by Paul Abelson, Technical Editor

When you think about it, you realize what an impressive job the fifth wheel and its intimate companion, the kingpin, have to do. They link together seven to 10 tons of tractor and 30 tons or more of trailer and cargo. They transmit traction and cornering forces, and resist some braking force. They absorb pitching, rocking and twisting motions. And despite these enormous forces, they maintain the strength to suspend a tractor solely by the kingpin-fifth wheel connection. This was demonstrated dramatically a few years ago in a photograph of a rig that had been in an accident. It was hanging over the side of a bridge, with the tractor in mid air, held only by its fifth wheel locking mechanism.

If that show of strength wasn’t enough, today’s fifth wheel assemblies must also slide to adjust weight distribution, and in some cases, rock side-to-side to relieve stresses, especially with frameless dump trailers and tankers.

Here we’ll examine fifth wheels: what the major components are, how they operate, and how you should maintain them. We’ll also look at some newly developed hardware to improve coupling durability and safety.

One sign of a complex and sophisticated product is the number of manufacturers supplying it. The easier a product is to engineer and build, the more companies will manufacture the product. The more demanding the engineering and manufacturing process, the fewer manufacturers there are. There are more than 200 makers of trailers in North America, but only three manufacturers of fifth wheels: ASF, Fontaine, and Holland. Each company has its proprietary way of activating and securing the mechanism that grips the kingpin, but the general principles of operation are similar.

The basic components of a fifth wheel are the top plate, the jaws, the locking/releasing mechanism and the mounting system. The top plate is the part that bears the weight of the trailer and its load. It transmits the load through its reinforcing structure to large diameter pins on either side. The top plate also has pockets and channels to hold and distribute the grease needed to provide lubrication. The grease lets the trailer’s upper coupler plate pivot while resting on the top plate. The pins allow the fifth wheel to rotate about its transverse axis, to allow pitching motion, which compensates for relative motion between tractor and trailer when going up and down hills. The pins also allow the fifth wheel to be tilted back for coupling. Brackets support the top plate pins, provide a bearing surface for the pins to rotate, and transmit all pulling, braking and cornering forces from fifth wheel to chassis.

There is an open notch in the rear of the top plate, so that when the tractor backs under the trailer, the kingpin is centered at the locking mechanism. Since the tractor pulls the trailer and the notch is open to the rear, some other mechanism is needed to securely hold the kingpin, and thus the trailer, in place. That mechanism consists of jaws and a positive locking device to keep the jaws from opening. The term, "jaws," as used here, refers to any device that holds the kingpin in place. Some use C-shaped clamps that open and close from the sides. Some use an S-shaped jaw that swings around from one side, and a third device is a notched bar that slides in from one side. All prevent the kingpin from pulling out, and all are secured by additional locking mechanisms that prevent accidental opening of the jaws. Various devices take up slack to prevent impact damage to jaws and kingpins. 

As with any complex mechanical device, preventive maintenance (PM) is a key to long operating life and minimal repair cost. Manufacturers recommend that every three months or 30,000 miles, and certainly no longer than every six months, each fifth wheel be thoroughly steam cleaned and examined. Start by checking for obvious problems: cracks in welds or brackets; loose or missing bolts or nuts; missing cotter pins and clinch pins. You should repair or replace them as needed.

Then thoroughly lubricate the assembly, following manufacturer’s recommendations. Using a lithium chassis grease or general-purpose high temperature multi-purpose grease, lube each Zerk fitting. Make sure grease evacuates all around. Rock the top plate to help distribute the grease evenly. Use a pry-bar to raise the top plate to unload the pin bearings in the bracket, and grease the bracket until you see new grease all around. Brush grease onto all sliding parts. In extremely cold climates, it may be necessary to use a 90-weight gear oil in certain locations, to allow for free movement. Again, follow the recommendations for your specific make and model. If you’ve lost your owner’s manual (or never got one from your dealer) try contacting the manufacturer online to download service information for your make and model.

