Chapter 34

Fifth Wheels and

Coupling Systems

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.

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.

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.

Semi-oscillating Fifth Wheel

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.

Sliding Fifth Wheel

Compensating Fifth Wheel

Compensating Fifth Wheel

Elevating Fifth Wheel

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.

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.

A-Lock

B-Lock

Conmet (now Holland) 400

Fontaine No-Slack

Locking Mechanism Adjustment

Slide Mechanism Adjustment

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.

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!

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.

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!

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.

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!

Air Slide

Manual Slide

Caution

• Always consult the manufacturer for the correct procedure when welding kingpins to upper coupler assemblies.

• Welding diagrams often are provided with replacement kingpins.

Pintle Hook

Drawbar

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.

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.

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.

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.

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.