Mercedes-Benz SLT: heavyweights for the biggest transport tasks

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Actros SLT and Arocs SLT redefine heavy-duty trucking

Stuttgart / May 20, 2014

• The new SLT heavy-haulage vehicle: top-class specialist for toughest operations
• Design: imposing appearance
• Cab: functional workplace and comfortable living quarters
• Engine: Mercedes-Benz OM 473 with 3000 Nm
• Power transmission: the only heavy-duty tractor with 16 gears
• Turbo retarder clutch: unique start-up and braking system

The new range of SLT heavy-haulage tractors and severe-service vocational chassis has been developed by Mercedes-Benz to meet the requirements of this highly demanding transport segment. Customers opting for the new heavy haul models will benefit from highly specialized trucks that are custom built in-house by Mercedes-Benz with perfectly matched components: engines, transmissions and axless all bearing the Mercedes star that deliver ultimate performance.

The base SLT chassis is manufactured at the Mercedes-Benz plant in Wörth, Germany, and completed at the company’s CTT (Custom Tailored Trucks) subsidiary in Molsheim, France.

Made-to-measure heavy-duty tractor unit – ready for any application

The new SLT heavy-haulage range is available in a wide range of drive and axle onfigurations. The air-sprung Actros SLT tractor with the spacious GigaSpace and BigSpace cabs measuring 2.5 m in width.

The steel-sprung Arocs SLT tractor based on the robust Arocs “Grounder” platform is also available with the BigSpace cab as well as the StreamSpace cab measuring 2.3 m in width.

The air-sprung Actros SLT is available as:

Actros SLT 6x4, wheelbase 3400 mm or 4000 mm

Actros SLT 8x4, wheelbase 4000 mm

The steel-sprung Arocs SLT is available as:

Arocs SLT 6x4, wheelbase 3300 mm or 3900 mm

Arocs SLT 6x6, wheelbase 3900 mm

Arocs SLT 8x4, wheelbase 3900 mm

Arocs SLT 8x6, wheelbase 4200 mm

Arocs SLT 8x8, wheelbase 4850 mm

The four-axle variants form the core of the range. In this configuration, the technically permissible gross vehicle weight is 41 t, while the individual axle loads (from front to rear) are nine, eight and 2x13 t. For export markets, higher tractor GVWs up to 48 t for the Arocs 8x8 are possible. The permissible gross combination weight for all models stands at 250 t.

The SLT is an extremely robust high-tech heavy-haulage vehicle designed to cope with the highest levels of stress and strain. For operators who don’t contend with such extremes and who operate with gross combination weights under 120 t in predominantly easy-going terrain, the SLT is also available as a semi-heavy version without turbo retarder clutch and without an auxiliary radiator system. A total truck and trailer weight of 150 t is then also possible, subject to certain restrictions in the course of individual approval procedures, depending on the equipment on board and the area of operation.

Elegant but powerful design

A defining feature of the new heavy-haulage vehicle's impressive appearance is its powerful design. Both attractive and functional are the side panels on the cooling tower behind the cab, with large cooling air intakes on the right-hand side. These flaps in glass fibre-reinforced plastic take up the lines of the cab, lending it a longer appearance. In addition to the cooling system for engine and turbo retarder clutch, the cooling tower also accommodates the hydraulic system, compressed air tank and fuel tank.

While unmistakeably based on the Actros and Arocs, the SLT heavy-haulage vehicle optionally sports a distinctive characteristic in the form of a bumper centre section featuring an attachment bracket for a heavy-duty coupling to be deployed in pushing mode. Hydraulic and brake connections are provided at this point for such applications. Steps are additionally integrated for servicing work.

SLT cab: functional yet comfortable

The heavy haulage vehicle's developers have chosen three spacious long-haul cabs for the SLT from the diverse range of cabs available for the Actros and Arocs. Mercedes-Benz supplies the air-sprung Actros SLT with the GigaSpace and BigSpace cabs in a width of 2.5 m. The GigaSpace cab boasts a particular abundance of space inside, with up to 2.13 m of headroom, ample freedom of movement and impressive stowage capacity. The steel-sprung Arocs SLT is available with the spacious BigSpace cab offering 1.99 m of headroom. The Arocs interior is especially functional and easy to care for.

Like all Mercedes-Benz trucks belonging to the new generation, the Actros SLT and Arocs SLT benefit from an ergonomically perfect and driver-oriented cockpit with readily legible instruments, an informative colour display and a multifunction steering wheel. The driver's comfort suspension seat boasts a large seat surface and a backrest with shoulder support. The seat features diverse adjustment options. A climatised suspension seat with massage function is optionally available.

The comfortable beds on board the heavy-haulage vehicle fit the bill for restful sleep. The beds are 750 mm wide and up to 2200 mm long. A second bed of the same quality is optionally available on the upper level in the heavy-haulage vehicle.

