Semi-trailer

File:Sattelschlepper.jpg

A truck pulling a 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.

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 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 easily.
• 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 coupling close on to it. The driver has to raise the trailer legs manually and couple the airbrake lines and electric light cables.

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.

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.

See also

• Air brake (road vehicle)
• Articulated vehicle
• Brake
• Jackknifing
• Semi-trailer truck
• Trailer