Semi-trailer truck
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A semi-trailer truck or tractor-trailer (colloquially known as a transfer truck, 18-wheeler, semi, or big-rig in the U.S.; as a semi in Australia, and Canada; and as an articulated lorry (artic), or truck and trailer in the UK, Ireland, and New Zealand) is an articulated truck or lorry consisting of a towing engine (tractor in the U.S., prime mover in Australia, and truck in the UK, Canada and New Zealand), and a semi-trailer (plus possible additional trailers) that carries the freight.
In the UK, the term juggernaut is sometimes used for especially large articulated lorries.
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[edit] Regional Configurations
[edit] North America
In North America, 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 "dualies" (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.
The maximum weight for a tractor-trailer in this configuration, in the U.S., is usually 80,000 pounds (36.2 T), although some states allow up to 99,000 pounds (44.9 T).
Although dual wheels are most common, use of a single, wider tire (known as "super singles") on each axle is becoming 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, 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.
The United States also allows 2-axle tractors to tow two 1-axle 28.5-foot (8.7 m) semi-trailers known officially as STAA doubles and colloquially as doubles, a set, or a set of joints on all highways that are part of the National Network. The second trailer in a set of doubles uses a converter gear, also known as a con-gear. This apparatus supports the front half of the second trailer. Individual states may further allow longer vehicles (known as "longer combination vehicles" or LCVs), and may allow them to operate on roads other than those part of the National Network.
LCV types include:
- Triples: Three 28.5 foot (8.7 m) trailers; maximum weight up to 129,000 pounds (58.5 t).
- Turnpike Doubles: Two 48 foot (14.6 m) trailers; maximum weight up to 147,000 pounds (66.7 t)
- Rocky Mountain Doubles: One 40 foot (12.2 m) trailer and one 28.5 foot (8.7 m) trailer (known as a "pup"); maximum weight up to 129,000 pounds (58.5 t)
- In Canada, a Turnpike Double is two 53 foot trailers and a Rocky Mountain Double is a 50 foot trailer with 24 foot "pup"
Regulations on LCVs vary widely from state to state. No state allows more than three trailers without a special permit. Reasons for limiting the legal trailer configurations include both safety concerns and the impracticality of designing and constructing roads that can accommodate the larger wheelbase of these vehicles and the larger minimum turning radii associated with them.
The long-haul tractors 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.
[edit] Manufacturers used in the U.S.
These are just for semi-trailer tractors, not straight, rigid, box or other heavy vehicles.
See also: List of dump truck manufacturers.
[edit] Europe
The noticeable difference between tractor units in the U.S. and Europe is that most European models are "cab over engine" (COE or forward control), while most U.S. trucks are conventional (or normal control). In European design, the driver's cab is normally positioned above the engine. For repairs, the entire cab hinges forward to allow maintenance access. European trucks, whether small rigid or fully articulated, have a sheer face on the front. This allows greater manoeuvrability, 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 U.S. the cabin of the truck is normally not part of the measurement.
From 2006, 25.25m truck trailer combinations are to be allowed on restricted routes within Germany, following a similar (on-going) trial in The Netherlands. Like in Scandinavia, these vehicles in The Netherlands will run a 60 tonne weight limit. Two types are to be used: 1) a 26 tonne truck pulling a dolly and semi-trailer, or 2) an articulated tractor unit pulling two semi-trailers linked by a dolly. The UK government has so far decided not to have its own trial of these 60 tonne vehicles, but to keep an eye on the other countries' trials.
Using a dolly, which generally has to be equipped with lights and a license plate, rigid trucks can be used to pull semitrailers. The dolly is equipped with a fifth wheel to which the trailer is coupled. Because the dolly attaches to a pintle hitch on the truck, maneuvering a trailer hooked to a dolly is different from manuvering a fifth wheel trailer. Backing the vehicle requires same technique as backing an ordinary truck/full trailer combination, though the dolly/semi setup is probably longer, thus requiring more space for maneuvering. The tractor-semitrailer configuration is rarely used on timber trucks, since these will use the two big advantages of 1) having the weight of the load on the drive wheels, and 2) the loader crane used to lift the logs from the ground can be mounted on the rear of the truck behind the load, allowing a short (lightweight) crane to reach both ends of the vehicle without uncoupling. Also construction trucks are moer often seen in a rigid + midaxle trailer configuration instead of the tractor + semitrailer setup.
The largest trailer manufacturer in Europe is Schmitz Cargobull.
