Mallet Articulated Locomotives

THE maximum tractive force which can be developed by a locomotive depends primarily upon the weight carried on the driving wheels. With given track conditions, there is a maximum load per wheel which cannot be safely exceeded; hence the number of driving wheels used must be such that the weight necessary for adhesion can be carried without overloading the rails. Because of wheelbase limitations it is not practicable to couple more than five pairs of driving wheels in one group; and on some roads even this would require a rigid wheelbase of prohibitive length. If, therefore, additional wheels must be used to carry the required weight, it is necessary to divide them into two groups and to arrange at least one group in the form of a truck in order to keep the rigid wheelbase within reasonable limits. With such a plan a locomotive of high tractive force can be designed with a long total wheelbase and moderate wheel loads, and can at the same time traverse curves without difficulty.

The most successful type of articulated locomotive at present in use on American railways is the Mallet. These locomotives were first introduced on European railways, in 1889, by M. Anatole Mallet, a noted French engineer. They were first built by The Baldwin Locomotive Works in 1904, for the American Railroad of Porto Rico, a metre gauge line; but it was two years later before they were employed to any extent on railways in the United States.

The use of Mallet locomotives in this country was at first restricted to pushing service on mountain grades. For this they are admirably adapted, as because of their great tractive force the number of helper engines required to perform a given service is reduced to a minimum, as is also the amount of tonnage reduction necessary when transferring a train from a level to a mountain division.

With a given allowable weight per axle, a Mallet locomotive can be built to develop twice as much tractive force as an engine of the ordinary type, because twice as many pairs of driving wheels can be placed under it. The weight necessary for adhesion can thus be safely carried, and a proportionate increase in the tractive force developed. by building the locomotive of suitable dimensions. This high tractive force, however, can be utilized only at moderate speeds, such as are maintained in heavy freight and mountain service. As in other types of locomotives, the tractive force of a Mallet falls off with an increase in speed; and hence a point is soon reached where the large adhesion weight cannot be utilized. It is important that Mallet locomotives be specially designed for the required service, and used in that service to obtain the most satisfactory results.

In certain cases, Mallet locomotives are used to advantage in road service; as, for example, where heavy tonnage trains are hauled over long grades at moderate speeds. Such work requires locomotives capable of exerting a high tractive force for sustained periods of time. The locomotives for the Clinchfield road are operating under these conditions. Several other locomotives used in road service, are described. In each case the engines were specially designed for the particular service required.

The general features of the Mallet type of locomotive are well known, and only brief reference need be made to them in this connection. The cylinders are four in number, and they are arranged on the compound system; the high-pressure cylinders driving the rear group of wheels, and the low-pressure the forward group. The front frames are hinged to the rear frames in such a way that when the engine enters a curve, the front group of wheels swings about a hinge pin located on the center line of the engine between the high-pressure cylinders. The boiler is held in rigid alignment with the rear frames, and is supported on the front frames by sliding bearings. Flexible pipes convey the steam from the high to the low-pressure cylinders and from the latter to the smoke-box. Large locomotives of this type can be designed to traverse the sharpest curves usually encountered in trunk line operation.

The first standard gauge Mallet locomotives built by The Baldwin Locomotive Works were completed in 1906, for the Great Northern Railway. These locomotives are of the 2-6-6-2 type and the general arrangement of the steam piping is like that shown in Figure 1. The steam dome is placed immediately above the high-pressure cylinders and the live steam is conveyed from the throttle to the steam chests through external, rigid pipes. The receiver pipe connecting the high and low-pressure cylinders, is placed on the center line of the locomotive between the frames, and is fitted with a ball joint at the back end. The center line of the ball joint coincides with the center of the articulated frame connection, so that the length of the receiver pipe is constant regardless of the relative positions of the front and back frames. A slip joint is placed near the front end of the pipe, to compensate for expansion and contraction. The exhaust pipe is fitted with a ball joint at each end and a slip joint in the middle.

