Heavy Duty: Inspect That Upper Coupler

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

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.”

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