CDL · Combination Vehicles · 6 min read
Separating a Tractor and Semitrailer
- Identify the correct steps for positioning the rig, securing the brakes, and relieving pressure on the fifth wheel locking mechanism.
- Explain the proper procedures for operating the landing gear on empty versus loaded semitrailers and safely stowing disconnected lines.
- Describe the partial pull-out safety stop and final inspection required before clearing the semitrailer completely.
Positioning the Rig and Securing the Brakes
Knowing how to couple and uncouple correctly is basic to the safe operation of combination vehicles. Incorrect coupling and uncoupling can be very dangerous to the driver, the equipment, and other road users. Because equipment designs vary between rigs, drivers must learn the specific coupling and uncoupling details for the trucks they operate. Even experienced drivers must approach each uncoupling operation with care, as skipping steps or rushing through the process can lead to serious equipment damage or severe injuries.
The first step in safely uncoupling a tractor-semitrailer is to position the rig properly. When positioning the rig to uncouple, you must make sure the parking area surface can support the weight of the trailer. If the surface is too soft, the trailer landing gear could sink once the tractor is pulled away. In addition to selecting a firm surface, the tractor must be lined up straight with the semitrailer when uncoupling. Pulling the tractor out from under a trailer at an angle can damage the trailer landing gear, creating costly structural problems and unsafe operating conditions.
Once the rig is positioned straight on a supportive surface, you must secure the trailer and ease pressure on the fifth wheel locking jaws before attempting to release the fifth wheel. Pressure must be eased on the fifth wheel locking jaws because tension from the trailer kingpin can bind the locking mechanism tightly. To lock the trailer brakes before uncoupling, shut off the trailer air supply. The trailer air supply can be shut off from the cab by pulling out the trailer air supply knob or moving the tractor protection valve control to emergency.
After shutting off the trailer air supply to lock the trailer brakes, ease pressure on the fifth wheel locking jaws by backing the tractor up gently against the trailer kingpin. Backing gently against the kingpin relieves pressure so the fifth wheel locking lever can be released. While the tractor is pushing against the kingpin, apply the tractor parking brakes. Applying the parking brakes while pushing against the kingpin holds the vehicle in place with pressure off the locking jaws, keeping the mechanism relaxed so it can unlatch smoothly.
Securing the trailer also requires evaluating the need for wheel chocks. Trailer wheels must be chocked if the trailer is not equipped with spring brakes, or if you are not sure whether the trailer has spring brakes. If a trailer does not have spring brakes, air can leak out of the trailer air tank and release its emergency brakes. Without wheel chocks, a trailer can roll and move if emergency brake air pressure leaks out. Therefore, always chock the trailer wheels if the trailer does not have spring brakes or if you are not sure; without chocks, the trailer could move.
Common mistakes
- Pulling the tractor out from under the trailer at an angle, which places sideways stress on the landing gear and can cause severe structural damage.
- Failing to chock the trailer wheels when a trailer lacks spring brakes or when unsure of the brake system, allowing the trailer to roll away if air leaks out.
- Cranking a loaded trailer's landing gear so high that the trailer lifts completely off the fifth wheel plate, rather than just lifting some weight off the tractor.
- Driving completely away from the trailer in a single pull without stopping with the tractor frame underneath to verify that the landing gear is holding.
Key points
- Position the rig straight on a surface that can support trailer weight, as uncoupling at an angle can damage the landing gear.
- Lock the trailer brakes by shutting off the trailer air supply, ease jaw pressure by backing gently against the kingpin, apply tractor parking brakes while pushing, and chock wheels if the trailer lacks spring brakes or if you are unsure.
- Lower the landing gear to firm contact for an empty trailer, or add a few extra turns in low gear for a loaded trailer to ease unlatching and future coupling without lifting off the fifth wheel.
- Stow disconnected air glad hands in dummy couplers or together, and hang the electrical plug facing down to prevent moisture from entering.
- Keep legs and feet clear of tractor wheels when pulling the release handle, pull forward only until the fifth wheel plate clears, and stop with the tractor frame under the trailer.
- Secure the tractor in neutral with parking brakes applied, inspect the landing gear and ground support, then release brakes, check the area, and drive completely clear.
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Sample lesson: Controlling Articulated Rigs25 practice questions on this exam
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