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A locking differential 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 available traction or differences in resistance seen at each wheel.
A locking differential is designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction (or lack thereof) available to either wheel individually.
When the differential is unlocked (open differential), it allows each wheel to rotate at different speeds (such as when negotiating a turn), thus avoiding tire scuffing. An open (or unlocked) differential always provides the same torque (rotational force) to each of the two wheels, on that axle. So although the wheels can rotate at different speeds, they apply the same rotational force, even if one is entirely stationary, and the other spinning. (Equal torque, unequal rotational speed).
By contrast, a locked differential forces both left and right wheels on the same axle to rotate at the same speed under nearly all circumstances, without regard to tractional differences seen at either wheel. Therefore, each wheel can apply as much rotational force as the traction under it will allow, and the torques on each side-shaft will be unequal.(Unequal torque, equal rotational speeds). Exceptions apply to automatic lockers, discussed below.
A locked differential can provide a significant traction advantage over an open differential, but only when the traction under each wheel differs significantly.
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Types
There are two main types of lockers: automatic and selectable.
- Automatic lockers lock and unlock automatically with no direct input from the driver. Some automatic locking differential designs ensure that engine power is always transmitted to both wheels, regardless of traction conditions, and will "unlock" only when one wheel is required to spin faster than the other during cornering. They will never allow either wheel to spin slower than the differential carrier or axle as a whole. The most common example of this type would be the famous "Detroit Locker," also known as the "Detroit No-Spin," which replaces the entire differential carrier assembly. Others, sometimes referred to as "lunchbox lockers," employ the stock differential carrier and replace only the internal spider gears and shafts with interlocking plates. Both types of automatic lockers will allow for a degree of differential wheel speed while turning corners in conditions of equal traction, but will otherwise lock both axle shafts together when traction conditions demand it.
- Pros: automatic action, no driver interaction necessary, no stopping for (dis-) engagement necessary
- Cons: intensified tire wear, noticeable impact on driving behaviour (most often tendency to understeer)
Other automatic lockers operate as an "open," or unlocked differential until wheelspin is encountered and then they lockup. This style generally uses an internal governor to sense a difference in wheel speeds. An example of this would be GM's "Gov-Lok."
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- A "selectable" locker allows the driver to lock and unlock the differential at will from the driver's seat. This can be accomplished via compressed air (pneumatics) like ARB's "Air Locker" or vacuum, electronic solenoids (electromagnetics) like Eaton's "ELocker" and Nissan Corporations electric locker found as optional equipment on the Frontier (Navarra) & Xterra, or some type of cable operated mechanism as is employed on the "Ox Locker."
- Pros: allows the differential to perform as an "open" differential for improved driveability, maneuverability, provides full locking capability when it is desirable or needed
- Cons: needs human interaction and forward-thinking regarding upcoming terrain, mechanical complex with more parts to fail, some lockers require vehicle to stop for engagement, puts massive stress on driveline components when left in operation on terrain not requiring a locker
The internal spider gears of an open differential may also be welded together to create a locked (spooled) axle; however, this method is not recommended as the welding process seriously compromises the metallurgical composition of the welded components, and can lead to failure of the unit under stress. If it is desirable to have a spooled axle, the better option is to install either a mini-spool, which uses the stock carrier and replaces only the internal components of the differential, similar in installation to the lunchbox locker, or a full spool which replaces the entire carrier assembly with a single machined piece. A full spool is perhaps the strongest means of locking an axle, but has no ability to differentiate wheel speeds whatsoever, putting high stress on all affected driveline components.
Disadvantages
Locking differentials do have some disadvantages. Because they do not operate as smoothly as standard differentials, they are often responsible for increased tire wear. All automatic locking differentials are known for making a clicking or banging noise when locking and unlocking as the vehicle negotiates turns. This is annoying to many drivers. Also, locking differentials will affect the ability of a vehicle to steer, particularly if a locker is located in the front axle. Aside from tire scuffing on solid surfaces, locked axles provoke understeer and, if used on the front axle, will increase steering forces required to turn the vehicle.
Alternatives
Limited slip differentials are considered a compromise between a standard differential and a locking differential because they operate more smoothly, and they do direct some amount of torque to the wheel with the most traction, but they are not capable of 100% lockup.
Traction control systems are also used in many modern vehicles either in addition or as a replacement of locking differentials.
Volkswagen offers an optional Traction control system under the name of electronic differential lock (EDL). This EDL is not - as the name suggests - a differential lock. Sensors monitor wheel speeds, and if one is rotating 100 rpms or more than the other (i.e. slipping) the EDL system momentarily brakes it. This effectively transfers all the power to the other wheel,[1] but still employs the open differential, which is the same as on cars without the EDL option.
Applications
- Race cars often use locking differentials in order to maintain traction during high speed maneuvers or when accelerating at extreme rates.
- Some utility vehicles such as tow trucks, forklifts, tractors, and heavy equipment use locking differentials to maintain traction, especially when driving on soft, muddy, or uneven surfaces. Lockers are common in agricultural equipment and military trucks. On some farm tractors, there is a pedal that can be stepped on with the operator's heel to lock the differential as needed.
- Four-wheel drive vehicles that drive off-road often use locking differentials to prevent from getting stuck when driving on loose, muddy, or rocky terrain. Locking differentials are considered essential equipment for serious off-road driving.
- Differential locks are also used on some non-utility four-wheel-drive vehicles (such as the Mitsubishi Shogun) to compensate for a relative lack of axle articulation (vertical wheel movement). High amounts of articulation are desirable for off-road driving, to allow the wheels to maintain ground contact over uneven surfaces, but this can lead to excessive body-roll at high speeds on the road, as well as vague steering. Such 4x4 often have suspension systems geared towards more of a compromise between articulation and handling. A rear locking differential is often supplied to make up for this compromise- if a wheel is lifted off the ground, the locking differential can be brought into play to negate this.