Stowage compartments with a capacity of up to 558 liters are available under the bed. These are accessible from outside via large stowage flaps on the left and right. To this end, the air intake for the engine runs behind the cab along the right-hand side of the vehicle. A refrigerator can optionally be integrated in the spacious drawers of the centre section under the bed. Large, locking stowage compartments above the windscreen can hold up to 332 liters of luggage.

Top engine for the SLT: Mercedes-Benz OM 473

The Mercedes-Benz SLT is powered by the Mercedes-Benz OM 473 in-line six-cylinder engine. 15.6 liter displacement, maximum 460 kW (625 hp) of output and up to 3000 Nm of torque are the credentials of the BlueTec 6 engine. Superlative performance and robustness are outstanding characteristics of this engine.

Around 2500 Nm of torque are on tap directly from just above idling speed on all variants.

The engine's rated speed is exceptionally low, at just 1600 rpm. This means vast power in the main operating range. This gives rise to unusual flexibility and pulling power – a major benefit above all in heavy-duty transport. The engine's outstanding performance also extends to its spontaneous and powerful response to accelerator movements.

OM 473 (DD16 variant) in three power ratings

Output / Maximum torque

380 kW (517 hp) at 1600 rpm / 2600 Nm at 1100 rpm

425 kW (578 hp) at 1600 rpm / 2800 Nm at 1100 rpm

460 kW (625 hp) at 1600 rpm / 3000 Nm at 1100 rpm

Among the technical treats that the engine holds in store is the turbo-compound system. This is one of the prime reasons for the engine's high performance. The term "turbo-compound" refers to a second turbine downstream of the exhaust gas turbocharger.

To achieve a further increase in efficiency, this second turbine uses the energy which is still available in the exhaust gas after passing through the turbocharger. The power recovered from the energy contained in the hot exhaust gases is transmitted via a shaft and a hydrodynamic clutch to the engine's gear drive and thus directly to the crankshaft.

The turbo-compound system is directly tangible to the driver in the form of yet more spontaneous engine response right from the low rev range. At the same time, the system also results in fuel savings when operating at the high loads which are typical for heavy-duty tractors.

The supercharged decompression engine brake on the OM 473 which goes by the name of "High Performance Engine Brake" is a boon in heavy-duty transport. The driver activates the brake in two stages via the right-hand steering-column lever. The engine brake attains an impressive output of up to 475 kW (646 hp). This enables the driver to keep even the biggest masses under control on downhill gradients without incurring any wear.

Unique: heavy-duty tractor with 16 gears

The engine power is transferred by the Mercedes G 280-16 transmission with PowerShift automatic gearshift – the only automated transmission (AMT) with 16 gears on a heavy-duty tractor. 16 gears with close ratios ensure that the appropriate gear is available at all times.

Ratios of 11.70 in first gear and 0.69 in top gear result in a large spread of 16.96. For use in the SLT the developers have beefed up the transmission so that the full tractive force of 3000 Nm delivered by the engine also can be transmitted in the lower gears.

The heavy-haulage vehicle benefits from the latest Mercedes PowerShift 3 generation offering extremely simple operation by steering-column selector lever. The automatic gearshift operates with speed and precision – shifting takes place up to 20 percent more quickly than on the previous model.

Particularly when handling heavy loads on steep gradients, swift gear-shifting ensures the appropriate follow-on speed in the next-highest gear. PowerShift 3 also detects when the vehicle is operating in deceleration mode on a downhill gradient, maintaining the engaged gear in the interests of the maximum braking effect for the engine and the turbo retarder clutch.

Transmission mode "Heavy"

Various driving programmes and driving modes are tailored specifically to the heavy-haulage vehicle's areas of operations. Buyers of the SLT can choose between the driving programmes "Power" (gear-shifting takes place at an increased engine speed) or "Heavy" (swift gear-shifting with a further increased engine speed level). Various driving modes are available within the respective driving programmes.

Mercedes-Benz has developed the "Heavy" driving mode especially for the heavy-haulage vehicle. Here the shift points are set even higher compared with "Power". Shifting takes place particularly quickly in accordance with the prevailing torque requirements, and gears are not generally skipped. In this mode, the heavy-haulage vehicle copes effortlessly with even the most difficult driving situations, such as moving off on ten percent uphill gradients with a gross truck and trailer weight of 250 t.

The "Standard" driving mode in the driving programme "Heavy" stands for a particularly economical style of driving, on empty runs or when transporting light loads, for example. This driving mode already adapts to the high gross vehicle weights of a heavy-duty tractor. In "Manual" driving mode, drivers are able to select the gears themselves. The driver can switch off the transmission's standard-specification EcoRoll function.

