How to adjust a Jost JSK fifth wheel

It was inevitable that the push to increase payloads through higher productivity freight vehicles and PBS combinations would push the dynamic loads on couplings beyond the current relevant standards.

Many in the industry have welcomed the recent announcement of the Commonwealth Government funded “Safety of Couplings” project to be managed by the National Heavy Vehicle Regulator’s (NHVR) Heavy Vehicle Safety Initiative to fill a knowledge gap and provide evidence to update relevant Standards and Rules.

For well over a century, coupling devices using a receiver plate and kingpin have been the industry standard for connecting trailers to prime-movers.

Although in the past century, where technology advancement in every aspect of life and business has evolved at a staggering rate, the humble fifth wheel may seem distinctly low-tech. After all, they haven’t changed all that much since their inception.

Jost Australia’s Bob Martin disagrees: “You would be amazed at how much difference there is between fifth wheels,” Bob explained.

“Even within our Jost range of products, there are many options and factors that require consideration when specifying a fifth wheel for your combination.”

Luckily, technology plays an ever-growing role, and the benefits are tangible. For instance, Jost fifth wheels can be specified with numerous additional aids and sensors, all designed to increase vehicle combination safety further.

“While Jost fifth wheels are designed to be user friendly, they have the critical function of making sure the prime-mover and trailer stay coupled,” Bob emphasised.

“Because all coupling circumstances are not perfect. For instance, considering variation in the places trailers are un-hitched, disparities in trailer age and condition. Also, differences in driver techniques and speed, Jost spends enormous time developing researching our locks to give a secure connection in less-than-ideal situations.”

Nevertheless, while technology is fantastic, the human component is still critical. “I cannot put enough importance that the drivers have the crucial task of ensuring that the trailer’s kingpin has properly coupled to the prime mover’s fifth wheel before it leaves the depot,” said Bob.

“This process entails not only conducting a tug test but getting out of the cabin to visually confirm there is no air gap between the trailer’s skid plate and fifth wheel and ensuring that there is a secure couple,” Bob added.

“Sadly, we’re hearing of more and more incidents where drivers leave depots with the kingpin either sitting in front of or on top of the fifth wheel. And, consequently, the trailer parts company with the prime-mover.”

While Jost’s sensor-fifth wheel goes a long way in alerting driver’s, there is a potential problem with the fifth wheel and kingpin connection. Bob reiterates that there is no substitution for getting out of the truck to visually check the locking bar is securely locked around the kingpin.

“It’s vitally important to remember that the fifth wheel is a critical safety item on any combination, and that includes converter dollies and B-Double lead trailers. Its role is to keep the trailer coupled to the prime-mover, dolly or lead trailer. In other words, if your fifth wheels aren’t correctly maintained, your combination is looking for trouble.”

Bob kindly shared some tips on checking the locking mechanism adjustment on their popular Jost JSK fifth wheel and, importantly, how to perform the adjustment to return the fifth wheel to the manufacturer’s recommended specification.

As a rough rule of thumb, the perfect time to check the adjustment of the fifth wheel is while servicing the truck. Jost’s service bulletin (LT SK37U-03 REV.K) recommends that inspection and adjustment get carried out every three months or 50,000km.

But first, let’s look at the construction of the Jost JSK. It has four moving parts; the release handle, release arm, the lock bar and the lockjaw. These locking components are connected together.

When the release handle is in the extended position, the fifth wheel is unlocked and locked when the handle is in the “in” position.

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The lock bar is tapered and determines the lock’s tightness. Note that the lockjaw position moves forward and backwards as the lock bar is adjusted. This is because the sole purpose of the adjustment screw is to prevent the lock bar from jamming the lockjaw too tightly around the kingpin.

When the lockjaw is closed, the adjustment screw rests against the lock bar maintaining proper clearance (0.3mm) around the kingpin.

The lock adjustment screw is located on the left-hand side of the fifth wheel when the fifth wheel is mounted correctly to the truck.

Jost has special testing tools available from most reputable parts outlets to simplify the inspection and adjustment procedure. All the tools are available in either a 2-inch (50mm) or 3.5-inch (90mm) version.

The lock test tool is essentially a flat plate with a new kingpin fixed squarely to the underside of the flat plate.

Another handy tool is the ‘gauge lock jaw’, which is fundamentally a no-go gauge used to measure the wear of the lockjaw. If the gauge slides over the jaw, then the jaw is worn out and will need replacing.

To use the ‘gauge lock jaw’ checking tool, push the jaw forward by hand, then try to slide the tool over the jaw. If the device does not fit over the jaw, then the jaw is within servable tolerance. However, if the tool does slide over the jaw, then the jaw will need to be replaced.

Start by having the fifth wheel in the fully open position. In this position, the jaw is wide open and the handle all the way out.

Place the lock test tool on top of the fifth wheel’s top plate with the kingpin in the throat, then place the hook under the front skirt of the fifth wheel. Push the tool’s lever forward till the fifth wheel engages the lockjaw entirely around the kingpin in the tool.

It is essential to confirm that the kingpin sits squarely in the jaw on greaseless fifth wheels. To do this, you will need to rotate the tool slightly to ensure that tool is seated firmly on the greaseless pads at both the front and rear of the top plate.

To check the adjustment, try rocking the tool’s lever forward and backwards to check for movement in the lock. The lock is adjusted correctly if there is no fore or aft movement.

Next, disengage the hook from the front of the skirt and rotate the lock test tool from left to right; the lockjaw is appropriately adjusted if the tool rotates freely without any fore and aft movement.

The adjustment screw is located on the left-hand side of the fifth wheel. You will need two 24mm ring spanners or a 24mm ring spanner and a 24mm socket to complete the adjustment.

First, loosen the locknut on the adjustment screw. Then wind the adjustment screw outwards till it becomes loose.

This allows the lock bar to travel all the way inwards. However, now the lock bar is tight in behind the lockjaw. In fact, it is actually overtight, meaning there is not enough clearance for the kingpin to rotate freely in the lockjaw.

The next part of the adjustment procedure is to rewind the adjustment screw in until you feel the screw tighten. This indicates that the adjustment screw is now in contact with the lock bar.

At this point, you will need to get an offsider to raise the fifth wheel’s secondary safety latch, then move the release handle forward towards the front of the fifth wheel and hold it in that position while completing the adjustment procedure.

Now wind the adjuster screw in a further one and half turns. This will set the recommended clearance of 0.3mm.

Securely hold the adjustment screw with one spanner or the socket, then tighten the lock nut with the second spanner to fix the adjustment screw in position.

That completes the adjustment of the fifth wheel.

However, it is essential to repeat the adjustment check by testing that the tool rotates freely and there is no fore and aft movement to confirm the adjustment was successful.