A strong connection

At first glance, all fifth wheel couplings look very similar. But appearances can be deceiving: different requirements and types of use call for different sizes, designs and materials, while differences can also be seen in the equipment and handling of the fifth wheel couplings.

Black, covered in grease and largely ignored - fifth wheel couplings lead a shadowy existence in everyday transportation. But when in operation, the fifth wheel coupling and its counterpart, the king pin, have a particularly important task: their connection is the only thing holding the tractor and trailer together. Fifth wheel couplings are thus safety-relevant vehicle components, just like brakes, axles and steering, and require design approval.

In strenuous static and dynamic strength and wear tests, manufacturers must prove that their fifth wheel couplings can withstand tough everyday conditions over long periods and have sufficient safety reserves. The approval conditions include both testing by an independent testing institute accredited by the German Federal Motor Vehicle Office (KBA) and certification of the manufacturer and all its production facilities by the KBA. One of the prescribed tests, for example, simulates a toppling fifth wheel coupling. The trailer must not be separated from the tractor when this happens. José Algüera, Head of Development at fifth wheel coupling manufacturer JOST, explains the procedure: "In the test facility, we use very high forces to try to rip the king pin upwards out of the coupling." In this, a force of 490 kilonewtons is prescribed for a fifth wheel coupling with an imposed load of 20 tonnes, which corresponds to 50 tonnes of load. At JOST, however, the test engineers always go beyond this. "We attach great importance to the durability and safety of our products, even in unusual situations," says Algüera. "That's why we test all our standard couplings under 60 tonnes of tensile load." Distortion of the coupling is allowed in this, but the connection must not come undone under any circumstances. Further tests are directed at characteristics such as fatigue strength, wear resistance and safe and faultless operability.

Each new type of fifth wheel coupling must be subjected to all these tests, as well as safety testing. And there are plenty of different types and designs. The range offered by the manufacturers mirrors the full spectrum of applications - from six-tonne minitrailers to 200-tonne heavy goods trailers, from long-distance transport to off-road use. The range offered by the world market leader JOST alone covers eight different imposed loads from 6.0 to 50 tonnes, as well as three different bearings, four types of fastening and a large number of additional heights and equipment variants. How do you find the right fifth wheel coupling among such a wide range? What is especially worth looking out for?

The load makes the difference
The diameter of the king pin is an important feature for distinguishing between fifth wheel couplings of different sizes. Because the fifth wheel coupling as we know it today originated in America and dominated the world market in the early 20th century, the key measurements are still given in inches (1 inch = 2.54cm) rather than metric today. The king pin, forged from high-strength steel, and the fastening eye of the fifth wheel coupling therefore generally measure exactly two inches in diameter. For heavy goods applications, however, king pins with a diameter of 3.5 inches are used, since they have much higher tensile strength. Further classification is possible using the imposed load and the so-called D value, which gives the load-bearing capacity of the fifth wheel coupling and is calculated from the imposed load, the maximum total weight of the tractor and the maximum total weight of the trailer. For example, a minitrailer with a trailer based on a transporter reach a D value of 50 kilonewtons under a maximum load of 6.0 tonnes on the fifth wheel coupling. A light, simple fifth wheel plate made from pressed sheet metal, such as the JOST JSK 26, is suitable for this purpose.

