Transfer Case explained

Explanation

The transfer case also known as a transfer gearbox or transfer box is a mechanical housing which contains gear ratios and is used in some four wheel drive vehicles. It is separate from but connected to the transmission.
While there are some transfer cases that are not used for off roading I would like to explain the benefits of the transfer case as it relates to off road vehicles.

There are three main reasons for a transfer gearbox:

As the torque and horsepower is sent from the transmission, the transfer case splits that power and sends some to the front wheels and some to the rear wheels (in 4WD mode) or it can just send it to the rear wheels in 2WD mode.
The transfer box can also act as a locking center differential as it can lock the entire driveshaft and allow the front and rear wheels to rotate at the same speed during low traction conditions enabling all the wheels to receive power. If the vehicle is a part time 4X4 then once 4WD mode is selected the front and rear wheels should be automatically locked.
A vehicle with a transfer case can have a high and several low range gears while it is in 4WD mode. This reason separates transfer cases from differentials.

A brief explanation of how high and low range works will be that the high range can be considered to be the default 4X4 setting. The vehicle will be in 4WD mode with the same gears and torque as normal. Low range uses gear ratios in the transfer gearbox to reduce the speed of the vehicle but increase the torque output at all wheels.
The low range ratios of various manufacturers will vary but they are usually around half of the ratios of the gears in the high range. Low range is useful for driving over rocks, steep hills and other extreme off roading.
Imagine the gears on a mountain bike, you could never achieve the fastest speed of the bike in the softest gear (smallest gearwheel & largest sprocket) but you will climb hills easy even if the hill is rough with small rocks and deep holes. The first gear of a vehicle uses the same principle and the transfer case then doubles the ability.
Traditionally the transfer case was operated manually. There are two gear levers, one for the regular manual or automatic transmission and a slightly smaller lever to the transfer box by selecting between 2WD and 4WD and then high and low range when in 4WD mode.
It is now common to have electronic control and switches to operate the transfer case. Some vehicles such as AWD sports cars can have transfer cases that cannot be controlled by the driver and is permanently in 4WD.
The gear ratios in the transfer case can be achieved by using traditional gears but the popular alternative is chains to operate the ratios. It is also common for very large off roading vehicles such as trucks to have more than one low range gear in their transfer case.

External Links

Detailed Explanations

Videos

A youtube video showing a transfer case animation

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Transfer Case - Transfer Gearbox - Low Range