Once the assembly is lubricated, operate it. Use a test kingpin to test the jaws’ action. You can get a test kingpin from any of the fifth wheel manufacturers, or you can make one by mounting a kingpin on a metal plate. Put two handles on the opposite side of the plate, so you can push the kingpin in and out of the socket. The jaws should open smoothly and easily, and should lock firmly around the test kingpin. You should be able to pivot the kingpin, but not pull it out. The pull handle or release lever should have returned to the closed position.

Proper operation during coupling and uncoupling is as important to long fifth wheel life as are inspection and lubrication. Before coupling, make sure there’s an even coating of grease on the top plate, and that all reservoir pockets and grooves are full. All contact should be easy and gentle. When coupling, make sure the trailer and tractor are lined-up. Any angle will cause one side to lift before the other, stressing the trailer, fifth wheel and landing gear. If the kingpin is forced to ride along one side of the fifth wheel notch, it can get damaged, and can damage the fifth wheel jaws. If the trailer is forced sideways, it can damage the landing gear.

All backing should be in the lowest, slowest reverse gear. Stop a few inches before making contact with the trailer. Make sure the top plate is tilted all the way back. Then adjust trailer height so the nose of the trailer contacts the top plate about 10 inches behind the line between the pins. Continue coupling, making sure the jaws are firmly in place and the handles are secure. Get under the trailer and look forward to make sure you see the jaws and locking device behind the kingpin, and that the trailer upper coupling plate is resting securely on the fifth wheel top plate. If you can see any space between the top plate and the trailer, the kingpin is not being held securely. The tractor and trailer could separate. Double check by setting the trailer brakes with the trolley valve and gently trying to move the tractor just a few inches. You should feel resistance if the connection is secure.

When uncoupling, make sure the trailer won’t fall. You should pull out until the nose of the trailer rests on the top plate, then crank the landing-gear down to lift the trailer clear of the tractor.

Since the trailer pivots on the top plate whenever the tractor turns, the motion between the two surfaces should be as smooth and effortless as possible. Countless incidents and accidents have been reported where the driver unexpectedly lost control while trying to turn. Many have been traced to lack of lubrication or high friction, resulting in the trailer pushing the tractor straight, even though the steering wheel was cranked over.

To eliminate the need to lubricate the top plate and to reduce friction, fifth wheel cover plates and upper coupler plates are available from numerous sources. These are made of high-density or ultra-high density polyethylene. Both are extremely slippery. The more expensive ultra-high density material is slightly slicker and will withstand more abuse. When using these additional plates, whether attached to the fifth wheel or trailer, a special kingpin must be used to compensate for the thickness of the material. Using a plate with a standard kingpin will prevent the jaws from closing properly. Using these plates improperly will void most warranties, and can be extremely dangerous.

Holland Hitch introduced a new fifth wheel, their 3500 Low Lube, at the most recent Mid America Trucking Show. It has replaceable polyurethane segments fastened to the top plate. The urethane family of engineered polymers is known for both low coefficient of friction and high resistance to abrasion and wear. The fifth wheel works with all trailers.

The Revolver™ from Direct Dimensions, Inc., is an ultra-high density polyethylene turntable that takes the place of the trailer upper coupler plate. It has its own kingpin, so length is no problem. Its unique design moves the rotating surface away from the fifth wheel top plate and into a section of the trailer upper plate enclosed by a lip. That keeps dirt, gravel, salt and other contamination from interfering with the free rotation of the turntable. Because there is no relative motion between the fifth wheel top plate and the bottom of the trailer, fifth wheel life is greatly extended.

When using any low-lube or no-lube polymer, make sure to grease the jaws, lock and slider mechanisms regularly. They are designed to be lubricated when grease is scraped or squeezed from between the top plate and trailer during normal operations. When no grease is used, this lubrication is lost and you must make up for it.

Like anything else involving your truck and its component parts, fifth wheels require their share of care and attention. The care needed is intuitive. With a little discipline, it quickly becomes a habit. How important is fifth wheel care and maintenance? Just ask the driver who was rescued from the dangling tractor.