TM 5-5420-278-24&P
0005 00
0005 00-1
DESCRIPTION AND THEORY OF OPERATION
IMPROVED RIBBON BRIDGE (IRB)
RAMP BAY M16
NSN 5420-01-470-5825 P/N 12478918;
INTERIOR BAY M17
NSN 5420-01-470-5824 P/N 12478919.
Section II. THEORY OF OPERATION
GENERAL
This section explains how components of the IRB-I and IRB-R work. A functional description of the IRB
components and their operation is covered in the following paragraphs.
HOW THE BRIDGE IS LAUNCHED
Unfolding and folding operations involve the action of the bay’s two inner and outer pontons and the
cables, levers, and springs or torsion bars that make up the unfolding/folding mechanism. Once the bay is
launched, the actual movement or unfolding is caused by the forces of gravity and buoyancy generated as
the bay is released and begins to float. The unfolding mechanism’s function is to assist and dampen the
movement of the pontons about their hinged joints. The unfolding of the bay is automatic, whereas, to fold
the bay it must be lifted from the end at the center. The lifting force (provided by the transporter) causes
the inner pontons to move about their hinged joints, and their movement forces the unfolding/folding
mechanism’s cables, levers, and springs (IRB-I) or torsion bars (IRB-R) to simultaneously pull the outer
pontons up, about their hinged joints, thus folding the bay.
FUNCTION OF THE LOCKS
The ponton locks are designed to hold the bay in the folded or unfolded position. When an IRB-R or
IRB-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, and
ground storage of the bay, but are not engaged when the bay will be used in the unfolded position. To
secure the IRB-R or IRB-I in the unfolded position, the bay’s inner pontons are locked by two manually
engaged couplings. The bay’s outer pontons are then secured by manually engaging four swivel locks on the
IRB-R or four outer ponton locks on the IRB-I. Once secured in the unfolded position, the bay is ready for
bridge or raft construction.
FUNCTION OF THE COUPLINGS
The ponton couplings are the connection points for locating and holding IRB bays together in the
construction of a bridge or raft. The upper couplings are designed to secure the bay to another bay until the
lower lock pins can be engaged. The lower main coupling at each end of the inner ponton receives the lower
lock pin. The lower main coupling is designed as a connecting eye, is made of steel, and is bolted to the
inner ponton and a steel tie. The steel tie extends longitudinally the full length of the inner ponton. In a
bridge or raft configuration, the loads placed on the adjoining bays are carried by the steel tie rather than
the aluminum structure of the inner ponton itself. The upper coupling is made of high-tensile steel and
utilizes steel insert blocks bolted to the inner pontons. During bridging operations, the longitudinal upper
couplings, except for those on the ramp bay, are opened to allow the bays to hinge at the lower main
couplings. During rafting operations, the longitudinal upper couplings are closed for adequate rigidity.
