A tractor unit (or a truck-tractor) is a truck for heavy and bulky load transportation on a semitrailer. An articulated truck-tractor is called a long-haul truck. A tractor unit is a vehicle which consists of the following structural components that determine its functionality and application field:
Cab. Cabins can be conventional, but cab-over tractors also exist. Conventional cabins are well-spread mainly in the USA and Australia where there are no length restrictions for long-haul trucks. In Europe cab-over tractors and tilt cabins are produced and used thereafter. The engine is located in the front part (as for conventional cabs) right under the hood, what makes the access to it, when repairing is needed, easier. As for tilt cabins, the engine is located under the cabin over the front axle. The difference betweensuch cabin constructions determines the weight distribution and the gravity center, so the impact is being laid on the vehicle composure and the response of the whole tractor unit and the articulated truck-tractor.
Depending on the purpose and the vehicle class, a cab can be of various dimensions and versions. As a rule, cabins are divided into short, daily (with a bed inside), sleeper cabs and special cabs. Tilt sleeper cabs are usually up to 2,3 meters at length (the most well-spread model Scania is 2,26 meters long), but can also reach up to 3,42 meters (like Long Cre Cab). This makes possible to equip cabins with sleeping berths and other specialty equipment. Such cabins are from 1, 5 to 2,3 meters in height. Daily cabins are a bit shorter and not that high, they are not fitted with additional options and facilities and they are from 1, 9 to 1,5 meters in size. The width of cabins is usually limited by 2, 43 meters. Generally, the length is greater than the height at least 10-15 cm less (as applied to Scania). However, the biggest cabins can be in the shape of a cube (2, 3x2, 3x2, 3 meters in size).
Engine, if it’s possible to say so, is the most important functional part of any tractor unit. Its gross weight, payload and fuel consumption depends on the engine power. Truck-tractors are equipped with diesel engines. Contemporary fuel injection technologies, as well as a cooling system and an exhaust systemprovide the opportunity to advance the efficiency and to lower fuel consumption several-fold, if to compare with the predecessors. Besides that, stricter eco-standards make mounting engines with exhaust gas post-processing system necessary. The engine capacity of tractor-trucks may vary from 9 to 16 liters, and therefore the engine power reaches about 230-750 HP (as for Volvo and Scania trucks). These engines produce the quite impressive torque of about 3550 N/m2 along with 1000 rpm. Such characteristics of engines let increase the GVW up to 100 and more tons (the gross weight may reach up to 180 tons when gear reduction is equipped). That is how functionality of tractor units is enhanced, especially during heavy loads transportation, moving uphill and off-road driving.
Gearing system transmits the engine power to the wheels and determines the working performance of the entire articulated truck. The gearing system consists of a gearbox, driving axles and a PTO. A gearbox can be manual, semi-automatic and totally automatic. Classical mechanics is what a mechanical gearbox bases on. However, alongside with automatic clutch options and programming support they start to operate like automatic gearboxes, and the clutch foot pedal may even be lacking. Dual-control gearboxes have close gear transmission ratios, what increases the number of gears up to 12-14 and enhances the off-road performance of tractor units to the greatest possible extent. Many of the gearboxes are being equipped with retarder brakes which produce the braking power on the power shaft or the gearbox. It makes braking process more gradual and rapid.
Axles and reduction units are responsible for both weight distribution and load-carrying capacity. Axles can be fitted with standard mechanical reduction units or be supplemented by wheel reduction gears that increase the GVW. For example, single axle tractor units with the engine power of 250 HP are equipped with a standard reduction gear. A wheel reduction gear is needed if the engine power reaches up to 750 HP and the GVW amounts to more than 150 tons.
A swivel connection is a mechanical hooking device located upwards from the axels which provides coupling with a semi-trailer. A swivel looks like a rounded metal plate with a hole for hooking a semi-trailer, which can be 50, 8-88, 9 meters in size. This equipment is designed to provide a high cornering ability on roads of the entire articulated vehicle.
The principal producers of tractor units are:
- Daimler AG (and its Mercedes-Benz, Freightliner Trucks, Sterling Trucks, Unimog, Western Star, Fuso and BharatBenz vehicle brands)
- Volvo Group and its Volvo, Mack, Renault, UD Nissan Diesel brands
- PACCAR and its DAF Trucks, Kenworth, Peterbilt, Leyland Trucks brands
- Volkswagen Group and its Scania, MAN and Dongfeng Motor vehicle brands
- Tata Group and its Tata Motors, Daewoo Commercial Vehicle brands
- Hyundai Kia
- Toyota Group with its Hino Motors and Isuzu brands
- Iveco Group and its Magirus, Astra, Seddon Atkinson and Yuejin vehicle brands .