Transfer case
Encyclopedia
A transfer case is a part of a four-wheel-drive system found in four-wheel-drive and all-wheel-drive vehicles. The transfer case is connected to the transmission
and also to the front and rear axle
s by means of drive shafts. It is also referred to as a "transfer gearcase", "transfer gearbox","transfer box" or "jockey box" (Australia
).
, rock-crawling vehicles, and some military vehicles generally allow the driver to select 2WD versus 4WD as well as high versus 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 can use sets of gears to drive either the front or both the front and rear driveshafts. These are generally strong, heavy units that are used in large trucks, but there are currently several gear drive cases in production for passenger cars.
Chain-driven transfer cases use a chain to drive most often only one axle, but can drive both axles. Chain-driven transfer cases are quieter and lighter than gear-driven ones. They are used in vehicles such as compact trucks, full size trucks, Jeeps and SUVs. Some off-road driving enthusiasts modify their vehicles to use gear-driven transfer cases, accepting the additional weight and noise to gain the extra strength they generally provide.
Transfer cases are also classified as either "independent" 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, as is commonly found on recent Subaru
products and some other four-wheel-drive cars. An "independent" transfer case is completely separate from the transmission; it is bolted to the transmission output shaft and a short driveshaft travels from the transfer case to the front and rear differentials. Married transfer cases are by far the most common found in cars. Most pickups and SUVs use an independent transfer case. In fact, this is so common that the term "married" is rarely used. Independent transfer cases are used on very long wheelbase vehicles, such as commercial trucks or military trucks.
Manual transfer cases have a selector lever on the driver's side floor transmission hump and may also have either two sealed automatic front axle locking hubs
or two manual front axle hub selectors of "LOCK" and "UNLOCK" or "FREE". To engage the four-wheel-drive system the vehicle must be moving at a slow speed less than 30 miles per hour, this is only for the four-wheel-drive high setting. To engage the four-wheel-drive low setting, the vehicle must be stopped and the transmission must be shifted to neutral, then the four-wheel-drive low can be selected.
Electronic Shift On-the-Fly (ESOF) transfer cases have a dash-mounted selector switch or buttons with front sealed automatic locking axle hubs. Some models also have what is called selec trac, which has a slider switch on the center console. Unlike the manual transfer case, this system has a transfer case motor and vacuum-operated front hubs. To engage or disengage, the vehicle can be completely stopped or moving at any speed under 55 mph (24.6 m/s).
Most mud trucks are set up with a transmission and dual transfer cases because of the high relocation after a massive body lift is installed. This saves money on custom drive shafts for high pinion angles. Most of the time, monster trucks and mud trucks are full-time four-wheel drive and are not set up with a transmission at all, just an independent three-speed transfer case. These trucks are set up to be run at one constant speed and only need to have 1 to 2 gears plus reverse.
Transmission (mechanics)
A machine consists of a power source and a power transmission system, which provides controlled application of the power. Merriam-Webster defines transmission as: an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a...
and also to the front and rear axle
Axle
An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...
s by means of drive shafts. It is also referred to as a "transfer gearcase", "transfer gearbox","transfer box" or "jockey box" (Australia
Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
).
Functions
- The transfer case receives power from the transmission and sends it to both the front and rear axles. This can be done with a set of gears, but the majority of transfer cases manufactured today are chain driven. 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 is sometimes accomplished by means of a shifterShifterShifter may mean:* Shifter , or gear lever, a bicycle part that selects which gear the chain rests on* Shifter , an adjustable wrench/spanner...
, similar to that in a manual transmissionManual transmissionA manual transmission, also known as a manual gearbox or standard transmission is a type of transmission used in motor vehicle applications...
. 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 permanently "locked" into all-wheel-drive mode.
- An on-road, transfer case synchronizes the difference between the rotation of the front and rear wheels, in much the same way the differential acts on a given axle. This is necessary, because the front and rear tires never turn at the same speed when front and rear tire sizes differ.
