An overview of differential function and gearing

When it comes to a vehicle’s driving axle, the spool and differential are just the starting point for other options.


This month’s article is going to take a slightly different point of view and discuss the use of a particular type of gear compared to the typical article focusing on gear-specific details. In its basic concept, when it comes to the driving axles of the vehicle, there exist two choices in the way in which each driven wheel at either end of the axle housing rotates in respect to the another. The first choice is to have both wheels always rotating the same speed as one another, by being mated to a common driving shaft (in effect). The second choice is to allow the wheels to rotate independently of one another by using some form of device that controls the rotational difference between the two to some extent. The two common devices at either end of this spectrum would be the spool and the differential.


The spool is a device that bolts directly to the ring gear. The spool is driven by the ring gear and rotates on bearings within the axle housing. It has common internal splines that both axle shafts mate into. It effectively locks both axles together as if they were one, allowing for no difference in rotational speeds between these two shafts. It mimics the act of welding the two axle shafts together where their ends meet near the center of the axle housing. The spool acts as a single solid connecting point between the axles and this connection will cause tires to scrub when the vehicle is turned through a corner. The operator of the vehicle has no control over the operation of the spool.


The differential (generally referred to with more specificity as open differential) is made up of several parts, usually as an assembly within a case. The ring gear bolts to this case, or assembly, which in turn rotates in bearings the same as the spool above. The differential has independent internal gears, each with internal splines that the respective axle mates into. These gears, in combination with other internal parts, allow a difference in speeds and even different rotational directions between the two axle shafts. If one axle is forced to rotate 10 percent faster than the ring gear (for whatever reason), the other will be forced to rotate 10 percent slower than the ring gear. If one axle shaft is forced to rotate 100 percent faster than the ring gear, the other will be forced to rotate 100 percent slower, or essentially not rotate. At any percentage above 100, the second axle will actually be rotating in the reverse direction from the first. Unless broken, the differential never acts as a solid connecting point between the axles and, as such, eliminates any tire scrub through corners. The negative side to this is that any one tire can lose traction and the differential can deliver 100 percent of the torque to the axle driving that tire and wheel, stopping vehicle motion. The driver of the vehicle normally has very little control over the operation of the differential. However, a limited amount of control of the differential can be gained through simultaneous application of parking brake and throttle. Even more control can be gained with the introduction of steering brakes.

Selectable Locker

The selectable locker is usually found as an assembly within a case and, like the others, is driven with a ring gear and rotates in the same bearings in the axle housing. The selectable locker is sort of like having both a spool and a differential at the same time. It allows the operator to select one of two or more settings on how they want the locker to behave. It can be selected to cause a “spooled effect” between the axle shafts in a common housing, effectively locking the axle shafts together to rotate as one. It can also be selected to cause a “differential effect” between the axle shafts in a common housing. The unit employs many of the same parts as an open differential unit. The selectable locker requires operator input and is always in one or the other settings, a spool or a differential; there is generally no in-between condition. The selectable locker has no provisions for sensing wheel spin and automatically compensating.

The differential has independent internal gears, each with internal splines that the respective axle mates into. These gears, in combination with other internal parts, allow a difference in speeds and even different rotational directions between the two axle shafts. (Courtesy: Shutterstock)

Limited Slip Differential

The limited slip differential is usually found as an assembly within a case and driven with a ring gear as the others. The limited slip differential is more closely related to the open differential than to the spool. It has many internal parts that sense when a tire is losing traction and directs torque to the appropriate axle to overcome this. One of the main aspects of many limited slip differentials is the presence of clutches within the unit. These clutches allow a certain amount of slip. The limited slip can (depending upon the design) require varying amounts of driver input through the prudent use of brake and throttle to operate effectively. This is especially true with those units showing their age. Age and wear are the biggest factors that tend to make limited slip differentials less than effective. The correctly functioning limited slip differential is generally more responsive to operator input than an open differential. A limited slip differential can never act like a spool unless something is broken inside.

Positive Locking Differential

The same design consideration, ring gear, case, bearings also apply to the positive locking differential. The positive locking differential should not be called a “differential” at all in my opinion. It has none of the characteristics (and usually none of the parts) found within an open differential. A better term would be something along the lines of “automatic positive locking traction-aiding device.” The positive locking differential is more closely akin to the spool. It keeps both axle shafts locked together, all the time, with one exception — it has the ability to let one (and only one) axle unlock from the other when that axle receives force from the surface the vehicle is traveling on, causing that axle shaft to want to rotate faster than the other shaft. This action works on the same principle as an overrunning clutch. The positive locking differential does have clutches, but unlike a limited slip differential, they are not the kind of clutches with friction plates that can slip. They are called “dog clutches” and (again, in my opinion) should not even be called clutches. A more appropriate term might be “axial gears”. They have teeth that mate together not unlike any other gear set. The positive locking differential requires no input from the vehicle operator in order to switch between locked and unlocked modes. It is fully automatic. It has no provisions to unlock both axles from the unit at the same time. It cannot transfer all of the torque to another driving axle assembly unless both tires of the assembly it is installed in lose all traction. It has no provisions to operate as an open differential. It operates equally well whether the vehicle is moving forward or rearward, straight ahead, turning left, or turning right. The positive locking differential can be noisy and harsh as it allows axles to lock and unlock from driven case. This tendency can be overcome somewhat by modifying operator-driving styles.

All traction-aiding devices can cause unpleasant results when a vehicle so equipped is driven in a side-hill condition. In an open differential, usually one wheel will spin out while the other acts as a sort of rudder and helps to keep the vehicle headed straight. With a traction-aiding device, in a driving axle, both wheels can lose traction in unison causing the rudder effect to be lost. This action can cause the end of the vehicle with the driving axle to swing to the downhill direction. This tendency is true even on relatively flat conditions if the surface is slick enough. Icy crowned roads can be especially treacherous while driving a vehicle that is being driven with only one axle when that axle is equipped with a traction-aiding device that cannot be fully unlocked by the operator. 

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Dr. William Mark McVea, P.E., is President and Principal Engineer of KBE+, Inc. which develops complete powertrains for automotive and off-highway vehicles. He is the Principal Engineer with Kinatech, a joint venture with Gear Motions / Nixon Gear. He has published extensively and holds or is listed as co-inventor on numerous patents related to mechanical power transmissions. Mark, a licensed Professional Engineer, has a B.S. in Mechanical Engineering from the Rochester Institute of Technology, a Ph.D. in Design Engineering from Purdue University.