Truck News

Feature  May 1, 2000 12:00AM

First approaches to fifth wheel problems

As simple as a fifth wheel can appear to be, problems experienced as you couple or uncouple to a trailer can be linked to an array of issues.One source of trouble can be a jaw pin that has over-adjust...



As simple as a fifth wheel can appear to be, problems experienced as you couple or uncouple to a trailer can be linked to an array of issues.

One source of trouble can be a jaw pin that has over-adjusted for kingpin slack, although the make and model of fifth wheel will determine the cause of this problem. For example, you can over-adjust for trailer kingpin slack on an ASF Simplex Series Slack-Free design. (The remedy here is to check the adjustment using a new SAE kingpin or special test gauge.)

But an inexperienced mechanic, a worn kingpin, or even problems with a fifth wheel’s adjustment mechanism could all be the cause of initial trouble. So too can you have problems if the jaw opening has spread too far apart because someone tried to couple to the trailer when the kingpin was sitting too high.

Then again, bumps, bends and jams are other sources of problems. Just try to couple smoothly if you’ve a bent lever bar, lock arm, or operating rod. A bent cover plate can interfere with the movement of the lock, while a jammed safety latch can also cause grief.

Packed snow, ice or dirt can also restrict the workings of the lock, and be a problem around the fifth wheel shims, backing plate and buffing rubber. Other problems can include a top plate that has been damaged just below the jaw pin; a rotary operating rod that has slipped out of its track; a shim which is far thicker than the recommended spec’s; or a missing handle accelerating spring.

But don’t think that the issues are limited to coupling. Other issues can surface and make it difficult to uncouple the trailer.

Consider cases when the kingpin applies longitudinal pressure to the jaw. This can be caused when you park on an incline, and the mass of the trailer and its kingpin applies pressure against the lock jaw of the fifth wheel. Here, all you need to do is slowly back a tractor into the trailer to relieve the pressure.

You can also face problems if the jaws are adjusted too tight. Since we don’t want to lose trailers on the road, the fifth wheel jaws have to be adjusted to ensure a secure clamping force to provide minimal slack between the fifth wheel and the trailer. The jaw must retain its hold and allow rotation around the kingpin for turns. If there was a lot of slack, a trailer could be dropped as the truck bounces down the road. At the very least, the constant to-and-fro movement between the fifth wheel jaws and the trailer kingpin would buck the connection, wearing the kingpin and jaws.

Kingpins can also apply upward pressure on the jaws if air bags on a tractor aren’t properly inflated. The end result affects the tractor’s ride height, which thrusts up against the nose of the trailer.

Sometimes a tight lock is bad news. This can be caused when a jaw opening is spread too far apart in a high-coupling attempt and a driver still forces the kingpin lock in place.

Other problems could include a rotary operating rod that has slipped out of its track, a rod assembly that has not been properly adjusted, or lock arm indentations that are worn at the mating surfaces of the rear jaw.

When a trailer’s upper coupler plate and front underbody area displays excessive damage, you can often blame an incorrect fifth-wheel pick-up ramp.

The ramp is the welded steel component that slopes upward from the rear of the tractor frame toward the fifth wheel plate. This allows the reversing tractor to contact the trailer bolster plate, to permit it to slide up onto the ramp, to raise the nose of the trailer, and allow the kingpin to be aligned with the access opening into the open jaws of the fifth wheel.

These ramps are often used to help a trailer slide on and off the fifth wheel during coupling and uncoupling. If the fifth wheel ramp is too short (does not extend beyond the ears of the fifth wheel), it will act like a cutting tool as the trailer slides up the ramp. The end result is that most of the trailer’s weight is supported on the sharp edges of the ramp located bewteen the ramp and the ears of the fifth wheel.

The trailer bolster plate will be gouged, and metal could become lodged between the trailer and the fifth wheel plate coupling surfaces.

Always ensure that the fifth wheel ramp is long enough to ensure a smooth transition from one surface to the other. If it isn’t, you can weld on an extension or replace the ramp assembly.

Other concerns to consider are the shock loads that can be created by gunning a tractor under a trailer that has low landing gear. The trailer’s resistance will cause the turning components of the tractor’s drivetrain to stop while the engine is still applying torque in an attempt to keep them rotating. That can damage your drivetrain.

Other problems that can directly affect the wear rate on a fifth wheel – particularly in severe-service applications – can be traced to the use of an engine brake or driveline retarder. The resulting retarding effect on steep downhill runs may pound the trailer’s kingpin against the fifth wheel locks, creating a pattern of reverse wear.

In severe service applications, the effect will cut fifth wheel life in half. (Over-the-road tractors aren’t immune, but the effect shortens their life by a mere 10 per cent.)

The wear itself is caused by a jack-hammering effect that originates in the engine. It can be traced back to the piston motion that tends to send shock waves through the chassis, which are in turn transferred to the fifth wheel locks as well as the other components as the mass of the loaded trailer is forced against them. The frequency of these oscillations is related to the speed of the engine while the compression brake is on. n

- Bob Brady is the president of Hi-Tech consulting in Burnaby, B.C.


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1 Comment » for First approaches to fifth wheel problems
  1. Dan B says:

    I had backed in to a 53′ trailer like I usauley do, did my tug foward, checked under to make sure the slide bar was across, didn’t look like it was all the way across, so I repeted the process , put the truck in reverse with the trailer brakes still on , and pulled forward again, and checked under to see that the slide bar had gone across, it looked fine.I as I took off I first made a right hand turn, than another right hand turn, than a left hand turn and thats when it came undoneand came off the tractor, thank god I was still in the yard going only 10 mph. the trailer was loaded, it was 5 degrees out, this has never happened to me and I am trying to figure out what else I could of done to prevent this from happening.

    Thank you , Dan B

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