Turbo retarder clutch: unique start-up and braking system

The turbo retarder clutch is one of the special features of the Mercedes Benz Actros and Arocs SLT. It combines a hydraulic start-up clutch and a retarder in a single component. The turbo retarder clutch is more compact in design than other start-up systems. It is lighter while at the same time serving as a highly effective permanent brake. The turbo retarder clutch enables precision and wear-free starting up as well as manoeuvring at the lowest speeds – both of which merits are of outstanding importance for a heavy-duty tractor.

Starting up at the maximum gross weight requires engine speeds in the best traction range, around 1100 to 1300 rpm for a heavy-duty tractor. When the driver steps on the accelerator, compressed air pumps fluid into the turbo retarder clutch, establishing a frictional connection between engine and transmission input shaft.

This form of power transmission offers the benefit of fast yet gentle and wear-free transmission with high slip at full engine torque of up to 3000 Nm. The quantity of fluid is controlled, enabling torque transmission to be adapted to the individual driving situation and the accelerator position.

Directly after the start-up procedure, the turbo retarder clutch is bypassed and the fluid is discharged from the housing by means of centrifugal force – frictional connection between engine and transmission is now effected with maximum efficiency via a conventional friction clutch.

Depending on the load, gradient and selected programme, the laden heavy-duty tractor will now operate in first or second gear. As there is no starting-up with a slipping clutch on the heavy-haulage vehicle, a single-plate dry clutch is employed. On the semi-heavy version without turbo retarder clutch a two-plate dry clutch is used.

New turbo retarder clutch generation with enhanced dynamics

The new heavy-haulage vehicle features the second generation of the turbo retarder clutch, boasting a further increase in dynamic performance and enhanced stability. A significant reduction in the time required to discharge the fluid in the turbo retarder clutch leads to faster subsequent shifting and increased acceleration of the laden vehicle combination. The fast build-up of torque prevents the vehicle from unintentionally rolling back when moving off on uphill slopes.

The turbo retarder clutch additionally incorporates sophisticated control functions. Pre-filling of the hydrodynamic clutch housing with fluid takes place in maneuvering mode – for example when the vehicle is stationary on an uphill gradient with a gear engaged, in the interests of dynamic starting.

On uphill gradients with a forward gear engaged, the sensitive controls enable the laden vehicle combination to be rolled back deliberately, controlling the intensity of this function with the accelerator pedal. "Abseiling" in this way enables the vehicle to be rolled back as appropriate without using the brakes and thus without consuming compressed air. At the same time, the heavy-haulage vehicle is also able to move off again quickly.

Turbo retarder clutch brake power 350 kW (476 hp)

During braking, the turbine wheel is fixed in position and fluid is pumped into the housing once again – in this case the turbo retarder clutch acts as an effective engine-speed retarder.

With vehicle combinations weighing in at up to 250 t, in many driving situations the available brake power is actually more crucial than the engine output. The heavy-haulage vehicle benefits firstly from its engine's vast brake power of up to 475 kW (646 hp) with the High Performance Engine Brake. The integrated engine-speed retarder of the turbo retarder clutch contributes an additional 350 kW (476 hp).

To ease the strain on the transmission, the sum total power of the two high performance brakes on the heavy-haulage vehicle is limited to a system brake power of 720 kW (979 hp). This power is not limited to the rated speed, however, but is available across a broad rev range, guaranteeing the utmost in safety and the minimum possible strain on the service brakes even when driving downhill at the maximum gross combination weight.

The driver can firmly rely on the vehicle's high continuous braking power in practice, as the installed cooling system with the cooling tower behind the cab in conjunction with the standard cooling system under the cab discharges the generated heat with ease. Even outside temperatures of 50 degrees Celsius in conjunction with maximum engine output at low speed fail to push the cooling system to its limits.

Cooling tower with exclusive special equipment

The elegantly enclosed rear cooling tower behind the cab houses an array of special heavy-duty equipment. The cooling system combines the cooling for the engine and the turbo retarder clutch in a single water circuit.

This boosts the cooling capacity and, in turn, the performance capabilities of the heavy-haulage vehicle. Conversely, the common circuit enables the engine cooler to be used during braking. The cooling system is controlled via a CAN data bus system.

The cooling system demonstrates once again the advantages of fully integrated development from a single manufacturer: the radiators and fans have been adopted from Mercedes-Benz buses, while the water pump and compensation tank derive from Mercedes-Benz trucks. This reduces the number of complex special components and simplifies the stocking of replacement parts.

For optimum operation of the cooling system, the cooling air intake is integrated in the side paneling on the right-hand side. This ensures that even when travelling at low speeds air is drawn in at ambient temperature, rather than taking in the vehicle's own exhaust air.

