What drive axle pulls constant on a semi truck?

Answer 1

Most semi tractors with three axles are a 6x4 configuration, also referred to as 'twin screw'. On these, power is transmitted from the engine to a power divider, which is basically a differential. The power divider has outputs going to both drive axles, and supplies power to both drive axles constantly. Many drivers think a power divider lock engages one of the axles, but this is incorrect - power in constantly provided to both axles, and the PDL simply locks the outputs to rotate at the same speed to both axles (when unlocked, the power divider can account for differences in tire height at each axle). Some three axle tractors have only one live axle, but this is a bit rare in North America. In Europe, where 4x2 was the norm, and you could put 40,000 lbs. on the single drive axle, some have added an additional lift axle to reduce damage to roadways, but this axle is not powered.