A final drive is the last stop in the powertrain that gets a machine moving by making its tracks or wheels move.
But the path of power from the engine all the way down to the final drive isn’t that simple. Even the inner mechanisms of the final drive alone and how they affect speed and torque are a long story.
That long story is what this detailed guide covers.
In this guide, you’ll learn:
- How the final drive fits into the transmission system
- Components of a final drive
- How gear ratios of the final drive impact performance
- What types of final drives are used across different machines
Let’s get started.
Introduction to Vehicle Transmission System
Every vehicle takes its power from the engine, obviously, when it burns fuel and creates power.
However, that power doesn’t just magically show up at the wheels.
There’s an entire chain of components in between to make sure the energy from those little controlled explosions ends up pushing your vehicle forward or backward.
That system is called the drivetrain, and it includes the:
Most of us know whether our vehicle’s transmission is automatic or manual, and that it’s an expensive repair when it fails.
But that’s very superficial knowledge. If we go into technical details, a transmission is a setup of gears, clutches, and shifting components, all of which play their role in regulating how much of the engine’s power actually makes it down the line.
Now when it comes to trucks or heavy-duty equipment, there are up to eight different types of transmissions for different performance needs, each with its own way of controlling torque and speed.
Anyway, once power passes through the transmission, it travels along the drive shaft.
The drive shaft is a long metal rod spinning under your car or truck and sending that rotational energy toward the rear (or front, or all) wheels.
But that energy doesn’t jump straight to the wheels, either. That’s a component called final drive in between to enable the transmission of energy.
The final drive is exactly what it sounds like: the final stage in the power transfer chain.
It takes the rotation coming down the drive shaft and adjusts it again through gear reduction.
This reduction of RPM while boosting of torque is how your wheels get just the right balance of force and speed for proper acceleration without wrecking the engine or snapping anything important.
In setups like construction equipment or excavators, the final drive includes a planetary gear system that slows things way down to get even more torque. That extra torque is helpful when you’re moving tons of steel and dirt instead of just running errands.
Components of the Final Drive
Now that we’ve followed the power trail from the engine all the way down to the final stage, where everything hits the wheels, let’s get inside the final drive.
Hydraulic Power Motor
Inside a final drive, first up is the hydraulic power motor.
This is the back half of the final drive, where you’ll find hydraulic hoses connected.
It gets its juice from the main pump on the machine, which sends high-pressure hydraulic fluid directly into the motor. Inside, there’s a rotating group of pistons mounted on a swash plate which lets the incoming fluid spin the entire group.
This rotation engages a hardened, high-strength shaft running through the center, linking the hydraulic motor to the gearbox on the other side.
Let’s get into that gearbox.
Planetary Gear System
Next to the hydraulic motor sits the planetary gear system, which is probably the most crucial mechanical setup inside the final drive.
This configuration, also known as an epicyclic gear train, consists of a sun gear at the center, surrounded by multiple planet gears, and all of that enclosed within a large ring gear.
The sun gear brings the force in, the planet gears distribute and convert it, and the ring gear forms the housing that holds it all together.
[Image description: This planetary gear train consists of a sun gear (yellow), planet gears (blue), and carrier (green) inside a ring gear (red)]
[Alt text: Image showing a planetary gear train. Source]
In many modern final drives, you’ll find two sets of planetary gears, an inner and an outer, each made up of three evenly spaced planet gears and held together by a carrier.
[Alt text: Image showing the two layers of planetary gear.]
The shaft from the hydraulic motor turns the sun gear at a very high speed, and this speed gets passed into the planetary set. Because the planet gears rotate more slowly as they orbit the sun gear, they bring the input speed down.
And when speed decreases while power stays constant, torque increases to levels strong enough to move heavy equipment.
Final Drive Housing
After the hydraulic motor initiates rotation and the planetary gear system transforms that into usable torque, everything needs to stay in one solid piece.
And that piece is the housing.
The final drive’s housing fully encloses all the internal elements of the final drive, like the gears, bearings, and rotating shafts.
This enclosing creates a sealed environment for lubrication, which helps keep the system running smoothly and efficiently under heavy load.
But lubrication isn’t its sole purpose. The housing’s tough build also helps the final drive to bear both the internal stresses from gear operation and the external impact from jobsite conditions.
In fact, the housing on heavy equipment final drives can get so big and heavy that you’ll need cranes or forklifts just to handle it during maintenance.
Sprocket
While the sprocket isn’t technically inside the final drive unit, it’s still directly connected to it, and it’s where all that torque finally meets the track.
The sprocket is a toothed wheel that mounts to the output shaft of the final drive. Its teeth interlock with the inner links of a machine’s tracks, which is how they turn rotational force into forward (or reverse) motion.
In that sense, it acts a lot like a gear, just one that works with tracks instead of other gears.
Gear Ratios and Performance
With all that said, let’s look at how the mechanical components we just discussed work together to influence performance.
The movement of power through the gears is what we call the final drive ratio.
Remember you learned that planet gears spin at a slower rate than the sun gear? This setup naturally reduces speed, which is what helps increase rotational torque. And that’s where performance changes start to happen.
To find the final drive ratio, you divide the number of teeth on the ring gear by the number of teeth on the pinion gear (or the sun gear, depending on the layout).
For example, if your ring gear has 41 teeth and your pinion has 10, your final drive ratio is 4.10. This means the driveshaft needs to rotate 4.1 times to get the wheels (or differential) to complete one full turn.
Now, the range of typical final drive ratios depends on the vehicle type.
Passenger vehicles usually deliver a final drive ratio between 3:1 and 5:1.
Trucks, on the other hand, typically have ratios ranging from 5:1 up to 11:1, because they need more torque for hauling, towing, or navigating rough terrain.
The general trade-off of a higher ratio is that there will be more torque and better pulling power, but it’ll decrease rpm efficiency at cruising speeds and burn more fuel.
A lower ratio offers better speed and economy, but at the expense of rotational torque.
Types of Final Drives
The layout of the final drive components you saw above is the most common one found in heavy-duty equipment.
But we have other types of final drives as well, with a different configuration of gear pairs.
Let’s discuss one of its major types.
Double-Reduction Final Drive
This configuration is most commonly found in heavy-duty trucks and specialized military vehicles where extreme load handling and slow-speed operation are routine.
A double reduction uses two stages of gear engagement to bring down the speed while significantly boosting torque.
The first reduction stage is similar to what you’d find in a standard pinion-engaging-with-a-ring-gear setup.
Attached to that first ring gear is a secondary pinion, and this pinion connects to a much larger helical gear in the second stage.
That second gear is directly linked to the differential case. Together, these two stages reduce the input speed twice before the final output reaches the wheels.
This layered design delivers far more torque than a single-reduction system could provide, and it does so without needing overly large gear components.
Get Replacement Travel Reducing Drive Motors From ZHIHE
When your final drive wears out or stops delivering the torque your machine needs, a patchwork fix isn’t going to cut it. It usually requires the replacement of parts.
And when you’re buying parts from ZHIHE, low quality is already out of the window.
We manufacture high-performance travel reducers (another term for drive motors) for compact loaders, excavators, crawler cranes, and other mobile machinery.
Thanks to top-notch engineering, our units can bear extreme operating conditions while delivering stock-crushing torque outputs, from 2,800 to 110,000 Nm.
Reach out to ZHIHE today and get your machine back on track, built stronger and better than before.
