Semi-trailer - AbsoluteAstronomy.com

A semi-trailer is a trailer

Trailer (vehicle)

A trailer is generally an unpowered vehicle pulled by a powered vehicle. Commonly, the term trailer refers to such vehicles used for transport of goods and materials....

without a front axle. A large proportion of its weight is supported by a road tractor

Tractor unit

A tractor unit, prime mover , road tractor, or traction unit is a heavy-duty commercial vehicle within the large goods vehicle category, usually with a large displacement diesel engine, and several axles. The tractor unit serves as a method of moving trailers...

, a detachable front axle assembly known as a dolly

Dolly (trailer)

A dolly is a small trailer that can be coupled to a truck or trailer so as to support a semi-trailer. The dolly is equipped with a fifth wheel to which the semi-trailer is coupled...

, or the tail of another trailer

B-Train

In the simplest terms a B-Train consists of two trailers linked together by a fifth wheel, and are up to 26 m long. The fifth wheel coupling is located at the rear of the lead, or first trailer and is mounted on a "tail" section commonly located immediately above the lead trailer axles...

. A semi-trailer is normally equipped with landing gear (legs which can be lowered) to support it when it is uncoupled.

A road tractor coupled to a semi-trailer is often called a semi-trailer truck

Semi-trailer truck

A semi-trailer truck, also known as a semi, tractor-trailer, or articulated truck or articulated lorry, is an articulated vehicle consisting of a towing engine , and a semi-trailer A semi-trailer truck, also known as a semi, tractor-trailer, or (in the United Kingdom and Ireland) articulated truck...

or semi, or in the UK an articulated lorry.

In Australian English, the tractor unit is usually referred to as a prime-mover; and the combination of a prime-mover and trailer is known as a semi-trailer or semi. Semi-trailers with two trailer units are B-Doubles or road trains. A B-double consists of a prime mover towing two semi-trailers, where the first semi-trailer is connected to the prime mover by a fifth wheel coupling and the second semi-trailer is connected to the first semi-trailer by a fifth wheel coupling. A road train
Road train
A road train or roadtrain is a trucking concept used in remote areas of Argentina, Australia, Mexico, the United States and Canada to move freight efficiently. The term "road train" is most often used in Australia. In the U.S. and Canada the terms "triples," "turnpike doubles" and "Rocky Mountain...

means a combination, other than a B-Double, consisting of a motor vehicle towing at least two trailers (counting as a single trailer a converter dolly supporting a semi-trailer).

Advantages

In road haulage, semi-trailers predominate over full-trailers because of their flexibility.

The trailers can be coupled and uncoupled quickly, allowing them to be shunted for loading and to be trunked between depots.
In the event of a breakdown, a tractor unit can be exchanged quickly and the load delivered to its destination without undue delay and without having to trans-ship.
It is also possible to use a dolly to tow a semi-trailer behind a rigid truck
Truck
A truck or lorry is a motor vehicle designed to transport cargo. Trucks vary greatly in size, power, and configuration, with the smallest being mechanically similar to an automobile...

, or behind another semi-trailer.
Special tractors known as Shunt truck
Shunt truck
A Shunt truck, also known as a Spotting tractor, Yard truck, Yard Dog, Yard jockey, mule, etc. is an articulated vehicle consisting of a towing engine , and a semi-trailer hitch to carry the freight...

s (also as tugs, yard dogs, switchers, jockey horses, yard trucks or just shunts) are often used for internal transport, for example, maneuvering semi-trailers at a depot or loading and unloading ferries. These tractors may lift the coupling so that the trailer legs clear the ground.
Compared with a full scale trailer, a semi-trailer attached to a tractor unit is easier to reverse, since it has only one turning point (the coupling), whereas a full trailer has two turning points (the coupling and the drawbar
Drawbar (haulage)
A drawbar is a solid coupling between a hauling vehicle and its hauled load. Drawbars are in common use with rail transport, road trailers, both large and small, industrial and recreational, and with agricultural equipment.-Agriculture:...

attachment).
Compared with a rigid vehicle, a semi-trailer truck has a turning circle smaller than its overall length making it more maneuverable.
Because of the longer overall length of the cargo bed, a semi-trailer can haul longer objects (tree trunks, pipings, beams, railway track) than a full trailer.
Given equal lengths of the composition, a semi-trailer has greater load capacity, since the drawbar adds to the overall length of the composition.
A semi-trailer has a better ratio between tare and cargo weights.
A semi-trailer leads to more weight on the driven axles of the tractor than a truck plus trailer combination, an advantage on snow or mud.

Disadvantages

Since a semi-trailer rests on top of a tractor it has a higher centre of gravity which makes it more prone to tipping than a rigid vehicle.
Articulated vehicles can be more difficult to drive in snow and ice since the trailer loses traction more easily than a rigid truck. Using the retarder and/or engine brake only worsens the situation, making the composition more prone to jackknifing
Jackknifing
Jackknifing means the folding of an articulated vehicle such that it resembles the acute angle of a folding pocket knife. If a vehicle towing a trailer skids, the trailer can push it from behind until it spins round and faces backwards. This may be caused by equipment failure, improper braking, or...

.
A tractor-trailer composition is more likely to jackknife on ice because of the of the semi-trailer weighing significantly more than the tractor .
A rigid truck can be used without a trailer, whereas a semi-tractor has no use on its own.

Couplings

There are two types of couplings: fifth wheel coupling
Fifth wheel coupling
The fifth wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. Some recreational vehicles use a fifth wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle...

and automatic. In some applications, no separable coupling is fitted, and the trailer is bolted to the tractor unit, using a bearing and rocker feet as are used under a fifth wheel skid plate.

