Super Cool

Interstate general freight specialist Palm Trans is trialling pumpless versions of Meritor’s popular MT21-165G tandem drive axle developed specifically for linehaul applications. The company has realised numerous benefits including lower operating temperatures and reduced parasitic losses.

Modern trucking operations demand equipment that is ideally matched to the application in order to derive the utmost in efficiency while minimising operating costs. Interestingly, advancements in technology and processes don’t always equate to the best outcome for all applications.

This means that astute operators must be made aware of all options and choose the one that best matches their requirements.

For heavy-duty drive axle manufacturing specialist Meritor, the quest to provide its customers with products precisely tailored to their individual requirements is ongoing. This is the reason why its latest line up of MT drive tandems, which supersede the previous RT series, are offered in three distinct versions including the MT21-165G Linehaul Series, MT21-165GP Vocational Series and MT23-168GP Severe-Duty Vocational Series.

Designed in conjunction with PACCAR and it’s Kenworth K200 – which is the most popular linehaul B-double prime mover in Australia at present – Meritor’s pumpless MT21-165G is aimed squarely at long distance interstate operators looking to maximise fuel economy and minimise maintenance costs.

As the name suggests, one of the key differences in this design is the absence of an oil pump and filter in the front diff that are standard features in the other two versions.

Before extolling the merits of the pumpless MT21-165G product for linehaul operations typified by Palm Trans, it’s worth taking a peek back through the annals of history at the differentials used in the early days of trucking.

Truck differentials of old relied solely on splash lubrication which was effected by the crownwheel (also known as ring gear) teeth as they dipped into the oil like a paddlewheel and distributed it to the farthest reaches of the axle housing.

However, for this to happen successfully, the crownwheel must be turning at a decent speed with the oil sufficiently warmed and fluid to be effectively ‘flung’.

A good example of how splash lubrication can be inadequate in some circumstances was borne out in early single-drive trucks such as Austin tippers used to haul coal up the treacherously steep grade out of the Burragorang Valley southwest of Sydney in the 1940s and ‘50s.

Having been loaded the previous afternoon and then started up stone cold the next morning, it was discovered that the trucks whose drivers gave them a thrashing up the mountain had no diff problems while those who nursed them thinking they were doing the right thing ended up blowing diffs.

It was therefore deduced that the drivers who took it easy on the trucks were unwittingly starving the diff gears and bearings of sufficient splash lubrication under the high load conditions – largely due to the cold and therefore thicker oil – compared with those who gave them the gun right from the start, thus heating the oil quicker and enabling it to be splashed around sooner and more effectively to protect the moving parts.

This scenario was potentially exacerbated further with the proliferation of tandem drive trucks in the ‘60s and ‘70s which feature a power divider located high up in the front differential housing. This transmits drive to the rear diff via a through or interaxle shaft, commonly called a jack shaft.

The power divider functions like a ‘third differential’ dividing the torque from the transmission between front and rear axles while also allowing for the slight variations in speed between front and rear axles.

Again, in certain applications,, particularly where there was a possibility of one of the axles in the tandem set losing traction and spinning, it was found that splash lubrication was not entirely effective.

Under these ‘spin out’ conditions the rapidly turning power divider components could quickly become starved of adequate lubrication and fail.

This was a catalyst for the development of the diff oil pump which draws oil from the bowl of the diff, pushes it through a spin-on filter before supplying it under pressure to the bearings and cogs in the power divider gear set.

For applications such as logging where low speeds and heavy loads off-road, often in cold conditions, are widely considered the norm, this is an ideal solution.

But for operators at the other extreme – operating trucks for long distances and maintaining 100km/h as much as possible – the aforementioned oil pump that was touted as a saviour in some applications actually became an impediment to efficiency and oil longevity.

Enter Palm Trans, a privately owned family company managed by Darren and Julie Palmer.

The company was established in 1989 with the goal of providing local haulage around Melbourne, Sydney, Brisbane and Adelaide, along with express interstate and container cartage.

Today, Palm Trans has around 70 prime movers and 150 trailers operating throughout the eastern seaboard and South Australia.

The all-Kenworth fleet is serviced and maintained by the company’s own workshop which is managed by Chris Mifsud who has been with Palm Trans since 2000.

