One thing I'll never forget working around the C-5...that floor of the thing can handle *incredible* high weight pressures upon it.
The secret behind it was the width of a highly reinforced floor coupled with minimal weakness points in it. These weakness points are primarily the "skinny" main floor "roller-tray system" which are long, detachable metal plates with small, but quite numerous rollers on one side, flat surface on the other that snap down with locks built in to provide either palletized cargo or floor load cargo capability or combinations of both.
The other weakness points are the forward and rear ramp areas, which have weight restrictions on cargo secured there due to the hydraulics systems that raise and lower the ramps there.
Getting back to the roller trays of the 463L system, each aircraft posesses a differing configuration. The earlier C-130 and C-141 have much wider "trays" of these same type of rollers. The C-130 possessing a roller system that did not involve cutting holes into the floor to accomodate them, you just take the roller trays, which look like a metallic box with rollers inside of them and a lock device at each end, which then is slapped into a position on the floor (which is universally flat and "clean".
The C-141-B model uses a slightly updated roller system versus the C-130 that employed "sunken trays" of rollers that fit flush into the floor. These are almost as wide used on the C-130, but they fit into the floor for palletized cargo so that it was flush with the locks and detents in the rail system, or they were flipped over and smooth with the rest of the floor surface. This created a weight restriction and weakness area for certain widths of the C-141 floor as they were now weaker with these cutouts for these trays in them. Another feature is the passenger seat detent system in the floor which allows standard airline "trip seats" to be hammered into place for combi cargo/pax configuration.
The later C-5 uses same system in concept to the C-141 but MUCH narrower tray system affording much higher cabin weights to be lifted. (As earlier described, C-5 passenger seating -mostly- handled with 75 seats up top aft of the wing, although the C-5 does have a rare to be used "airbus config" using palletized airline seats that roll into place with 2 "comfort pallets" used to handle the extra lavatory and inflight feeding needs, I think the seating can be brought up to around 400 pax? I never saw this config used while I was in though, saw it mentioned in C-5 "-9 and -5 Tech orders" but heard it was only used during Vietnam's "Operation BabyLift", which tragically lost a C-5A at Saigon shortly after takeoff killing all aboard. Debate still exists whether it was the pressure bulkhead that blew out due to a manufacturers defect or a shoulder launched SAM that took it out.)
If one were to try and load an M-1 Abrams or Bradley on a 747 Frieghter, the thing would fall right through the floor and wreck the aircraft. It just cant handle it.
The M-1 weighs around 65 tons if my memory is right. That bad-boy is going to -sail- right through the floor! On a C-5, kneel her and chain them down no problem. The weight and stress the C-5 main floor can handle is -just incredible-.
Can anyone tell me which roller/floor system they use on the C-17? I never had a chance to work on them as I left the USAF before they were in operational service. Would be appreciated.