In continental Europe, most semi tractors have 2 axles, again with the front, steer, having two wheels, and rear, drive, having twin wheels on each side. Thus, the most common configuration has 6 wheels. The cargo trailer usually has three axles at the rear, with single wheels, or 6 wheels in total. The entire vehicle thus usually has 5 axles and 12 wheels in total, although the trailers can vary in number of wheels.
[edit] Sweden and Finland
In Sweden the allowed length is 24 metres (78.7 ft) for all vehicles and 25.25 metres (82.8 ft) for trucks with two trailers. In 1997 the rules were changed, following the European Economic Area rules, allowing trucks to pull two trailers with a total length of 25.25 metres (82.8 ft), assuming certain conditions were met, like ABS on all vehicles. In Finland most trucks can tow any trailer as long as total length stays within 25.25 metres (82.8 ft). The exception to this is a tractor unit pulling a semi-trailer, which can be only 16.5 metres (54.1 ft) long. The allowed gross weight in both countries is up to 60 tonnes (132,277 lb) depending on the distance between the first and last axle. In Sweden the old style tractor-trailer is still the most common, but in some areas, especially container haulage, 25.25 metre (82.8 ft) vehicles are available. In Finland most new trucks and trailers are built with 25.25 metre (82.8 ft) in mind.
[edit] UK
In the UK, to carry the maximum permitted gross weight of 44 tonnes, both tractor and semi-trailers must have 3 or more axles each [1]. No heavier vehicles are permitted on the UK road network, except for indivisible loads which would be classed as abnormal (or oversize).
Most UK articulated trucks have 10 wheels on the tractor unit in a similar configuration to US trucks (but with a cab over engine layout), while the trailer unit has 3 axles with 2 wheels each, giving a total of 16 wheels.
[edit] Australia
Australian road transport has a reputation for using very large trucks and road trains. This is reflected in the most popular configurations of trucks generally having dual drive axles and three axles on the trailers, with 4 tires on each axle. This means that Australian single semi-trailer trucks will usually have 22 wheels which is generally more than their counterparts in other countries. Long haul transport usually operates as B-doubles with two trailers (each with three axles), for a total of nine axles (including steering). True road trains only operate in remote areas, regulated by each state or territory government.
In total, the maximum length that any articulated vehicle may be (without a special permit and escort) is 53.5 metres (175.5 ft), its maximum load may be up to 164 tonnes (361,558 lb) gross and may have up to 4 trailers. However heavy restrictions apply to the areas where such a vehicle may travel in most states. 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 most areas a truck is generally limited to two trailers and a total length of 26 metres (85 ft) and in urban areas this length limit is further reduced to 19 metres (62 ft). 25 metre (82 ft), 62.5 tonne (137,788 lb) B-doubles are very common in all parts of Australia including state capitals and on major routes may outnumber single trailer configurations.
In Australia, both conventional tractor units and cabovers are common, however cabovers are most often seen on B-Doubles on the eastern seaboard where the reduction in total length allows the vehicle to pull longer trailers and thus more cargo than it would otherwise.
Super single tires are sometimes used on tri-axle trailers. The suspension is designed with travel limiting, which will hold the rim off the road for one blown or deflated tyre for each side of the trailer, so a trailer can be driven at reduced speed to a safe place for repair. Super singles are also often used on the steer axle in Australia to allow greater loading over the steer axle. The increase in loading of steer tyres requires a permit.
[edit] Construction
[edit] Coupling and uncoupling
The cargo trailer is, by means of a nipple, called the king pin, hooked to a horseshoe-shaped quick-release coupling device called a fifth wheel at the rear of the towing engine that allows easy hook up and release. The truck 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 called a jack-knife, or jackknifing.
[edit] Braking
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 brakes" of the trailer are spring brakes that require air pressure in order to be released. They are applied when air pressure is released from the system, 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 (also known as "palm couplings," and sometimes referred to as "pig tails") are air hose connectors, 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 if they are pulled, as may happen when the tractor and trailer are separated without first uncoupling the air lines. These connectors are similar in design to the ones used for a similar purpose between railroad cars.
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, or "Jacobs brake" (usually shortened to "Jake brake"). This feature controls hydraulic actuators on the exhaust valves of the truck's diesel engine. When activated, the actuators hold the valves slightly open, virtually turning the engine into a giant compressor. However, it produces a loud and distinctive exhaust note, and owing to noise concerns, some locales have prohibited or restricted the use of engine brake systems inside their jurisdictions, particularly in residential areas. The advantage to using this instead of conventional brakes is that a truck can travel down a long grade without overheating its wheel brakes.