The illustration in Figure 2, shows the arrangement of piping used when a Baldwin reheater is installed in the smoke-box. The high-pressure exhaust is carried forward through external pipes, which discharge the steam into the lower reheater drums. The reheater is arranged like a Baldwin superheater. The steam passes from both sections of the reheater into the flexible receiver pipe, which is placed on an angle under the smoke-box. This pipe is necessarily fitted with a ball-joint at each end and an intermediate slip joint.

On many of the larger Mallet locomotives built by The Baldwin Locomotive Works, the boiler is constructed in two sections, which are united by a separable joint. The front boiler section contains a feed-water heater. An open chamber is placed between the feed-water heater and the boiler proper, and the separable joint surrounds this chamber. The joint is formed by two external rings, which are riveted to the front and rear boiler sections respectively. These rings are butted, with a V-shaped fit, and are held together by horizontal bolts which are placed at close intervals around the entire circumference.

The above illustration shows the forward section of a boiler of this type. The arrangement is that applied to the passenger locomotives for the Southern Pacific Company. In this case the high-pressure exhaust enters a horizontal pipe, which traverses the feed-water heater through a large flue. This pipe communicates with the flexible receiver pipe, by means of a cast iron elbow located in the smoke-box. In this way, although the reheating surface is small, the piping is exposed as little as possible to the cooling effects of the atmosphere.

The illustration further shows the construction of the feed-water heater, which is traversed by horizontal fire-tubes and is kept constantly filled with water. The injectors, when in operation, force water into the bottom of the heater, and the water is discharged through an outlet at the top, and is then forced into the evaporating section of the boiler. By this means the feed is heated to a temperature of about 250 degrees. This arrangement of water-heater has been applied, with marked success, to a large number of locomotives.

In the case of the locomotives for the Virginian Railway and the Duluth, Missabe and Northern Railway, the single pipe in the water heater is replaced by a nest of small tubes, which provide considerable reheating surface.

The boilers of two of the locomotives for the Atchison, Topeka and Santa Fe Railway, embody several special features of construction. The first locomotive has a rigid boiler of the separable type. The front boiler section contains, in the following sequence, a smokebox, feed-water heater, intermediate chamber, reheater and superheater. The reheater and superheater are of the Buck-Jacobs type. They are built into the boiler shell, and are separated by a tube sheet; while one set of tubes traverses bofh heaters. The steam, guided by internal baffle plates; circulates through the heaters and absorbs heat from the furnace gases which are flowing through the tubes. The steam pipes used on this locomotive are all placed outside the boiler shell, so that the pipe joints are easily accessible. Another special feature is the firebox, which is of the Jacobs-Shupert sectional type as developed by the mechanical officers of the Santa Fe System. The inside and outside shells of this firebox are each composed of a series of channel sections, which are bent to a horseshoe form. The usual stay-bolts are replaced by plates, which have openings cut in them to permit the free circulation of steam and water, and are riveted between the adjacent channels. The construction of this firebox has been fully explained in the technical press.

The second locomotive has an articulated boiler, with each section rigidly mounted on its respective frames: The flexible joint consists of fifty rings of high carbon steel, which are ten inches wide and formed with a slight set, so that when placed adjacent to one another they form a series of V-shaped joints. The rings have an outside diameter of seventy-five and one-half inches. They are riveted together at their inner and outer edges, and form a bellow’s-shaped structure which is forty-four and three-quarters inches in length. This is bolted into place between the front and rear boiler sections. An internal flue, forty-four inches in diameter, traverses the flexible connection, and is flared out at the back to fit into the rear boiler section. This flue prevents ashes and cinders from lodging in the flexible joints.

The superheater, in this boiler, is built into the rear section while the reheater is built into the front section. Both heaters are of the Jacob’s type, arranged for outside steam pipe connections. The superheater is traversed at its center by an eighteen-inch flue, and the reheater by a six-inch flue; these flues being provided to facilitate the removal of the boiler tubes. The tube ends are all accessible by means of chambers in the boiler, and these chambers can be entered through suitable man-holes.

