Doubles and Triples: Coupling Devices

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

Sources: 49 CFR 393.70, TM 9-2330-203-14&P para 2-10, TM 9-2330-203-14&P para 2-14

What you need to know

393.70(d) requires safety devices on every full trailer and every converter dolly used to convert a semitrailer to a full trailer.

In a double or triple that is every connection behind the first. 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 lets a separated trailer gather speed before the chain takes up, which is when chains snap.

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)(6)(i) relaxes the usual two-chain or bridle requirement for a solid-tongue dolly with no hinge or swivel, allowing a single chain used alone.

A single device must then be in line with the centreline of the trailer tongue. A single chain may be used alone.

If the towed vehicle is a converter dolly with a solid tongue and without a hinged tow-bar or other swivel between the fifth wheel mounting and the attachment point of the tongue eye or other hitch device

49 CFR 393.70

393.70(d)(6)(ii) requires alignment with the tongue centreline.

Off-center, a single device would pull a separated dolly sideways instead of straight.

A single safety device, including a single chain or cable used alone as the safety device, must be in line with the centerline of the trailer tongue

49 CFR 393.70

393.70(d)(5) requires either two separate chains or a bridle attached at two widely spaced points.

Width is what keeps a separated trailer tracking straight rather than yawing. A single-chain bridle must attach at two points as far apart as the frame or axle allows. A bridle attaches at two points as far apart as the frame allows.

the towed vehicle must be equipped with either two safety chains or cables or with a bridle arrangement of a single chain or cable attached to its frame or axle at two points as far apart as the configuration of the frame or axle permits

49 CFR 393.70

393.70(d)(7) requires equivalence on three counts, including directional stability.

A device can be strong enough and still let the trailer yaw, which is why stability is named separately. Security of attachment is held to the same standard.

Safety devices other than safety chains or cables must provide strength, security of attachment, and directional stability equal to, or greater than, safety chains or cables installed in accordance with paragraphs (d) (5) and (6) of this section.

49 CFR 393.70

393.70(d)(8)(i) requires symmetry about the centreline, so the pull on a separated trailer is balanced.

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

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

The rightward bias steers a breakaway toward the shoulder rather than into opposing traffic.

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

Each coupling is marked and each must go to its like.

Cross them and the brakes no longer do what the pedal says: the service line follows your foot, the emergency line keeps the trailer supplied and its spring brakes released.

Ensure that towing vehicle air coupling marked SERVICE is connected to dolly air coupling marked SERVICE and that towing vehicle air coupling marked EMERGENCY is connected to dolly air coupling marked EMERGENCY.

TM 9-2330-203-14&P para 2-10

Crossed under the lunette.

If the pintle lets go the crossed chains form a cradle that catches the tongue, instead of letting it drop and dig into the road. The same crossing rule appears for pintle-connected trailers in TC 21-305-20.

Cross two safety chains (9) under lunette (7) and connect to towing vehicle eyebolts (8).

TM 9-2330-203-14&P para 2-10

Bleeding the pressure tank releases spring brakes that are holding because the dolly has no air.

Closing the draincock again matters, or the system cannot be recharged once connected. That releases spring brakes holding because the dolly has no air.

If dolly is to be moved to towing vehicle, and dolly brakes are locked, open draincock on pressure tank to release brakes. Close draincock.

TM 9-2330-203-14&P para 2-10

Counterclockwise raises it.

Note the direction reverses when lowering during uncoupling, which is the kind of detail that is easy to get backwards under time pressure.

Raise landing leg (12) by turning handcrank (10) counterclockwise until shoe (15) is off ground.

TM 9-2330-203-14&P para 2-10

Raised and locked, every time.

A leg that is up but unlatched can drop under vibration and catch on a curb or a rough crossing.

Landing leg and leg brace (13) must always be raised and locked in traveling position before dolly is towed.

TM 9-2330-203-14&P para 2-10

Nobody.

The manual carries this as a WARNING, its highest category, for both coupling and uncoupling, and states the consequence as serious injury or death. A spotter directs from where they can be seen, not from between the units.

All personnel must stand clear of towing vehicle, dolly, and trailer during uncoupling operation.

TM 9-2330-203-14&P para 2-14

Check yourself

• What must back up your tow-bar on your converter dolly?

• How much slack may your safety chain have?

• What is special about safety device requirements for a solid-tongue converter dolly?

• Where must a single safety device on a solid-tongue dolly be positioned?

• The normal safety device arrangement on a towed vehicle is ______.

• What standard must your safety device other than chains or cables meet?

• Where must two separately attached safety chains connect on your towing vehicle?

• Where may a single or bridled safety device attach on your towing vehicle?

• You are connecting the air lines between the towing vehicle and a converter dolly. What must match?

• How are your dolly's safety chains routed?

• The dolly's brakes are locked and you need to move it. What does the procedure say?

• Which way does your handcrank turn to RAISE your dolly's landing leg?

• What must be true of your landing leg and leg brace before your dolly is towed?

• Who may be between the units during coupling or uncoupling?

Answers are in the practice test for this topic.