Loose soil and inclines are challenges for construction vehicles when starting, after tipping or when driving uphill. SAF-Holland has developed the SAF Intra CD Track hydraulically driven axle for application when the drive axle of the tractor unit spins or semi-trailers get stuck. The driven trailer axle uses the hydraulic power already available in the vehicle for added thrust. Thus, it brings the drive to the trailer.
The axle, which has been manufactured as a standard product since 2018, supports the tractor unit in difficult terrain via a hydraulically driven motor. The SAF Intra CD Track axle carries the full axle load and transmits all of the drive torque. According to SAF-Holland, this makes the integrated additional drive an economically viable alternative to 4x4 tractor units, which are expensive to purchase and operate.
Elmar Weber, product manager, SAF-Holland, says: “Hydraulic motors provide a boost in tough terrain because of their very high torque. Depending on the system pressure, our trailer drive supplies up to 14,000 Newton meters of additional drive torque.”
The axle is designed for specific transport requirements in the construction industry and is optimised for trailers in the 9-tonne standard range. The system consists of the SAF Intra CD suspension system with SAF-Integral technology and a control system that is precisely adapted to the integrated motor.
Depending on the application, the additional drive is usually fitted on the rearmost axle of construction vehicles. If a tipper is fully loaded, then this axle has to bear a lot of weight. The trailer drive can therefore push the entire vehicle, in order to cope with an incline or drive safely over loose terrain.
Elmar Weber, product manager, SAF-Holland.
“With the SAF Intra CD Track, you can start on slopes or after tipping, even on soft ground. On-road and off-road vehicles, particularly tippers and walking floor vehicles used at construction sites or landfills, benefit from the hydraulically driven axle. In practical applications, drivers and hauliers have appreciated the extra power, particularly in rough terrain. Now, series production enables us to support even more customers who need to negotiate challenging routes on a regular basis,” says Weber.
To push the semi-trailer through loose terrain or pull it out of muddy ground, drivers activate the additional drive in the tractor at the push of a button. The extra boost is ensured by two hydraulic radial piston motors located in the hub units of the axle. The drive system uses the same hydraulic pressure that the tractor unit uses to raise the cart body. This pressure drives ten pistons which, directed outwards, act on a specially shaped tread surface and set the hub in rotation.
A valve block disperses the hydraulic oil as required for tipping or powering. A second valve block controls the direction of travel for the trailer drive. An integrated flow divider takes on the function of the differential, which is useful if one of the wheels is unable to transmit torque on slippery terrain.
Once the system is activated, it automatically detects the direction of travel and reacts independently: If the vehicle brakes are activated or the maximum speed of 13 km/h is exceeded, free-wheeling mode is enabled. A spring pulls the pistons back in the hydraulic motor to minimise drag losses. When the speed is reduced, for example due to a lack of traction, the drive automatically switches on again.
“The hydraulically driven motor from Poclain is durable and maintenance-free, ensuring that the trailer axle enjoys a long service life. It can be retrofitted onto existing trailers and combined with tractor units that have been prepared for this type of application. Its packaging space corresponds to that of the standard trailer axles from SAF-Holland. Wear parts such as brake pads and discs, air bags and wheel bearings are identical to other suspension systems in the Intra series,” says Weber.
Weber explains the conditions when it’s worthwhile for a contractor to equip trailers with the SAF Intra CD Trak instead of relying on an all-wheel drive tractor unit.
“We will not be able to replace a classic 4x4 tractor unit with the SAF Intra CD Trak. However, we still see good opportunities for our driven trailer axle, because all-wheel-drive tractor units have become fewer and fewer in recent years. In contrast to the pulling additional drives in the front axle, the SAF Intra CD Trak also implements traction more efficiently as a pushing axle. Articulated trucks benefit from these physical advantages, particularly when starting off,” says Weber.
The operating principle of the hydraulic motor in the hub unit is the same, and the manufacturer is also the same in many cases. However, there are two differences in the motor: the engine displacement volume on the SAF Intra CD Trak is greater, which increases the torque, but the possible speed is lower overall at a maximum of 13 km/h. In practice, this means that when tipping or pushing off, the load is shifted to the rear and the permitted axle loads can be exceeded. The torque that the engine then brings to the driven axle is higher than would be the case on a front axle of a truck.
“Our intention when developing this was to establish power vs. speed or to push powerfully from standstill rather than drive fast. Another difference is the fact that the wheel bearings in the SAF Intra CD Trak axle are grease-lubricated, as is standard on European trailer axles. Our goal was to be able to use the same components for wear parts such as brake pads or brake discs as for the other axles from the SAF-Holland Group. This makes it easier for workshops to store and work on the trailer. Semi-trailers with the SAF Intra CD Trak axle will remain in the fleet for a long time, and after more than six years the wheel bearings will certainly need replacing. Although wheel bearings are not classic wear parts, we can still rely on standard parts from SAF-Holland. With regard to the control system of the SAF Intra CD Trak axle, it functions differently from a tractor unit with a hydraulically driven front axle. It is essential that the control system in the trailer is matched to the hydraulic equipment of the tractor unit,” says Weber.
For the additional drive in the trailer to function smoothly, the tractor unit needs a hydraulic system with forward and reverse, a switch plus status and fault light and must be able to transmit CAN bus signals to the trailer. Furthermore, it should have a two-stage tipping valve, which is additionally controlled by a solenoid valve.
“It is important to understand the combination of the SAF Intra CD Trak and tractor unit on an individual basis. Scania Germany, for example, is currently preparing a complete equipment package for our driven axle,” says Weber.
“There are different views about which of the trailer axles should be equipped with the additional hydraulic drive. Most users opt for the rear axle, but a third of customers have also chosen the middle axle. The advantage of the middle axle may be that the third axle might no longer have ground pressure if it is frequently necessary to tip at soft breaklines. Ultimately, it’s the haulier who decides on the positioning of the driven axle when configuring the vehicle,” says Weber.
The driver can activate the SAF Intra CD Trak additional drive at the touch of a button, but the decision of when to use the feature depends on the power take-off of the motor vehicle, when and how it can be switched on.
“The advantage of a power take-off on the engine side is that it can be engaged while the vehicle is in motion. The SAF Intra CD Trak system can be switched on up to a speed of 8 km/h. Once the maximum possible 13 km/h is reached, it switches off, but remains active. If the speed of the vehicle falls below 11 km/h, it switches on again. If an articulated truck is travelling at more than 50 km/h, the system is completely deactivated. Prerequisites for this are an automatic transmission and a power take-off on the engine side,” says Weber.
“The trailer has a speed surplus to the tractor in order to be able to push at all. On dry, non-slip ground, wear is generated, therefore the drive should only be used on loose soil. The tread pattern should be adapted to the application; a coarse tread, for example, does not clog so quickly,” he adds.
Weber points out that retrofitting is technically feasible, but should also make economic sense. “Investment in the driven axle pays off best if the system is already installed ex-works and can be used longer than a comparable tractor unit,” he says.