Heavy Duty: Inspect That Upper Coupler

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.”

Corrosion concerns will likely force some changes in how OEMs build upper couplers in the future, Nissen says.

“They are going to be forced to put some kind of access holes in there to where these guys can stick this snake-eye up into the holes, and they can actually look at the cross-members and the corrosion on the inside of the trailers,” he says. “And the guys who are running tankers, they are really going to be hammered, because these guys are hauling, obviously, gasoline, jet fuel, fuel oil, hazardous waste, chemicals, they are going to be hammered on upper coupler inspections, and they should be.”


These days, if upper couplers are left uninspected, fleets can be in for a very rude awakening. Nissen says in some cases, an “overturn moment” with a bad coupler could quickly turn into a much larger problem.

“Let’s just say (the driver) comes into a curve or an off-ramp a little quick, and he’s got a commodity that shifts, like liquids,” he says. “If he’s in an overturning moment and the upper coupler is in some kind of under-maintained or corroded status, the fifth wheel will pull the upper coupler right out of the trailer.”

Nissen spends nearly all his time on the road, talking to dealers and fleet officials and hearing the real-life stories of the road. He says an accident he heard about a few months ago involving one of SAF-HOLLAND’s fifth wheels, but a different kingpin and upper coupler, brought the message home about corrosion and the need to inspect the upper coupler and all related components.

“You have three dimensions on the kingpin—a 2 7/8-inch, a 2-inch and a 2 13/16th (inch) on the bottom, and when it’s hooked to the fifth wheel, the fifth wheel just grabs the 2-inch diameter, so it’s in between the 2 7/8 and the 2 and 13/16th,” he says. “So this guy has a dry bulk tanker and he goes around the curve—his story is he was taking the curve at 10 miles an hour and looked in his mirror and saw the tanker going over. The fifth wheel stayed intact and locked and it severed the lower diameter (head) of that kingpin apart—it snapped it right in half, and the tanker went over. But when you looked at the condition of the kingpin after the accident, it probably should have been changed five years ago, because it was shot.

“The fleet checked the rest of their trailers and found six more pins that had to come off,” Nissen says. “There’s a perfect example of an under-maintained upper coupler, because nobody was checking those kingpins. Nobody.”

Wahlin says when inspecting the upper coupler, other load-bearing components like the kingpin must be included, and there are some very simple, quick tests that work just fine.

“One of those recommended by the kingpin manufacturers is a physical test by pulling on the kingpin with your hands to see if it can move,” he says.


Making sure your fleet’s fifth wheels and other related components are properly lubricated can go a long way to ensuring that you won’t have problems with the upper coupler system. ConocoPhillips Commercial Lubricants director of technical information and training Harold Tucker saw for himself what kind of damage can occur when he worked in the trucking industry.

“We had trucks hauling product from a factory and those trucks would come in and (technicians) would lube the fifth wheels and the tractors would be sitting there in wait on a gravel parking lot while other trucks were coming and going,” he says. “So this lubricated fifth wheel is gathering up all this dust and grit and then at the appropriate time, they jump in that tractor and slide it up under the trailer, so you’ve got a great combination of lubricant with abrasive.

“So it’s important when the fifth wheels are not in use to keep them lubricated with a good, tenacious grease that has excellent weather-water resistance; a good heavy base oil—something in the range of ISO 220 or 320 base oil for the fifth wheel, if not something heavier and more tenacious than that.”

Of course, the first step is to make sure the surface of the fifth wheel is clean, so you are not just grinding down the surface after coupling.

“(Fifth wheels) are pretty tough and survive many years the way they are, but cleaning them off periodically and making sure you have fresh grease that’s not too loaded up with fresh lubricant on there, or not too loaded up with dirt and grit is pretty important,” he says. “You can eyeball it to make sure the surface is completely covered and no exposed metal surface is going to rust and corrode.

“That’s about the quickest kind of down-and-dirty inspection to make sure the fifth wheel is not exposed to the elements,” Tucker says. “When you’re driving a truck down the Interstate and they’re continually in this super-fog of water, it’s just about comparable to driving underwater; inundated in water and moisture. It’s trying to get into every little crack and crevice, and the grease you use needs to be very water-resistant.”

Many technicians get hung up the color of grease, Tucker says, instead of the more important questions about its properties.

“(What is the) base oil viscosity?” he says, “Is the thickener resistant to oxidation? Is it resistant to water and the elements? Does it have good corrosion protection? Is that applicable for this application?’ Rather than saying, ‘Well, I’ve always used the black grease.’

“The cost of downtime is incredible for trucking, so you don’t want to short the maintenance or the frequency or the product quality in order to minimize the downtime,” Tucker says. “You don’t want any additional maintenance above and beyond what’s normal. So picking the right product and applying it in the proper amount, with the proper frequency, is extremely important to maintaining the vehicle, and the longevity of the parts.”


The introduction of magnesium and calcium chlorides as road de-icing agents have changed the game as far as maintaining upper couplers and related components. The sooner fleet officials get on board and realize the importance of properly maintaining and inspecting their upper couplers, the better off their bottom lines will be, says Wahlin, and that is the idea behind RP750.

Like anything maintenance-related, staying on top of problems is always more cost-effective than dealing with the after-effects, and while upper couplers might not be the sexiest subject, he says this new Recommended Practice can save money, which always looks good.

“Fleets are trying to protect their investment, and this is a cost,” he says. “If they can do things and watch for things that might be happening and stop things before they get too far, or at least identify problems that might be unique to a particular fleet, depending on the freight that they haul, they may be able to work with an OE such as ourselves and improve the life of that coupler assembly.

“So a lot of this is certainly safety-related, but it’s also a tool that the fleets can use to improve the cost performance of their equipment, and the way they specify a trailer in the future.”


A. Front of Coupler: The leading edge of the upper coupler should allow smooth contact with the fifth wheel. Any irregularity that interferes with smooth coupling (gouges, dents, corrosion) should be repaired or replaced.

B. Bottom of Coupler: Should be smooth and even—any welds that extend below the surface of the bottom plate should be ground smooth. Check for cracks and corrosion, inspect bottom-plate-to-main beam welds, and check for damaged, loose or missing fasteners.

C. Kingpin Inspection: Attempt to move by hand and check for wear—.125 inches or more requires the trailer to be put out of service. Any wear on the kingpin head is not acceptable.

D. Rear of Coupler: The rear edge of the bottom plate should be smooth, beveled or rounded, and sharp edges removed. Check the attachment and condition of any stanchion or tire plates. Check for cracked welds between the bottom plate and upper coupler cross member and top plate.

E. Side of Coupler: For van trailers, inspect the connection to the sidewall, the fasteners that secure the sidewall to the upper coupler and the welds of the upper coupler end plates for excessive wear. For flatbeds, inspect the connection to the main beam, any fastened or welded connections between the coupler and main beams and check for corrosion.

F. Upper Coupler Cavity: Remove debris if practical, but not by spraying water. Inspect for cracks, corrosion or damaged members. Scrape accessible members to determine soundness.

G. Top of Coupler: Check for sagging, and check welds for cracks. Repair any cracks or tears.

For more information on the Technology and Maintenance Council’s list of Recommended Practices, call TMC at (703) 838-1763.