LOCKERS

Not the kind you put your gym clothes in!!!

By Jake Scarbrough-2/4/2004

Figure 1:  A locking system for a differntial-courtesy of How Stuff Works

Have you ever driven on ice or been stuck in the snow, and realized that one tire was just spinning extremely fast, while the tire on the opposing side just sat there and watched?  This problem is very common with automobiles, and happens on many different types of terrain, including: ice, snow, mud, or sand.  The problem can be fixed with a product that has become a best friend to off-road enthusiasts throughout the world.  This product is known as a “locker.”  To grasp what a locker actually is, what it does, and how it works, one must have a slight knowledge of automobiles, most specifically the differential. 

A differential is found in nearly every consumer automobile, and is most easily identifiable as a large round object on the axle-housing between the wheels of the vehicle.  The general location of the differential can be seen in the image below (colored red).  The purpose of the differential is to transfer rotational energy from the driveshaft, which receives its energy from the engine, to the axle-shafts located in the axle-housing (the axle-housing is what you see from the outside).  These axle-shafts then transmit their rotational energy to the wheels, and you know what happens after that.  The most important thing about the differential is that it allows the axle-shafts to spin at different RPM’s (rotations per minute) so the wheels can move at different speeds while turning.  When a vehicle turns a corner, the outside wheel must travel a greater distance, and therefore will have a greater rotational speed than the inside wheel.  The differential allows the wheels to spin at different speeds.  If the inside wheel tried to rotate with the same RPM as the outside wheel, and there was no loss of traction, turning would be impossible.

Figure 2: General drivetrain layout showing differential and axle-housing-courtesy of How Stuff Works.

Most automobiles are equipped with what is known as a “open differentials.”  An open differential averages the rotational energy from the driveshaft between the two axle-shafts.  If one wheel is spinning in the air, and the other wheel isn’t moving, than the wheel spinning is spinning twice as much as it would if both wheels were spinning equally.  Though the open differential is great for normal driving, it is a major setback in conditions where traction is essential.  The reason for this is that the open differential transmits power to the wheel that has the least amount of resistance (the easiest to spin).  So imagine the often occurring situation of being stuck in the mud.  You somehow drive halfway into the mud – your passenger side is in the mud, and driver side is on dry land or even cement for this matter.  When you punch the gas, if you were to look at the rear wheels for example, you would most likely notice that the wheel in the mud would be spinning very fast, while the wheel on dry land just sits there motionless.  This is because the wheel in the mud has the least resistance, and is easiest to spin, so the differential transfers the power to it instead of the wheel with the most traction.  The same thing can be illustrated if you think of the vehicle in a ditch with a wheel in the air.  The wheel in the air is the easiest to spin, so it gets all of the power.  Of course a wheel spinning in the air is not going to move anything.  Some people think that 4-Wheel-Drive, solves this, but that belief is false.  4WD tries to spin all 4 wheels, but if you get stuck, there is still usually only one wheel in the front, and one wheel in the rear actually spinning.  The solution to the problem: the “locking” differential, most commonly known as a “locker.”

You can now probably guess somewhat closely to what a “locker” does.  A locker locks the axle-shafts together so that they rotate at the same RPM, and the wheels spin at the same speed.  The locker replaces the “spider gears” within the differential with a device which locks and unlocks the axle-shafts.  There are several different types of lockers: automatic, pneumatic, electric, cable, and Lincoln lockers.

Automatic lockers use purely mechanical systems, which detect wheel speed differentiation, and send power or torque to the wheel where it is need most.  Many times this is all of the power to the wheel with traction, or at least more power to the one with traction than the one without, or both the same amount.  The advantage of the automatic locker is that it knows when to activate.  When driving around a corner, the automatic locker will not engage unless rapid acceleration occurs in which the tires gain great amounts of traction.  You really have to punch it around a corner.  The automatic locker provides excellent street and off-road capabilities, and is cheaper than other lockers, yet not as great in performance.

The pneumatic, electric, and cable lockers are all built very similarly to the automatic locker and act the same besides the source of activation: compressed air, electricity, and a manual pull cable respectively.  The thing that sets these apart from the automatic locker is that when activated they totally lock the axleshafts into moving at the same RPM as the driveshaft and each other until deactivated.  So when driven on the road, the differential acts just like an open, normal differential, but as soon as a switch is activated or a cable pulled, both wheels will spin at the same RPM no matter what, unless something breaks.  This is very useful in off-road conditions, and provides the ultimate in traction.

The “Lincoln locker” gets its name after the Lincoln Welding Co. This is because the gears in the differential are just permanently welded up so that the axle-shafts cannot move at different speeds, and you essentially have one solid axleshaft.  This is also known as a “spool” and actual locker systems can be purchased which work the same as the Lincoln locker, but instead of welding the gears, you just replace them with a spool.   This is great for a vehicle which spends all of its time offroad, and needs extreme traction, but horrible for street driving.  It is not advisable to do this in the front axle of a vehicle because turning would be very difficult as I am sure you can imagine.  With the other lockers, you can at least turn them off by a flick of a switch if you really can’t make a turn.  

Lockers are not to be confused with limited-slip differentials (LSD), also known as “posi-traction.”  LSD comes stock with many automobiles and trucks.  It usually uses somewhat the same design as an automatic locker, but uses heavy duty clutch disks and a a spring.  The clutches and spring try to keep the axle-shafts spinning at the same RPM, until whatever it is keeping the tire from spinning overcomes the power of the clutch disks and spring, which really doesn’t take much to do.  Torsen, short for Torque Sensing, differentials are somewhat like a LSD but no clutches or discs are used.  It is purely mechanical.  It works on a torque bias ratio of  5:1, offering up to five times as much torque to the wheel with good traction.  Of course, if one wheel is in the air just spinning than there is nearly no torque to that wheel, so five times no torque to the other wheel that has traction is not very helpful.

So next time you are stuck somewhere and wish that both of your tires would spin, think about a locking differential, or maybe a light duty limited slip.