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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.^[1] 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.

In 1957 the first every hydraulic modular trailers were made by Willy Scheuerle a Germany based trailer specialist which were four axles 32 wheeled modules for Robert Wynn and Sons Ltd, a Shaftesbury-based Guinness Book of Record-winning heavy haulage company.^[2] Wynns were also the first to use pneumatic tires for loads weighing more than 100 tons and also to use hydraulic suspension trailers which were manufactured by Cranes Trailers limited from Dereham.^[3]

In 1962 Cranes Trailers limited developed two four-axle 32-wheel modules for Pickfords, a London based heavy haulage company, with combined payload capacity of 160 tons on a total of eight 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 either of both ends or even both for push-pull combination.^[4]

In 1963 Goldhofer developed modular trailers in Europe for heavy haulers.^[5] In the same year, Cometto developed a 300-ton capacity module in 14-axle, seven-row configuration.^[6] Scheuerle also demonstrated its modules at events in 1967^[7] and later King Truck Equipment Ltd signed an agreement with Scheuerle which gave them exclusive manufacturing rights to produce their trailers in the UK.^[8]

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 six 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.^[9]

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,^[10] 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.

The number of axles on a HMT is not specified; two-, three-, four-, five-, six-, and eight-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.^{[citation needed]}

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 two turn tables, two knees, two suspension cylinders and four to eight 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.^{[citation needed]}

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.

^{[citation needed]}

Gooseneck is the most common coupling used in the industry. 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,^[11] 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. Additionally, 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 consists of an A-shaped frame with an 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^[12] 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.^[13] Draw bars and tow hitches are manufacture red by companies like jost and Ringfeder.^{[citation needed]}

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 feet (4.0 m) to 20 feet (6.1 m) wide to make transport of empty trailers easy and also comply with state regulations when required.^[14]

• Gooseneck
• Draw bar
• Drop Deck
• Vessel Bridge^[15]
• Intermediate spacer
• Excavator deck
• Extendable spacer
• Turntables (bolster)
• Blade Lifter^[16]
• Tower adapter^[17]
• Girder frame^[18]
• Trailer power assist^[19]

• 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

• ALE^[20]
• Sarens^[21]
• Mammoet^[22]
• Lampson International^[23]
• Pickfords^[24]
• Alstom
• ALE^[25]
• CLP Group
• Omega morgan
• United States Army
• Indian Army
• British Army
• Republic of Korea Armed Forces
• French Army
• Italian Army
• Turkish Armed Forces

• 
  Windmill tower section being transported using tower adapter configuration
• 
  DOLL self steering blade transporter dolly
• 
  Goldhofer FTV blade lifter being pulled by a Mercedes-Benz SK 3553 tractor
• 
  ALE girder bridge with trojan tractors in push and pull configuration
• 
  Volvo tractor with drop deck configuration moving oversize load
• 
  MAN tractors with vessel deck configuration

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