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A converter gear on a dolly is a coupling device of
one or two axles and a fifth wheel by which a
semitrailer can be coupled to the rear of a tractor-
trailer combination forming a double bottom rig. See
Figure 7.1.
Figure 7.1
Position Converter Dolly in Front of Second
(Rear) Trailer
Release dolly brakes by opening the air tank
petcock. (Or, if the dolly has spring brakes, use the
dolly parking brake control.)
If the distance is not too great, wheel the dolly into
position by hand so it is in line with the kingpin.
Or, use the tractor and first semitrailer to pick up the
converter dolly:
Position combination as close as possible to
converter dolly.
Move dolly to rear of first semitrailer and couple it
to the trailer.
Lock pintle hook.
Secure dolly support in raised position.
Pull dolly into position as close as possible to nose
of the second semitrailer.
Lower dolly support.
Unhook dolly from first trailer.
Wheel dolly into position in front of second trailer in
line with the kingpin.
Connect Converter Dolly to Front Trailer
Back first semitrailer into position in front of dolly
tongue.
Hook dolly to front trailer.
Lock pintle hook.
Secure converter gear support in raised position.
Connect Converter Dolly to Rear Trailer
Make sure trailer brakes are locked and/or wheels
chocked.
Make sure trailer height is correct. (It must be
slightly lower than the center of the fifth wheel, so
trailer is raised slightly when dolly is pushed
under.)
Back converter dolly under rear trailer.
Raise landing gear slightly off ground to prevent
damage if trailer moves.
Test coupling by pulling against pin of the second
semitrailer.
Make visual check of coupling. (No space between
upper and lower fifth wheel. Locking jaws closed
on kingpin.)
Connect safety chains, air hoses, and light cords.
Close converter dolly air tank petcock and shut-off
valves at rear of second trailer (service and
emergency shut-offs).
Open shut-off valves at rear of first trailer (and on
dolly if so equipped).
Raise landing gear completely.
Charge trailer brakes (push "air supply" knob in),
and check for air at rear of second trailer by
opening the emergency line shut-off. If air pressure
isn't there, something is wrong and the brakes
won't work.
7.2.2 – Uncoupling Twin Trailers
Uncouple Rear Trailer
Park rig in a straight line on firm level ground.
Apply parking brakes so rig won't move.
Chock wheels of second trailer if it doesn't have
spring brakes.
Lower landing gear of second semitrailer enough
to remove some weight from dolly.
Close air shut-offs at rear of first semitrailer (and
on dolly if so equipped).
Disconnect all dolly air and electric lines and
secure them.
Release dolly brakes.
Release converter dolly fifth wheel latch.
Slowly pull tractor, first semitrailer, and dolly
forward to pull dolly out from under rear semitrailer.
Section 7 - Doubles and Triples Page 7-2
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Uncouple Converter Dolly
Lower dolly landing gear.
Disconnect safety chains.
Apply converter gear spring brakes or chock
wheels.
Release pintle hook on first semi-trailer.
Slowly pull clear of dolly.
Never unlock the pintle hook with the dolly still under
the rear trailer. The dolly tow bar may fly up,
possibly causing injury, and making it very difficult
to re-couple.
7.2.3 – Coupling and Uncoupling Triple Trailers
Couple Tractor/First Semitrailer to
Second/Third Trailers
Couple tractor to first trailer. Use the method
already described for coupling tractor-semitrailers.
Move converter dolly into position and couple first
trailer to second trailer using the method for
coupling doubles. Triples rig is now complete.
Uncouple Triple-trailer Rig
Uncouple third trailer by pulling the dolly out, then
unhitching the dolly using the method for
uncoupling doubles.
Uncouple remainder of rig as you would any
double-bottom rig using the method already
described.
7.2.4 – Coupling and Uncoupling Other
Combinations
The methods described so far apply to the more
common tractor-trailer combinations. However,
there are other ways of coupling and uncoupling the
many types of truck-trailer and tractor-trailer
combinations that are in use. There are too many to
cover in this manual. You will need to learn the
correct way to couple and uncouple the vehicle(s)
you will drive according to the manufacturer and/or
owner specifications.
7.3 – Inspecting Doubles and Triples
Use the seven-step inspection procedure described
in Section 2 to inspect your combination vehicle.
