An SPMT (Self-Propelled Modular Transporter) is a modular transporter that moves under its own power unit, with each axle line steered electronically and independently and hydraulic suspension that balances the load. Modules can be coupled lengthwise and side by side to carry loads of thousands of tonnes. Because it positions loads with centimetre precision at low speed, it is used in ports, shipyards and project sites.
How does an SPMT work?
An SPMT consists of three main parts: axle-line modules, a power pack unit (PPU) supplying hydraulic power to the modules, and an electronic control system. A diesel or electric engine in the power unit drives the hydraulic pumps; hydraulic motors on the driven axles turn the wheels. The operator usually controls the vehicle by remote control while walking alongside it.
Manufacturer data show the capacity range. On Goldhofer PST/SL-E modules, the technical load per axle line is 45 tonnes at 1 km/h, the steering angle ±135°, the axle compensation 600 mm and the tractive force 180 kN per driven axle line; power units range from 155 to 390 kW. Cometto's EVO3 series goes up to 70 tonnes per axle line; the power unit can be diesel, hybrid or electric.
Steering modes
- Normal and double-ended: Front and rear axles turn in opposite directions, so the vehicle turns with a short radius at tight junctions.
- Crab steering: All axles turn to the same angle and the vehicle moves diagonally.
- Transverse movement: The axles turn 90° and the vehicle moves sideways.
- Carousel: The vehicle rotates on the spot around its own centre; the ±135° steering makes this mode possible.
Where is it used?
- In ports, driving heavy cargo from the quay onto a vessel over a ramp and taking it off the vessel (handling oversize cargo in port).
- Moving modules up to their foundations at refinery, power plant and factory sites (on-site final positioning).
- Driving bridge segments and shipyard blocks into place.
- Transferring heavy parts within a facility to points a crane cannot reach.
Capacity and speed limits
Theoretical capacity is the number of axle lines multiplied by the permissible line load. On site, the limit is usually set not by the vehicle but by the ground: the ground must be able to bear the pressure transferred from the wheels. For this reason, earth sites are prepared with compaction, steel plates or temporary concrete roads. An SPMT is designed for precision, not speed; when loaded it moves at roughly walking pace and is not suitable for long road distances.
How it differs from a towed modular trailer
| Feature | SPMT | Towed modular trailer |
|---|---|---|
| Drive | Own power unit and hydraulic drive | Tractor, with a pusher at the rear if needed |
| Steering | Electronic; carousel and transverse movement | Mechanical or electronic; e.g. ±55° on the THP/SL |
| Speed | Very low, precise positioning | Higher on public roads, subject to permit conditions |
| Typical job | Port, site, in-plant; short distance | Long-distance road transport |
Details of the towed system are in modular hydraulic trailer.
Who is responsible?
SPMT operations are carried out by the heavy haulage company's engineers and certified operators. Before the move, stability, ground and ramp calculations are made; the steps are set out in writing in the method statement. The cargo owner is responsible for the accuracy of the weight and centre-of-gravity data. The division of responsibility between the parties is discussed in responsibility in oversize transport. The SPMT leg is often the final link in project transport.
Common mistakes
- Entering an earth site without calculating ground pressure; settlement under the wheels causes the load to tilt.
- Not discussing the ramp angle and the vessel's ballast adjustment in advance when loading onto a ship.
- Not setting a wind speed limit for moving modules with a large wind-exposed surface.
- Not confirming the support frame on which the load will sit and the blocking points with the manufacturer.