Heavy Duty: Inspect That Upper Coupler

MC RP 750: Corrosion issues prompt guidelines for upper coupler inspections.

Inspecting the “upper coupler structure of commercial van and flatbed trailers equipped with two-inch kingpins in a fixed-position coupler” is probably not the first thing on your technicians’ to-do list. In fact, it may not even be on the list.

Not any more. The folks at the Technology & Maintenance Council (TMC) leave no stone unturned when it comes to maintenance, and in March, after more than two years of research, its Task Force on upper couplers released RP 750—its Recommended Practice for inspecting these components. Task Force co-chair Bill Wahlin, assistant vice-president of engineering for Stoughton, WI-based Stoughton Trailers, says it was time to do something about the growing problem.

“I’ve had some comments from people who have said that we need to print this immediately and get this out in the field,” he says. “I don’t think that there’s anything that’s really Earth-shaking here, but by simply being aware of it, some fleets are going to use this as a tool to train their technicians and their drivers what to look for, and that goes a long ways. I suspect this will be in their tool kit, if you will, of documents for avoiding problems in the future.”


The upper coupler and adjoining fifth wheel are two of the least-maintained components on a truck, says Wahlin’s Task Force co-chair Rob Nissen, Holland, MI-based SAF-HOLLAND’s manager of technical service/training. A big reason why is they have generally not given technicians many problems to worry about in the past.

“The industry is in the frame of mind that it’s pretty much maintenance-free—it’s put together by the OEM, and once it’s put together it’s like there forever,” he says.

That is quickly changing, though. While upper couplers and related components might have gotten short shrift in years past, Nissen says the introduction of sodium chloride and magnesium chloride as de-icing agents on roadways across the country in the past decade has changed the game completely, and fleets need to adapt their maintenance programs to compensate.

“This stuff is getting up on the side of the upper coupler, between the floor and the upper coupler, where the substructure is put together with the cross-members,” he says. “It’s getting in there and eating the support system away and these upper couplers are coming apart. They’re just eaten away from the trailer.”

Wahlin says while the new de-icing chemicals are not any more corrosive than older mixtures, they are difficult to remove

“They want to grab onto materials—steel, aluminum—and you really need to rub them off,” he says. “The other thing that’s insidious about those two chemicals is they don’t need a lot of moisture to start reacting; they can react with high humidity. They just don’t wash off, and they get into places that sodium chloride may not be able to get into.”

There is more than one type of corrosion for technicians to worry about, as well.

“It’s not only the simple red rust that you see from corroding steel, but the electrolysis that happens between dissimilar metals,” Wahlin says. “That’s where an aluminum side rail on a van trailer and the interface with the normally steel coupler assembly; that’s where electrolysis comes into play. Flatbeds are similar, in that there are designs that make use of an aluminum substructure with some steel components, so there are common threads between the van trailer and the coupler assembly on a flatbed.”

Kingpins are also vulnerable to corrosion, Nissen says.

“People think it’s going to be there forever, and they don’t understand with this chloride and the use of grease on the fifth wheel, is that the road dirt gets into the grease and it mixes itself and kind of turns into a compound, like a rubbing compound, and it gets between the locks and the pin and it actually wears the pin as well as the locks,” he says. “You never see anybody putting grease on an upper coupler, and you very, very seldom see anybody taking a kingpin gauge and gauging the wear on the kingpin. At a minimum on kingpins, they should be checked annually, and if they run a rail operation, those rail stanchions on those rail cars, those are not maintained, and they just eat kingpins alive.”

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