Mercedes-Benz Arocs Vocational Trucks with HAD - All-wheel drive at the push of a button

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Press Release / May 26, 2015

Video: https://www.youtube.com/watch?v=t3lqxab8Afs

Some need it all the time, others never and many every now and then. We are referring to all-wheel drive for construction vehicles.

As a specialist among construction vehicles, the Mercedes-Benz Arocs is as individual as the requests of its customers. This applies to its numerous axle configurations, engines and cabs, but to its drive system as well:

Mercedes-Benz offers the Arocs range without all-wheel drive for predominant use on the road and in mild terrain, with on-demand all-wheel drive for terrain of medium difficulty and with permanent all-wheel drive for heavy-duty off-road use.

The Arocs with Hydraulic Auxiliary Drive (HAD) now rounds out the product line-up as another variant. HAD is an ingenious hydraulic auxiliary drive of the front wheels for occasional off-road use that is setting new standards in its segment: it is powerful, lighter than an all-wheel-drive model, easy to operate, maintenance-free and has a long service life. It lowers fuel consumption and consequently protects the environment as the result of low emissions.

Construction vehicles: extremely broad application spectrum

There are construction vehicles that rarely leave the pavement during their service life. Others plough through rough terrain day in, day out. For both specialists the question of what kind of drive system is best is quickly answered.

But there is a third and especially common variant between on-road and off-road: construction vehicles that occasionally venture into terrain or that have to master a route with demanding terrain or steep ramps from time to time and therefore need an additional drive system.

These are solo vehicles with tipper bodies, roll-on/roll-off tippers, tractor vehicles with trailer, tractor units with tipper semitrailer and sliding floor semitrailers, as well as tractor units with low-bed trailers for heavy-duty construction machines or construction vehicles in areas with heavy snowfall.

The range of applications is very broad. There are classic construction vehicles carrying bulk materials. While payload is paramount for these vehicles, loading and unloading requires high traction. Roll-on/roll-off tippers require a traction aid when unloading on unpaved or slippery ground. Lumber transports have to navigate challenging routes in the forest, agricultural transports have to do the same in fields. Most of these trucks amass the majority of their kilometres on the road, but without outstanding traction they are unable to do their job. This is especially true when the truck is also used on the side for winter road maintenance during the cold months. And then there are the regions with heavy snowfall or areas with poor roads. For all these uses that require an all-round talent, the Mercedes-Benz Arocs HAD is the tailor-made vehicle.

Hydraulic Auxiliary Drive: additional drive system at the push of a button

The abbreviation HAD stands for Hydraulic Auxiliary Drive, an additional drive system. It is only used when needed and its technology is very different from a classic mechanical all-wheel-drive system with transfer case, drive shaft, differential and half shafts on a permanently or on-demand driven front axle.

The advantages of the hydraulic drive are the comparatively low added weight, benefits in terms of fuel consumption and emissions, and great versatility in the configuration of the vehicle. In the case of Mercedes-Benz, these qualities are complemented by freedom from maintenance, low fuel consumption with activated hydrostatic drive and, above all, the combination with the Mercedes PowerShift 3 transmission. Hydraulic Auxiliary Drive from Mercedes-Benz is a proprietary development using components from Poclain, a globally leading manufacturer of hydraulic drive systems. The appropriate components were adapted and integrated into the vehicle. In particular the crucial control technology was created by Mercedes-Benz.

Simple operation: a push of a button will do

The operation of Hydraulic Auxiliary Drive is quite simple for the driver: all it takes is a push of a button in the cockpit and the auxiliary drive is active. This can be done while still on the road before entering the construction site or at any time in terrain. In this way, the driver is able to master even unforeseen situations with the Arocs HAD.

The drive system offers extremely flexible use: the driver is able to engage differential locks independent of activating the Hydraulic Auxiliary Drive. This makes it possible to operate solely with the rear differential lock engaged and without auxiliary front-wheel drive as the situation allows.

Wide range of variants with two and three axles

Hydraulic Auxiliary Drive is available for the Arocs in a multitude of variants. First of all, for the axle configurations 4x2, 6x2 with trailing axle and 6x4, each equipped with air-sprung rear axles.

