% in 8 ins. |
| Steel crank axles | not less than 30 | 20% in 2 ins. |
| Steel crank axles, oil treated | not less than 35 | 20% in 2 ins. |
| Steel straight axles | 25 to 40 | 25% to 20% in 2 in. |
| Steel tyres, class C | 50 to 55 | 13% to 11% in 2 ins. |
| Steel tyres, class D | 56 to 62 | 10% to 8% in 2 ins. |
| Steel castings with wearing surfaces | not less than 35 | 10% in 2 ins. |
| Cast steel wheel centres. | not less than 26 | 15% in 2 ins. |
| Copper plates for fireboxes | not less than 14 | 35% in 8 ins. |
| Copper stays for fireboxes | not less than 14 | 40% in 8 diameters |
| Mild steel tubes for boilers | not more than 24 | 28% in 8 ins, |
| “Best Yorkshire” iron bars, 1 in. to 4 ins. diameter |
21 to 23½ | varies according to bars. |
Spring steel is not usually tested for tensile strength, but the springs are subjected to rapid deflection tests.
Bending Tests. Other tests specified include “cold bend” and “temper bend” tests. In the former, specimens cut from plates are doubled over cold, until the internal radius is not greater than the thickness of the plate, and the test piece must withstand this without fracture. The “temper bend” test is similar, but the piece is heated beforehand to a red heat and quenched in water at a temperature not exceeding 80°F. Rivets are tested by being bent cold until they are completely doubled up without fracture, and the heads must be hammered flat without cracking.
The specified tensile and bending tests differ according to the material concerned. In the case of plates there is always sufficient excess material at the sides from which a test specimen can be cut, so that each plate can be tested if required. In the case of straight axles and tyres which are tested to destruction the inspector selects any two per cent. of the number ordered, and the quality of the whole lot is decided by the behaviour of those so selected.
Impact Tests. In addition to tensile and bend tests, axles and tyres are also subjected to the “falling weight test.” The test axle is placed upon bearings which are placed at a distance apart varying from 3 ft. for an axle 4 ins. diameter at the centre, to 5 ft. for axles 6 ins. or more in diameter, and a weight of 1 ton is dropped on to the centre of the axle from a height varying from 16 ft. for the 4 ins. axle to 35 ft. for the 6 ins. and larger axles. The axle must withstand five such blows without breaking. Under the first blow it bends, and is then turned over to receive the next blow, this being repeated each time. If it stands this test the axle is finally broken.
Tyres are tested in a similar manner by being placed on edge, a 1-ton weight being then dropped on to them from heights of 10, 15, and 20 ft. until the tyre deflects under the blows a specified amount without breaking.
Chemical Tests. Each railway company has, in addition to a mechanical testing shop, a well equipped laboratory in which samples of the above materials are analyzed. Chemical tests are not often specified for the materials, beyond limiting the amounts of sulphur and phosphorus in steel, both of which are deleterious. Thus boiler plates must not show more than 0·05 per cent. of either of these elements, but in the case of springs, tyres, and axles, these limits are reduced to 0·035 per cent. Copper firebox plates are specified to contain from 0·25 to 0·55 per cent. of arsenic, which has the valuable property of reducing wear in service.
Trial Trips of Completed Locomotives. Contrary to what might be supposed, new locomotives do not, except under special circumstances, undergo any elaborate tests, such as are usual with large marine or mill engines. An engine built at a railway company’s works is immediately sent out on a trial run of some 20 to 25 miles without a train. During the trial the working of the motion, springs, brakes, injectors and sandgear is noted carefully. The big end and axlebox bearings must run perfectly cool, and there must be no leakages at any of the joints, to which the fittings and mountings on the boiler are attached. Everything that is not satisfactory is noted for rectification when the engine returns. A second trial may occasionally be necessary, especially in the case of an engine of a new class, and finally, when everything is satisfactory, the engine is sent to the paint shop to be painted.
An express engine is not at once set to work on fast trains, but for about three weeks it is employed on local stopping passenger trains at the shed where the works are situated. This gives the engine an opportunity of “finding its bearings,” and any defects which may show themselves are rectified. Afterwards the engine is tried for a week or two on fast trains, and is then ready for regular service and is sent away to its allotted station. An engine which has come out of the works after repairs undergoes exactly the same routine.
Special Tests in Service. These are usually “road tests,” that is, they are made in service on the line when the engine is working its usual trains. Generally such tests consist of a long daily series, extending over a period of several weeks, and they are generally undertaken, when it is desired to estimate the coal and oil consumption of a new or altered class of engine in order to compare the results with those given by an engine of an existing class, which has hitherto been working the same trains.
For this purpose records are kept of the weight of each train; of the miles run, and the time gained or lost, from which the actual average speeds can be calculated; and of the amounts of coal burnt and oil used. The data obtained are tabulated each day, and the results are compared carefully. Minor alterations in the new engine may be made as a result of these trials. British engineers generally prefer this form of extended road test, since it gives results under commercial working conditions. The tests may be elaborated by including a dynamometer car in the train, attached, immediately behind the tender, so as to secure a continuous record of the pull of the engine on the tender draw bar. At the same time indicator diagrams are taken at intervals. The indicated horse power calculated from the indicator diagrams, when compared with the useful horse-power calculated from the draw-bar pull registered by the dynamometer car, gives a measure of the efficiency of the engine.
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Fig. 45.—Locomotive Testing Plant.
G.W.R. Works, Swindon.
Coal Consumption. The coal consumption varies greatly with the class of engine, weight of train, gradients, weather, and speed. An express engine may burn anything from about 23 to 50 lbs. of coal per mile. Experiments on the Midland Railway, the main line of which has heavy gradients, showed an average consumption during certain tests of from 0·07 to 0·16 lbs. per ton-mile with different engines and trains.
Testing Plant. Another and modern form of test may be mentioned briefly. This is made in the works on a test plant, the conditions being such as to approximate as nearly as possible to a road test. The engine is placed over a special pit in such a way that its coupled wheels drive by friction another set of wheels and axles fixed across the pit. The engine can be run under its own steam at any speed desired, and the draw-bar pull is measured by a special apparatus at the back. Coal, water and oil consumptions at certain speeds over a definite length of time are measured and recorded. This form of test plant, which is extremely valuable for experimental purposes is of American origin, but there is only one in this country, at the Great Western works at Swindon. An illustration of this is given in Fig. 45.
- ↑ An explanation of the term “tractive force,” is given in the primer The Steam Railway Locomotive.