A modular hydraulic trailer is a heavy-load platform that can be extended to suit the weight of the cargo by coupling modules made up of axle lines lengthwise and side by side. Each axle line shares the load evenly through hydraulic suspension and can be steered. This makes it possible to move transformers, reactor equipment or bridge girders weighing hundreds of tonnes by road with a tractor unit and, where necessary, a pusher vehicle at the rear.
How does it differ from a conventional trailer?
A lowbed trailer is limited by the number of axles and the capacity it was built with. A modular trailer, by contrast, is made up of building blocks: modules carrying two to eight axle lines are coupled together, and if necessary a second row is added alongside. In Goldhofer's THP/SL series, for example, the module width is 3.0 m, the axle spacing 1.5 or 1.8 m, the steering angle ±55° and the axle compensation 600 mm. The manufacturer gives the technical load capacity per axle line as 45 tonnes at a speed of 1 km/h. This value is an engineering limit; the value permitted on the road is much lower.
How does the hydraulic suspension work?
Each axle line rests on a pendulum axle pivoted at its centre and is connected to a hydraulic cylinder. Because the cylinders in the same group are connected to a common oil line, the pressure is equalised across all of them. Even if the road dips at one point, that axle drops down, the others continue to share the load, and roughly the same load bears on each wheel. The platform height is also adjusted hydraulically; the vehicle is raised over bumps and ramps and lowered under low passages.
Three-point and four-point support
When the cylinders are divided into three hydraulic groups, the platform is supported at three points, like a tripod. This arrangement makes the load statically determinate and prevents the chassis from twisting on uneven ground. A four-group arrangement, on the other hand, provides a wider support area against tipping. The choice is made by assessing the height of the cargo's centre of gravity together with the cross slope of the road.
How is the combination calculated?
- Find the total weight: Cargo, trailer tare weight, gooseneck, bridge girder and chocks.
- Find the permitted axle line load: The calculation is based not on the technical capacity but on the value permitted by the roads and bridges along the route; this value is set in the permit conditions.
- Work out the number of lines: The total weight is divided by the permitted line load and rounded up.
- Choose the width: A single row gives a width of 3 m, a double row 6 m; the double row is more stable but blocks more of the road.
- Check the traction: The number of tractor units is determined by the total train weight on the steepest gradient.
A simple example: a 180-tonne transformer is to be moved, the tare weight per axle line is about 3.5 tonnes and the permitted line load is 12 tonnes. Since each line can carry 12 − 3.5 = 8.5 tonnes of net load, 180 / 8.5 ≈ 21.2; the combination needs at least 22 axle lines. When the gooseneck and intermediate equipment are added, the number rises. Details of the distribution are in the entry on axle load calculation. For very long and heavy loads, the cargo is suspended between two trailer groups to form a girder bridge trailer arrangement.
When is a pull-push arrangement needed?
When the train weight reaches several hundred tonnes, a single tractor unit may not be enough for traction on gradients and for braking on descents. In that case a second tractor unit is coupled at the rear: it pushes uphill and assists braking downhill. The two drivers work in sync by radio. In long combinations, the rear modules are additionally steered by an operator so that the train keeps its track through tight junctions.
Not to be confused with an SPMT
A towed modular trailer is designed to cover kilometres on public roads. The vehicle that moves under its own power pack and positions loads with millimetre precision on site is the SPMT. On large projects both are used in the same operation: the road leg with towed modules, the port and on-site leg with SPMTs. Vehicle selection in oversize and heavy cargo transport is made together with the permit and route survey.