Definition for the Freight Industry presented
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A semi-trailer truck
(colloquially known as an
US, as a semi
Australia, US, and Canada,
and as an articulated
or truck and trailer
in the UK, Ireland, and
New Zealand) is an articulated
consisting of a towing engine
in the US,
in Australia, "truck"
in the UK and New Zealand),
trailer that carries
the freight. In the UK,
the term juggernaut
is sometimes used for especially
large artics. (See below
for the etymology of the
In United States, semi
tractors usually have 3
axles, the front, or "steer"
axle having two wheels,
and each of the two rear "drive"
axles having a pair of "dual"
(double) wheels on each
side. Thus, the most common
configuration of tractor
has 10 wheels. The cargo
trailer usually has two "tandem"
axles at the rear, each
of which has dual wheels,
or 8 wheels on the trailer.
Although dual wheels
are most common, use of
a single, wider tire on
each axle is becoming increasingly
popular, particularly among
bulk cargo carriers and
other weight-sensitive operators.
The advantages of this configuration
are two: The lighter weight
allows a truck to be loaded
with more weight of product,
and the single wheel covers
less of the brake unit,
which allows faster cooling.
The biggest disadvantage
is that when a tire becomes
deflated or destroyed, it
is not possible to drive
the vehicle to a service
location without risking
damage to the rim, as it
is with dual wheels.
However, the United States
also allows 2-axle tractors
to tow two 1-axle 28-foot
(8.5 m) semi-trailers known
colloquially as doubles,
a set, or a set
of joints. Some states
also allow towing up to
three 28-foot trailers known
coloquially as triples
or road trains. A
2-axle full-sized semi-trailer
pulling a 28-foot "pup"
trailer known as a Rocky
Mountain Double is also
permitted in some regions.
Very few states allow two
full-sized semi trailers
which are similar to the
Australian road trains.
Reasons for limiting the
legal trailer configurations
include both safety concerns
and the impracticality of
designing and constructing
roads that can accommodate
wheelbase of these vehicles
and the larger minimum turning
radii associated with them.
Overall lengths often
range from 50 to 70 ft. (15
to 25 m) in the US, and
most US states limit the
overall weight to 80,000
lb (36 tonne) The long-haul
towing engines used in interstate
travel are often equipped
with a "sleeper"
behind the driver's cab,
which can be anything from
a small bunk to a rather
elaborate miniature apartment.
Europe in general
In Europe, most semi tractors
have 2 axles, again with
the front, steer, having
two wheels, and rear, drive,
having a pair of double
wheels on each side. Thus,
the most common configuration
has 6 wheels. Conversely,
has three axles at the rear,
with single wheels, or 6
wheels in total. One way
or the other, the entire
vehicle thus usually has
5 axles and 12 wheels in
total, although the trailers
can vary in number of wheels.
The noticeable difference
between trucks in the US
and lorries in Europe is
the lack of a nose on European
models. While some US trucks
are built without a nose,
they are not as common.
In European design, the
driver's cab is positioned
above the engine. For repairs,
the entire cab hinges forward
to allow maintenance access.
European lorries, whether
small or fully articulated,
have a sheer face on the
front. This allows greater
manoeuvrability when steering,
as the driver need only
gauge distances behind his
seating point, and this
allows for shorter trucks
with longer trailers (with
larger freight capacity)
within the legal maximum
total length. In Europe
the entire length of the
vehicle is measured as total
length, while in US the
cabin of the truck is normally
not part of the measurement.
Sweden and Finland
In Sweden the allowed length is 24 meters
for all vehicles and 25.25 meters for trucks
with two trailers. In 1997 the rules were
changed, under pressure from the EU, allowing
trucks to pull two trailers with a total
length of 25.25 meters, assuming certain
conditions were met, like ABS on all vehicles.
In Finland most trucks can tow trailer as
long as total length stays within 25.25
meters. The exception to this is a tractor
unit pulling semi-trailer, which can be
only 16.5 meters long. The allowed gross
weight in both countries is up to 60 metric
tons depending on the distance between the
first and last axle. In Sweden the old style
tractor-trailer is still the most common
overall, but in some areas, especially container
haulage, 25.25 meter vehicles are coming
strong. In Finland most new trucks and trailers
are built with 25.25 meter in mind.
Using a dolly, which has to be equipped with lights and
a license plate, rigid trucks can be used to pull semitrailers.
The dolly is equipped with fifth wheel to which the trailer
is coupled. The dolly and trailer together act like a regular
trailer, so driving it and backing up is usually no different.
The old truck-trailer configuration is almost the only style
used on timber trucks. There are at least two big advantages
with this, one is the weight of the load on the drive wheels,
and two, that the crane used to lift the logs from the ground
can be mounted on the rear of the truck behind the load,
instead of behind the cab which would make it difficult
to reach to the end of the trailer.
Australia has a reputation of having very large trucks.
