Massachusetts CDL Combination flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Combination vehicle

A power unit towing one or more trailers, such as a tractor with a semi-trailer. The tractor carries the coupling device; the trailer carries the matching kingpin or lunette eye.

Tractor protection valve

A valve that closes when trailer air lines separate or pressure drops, sealing the tractor's air supply so the tractor keeps braking and the trailer emergency brakes apply.

Trailer hand valve

A control that applies only the trailer brakes. It is used to test trailer brakes and check for air leaks, never as a normal service brake while driving.

Fifth wheel jaws

The locking mechanism that grips the trailer kingpin. The jaws must close completely around the kingpin shank with no visible gap, and the release handle must be secured.

Air loss limit combination

With the engine off and full pressure, a combination vehicle may lose no more than 3 psi in one minute with brakes released, or 4 psi per minute with the brakes applied.

Tug test

After coupling, pull forward gently in low gear with the trailer brakes set. If the tractor moves out from under the trailer, the kingpin was not locked.

Trailer wheel chocks

Blocks placed against the trailer tires before coupling or uncoupling. They prevent the trailer from rolling when the tractor is removed or the air lines are disconnected.

High hook

A coupling failure in which the fifth wheel jaws close above or behind the kingpin instead of around it. The tug test and a visual jaw check catch it.

Landing gear

The trailer's support legs. Lower them to the ground and give a few extra turns to lift the tractor slightly free before pulling out from under the trailer.

Emergency line color

The emergency or supply air line is red; the service line is blue. Crossed connections can leave the trailer without working emergency brakes.

Sliding fifth wheel

An adjustable fifth wheel that shifts weight between tractor axles. Its locking pins must be fully engaged in a notch before the vehicle is driven.

Pintle hook

A hook-and-eye coupling used on heavy trailers. The lunette eye must sit fully in the hook with the latch closed and a safety pin installed.

Safety chains

Chains that catch a trailer if the pintle hook or coupler fails. Cross them under the tongue, leave enough slack to turn, and never twist them to shorten.

Glad hands

Couplers that join the tractor and trailer air lines. Check the rubber seals for cracks, press the halves together, turn the locking rings, and listen for leaks.

Trailer spring brakes

Brakes that apply automatically when the emergency line loses pressure. This is why trailer wheels must be chocked before the red line is disconnected.

Governor cut-out

The pressure at which the compressor stops pumping, about 120 to 125 psi. Pressure below this at the start of a leak test means the test is invalid.

Low-air warning

A warning that must activate before air pressure falls below 60 psi. The tractor protection valve should close between 20 and 45 psi.

Compressor build rate

With the engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds on a combination vehicle. Slower build indicates a compressor or leak problem.

Trailer ABS lamp

The trailer ABS indicator should cycle on and then off at start-up. A lamp that stays on means the trailer ABS is not working and braking may be reduced.

Slack adjuster check

With brakes released, pull each brake chamber pushrod. Travel should be about 1 to 1-3/4 inches for a type 30 chamber, with the pushrod near 90 degrees when applied.

Off-tracking

The tendency of a combination vehicle's rear wheels to follow a shorter path than the front wheels. A rig can cut a corner by about 2 feet per 10 feet of wheelbase.

Following distance rule

Allow at least one second per 10 feet of vehicle length, plus one second at speeds under 40 mph. A 60-foot rig at 55 mph needs about 7 seconds.

Bobtail braking

Driving a tractor without a trailer. The light rear axle locks easily, so brake gently, allow extra stopping distance, and expect different handling.

Downgrade gear rule

Select a gear low enough to control speed before starting down a grade, and use the same gear going down that you needed going up.

Swing-out

The outward movement of the tractor's rear and the trailer's front during a turn. It grows with wheelbase and trailer length, so allow extra room at curbs and poles.

Right turn procedure

Swing wide to the left before turning without crossing the center line, keep the tractor's rear wheels close to the curb, and turn slowly while watching both mirrors.

Backing direction rule

Steer the tractor's rear wheels toward the direction you want the trailer to go. If the trailer turns the wrong way, stop, pull forward, and correct.

Get out and look

A backing practice of stopping and walking around the vehicle when visibility or clearance is uncertain. Never back a trailer you cannot see.

Rollover cause

Rollovers come mainly from excessive speed in turns, abrupt steering, and a high center of gravity. Curve speeds posted for cars are too fast for a loaded combination.

Liquid load surge

Partially filled tankers and bulk trailers shift weight during turns and stops. Brake and steer gently and allow longer stopping distance.

Skid recovery

Release the brakes to let the wheels roll again and steer in the direction you want to go. Braking harder deepens the skid.

Brake lag distance

The delay of about 0.4 to 0.6 seconds before air brakes apply. At 55 mph that delay alone covers roughly 32 to 48 feet before braking begins.

Brake fade

Loss of braking force from overheated drums caused by continuous or repeated braking. Control speed with gears and use stab braking on long downgrades.