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Locomotive Frames. Among other details of importance in the construction of a locomotive, none is more important than the frame. The frame is the supporting element and the tie bar that connects all the various moving and fixed parts. Its present form and proportions are due most largely to development rather than to pure design. It would be extremely difficult to analyze all the various forces to which the frames are subjected. There are two principal classes of locomotive frames, namely, the single front rail and the double front rail. The single front rail is illustrated in Fig. 75. At first the joint between the main frame and the front rail was made as shown at A in Fig. 75. The rear end of the front rail was bent downward with a T-foot formed thereon by means of which it was connected to the main frame. The top member of the main frame was bent down and extended forward and connected to the front rail by means of bolts and keys. The T-head was fastened to the pedestal by two countersunk bolts. As locomotives grew in size, much trouble was experienced due to the countersunk bolts becoming loose or breaking. To overcome this difficulty, the form of joint shown in B, Fig. 75, was developed. Here the pedestal had a member welded to it which extended forward and upward to meet the front rail. The top member extended outward and downward as before. The front rail fitted between these two members and had a foot which rested against the pedestal. This latter form was used for many years, being changed in details considerably but retaining the same general arrangement. These forms of single bar frames continued to be used for many years and are employed at the present time for light locomotives. When the heavier types of locomotives, such as the Consolidation made their advent, it became necessary to improve the design of the frame. To meet this necessity, the double front rail frame was developed. Fig. 76 illustrates one of the earlier forms of this frame. The top rail was placed upon and securely bolted to the top bar of the main frame and the lower front rail was fastened to the pedestal by means of a T-foot with countersunk bolts. The same difficulty was experienced with this design as with the first form of the single front rail type, namely, the breaking of the bolts fastening the lower bar to the pedestal. This led to experiments being tried which resulted in many stages of advancement until a heavy and serviceable design was developed, as shown in Fig. 77. In this design the pedestal has a bar welded to it on which the lower front rail rests and to which it is connected by means of bolts and keys. The top front rail rests on top of the top main frame and extends back beyond the pedestal, thus giving room for the use of more bolts. The design shown in Fig. 77 is the one largely used on all heavy locomotives, it being slightly changed in detail for the various types.

In addition to the two general types of bar locomotive frames which are made of wrought iron or mild steel, a number of cast-steel frames are being used. The general make-up of the cast-steel frame does not differ materially from that of the wrought iron except in the cross-section of the bars. The bar frame is rectangular or square in cross-section whereas the sections of cast-steel frames are usually made in the form of an I.

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