In a locomotive of this type, it is not necessary to have sliding supports under the forward part of the boiler; and the locomotive curves with a minimum amount of resistance, and its stability, when traversing a curve, is not affected. Only one flexible steam pipe is required, and that connects the high-pressure cylinders with the reheater. Metallic flexible joints are used in the feed pipes which pass the boiler articulation.

Figure 5 shows the latest design of articulated frame connection used on Mallet articulated locomotives built by The Baldwin Locomotive Works. In this construction the high-pressure cylinder saddle serves as a support for the cylinders and boiler, and constitutes part of the main frame of the locomotive. This saddle also carries the hinge pin, and contains a pocket in which is placed the ball joint of the receiver pipe. The saddle is of cast steel, made in two pieces. The upper piece is riveted to the boiler shell, as a tighter joint can be secured in this way than when the parts are bolted together. The main frames are bolted to the lower section of the saddle, and the joint is arranged with a deep slab fit and is locked by a pair of keys which are driven into a long vertical key-way with their tapered faces in contact. A similar plan is used for keying the cylinders to the saddle. The frame connection is effected by a single radius bar, which also constitutes a strong transverse brace for the rear end of the front frames. The hinge pin is inserted from below, and is held in place by a supporting bolt. The holes for the hinge pin in both the radius bar and saddle are bushed. As shown in the drawing, the center of the hinge pin coincides with that of the ball joint in the receiver pipe. The saddle is cored with a suitable passage, through which steam passes from the high-pressure exhaust pipes to the receiver pipe.

The saddle is made with a forward extension on each side, and this extension has a slight amount of vertical clearance between the upper and lower rails of the front frames. Any transference of weight at this point takes place by actual contact between the frames. The drawing shows the joint in the reach rod which connects the reverse shafts of the front and rear engines. This joint is arranged like a crosshead, and the guides supporting it are bolted to the inner walls of the saddle. This crosshead, throughout its entire travel, is so near the center of the hinge pin, that there is practically no distortion to the movement of the low-pressure valves when the engine is traversing curves.

Figure 6 shows the construction at the forward end of the front frames. The arrangement is in many respects similar to that described above, except that no support is provided for the boiler. The casting between the cylinders has cored in it suitable passages for conveying the steam from the receiver pipe to short elbow pipes which lead to the low-pressure steam chests.

The arrangements for bolting together the frames, cylinders, and cylinder saddles, as described above, are exceptionally strong, and the various parts have such large bearing surfaces that the chances of their working loose are reduced to a minimum. The cylinders are so arranged that they can be easily removed without dismantling the saddles, frames or principal steam pipes. These designs constitute an interesting illustration of the use of cast steel in heavy locomotive construction, as they would be impracticable unless the larger parts (with the exception of the cylinders) were made of this material.

Figure 7 shows a section through the low-pressure cylinder of a Mallet articulated locomotive. The steam distribution is controlled by a new design of double ported piston valve, which in the illustration referred to, is arranged for outside admission. The valve has the same effect as an Allen-ported slide valve. This is of special advantage when the engine is linked up, as the port opening is far greater than could be obtained in a valve of the ordinary type. The arrows on the drawing show the course of the steam. This same style of valve, when used on the high-pressure cylinders, is usually arranged for inside admission.

A reliable form of power reverse mechanism is an essential feature of a Mallet locomotive, in order that the engine may be handled without undue effort on the part of the enginemen. Figure 8 represents the Ragonnet power gear, a patented device which is regularly applied to Mallet locomotives built by The Baldwin Locomotive Works. This device consists of an air cylinder, whose piston is connected to the reverse shaft of the rear engine by means of a suitable crosshead and link. Admission of air to the cylinder is controlled by a slide valve, and the mechanism is operated by a small hand lever which is placed in the cab. When the hand lever is moved to change the cut-off, the valve is displaced from its central position, air is admitted to one end of the cylinder, and a movement of the piston follows. The crosshead and valve rod are both connected to a combining lever, and the arrangement is such that when the piston moves, the swing of this lever moves the valve back to its middle position. The mechanism is so adjusted that when the link blocks have been shifted to give the cut-off desired, the slide valve closes the admission port and further movement of the piston ceases. The construction is such that when the gear is set for any particular point of cut-off, the valve is in its middle position. Should air leak in at one end sufficiently to cause a movement of the piston, the valve will at once move, admitting air to the other end of the cylinder and locking the mechanism. The inside, or exhaust lap of the valve is considerably greater than the outside lap, so that air pressure can be held on both sides of the piston simultaneously.