This is reflected in that the most popular configurations
of trucks generally have axles in groupings of 3 rather
than 2, with either 4 or 6 tires on each axle. This means
that Australian semi-trailers will often have 26 or even
32 wheels which is generally more than their counterparts
in other countries. In total, the maximum length that any
articulated vehicle may be is 53.5 metres, its maximum load
may be up to 115.5 tonnes gross and may have up to 4 trailers.
However heavy restrictions apply to the areas where such
a vehicle may travel in many of the more densely populated
states. In less remote areas a truck is generally limited
to two trailers to 25 metres long and in urban areas this
length limit is further reduced to 19 metres. 25 metre,
62 tonne B-doubles are very common in all parts of Australia
including state capitals and on certain roads actually outnumber
single trailer configurations, however these vehicles typically
travel at night and by law stay on main roads so are not
encountered as often by passenger vehicle drivers. In remote
areas such as the
Northern Territory great care must be taken when sharing
the road with longer articulated vehicles that often travel
during the day time, especially 4 trailer road trains.
In Australia, both semi-trailers with and without "noses"
are common, however those without noses are most often seen
on B-Doubles on the south east coast where the reduction
in total length allows the vehicle to pull longer trailers
and thus more cargo than it would otherwise.
The cargo trailer is hooked to a horseshoe-shaped coupling
device called a fifth wheel at the rear of the towing
engine that allows easy hook up and release. The trailer
cannot move by itself because it only has wheels at the
rear end, hence the name semi-trailer: it only carries half
its own weight. The vehicle has a tendency to fold at the
pivot point between the semi and the trailer when braking
hard at high speeds. Such a truck accident is appropriately
called a jack-knife, or jack-knifing.
Semi trucks use air pressure, rather than hydraulic fluid
to actuate the brakes. This allows for ease of coupling
and uncoupling of trailers from the tractor unit, as well
as reducing the potential for problems common to hydraulic
systems, such as leakage or "brake-fade" caused
when overheated brake fluid vaporizes in the hydraulic lines.
(Brake fade may also occur when the lining of the braking
unit becomes severely overheated. This has no connection
to the fluid lines.)
The "parking brake" of the tractor unit and
the "emergency brake" of the trailer are applied
when air pressure is released, and disengaged when
air pressure is supplied. This is an emergency feature which
ensures that if air pressure to either unit is lost, that
unit will not lose all braking capacity and become uncontrollable.
The trailer controls are coupled to the tractor through
two "glad-hand" connectors, which provide air
pressure, and an electrical cable, which provides power
to the lights and any specialized features of the trailer.
"Glad-hand" connectors are air couplers, each
of which has a flat engaging face and retaining tabs. The
faces are placed together, and the units are rotated so
that the tabs engage each other to hold the connectors together.
This arrangement provides a secure connection, but allows
the couplers to break away without damaging the equipment
when they are pulled, as may happen when the tractor and
trailer are separated without first uncoupling the air lines.
Two air lines control the trailer unit. An "emergency"
or main air supply line pressurizes the trailer's air tank
and disengages the emergency brake, and a second "service"
line controls the brake application.
Another braking feature of semi-trucks is the "engine
brake", colloquially known as the "Jake
brake". This feature uses the engine to slow the
vehicle, which allows trucks to travel down long grades
without overheating their wheel brakes. Due to noise concerns,
some locales have prohibited or restricted the use of engine
brake systems inside their jurisdictions
Because of the wide variety of loads the semi may carry,
they usually have a manual transmission to allow the driver
to have as much control as possible. A special driver's
license is required to operate a semi-trailer in most countries.
In Australia, semi-trailers with more than one trailer are
road trains. In certain areas "B-doubles"
are permitted. These include a modified trailer with a turntable
at the rear to allow a second trailer to be tightly coupled
to the rig without the extra cost and handling problems
of a dolly.
On some interstate highways in the US, long-haul
semi-trailer trucks can tow another full trailer at the
end, which makes the vehicle look like a two-car small train.
Some of the second cars are full trailers with wheels on
both ends, while others are just regular semi-trailer cars
hooked to the standard coupling device on another set of
wheels in tow (sometimes referred to as a "dolly").
Some states further allow a third trailer to be added to
the vehicle, against the objections of some car drivers
who must share the highways with these longer trucks.
Role in Industry
Modern day semi-trailer trucks often operate as a part
of an international
transport infrastructure to support
containerized cargo shipment. Some flat bed train cars
are modified to hold the cargo trailer with wheels and all.
This is called "piggy-back"
in North America. The system allows the cargo to switch
from the highway to railway or vice versa with ease.
The large trailers pulled by a semi come in many styles,
lengths, and shapes. Some common types are: vans, reefers,
containerlifts and tankers. These trailers may be refrigerated,
heated, ventilated, or pressurized, depending on climate
and cargo. Some trailers have movable wheel axles that can
be adjusted by moving them on a track underneath the trailer
body and securing them in place with large
pins. The purpose of this is to help adjust weight distribution
over the various axles, to comply with local laws.
Semi-trailer is the name of the kind of trailer that
a prime mover pulls, but in general use refers to the whole
assembly. "Semi" refers to the fact that when
attached to the tractor unit the cargo is half supported
by the rear wheels of the tractor.
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