Coupling, Landing Gear, and Towing Systems
This domain covers the mechanical link between tractor and trailer and the systems that support a parked trailer. Expect questions on fifth wheel adjustment, kingpin wear, landing gear operation and inspection, and hitch hardware.
Fifth wheel and kingpin
The fifth wheel's jaws lock around the kingpin; a worn jaw, worn kingpin, or loose mounting allows chucking and can cause separation. Inspect jaw and lock condition, lubrication, mounting bolts, and the upper coupler plate for cracks. Kingpin diameter and wear are checked with a gauge against the OEM limit - a worn kingpin requires replacement, not adjustment. Slider fifth wheels and trailer tandems rely on locking pins that must fully engage.
Landing gear
Landing gear is a two-speed screw-and-nut system: high gear for rapid travel, low gear for lifting under load. It supports a parked trailer, not a trailer being worked under. Inspect legs for bends, shoes for cracks and wear, cross shafts and gears for damage, and lubricate per the OEM schedule. Internal clutches and shear pins protect the gearbox from overload.
Hitches and safety hardware
Pintle hooks and lunette eyes, ball hitches, drawbars, safety chains, and breakaway cables. Chains should cross under the tongue to cradle a disconnected coupler; breakaway systems must apply trailer brakes if separation occurs. Rear underride guards and mud flaps are regulated equipment, not accessories.
Worked example
A driver reports clunking during acceleration and braking. Reasoning: chucking points to looseness in the coupling. Inspect the fifth wheel jaws for wear, check the kingpin with a gauge, verify mounting bolt torque, and confirm the fifth wheel is properly lubricated and adjusted. A worn jaw or kingpin requires replacement of the worn part; adjusting the jaw to compensate for a worn kingpin just moves the play elsewhere.
Traps that cost marks
Using landing gear as a jack, greasing a kingpin without checking wear, repairing rather than replacing a worn kingpin, and leaving safety chains uncrossed. Also watch for slider pins that are only partly engaged.