Prime Mover Magazine

Upgrading for downspeeding

Upgrading for downspeeding

In the continual drive for optimised fuel economy and emission reductions, the latest heavy-duty trucks are tuned and geared to produce mountain-munching peak torque outputs at lower RPM than ever before. To cope with this increased demand, highly experienced driveline manufacturer Dana has released the SuperDuty 300, its sturdiest on-highway driveshaft by a country mile.

Many transport operators who grew up in regional areas had their first taste of lorry driving at a very early age on farms scattered all over Australia. Back in the ‘60s, ’70s and early ‘80s the typical farm truck was either an Austin/ Leyland, Bedford, International or Dodge.

At that stage Japanese domination of the medium-duty truck market led by Isuzu and Hino was still a very distant blip on the horizon.

As a rule of thumb, these poor old single-drive five-tonne rated relics were regularly loaded with ten tonnes of grain before ‘Farmer Brown’ would scurry off to the local silos, all the while hoping the Department of Main Roads Inspectors had knocked off early for the day.

Many a youngster would have been intrigued by the tinkling sound, not unlike the call of bellbirds, that would emanate from beneath the overloaded farm trucks at clutch take-up point just before they started to move.

This came from the flexing of the thin-walled steel tube in the lengthy drive shaft – centre bearings were rare in those times – as it desperately struggled to initiate gear rotation inside the diff on the vastly overloaded truck.

Thanks largely to the generally flat country on and around grain properties and the woefully feeble torque outputs from the typical six-cylinder petrol ‘power’ plants of the day, most driveshafts managed to survive.

But it really is a wonder considering their light-weight construction that more of these didn’t wind themselves into something resembling a stick of liquorice.

This brings us to the Dana Superduty 300 and the reason why most old farm trucks of that era, apart from those lovingly restored and residing in museums for the benefit of posterity, have long since been relegated to the junk yard.

Put simply, today’s trucks are engineered for vastly superior performance compared to the relics of the past. Roads are far better, speeds are much faster and proportional to the number of axles, loads are a lot heavier, which all adds to an immense demand on the entire powertrain. And, of course, the driveshaft, along with the transmission and drive axles, is a critical component in delivering the turning effort to the tyres.

As previously mentioned, the current trend towards downspeeding – where cruising RPM is reduced to a minimum in the quest for optimum fuel efficiency – is placing even more strain on driveline components like the driveshaft. It’s for this reason that driveline component manufacturer Dana has risen to the challenge of designing and producing a super-duty driveshaft that is well up to this formidable role.

As forerunners in the fuel efficiency charge, European truck and engine manufacturers are well versed in downspeeding and have been developing the principle for many years.

For North American manufacturers like Cummins, however, this is a concept that is relatively new and one that the company has embraced with gusto as it seeks to improve fuel efficiency and reduce emissions from its engines.

Cummins understands that to get the desired outcomes for its customers, it must ensure the complete drivetrain from engine to diff is well up to the task of managing the increased engine torque at lower RPM developed by its downsped engines. In other words, the last thing Cummins wants is for a jack-shaft to come adrift from a customers’ truck because it can’t handle the torque.

As such, Cummins has developed what it calls Cummins Integrated Powertrain which involves the development of the engine, transmission and driveline as a complete package.

In terms of integration between the Cummins X15 engine and Eaton UltraShift Plus automated transmission, much work has been done through collaboration between the companies to enable optimum electronic communications in terms of transmission shift points to suit the higher torque output produced at lower revs from the Cummins X15 engine.

The flow-on effect is that the other driveline components like driveshafts and axles need to be beefed up to handle the extra torque or turning force exerted on them.

To put it in simple terms, it’s like having a short lever and a long lever to move a given load. The long lever moves the load with less effort, but it must be moved a greater distance – this represents an engine geared to cruise at higher RPM.

The short lever requires greater effort exerted over a shorter distance to move the load which represents an engine geared to cruise at lower RPM.

Somewhat confusingly, the diff or final drive ratios that are closer to 1:1 and therefore enable cruising speed at lower RPM are called ‘tall’ while the ‘higher’ ratios such as 4.44:1 are called ‘short’. This terminology is also at odds with the previously mentioned lever principle where the ‘short’ lever relates to a ‘tall’ final drive ratio and vice versa.