Separate hydraulic fluid circuit for tractor and semitrailer

The cooling tower also houses the hydraulic system. This is operated by the engine's PTO. The new heavy-haulage vehicle incorporates separate circuits for the hydraulic fluid of the tractor (auxiliary water pump, fan drive) and the semitrailer respectively. This prevents unspecified or even contaminated fluid from a semitrailer from entering into the tractor's circuit. At the same time, this configuration also ensures that the two circuits can be operated at full load independently of one another. Three inspection windows show the fluid levels.

An aluminium fuel tank with a capacity of 900 l is accommodated in the bottom level of the cooling tower. This ensures a large range – mindful that it is no simple matter for a bulky and heavy heavy-duty vehicle combination to pull up at a filling station. The formula is simple: the vehicle combination must be capable of completing a night-time stretch without calling in at a filling station.

The compressed air tank, a fuel cooler and a cooler for the hydraulics are also housed here. Three floodlights at the top of the cooling tower illuminate the rear of the vehicle at night.

The robust backbone of the SLT

A reinforced frame is the robust backbone of the new heavy-haulage vehicle. Depending on the variant concerned, the frame is 834 mm wide overall with longitudinal members measuring 8 mm in thickness (Actros SLT with air suspension) or 744 mm wide with longitudinal members measuring 9 mm in thickness (Arocs SLT with parabolic suspension). Reinforcements developed especially for the heavy haulage vehicle are additionally incorporated. The longitudinal members are unwelded and consist of cold-worked fine-grained steel, guaranteeing maximum loading capacity and stability.

There is space on the left side of the frame for the AdBlue tank with a capacity of 60 l (Arocs) and 75 l (Actros), and on the right for the exhaust system. Between AdBlue tank and leading axle there is room for a spacious stowage box. The so-called "catwalk", a platform behind the cooling tower, provides a safe and convenient means for the driver to walk on the frame in front of the fifth-wheel coupling.

Flexible SLT: adjustable fifth-wheel coupling, two heavy-duty couplings

The new heavy-haulage vehicle is available with various fifth-wheel couplings. An adjustment device is also optionally available to optimize the axle loads according to the given payload. For pushing and towing operations – when in use as part of a heavy-duty train, for example – the new heavy-haulage vehicle can be equipped with attachment brackets for heavy-duty couplings at front and rear. In this case, the Rockinger 56 E coupling is used.

The heavy-haulage vehicle is also optionally available with a ballast platform. This ensures the necessary traction when using drawbar trailers or when the heavy-haulage vehicle performs a towing and pushing function as part of a train comprised of several heavy-duty tractors.

16 tons axle load for the drive axles

The drive axles on the heavy-haulage vehicle take the form of extremely robust planetary axles produced in-house at Mercedes-Benz. Their standard of engineering and technology has been tried and tested in millions of applications in the toughest conditions.

The standard ratio is 5.333. With the commonly employed tyres in size 315/80R22.5 for the drive axles, this results in an engine speed of 1500 rpm at a vehicle speed of 80 km/h. This predestines the heavy-haulage vehicle for a high level of tractive power. Additional final drive ratios between 4.833 and 6.000 enable the heavy-haulage vehicle to be tailored precisely to the individual customer's wishes and operational needs. The drive axles are designed for a technically permissible axle load of up to 16 t.

The rear axles on the Actros SLT are air-sprung as standard. The hallmark is the four-bellows air suspension which is familiar from the Actros. In conjunction with the wide frame, this suspension ensures good road holding, facilitates loading and unloading, reduces noise levels when the vehicle is unladen and, not least of all, provides for enhanced ride comfort. The Arocs SLT features parabolic springs on all axles.

These can be fitted with various spring packs, according to the type of deployment for the vehicle.

Front axle with three load capacity ratings and Servotwin steering

The front axle is identical in design on the Actros SLT and Arocs SLT (6x4 and 8x4), with parabolic springs and disc brakes all-round (with the exception of the all-wheel-drive variant). This axle is available for the Actros and Arocs with a load capacity of 7.5, 8.0 or 9.0 t.

The special technical features of the SLT 8x8 include its steering, the new electro-hydraulic Servotwin steering system for four-axle trucks, which provides speed-sensitive steering power assistance and active steering return. In addition to offering unprecedentedly smooth, easy and precise steering, this also ensures that the heavy-haulage vehicle is fully steerable when stationary, for perfect manoeuvring.

Leading axle: air-sprung and steered

Other special features of the new heavy-haulage vehicle include the electro-hydraulically steered and air-sprung leading axle which is produced in-house for the four-axle models. This axle, including its suspension, is based on a front axle from the trucks range. It has a load capacity of up to 8.0 t.

Both the front axle and the leading axle can be factory-fitted with tyres in size 385/65R22.5. The wheelbases of the three- and four-axle trucks have been kept very short in the interests of maneuverability, with the installation space requirement of the exhaust system for compliance with the Euro VI emissions standard imposing the sole restriction.