Two engines, each with four closely staggered output levels, are available to choose from: the Mercedes-Benz OM 470 with a displacement of 10.7 litres and outputs from 240 kW (326 hp) to 315 kW (428 hp) and the OM 471 with a displacement of 12.8 litres and outputs from 310 kW (421 hp) to 375 kW (510 hp). The power is sent to the wheels in by fully automated Mercedes PowerShift 3 transmissions with 12 and 16 gears.

With ClassicSpace cabs in three lengths, CompactSpace, StreamSpace and BigSpace cabs as well as cab widths of 2.3 and 2.5 metres, the line-up of cabs also leaves nothing to be desired.

Hydraulic Auxiliary Drive is also available for the Actros and Antos in all these specifications. While there is less demand for all-wheel drive in the case of these model series, there is a call for a powerful additional drive system in certain types of operation, for example in heavy-duty distribution transport. Where an additional road sign to that effect exempts all-wheel-drive vehicles from snow chain laws, HAD eliminates the need for the time-consuming mounting of snow chains – for example, a case for distribution trucks with tyres of size 385/65 R 22.5 on demanding mountain roads.

The sophisticated technology of the hydraulic drive

The essential components of Hydraulic Auxiliary Drive comprise a high-pressure pump, a front axle with wheel hub motors, a side module and a valve block. The hydraulic components are interconnected by a system of hydraulic lines with different pressures.

The high-pressure pump is the heart of HAD

A powerful high-pressure pump is the heart of Hydraulic Auxiliary Drive. It is positioned in a central location on the engine and driven directly by its gear drive. The pump has an output of up to 112 kW and delivers a flow rate of up to 350 litres per minute at a maximum pump pressure of 450 bar.

The front axle of the Arocs HAD is the familiar steel-sprung axle with disc brakes. From the outside, HAD can be identified by the modified wheel ends compared with a conventional drive system. The axle is available with two offsets, which are added automatically in dependence of the model series. For the integration of the wheel hub motor on the left and right, the stub axle and the wheel end were newly developed.

Powerful radial multi-piston motors with ten cylinders

The hydraulic wheel hub motors convert hydraulic pressure into mechanical work. They are radial multi-piston motors with centrally positioned power unit and ten cylinders arranged in a circle. Their pistons with rollers at the tip are alternatingly pressed outward against a cam ring by hydraulic pressure. The counterforce creates a torque that drives the front wheels. The wheel hub motors have an intake capacity of 934 cubic centimetres, a power output of 40 kW each and a peak torque of 6250 Nm per wheel.

The wheel hub motors are supplied with hydraulic oil directly through the axle bolt and the axle journal. A rotary distributor in the axle stub protects the high-pressure hydraulic lines against twisting when steering. The system only has to compensate for the suspension motions and not for steering motions – a major plus in favour of a long service life.

Hydraulic system generates a maximum pressure of 450 bar

The hydraulic system consists of three circuits. The high-pressure system supplies the working pressure of up to 450 bar for driving the wheel hub motors in the front axle. The low-pressure system operates with a pressure of up to 30 bar and serves to control the system. It holds the pistons of the wheel hub motors in rest position when they're not needed. The low-pressure circuit is furthermore connected to a separate cooler. The third component is a non-pressurised leakage line that returns excess hydraulic fluid to the reservoir.

All in all, 32 litres of oil circulate through the hydraulic system. Mercedes-Benz uses the synthetic transmission fluid familiar from its trucks. Its operating range from minus 40 to plus 90 degrees Celsius covers even extreme job conditions. When it is cold outside, a special HAD software routine automatically heats the oil to the required minimum system temperature of about 15 degrees Celsius when HAD is activated. This ensures a high level of system availability.

A cooling module prevents excessively high temperatures. It is installed in a side module on the right side of the frame. The cooling module has a high cooling capacity of about 20 kW. It consists of an oil cooler and a fan in one unit mounted in an upright position, the hydraulic reservoir and an oil filter.

