How Are Semi Trailers Made?

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    Frame Types

    • Trailer frames come in four basic varieties: straight frames (the most common), drop-deck frames (where the trailer floor drops as low as possible to the ground), double-drops (a variation on the drop deck) and specialty frames designed for car carriers and tankers. Every frame has a different shape and type of main longitudinal railing, but the manufacturing procedures vary little.

    Framerail Manufacturing

    • Frames come into the main production shop as either pre-formed sections of I-beam, box steel or C-shaped open tubes or as thick steel panels. For tube-based frames, the frame shop slices the sections to length and proceeds to cut mounting holes and weld flanges to the framerails. For drop-deck trailers, the frame shop will cut the steel tubes into sections and re-weld them with the appropriate angles. For specialty trailers, the frame shop will cut long, thin sections of plate for the top, bottom and sides of the framerail. Once those sections have the right bends, the framers set them on a jig and weld the sections together to form a boxed frame.

    Finishing The Frame

    • Once they have finished and welded framerails, the framers move them to another assembly jig with an enormous crane. They then begin to weld in the lateral crossmembers to tie the framerails together, riveting and bolting them together. From here the finished frame goes to "Assembly."

    Frame Assembly

    • Assemblers put all the mechanical components on the frame, which are either manufactured on-site or outsourced from another shop. These add-ons include the sliding tandem wheel carrier/axle assembly (called the "carriage"), the landing gear, trailer kingpin, bumper, braking system components, and mudflap brackets, floor supports and flooring. For boxed trailers, assemblers install the walls, doors, refrigeration unit and appropriate ducting.

    Final Assembly

    • The trailer becomes a mostly-functional unit in "Final Assembly." In many factories these final assemblers are called "riggers," which is a throwback to those ship-builders whose job it was to install ropes for the sails. Riggers install the trailer insulation and interior panels (if refrigerated), air lines and connectors, electrical wiring, lights and anything else the trailer needs to function. From here, the trailer may go on to the Body Shop for painting, polishing and last minute finishes, and then to "Quality Control" for testing, evaluation and certification.

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