Unlocking Potential

The differential lock plays a vital role in maintaining drivability of a vehicle in adverse conditions. Without it the vehicle would be unable to traverse tricky environments often found in off-road situations.

One of the ground-breaking inventions associated with early industrial and automotive development was the differential, or diff for short.
Initially the engineers and inventors of the day came up with crude methods to deliver the torque from the power source to the driving wheels, including chain drive and worm gear drive systems.

In 1827 a Frenchman by the name of Onésiphore Pecqueur is said to have invented a differential that was first used on steam-driven vehicles.

By the time internal-combustion engines started appearing towards the end of the 19th century, it was reportedly a well-established component.

Logically named from its ability to provide a ‘differential’ in speed between the two axles, the differential concept was devised by incorporating a pinion and crown wheel to enable a 90-degree turn in the torque delivery path, along with a planetary gear set.

This enabled the left and right axles to revolve at different speeds when the vehicle was turned from the dead ahead course. The sharper the turn the more difference in speed between both axles.

This, however, created an issue in that if a wheel lost traction due to slippery conditions it automatically received most of the engine’s torque due to the ‘path of least resistance’ principle.

The other wheel receiving next to no torque was therefore rendered useless even if it was on a hard surface.

To overcome this problem a number of measures were devised including the limited-slip differential (LSD) and the diff lock.

The LSD is typically considered something of a compromise between an open (standard) diff and a fully lockable diff.

It often features a series of clutch friction discs and plates that are held apart by springs and forced together by centrifugal force when a wheel breaks traction, thus diverting the torque to the other wheel which has more traction.

Diff locks, on the other hand, come in two distinct styles: Driver Controlled and Automatic Locking — of which the venerable Detroit Locker is arguably the most auspicious.

The Detroit Locker is an automatic locking differential that is designed to lock both rear axles together when torque is applied and the vehicle is travelling in a straight line.

During turns, the unequal speed of the wheels forces disengagement, thus allowing them to turn at different speeds.

Originally named the Thornton NoSpin Differential, it was patented by Ray Thornton in 1941, and initially developed for 6×4 (tandem drive) truck applications. It was manufactured by the Detroit Automotive Product Corporation and used in American military vehicles during WWII.

After the war and into the 1960s, the Thornton NoSpin was original equipment on many American light and medium duty trucks – earning it the nickname Detroit Locker – and at this point drag racers and high-performance car enthusiasts started using it.

The Detroit Locker’s major internal pieces are the side gears, outer springs, driven-clutch assemblies, spider assembly and centre cam.

The centre cam is held in the spider assembly with a snap ring and is free to rotate. The spider assembly takes the place of spider gears found in a conventional open differential.

The locking effect comes from a series of teeth on the spider assembly and the driven clutches on either side of the spider. The driven clutches are located between the side gears, which are splined to the axles and therefore turn at the same speed as the axles.

When the vehicle is under power both axles are locked in position and turn at the same speed.

During a turn, the centre cam inside the spider gear locks into position and acts as a ramp to disengage the driven-clutch teeth from the spider gear. This allows the outside wheel to rotate faster around the corner. When both wheels are again travelling at the same speed, the exterior springs force the teeth to mesh and resume their locked status.

Early Detroit Lockers had a reputation for loud mechanical noise when turning corners, a feature that has been moderated on later versions.

In 2005, powertrain giant Eaton Corporation purchased Detroit Locker’s parent company and greatly expanded the offering.

The Driver Controlled Diff Lock (DCDL), sometimes called cross lock, is widely used on heavy vehicles and uses a dog clutch arrangement – usually activated by a pneumatic solenoid – that locks the differential gears and axles together as a solid unit.

Tandem drive units have an additional locking device within the power divider or inter-axle differential (IAD). This mechanism is mounted on the forward diff which enables varying axle speeds to occur between the forward and rear drive axles during turns.

With the IAD locked and both DCDL units engaged, all drive wheel groups in a tandem drive truck are compelled to turn in unison.

This is extremely useful when traversing muddy or boggy ground, but drivers must be sure to disengage all locks before resuming dry bitumen road driving to avoid damaging the diffs, driveline and tyres. Furthermore, if diff locks are engaged the vehicle will not turn corners easily, which can pose a safety risk.

The general consensus from prominent truck axle manufacturers is that the IAD lock and DCDL should only be used at speeds under 40km/h during adverse road conditions such as rain, snow or gravel/dirt roads, and then only until conditions improve to enable sufficient traction with the locks disengaged.

An important point to note is that cross locks and the IAD lock should never be engaged while there is wheel slip or spin occurring as this can seriously damage the mechanism.

It’s also recommended that DCDL are not engaged when the vehicle is descending steep grades as potential loss of vehicle stability could result in a jackknife of the prime mover and trailer.

The trick prior to travelling on adverse road surfaces is to flick the switch, either while stationary or travelling at a slow constant speed, and then ease up on the accelerator momentarily to reduce torque on the gearing and let the locks engage.

It’s the same procedure for unlocking, with the diff locks unlocked first while still at slow speed, followed by the IAD lock.

With most trucks, an indicator light and in some cases an additional audible warning come on to remind the driver that the locks are engaged. While for many interstate trucks diff locks are rarely needed, for those in vocational applications involving off-road work such as tippers and logging trucks, they are relied upon on a daily basis.

In these applications the modern marvels of diff locks and central tyre inflation systems enable powerful trucks to maintain traction while hauling massive weights up steep grades on dirt or gravel tracks.

Without them, these feats simply wouldn’t be possible.

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