Speaking about the latest pumpless Meritor tandem the company has on trial, Chris says it is the best diff development he has seen in the two decades he has been working for Palm Trans.

“We have always found the Meritor front diffs in the Kenworths to run hot due to the large volume of oil that the pump pushes around which generates quite a bit of heat,” he says.

“It’s a bit of a double-edged sword really, because there are also parasitic losses associated with driving the pump at such a high rate at highway speeds which impacts negatively on fuel efficiency.”

Somewhat ironically, Chris explains that the issue of high front diff operating temperatures has become more pronounced on the Melbourne-Sydney and Sydney-Brisbane runs in recent times due to the massive upgrading of the freeways between the capital cities.

“In earlier times the trucks would periodically slow down to 50 or 60km/h going through towns which allowed the diff oil temp to drop a bit whereas now with virtually all of the towns bypassed they are sitting on 100km/h consistently for much of the journey,” he says.

Chris maintains that the sustained high speeds and long cycle times of the company’s linehaul trucks, particularly during the hot summer months, leads to the front diff oil temperature creeping above 100 degrees C, which can be detrimental to oil integrity and the O-rings and seals in the diff.

It was for these reasons Palm Trans jumped at the opportunity to test the pumpless unit.

“We were keen to jump onboard with this advancement and we’ve definitely seen benefits in increased longevity of the oil and reduced hardening of O-rings and seals caused by excessive operating temperatures,” he says.

While acknowledging that the pump was originally incorporated to improve the lubrication of the power divider and upper areas of the differential, Chris says that as part of the pumpless design Meritor has redesigned the housing to create small dams and wells in the casting which help retain and feed oil into the components that need it.

“It must be stressed that all of our trucks are on long distance work,” he adds. “For on-highway applications where there’s good splash lubrication provided by the crownwheel, its fine. We’ve had no issues in terms of longevity of power dividers and through shaft bearings – they seem to be standing up very well.”

In fact, not having to drive an oil pump when it isn’t required makes mechanical sense to Chris as it reduces the parasitic loss.

“If you’re driving less componentry, you’re wasting less horsepower and fuel and transmitting more power to the rear wheels which is where you want it,” he says.
Chris’s comments are backed up by research carried out by Meritor that verifies the lower operating temperatures and reduced parasitic loss.

According to Meritor, the parasitic loss attributed to the oil pump equates to 3.16kW (4.2hp) and bench testing has realised a potential improvement in fuel efficiency of up to 1.47 per cent for the pumpless unit compared with its pumped brethren.

Potential fuel savings, Meritor claims, could be up to $3,300 per year, assuming a fuel price of $1.46/L and 250,000km annual distance travelled.

These figures are based on a nine-axle B-double combination loaded to 68 tonnes gross combination mass pulled by a prime mover running a 4.3:1 final drive ratio with a cruising speed of 100km/h. Significantly, Meritor says its bench testing has shown that the pumpless carrier operates at up to 20oC lower than the carrier with the pump, while field tests revealed a drop of up to 15oC.

In Palm Trans’s case, this suggests that under the most arduous linehaul operating conditions a desirable front diff oil temperature of between 85 and 90 degrees C should be maintained.

Interestingly, Chris explains that there was no need to vary the oil change intervals with the pumpless compared with the pump equipped diffs. This is despite the fact that the pumpless units don’t have an oil filter.

“We run the same 85W-140 gear oil and not having to change a spin-on filter is an added bonus in reducing maintenance costs and time,” he says.

When Meritor and Kenworth approached Palm Trans to try the new pumpless diff there was no hesitation as the high front diff oil temperatures had been a concern the whole time Chris has worked for the company.

“We are always keen to try new products that we can see will be beneficial to the business,” he says. “I would say this is the biggest advancement I have seen in the Meritor drive axle in the 20 years I’ve been working for Palm Trans.”

Given the pumpless Meritor MT21-165G drive tandem has been developed specifically to address the issue of excessive front diff oil temperature in on-highway applications, it’s refreshing to see a solution that actually makes the product eminently more fit for purpose while using fewer moving parts, and at the same time removing a parasitic loss and maintenance costs from the equation.