Combination Vehicles: Coupling Uncoupling

39 questions in the bank cover this topic. Everything below is drawn from the sources those questions cite.

Sources: 49 CFR 393.70, TC 21-305-20 para 13-23, TC 21-305-20 para 13-24, TC 21-305-20 para 13-25, TC 21-305-20 para 13-26, TC 21-305-20 para 13-27, TC 21-305-20 para 13-35

What you need to know

393.70(b)(2) requires the lock to hold the halves together until someone deliberately works a positive manual release.

Nothing about load, pressure or motion may release it. It is the upper and lower halves it holds together.

Every fifth wheel assembly must have a locking mechanism. The locking mechanism, and any adapter used in conjunction with it, must prevent separation of the upper and lower halves of the fifth wheel assembly unless a positive manual release is activated.

49 CFR 393.70

393.70(b)(2) requires automatic application on coupling for a readily separable fifth wheel.

You still test the connection afterwards, because an automatic lock can fail to seat on a high or low trailer.

If a motor vehicle has a fifth wheel designed and constructed to be readily separable, the fifth wheel locking devices shall apply automatically on coupling.

49 CFR 393.70

393.70(b)(1)(i) specifies proper brackets, plates or angles with correctly tightened bolts of adequate size and grade, or equivalent security.

The installation must not crack, warp or deform the frame. Mounting plates or angles are equally acceptable, with bolts of adequate size and grade, or an equivalent method.

The lower half of a fifth wheel mounted on a truck tractor or converter dolly must be secured to the frame of that vehicle with properly designed brackets, mounting plates or angles and properly tightened bolts of adequate size and grade, or devices that provide equivalent security.

49 CFR 393.70

393.70(b)(1)(i) requires a positive anti-shift device.

A sliding fifth wheel that crept on the frame under braking would change weight distribution and could overload an axle.

The installation must include a device for positively preventing the lower half of the fifth wheel from shifting on the frame to which it is attached.

49 CFR 393.70

393.70(b)(1)(ii) holds the upper half to at least the same standard as the lower half.

Both halves carry the full coupling load, so neither is the weaker link by design.

The upper half of a fifth wheel must be fastened to the motor vehicle with at least the same security required for the installation of the lower half on a truck tractor or converter dolly.

49 CFR 393.70

393.70(b)(3) ties fifth wheel location to weight distribution across the axles of both vehicles, and to steering, braking and maneuvering.

Sliding it changes axle loads, which is how you correct an overweight drive or trailer axle. That in turn affects steering, braking and maneuvering.

The lower half of a fifth wheel shall be located so that, regardless of the condition of loading, the relationship between the kingpin and the rear axle or axles of the towing motor vehicle will properly distribute the gross weight of both the towed and towing vehicles on the axles of those vehicles, will not unduly interfere with the steering, braking, and other maneuvering of the towing vehicle

49 CFR 393.70

The fifth wheel on the tractor receives the kingpin on the trailer, and the fifth wheel's coupling jaws hold them together.

A pintle hook and tow-bar is the arrangement used for a full trailer, not a semitrailer.

The tractor and semitrailer are separate units joined together by the tractor’s fifth wheel and the semitrailer kingpin.

TC 21-305-20 para 13-25

The landing gear carries the front of the trailer when it is not coupled.

It must be fully raised before moving off and fully lowered and supporting the load before uncoupling.

When not joined to the tractor, the semitrailer is held upright by two legs that support the front end.

TC 21-305-20 para 13-25

Closing the shut-off valves before disconnecting the lines stops the tractor's air escaping through open couplings.

The trailer's own air is held by its relay emergency valve, which is what keeps the spring brakes set.

Turn the air brake hose shut-off valve levers to the closed position.

TC 21-305-20 para 13-27

Setting the trailer brakes holds it still so the tractor can be pulled gently against the locked fifth wheel.

If the jaws have not closed on the kingpin, the tractor pulls out and you find out at walking pace rather than on the road. The connection cannot be tested safely against a moving trailer.

Pull down the trailer air brake hand control lever (#6) to engage the semitrailer brakes

TC 21-305-20 para 13-26

393.70(c) requires a tow-bar for a full trailer.

A full trailer carries its own weight on its own axles, so it does not rest on a fifth wheel the way a semitrailer does.

A full trailer must be equipped with a tow-bar and a means of attaching the tow-bar to the towing and towed vehicles.

49 CFR 393.70

393.70(c)(4) requires a locking device against accidental separation, and the hitch mounting must include frame reinforcement or bracing sufficient to prevent undue distortion of the frame.

Be provided with a locking device that prevents accidental separation of the towed and towing vehicles.

49 CFR 393.70

393.70(c)(3) requires enough articulation for the vehicles to turn and ride, but not excessive slack.

Slack lets the trailer surge against the tow vehicle, which is hard on the hitch and unsettling to drive.

Provide for adequate articulation at the connection without excessive slack at that location

49 CFR 393.70

393.70(d) requires safety devices, typically chains or cables, attached to the frame or a frame extension.

They are the last defense if the tow-bar fails or comes undone. A frame extension counts, and one or more devices may be used.