In contrast, much higher imposed loads occur in long-distance transport with triple axle standard semi-trailers and total towing weights or 40, 44 or even 50 tonnes. The imposed load is limited to twelve tonnes in Germany, but imposed loads of 15 tonnes or even more are very common in other countries and regions. JOST therefore approves all fifth wheel couplings for this use with up to 20 tonnes of imposed load. This category includes the robust and worldwide best-selling fifth wheel coupling, with over 1.5 million sold, JSK 37C, the weight-optimised long-distance transport coupling JSK 42 and the JSK 36D, a light and robust plate coupling. Higher loads often occur during use on construction sites, or in log transport in Scandinavia. For this, JOST offers the JSK 38, which is approved for an imposed load of up to 28 tonnes with a king pin diameter of 2 inches, and for 36 tonnes in the 3.5-inch design. Using an appropriate conversion kit, the JSK 38 heavy duty hitch can also be converted for a 2-inch fastening in just 15 minutes, so that heavy duty tractors can also pull standard semi-trailers. In addition, the JSK 38 is also available with a cardanic bearing, which allows the fifth wheel plate to swivel in all directions - an indispensable feature in off-road use. Imposed loads of over 40 tonnes and tensile loads exceeding twice that of a normal fifth wheel coupling are common in quadruple or quintuple axle tractors. Special heavy duty hitches are needed for this purpose and are usually equipped with a strong 3.5-inch king pin. At JOST, the flagship fifth wheel coupling is the JSK 50. It can bear up to 50 tonnes and has a D value of 300 KN, theoretically allowing a 41-tonne tractor to pull over 500 tonnes. However, at 290 kg, the JSK 50 also weighs around twice as much as a "normal" fifth wheel coupling.

Spoilt for choice in material and equipment

In the best-selling 20-tonnes imposed load category, manufacturers offer fifth wheel plates made from both pressed sheet metal and cast iron. Till Franckenstein, Truck Project Manager at JOST, explains the difference: "Plate couplings are somewhat lighter and their simple construction makes them popular for use in countries outside Europe. In addition, the JSK 36 plate coupling from JOST is equipped with a pivot bearing as standard, allowing dumping with the fifth wheel plate - a basic requirement when used on vehicles with leaf springs. However, fifth wheel couplings made from cast steel are much more wear resistant and convenient than plate couplings." Furthermore, JOST offers these in a range of equipment variants, which is why JOST produces around 90 percent of all cast fifth wheel couplings sold worldwide today.

While the robust JSK 37 - optionally equipped with convenient wear-reducing rubber bearings or pivot bearings - leads sales figures worldwide, in Europe, the JSK 42 fifth wheel coupling with slide bearing is becoming ever more established as the standard coupling in Europe. Advantage: the weight-optimising construction means that the JSK 42 weighs around 20 kilograms less than a comparable JSK 37. In addition, the king pin can be unlocked with one hand as standard, making it very convenient. Both couplings are also available in low-maintenance designs with top plate liners in the fifth wheel plate and a wearing ring with a plastic insert in the lock. The top plate liners have a very low friction coefficient. This solution therefore makes it unnecessary to apply grease to lubricate the sliding surfaces, although it is not a bad idea, as Product Manager Franckenstein explains: "It's not true that low-maintenance couplings should be kept completely free of grease. Quite the opposite - we would even recommend a thin layer of grease in order to prevent corrosion on the trailer plate. Low-maintenance couplings can thus also be used when driving trailers which previously had conventional greased couplings." Low-maintenance couplings are particularly suitable for long-distance transport, where long journeys and only infrequent coupling and decoupling are usual.

Although there is no need to apply grease regularly to the sliding surface of low-maintenance fifth wheel couplings, the lock jaw definitely needs lubrication. In order to improve safety over many years of operation, as well as to make maintenance and operation more convenient, manufacturer JOST has been equipping all low-maintenance fifth wheel couplings in the JSK 37 and JSK 42 series with the LubeTronic 1Point automatic lubrication system as standard since 2010. This system, including a supply of grease and the battery, are integrated into the fifth wheel plate and supplies grease to the lock jaw regularly over a three-year period. Manual lubrication becomes unnecessary. LubeTronic 1Point can be retrofitted to older fifth wheel couplings in this series at any time.

