Chapter 34 Fifth Wheels and Coupling Systems Objectives

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 Con. Met 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 properation 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 weightover-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.