[edit] Transmission
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. However, some manufacturers now offer automated manual transmissions as well as fully automatic transmissions.
[edit] Lights
In most countries a trailer or semi-trailer must have minimum
- 2 rear lights (red)
- 2 stop lights (red)
- 2 turning lights; one for right and one for left, flashing (yellow, orange or red)
- 2 marking lights behind if wider than certain specifications (red; 3 lights in North America)
- 2 marking lights front if wider than the truck or wider than certain specifications (white; 3 amber lights in North America)
[edit] Drivers license
A special driver's license is required to operate various commercial vehicles.
[edit] United States
Drivers of semi-trailer trucks require a Class A commercial driver's license to operate any combination vehicles with a Gross Vehicle Weight Rating (or GVWR) in excess of 26,000 pounds (11.8 t).
In addition, Endorsements are necessary for certain cargo and vehicle arrangements and types;
- H - Hazardous Materials (HazMat or HM) - necessary if materials require HM placards.
- N - Tankers - the driver is acquainted with the unique handling characteristics of liquids tankers.
- X - Signifies Hazardous Materials and Tanker endorsements, combined.
- T - Doubles & Triples - the licensee may pull more than one trailer.
[edit] Taiwan
The Road Traffic Security Rules (zh:道路交通安全規則) require a combination vehicle driver license (Traditional Chinese: 聯結車駕駛執照) to drive a combination vehicle (Traditional Chinese: 聯結車). These rules define a combination vehicle as a motor vehicle towing a heavy trailer, i.e., a trailer with a gross weight of more than 750 kilograms (1,653 lb).
[edit] European Union
A category EC driver's license is required to drive an articulated lorry.
[edit] Australia
Truck drivers in Australia require an endorsed license. These endorsements are gained through training and experience. The minimum age to hold an endorsed license is 18 years, and/or must have held open (full) driver's license for minimum 12 months. The following are the heavy vehicle license classes in Australia:
- LR- A Light Rigid 'Class LR' covers a rigid vehicle with a GVM of more than 4.5 tonnes but not more than 8 tonnes. Any towed trailer must not weigh more than 9 tonnes GVM. Also includes vehicles with a GVM up to 8 tonnes which carry more than 12 adult including the driver and vehicles in class 'C'.
- MR- A Medium Rigid 'Class MR' covers a rigid vehicle with 2 axles and a GVM of more than 8 tonnes. Any towed trailer must not weigh more than 9 tonnes GVM. Also includes vehicles in class 'LR'.
- HR- A Heavy Rigid 'Class HR' covers a rigid vehicle with 3 or more axles and a GVM of more than 8 tonnes. Any towed trailer must not weigh more than 9 tonnes GVM. Also includes articulated buses and vehicles in class 'MR'.
- HC (Heavy Combination)- A 'Class HC' Licence covers heavy combination vehicles like a prime mover towing a semi-trailer, or rigid vehicles towing a trailer with a GVM of more than 9 tonnes. Also includes vehicles in class 'HR'.
- MC (Multi Combination)- A 'Class MC' Licence covers multi-combination vehicles like Road Trains and B-Double Vehicles. Also includes vehicles in class 'HC'.
[edit] Road trains
In Australia and other countries, semi-trailers with more than one trailer are known as road trains. In most 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.
In some states 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 dolly. Some states further allow a third trailer to be added to the vehicle.
[edit] 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" or "piggyback". The system allows the cargo to switch from the highway to railway or vice versa with relative ease by using gantry cranes.
The large trailers pulled by a tractor unit come in many styles, lengths, and shapes. Some common types are: vans, reefers, flatbeds, 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.
See : dump truck.
[edit] Media
- NBC ran two popular TV series about Truck drivers in the 1970's. Movin' On (1974-1976) and B.J. and the Bear (1978-1981)
- The "trucker's movie" Smokey and the Bandit was second only to Star Wars in box office gross when released in 1977.
- Stephen King's 1986 film Maximum Overdrive featured big rigs among its homicidal villains.
- Tractor-trailers appear in the 1980s cartoon The Transformers as the Autobots' leader Optimus Prime (Convoy in Japanese version), their second-in-command Ultra Magnus, and as the Stunticons' leader Motormaster. (The latter considers himself Optimus Prime's rival for the title "King of the Road".)
- The eighteen-wheeled truck was immortalized in numerous country music songs.
- William Henkin, actor from a suberb of Chicago, had the stage name " Big Rig" when he appeared in various featurettes on Broadway
