In the transportation and logistics services industry involving the lifting and transit of massively heavy and bulky cargoes, the common types of vehicles used are the self-propelled modular transporter and the modular trailers.

Detailed in here are the main differences between them based on the structure, function and level of performance.


1. What is a SPMT? and What is Modular Trailer?

So, what is a SPMT? A Self-Propelled Modular Transporter (SPMT) is a platform vehicle consisting of a large arrangement of wheels. It is also known as a self-propelled modular trailer.

These types of vehicles are basically used for the transportation of heavyweight and bulky objects such as industrial equipment and

project parts such as huge bridge sections among others that cannot be handled safely through the use of conventional trucks.

However, the trucks can be used to provide braking and traction for the SPMTs when on descents and inclines.

These types of vehicles are commonly found in such industries as the oil and construction industries. They are also applicable in the shipyard and offshore industry, on plant construction sites, for road transit and also for the transportation of oil platforms.

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1.1 Main features of a SPMT

In summary, a SPMT is made of a grid of axles controlled through a PPU. It is used to transport heaviest cargo in the world.

Two or more axles lines can be arranged side-by-side in what is normally referred to as an axle line. The most important thing to note is that each of these axles gets controlled individually.

This helps in the even distribution of weight and in achieving accurate steering.

All axles can swivel at angles of ±110°. The coordination of the axles is usually done through the system of controls to make it possible for the SPMT to turn, move in a sideways manner or as desired, spin from a fixed point.

With some SPMTs, the axles can telescope autonomously without causing the others to adjust and that makes it possible to have the load lay flat and most importantly be distributed evenly as transport is being made over a rugged terrain.

Each of the axles can also be provided with a hydrostatic drive unit.

modular transporter


Read More: SPMT Ultimate Guide – Self-propelled modular transporter Design & Specs

1.2 What is a Modular Trailer?

A modular trailer accounts for various special trailers or semi-trailers that are normally used in the transportation of bulky and heavy cargoes that cannot easily be disassembled for ease of transportation.

They are also used to transport products that are over-length. The tractor trucks are used to provide traction for the Modular trailers. 

Modular trailers are commonly used in the chemical industry, power stations, construction industry and in the iron and steel industry. Due to their excellent lateral stability, they can be used for mining operations.

The loading platform of all modular trailers is usually kept at a balanced position during the transportation of products on rugged surfaces due to their excellent dumping characteristics.

The brace kit of the modular trailer vehicle can attain either 3 or 4 brace points that make sure that there is a uniform loading on each point.

heavy duty transporter

Modular Trailer

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Read More: What is a Modular Trailer?

2. What is Difference Between SPMT and Modular Trailers

Conventionally, the main difference between an SPMT and a Modular Trailer is the SPMT’s power pack unit (PPU).

While this is the main steering system of the SPMT, the Modular Trailer has a different steering system that is mechanical.

The mechanical steering system in a modular trailer is contained in a tractor that tows the trailer during transportation. For modular trailers, the trailer can have a combination with the use of a gooseneck and a drawbar.

2.1 Different Drive Modes

2.1.1 Self-Propelled Modular Transporter (SPMT)

The SPMT is driven by a part called PPU. This system of steering controls comprises various parts that include the steering hydraulic pump, angular sensor, a proportional sandwich valve and the oscillating cylinder.

spmt ppu

spmt ppu

Other parts of the steering system for the SPMT include a rack and pinion steering device and the electronic calculating unit to facilitate the controls.

The rack-and-pinion device has the rack, the pinion, oscillating cylinder cases, angular sensor and the oscillating cylinder.

The cases for the oscillating cylinder acts as a cross girder. To achieve this function, the cases are welded together making the frame one unit with a reliable framework and offering great manufacturability.

The angular sensors are normally installed on the trailer’s wheel bogies. Each of the bogies has an angular sensor.

When there is a steering signal released from the remote controller, a theoretical angle is usually calculated by the electronic calculating unit based on the mode of steering.

The electronic unit then makes a comparison of the feedback coming from all the angular sensors on each of the wheel bogies.

With the use of the feedback comparison, the electric current of the proportional sandwich valves of that control and manage the opening can be determined.

Hydraulic steering cylinder causes the wheel bogies to steer to the desired positions. With this kind of arrangement, the self-propelled modular transport can assume 8 various steering modes as follows:

  • Diagonal steering cross mode
  • Reset mode
  • Diagonal steering cross mode
  • Front wheel steering mode
  • All-wheel steering cross mode
  • Rear wheel steering mode
  • All-wheel steering along mode
  • Circle steering mode

The multi-coupling trailers have the same modes of steering just like the single transporters. When there is a discrepancy in the angle of steering of the wheel bogie set above 8o, then the system will shut down automatically.

