A semi-trailer truck or tractor-trailer (colloquially known as a transfer truck, 18-wheeler, semi, Diesel, or big rig in the U.S., and Canada; as a semi in Australia; and as an articulated lorry (artic) in the UK, Ireland, and New Zealand) is an articulated truck or lorry consisting of a towing engine (tractor in the U.S., prime mover in Australia, and truck in the UK, Canada and New Zealand), and a semi-trailer (plus possible additional trailers) that carries the freight. In the UK, the term juggernaut is sometimes used for articulated lorries.
Regional Configurations
North America
In North America, semi tractors usually have 3 axles, the front, or "steer", axle having two wheels, and each of the two rear, "drive", axles having a pair of "dualies" (double) wheels on each side. Thus, the most common configuration of tractor has 10 wheels. A smaller tractor having a single drive axle (six wheeler) is often used to pull shorter trailers in tight urban environments, such as downtown areas where a 60-foot rig would be too difficult to maneuver.
The cargo trailer usually has two "tandem" axles at the rear, each of which has dual wheels, or 8 wheels on the trailer. Many trailers are equipped with moveable tandems that can be set to balance the weight of the trailer to stay within legal limits.
Although the cargos weight added to the semi's weight can equal a certain amount of gross some roads are marked with a different gross restriction so the roads are not damaged. Cargos that exceed allowed weights are usually marked with overweight load and must obtain a permit to use certain roads.
Rules governing the maximum size and weight of vehicles differ among states in the US. However, since the majority of hauling is done on the interstate system, the vast majority of trucks and trailer made in the US are built to the specifications of the Department of Transportation which governs the use of the interstate system. The D.O.T. has established vehicle limits of: 102 inches wide, 13.5 feet in height, and 80,000 lbs gross weight. [1] These limits can be exceeded as individual states have the right to issue temporary oversize and/or overweight permits.
Trailer dimensions vary greatly depending on amount and type of cargo it was designed to haul. See types of trailers under Construction below.
Although dual wheels are most common, use of a single, wider tire (known as "super singles") on each axle is becoming popular, particularly among bulk cargo carriers and other weight-sensitive operators. The advantages of this configuration are two: the lighter tire weight allows a truck to be loaded with more freight, and the single wheel covers less of the brake unit, which allows faster cooling. The biggest disadvantage is that when a tire becomes deflated or destroyed, it is not possible to drive the vehicle to a service location without risking damage to the rim, as it is with dual wheels.
The United States also allows 2-axle tractors to tow two 1-axle 28.5-foot (8.7 m) semi-trailers known officially as STAA doubles and colloquially as doubles, a set, or a set of joints on all highways that are part of the National Network. The second trailer in a set of doubles uses a converter gear, also known as a con-gear. This apparatus supports the front half of the second trailer. Individual states may further allow longer vehicles (known as "longer combination vehicles" or LCVs), and may allow them to operate on roads other than those part of the National Network.
LCV types include:
- Triples: Three 28.5-foot (8.7 m) trailers; maximum weight up to 129,000 pounds (58.5 t).
- Turnpike Doubles: Two 48-foot (14.6 m) trailers; maximum weight up to 147,000 pounds (66.7 t)
- Rocky Mountain Doubles: One 40-foot (12.2 m) trailer and one 28.5-foot (8.7 m) trailer (known as a "pup"); maximum weight up to 129,000 pounds (58.5 t)
- In Canada, a Turnpike Double is two 53-foot trailers and a Rocky Mountain Double is a 50-foot trailer with 24-foot "pup"
Regulations on LCVs vary widely from state to state. No state allows more than three trailers without a special permit. Reasons for limiting the legal trailer configurations include both safety concerns and the impracticality of designing and constructing roads that can accommodate the larger wheelbase of these vehicles and the larger minimum turning radii associated with them.
The long-haul tractors used in interstate travel are often equipped with a "sleeper" behind the driver's cab, which can be anything from a small bunk to a rather elaborate miniature apartment.
Manufacturers used in the U.S.
These are just for semi-trailer tractors, not straight, rigid, box or other heavy vehicles.
- Ford
- Freightliner
- Kenworth
- International
- Mack
- Peterbilt
- Sterling
- Volvo
- Western Star
Construction
Types of trailers
There are many types of semi-trailers in use, designed to haul a wide range of products. See Semi-trailer for more detail.
- Box
- Reefer - see Refrigerator truck
- Tanker - see Tank truck
- Dry Bulk
- Flatbed
- Lowboy
- Car hauler
Coupling and uncoupling
The cargo trailer is, by means of a nipple, called the king pin, hooked to a horseshoe-shaped quick-release coupling device called a fifth wheel at the rear of the towing engine that allows easy hook up and release. The truck trailer cannot move by itself because it only has wheels at the rear end, hence the name semi-trailer: it only carries half its own weight. The vehicle has a tendency to fold at the pivot point between the semi and the trailer when braking hard at high speeds. Such a truck accident is called a jack-knife, or jackknifing.
Braking
Semi trucks use air pressure, rather than hydraulic fluid, to actuate the brakes. This allows for ease of coupling and uncoupling of trailers from the tractor unit, as well as reducing the potential for problems common to hydraulic systems, such as leakage or "brake fade" caused when overheated brake fluid vaporizes in the hydraulic lines. (Brake fade may also occur when the lining of the braking unit becomes severely overheated. This has no connection to the fluid lines.)
The "parking brake" of the tractor unit and the "emergency brakes" of the trailer are spring brakes that require air pressure in order to be released. They are applied when air pressure is released from the system, and disengaged when air pressure is supplied. This is an emergency feature which ensures that if air pressure to either unit is lost, that unit will not lose all braking capacity and become uncontrollable.