Every full trailer and every converter dolly used to convert a semitrailer to a full trailer must be coupled to the frame, or an extension of the frame, of the motor vehicle which tows it with one or more safety devices to prevent the towed vehicle from breaking loose in the event the tow-bar fails or becomes disconnected.

49 CFR 393.70

393.70(d)(2) allows only the slack needed for proper turning.

Excess slack means the trailer travels further and gathers more speed before the chain takes up, which is when chains break.

The safety device must have no more slack than is necessary to permit the vehicles to be turned properly.

49 CFR 393.70

393.70(d)(8)(ii) specifies the centreline or within 152 mm, which is 6 inches, to the right of it.

The right-hand bias keeps a breakaway trailer swinging toward the shoulder rather than into opposing traffic. Longitudinal centreline of the vehicle.

Where two chains or cables are attached to the same point on the towing vehicle, and where a bridle or a single chain or cable is used, the point of attachment must be on the longitudinal centerline or within 152 mm (6 inches) to the right of the longitudinal centerline of the towing vehicle.

49 CFR 393.70

393.70(d)(8)(i) requires symmetry about the centreline.

Unequal attachment would pull a breakaway trailer to one side.

When two safety devices, including two safety chains or cables, are used and are attached to the towing vehicle at separate points, the points of attachment on the towing vehicle shall be located equally distant from, and on opposite sides of, the longitudinal centerline of the towing vehicle.

49 CFR 393.70

The manual color codes the emergency line red.

The service line is yellow or blue. Getting them crossed means the brakes do not behave as the pedal says, which is why the manual carries a specific caution about it.

Attaching the emergency hose on the tractor (color coded red) to the emergency coupling on the semitrailer.

TC 21-305-20 para 13-26

The service line is color coded yellow or blue, against red for the emergency line.

The service line carries the pressure that follows your pedal; the emergency line keeps the trailer supplied and its spring brakes released.

Attaching the service hose (color coded yellow or blue) to the service coupling on the semitrailer.

TC 21-305-20 para 13-26

Cross the lines and the trailer's braking no longer matches the driver's inputs: the pedal may not brake the trailer and the emergency function may not work when it is needed.

The couplings can physically be crossed, which is why the caution exists. Crossing the service and emergency lines is what causes it.

Be sure the service air hose from the tractor is connected to the service coupling, the emergency air hose, to the emergency coupling on the semitrailer.

TC 21-305-20 para 13-26

The manual states this as a warning: open at all times during normal operation, with the semitrailer brakes functional.

Leaving a shut-off valve closed after coupling means driving a combination whose trailer cannot brake. Otherwise the semitrailer brakes are not functional.

Air brake hose shut-off valves must be open at all times during the normal operation of the tractor and the semitrailer. The semitrailer brakes must be functional. Failure to do this may cause injury or death.

TC 21-305-20 para 13-26

The jaws must meet the kingpin squarely.

A crooked approach can let the fifth wheel strike the trailer's apron or ride up over the kingpin instead of capturing it.

Position the tractor so that the fifth wheel coupler jaws line up with the semitrailer kingpin.

TC 21-305-20 para 13-26

The manual requires chock blocks properly placed at the semitrailer wheels before coupling.

Backing a tractor into an unchocked trailer can push it away or let it roll.

Be sure chock blocks are properly placed at semitrailer wheels. If chock blocks are not in place, properly position them

TC 21-305-20 para 13-26

Slightly lower means the fifth wheel lifts the trailer as it slides under, which seats the kingpin properly.

Too high and the fifth wheel passes underneath; too low and it strikes the trailer's nose.

Adjust the height of the semitrailer to the height of the tractor’s fifth wheel. Use the crank handle on the semitrailer to raise or lower the landing legs so the semitrailers approach ramps are slightly lower than the tractor’s fifth wheel.

TC 21-305-20 para 13-26

The sequence is stop, neutral, parking brake.

You are about to work between the units, so the tractor must be incapable of moving.

Stop the tractor in front of the semitrailer. Place the transmission shift lever in neutral. Apply the tractor’s parking brake.

TC 21-305-20 para 13-26

The manual requires the moving parts and the fifth wheel top plate to be lubricated.

A dry fifth wheel binds in turns, which loads the coupling and can make the combination steer unpredictably.

Be sure all moving parts and the top of the fifth wheel (semitrailer coupler) are properly lubricated.

TC 21-305-20 para 13-26

The procedure opens with an inspection of the coupling devices on both units.

Everything that follows assumes the hardware is sound, so this is the step that must come first.

Check all coupling devices on the tractor and semitrailer for condition and working order.

TC 21-305-20 para 13-26

Both sides, because a trailer left standing could roll either way depending on the ground.

Chocking one side only leaves the other direction open.

Place wheel chocks in front of and behind the semitrailer wheels.

TC 21-305-20 para 13-27

The procedure has you place the pads then crank the landing gear down until it firmly contacts them.

The pads spread the load, so the legs do not sink into soft ground and leave the trailer leaning. Otherwise the gear sinks into a soft surface.

Place the landing gear float pads on the ground under the semitrailer landing gear.