- Transfer cases designed for off-road use can mechanically lock the front and rear axles when needed (e.g. when one of the axles is on a slippery surfaces or stuck in mud, whereas the other has better traction). This is the equivalent to the differential lockLocking differentialA locking differential, diff-lock or locker is a variation on the standard automotive differential. A locking differential may provide increased traction compared to a standard, or "open" differential by restricting each of the two wheels on an axle to the same rotational speed without regard to...
.
- The transfer case may contain one or more sets of low range gearsGear ratioThe gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...
(generally for off-road vehicles). 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. 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. This feature is often absent on all-wheel-drive cars. Some very large vehicles, such as heavy equipment or military trucks, have more than one low-range gear.
Types
Those used on "part-time four-wheel-drive" system off-road vehicles such as trucks, truggiesTruggy
A truggy or truggie is a specialized off-road vehicle. Truggy is a portmanteau of the words truck and buggy. The truggy nomenclature is derived from using a truck frame as the basis for the vehicle, whereas a "buggy" would have started from the chassis of a Volkswagen Bug.Truggies are most...
, rock-crawling vehicles, and some military vehicles generally allow the driver to select 2WD versus 4WD as well as high versus 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 can use sets of gears to drive either the front or both the front and rear driveshafts. These are generally strong, heavy units that are used in large trucks, but there are currently several gear drive cases in production for passenger cars.
Chain-driven transfer cases use a chain to drive most often only one axle, but can drive both axles. Chain-driven transfer cases are quieter and lighter than gear-driven ones. They are used in vehicles such as compact trucks, full size trucks, Jeeps and SUVs. Some off-road driving enthusiasts modify their vehicles to use gear-driven transfer cases, accepting the additional weight and noise to gain the extra strength they generally provide.
Transfer cases are also classified as either "independent" 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, as is commonly found on recent Subaru
Subaru
; is the automobile manufacturing division of Japanese transportation conglomerate Fuji Heavy Industries .Subaru is internationally known for their use of the boxer engine layout popularized in cars by the Volkswagen Beetle and Porsche 911, in most of their vehicles above 1500 cc as well as...
products and some other four-wheel-drive cars. An "independent" transfer case is completely separate from the transmission; it is bolted to the transmission output shaft and a short driveshaft travels from the transfer case to the front and rear differentials. Married transfer cases are by far the most common found in cars. Most pickups and SUVs use an independent transfer case. In fact, this is so common that the term "married" is rarely used. Independent transfer cases are used on very long wheelbase vehicles, such as commercial trucks or military trucks.
Manual transfer cases have a selector lever on the driver's side floor transmission hump and may also have either two sealed automatic front axle locking hubs
Locking hubs
Locking hubs, also known as free wheeling hubs are an accessory fitted to many four-wheel drive vehicles, allowing the front wheels to be manually disconnected from the front half shafts....
or two manual front axle hub selectors of "LOCK" and "UNLOCK" or "FREE". To engage the four-wheel-drive system the vehicle must be moving at a slow speed less than 30 miles per hour, this is only for the four-wheel-drive high setting. To engage the four-wheel-drive low setting, the vehicle must be stopped and the transmission must be shifted to neutral, then the four-wheel-drive low can be selected.
Electronic Shift On-the-Fly (ESOF) transfer cases have a dash-mounted selector switch or buttons with front sealed automatic locking axle hubs. Some models also have what is called selec trac, which has a slider switch on the center console. Unlike the manual transfer case, this system has a transfer case motor and vacuum-operated front hubs. To engage or disengage, the vehicle can be completely stopped or moving at any speed under 55 mph (24.6 m/s).
Most mud trucks are set up with a transmission and dual transfer cases because of the high relocation after a massive body lift is installed. This saves money on custom drive shafts for high pinion angles. Most of the time, monster trucks and mud trucks are full-time four-wheel drive and are not set up with a transmission at all, just an independent three-speed transfer case. These trucks are set up to be run at one constant speed and only need to have 1 to 2 gears plus reverse.