Contractors Hot Line is part of the Catalyst Communications Network publication family.
Contractors Hot Line is part of the Catalyst Communications Network publication family.
To virtually everyone in the drivetrain industry, it’s a well-known part, but to most people outside this industry, or even on its fringe, it’s almost unheard of.
In this article, we will explain what a final drive is, why heavy equipment can’t live without it and why we should remember it … regularly.
What Is It?
In its simplest form, the final drive is the last gearset in the drivetrain that delivers power from the engine to the ground. The way this power is delivered is by converting engine speed into torque through a series of speed reductions. These are accomplished by a number of gearsets at various “work stations” along the drivetrain.
The very last, or final, one of these workstations is the final drive, that is, it is the last point of contact before the power enters the wheels or the tracks and moves them on the ground.
The reason these series of step-downs are necessary is because even a large, strong engine cannot, on its own, turn the wheels or tracks. Trying to do so is analogous to trying to cut through a grade 5 bolt with a pair of garden pruners. It just wouldn’t happen. But at 4,500 foot-pounds, a pair of bolt cutters can leverage the power of a person’s arms through very long handles and focus all that power down into a tiny set of blades, barely big enough to fit around a bolt, and slice through it with relative ease.
Similarly, gear reductions are used to convert a bulldozer’s 400-plus horsepower into enough torque to move that 100,000-pound machine, and to do so with relative ease.
The final drive is a critical component in that conversion, it being the part that actually turns the wheels or moves the tracks. However, it often gets overlooked when it comes to care and maintenance, due in part to the fact that many operators who are not mechanics don’t really know what it is, let alone where.
Where Is It Located?
While some can be standalone components, most are integrated into the axle housing of the machine. For example, they are in the hubs of the drive (rear) sprockets of an excavator, the front wheels of a skip loader and in the sprocket which gives a track loader the “hump” in its track.
How Does It Work?
The most common final drives are enclosed boxes that contain two, three or more planetary gears that are turned by a much smaller sun gear and held in place by a stationary toothed ring gear. The planetary system gets its name from its analogy to orbiting planets circling around the sun. It is also known as an epicyclic gear system.
By the time the power has reached the sun gear, it has already been stepped down considerably and at the same time the torque has exponentially increased. However, this last piece of the drivetrain puzzle gives that powertrain a tremendous amount of work-producing torque. The relatively tiny sun gear spins very rapidly as it turns the much larger planet gears, which in turn rotate the wheels or move the track. The more planet gears, the more driving power is delivered without overtaxing the drivetrain.
Signs of Failure
As with all components of all machines, final drives are subject to failure. What are some of the signs to look for? The most common indicators are heat, noise and leakage. If you throw some water on the exposed part of the final drive and it evaporates immediately, you have a problem with excessive heat. The motor itself may not be bad; it could be the result of hot hydraulic fluid. In either case, the drive should be serviced right away in order to forestall an on-the-job breakdown. Most heavy equipment is noisy.
In the case of high-end excavators which can produce up to 512 horsepower, for example, the noise may even seem excessive. But if you can hear noise even above that, don’t just chalk it up to the machine’s working hard. Get it inspected as soon as possible.
Leakage is also a common indicator of imminent failure. Leaks themselves are not necessarily an issue, but they may be a visible sign of a more serious internal problem. If discovered early on, a leak may not prevent the machine from finishing its job, but it should not go unresolved for long.
Purchasing Tips
Companies that sell aftermarket final drives will often cite gear ratio as the end-all-be-all criteria for making a good purchase. Although there is some merit in that, oftentimes that reasoning is used simply to sell their own products. Todd Resser, owner of Southwest Equipment Parts and 20-year veteran working with final drives, says there is more to purchasing the right drive than gear ratio.
“Overall speed is the critical data,” argues Todd. “You can have the exact same gear ratio and still have a machine that spins in circles due to different motor displacement.”
Installation Tips
ConEquip Construction Equipment Parts offers these six tips for performing the proper installation of a final drive.
Giving the operation due diligence, you should research each of these in more depth before undertaking the installation.
Maintenance Tips
H&R Construction Equipment makes this solid recommendation regarding final drive maintenance: Inspect it often. Give your equipment a quick once-over at least once a day so as to catch problems early on. Then follow these maintenance suggestions for optimal performance.
The underrated and understated final drive is a very elegant system that gets very little press and has one of the dirtiest jobs on earth, but without it, our monstrous machines could not accomplish the herculean tasks that they do. We need to show it some respect by giving it the same care and attention as its more well-known workmates. Whether you are putting your equipment through its paces while working or giving it some much-needed maintenance, don’t forget the final drive!