This will consequently cause the vehicle to stop or sound an alarm. Additionally, the SPMT can assume various other modes of steering based on the needs of a given operation.

The electronic compound multi-mode system of steering is designed to achieve a steering angle of ±110°.

double transporters steering

single transporter steering

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Read more: SPMT Ultimate Guide – Self-propelled modular transporter Design & Specs

2.1.2 Modular Trailer

The drive mode of the modular trailer is a set up where the trailer is towed by heavy tractor truck. Modular trailers have a steering system with a hydraulic planar pitman driver.

That way, the vehicle can attain a minimum turning diameter and at the same time have a normal drive by making adjustments to the hydraulic steering system and with the use of various logical pitman layouts.

The supporting members for a trailer part have a strong solid box beam framework. It is mainly made of high-performance welding steel especially the main frame longitudinal girder, steering arm, the bogie frame and the platform in general

2.2 Power Pack Unit (SPMT) and Power Pack (modular trailer)

2.2.1 The Power Pack Unit (PPU) for the SPMT

The power pack unit is the one that provides power for driving the SPMT and help in steering the axles. It is the most important feature of the SPMT, particularly one that differentiates it from the modular trailer.

In details, the PPU is the main power source for the electric system of steering for the SPMT.

Most important to note is the fact that the PPU is responsible for the supply of power to the hydraulic systems used for controlling the axle height and the braking system.

The PPU parameters usually vary based on the type of transporter. Take, for example, a self-propelled modular transporter with a load carrying capacity of 10,000 Kg.

The higher capacity power pack unit has capabilities of managing lift-able angles of 12o and a dead weight to the tune of 7,000 kg. The capacity of the fuel tank is 400 litres while the hydraulic one can take up to 850 litres.

The electrical system voltage powered by the PPU is 28V. for the driving systems, they can take a pressure to the tune of 400 bars. The max pressure for the steering and lifting system is pegged at 300 bars. 

Spmt PPU specs

Spmt PPU specs

This type of PPU is best suited for an SPMT with a total of 30 driving axle lines. However, the PPU parameters are not any different from one another. For example, the voltage of the electrical system varies from 24V to 28V for most of the PPUs.

The main difference between the various PPUs is the capacity of power contained in each one of them. The smallest PPUs have between 132 and 155Kw while the largest ones have between 350 and 375Kw.

When searching for an SPMT, the power capacity is the most important consideration to make for each of the PPUs you will come across in the industry.

The transporter parameters may also be different and therefore should be checked prior to purchase.


2.2.2 Power Pack for the Modular Trailer

The power pack (PP) unit of the modular trailer is only used to provide power to help in steering the axles. The PP has smaller power compared to that of the PPU for the self-propelled modular transporter.

The hydraulic power pack and the devices of control for the modular trailer can be used to carry out a number of functions that include making adjustments, steering and sustaining the combination of the vehicle.

Modular Trailer Power Pack

Modular Trailer Power Pack

Ideally and based on the details provided in here, it is obvious that the power pack differs from the PPU both in performance and in function.

It is unlikely that the power pack will produce the same power as is the case with the power pack unit. All that the PP can do is to provide power for steering the modular trailer but not driving the axles for the trailer.

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2.3 Different Steering Structure

2.3.1 SPMT

The SPMT is by nature an electronic compound multi-code steering system designed to achieve ±110° angle of steering.

These SPMTs that are electronically and remotely controlled usually come in different features, dimensions and load-bearing capacities.

However, the SPMT, in general, has a huge platform that has many wheels beneath it. It varies from the mechanically steered modular trailers due to the fact that it has a multi-way electronic steering stem.

Self-propelled modular trailers are different in the way their bolt-joint combinations are designed.

Some of them have both longitudinal and lateral mechanical bolt joint combinations unlike with most other types of modular trailers you will find in the market.

Other types of SPMTs have electronic modes of combination that don’t have bolt joints. Primarily, the SPMT has along with the ±110° turning radius with a rated load capacity of 216 tons and again with a dead weight of 30 tons.

SPMTs usually have 6 axle lines, 4 drive axles and 8 axles for braking. Their grade ability while on full load capacity is 7% compared to that of mechanically steered modular trailers that range between 8.5% and 20%.


2.3.2 Modular Trailers

For the Modular Trailer, the steering mechanism is purely mechanical making use of a steering central panel, steering cylinders, steering orifice plate and steering rods.

steering rods

steering rods

The steering system can turn an angle of ±55° during operations. The steering orifice plate is normally welded together with trailer’s bogie frame. The plate has a number of assembling holes that are used in the installation of the steering rods.

The positions in which the steering rods are installed on the steering orifice plate can be converted to meet the needs in different modes of combination for the modular unit.

