The simple answer is a differential lock (or diff lock) locks both wheels on the same axle in order for the wheels to spin at the same rpm. A power divider (also known as an Inter-Axle Differential or IAD) is a separate differential box that locks the first and second differentials in a tandem truck into place. This allows all drive wheels to movie at the same speed. So, the difference is between one set of wheels moving at the same speed (diff lock) or all drive wheels moving at the same speed (IAD).Large offroad tire and the underside of a heavy-duty truck

Before we go any further though, let’s do a quick recap of what a differential is and what it does for trucks. If you are already savvy to the wonderful workings of the differential, go ahead and skip to the next section for more complete information on the difference between diff locks and power dividers.

Let’s start with the differential

The differential is basically a gearbox that commonly consists of a pinion gear, a ring gear and a side gear. A differential transmits power from the transmission and engine to the wheels of a vehicle and allows them to spin at different rpms when needed. This is important because a vehicle’s wheels turn at different speeds when making a turn. The inside wheel in a turn spins slower and has shorter distance to travel than the outside wheel, which has to spin faster and has a longer distance to travel. Without a properly functioning differential, tires would experience excessive wear or “scrubbing” and the axle could be overstressed and damaged.

Drawing of tire tracks weaving into the distance
A standard differential or “open” differential always supplies an equal amount of torque to each wheel, but it has a major drawback. If there is a reduction or loss of traction for one of the wheels, all torque is transferred to the wheel with the least traction. In essence, power travels the path of least resistance with an open diff. So, in situations of snow, ice, mud or loose dirt, you end up with one wheel spinning and the other not getting enough power to help move the vehicle. The answer to the open diff drawback is the locking differential.

As stated above, the locking differential “locks” both wheels on an axle so that they spin at the same speed. This allows for better traction in slippery or uneven surface situations.

A Deeper Dive Into Diff Locks

Activated or engaged via a manual switch, the differential lock or diff lock should be turned on when you’re at a stop or traveling at a slow speed. You should never engage it when you are actively experiencing a spin out with one of the tires. And, while opinions seem to vary on how fast you can travel with the diff lock “on,” generally, you shouldn’t go faster than 25 mph. It’s important to note that steering can be adversely affected and your turning radius will increase with the diff lock engaged.

As you probably know, diff locks help you in poor traction conditions. When you’re out of the situation that was causing poor traction, it’s best to disengage your diff lock. You can do this while stopped or traveling in a straight line at a slow speed.

Understanding the Power Divider

While a differential divides power to the wheels on an axle, the power divider or inter-axle differential (IAD) equally distributes torque between both axles of a tandem truck. In the “unlocked” position, power is equally provided to the front and back axles, but speed differences in the two axles is still allowed.

When “locked,” the IAD causes the front and rear axles to spin at the same speed. This allows for greater traction but also prevents them from being able to compensate for differences in speed, so the potential for a “spin out” is significantly minimized. It should be noted that spin out poses the greatest threat to an IAD. According to a great article

“Spin Out” or wheel spin creates the greatest difference in speed between the axles, and is the number one enemy of the IAD. Spin out occurs when one or both wheels of an axle lose traction and spin while the wheels of the other axle are stationary or moving slowly. The worst-case spin out is when the rear drive axle loses traction and spins while the front axle is stationary, resulting in low or no lube to the nest. Having the IAD in the locked position would prevent this worst-case scenario.

The Quick Power Divider Lock Points to Remember

  • The PDL is activated by the driver via a switch
  • The PDL helps protect power divider parts from spin out damage
  • The PDL directs available power to both axles
  • The PDL can be utilized at all speeds, but to reduce axle stress and excess tire wear, should be disengaged when not needed for added traction

Using the Diff Lock and PDL Together

Now you may be thinking that if you have a diff lock, why would you need the PDL? The answer is simple: to give you the maximum amount of traction possible in a slippery or loss of traction situation. Your diff lock ensures the tires on a single axle are turning at the same rpm and your PDL ensures all the tires on the front and rear axles are moving at the same speed.

Just remember that when using a diff lock by itself or in conjunction with the PDL, don’t exceed 25 mph.

Need More Insight on Diff Locks and PDLs?

The professionals at Inland can answer all your questions about differentials, power dividers and more. Just give us a call or stop in at your nearest Inland location today for great service, quality parts and over 75 years of experience in getting and keeping you on the road.