This gear has proved so reliable in service that ordinarily no hand reversing mechanism is applied.

Figure 9 illustrates the flexible exhaust pipe of a Baldwin Mallet locomotive. Special attention is called to the construction of the ball joint at the smoke-box end of this pipe. This joint is placed immediately under the exhaust nozzle, and is kept tight by a coiled spring. The spring is always in compression, and is confined within a suitable casing; so that, when the parts are being dismantled, it cannot suddenly extend to its free height and thus cause damage. The construction of the casing is shown in the drawing. The upper and lower sections are provided with a series of projections, which interlock and are surrounded by a steel wire ring. Each projection has a lip extending outwardly, and these lips engage the ring and hold the sections together. The effectiveness of the spring in keeping the joint tight is not impaired by reason of the casing; and the removal and replacement of the spring, when making repairs, are easily effected.

The drawing also shows the construction of the ball joint at the front end of the pipe. The ball is seated on two babbitt lined rings of brass, and these can be adjusted by a packed gland. The slip joint in the middle of the pipe has a long sliding fit, and is kept tight by a pair of snap rings and a series of leakage grooves.

The ball joints in the receiver pipe are similar in construction to that used at the forward end of the exhaust pipe. The slip joint in the receiver pipe is fitted with a packed gland, as the pressure here is sufficiently high to require this form of construction.

The Handling of Mallet Articulated Locomotives

The handling of a Mallet articulated locomotive presents no special difficulties. As the high and low-pressure engines each operate like a single expansion locomotive, it is unnecessary to introduce complicated features because of the application of the compound principle. In order to enable the locomotive to develop full power at starting, it is necessary to provide means for admitting steam direct from the boiler to the low-pressure cylinders. In the Baldwin engine, a small pipe is run from a starting valve in the cab, to the receiver pipe connecting the high and low-pressure cylinders. By opening the starting valve, steam will pass direct from the boiler to the receiver pipe and thence to the low-pressure cylinders, and the locomotive will develop a tractive force up to the limit of its adhesion. This device is recommended because of its simplicity and reliability in service.

The flexible pipes which convey the steam from the high to the low-pressure cylinders, and from the latter to the smoke-box, should frequently be inspected and tested for leakage; as it is important that the ball and slip joints be kept tight. The sliding bearings supporting the boiler on the front frames should be regularly oiled; also the hinge pin connecting the front and rear frames, and the joint in the reach rod connecting the front and back reverse shafts. This joint as usually arranged on Mallet locomotives built by The Baldwin Locomotive Works, is provided with a crosshead, which is placed between the inner walls of the high-pressure cylinder saddle; and is so arranged that the reach rod can accommodate itself to the swing of the front frames when the engine is traversing curves.

Reference has been previously made to the separable type of boiler used on large Mallet locomotives. A boiler of this type should be fed in the same manner as one of the ordinary design, as water is simply forced over from the feed-water heater to the evaporating section when the injectors are in operation. The temperature of the feed-water is raised to about 250 degrees; hence a certain amount of scale, depending upon the quality of the water, will be deposited in the heater, with a corresponding reduction in the quantity deposited in the boiler. The heater, therefore, should be cleaned at regular intervals.

In a long boiler of this type the smoke-box gases are comparatively cool, tests having shown temperatures approximating 450 degrees. The result is a relatively high boiler efficiency, due to the large amount of heat absorbed by the water before the gases escape up the stack.

