Editing Transfer case (section)

==Functions==
The transfer gearbox (a secondary transmission system) receives power from the [[transmission (mechanical device)|transmission]] and sends it to both the front and rear axles, or just one (usually the rear.)  This can be done with gears, hydraulics, or chain drive.  On some vehicles, such as four-wheel-drive 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 was traditionally accomplished by means of a [[Gear_stick#Secondary_gear_levers|shifter]], similar to that in a [[manual transmission]].  Increasingly it is electronically operated by a dashboard, center console, or shift lever switch.  

A transfer case that allows alternating between 2-wheel drive and 4-wheel drive modes but lacks a center differential for coordinating axle speeds is known as "[[Four-wheel drive#Part-time|part-time]]". Some vehicles, such as [[all-wheel-drive]] (AWD) sports cars, have transfer cases that are not selectable, known as "full-time".  Such a transfer case is permanently "locked" into AWD mode. In recent decades hybrids have been developed that share properties of each.<ref name=MT>{{Cite web|last=Hardin|first=Drew|date=March 1, 2002|title=How Transfer Cases Work|url=https://www.motortrend.com/how-to/68958-how-transfer-cases-work|access-date=2024-03-30|website=motortrend.com}}</ref>

Transfer cases also perform other functions.  Some are common to all types, others vary by type:

* The transfer case may contain one or more sets of [[gear ratio|low range gears]] for off-road use.  Low range gears are engaged with a shifter or electronic switch. In many transfer cases, this shifter is the same as the one that selects 2WD or 4WD operation.  Low range gears allow the vehicle to drive at much slower speeds while still operating within the usable power band / RPM range of the engine.  This also increases the torque available at the axles.  Low-range gears are used for very inclement road conditions, towing a heavy load slowly, driving on rough, unimproved roads slowly, and extreme off-road maneuvers such as [[rockcrawling]].  This feature is often absent on all-wheel-drive cars.  Some very large vehicles, such as heavy equipment or military trucks, may have more than one set of low-range gears.
* The transfer case may require stopping entirely to engage four-wheel drive (or just low-range 4WD), slowing to a designated speed to engage or disengage an extra axle, or may permit full "shift-on-the-fly" engagement of high-range 4WD at any speed.
* Transfer cases that are designed to allow for normal road use synchronize the difference between the rotation of the front and rear wheels,<ref>{{cite web |url=http://www.cdxetextbook.com/trans/finalDrives/allWheel/transfercasediff.html |title=Transfer case differential action |access-date=2011-02-22 |url-status=dead |archive-url=https://web.archive.org/web/20071225131043/http://www.cdxetextbook.com/trans/finalDrives/allWheel/transfercasediff.html |archive-date=2007-12-25 }} Transfer case differential action</ref> in much the same way the [[Differential (mechanical device)|differential]] acts on a given axle. This is necessary because the front and rear tires never turn at the same speed.  Different rates of tire rotation are generally due to different tire diameters (since the front and rear tires inevitably wear at different rates) and different gear ratios in the front and rear differentials since manufacturers will often have a slightly lower ratio in the front vs. the rear to help with control (such as a 3.55:1 in the rear differential and a 3.54:1 in the front differential).  If the transfer case did not make up the difference between the two different rates of rotation, binding would occur and the transfer case could become damaged.  This is also why a transfer case that is not designed for on-road use will cause problems with [[driveline windup]] if driven on dry pavement.
* Most traditional transfer cases designed for off-road include the ability to mechanically lock the front and rear [[drive shaft]]s to evenly distribute engine torque between them.<ref name=MT/> This is similar to the [[Locking differential|differential lock's]] ability to force both wheels of an axle to turn simultaneously by locking them together, only between axles instead of wheels.
* A part-/full-time hybrid transfer case enables a vehicle to operate in two-wheel drive, full-time four-wheel drive (with the center differential open) and part-time four-wheel drive (with the center diff locked) in both high- and low-ranges.<ref name=MT/> 

[[File:Kubota B6200 Transfer Case Controls.jpg|thumb|The levers on a [[Kubota]] tractor for selecting between 2WD and 4WD (left) and high and low gear ranges (right) are connected to the transfer case . The transfer case is currently in high-range, rear-wheel-drive.]]