JOST also has an extremely convenient solution for lubricating fifth wheel couplings with conventional greased sliding surfaces: the LubeTronic 5Point. This automatic lubrication system is also integrated into the coupling and uses five lubrication nozzles to supply both the lock jaw and the top plate with grease at regular intervals. An electronic control doses the distributed amount of grease precisely at regular intervals. The lubricant in the lubrication cartridge on the fifth wheel coupling will last for around twelve months. It can be easily refilled with a lubrication nipple. The electronically controlled dosing ensures that there is neither too little nor too much grease on the fifth wheel plate and lock jaw. This saves grease and protects the environment. MAN is the first vehicle manufacturer to have recognised the benefits of this automatic lubrication system and has been installing the JOST LubeTronic 5Point on its tractors since Spring 2011. JOST fifth wheel couplings with the LubeTronic 5Point automatic lubrication system are available in the JSK 42, JSK 37 C and JSK 38 series.

Sensors provide safety

Whether the fifth wheel coupling is heavy or light, plate or cast, low-maintenance or conventional: the coupling is only connected to the king pin safely when the king pin sits correctly in the fastening and the fastening is completely locked. For this to happen, the king pin must be inserted into the fastening at the correct height during coupling. JOST Head of Development José Algüera explains the special safety features for coupling: "Our fifth wheel couplings have tappets in the wedge which guide the king pin into the fastening at precisely the right height and direction. We use a patented lock jaw to prevent dangerous false locking. This is shaped in such a way that it is only triggered when the king pin is inserted into the fastening properly." When the lock jaw is engaged, a bar resting on both sides slides behind the lock jaw and thus guarantees an extremely stable lock. The handle only springs back to its original position once the lock jaw and bar have reached their correct locking position. To allow the driver to see more easily when it is correctly locked and recognise unwanted unlocking at a quick glance, JOST offers the Flashtronic sensor locking check for all fifth wheel couplings in the JSK 42 series. This small device and the handle needed for it can be retrofitted easily on any JSK 42. The Flashtronic uses a flashing LED display directly on the coupling to signal that the fastening is sitting correctly. An integrated battery guarantees three years of operation. After that, the entire sensor unit is simply replaced.

Lock monitoring is safer and more convenient with a sensor coupling. Sensors check the locking position, the contact between the fifth wheel and trailer plates and that the king pin is sitting correctly. A remote display processes the signals and displays the locking position in the driver's cab. In the JOST SKS sensor coupling system, this is supplemented by pneumatic unlocking, which allows the fifth wheel coupling to be operated from the driver's seat. But that's not all. JOST presented the dream of safe coupling and uncoupling without leaving the driver's cab at the IAA as early as 2008 - the KKS Comfort Coupling System. "This system offers the very best in safety and convenience," enthuses Head of Development Algüera. As well as the sensors and a remote control for unlocking, the KKS also has an automatic interface for compressed air and electrical connections, as well as remote control for the telescopic landing gears. "Needless to say, vehicles equipped with KKS can also be combined with other vehicles," explains Algüera. The full range of benefits offered by KKS really comes into play in transports requiring frequent coupling and uncoupling.

Fixed solidly to the frame

However safe and convenient the fifth wheel coupling is, it can only guarantee optimum safety and tensile strength if it is fastened to the vehicle frame correctly. Manufacturers offer a wide range of options for connection to the vehicle frame: the range stretches from simple yet heavy planar mounting plates and corrugated mounting plates for varying mounting heights, to lightweight construction bearings and manufacturer-specific frame mounting components, such as in an angle mount design. This allows an appropriate and safe solution to be found for any frame width and height. A dual height fifth wheel system is a convenient and safe solution for low-deck tractors intended to take not just megatrailers, but also trailers with other coupling heights. The JSK DH, for example, not only lifts or lowers the fifth wheel coupling to the desired coupling height pneumatically, but a compressed air cylinder is also used for locking in the correct height position. A slider enables the position of the fifth wheel coupling to be changed lengthways. This allows the driver to optimise the axle load distribution or shorten the total length, such as when loading onto a ferry. Sliders are very popular in the USA in particular, due to the special regulations on axle load in several American states