Heavy Duty: Inspect That Upper Coupler

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

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.

We Recommend