Tractor unit - AbsoluteAstronomy.com

A tractor unit, prime mover (Australian English

Australian English

Australian English is the name given to the group of dialects spoken in Australia that form a major variety of the English language....

), road tractor, or traction unit is a heavy-duty commercial vehicle

Commercial vehicle

A commercial vehicle is a type of motor vehicle that may be used for transporting goods or passengers. The European Union defines "commercial motor vehicle" as any motorised road vehicle, which by its type of construction and equipment is designed for, and capable of transporting, whether for...

within the large goods vehicle

Large Goods Vehicle

A large goods vehicle , is the European Union term for any truck with a gross combination mass of over...

category, usually with a large displacement

Engine displacement

Engine displacement is the volume swept by all the pistons inside the cylinders of an internal combustion engine in a single movement from top dead centre to bottom dead centre . It is commonly specified in cubic centimeters , litres , or cubic inches...

diesel engine

Diesel engine

A diesel engine is an internal combustion engine that uses the heat of compression to initiate ignition to burn the fuel, which is injected into the combustion chamber...

, and several axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

s. The tractor unit serves as a method of moving 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....

s (most often semi-trailer

Semi-trailer

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...

s). Different trailers can be swapped between tractor units quickly, so the tractor unit does not stand idle whilst the load

Cargo

Cargo is goods or produce transported, generally for commercial gain, by ship, aircraft, train, van or truck. In modern times, containers are used in most intermodal long-haul cargo transport.-Marine:...

is unloaded or loaded, unlike a rigid, and is not restricted to one type of goods as trailer types can be swapped e.g. bulk tipper

Dump truck

A dump truck is a truck used for transporting loose material for construction. A typical dump truck is equipped with a hydraulically operated open-box bed hinged at the rear, the front of which can be lifted up to allow the contents to be deposited on the ground behind the truck at the site of...

to a box van

Box truck

A box truck, also known as a cube truck, cube van, bob truck, box van, or straight truck, is a truck with a cuboid-shaped cargo area.-Road vehicles:...

. The tractor-trailer combination also means that a load can be shared across many axles, yet be more maneuverable than an equivalently sized rigid truck (see 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...

s). The tractor unit couples to the trailer using some sort of mechanical lock system, usually 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...

. A tractor unit is primarily designated a freight vehicle, and is most common in the heavy goods vehicle

Large Goods Vehicle

A large goods vehicle , is the European Union term for any truck with a gross combination mass of over...

(HGV) class of vehicles, although smaller van

Van

A van is a kind of vehicle used for transporting goods or groups of people.In British English usage, it can be either specially designed or based on a saloon or sedan car, the latter type often including derivatives with open backs...

based tractor units do exist.

Axle configuration

A tractor unit can have many axle

Axle

s depending on axle load legislation

Gross axle weight rating

The gross axle weight rating is the maximum distributed weight that may be supported by an axle of a road vehicle. Typically, GAWR is followed by either the letters FR or RR, which indicate front or rear axles respectively.-Importance:...

. The most common varieties are those of 4x2, 6x2 and 6x4 types. However, some manufacturers offer 6x6, 8x6, 8x8, 10x8, and 10x10 axle configurations. A 6x4 has three axles, with two of the axles driven. 6x4 units are more common in long distance haulage in larger countries such as the USA and Australia

Australia

Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. In Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, the 4x2 and 6x2 variants are more commonplace. Those with three axles or more can have more than one steering

Steering

Steering is the term applied to the collection of components, linkages, etc. which will allow a vessel or vehicle to follow the desired course...

axle, which can also be driven. Most 6x2 units allow the undriven rear axle to be raised when lightly loaded, or running without a trailer, to save tyre

Tire

A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...

wear, and increase traction

Traction (engineering)

Traction refers to the maximum frictional force that can be produced between surfaces without slipping.The units of traction are those of force, or if expressed as a coefficient of traction a ratio.-Traction:...

. 6x6 units have three axles, all driven. 8x6 units have four axles, with the rear three usually driven and the front axle for steering. 8x8 units also have four axles, but with all of them driven. 10x8 units have five axles with the rear four usually driven and the front axle for steering. 10x10 units have five axles which are all driven. The front two axles are usually both steer axles. The axle configurations are usually based upon axle load legislation, and maximum gross vehicle weight rating

Gross vehicle weight rating

A gross vehicle weight rating is the maximum allowable total weight of a road vehicle or trailer when loaded - i.e., including the weight of the vehicle itself plus passengers, and cargo....

s (GVW). Heavier versions of tractor units, such as those used in heavy haulage and 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...

s, tend to have four or more axles, with more than two axles driven. In certain countries (such as Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

), a certain amount of weight must be spread over driven axles, which lead to heavier varieties having six-wheel drive

Six-wheel drive

Six-wheel drive, 6WD or 6x6 is a drivetrain configuration of six wheels, all of which are driven simultaneously by the vehicle's engine. Unlike four-wheel drive drivetrains, the configuration is largely confined to off-road and military vehicles, particularly heavy-duty ones...

