The United States 3-ton frame (Fig. 190) is an illustration of the structural or rolled channel frame, combined with steel castings and a construction in which each unit is mounted as flexibly as possible. The forward three cross members are steel castings, the fourth of I-beam section rolled steel and the rear of channel shape rolled steel. The front-spring hangers are formed integral with the front cross member, which is in two halves, bolted together at the center.
Fig. 191. Knox Tractor Frame.
The Knox tractor frame (Fig. 191) is made from rolled steel of channel shape; however, it is comparatively short, as the rear axle is attached to the frame by means of cantilever rear springs. Thus the frame merely extends far enough to carry the support on which the spring pivots. The illustration shows the combined jackshaft and brake support bracket and other parts which are riveted to the frame. Large gusset plates are used at the front and rear end, while the front cross member extends to each side and is curved to form the bumper.
The 3-ton Velie frame (shown in Fig. 192) is built up from rolled steel of I-beam section, with subframe members of channel section. It is of the rigid type well braced, and provided with heavy gusset plates. The front cross member is bolted in position for easy access to the power plant. Diagonal braces of channel section are placed between the third and fourth cross members for stiffening the frame. It is claimed that I-beam section provides a much heavier and stronger frame, due to greater width of the flanges.
To eliminate the braking of frames, there seems to be a general movement against- drilling any rivet or bolt holes in the bottom flanges of the side rails. Instead of drilling the flanges in attaching the body and frame brackets, the vertical section of the frame is drilled. There is less weakening of the frame by this process. There is also a decided tendency toward the use of straight-side rails. A novel feature which accomplishes this is used on the Nash trucks and illustrated in Fig. 193. Instead of tapering the frame at the front end as is usual by carrying the top flange straight, the taper is accomplished by keeping thie bottom flange straight and tapering the top gradually down to the spring horn. Bolting instead of riveting frame members and castings is also receiving serious consideration at present.
Structural steel of channel or I-beam section is bought from the steel mills in stock lengths. It is usually manufactured from Bessemer steel, afterward subjected to open hearth process in which it is saturated with carbon, to certain specifications for certain uses.
Pressed steel is purchased in sheet form, cut to the proper shape in the flat and then pressed into channel form under great pressure. It is made of steel rolled into sheets; it is made somewhat closer grained, and there is no breaking of the flake in the rolling operation. The pressed steel frame permits of greater simplicity in assembling, since parts can be easily bolted or riveted to it.
Fig. 192. Velie Structural l-Beam Frame.
Fig. 193. Nash Frame showing Taper of Upper Flange.