It allows the vehicle to carry more weight.
Varies by state. In North Carolina, a tri-axle is generally good for 15 - 17 tons, depending on the tare weight of the vehicle.
No. A tri-axle has one additional lift axle; a quad axle has two lift axles.
A tri-axle has two live axles and a lift axle.. if it's located in front of the drive axles, it's known as a pusher... if it's located behind the drive axles, it's known as a tag axle. With a tandem axle setup (which is what tri-axles, quad axles, etc. are, albeit with the addition of dead axles), the driveshaft runs from the transmission output to the power divider. The power divider is a differential which transmits power evenly to both live axles.
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.
When you're loaded to the point where your weight will exceed your legal weight as a tandem truck. Honestly, you should probably have it lowered any time you're loaded, since a motor carrier officer who sees a tri axle, quad axle, etc. with a raised lift axle is likely to question it... even if you are legal, it's time wasted and another annoyance to deal with. If you mean when after you get loaded, you do it while the vehicle is not in motion - don't lower your lift axle while the vehicle is moving.
You mean third axle. There'll be some sort of switch on the vehicle somewhere.. it may be in the cab, or it could be outside of the cab.
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.
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".
Five, all told. The steer axle isn't included in the count, so a single axle with have two - a steer and a single drive axle, a tandem will have a steer axle and two drive axles, a tri-axle will be a tandem with an additional lift axle, a quad axle will have two additional lift axles, a quint will have three additional lift axles, and a centipede will have four additional lift axles. A superdump is usually a quint with an additional Strong Arm mounts to the top of the dump body which extends out to maximize the vehicle's wheelbase.
A trans axle is a transmission and differential rolled into one in a front wheel drive vehicle. The trans axle delivers power from the engine to the drive wheels.
20,000 on the steer axle, 34,000 on the drive tandems together.