the number of shafts provided in heavy duty vehicles gear box is normally
Transmission (mechanics) Many transmissions and gears used in automotive and truck applications are contained in a cast iron case, though more frequently aluminium is used for lower weight especially in cars. There are usually three shafts: a mainshaft, a countershaft, and an idler shaft.
Semi-trailer truck The cargo trailer usually has tandem axles at the rear, each of which has dual wheels, or eight tires on the trailer, four per axle. In the US it is common to refer to the number of wheel hubs, rather than the number of tires; an axle can have either single or dual tires with no legal difference.[5][6] The combination of eight tires on the trailer and ten tires on the tractor is what led to the moniker eighteen wheeler, although this term is considered by some truckers to be a misnomer (the term "eighteen-wheeler" is a nickname for a five-axle over-the-road combination). Many trailers are equipped with movable tandem axles to allow adjusting the weight distribution.