Hydraulic modular trailer
A hydraulic modular trailer (HMT) is a special platform trailer unit which feature swing axles, hydraulic suspension, independently steerable axles, two or more axle rows, compatible to join two or more units longitudinally and laterally and uses power pack unit (PPU) to steer and adjust height. These trailer units are used to transport oversized load, which are difficult to disassemble and are overweight. These trailers are manufactured using high tensile steel, which makes it possible to bear the weight of the load with the help of one or more ballast tractors which push and pull these units via drawbar or gooseneck this combination of tractor and trailer is also termed as heavy hauler. Typical loads include oil rig modules, bridge sections, buildings, ship sections, and industrial machinery such as generators and turbines also many militaries uses HMT for tank transportation. There is a limited number of manufacturers who produce these heavy-duty trailers because the market share of oversized loads is very thin when we talk about the over all transportation industry. There are self powered units of hydraulic modular trailer which are called SPMT which are used when the ballast tractors can not be applied due to space. History 

The first hydraulic modular trailer modules can be traced back to 1962 when heavy trailer specialist Cranes Trailers limited from Dereham developed two 4 axle 32 wheel modules for Pickfords a London based heavy haulage company with combined payload capacity of 160 tons on a total of 8 axles and 64 wheels the modules incorporated hydraulic suspensions and each axle interlinked with mechanical steering system at an operational height varied from 2.9 to 3.11ft. The modules had drawbar coupling which could be coupled at any of both ends or even both for push-pull combination. In 1963 Goldhofer developed modular trailers in Europe for heavy haulers. In the same year, Cometto developed a 300-ton capacity module in 14-axle, seven-row configuration. Scheuerle also demonstrated its modules at events in 19867 and later King Truck Equipment Ltd signed an agreement with Scheuerle which gave them exclusive manufacturing rights to produce their trailers in the UK. In 1971, King Truck Equipment Ltd demonstrated two units that were custom-built for Pickfords . A single unit was able to carry 150 tons on 6 axle rows and 48 wheels in total. Who would use them mostly with their Scammell ballast tractors via a drawbar coupling. These trailers had independent suspension and steering abilities via the Petter twin-cylinder diesel engine used as a PPU. The same year, Robert Wynn and Sons Ltd, a Shaftesbury-based Guinness Book of Record-winning heavy haulage company, received two 10-axle row HMT from Nicolas Industrie which had a gross capacity of 600 tons when coupled with a gooseneck coupling. With seven rows, the unit had a total capacity of 350 tons. In the 1970s, many manufacturers started to developed HMTs as the industry believed that the conventional low loaders had various limitations. To comply with new regulations and keeping safety in mind, the industry knew that they needed more axles to distribute the payload and the ultimate solution for the demand would be HMTs. Manufacturers opted hydraulic suspension instead of mechanical leaf springs and air suspension due to its efficient size and adjustable characteristics. Manufacturers chose high-tensile steel instead of aluminum because when it comes to HMTs and oversize loads, the minimizing the weight of the HMT is not relevant when they have their own payload capacity excluding the ballast tractor. The only weak point that existed on a HMT were the tires, which are still a significant weakness till today, that's the reason why SPMTs have solid tires. HMTs operate at a higher speed then SPMts that's why solid tires are not an option for HMTs. Specifications 

The number of axles on a HMT is not specified; 2, 3, 4, 5, 6, and 8 axle units are manufactured. Multiple units can be coupled longitudinally and laterally to transport a heavier load; each axle has a lifting capacity ranging from 18 tons to 45 tons. With a steering capacity of 50 to 60 degrees. Some combinations require a trailer operator who controls steering and height adjustments of the trailer via a controller which is modular and can be mounted at the frontend or rear end of the trailer. Huge combinations may also have a cabin for the operator, while typical combinations have a seat attached to the controller. Hydraulic cylinders are used for steering and suspension of the trailer each axle has an individual suspension cylinder, steering rod which is connected to the main steering cylinder which is at the frontend of the trailer which makes all the axles steer at once in the same direction one row of axle consist of 2 turn tables, 2 knees, 2 suspension cylinders and 4-8 wheels attached to a high strength metal platform. Steering and suspension cylinders are hydraulically operated using hydraulic fluid through hose pipe from the hydraulic tank, which is located near the PPU. PPU, which powers the steering, suspension  to and fro flow of hydraulic fluid from hydraulic tank to suspensions and steering cylinders, puts out about 18 to 25 hp of power and are available in both diesel and petrol variants manufactured by renowned brands like Kohler, Yanmar and Hatz. Multiple units of HMT can be interconnected longitudinally by pins and interconnecting couplings mounted in the centre of the chassis in the front and rear to interconnect them laterally they are bolted on the side wall of the chassis. HMTs can not move themselves, so There are two ways by which  a HMT can be coupled with a tractor unit which can push and pull the trailer, these are gooseneck and drawbar. Gooseneck is the most common coupling used in the industry as the name suggest a swan shaped coupling is coupled to the trailer and the tractor via connection of trailer pin and tractor fifth wheel. This coupling can be hydraulically adjusted to suit the tractor's height also the steering controls are connected to the coupling. Goosenecks are easy to use and gives benefit to using conventional tractors, but this coupling has two huge drawbacks this coupling can not be applied in a two file or side by side HMT configuration which limits the payload, and also it can not be applied in push and pull configuration. Goosenecks are manufactured by the trailers manufactures themselves. Drawbar is the most efficient and economical coupling which consist of  A shaped frame with a i shaped loop which is coupled to the trailer and connected to a ballast tractor via a towing hitch of the tractor. This coupling is widely used in developing countries because of its economical cost. Unlike gooseneck, this coupling can be applied to side by side and push & pull configuration which, but this coupling can not be connected to a typical tractor, it requires a ballast tractor which has a ballast box instead of a fifth wheel and tow hitches in the rear and front. Draw bars and tow hitches are manufacture red by companies like jost and Ringfeder. Since 2005 in the United States of America, HMT have extra features and design changes which include widening axles, and half way folding system. Due to different road regulations in different states, almost all manufacturers have adopted the US design and developed a product for the US market. These HMT trailers are named dual lane trailers, which comes from the widening characteristic of the trailer. Dual lane trailers have capability to change its width from 13 ft to 20 ft wide to make transport of empty trailers easy and also comply with state regulations when required. Accessories 

 Gooseneck
 Draw bar
 Drop Deck
 Vessel Bridge
 Intermediate spacer
 Excavator deck
 Extendable spacer
 Turntables (bolster)
 Blade Lifter
 Tower adapter
 Girder frame
 Trailer power assist

Manufacturers 

 Goldhofer
 Scheuerle
 Nicolas
 Kamag
 Tiiger
 Tratec
 Faymonville
 Cometto
 Drake Trailers
 Kennedy Trailers
 Trabosa
 Taskers of Andover
 DOLL Fahrzeugbau
 TRT Australia
 Broshuis
 Tuff Trailers
 Colombo
 Capperi
 Modern Transport Engineers
 King Trailers Limited
 Leonardo DRS
 BEML

Operators 

 ALE
 Sarens
 Mammoet
 Lampson International
 Deugro
 Pickfords
 Alstom
 ALE
 CLP Group
 Omega morgan
 United States Army
 Indian Army
 British Army
 Republic of Korea Armed Forces
 French Army
 Italian Army
 Turkish Armed Forces

Gallery

See also 

 Heavy hauler
 Tractor unit
 Ringfeder
 Ballast tractor
 Transporter Industry International
 Faymonville Group

References 

Trailers
Heavy haulage
Engineering vehicles
Modularity
Machines