September 15, 2017
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Caterpillar, High Drives, and Integrated Gear Reduction Systems

The drive sprocket is set high (elevated above the ground line) on many tracked machines like CTLs and bulldozers. The reasoning behind that design choice is the topic of this Shop Talk Blog post. We’ll discuss the pros and cons of this design as well as how it led to hydraulic motors with an integrated gear reduction system.

High Drive Design on Bulldozers and CTLs

If you look at the tracks on a Caterpillar D8 bulldozer you’ll notice that the drive sprocket is set high compared to older model track loaders. Most compact track loaders and multi-terrain loaders are designed the same way: the final drive and drive sprocket are set above the ground. The tracks then follow a triangular shape  instead of a elongated rectangular shape. This design is can seen in the photo of the CAT D8R bulldozer shown below, and is often referred to as a “high drive” design.


Original Final Drive Design

In much older designs of bulldozers and crawler tractors (long before the advent of CTLs and MTLs), the drive train would be mounted rather high in the machine and the cross shaft would exit about where you would see the sprockets in modern CAT bulldozers. This cross shaft had a pinion gear which would drive a larger gear, mounted below it, to achieve the needed torque multiplication.  This would mean that when the bulldozer was pushing a load, all the force necessary to achieve this would be resting on two gear teeth (one on each side). Failure of the gears was very common back in the day.

High-Drive Design

Modern Caterpillar dozers, as well as various brands of CTLs and MTLs, opt for the high-drive design.  Caterpillar actually holds the patent for the high-drive track-type vehicle design, assigned to them back in 1974.  The image you see below is taken from that patent, and shows the shape of the tracks and the placement of the sprocket.


The main drawback of the high-drive design involves the triangular shape of the tracks, which means that there is an additional bend in them. This leads to more wear and tear on the tracks, which can prove very expensive (as most tracked machine owners are all too aware). There must be some major benefit to risking the additional wear on the tracks.

When the drive is raised, it is physically separated from the suspension and mounted directly but rotatably on the frame. If it is no longer connected to the suspension, it no longer has to help support the weight of the machine. The suspension will need to be designed to provide the weight support, and the drive system (including the final drive motor (or track motor) and the drive sprocket) can focus entirely on providing the necessary torque to move the dozer or CTL and its load. This design allowed for the development of the final drive motor, a hydraulic motor with an integrated speed-reducing planetary gear system

Modern Final Drive Hydraulic Motors

The design of final drive motors as we know them today solved the problem of placing the entire load on two gear teeth. The modern speed-reducing gear system is a planetary gear design, with a ring gear, sun gear, and usually three planetary gears. This distributes the load over multiple gear teeth, places the drive motor up higher, allowing it to be separated from suspension, and multiples the torque.

High Drive Machine Design

The high drive design for tracked vehicles enabled the machines to function more reliably, facilitated greater torque multiplication, and isolated the gearing system from the elements by sealing it inside the drive motor hub.  It also makes it much easier to access and repair the drive system, including the track drive motor.

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