The valve control block plays a crucial role. It is also housed in the side module below the frame and contains all valves required for controlling the hydraulic drive. It also transfers a constant amount of oil from the high-pressure circuit to the low-pressure circuit for cooling. The pressure sensors of the different circuits and a temperature sensor are also integrated into the valve control block. The cast steel design is weight-optimised. Compared with milled valve blocks, it offers weight savings of about 35 kilograms.

Transmission control unit – the brain of HAD

"The brain" of Hydraulic Auxiliary Drive is the control module with the transmission control unit (TCU). It controls the pump, the valve control block and the fan, ergo the entire drive system. A major advantage of the control system developed by Mercedes-Benz is that in dependence of the driving situation it only builds up as much torque on the front axle as is actually needed for traction. The basis for this control system is provided by the vehicle sensors. Among other things, the system knows how much slip there is, how heavy the vehicle is and the tilt angle of the truck.

Compared with drive systems with simple on-off control, the fine control of Hydraulic Auxiliary Drive offers traction advantages because it only generates as much power as needed in every situation. At the same time, this strategy increases the service life of the drive technology and noticeably lowers fuel consumption and consequently also CO₂ emissions.

Driving made easy: drivetrain with Mercedes PowerShift 3

The trend towards automated transmissions has long since made it to construction transport as well: more than 90 percent of the construction vehicles from Mercedes-Benz are equipped with the fully automated Mercedes PowerShift 3 transmission (aka. Detroit DT12). It is part of the standard equipment of the current vehicle generation.

Among the major advantages of Hydraulic Auxiliary Drive from Mercedes-Benz is therefore that, contrary to similar systems, the auxiliary drive is coupled with the fully automated Mercedes PowerShift 3 transmission. Especially in off-road operations this protects the major powertrain components from the engine and clutch to the transmission and the drive axles. In addition, drivers encounter their accustomed operating concept and can fully concentrate on the job of steering the truck through rough terrain.

HAD also makes wear-free braking easier. Because the pump of the hydraulic system is driven by the engine PTO, vehicles with HAD can optionally be equipped with the equally powerful and lightweight secondary water retarder.

The big plus is the torque-controlled drive system

Contrary to comparable systems, Hydraulic Auxiliary Drive is controlled by torque. It always supplies precisely the right amount of torque that is needed. The corresponding request is controlled via the pressure on the accelerator. This makes spectacularly and pointlessly spinning wheels a thing of the past. Furthermore, HAD intervenes immediately, dependent on any potential slip of the rear wheels.

In addition, the pressure in the system is increased automatically when starting. This ensures the necessary breakaway torque in rough terrain or on uphill grades.

The auxiliary drive is available up to a speed of 25 km/h. At higher speeds, no additional drive system or traction boost is required. Activated Hydraulic Auxiliary Drive then goes into passive mode for this reason. Once the speed drops back below 25 km/h, HAD again automatically intervenes and ensures forward progress. This strategy also makes the driver's job easier. HAD is also active when reversing in gear one and two.

Full tractive force even during gear changes

To ensure maximum traction, the transmission will never skip any gears with activated HAD. Another advantage of HAD becomes apparent during gear changes: by driving HAD from the engine PTO, interruptions in tractive force are prevented during gear changes. The full tractive force is always available on the front axle.

Once the truck travels faster than 60 km/h, HAD is automatically deactivated completely. This protects the major components and minimises fuel consumption and emissions. The auxiliary drive is also deactivated automatically after the ignition is switched off. This ensures that the truck isn't started unnecessarily with activated HAD the next day.

The central display in the instrument panel keeps the driver always informed about the current status of the front-wheel drive: if the appropriate indicator shines white, Hydraulic Auxiliary Drive is activated but no torque is being commanded at the moment. If the indicator shines blue on the other hand, HAD is activated and boosts forward progress. The system status is also indicated by an LED in the switch.

The high cooling capacity of 20 kW ensures continuous operation in terrain at speeds of up to 15 km/h. At higher speeds, only a small amount of torque is transmitted so that HAD is fully operational for about five minutes. As a result, virtually all conceivable operations of construction vehicles in mixed use are covered.

