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.
The exact same thing as the first drive axle.
#1 axle is the steer axle, #2 axle is the front drive axle, #3 axle is the rear drive axle.
From the center of steer axle to center of rear axle on trailer. If you want just the truck it's the center of steer axle to center of rear drive axle.
There's no such thing as a one axle truck. There has to be a minimum of two axles. "Single axle" if a term referring to a truck with only a single drive axle, and doesn't count the steer axle. Same with "tandem axle", "tri axle", "quad axle", and "quint axle".
The power unit has a steer axle, and only one drive axle.
Go to a truck driving school. You're not going to learn how to drive a Class 8 truck by reading about it on this website.
It's a dump truck with four axles--the steer axle in front and three axles in back. One of them moves up and down via a control in the cab, so the tires aren't on the road if the truck's not loaded. A tri-axle dump truck carries more weight than a one-axle or two-axle truck.
The length from the middle of the steer wheels to the center point between the drive tandems (for a tandem truck) or center of the single drive axle (on a single axle truck).
Those designations actually exclude the steer axle, and only count axles behind the steer axle, so what you're talking about would actually be a truck with two axles - a steer axle, and a single drive axle.
It depends if it is a 4 wheel drive truck. A rear driveshaft is used to drive the rear axle. A front driveshaft is used to drive the front axle on a 4X4.
20,000 on the steer axle, 34,000 on the drive tandems together.
They both do. With the differentials unlocked and the truck in gear, if you raised just one of those wheels off the ground, all the torque would transfer to that wheel (which would be the path of least resistance), and the truck would be going nowhere.. that wheel would just be spinning in the air. The power divider lock doesn't engage the second axle - the second axle is already engaged. The power divider lock simple ensures that the driveshaft to the rear axle continues to rotate at the same speed at the driveshaft going to to forward drive axle.