There are more things to inspect on a combination
vehicle than on a single vehicle. Many of these
items are simply more of what you would find on a
single vehicle. (For example, tires, wheels, lights,
reflectors, etc.) However, there are also some new
things to check. These are discussed below.
7.3.1 – Additional Checks
Do these checks in addition to those already listed
in Section 2, Step 5: Do a Walk-around Inspection.
Coupling System Areas
Check fifth wheel (lower).
Securely mounted to frame.
No missing or damaged parts.
Enough grease.
No visible space between upper and lower fifth
wheel.
Locking jaws around the shank, not the head of
kingpin.
Release arm properly seated and safety latch/lock
engaged.
Check fifth wheel (upper).
Glide plate securely mounted to trailer frame.
Kingpin not damaged.
Air and electric lines to trailer.
Electrical cord firmly plugged in and secured.
Air-lines properly connected to glad hands, no air
leaks, properly secured with enough slack for turns.
All lines free from damage.
Sliding fifth wheel.
Slide not damaged or parts missing.
Properly greased.
All locking pins present and locked in place.
If air powered, no air leaks.
Check that fifth wheel is not so far forward that the
tractor frame will hit landing gear, or cab will hit the
trailer, during turns.
Landing Gear
Fully raised, no missing parts, not bent or
otherwise damaged.
Crank handle in place and secured.
If power operated, no air or hydraulic leaks.
Double and Triple Trailers
Shut-off valves (at rear of trailers, in service and
emergency lines).
Rear of front trailers: OPEN.
Rear of last trailer: CLOSED.
Section 7 - Doubles and Triples Page 7-3
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Converter dolly air tank drain valve: CLOSED.
Be sure air-lines are supported and glad hands are
properly connected.
If spare tire is carried on converter gear (dolly),
make sure it's secured.
Be sure pintle-eye of dolly is in place in pintle hook
of trailer(s).
Make sure pintle hook is latched.
Safety chains should be secured to trailer(s).
Be sure light cords are firmly in sockets on trailers.
7.3.2 – Additional Things to Check during a
Walk-around Inspection
Do these checks in addition to subsection 5.3,
Inspecting Air Brake Systems.
7.4 – Doubles/Triples Air Brake Check
Check the brakes on a double or triple trailer as you
would any combination vehicle. Subsection 6.5.2
explains how to check air brakes on combination
vehicles. You must also make the following checks
on your double or triple trailers
7.4.1 – Additional Air Brake Checks
Check That Air Flows to All Trailers (Double and
Triple Trailers). Use the tractor parking brake
and/or chock the wheels to hold the vehicle. Wait for
air pressure to reach normal, then push in the red
"trailer air supply" knob. This will supply air to the
emergency (supply) lines. Use the trailer handbrake
to provide air to the service line. Go to the rear of
the rig. Open the emergency line shut-off valve at
the rear of the last trailer. You should hear air
escaping, showing the entire system is charged.
Close the emergency line valve. Open the service
line valve to check that service pressure goes
through all the trailers (this test assumes that the
trailer handbrake or the service brake pedal is on),
and then close the valve. If you do NOT hear air
escaping from both lines, check that the shut-off
valves on the trailer(s) and dolly(ies) are in the
OPEN position. You MUST have air all the way to
the back for all the brakes to work.
Test Tractor Protection Valve. Charge the trailer
air brake system. (That is, build up normal air
pressure and push the "air supply" knob in.) Shut
the engine off. Step on and off the brake pedal
several times to reduce the air pressure in the tanks.
The trailer air supply control (also called the tractor
protection valve control) should pop out (or go from
"normal" to "emergency" position) when the air
pressure falls into the pressure range specified by
the manufacturer. (Usually within the range of 20 to
45 psi.)
If the tractor protection valve doesn't work properly,
an air hose or trailer brake leak could drain all the
air from the tractor. This would cause the
emergency brakes to come on, with possible loss of
control.
Test Trailer Emergency Brakes. Charge the trailer
air brake system and check that the trailer rolls
freely. Then stop and pull out the trailer air supply
control (also called tractor protection valve control
or trailer emergency valve) or place it in the
"emergency" position. Pull gently on the trailer with
the tractor to check that the trailer emergency
brakes are on.
