Differentials, Lockers, etc.


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Posted by Clint Dixon on Thursday, March 15, 2007 at 13:38:14 :

I am placing this post here as the start of a new thread although it is in reference to an ongoing thread lower on this Forum, and likewise, on past Forums. I am placing it here, as the beginning of a new thread, for three reasons:

1) I don’t know where the safest place to post it would be in the aforementioned thread below. I don’t want to post it under someone else’s post and risk sounding like I am targeting anyone in particular, being disrespectful, or making acquisitions. Those are definitely not my intentions and I apologize to everyone here if I inadvertently made it sound like that was what I was doing.

2) I think this topic is better handled as a new thread as it kind of got lost in the context of a semi-related thread below to the point of almost hijacking that thread.

3) I think this topic may be of interest to a lot of folks who may otherwise miss reading it if it were left to continue buried below.

There seems to be some basic confusion relating to the design and operation of various traction aiding devices available for the axle housings of our trucks. I will be the first to admit that I don’t know everything about any one thing, a little about a lot of things, and just enough to get myself into trouble on most things. I have owned several vehicles with standard differentials in the axle housings. I have owned two vehicles with limited slip traction aiding devices in the axle housings. And, I have owned one vehicle with a positive locking traction aiding device in the axle housing. I have never owned a vehicle having no means within the axle housing to compensate for different rotational speeds between the two axle shafts located at either ends of the common housing.

So, with that in mind, I have been trying to help where I can, with what information I have, to help clear up the basic confusion that exists on this subject. I have also been struggling to pinpoint and to understand just what exactly the confusion is. I do not feel that I have completely realized the whole scope of the confusion so my premature attempts at answering bits and pieces of the whole have probably created even more confusion because of this. Sorry if I have done that. If so, I believe that my answers have fallen short of the big question and in that context should be considered at worst as wrong and at best as incomplete. I am a firm believer that wrong information is worse than no information. After reading through all of the posts, I now believe that we are comparing apples to oranges so I will give it another try to clear up things a little:

Basically when it comes to a driving axle, you have the choice of both wheels at either end of the axle housing always turning the same speed as one another, by being mated to a common rotating shaft (in effect), or turning independently of one another by some device that controls that difference between the two to some extent. The two devices at either end of this spectrum would be the Spool and the Differential.

The Spool is a device that bolts right to the ring gear. The Spool rotates on bearings in the axle housing. It has common splines that both axle shafts mate into. It allows for no difference in the speeds between these two shafts. It mimics both wheels being attached to one common axle shaft that extends between both. It is also mimics welding the two axle shafts together where their ends meet near the center of the axle housing.

The 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 on bearings like the same as the spool above. The Differential allows different speeds and even different rotational directions between the two axle shafts. If one axle is forced to rotate 10% faster than the case ring gear (for whatever reason) the other will be forced to rotate 10% slower than the ring gear. If one axle shaft is forced to rotate 100% faster than the ring gear (for whatever reason) the other will be forced to rotate 100% slower and will actually be rotating the reverse direction from the first.

The Selectable Locker is kind of both a Spool and a Differential all at the same time. It allows the operator to select whether they want the “spooled effect” between the axle shafts in a common housing, or whether they want the “differential effect” between the same. It requires operator input and is one or the other – a Spool or a Differential – no in-between.

The Limited Slip Differential is more closely related to the true Differential. It has many parts that sense when a tire is loosing traction and directs torque to overcome this. One the main aspects of many Limited Slip differentials are the presence of clutches in the unit. These allow a certain amount of “slip”. They can (depending upon the design) require varying amounts in driver input through the prudent use of brake and throttle. A Limited Slip Differential can never act like a Spool unless something is broken inside.

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 a true Differential. A better term would be something like: “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 of 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 that causes 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 should not even be called “clutches” in my opinion. A more appropriate term might be “axial gears”. The have teeth that mate together not unlike any other gear set. The Positive Locking Differential requires no input from the vehicle operator. It is fully automatic. It has no provisions to unlock both axles from the unit at the same time. It cannot transfer all torque to another driving axle assembly unless both tires of the assembly it is installed in loose total traction. It has no provisions to operate like a true differential. It operates equally well whether the vehicle is moving forward or rearward, straight ahead, turning left, or turning right.

I sure hope this helps. ;^)


Junior



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