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Transfer Case General Knowledge - Transfer Case Primer

Posted in How To on May 1, 2010
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Photographers: Advance AdaptersChrysler

The theme for this issue is transfer cases. And why not? Without a transfer case, your 4x4 would be a 2x4. More than any other part of your truck, the transfer cases defines it as a four-wheel drive. What exactly is a transfer case, though? And how does it work? Glad you asked.

The entire purpose of a transfer case can be summed up by saying that it splits engine power and sends it to all four tires via the front and rear axles. Transfer cases have been around nearly as long as the automobile. It wasn't too long after people started driving horseless carriages that they got stuck. The tow truck usually looked a lot like a pair of horses wrangled by a person who mocked the horseless transportation. Not wanting to withstand mocking, things like pavement, rubber tires, and, yes, the transfer case were invented.

Just like the car itself, transfer cases have changed dramatically since their early days. We'll depart from the history lesson at this point and dive into the details of the transfer cases that you are most likely to run into in your off-road adventures.

Steve Roberts and Vic Carroll of Advance Adapters provided many of the images in this article and gave us a brief history lesson on transfer cases. In addition to making adapters to mate popular transfer cases, transmissions and engines together, Advance Adapters also makes their own gear-driven transfer case, the Atlas. And it offers the Atlas as a conventional two-speed transfer case or an extremely cool four-speed unit.

Anatomy
If you dissect a transfer case, you'll find a few common components. There's an input shaft which is spun by the transmission. That input shaft is connected to two output shafts: One that spins the front driveshaft and one that spins the rear. Many transfer cases also have a differential. Just like the differential in the axles, this allows one output to spin at a different rate than the other to avoid driveline bind on hard surfaces. More on that in a minute....

The transfer case is the center of the drivetrain in a 4x4. Mounted to the back of the transmission, it splits engine power and sends it to the front and rear axles.

You might say that the only transfer cases worth talking about are two-speed transfer cases. That means that there is either a second set of gears or a planetary gear set that produces low-range gearing when engaged. This essentially gives you the opposite effect of the gearing in your transmission, slowing the speed of the driveshafts relative to the speed of the transfer case input shaft. This is an important aspect of 4x4s, as the gear reduction multiplies the power delivered to the tires. It also slows the wheel speed. The result is more power delivered to the ground at a slower wheel speed, giving you more control.

There are a bunch of names for various types of four-wheel drive, most of which depict a slightly different differential within the transfer case. Here's the who's who and what's what in the name game.

Part-Time 4WD: This is the most common type of transfer case. It allows you to operate the vehicle in two-wheel-drive, four-wheel-drive high-range (4Hi) and four-wheel-drive low-range (4Lo). The 4Hi operation utilizes a differential. Because the front driveshaft and axle can be cut off completely from the power, these systems offer smoother operation on pavement and better fuel economy. The strongest transfer cases are generally part-time systems because they were designed for real off-road use, often in a truck application.

One of the most revered factory transfer cases is the NP205. These work horses have a cast-iron case and all-gear transfer of power. They are extremely robust and have been used in applications ranging up to 1-ton trucks.

Full-Time 4WD: This is the simplest type of transfer case. It sends power to the front and rear axles all the time. To eliminate, or at least diminish, driveline bind on hard surfaces, these use an open T-case differential in high range. Some offer a 4Hi Lock position which locks the differential.

Active 4WD: Over the last decade or so, a variety of full-time and part-time systems have been developed that use electronic or mechanical means to adjust the amount of power delivered to the front axle according to wheel slip. They have a variety of names and levels of performance, but they provide some of the benefits of a part-time system without the owner every having to switch anything.

Because active 4WD was designed for smooth operation without any input from the driver, they can be found on everything from BMWs to Dodge 3500s.

Gear Reduction
There are two different methods of gear reduction used in a transfer case. The older method uses traditional gears mounted on shafts, very similar to the way a manual transmission works. The gears may be either straight-cut or helically-cut gears. Both are extremely strong, with helically-cut gears generally being quieter.

The other way to produce gear reduction, and the method that nearly every 4x4 made today employs, is with a planetary gear set. Planetary gear sets are used in automatic transmissions, and they include a ring gear, planet carrier and sun gears. Securing the sun gear or the planet carrier produces different ratios. They key advantage of a planetary gear set is that they can be engaged at nearly any speed.

PhotosView Slideshow

Chains vs. Gears
People are pretty passionate about their preference of chain-driven and all-gear-driven transfer cases. In this case, we're talking about the method that connects the front output shaft to the rear output shaft inside the transfer case.

In the beginning, there were only gears, and they were worked well. They were noisy and passed every bit of roughness from the road back up through the drivetrain. But they functioned well and were durable under a variety of loads. The transfer cases that use all gears also happen to be ones with cast-iron cases and a few other very desirable features. The NP205 specifically, was used in medium-duty vehicles all the way up to 1-ton truck applications, earning a reputation as an extremely durable transfer case.

This is the transfer case from a Nissan Titan. Even though it's a different design, it follows the same form as most other modern transfer cases: aluminum case, chain drive and planetary gearset for low-range.

A chain drive was introduced in the early '70s as part of an overall effort to make transfer cases lighter, quieter and smoother. One of the best known chain-driven cases is the NP231 which found its way into Dodge, GM and Jeep applications. The advantages of the chain-drive have already been mentioned. The primary disadvantage is that chains can stretch and even break. Chain-drives got a black-eye early on because of the cases they were mounted in. In the pursuit of a lighter transfer case, the housings were made from aluminum. When hard parts come apart in an aluminum case, it's catastrophic.

Cases
The material that the case is made from also has some key benefits. The two primary materials used in factory transfer cases have been cast iron and cast aluminum.

Two of the most cursed changes in transfer case history are the aluminum case and chain drive. Both have improved over time to the point where most people will use them.

Cast iron offers brute strength. It's rare to break one of these cases, but not impossible. The mounting points for the gear shafts don't move as much. And wear generally is not an issue with an iron case. They will also survive most internal component failures. The primary drawback is that cast-iron cases are extremely heavy.

Aluminum was introduced to save weight, shaving 20 to 30 percent of the overall transfer-case weight. That's really the only pro. The cons are accelerated wear of the case and the possibility of destroying the case if an internal part breaks.

In addition to iron and aluminum, some GM applications used an NV246 with a magnesium housing. Magnesium is lightweight like aluminum, but has better wear characteristics. It's far more expensive, so it was not widely used.

Now that you've had a primer on transfer cases, explore the other articles in this issue to see what upgrades are available for the model under your truck.

Decoding New Venture Naming
New Venture, through its various owners, has made the majority of transfer cases used in modern Dodge, GM and Jeep vehicles. You've probably heard some of the more common model numbers, such as NV231 and NV241. There's a naming system to the New Venture transfer cases that makes deciphering the alpha numeric code easy.

In addition to the explanation in the graphic, Some models had a "D" or "HD" after the standard designation. These were heavy duty versions of the transfer case, with some improved input shafts, chains or cases for extra duty.

Sources

Advance Adapters
Paso Robles, CA 93446
800-350-2223
www.advanceadapters.com

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