Test Trailer Service Brakes. Check for normal air
pressure, release the parking brakes, move the
vehicle forward slowly, and apply trailer brakes with
the hand control (trolley valve), if so equipped. You
should feel the brakes come on. This tells you the
trailer brakes are connected and working. (The
trailer brakes should be tested with the hand valve,
but controlled in normal operation with the foot
pedal, which applies air to the service brakes at all
wheels.)
Section 7
Test Your Knowledge
1. What is a converter dolly?
2. Do converter dollies have spring brakes?
3. What three methods can you use to secure a
second trailer before coupling?
4. How do you check to make sure trailer height
is correct before coupling?
5. What do you check when making a visual
check of coupling?
6. Why should you pull a dolly out from under a
trailer before you disconnect it from the trailer
in front?
7. What should you check for when inspecting
the converter dolly? The pintle hook?
8. Should the shut-off valves on the rear of the
last trailer be open or closed? On the first
trailer in a set of doubles? On the middle
trailer of a set of triples?
9. How can you test that air flows to all trailers?
10. How do you know if your converter dolly is
equipped with antilock brakes?
These questions may be on your test. If you can’t
answer them all, re-read Section 7.
Section 7 - Doubles and Triples Page 7-4
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
Section 8
TANK VEHICLES
This Section Covers
• Inspecting Tank Vehicles
• Driving Tank Vehicles
• Safe Driving Rules
This section has information needed to pass the
CDL knowledge test for driving a tank vehicle. (You
should also study Sections 2, 5, 6, and 9). A tank
endorsement is required for certain vehicles that
transport liquids or gases. The liquid or gas does not
have to be a hazardous material. A tank
endorsement is required if your vehicle needs a
Class A or B CDL and you want to haul a liquid or
liquid gas in a tank or tanks having an individual
rated capacity of more than 119 gallons and an
aggregate rated capacity of 1000 gallons or more
that is either permanently or temporarily attached to
the vehicle or the chassis. A tank endorsement is
also required for Class C vehicles when the vehicle
is used to transport hazardous materials in liquid or
gas form in the above described rated tanks.
Before loading, unloading, or driving a tanker,
inspect the vehicle. This makes sure that the vehicle
is safe to carry the liquid or gas and is safe to drive.
8.1 – Inspecting Tank Vehicles
Tank vehicles have special items that you need to
check. Tank vehicles come in many types and sizes.
You need to check the vehicle's operator manual to
make sure you know how to inspect your tank
vehicle.
8.1.1 – Leaks
On all tank vehicles, the most important item to
check for is leaks. Check under and around the
vehicle for signs of any leaking. Don't carry liquids
or gases in a leaking tank. To do so is a crime. You
will be cited and prevented from driving further. You
may also be liable for the clean-up of any spill. In
Check manhole covers and vents. Make sure the
covers have gaskets and they close correctly.
Keep the vents clear so they work correctly.
8.1.2 – Check Special Purpose Equipment
If your vehicle has any of the following equipment,
make sure it works:
Vapor recovery kits.
Grounding and bonding cables.
Emergency shut-off systems.
Built in fire extinguisher.
Never drive a tank vehicle with open valves or
manhole covers.
8.1.3 – Special Equipment
Check the emergency equipment required for your
vehicle. Find out what equipment you're required to
carry and make sure you have it (and it works).
8.2 – Driving Tank Vehicles
Hauling liquids in tanks requires special skills
because of the high center of gravity and liquid
movement. See Figure 8.1.
Figure 8.1
8.2.1 – High Center of Gravity
High center of gravity means that much of the load's
weight is carried high up off the road. This makes
the vehicle top-heavy and easy to roll over. Liquid
tankers are especially easy to roll over. Tests have
shown that tankers can turn over at the speed limits
general, check the following:
Check the tank's body or shell for dents or leaks.
Check the intake, discharge, and cut-off valves.
Make sure the valves are in the correct position
before loading, unloading, or moving the vehicle.
Check pipes, connections, and hoses for leaks,
especially around joints.
posted for curves. Take highway curves and on
ramp/off ramp curves well below the posted speeds.
8.2.2 – Danger of Surge
Liquid surge results from movement of the liquid in
partially filled tanks. This movement can have bad
effects on handling. For example, when coming to a
stop, the liquid will surge back and forth. When the
Section 8 - Tank Vehicles Page 8-1
Version: July 2017
Commercial Driver’s License Manual – 2005 CDL Testing System
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