Semi-trailer

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A tractor unit pulling a semi-trailer
A truck pulling a semi-trailer using a trailer dolly

A semi-trailer is a trailer without a front axle. A large proportion of its weight is supported by a road tractor, a detachable front axle assembly known as a dolly, or the tail of another trailer. 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 or semi, or in the UK an articulated lorry. The fifth wheel on a truck connects to a semi trailer Kingpin. Kingpins come in many guises, however the most common within the UK market is the 2.0" (50.8mm) EEC approved type. This Kingpin is fully interchangeable and, given a strict maintenance schedule, it should last the life of a trailer.

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 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).[citation needed]

Contents

Advantages and disadvantages

Advantages

A 1920 advertisement for semi-trailers

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

Disadvantages

Couplings

There are two types of couplings: fifth wheel coupling 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 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[citation needed] but are now quite rare. Automatic couplings were generally used for payloads of 12 tons or less, e.g. on the Scammell Mechanical Horse.

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.[citation needed]

Types

A curtain sider semi-trailer without truck

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

Common widths are 8 feet (2.44 m),[1] and 2.6 metres (8 ft 6.4 in).[2]

Box
The most common type of trailer. Also called a van trailer.
Standard lengths in North America are 28 ft 0 in (8.53 m), 32 ft 0 in (9.75 m), 34 ft 0 in (10.36 m), 36 ft 0 in (10.97 m), 40 ft 0 in (12.19 m), 45 ft 0 in (13.72 m), 48 ft 0 in (14.63 m) and 53 ft 0 in (16.15 m).[2]
Bus
A bus bodied trailer hitched to a tractor unit to form a trailer bus, 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.
Hopper Bottom
Usually used to haul grain, but can be used to haul other materials.
Live bottom
Has a conveyor belt on the bottom of the trailer tub that pushes the material out of the back of the trailer. The tub does not have to be raised to deposit the materials.
Livestock Trailer
Used to haul livestock such as cows, pigs, sheep, etc. Commonly have two levels to maximize capasity.
Lowboy
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 - see Refrigerator truck
Box trailer with a heating/cooling unit (reefer) attached. Used for hauling produce, ice cream, meat, flowers, 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
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 at oil wells.[3] It is shaped like a wedge, and when it is unhitched its bottom side lies flat on the ground.

See also

References

External links