AN EXTRA BOOST
SAF TRAK

Expanding market share in axle and suspension systems for trailers is a key component of “Strategy 2020”. This is why SAF- HOLLAND is concentrating on growth through innovation and new solutions for special customer requirements in its core market of Europe. In 2016, the axle specialist developed its first hydraulically driven trailer axle. These axles help tippers, which often find themselves on rough terrain, gain significant traction, which can otherwise only be achieved with the help of tractors with more than one driven axle line.

Energy from the tractor for the trailer

For added thrust, SAF TRAK uses the hydraulic power available, which is being held ready for use for the tipping cylinder.

A gravel pit in the Spessart hills. It’s autumn. A drizzle falls over the countryside softening the soil. Several engineers watch a tipper drive slightly uphill. The drive axle of the tractor starts to slip. The vehicle slows down and eventually gets stuck. “Usually at this point, the driver would ask for help, possibly opting to be towed by a wheel loader,” said Dr Stefan Wallmeier, Vice President of Engineering at SAF-HOLLAND. But in this case, Dr Wallmeier and his team have equipped the tipper with the SAF TRAK. At the push of a button, the driver can activate the wheels rotating on the trailer’s rear axle and boost the vehicle almost effortlessly out of the mud.

The driver can turn the pull-away assist on and off using a simple switch in the cab.

Approx.

10km/h

is the level currently set to trigger the pull-away assist function. The drive then switches off, and a frewheel function minimizes the drag losses of the hydraulic motors. SAF-HOLLAND's developers are currently working on optimizing the threshold speed which is also depending on the system components.

Only

200kg

of added weight is contributed by the drive system. The weight of the axle drive is negligible because it is based on SAF-HOLLAND’s existing suspension system technology.

A compact hyraulic motor in the inner wheel provides an extra boost – in this case for the rearmost trailer axl.

The crucial extra boost is ensured by two hydraulic radial piston motors located in the wheel hub of the powered trailer axle. The drive system uses the same hydraulic pressure already provided by the tractor to raise the cart body. A valve block disperses the hydraulic fluid as required either for tipping or powering.

“When developing the new trailer drive, our engineers were able to build on the experience already gained from hydraulically driven pendulum axles for vocational vehicles.” Dr Stefan Wallmeier, Vice President Engineering at SAF-HOLLAND

If the driver chooses the trailer’s drive, then hydraulic fluid is directed to a second valve block. This valve block synchronizes the direction of the axle drive with the tractor’s engine control. It can also power the right or the left wheel of the trailer axle separately if required – known as the differential lock function. This function is important if one of the wheels is on a slippery terrain and not able to transmit torque.

This is how a hydraulic motor works:
In a radial piston motor, the hydraulic pressure drives the pistons, which act outwardly on a specially curved track. This is how the pistons make the motor rotate.
The hydraulic motors are fitted on the inside of the wheels, usually on the rearmost axle of the trailer. If the tipper is fully loaded, then this axle must endure a particularly high amount of weight. This allows the trailer driver to push the entire vehicle in order to make it up a slope or to free the vehicle from muddy terrain.

Hydraulic motors are well-suited for providing a boost in difficult terrain because of their very high torque. SAF-HOLLAND’s axle drive supplies up to 14,000 Newton meters of additional torque depending on the system pressure. If this force is no longer needed at a speed of roughly 10 km/h after the start-up, SAF TRAK will then proceed to activate a freewheeling function in which a spring pulls the pistons back in the hydraulic motor to minimize drag losses.

“The core element of the hydraulic trailer drive is our proven INTRA CD suspension system equipped with disc brake technology," explains Wallmeier. “The unit only needs to be extended at the ends of the axle by adding another stub axle, hub unit prepared to receive the motor, a sealing system and controls.” This is why the hydraulic drive, which won the “Trailer Innovation 2017“ award at last year’s IAA, can also be retrofitted onto existing trailers and combined with numerous tractors. Another plausible use for these drives, aside from their use in tippers, is their use in other vehicles that provide a high level of hydraulic power – for example, agricultural machinery, forestry vehicles and vehicles with moving floor.

Energy from the tractor for the trailer
For added thrust, SAF TRAK uses the hydraulic power available, which is being held ready for use for the tipping cylinder.

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