TM 5-5420-278-24&P0005 000005 00-1DESCRIPTION AND THEORY OF OPERATIONIMPROVED RIBBON BRIDGE (IRB)RAMP BAY M16NSN 5420-01-470-5825 P/N 12478918;INTERIOR BAY M17NSN 5420-01-470-5824 P/N 12478919.Section II. THEORY OF OPERATIONGENERALThis section explains how components of the IRB-I and IRB-R work. A functional description of the IRBcomponents and their operation is covered in the following paragraphs.HOW THE BRIDGE IS LAUNCHEDUnfolding and folding operations involve the action of the bay’s two inner and outer pontons and thecables, levers, and springs or torsion bars that make up the unfolding/folding mechanism. Once the bay islaunched, the actual movement or unfolding is caused by the forces of gravity and buoyancy generated asthe bay is released and begins to float. The unfolding mechanism’s function is to assist and dampen themovement of the pontons about their hinged joints. The unfolding of the bay is automatic, whereas, to foldthe bay it must be lifted from the end at the center. The lifting force (provided by the transporter) causesthe inner pontons to move about their hinged joints, and their movement forces the unfolding/foldingmechanism’s cables, levers, and springs (IRB-I) or torsion bars (IRB-R) to simultaneously pull the outerpontons up, about their hinged joints, thus folding the bay.FUNCTION OF THE LOCKSThe ponton locks are designed to hold the bay in the folded or unfolded position. When an IRB-R orIRB-I is placed in the folded position, its inner and outer pontons are held by foldlocks and travel latches.These locks are manually engaged or disengaged during a controlled launch, retrieval, transport, andground storage of the bay, but are not engaged when the bay will be used in the unfolded position. Tosecure the IRB-R or IRB-I in the unfolded position, the bay’s inner pontons are locked by two manuallyengaged couplings. The bay’s outer pontons are then secured by manually engaging four swivel locks on theIRB-R or four outer ponton locks on the IRB-I. Once secured in the unfolded position, the bay is ready forbridge or raft construction.FUNCTION OF THE COUPLINGSThe ponton couplings are the connection points for locating and holding IRB bays together in theconstruction of a bridge or raft. The upper couplings are designed to secure the bay to another bay until thelower lock pins can be engaged. The lower main coupling at each end of the inner ponton receives the lowerlock pin. The lower main coupling is designed as a connecting eye, is made of steel, and is bolted to theinner ponton and a steel tie. The steel tie extends longitudinally the full length of the inner ponton. In abridge or raft configuration, the loads placed on the adjoining bays are carried by the steel tie rather thanthe aluminum structure of the inner ponton itself. The upper coupling is made of high-tensile steel andutilizes steel insert blocks bolted to the inner pontons. During bridging operations, the longitudinal uppercouplings, except for those on the ramp bay, are opened to allow the bays to hinge at the lower maincouplings. During rafting operations, the longitudinal upper couplings are closed for adequate rigidity.
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