Transfer case

TRANSFER CASE

Shortened form of "Transfer Gearcase". Also referred to as "Transfer Gearbox" or just "Transfer Box"

A Transfer Case is a part of four wheel drive drive system found in 4x4s and all wheel drive vehicles. The transfer case is connected to the transmission and also to the front and rear axles by means of driveshafts.

Functions

1. The transfer case receives power from the transmission and sends it to both the front and rear axles. This is usually done with a set of gears, though some transfer cases are chain-driven. On some vehicles, such as 4x4 trucks or vehicles intended for off-road use, this feature is controlled by the driver. The driver can put the transfer case into either "two wheel drive" or "four wheel drive" mode. This is sometimes accomplished by means of a shifter, similar to that in a manual transmission. On some vehicles this may be electronically operated by a switch instead. Some vehicles, such as all-wheel-drive sports cars, have transfer cases that are not selectable. Such a transfer case is "locked" into 4wd (awd) mode 100% of the time.

2. The transfer case may contain one or more sets of "low range gears". Low range gears are engaged with a shifter or electronic switch. On many transfer cases this shifter is the same as the one that selects 2WD or 4WD operation as described above. Low range gears slow down the vehicle and increase the torque available at the axles. Low-range gears are used during slow-speed or extreme off road maneuvers, such as rockcrawling, or when pulling a heavy load. Again, on all-wheel-drive sports cars this feature is often absent. Some very large vehicles, such as heavy equipment or military trucks, have more than one low-range gear.

Types of Transfer Cases

Those used on off-road vehicles such as trucks, truggies, rock crawling vehicles, and some military vehicles generally allow the driver to select 2 vs. 4 WD as well as high vs. low gear ranges. Those used in sports cars are usually "transparent" to the driver: there is no shifter or select lever.

There are two different types of "internal workings" found in most transfer cases. Gear-driven transfer cases use sets of gears to drive both the front and rear driveshafts. These are generally strong, heavy units that are used in large trucks. Chain-driven transfer cases use a wide chain to drive both front and rear axles. Chain driven transfer cases are generally weaker and lighter than gear-driven ones. They are used in smaller/lighter vehicles such as compact trucks, Jeeps, etc. Many off-road driving enthusiasts modify their vehicles to use gear-driven transfer cases due to the extra strength they generally provide.

Transfer cases are also classified as either "divorced" or "married". "Married" transfer cases are bolted directly to the transmission. Sometimes a "married" transfer case is an integral part of the transmission and the two components share the same housing. A "Divorced" transfer case is completely separate from the transmission. A short driveshaft travels from the transmission to the divorced transfer case. Married transfer cases are by far the most common. In fact this is so common that the term "married" is rarely used. Divorced transfer cases are used on very long wheelbase vehicles, such as commercial trucks or military trucks. Transfer cases found in cars are always of the married type.