Types of Trucks

Straight Truck - a truck in which all axles are attached to a single frame.

Articulated Truck - this kind of a truck consists of two or more separate frames connected by suitable couplings.

Truck Tractor - this is designed primarily for drawing truck trailers and is constructed to carry part of the weight and load of a semitrailer.

Semitrailer - a semitrailer is a truck trailer equipped with one or more axles, so constructed that the end of a substantial part of its own weight and that of its load rests upon a truck tractor.

Fulltrailer - it is equiupped with one or more axles and is so constructed that all of its own weight and that of its load rest upon its own wheels.

Fifthwheel - it is a device used to connect a truck tractor to a semitrailer. It also permits articulation between the units. It generally comprises of a lower half, consisting of a trunnion (pivot assembly) plate and latching mechanism. This latching mechanism is mounted on the truck tractor for connection with a kingpin that is again mounted on the semitrailer.

A semitrailer can be converted to a full trailer by

• A trailer-converted dolly
  
• An auxillary axle assembly equipped with the lower half of a fifth wheel
  
• A drawbar
  
• Other special parts

Axle assemblies - in heavy trucks, they can comprise of two or more axles, one out of which may be powered. They are usually so spaced that the distance between axle centers is not more than one and one- half times the overall diameter of the wheel and tire. In case of the axles being separated by a larger distance, the assembly is called a spread tandem.

Gross Vehicular Weight (GVW) - this is one of the methods by which trucks are categorised.

Light trucks - these have GVW ratings of up to 14,000 pounds (6.35 metric tonnes). 92% of all trucks sold belong to this category.

Medium trucks - their GVW ratings vary from 14,000 to 33,000 pounds and usually have straight designs. 4% of trucks belong to this category.

Heavy-duty trucks - these have GVW ratings of more than 33,000 pounds (15 metric tons) and are cross-country tractor-trailer combinations and off-road construction or mining trucks. The remaining 4% of all trucks come in this category.

Frames - it is made of two channel sections of alloy steel with a standardized width. Semitrailers and tank trailers in certain cases now employ the integral or unitized type of construction.

Suspensions - individual front suspension was introduced in 1960. Its most common form is a drop-forged, one section front axle attached to the frame through leaf springs and shock absorbers.

Axle - trucks with a single powered axle generally have their axle attached to the frame by leaf springs. The axle is always fully floating and the weight is carried by the axle housing. This ensures that the load continues to be supported by the axle housing even if a driving axle breaks. An axle might be of a single reduction type, meaning that it has one-gear reduction, or double reduction.

In the case of a two-speed axle, the gear change is built into the axle. This makes it possible to have two speeds for each transmission speed.

The tandem-drive axle has two powered axles. There is a possibility of there being two drive shafts, one to each axle. A system of levers is inbuilt between the two axles to equalize the load.

Other types of powered axle include the Hotchkiss type, in which all the driving and braking thrust is taken by the leaf springs and the torque-arm type, in which all the thrusts are taken by the rods. Maintenance is considerably reduced in this kind of an axle.

Steering - the Ackermann system is employed in the steering of trucks. It provides for a kingpin for each front wheel, the maximum cramp angle for which is around 35 degrees. The minimum turning radius depends on the wheelbase.

Engines and fuels
The diesel engine has gained popularity after the IInd World War for trucks that a reused for long distances. Other designs have also been made but have not been commercially successful. In 1952 an engine using liquid-propane gas was introduced. The diesel engine has the disadvantage of high initial cost while the propane-burning engine gived a much lower output for a given engine displacement.

Tyres
Till World War I, large trucks were run on hard rubber tyres. In the 1930s, cotton took the place of synthetics in the carcass and steel wire. Fibreglass plies were substituted later. The first few years have seen the use of a single wide tire to carry the same load as a dual wheel. Recapped tyres extend the life of the carcass by about three times and are therefore used by many trucking firms. Tubeless tires became available in large truck sizes since 1955.

Transmission
Trucks have been using automatic transmissions since the 1950s. It is imperative for some diesels to operate the engine within a relatively narrow speed range. These types of trucks need a large number of forward speeds. One recent diesel-powered unit has 16, obtained two by combining transmissions, each with four forward speeds.

Brakes
It was in 1918 that the first brakes were introduced. In 1925, four-wheel brakes were introduced on trucks, the internally expanding type was introduced by 1930 and the vacuum booster, or hydraulic brake in the late 1930s.

An electric brake system employs a floating armature that contract a rotating disk on the wheel current is applied and through a cam arrangement applies the shoes to the drums.

Some vehicles use air-over-hydraulic brakes.
Air is the most widely used of all these methods of application. The engine-driven compressor supplies air at a nominal pressure, regulated by an air governor. Air pressure is indicated by a pressure gauge and a low-pressure warning device, either audible or visual. Air is stored in the reservoirs and supplied to the brake valves; a foot valve supplies air to all brake chambers on the vehicle, including those being towed.

Another brake valve is hand-controlled and applies the brakes on the towed vehicle only. Both the foot pedal and hand valve supply air to the same service line, which extends back to the towed vehicles. The second, or emergency, line carries full air pressure when the vehicle is in operation. If this line is broken, the emergency brakes are applied on all towed vehicles from air reservoirs located on the towed vehicles. After reaching the brake chamber from the brake valves, the air acts on a diaphragm connected to a push rod, which in turn actuates a cam that moves the shoes against the brake drum. The cam was replaced by a wedge in the 1970s.

Safety has been the buzzword in the last few years. Much attention has been paid to the problem of brake failure arising due to loss of air pressure. If a failure in the service line occurs, the emergency system mentioned earlier can be utilized. Alternately, it can be remedied by the use of the tractor protection valve and control valve. The driver can therefore apply all brakes on the towed vehicles.

A parking brake is another safety device that can be as an emergency system in case of loss of air. Known as a DD3 Actuator, the driver has the option of using the axle on which the DD3 actuators are mounted as a parking system or as an emergency braking system by operating a push-pull button. A mechanical lock holds the brake on once this system is actuated, even if all air pressure is lost.

The spring brake is another emergency system. As long as normal air pressure is available, the spring brake actuator is inactive and the normal brake chambers apply the brakes. When air has been lost, springs are used to apply the brakes. The emergency system can be put to action if the driver wants to by means of a push-pull valve.

Air-cooled disc-type brakes were introduced in the 1990s on heavy-duty trucks. These units were combined with electronically controlled antilock air brake systems in order to reduce the possibility of jackknifing.