, otherwise another tractor unit would have to be used. The heavy haulage variants of tractor-units are often turned into a ballast tractor

Ballast tractor

A ballast tractor is a heavy haulage road vehicle designed to pull or push heavy or exceptionally large loads. Unlike the tractor unit from an articulated trailer, the ballast tractor is designed or adapted to pull or push loads from a drawbar...

by fitting temporary ballast, because their chassis allows a high GVW which may go beyond the legal limit, hence requiring special permits.

Crane

A tractor unit can sometimes have a mounted crane

Crane (machine)

A crane is a type of machine, generally equipped with a hoist, wire ropes or chains, and sheaves, that can be used both to lift and lower materials and to move them horizontally. It uses one or more simple machines to create mechanical advantage and thus move loads beyond the normal capability of...

located behind the cab

Cabin (truck)

The cab of a truck is an enclosed space in a truck where the driver is seated. The word originated as a short form of cabriolet, not cabin. Some may refer to the tractor unit of a semi-trailer truck as a cab, but this is not the subject of this article...

. The tractor unit can still pull a trailer using its fifth wheel, although it will carry less payload

Cargo

, due to the extra bulk of the crane. A crane truck is able to lift its load on site, and usually comes with stabilisers like those on mobile cranes. These cranes can come in sizes from a few tonne

Tonne

The tonne, known as the metric ton in the US , often put pleonastically as "metric tonne" to avoid confusion with ton, is a metric system unit of mass equal to 1000 kilograms. The tonne is not an International System of Units unit, but is accepted for use with the SI...

s - up to special heavy duty ones that can lift in excess of 80 ton/mtr - for lifting intermodal container

Intermodal container

An intermodal container is a standardized reusable steel box used for the safe, efficient and secure storage and movement of materials and products within a global containerized intermodal freight transport system...

s and office units. Unlike a rigid truck with a crane, the tractor and crane have the advantage that they can be separated and repositioned relative to the load if necessary.

Tanker haulage

In the European Union

European Union

The European Union is an economic and political union of 27 independent member states which are located primarily in Europe. The EU traces its origins from the European Coal and Steel Community and the European Economic Community , formed by six countries in 1958...

and non-EU countries that have signed the ADR agreement

European Agreement concerning the International Carriage of Dangerous Goods by Road

The European Agreement concerning the International Carriage of Dangerous Goods by Road, commonly known as ADR , governs transnational transport of hazardous materials.Launched in Geneva on 30 September 1957 under the aegis of the United...

, tractor units which haul tankers

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...

carrying flammable liquid

Flammable liquid

Generally, a flammable liquid is a liquid that can catch fire.In the USA, there is a precise definition of flammable liquid as one with a flash point below 100 degrees Fahrenheit. Less-flammable liquids are defined as combustible liquids...

s must be specially modified to reduce the fire risk. This includes using a fully insulated electrical system (neither side earthed to the frame), relocation of exhaust components; and enclosing all wiring in conduit. Additional requirements include mandatory ABS

Anti-lock braking system

An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...

on both the tractor and the trailer, additional 6 kilogram (12 lbs.) fire extinguisher

Fire extinguisher

A fire extinguisher or extinguisher, flame entinguisher is an active fire protection device used to extinguish or control small fires, often in emergency situations...

for putting out fire in the tanker, leak containment equipment, etc.

In countries that have signed the ADR agreement, a driver is required to have a special training for hauling dangerous goods (including flammable liquids) in tanks. This is a supplement to the main ADR training certificate.

Customization

In the USA and Australia in particular, owner drivers, and even fleet operators go to great lengths to customise their vehicle in a similar fashion to a custom car

Custom car

A custom car is a passenger vehicle that has been modified in either of the following two ways. First, a custom car may be altered to improve its performance, often by altering or replacing the engine and transmission. Second, a custom car may be a personal "styling" statement, making the car look...

. This involves fitting larger exhaust system

Exhaust system

An exhaust system is usually tubing used to guide reaction exhaust gases away from a controlled combustion inside an engine or stove. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes...

s (often highly polished stainless steel

Stainless steel

In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....

or chrome plating

Chrome plating

Chrome plating, often referred to simply as chrome, is a technique of electroplating a thin layer of chromium onto a metal object. The chromed layer can be decorative, provide corrosion resistance, ease cleaning procedures, or increase surface hardness.-Process:A component to be chrome plated will...

), extra lights and custom paint, among other things. Such tractor units attract lots of attention, and can be beneficial for promoting their business, such tractors can be matched with a customised trailer. Some companies have one custom truck, or at least one tractor unit that is different, or more powerful than the rest, to act as their flagship.

Axle configuration

Crane

Tanker haulage

Customization

See also