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How it Works: A Final Drive Unit

April 26, 2023

A backhoe digger up dirt on a construction site.

A final drive unit is the largest part of a transmission system in a vehicle. Because of this, a final drive unit can naturally be a complicated unit to understand. However, it’s important to understand a final drive, especially if you work in construction and rely on it to be working properly for heavy-duty equipment, like excavators. There are plenty of problems that you may come across with your final drive. However, having greater knowledge of how it works in not just construction equipment, but also in other vehicles, can help you find a far more efficient solution to any breakdown problems. Understand how a final drive unit works ahead.

Function in an Excavator

The final drive is a component of an excavator’s power transmission system and is located between in the rear of the machine’s undercarriage opposite of the blade. A final drive’s main function is to reduce the speed of the propeller shaft by decreasing RPMs and increasing the torque. This all begins when the final drive engine’s transmitted torque is split up with the two driving tracks and can turn the final drive at a 90-degree angle. This action moves the torque from the final drive’s propeller shaft to the driving sprockets.

Types Across All Applications

Spiral Bevel Gear

There are various final drives gears that are the key to its function. The first one that we will be looking at is the spiral bevel gears. A spiral bevel gear can typically be found in trucks and older vehicles. A spiral bevel gear consists of gear teeth and has a pinion and a ring gear line along with it. These parts are all typically located on the same center line and require a lot of heavy grade lubricants to run properly.

The Hypoid Gear

Newer trucks and rear-wheel drive vehicles will usually have a hypoid gear final drive, which is slightly different from the spiral bevel gear that was previously mentioned. More vehicles today use the hypoid gear instead of the spiral bevel gear because they can improve the gear-meshing action. Another major benefit of the hypoid gear is that it can improve a gear life and reduce the amount of noise it makes.

Double-Reduction Final Drive

Heavy-duty trucks that you will find in vehicles used for the military will typically utilize a double-reduction final drive. A double-reduction drive doesn’t require a large ring gear to get a gear reduction. Instead, a second pinion is placed on the primary ring gear shaft. The other gear reduction happens when the second pinion is roughly a part of the primary ring gear, which can now handle a large helical gear that is on a differential case.


In addition to final drives, it’s also important to know about differentials. With many vehicles, the rear wheels will not turn at the same speed. This occurs because the rear wheels may have a different diameter, so they will have to rotate at different speeds. The purpose of a differential is to prevent a situation when tires would slide, make unusual sounds, or wear down as the steering wheel is turned when there is a connection with the axles and the differential case.

When operating a vehicle, there are a few different ways it will work. If you are driving straight, the differential case, the pinion gears, the ring gear, and the differential slide gears will all turn together. However, the two pinion gears will now rotate on the pinion shaft. This allows for a vehicle’s rear wheels to turn at the same speed because the side gears will now turn at the same speed as the ring gear.

Another important function of vehicles is that they can turn. For this to happen, the pinion gears will rotate on the pinion shaft because the parts need to work around the slower turning differential side gear. Because of this, the pinion gears have extra rotary motion compared to the wheel on the outside that is turning faster as the entire vehicle turns.

There are also instances when one wheel may have less traction if a wheel comes across a slippery surface, such as ice. This becomes a problem because that wheel cannot deliver the torque it needs to. A way to give your vehicle more traction is by having a limited slip differential. This is something many vehicles take advantage of and it can prevent this issue. A slip differential is capable of doing this because it can send maximum torque to the wheel and drive your vehicle more traction.

Differential Lubricant Service

To keep your differential running, it’s important to get the used oil drained and replaced with new oil that meets equipment standards. Oils will naturally wear down the fluids over time due to high temperatures. So, it’s very important to refer to your manufacturer’s recommendations on when you should be getting this changed.

Potential Problems

With so many moving parts, there must be enough clearance and bearings put into place to ensure that everything runs properly. When parts are too close together, this can result in loud noises and can wear down your final drive. This issue is prevented with the ability for the gears in the final drive unit to be reset. The bevel gears tend to move away from each other, which is why they need to be restrained. Another issue your final drive may have is that there may be problems with the oil sealing, which can lead to leakage. A way to avoid this is to replace the seals and O-rings if the machinery is being overhauled.

There are a lot of moving parts that make a final drive unit work properly. It’s also the key to a vehicle’s transmission system and can create a lot of problems if you do start to experience problems with the piece of equipment. Now that you have a better understanding ofhow a final drive unit works, you can now get the right parts for any final drive you have, whether John Deere, Kobelco, Yanmar, or any others and make the necessary repairs that you need. By browsing the large selection at Mini Final Drives, you can exactly what your ride needs, all with great quality guaranteed.

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  • Manufacturers' trademarked names, models, part numbers, symbols, and related descriptions are provided for reference only to help you with finding the right aftermarket replacement final drive for your machine. This reference information does not imply that any items for sale are the product of those referenced manufacturers and does not imply that L&M HydraComm LLC is an authorized source nor agent of the referenced machine manufacturers.