1. Fifth Wheels and Coupling Systems
Chapter 34
Fifth Wheels and
Coupling Systems

2. Objectives (1 of 3)
Describe the different types of fifth wheels used on tractors.
Outline the operating principles of the Holland, Fontaine, and ConMet fifth wheels.
Understand the importance of correctly locating the fifth wheel on the tractor.
Describe the locking principles of each type of fifth wheel.

3. Objectives (2 of 3)
Outline the procedure required to couple and uncouple a fifth wheel.
Service the common types of fifth wheels.
Describe the procedure required to overhaul a fifth wheel.
Identify the overhaul procedure required of some common fifth wheels.

4. Objectives (3 of 3)
Define high hitch and outline what is required to avoid it.
Describe the operating principle of a pintle hook and draw bar.
Outline the function of the kingpin and upper coupler assembly.

5. Semi-oscillating Fifth Wheel

6. Shop Talk
Most truck and trailer coupling mechanisms are simple in design and operating principles.
However, because of the extreme consequences of a trailer breakaway, care must be practiced when doing any work on coupling devices.
Always consult OEM service literature and ask questions when you are unsure of a procedure.

7. Sliding Fifth Wheel

8. Compensating Fifth Wheel

9. Compensating Fifth Wheel

10. Elevating Fifth Wheel

11. Shop Talk
The fifth wheel location on the tractor affects the bridge formula, that is, how the load is distributed over the tractor and the trailer axles.
Double-check that the fifth wheel is correctly located when installing.
Incorrectly placed fifth wheels can result in weight-over-axle infractions and severely imbalanced dynamic braking.

12. Caution
Installing a jaw set into a fifth wheel is a simple, everyday shop procedure.
However, because of the serious consequences that result from a fifth wheel separation, it is essential that you carefully observe the OEM service literature during assembly and testing.
Double-check every step in the procedure. Test the operation of a completed fifth wheel rebuild by coupling and uncoupling a couple of times to a trailer.

13. A-Lock

14. B-Lock

15. Conmet (now Holland) 400

16. Fontaine No-Slack

17. Locking Mechanism Adjustment

18. Slide Mechanism Adjustment

19. Shop Talk
Do not attempt to inspect a fifth wheel without removing the grease.
The best way to do this is to first scrape off as much as possible, and then apply some solvent before hot pressure washing.
Check the turntable plate, throat, and saddle/pivot assembly for cracks.
Check the dynamic action of the locking mechanism and the secondary lock integrity.
Check all the mounting fasteners and welds.

20. Caution
Installation welding on fifth wheel assemblies should be performed by a qualified welder.
Improper welding procedures can result in a trailer separation—the consequences of this can be fatal!

21. Caution
The trailer must be stationary with its parking brakes applied to prevent damage to the tractor or trailer by uncontrolled sliding of the fifth wheel.

22. Shop Talk
On a sliding model fifth wheel, after locating it in the desired position, visually inspect the locking plungers to be sure that they are properly engaged.
Also, set the trailer brakes and rock the tractor fore and aft.
Do not attempt to shift a sliding fifth wheel when a trailer is in motion!

23. Caution
Proper adjustment of the sliding bracket locking plungers must be performed at installation and maintained at regular intervals by use of the adjusting bolts provided on both sides.
Proper adjustment is required for proper operation and for proper load transfer and distribution.

24. Caution Rebuilding of fifth wheels is a common shop practice.
Although it is a simple procedure, each type of fifth wheel uses distinct locking mechanisms.
Always use the manufacturer procedure included in every rebuild kit. A fifth wheel failure can have fatal consequences!

25. Air Slide

26. Manual Slide

27. Caution
Always consult the manufacturer for the correct procedure when welding kingpins to upper coupler assemblies.
Welding diagrams often are provided with replacement kingpins.

28. Pintle Hook

29. Drawbar

30. Summary (1 of 5)
The purposes of the fifth wheel are both to permit the truck/tractor to articulate around the trailer kingpin when turning corners and to support a percentage of the weight of a semi-trailer.
The semi-oscillating fifth wheel is the most common highway tractor fifth wheel.
A semi-oscillating fifth wheel articulates or oscillates about an axis perpendicular to the vehicle centerline: that is, on a fore-aft plane.

31. Summary (2 of 5)
Compensating fifth wheels are designed to provide both front-to-rear and side-to-side oscillation between the tractor and semi-trailer.
Side-to-side oscillation occurs below the fifth wheel plate surface and lessens the effect of trailer torque or twist acting on the frame of the tractor or lead trailer.
Fully-oscillating fifth wheels articulate on all planes and are used with gooseneck, low-bed trailers, especially those operated on rough, off-highway terrain.
Elevating fifth wheels are used on yard shunt tractors. They may be equipped with either air or hydraulic lift mechanisms.

32. Summary (3 of 5)
Many fifth wheels are built with a sliding mechanism that permits longitudinal adjustment placement of the fifth wheel on the tractor frame.
This allows the point at which the weight of the trailer is supported by the tractor to be altered according to load and legislative requirements.
Relocation of a sliding fifth wheel alters the weight-over-axle distribution on a tractor when coupled to a trailer.
As a fifth wheel is moved forward, the percentage of weight carried by the tractor steer axle increases.

33. Summary (4 of 5)
Fifth wheel mountings may be plate-mounted or angle-on-frame mounted.
The fifth wheel should be inspected every 30,000 miles or 3 months, whichever comes first.
Proper lubrication of the fifth wheel includes keeping a water-resistant, lithium-based grease applied to the fifth wheel plate, lubricating all moving parts with a light oil, and applying chassis grease to the grease zerks at the pivot points and the cam.

34. Summary (5 of 5)
Kingpins are usually welded to the trailer upper coupler unit and are used to couple a trailer to the fifth wheel.
Kingpins come in two SAE standard sizes, 2- inch and 3.5-inch.
They are manufactured in special alloy steels and usually are welded to the upper coupler assembly. Bolted and removable kingpins also are used.
A pintle hook and drawbar assembly is used to couple a full trailer to a truck or lead trailer.