SEMI-TRAILER

A 'semi-trailer' is a trailer without a front axle. A large proportion of its weight is supported either by a road tractor or by a detachable front axle assembly known as a dolly. A semi-trailer is normally equipped with 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''. Though most road trailers meet this definition, the term is most often applied to heavy trailers appropriate for use in such a rig.

Contents

Advantages and disadvantages

Advantages

Disadvantages

Couplings

Fifth wheel coupling

Automatic couplings

Battery semi-trailer

Types

See also

External links

Advantages and disadvantages

Advantages

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

★ 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, or behind another semi-trailer.

★ Special tractors (known as ''tugs'', ''yard dogs'' or ''yard trucks'') are often used for internal transport, for example, manoeuvring 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 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 attachment).

★ Compared with a rigid vehicle, a semi-trailer truck has a turning circle smaller than its overall length making it more maneuverable.

Disadvantages

★ Since a semi-trailer rests on top of a tractor it has a high centre of gravity which makes it less stable than a rigid vehicle.

★ Articulated vehicles are difficult to drive in snow and ice since they lose traction more easily than a straight truck.

★ They are also subject to jackknifing.

Couplings

There are two types of couplings: 'fifth wheel' 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 cable. On tractor-trailers that have different colored lines, the red and blue lines are air lines used in brake operations, and the green one is the electrical line.

Automatic couplings

Many years ago automatic couplings predominated 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 on 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.

Battery semi-trailer

A battery propulsion semi-trailer is a semi-trailer that contains the batteries that are used to power an electric truck or bus.

Types

Different types of semi-trailers are designed to haul different cargoes.
;Box
:The most common type of trailer can haul almost anything that can fit inside. Also called a van trailer.
;Reefer - see Refrigerator truck
:Box trailer with a heating/cooling unit (reefer) attached. Used for hauling produce, ice cream, etc.
;Tanker - see Tank truck
:Used for hauling liquids like gasoline, milk, etc.
;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
: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.
;Car carrying trailer
:Carries multiple cars; usually new cars from the manufacturer.

See also

★ Air brake (road vehicle)

★ Articulated vehicle

★ Brake

★ Jackknifing

★ Semi-trailer truck

★ Trailer

External links