The trailer controls are coupled to the tractor through two "glad-hand" connectors, which provide air pressure, and an electrical cable, which provides power to the lights and any specialized features of the trailer.
"Glad-hand" connectors (also known as "palm couplings,") are air hose connectors, each of which has a flat engaging face and retaining tabs. The faces are placed together, and the units are rotated so that the tabs engage each other to hold the connectors together. This arrangement provides a secure connection, but allows the couplers to break away without damaging the equipment if they are pulled, as may happen when the tractor and trailer are separated without first uncoupling the air lines. These connectors are similar in design to the ones used for a similar purpose between railroad cars.
Two air lines control the trailer unit. An "emergency" or main air supply line pressurizes the trailer's air tank and disengages the emergency brake, and a second "service" line controls the brake application.
Another braking feature of semi-trucks is the engine brake, "Retarder," or "Jacobs brake" (usually shortened to "Jake brake"). This feature controls hydraulic actuators on the exhaust valves of the truck's diesel engine. When activated, the actuators hold the valves slightly open, virtually turning the engine into a giant compressor. However, it produces a loud and distinctive exhaust note, and owing to noise concerns, some locales have prohibited or restricted the use of engine brake systems inside their jurisdictions, particularly in residential areas. The advantage to using this instead of conventional brakes is that a truck can travel down a long grade without overheating its wheel brakes.
Transmission
Because of the wide variety of loads the semi may carry, they usually have a manual transmission to allow the driver to have as much control as possible. However, all US truck manufacturers now offer fully automatic transmissions.
Since engine efficiency is a requirement and high acceleration is not, semi truck transmissions usually provide at least nine or ten gear ratios, but possibly as many as eighteen (e.g. Australian Road Trains). A large number of transmission ratios means the engine itself can operate within a narrow range of speeds. The range of speeds over which an engine is expected to perform well has implications for the design - the narrower the range, the more the engine can be optimised for that. Also having so many gears allows fine-grained control of engine braking for better control on downhills and in curves.
There is a safety requirement that an HGV not slide backwards if started up while pointing up a hill - it may have a clutch brake to prevent this.
A ten speed manual transmission is controlled via a six-slot H-Box pattern similar to that in five-speed cars - five forward and one reverse gear. Gears six to ten (and high speed reverse) are accessed by toggling a selector control - so that first gear becomes sixth, second becomes seventh, etc.
Lights
An electrical connection is made between the tractor and the trailer through a cable often referred to as a "pigtail." This cable is a bundle of wires in a single casing. Each wire controls one of the electrical circuits on the trailer, such as running lights, brake lights, turn signals, etc. A standard cable would break when the rig went around corners so it is coiled and retains these coils when not under tension. It is these coils that cause the cable to look like a pigtail.
In most countries a trailer or semi-trailer must have minimum
- 2 rear lights (red)
- 2 stop lights (red)
- 2 turning lights; one for right and one for left, flashing (yellow, orange or red)
- 2 marking lights behind if wider than certain specifications (red; 3 lights in North America)
- 2 marking lights front if wider than the truck or wider than certain specifications (white; 3 amber lights in North America)
Drivers license
A special driver's license is required to operate various commercial vehicles.
Canada
Regulations vary by province. A license to operate a vehicle with air brakes is required, normally a Class I commercial license. In Ontario, a "Z" endorsement[3] is required. Anyone holding an Ontario "Z" endorsement can drive a heavy truck with up to one towed vehicle, weighing no more than 4.6 metric tons if he or she also holds a Class A (tractor-trailer), B (school bus), C (regular bus) or D (heavy trucks other than tractor trailers) licence.[4] Anyone holding an Ontario Class A licence or equivalent can drive a tractor-trailer combination with at least 2 towed vehicles (Ibid.).
United States
Drivers of semi-trailer trucks require a Class A commercial driver's license to operate any combination vehicles with a Gross Vehicle Weight Rating (or GVWR) in excess of 26,000 pounds (11.8 t).
In addition, Endorsements are necessary for certain cargo and vehicle arrangements and types;
- H - Hazardous Materials (HazMat or HM) - necessary if materials require HM placards.
- N - Tankers - the driver is acquainted with the unique handling characteristics of liquids tankers.
- X - Signifies Hazardous Materials and Tanker endorsements, combined.
- T - Doubles & Triples - the licensee may pull more than one trailer.
Road trains
In Australia and other countries, semi-trailers with more than one trailer are known as road trains. In most areas "B-doubles" are permitted. These include a modified trailer with a turntable at the rear to allow a second trailer to be tightly coupled to the rig without the extra cost and handling problems of a dolly.
In some states in the US, long-haul semi-trailer trucks can tow another full trailer at the end, which makes the vehicle look like a two-car small train. Some of the second cars are full trailers with wheels on both ends, while others are just regular semi-trailer cars hooked to the dolly. Some states further allow a third trailer to be added to the vehicle.
Role in industry
Modern day semi-trailer trucks often operate as a part of a domestic or international transport infrastructure to support containerized cargo shipment. Various types of rail flat bed train cars are modified to hold the cargo trailer or container with wheels or without. This is called "Intermodal" or "piggy-back" or "piggyback". The system allows the cargo to switch from the highway to railway or vice versa with relative ease by using gantry cranes.
The large trailers pulled by a tractor unit come in many styles, lengths, and shapes. Some common types are: vans, reefers, flatbeds, containerlifts and tankers. These trailers may be refrigerated, heated, ventilated, or pressurized, depending on climate and cargo. Some trailers have movable wheel axles that can be adjusted by moving them on a track underneath the trailer body and securing them in place with large pins. The purpose of this is to help adjust weight distribution over the various axles, to comply with local laws.
(from Wikipedia - the free encyclopedia - subject to the GNU Free Documentation License)