Mack power divider: Explainer

The introduction of Mack's B-Model in 1953 bought a raft of new technology and design concepts that would prove a complete departure from conventional trucks of the time. One of these more noticeable design concepts was found at the truck's rear. Here, top-mounted dual-reduction carriers were married with Mack's Camelback suspension that Mack eventually marketed as ‘Balanced Bogies’.

Interestingly, Mack still offers its Balanced Bogies today in the Granite model. We pondered, some time ago, why Mack's Balanced Bogies are still so well-liked and why they're still in production after 70 years, when we explored the pros and cons of Mack's Camelback in an earlier article.

This article, however, delves into the distinguishing features of Mack's unique top-mounted axle carriers and explains how they work…

Tough lessons learned

Surviving two World Wars as the preferred heavy truck supplier to the military had taught Mack many lessons. One important one was integrating parts commonality across components. Component parts commonality in a war zone meant transport regiments only needed to carry minimal spares to maintain a large army convoy.

For this reason, Mack engineers made the design the front and rear carriers identical, except for the main input drive and through shaft arrangement of the front axle carrier, which differs from the input pinon mounting of the rear carrier head. The economic benefits of this clever design paid dividends for Mack in production and also for customers in the field, through reduced maintenance costs and reduced downtime.

Related reading:
Mack Camelback suspension: What you need to know
Advice: Bogie drive basics
Advice: Is a PBS combination right for you?

Before getting into the nitty-gritty of the Mack differential process, it’s probably best if we take a minute to understand what a power divider is and how a conventional one operates, so we can fully appreciate the unique operation of Mack's power divider.

It's a fact of life that all tandem or tri-drive trucks require a power divider, also referred to as the inter-axle differential, which allows the forward and rear axles to rotate independently of each other. Apart from relieving stress on all the axles and reducing tyre wear, it importantly enables the vehicle to corner more effectively.

Most trucks have a switch located on the accessory switch panel that enables the power divider to be locked or engaged. When the power divider is in the ‘engaged’ position, it causes the differential action of the power divider to act as a ‘through-drive’, transmitting power equally between both the forward and rear axles. This, in turn, provides better traction over slippery surfaces.

The Mack difference

Mack's power divider works a little differently and is called a 'torque proportioning inter-axle power divider' because of its ability to distribute torque according to the requirement to deliver the best possible traction.

The best way to describe how the system works is if you think of the Mack power divider as a third differential. The inter-axle power divider splits driving torque evenly or unevenly, as required between the axles. It does this autonomously, without any input from the driver, to maintain traction in the most challenging conditions.

Essentially, the torque proportioning power divider automatically provides a 3-to-1 torque biasing capability to the axle with the best traction. Consequently, in slippery conditions where there is a risk of wheel spin that could cause a loss of momentum, 75 per cent of the total available torque gets directed to the axle with the traction.

Conversely, a traditional hypoid-style differential will either apply torque to the axle or wheel with the least resistance or equally distribute torque if resistance is equal at all wheels. For instance, when driving into a depot with a deep gutter out the front, you may find the truck gets stuck when one wheel in the tandem group is hanging in the air, spinning. All the torque goes to the wheel hanging in the air, as in this situation it’s the wheel with the least resistance (ie none). Here the driver will need to engage the power divider lock to transfer some of the driving torque to the wheels still in contact with the ground, to get the truck rolling again.

On the other hand, in order to move the vehicle forward in the same situation with a truck fitted with Mack's torque proportioning power divider, the Mack power divider instantly detects the loss of traction to wheels hanging in the air. It automatically multiplies the driving torque by three to the remaining wheels with traction on the ground.

Most old Mack drivers knew that even when no traction was available at the spinning wheel, if they applied the brakes slightly, it created enough resistance at the wheel to direct torque to the axle with traction. It's a very clever system. So brilliant, in fact, that it has remained in production since its introduction in 1953.

Over the years, many rival axle manufacturers have tried to imitate Mack's top-mounted axle carriers, all with limited success. By the late ’60s and through the ’70s, Rockwell manufactured the 4640 family of top-mounted double-reduction tandem carriers known as the SFDD and SRDD models. However, after a quick glance at an SFDD parts schematic, you'll notice that the Rockwell model failed to match the benefits of Mack's torque proportioning power divider by opting for a traditional power divider instead. The famous Irish author and poet Oscar Wilde comes to mind here, when he wrote, “Imitation is the sincerest form of flattery that mediocrity can pay to greatness”…

However, much like a TV infomercial, ‘But wait, there's more!’… Mack also provides a power divider lockout designed to couple the forward and rear carriers together as one unit in a straight-through positive drive arrangement.

Typically, the power divider lockout's main advantage is when operating in exceptionally severe operating conditions. In situations where the road surfaces are so slippery that traction is difficult to achieve through the torque proportioning power divider action alone, the lockout device inhibits the operation of the power divider. It consists of an air-actuated, gear-type sliding clutch which, when engaged, couples the cage to the outer cam to effectively de-activate the function of the power divider differential.

To further enhance traction, each carrier incorporates an inter-wheel power divider that functions much like the inter-axle power divider by providing torque biasing from wheel to wheel. The inter-wheel power divider operates autonomously. It works simultaneously with the inter-axle power divider to provide increased manoeuvrability in poor traction situations.

Indeed, during their seven decades of production, Mack's top-mounted carriers have received many upgrades and design enhancements. Today the current top-mounted carriers enjoy a B10-life of one million miles. They are the only tandem drive on the market with high mounted straight shafts that provide narrow driveline angles to reduce torque vibrations and extend U-joint life. They boast an additional 50mm of axle ground clearance over traditional axles, and they are far lighter, allowing for a greater payload.

It's little wonder then that Mack's ‘Balanced Bogies’ are standard spec on the brand’s vocational construction models.