The already assembled unit can meet the minimum radius of steering and the max turning 55o angle. 

The steering rods can easily and quickly be adjusted to meet the desired angle of steering.

The modular unit comes with a hydro-mechanical all-wheel system that has a two-circuit-system ensuring that the steering system can maintain functionality despite the situation even when one or several steering circuits fail to function.

  • Wheel Bogie Steering Systems

The hydraulic modular trailer normally has two types of wheel bogie steering systems. The steering system that is included in a modular trailer determines the kind of operations and terrain that it can travel through.

These are the pivot bearing steering system and the ball bearing race ring option:

  • Pivot Bearing: The hydraulic modular trailers that come with pivot bearings are meant to take on heavy on-road operations of cargo transportation. The trailers come with reinforced steel frames that make it possible for them to take up axial loads. This type of trailer is most applicable for use in an in-plant operation.
  • Ball Bearing Race Ring: These types of modular trailers that have a ball bearing race ring systems of steering are the most suited for use in an off-shore road operation. Ideally, the trailers can take up heavy loads in a given site location. The framework for this kind of trailer is well-designed for top performance and safe heavy-duty operation procedures.
modular unit wheel bogie steering

modular unit wheel bogie steering

The steering mechanism can be used to steer the axles in two different ways. These include the tow-steering mode and the use of a remote control switch or valve switch.

  • Tow-steering: This occurs when the truck or the tractor is doing the steering. Here the gooseneck or the drawbar delivers a signal that causes the axles to begin steering concurrently.
  • The remote control: it is applied as a manual control to trigger the axle steering.

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2.4 Different Combination Form

The modular trailer has a widening framework of structures. Ideally, the half modular unit can be used to widen the trailer.

The structure and arrangement of all the axles align half modular unit is usually combined through the use of the right and the left half modular unit.

The technical aspects of the half modular unit are usually similar to the modular unit once they are put together.

The right and the left half modular sections cannot be used individually but can be applied when they get combined and again be used when they are combined with the modular unit as well. 

half modular unit

Half modular unit

Each of the modular units can be combined either from end-to-end or side-by-side. A hydraulic link-pin is normally used in the end-by-end combination of various modular units.

The combination of the hydraulic and brake pipeline needs fast coupling linkages or connectors that have a high level of performance. Again, the electric interface makes use of a reliable heavy connector.

The modular combination of these hydraulic modular trailers provides room for having two forms of towing.

The two different forms are possible when all the entities available for each vehicle are linked together with the use of different combination accessories and other auxiliary equipment.

These forms include the trailer, semi-trailer and the full-trailer. The modular hydraulic trailer unit can achieve two main combinations that are listed as follows:

  • Combination with Drawbar: This type of combination takes into account the vehicle entity, the drawbar and the side steering frame
  • Combination with STZ-V gooseneck: It involves both the STZ-V gooseneck and the vehicle entity.

The modular type of trailer in this combined form can replace the conventional vehicle entirely in a given environment of transportation.

This is particularly in a situation whereby there is a need for the transportation of heavy lift products at a speed of as low as 5km/h. Here is a detailed look of the said combination forms.

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Self-propelled modular trailers, on the other hand, make use of different combinations to help in the transportation of big equipment or any other product that would warrant this application.

The possibility of using various combinations include the T combination, the 4-fold combination,

the V combination and the circle combination as well for the purposes of making the SPMT efficient in its operations and economical to use at the same time.

Looking at the various combinations, there is no need purchase a customized special SPMT for given project. The same self-propelled modular trailers can take various projects and move any type of heavy-weight and bulky cargo.

All that is required is to change the different combinations to meet the cargo dimensions. In the event that the length of the SPMT is not sufficient to take up the complete load, the frame can be used to enhance the length of the transporter.

Flat beam for SPMT

Flat beam for SPMT

You just need to change the combinations to suit the dimensions of the cargo.

If the SPMT’s length is not enough to accommodate the full load, the frame is used to increase the transporter’s length.

Read More: What is Heavy Lift / Project Cargo Transportation?

What Is the Advantage of Using Modular Trailers in Project Cargo Industry

3. Conclusion

To make the best use of the heavy lift vehicles available in the market, it is important to have sufficient knowledge of the various specifications and the functionality of each one of them.

This helps a lot when it comes to making a choice of the best trailer model to use for your specific project operations.

More than that, standards of quality should also be checked and confirmed based on factory strength, the cost and the technology used by different suppliers.

When prepared with the right kind of information, you will likely find the best market for getting these modular trailers. You can source for modular trailer transporters that match other brands globally.

In the end, you will be in a position to find a modular transport that has a high loading capacity and a firm structure that will meet all your transportation needs.

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