Among the other strong points of the Arocs HAD is its great manoeuvrability. Contrary to a mechanical all-wheel-drive system, HAD does not limit the steering angle. It is identical to that of a truck with rear-wheel drive. The steering forces also are identical and due to the decoupled front-wheel drive no strain in the drivetrain is possible. If needed, drivers can additionally mount snow chains on the front wheels of a Mercedes-Benz truck with HAD.

HAD: high traction, low fuel consumption

As with all Mercedes-Benz Arocs trucks, the Arocs with Hydraulic Auxiliary Drive also benefits from its perfectly calibrated and highly efficient powertrain from a single source. And there is an added two-fold advantage: firstly, the on-demand additional drive system increases fuel consumption in standard operations only minimally. Secondly, the system developed by Mercedes-Benz offers another important specific benefit: the auxiliary drive operates in controlled mode. Contrary to comparable drive systems, the fuel consumption is only increased by the amount of power the driver requests.

The sample calculation for an Arocs 1843 LS – it is popular in construction site operations pulling a tipper semitrailer – looks as follows: on the road without activated auxiliary drive, fuel consumption with Hydraulic Auxiliary Drive is only 1.5 percent higher than that of a rear-wheel-drive truck with identical transmission and

hypoid axles – while offering enormous traction reserves at the same time. The basis for the calculation is a typical application profile with a large percentage of country roads and less use on the motorway and in the city.

Customers opting for a heavy-duty model of the Arocs HAD with hub reduction axles and overdrive transmission will see an increase in fuel consumption of about two percent under the same conditions over a base vehicle with HAD and driven hypoid rear axle. In contrast, with a conventional on-demand all-wheel-drive system, fuel consumption increases by up to eight percent and by up to ten percent with permanent all-wheel drive over HAD and a hypoid rear axle. That not only saves money, but also results in a better CO₂ balance.

Added weight less than half of permanent all-wheel drive

The bottom line when it comes to the weight of the Arocs with Hydraulic Auxiliary Drive also deserves special attention. It offers a much higher payload than a classic all-wheel-drive system.

The added weight of HAD is just 400 kilograms. The on-demand all-wheel-drive system of the two-axle Arocs weighs twice that much (an additional 425 kg), while permanent all-wheel drive even weighs in at an additional 575 kilograms – the transfer case, differential, drive shaft, the thicker frame and other features of a conventional all-wheel-drive system are no featherweights.

Sophisticated maintenance-free technology saves time and money

Practical people in the construction industry keep an eye on maintenance and repair expenses as well. The Arocs with Hydraulic Auxiliary Drive impresses in this respect too: its drive system is entirely maintenance-free. There are no downtimes or costs.

Hydraulic Auxiliary Drive is designed for the vehicle's service life

Buyers of an Arocs HAD can rely on a solid drive technology: in typical mixed operations as a construction vehicle on the road and in terrain, Hydraulic Auxiliary Drive is designed to last for the service life of the vehicle. This statement has recently been proven realistic for construction vehicles during testing of the Arocs Hydraulic Auxiliary Drive.

Mercedes-Benz has tested the technology extensively in real life on the basis of a wide range of different testing scenarios and application profiles. From trials along the Arctic Circle and customer field tests in rock quarries and underground to the tough test tracks in Wörth, the vehicle was put through its paces extensively during its development.

The Arocs Hydraulic Auxiliary Drive offers crucial advantages even in extreme situations. Should the typically rough use in terrain cause damage to the high-pressure circuit of the hydraulic system, the Arocs HAD unlike others will keep going: the valve control block immediately cuts off the oil supply to the damaged line, dials the high-pressure pump back to zero flow and consequently prevents a total loss of oil.

In addition, the driver is warned of the damage by a message in the instrument panel and an audible warning signal. Neither the wheel hub motors nor the high-pressure pump can run dry. As a result, the still operational conventional rear-wheel drive ensures continuing on to the nearest workshop.

Related reading: http://www.bigmacktrucks.com/index.php?/topic/39998-volvo-unveils-automatic-all-wheel-drive-truck-tech/?hl=hydraulic#entry289755