Turn the crank handle until the landing gear firmly contacts the float pads.

TC 21-305-20 para 13-27

The manual notes it pops back and disengages on its own.

That is the system protecting the tractor's air when the trailer lines open, the same function the tractor protection valve performs under 393.43(a).

The trailer air supply valve inside the vehicle cab will automatically pop back and disengage when the air couplings are disconnected.

TC 21-305-20 para 13-27

Pull the plunger handle forward, then out.

The same motion appears in the coupling procedure to set the jaws open before backing under.

Release the semitrailer kingpin by pulling the plunger handle forward, and then out to open the fifth wheel coupling jaws.

TC 21-305-20 para 13-27

Position, neutral, trailer hand brake, parking brake.

Both units are immobilised before anyone goes between them. Note the hand brake is used here to hold a stationary trailer, which is one of its two permitted uses. The air brake hand control goes on before the parking brake.

Place the semitrailer in the proper location. Place the transmission selector lever in neutral. Engage the air brake hand control lever. Apply the parking brake.

TC 21-305-20 para 13-27

Para 13-23 puts one ahead and one behind, on opposite sides.

That blocks movement in both directions, which a pair on one side would not. Opposite sides so the trailer cannot roll either way.

When on level ground, place one chock block in front of the rear duals. Place the other chock block behind the rear duals on the opposite side

TC 21-305-20 para 13-23

Behind on an upgrade, in front on a downgrade.

The chock goes on the side the trailer would roll toward, which is downhill in both cases.

place chock blocks firmly behind wheels on both sides of the semitrailer when parking uphill. Place the blocks in front of the wheels on a downgrade

TC 21-305-20 para 13-24

Crossed under the lunette.

If the coupling fails the crossed chains form a cradle that catches the tongue instead of letting it drop and dig into the road. Left chain to the right shackle and right chain to the left.

Pass the left safety chain under the trailer lunette and secure it to the right lifting shackle of the towing vehicle. Pass the right safety chain under the lunette and secure it to the left lifting shackle of the towing vehicle.

TC 21-305-20 para 13-35

The hook is opened before backing.

Reversing the sequence means backing into a closed hook, which at best wastes a maneuver and at worst damages the lunette. All three in that order.

Remove the cotter pin, raise the latch, and open the pintle hook lock.

TC 21-305-20 para 13-35

Three actions, and the cotter pin is the one that gets forgotten.

Without it the latch can work open under vibration, leaving the lock free to lift. The cotter pin is the step most often skipped.

Close the pintle hook lock, close the latch, and replace the cotter pin.

TC 21-305-20 para 13-35

Raised AND secured.

A leg that is up but not latched can drop under vibration and catch on a curb or a rough crossing.

Raise the landing (support) legs and secure properly.

TC 21-305-20 para 13-35

Sound the horn first.

It is the cheapest possible warning to anyone in the area you cannot see, and the manual repeats it for backing a semitrailer too.

Sound horn before backing (if tactical situation permits).

TC 21-305-20 para 13-35

Check yourself

• What must your fifth wheel locking mechanism do?

• On a readily separable fifth wheel, when must your locking devices apply?

• How must the lower half of your fifth wheel be secured to the tractor frame?

• What additional device must your fifth wheel installation include?

• What security standard applies to the upper half of your fifth wheel?

• Why does your fifth wheel position matter?

• What two components form the connection between your tractor and your semitrailer?

• What holds the front of an uncoupled semitrailer off the ground?

• During uncoupling, what must you do with the air hose shut-off valves?

• Why are your trailer brakes engaged before testing a new coupling?

• How is a full trailer connected to the vehicle towing it?

• What is a full trailer's tow-bar attachment required to include?

• What is required of the articulation at your tow-bar connection?

• What must back up a full trailer's tow-bar?

• How much slack may your safety chain or cable have?

• Where must a single or bridled safety chain attach on your towing vehicle?

• Where must two separately attached safety chains connect?

• What color identifies the emergency air hose?

• Which colors identify the service air hose?

• Why does the manual carry a specific caution about which hose goes where?

• What position must the air brake hose shut-off valves be in during normal operation?

• What is the aim when positioning your tractor before backing under your trailer?

• How do you prepare the semitrailer wheels before coupling?

• How should trailer height be set relative to your fifth wheel before backing under?

• Before adjusting trailer height and opening the jaws, what does the manual require of your tractor?

• What lubrication check does the coupling procedure include?

• What is the first step in the coupling procedure?

• Where do wheel chocks go when uncoupling?

• The purpose of your landing gear float pads is ______.

• What is the effect on the in-cab trailer air supply valve when the air couplings are disconnected?

• How are your fifth wheel coupling jaws opened?

• What four actions begin uncoupling?

• On level ground, how are two chock blocks placed at your semitrailer's rear duals?

• Where do chock blocks go when uncoupling on an upgrade?

• How are safety chains routed on your pintle-connected trailer?

• What is the first step in connecting your pintle-hook trailer?

• What is needed to complete your pintle hook connection?

• How do you deal with the landing legs after connecting?

• What should precede backing toward your trailer?

Answers are in the practice test for this topic.