The high-pressure cylinders of a Mallet articulated locomotive are lubricated from the cab in the usual manner. The low-pressure cylinders may be lubricated in the same way, through flexible pipes. Another method, which has proved convenient and satisfactory, is to use a pair of force feed oil pumps, which are driven from the valve motion of the front engine. Flexible piping is thus dispensed with. In a similar way, the front group of driving wheels can be supplied with sand from a box placed over the forward deck plate, and the sand delivered to the rails through rigid pipes. Pneumatically operated cylinder cocks are frequently used on the low-pressure cylinders, and the cylinder cock rigging is simplified by this arrangement.

Before starting a Mallet locomotive on a trip, care should be taken that the air pressure is fully pumped up, and that the valve admitting air from the main reservoir to the cylinder of the power reverse, is open. The sliding bearings under the boiler and the pin of the articulated frame connection, also the reach rod, should be inspected and properly lubricated. The receiver pipe between the high and low-pressure cylinders can be tested for leakage by opening the starting valve while the engine is standing with brakes set. The joints in the pipes are provided with glands for taking up wear in the packing, and any leakage should receive immediate attention.

In moving the engine to its train, the cylinder cocks should be kept open as considerable condensation is liable to occur. This is especially true of the low-pressure cylinders, and in cold weather it is advisable to warm these, before starting, by keeping the starting valve open for a short time.

A proper use of the starting valve is essential in order to secure the best results when operating a locomotive of this type. If the engine is at the head end of the train and the slack can be taken up, a successful start can usually be made with the main throttle alone, as by the time the entire train is under way the low-pressure cylinders will be receiving steam and the full tractive force of the locomotive can be developed. If, however, the slack cannot be taken up, as is usually the case when the locomotive is pushing, the starting valve should be opened. As soon as the wheels have made a few revolutions, and the low-pressure cylinders are receiving steam from the high-pressure, the valve should be closed.

Attention should be given to the slipping of the driving wheels in a Mallet locomotive. If the wheels of the forward group slip frequently, while those of the rear group do not, it is an indication that steam is leaking past the high-pressure valves, and these should be examined for blows. If the valves are in good condition, and the wheels of only one group slip, the unbalanced pressures resulting will tend to stop such slipping. Any continuous slipping can occur only in both groups of wheels simultaneously, and should be corrected by throttling the steam and using sand.

In running these locomotives, care should be taken not to attempt to operate them at too short cut-off, as this will result in very low terminal pressure. It is preferable to use a relatively long cut-off, and throttle the steam when too much power is developed. Furthermore, if the cut-off is too short, the compression resulting will cause the locomotive to ride hard. Experience will at once indicate how far the locomotive can be linked up at any given speed, without detrimental results.

In drifting on long grades, the starting valve should be opened at intervals in order to keep the low-pressure cylinders moist; otherwise they are liable to become hot. Vacuum valves are provided on all cylinders, and by-pass valves are usually placed on piston valve cylinders, to enable the locomotive to drift freely.

Economy in fuel and water consumption results from the use of compound cylinders, and on basis of coal and water per ton-mile Mallet locomotives show a marked reduction when compared with single-expansion locomotives of ordinary types. For this reason the firing of these engines presents no special difficulties.

In the event of a breakdown on the road, any one of the four cylinders can be cut out by disconnecting the valve gear, blocking the valve at mid-position and taking down the main rod. The engine can then be run with three cylinders. If one of the high-pressure cylinders is thus cut out, the starting valve should be left open, so that more steam is delivered to the low-pressure cylinders and additional power developed. If a high-pressure valve is broken, steam will leak into the receiver pipe and pass direct to the low-pressure cylinders. In such an event it will be necessary to throttle the steam closely in order to prevent excessive slipping of the low-pressure engine.

In all other respects, the rules applying to single expansion locomotives are applicable to the Mallet type, and the same care and attention should be given to the maintenance of the locomotives in order that satisfactory service results may be secured.

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