Fifth wheel coupling

The towing vehicle has a wide coupling plate known as a fifth wheel coupling

Fifth wheel coupling

The fifth wheel coupling provides the link between a semi-trailer and the towing truck, tractor unit, leading trailer or dolly. Some recreational vehicles use a fifth wheel configuration, requiring the coupling to be installed in the bed of a pickup truck as a towing vehicle...

bolted onto its chassis on which the semi-trailer rests and pivots. As the tractor reverses under the trailer, a king-pin under the front of the trailer slides into a slot in the skidplate, and the jaws of the fifth wheel close on to it. The driver has to raise the trailer legs manually, and couple the airbrake lines and electrical cables.

Automatic couplings

Many years ago, automatic couplings predominated but are now quite rare. Automatic couplings were generally used for payloads of 12 ton

Ton

The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...

s or less, e.g. on the Scammell Mechanical Horse

Scammell Lorries

Scammell Lorries Limited was a British manufacturer of trucks, particularly specialist and military off-highway vehicles, from 1921 to 1988.-History:...

.

There is no coupling plate on the tractor. There is a turntable permanently fixed to the underside of the trailer. This locks to the chassis of the tractor. When the tractor reverses under the trailer, its legs rise and the brake and electrical connections are made automatically. Almost the entire coupling and uncoupling procedure is operated by the driver from inside the cab, except that he or she has to descend to release (or apply) the trailer parking brake.

Types

Different types of semi-trailers are designed to haul different cargoes.

Common widths are 8 feet (2.44 m), and 2.6 metre.

Box

The most common type of trailer. Also called a van trailer.

Standard lengths in North America are 28 in 0 in (8.53 m), 32 in 0 in (9.75 m), 34 in 0 in (10.36 m), 36 in 0 in (10.97 m), 40 in 0 in (12.19 m), 45 in 0 in (13.72 m), 48 in 0 in (14.63 m) and 53 in 0 in (16.15 m).

Bus

A bus is a road vehicle designed to carry passengers. Buses can have a capacity as high as 300 passengers. The most common type of bus is the single-decker bus, with larger loads carried by double-decker buses and articulated buses, and smaller loads carried by midibuses and minibuses; coaches are...

A bus bodied trailer hitched to a tractor unit to form a trailer bus

Trailer bus

A trailer bus or articulated trailer bus is a bus formed out of a bus bodied semi-trailer pulled by a conventional tractor unit in the same way as a conventional articulated semi-trailer truck....

, a simple alternative to building a rigid bus.

Car-carrying trailer

Carries multiple cars; usually new cars from the manufacturer. In the U.S., car carriers often carry used vehicles as well.

Curtain sider

A curtain sider is similar to a box trailer except that the sides are movable curtains made of reinforced fabric coated with a waterproof coating. The purpose of a curtain sider is to allow the security and weather resistance of a box trailer with the ease of loading of a flatbed.

Drop-deck trailer

A drop-deck trailer is a trailer on which the floor drops down a level once clear of the tractor unit; the most common types of drop-deck trailer are flatbeds and curtain siders.

Double decker

Double deckers or deckers are trailers with either a fixed, hinged or moveable second floor to enable them to carry more palletised goods. In general a double decker can carry 40 pallets, as opposed to 26 for a standard trailer. Double deck trailers are generally a stepped box or curtain siders, with box trailers having either a fixed or movable (floating) deck, and curtain sides having either a fixed or hinged second deck; this hinged second deck generally swings into a position down the length of the trailer, and can be divided into 2 or 3 sections to allow greater load flexibility.

Dry Bulk

Resembles a big tanker, but is used for sugar, flour, and other dry powder materials.

Flatbed

Consists of just a load floor and removable side rails and a bulkhead in front to protect the tractor in the event of a load shift. Can haul almost anything that can be stacked on and strapped down.

Lowboy

Lowboy (trailer)

A lowboy or dolly in Australia is a semi-trailer with two drops in deck height: one right after the gooseneck and one right before the wheels. This allows the deck to be extremely low compared with other trailers...

Type of flatbed in which the load floor is as close to the ground as possible. Most commonly used to haul heavy equipment, cranes, bulldozers, etc.

Reefer

Reefer (container)

A refrigerated container or reefer is an intermodal container used in intermodal freight transport that is refrigerated for the transportation of temperature sensitive cargo....

- see Refrigerator truck

Refrigerator truck

A refrigerator truck is a van or truck designed to carry perishable freight at specific temperatures. Like refrigerator cars, refrigerated trucks differ from simple insulated and ventilated vans , neither of which are fitted with cooling apparatus...

Box trailer with a heating/cooling unit (reefer) attached. Used for hauling produce, ice cream, etc.

Sidelifter

Semi-trailer with hydraulic cranes mounted at both ends of the chassis allowing for the loading and unloading of shipping containers without the need of a forklift or other container handling equipment.

Tanker - see Tank truck

Tank truck

A tank truck or road tanker is a motor vehicle designed to carry liquefied loads, dry bulk cargo or gases on roads. The largest such vehicles are similar to railroad tank cars which are also designed to carry liquefied loads...

Used for hauling liquids such as gasoline, milk, orange juice, and alcohol.

Frac

A type of tank trailer with a single and fixed axle, typically used during hydraulic fracturing

Hydraulic fracturing

Considerable controversy surrounds the current implementation of hydraulic fracturing technology in the United States. Hydraulic fracturing, or fracking, is the process of utilizing pressurized water, or some other liquid, to fracture rock layers and release petroleum, natural gas, or other...

at oil well

Oil well

An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...

s. It is shaped like a wedge, and when it is unhitched its bottom side lies flat on the ground.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Advantages

Disadvantages

Couplings

Fifth wheel coupling

Automatic couplings

Types

See also

External links