Hey Matt, this is what I got off the interwebs. It explains a lot better than I can. Hope it helps.
"Unique among bulldozers is the high drive design shown in this photo. Caterpillar invented this design a number of years ago introducing it first on their largest machines and later on the smaller ones. As you can see the drive sprocket is elevated about the ground line. This has the disadvantage of proving one more place for the track chain to bend and presumably wear faster. This is critical because track wear is a very major operating expense on large dozers. Cat claims to have engineered around the problem with the development of sealed and lubricated (SALT) tracks in lieu of the traditional unlubricated steel on steel track chains of traditional times.
There are a multitude of advantages to the design, however. It provides for a physical separation of the drive sprocket from the suspension. This means that the drive sprocket (the elevated unit) does not need to also support the weight of the machine. This functional separation means that the drive mechanism can concentrate on managing turning forces and the suspension can be designed for weight support.
Most importantly, however, it gets rid of the dreaded 'final drive'. As long as their have been bulldozers there have been broken final drives. In the mid part of this century Caterpillar largely overwhelmed the domestic competition (Allis Chalmers, and International Harvester) due to the reputation of the others for broken final drives. The problem is that the drive sprocket needs to be relatively small in diameter to keep the ground speed down, but the dozer needs ground clearance. To accomplish this the drive train is mounted fairly high in the machine with the cross shaft exiting the sides of the dozer about where you see the "high drive" sprocket. In a traditional dozer there would be a small gear (a pinion) driving against a large gear (a bull gear) mounted below it which would provide the final gear reduction (hence final drive) and also the physical offset sufficient to provide the ground clearance. The problem with the little gear-big gear gear train is that the entire load of the vehicle is driven by a single gear tooth. the ability of a gear train to transmit power is a function of both speed and strength. The final drive by definition moves very slowly and must therefore be very strong. Every gear tooth must be strong enough to stall the engine or spin the track at a very low speed. Imagine the pressure developed on a gear when you take a diesel engine of several hundred horsepower, and run it through a series of speed reductions until it is down to just a few RPM and then concentrate that entire force on a single gear tooth.
So what is the Solution. The engineers solution is fairly simple. Just get rid of the final drive. The planetary final drive is the better idea. It is inherently much stronger, because it has 3 or 4 (or more) planet gears circling a sun gear providing the gear reduction. This means that there are always multiple gear teeth engaged at any one time making the drive mechanism inherently stronger than the 1 tooth solution. The problem is, of course, that the planetary drive reduction does not provide a physical offset. With the high drive design this is not a problem. The planetary final gear reduction is contained within the drive sprocket (behind the massive bolt circle) and all is well."
Geo