What is a final drive?

The final drive motor on your mini excavator, skid steer loader, compact track loader, or tractor is truly a marvel of modern technology. To properly appreciate it and take care of it, repair it or replace it, it’s important to understand its role in your machine. Whether you have a Caterpillar, Bobcat, Yanmar, Kubota, or another brand of final drive, they all run on the same principles which we’ve detailed below.

Gears, gears, gears

When you’re working with heavy machinery, even of the compact excavator variety, there’s a lot going on. Across a broad spectrum of all types of machines, the engine powers multiple systems that increase torque at different speeds in different places, including the final drive, the transmission, the torque converter or flywheel, the hydraulic pump/motor system, and/or the differential, depending on the design and application of the machine.

All of these gearing systems are made up, more or less, of pinion gears, ring gears, and hydraulic fluid and/or gear lubricant, and they’re all connected. The torque converter, flywheel, and/or hydraulic pump transfers engine power to the transmission device. The transmission device then provides power to the differential or final drive. In a standard axle-driven design, the differential gears then change the direction of that power in order to turn axle shafts. Alternatively, an excavator final drive has a planetary gearing system that slows things down from what is supplied from the attached hydraulic motor, increasing torque, and making it possible for the machine to do more work.

The final drive’s sun and planets

A relatively small device, your final drive motor is what powers the tracks on your mini excavator. It’s essentially divided into two halves. The first half is the hydraulic travel motor – that’s the back side where the hydraulic hoses are connected. It gets its energy from the main pump on your machine. The high pressure flow supplied by the main pump spins a rotating group of pistons on a swash plate in the hydraulic travel motor, which engages a super-strong shaft that runs through to the other half of the final drive, the final drive gearbox. Because it uses a planetary gear configuration, the final drive gearbox is able to reduce the speed supplied by the shaft, which increases the available torque to rotate the hub to which the track sprocket is mounted. This engages the tracks on your compact excavator with tremendous torque, moving the machine in the direction you want it to go. How does something so small manage to do something so big?

It’s the planetary gear configuration which makes it possible. Back when you took Physics class, you probably learned that:

Torque = Power / Speed

In other words, lowering the speed increases the torque. A planetary gearing system has a ring gear with a sun gear at its center and three planetary, or pinion gears, equal distances apart from each other that rotate around the sun gear. The shaft that connects the hydraulic travel motor to the final drive gearbox turns the sun gear at a super-high speed. The planetary gears mesh with the sun gear and turn while rolling around the ring gear. They spin at a slower rate than the sun gear, and that speed reduction increases torque.

These gearing relationships and the power they provide is known as your final drive ratio. By dividing the number of teeth in your ring gear by the number of teeth in your planetary gears, you arrive at that number. Generally speaking, a higher final drive ratio will mean more torque at the tracks but a lower top speed.

How it works in the wild

It’s amazing how much power and torque these drives can produce relative to their size. Consider for example, a John Deere 35D mini excavator, equipped with a pair of final drive motors that have a total mass weight of only 170 pounds (77 kilograms). These can rapidly move that JD 35D mini excavator — which weighs 7,672 pounds (3487 kg) by the way — at speeds of up to 2.8 mph (4.5 kph). Those little final drive motors are carrying nearly 50 times (5000%!) their own weight at those speeds, and they can do it up an incline, with their bucket full and their blade digging dirt! If that’s not a marvel of human engineering, I don’t know what is. About the only thing in the natural world that comes close to this feat are worker ants, who can carry up to 50 times their own body weight.

Perhaps even more amazing is that, with proper maintenance, your final drive motor can do all that impressive work for many years. Those worker ants may only survive a few weeks! As you can imagine, because of the engineering and workmanship that goes into these final drives, they can be quite expensive to repair or replace when they reach the end of their lives. That’s where we come in. We can help you save a ton of money and get back to work fast!

In future blogs, we’ll cover topics on how to properly care for your final drive motor and maximize the life of this incredibly valuable, hard-working, and too-often under-appreciated and overlooked component of your workhorse mini excavator!

If you want to learn more about how these marvels of modern technology work, or if you need help maintaining or repairing your excavator final drive motor, give Mini Final Drives a call today!

Our family at Mini Final Drives has been providing excellent Quality, Value, and Service for over 50 years, and you can continue to count on us for a lifetime of friendly, helpful support for all your final drive and travel motor needs. Call us today at 877-483-2806 or visit our online store at www.minifinaldrives.com to find and order what you need today for Fast and Free delivery to your door.