The diff or final drive ratios for highway trucks, depending on application, have traditionally been 4.33, 4.11 or 3.9:1 with the latter formerly considered ultra-tall. However, with recent on-road testing Cummins has been trialling the previously unheard of in truck circles 3.73 and 3.58 diff ratios behind respective Euro 5- and Euro 6-compliant engines to determine the optimum ratios for on-highway applications.

It’s important to remember that there is a ‘sweet spot’ in ratio selection and that down-speeding too far is in fact counterproductive.

From our experience during a recent drive of a Kenworth K200 with a Euro 6-compliant Cummins X15 and the 3.58:1 diff ratio, there would be no benefit in using a taller ratio than this in a full-weight east coast B-double application.

Relating all this to the Dana SuperDuty 300 driveshaft that the company has endowed with the necessary brawn for down-speeding applications, Mark Wallace, Executive Vice President of Dana, says the benefits of engine downspeeding are undeniable.

“We are partnering with industry leaders to advance this efficiency-boosting technology,” Wallace says. “Joint collaboration with Cummins and Eaton allows Dana to take a comprehensive systems approach, examining the integration of all components throughout the vehicle in an effort to fully optimise performance and make further drivetrain enhancements.”

As previously explained, an engine running at lower RPM needs faster axle ratios to maintain the same vehicle speed and performance in all driving conditions.

This, in turn, requires significantly higher torque output from the engine. The higher torque places added stress on axles, driveshafts and inter-axle shafts, with the potential to significantly reduce the life of these and other related components, especially universal joints.

It was for these reasons specifically that Dana set about developing what it describes as the most robust driveshaft it has ever manufactured for on-highway truck applications, the SuperDuty 300 (SD300).

Prior to the development of the SD300, the on-highway industry standard Dana driveshaft was the SPL250, which has a torque capacity of 1,850lbft (2,500Nm). In comparison the SD300 is rated to cope with 2,210lbft (2,996Nm) of torque which is well in excess of the 2,050lbft (2,779Nm) peak torque produced by the latest downsped Cummins engines.

The higher capacity is due to a number of changes, not least a 1.0mm increase in the shaft wall thickness from 5.0 to 6.0mm and the use of larger journal diameter universal joint said to be the highest torque-rated of their kind in the industry.

Importantly, the tube outer diameter remains the same at 130mm, providing a compact swing diameter which aids installation in space limited applications.

Other design features that enhance durability and minimise maintenance in high torque applications are a proven service-free slip joint, the splines of which are treated with Dana’s patented GlideCote process.

This is a permanent blue coloured surface coating that minimises friction between the moving parts. As with the universal joints, the slip joint is lubricated and sealed for life with synthetic grease at assembly.

Other features that bode well for a long service life under extreme operating conditions include cross-serrated flanges and a four-bolt attachment system.

With an appearance similar to the cross-hatch pattern on calamari served in a restaurant, the cross-serrated flange features something of a vestigial spline in that it positively aligns the male and female yokes to ensure correct engagement and provides the strongest possible mating interface, meaning the sole purpose of the bolts is to clamp the two yokes together.    

Another key point that will please many Australian operators is that the SD300 is manufactured locally at Dana’s Australian facility in Keysborough. This is fitting considering the Kenworth trucks to which it is installed are also manufactured in Australia at nearby Bayswater.

“Typically the driveshaft is seen as the weakest link in the powertrain so even though a suitably strengthened SPL250 would do the job in the downsped trucks its projected lifespan of eight to ten years could be reduced by two years due to the extra torque,” says Paul French Dana Australia OEM Sales Manager. “So in developing the SD300 our objective was to provide the same eight to ten year service life in the more demanding downsped applications.”

According to French, the SD300 was developed from the company’s GWB range of super heavy-duty industrial driveshafts produced in Europe, which have torque ratings ranging from 2,400Nm (1,770lbft) to a mind-boggling 16,300,000Nm (12,022,263lbft).

Dana’s GWB driveshafts have been produced since 1946 and are said to be the first that were developed specifically for diesel locomotives in the 1950s.

This was followed a few decades later by the introduction of the first maintenance-free driveshaft.

With credentials like that, it would seem the present and future driveshaft demands of heavy-duty downsped trucks like Kenworth are in safe hands.

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