Advice: How to couple and uncouple a semi trailer (2024)

Coupling and uncoupling a prime mover and semi-trailer is a skill that should be mastered before a driver gets their Heavy Combination licence. Yet there are an increasing number of photos posted on social media of trailers being dropped.

According to several fleet operations managers we questioned, two common factors emerged behind most incidents: complacency and distraction.

In the first instance, drivers were found to have taken short-cuts in the coupling procedure, while in the second the simple issue of become distracted during the procedure had serious ramifications.

Sadly, in recent times there has also been an increase in dropped trailers due to vandalism – where the fifth wheel's lever has been pulled while a driver has been taking a fatigue break.

According to Jost Australia's Bob Martin, this has caused several fleets to amend their operating procedures to include a visual inspection to ensure trailers are coupled at the end of every fatigue break.

"We are hearing of this more and more," Bob said.

"I can't emphasis enough to drivers that they need to walk around their truck and check their trailers are connected after each stop, especially when they've parked in a public truck stop."

Let's start with the basics and look at some definitions of terms you're likely to come across when looking at coupling and uncoupling…

Fifth wheel
A device used for coupling a prime mover to a semi-trailer. A fixed fifth wheel, as the term suggests, is fixed to the prime mover and the king pin and trailer skid plate rotate while resting on top of the fifth wheel.

Ballrace turntable
Has a base fixed to the prime mover and the top attached to the fifth wheel, and it rotates easily via a series of internally mounted ball bearings. On trailers fitted with a block, the trailer moves with the top plate of the fifth wheel.

King pin
A pin on a semi-trailer skid plate that locks into a prime mover's fifth-wheel jaw – this is the one we refer to in this document. However, it is also the pin around which a steerable wheel rotates in the vertical axis.

Skid plate
A thick steel plate fixed to the underside of the front of a semi-trailer and incorporating the king pin (and often a block to keep the skid plate from turning on the upper part of fifth wheels mounted on ballrace turntables). The front of the plate is usually curved upwards to enable the prime mover to slide more easily under the trailer during coupling.

Throat
The opening at the rear of the fifth wheel that allows the king pin to easily move into the jaw.

Jaw
The mechanism that wraps around the king pin to secure it to the fifth wheel.

Locking bar/wedge
The secondary mechanism to ensure the king pin is secure in the fifth wheel.

Secondary safety latch
The tab that ensures the release handle will not come loose during travel.

Release handle
The handle to release the jaw when uncoupling the trailer.

Non-metallic
Greaseless fifth wheels use non-metallic pads to reduce friction between the skid plate and the fifth wheel top. This means that grease is not required on the top plate, giving the obvious benefits of reduced grease costs and requiring less cleaning for maintenance.

Step 2: Connect
Reverse the prime mover into position straight in front of the trailer. Use your mirrors to line up the sides of the trailer with the outer sides of the prime mover's mudguards and reverse back so that the rear mudguards on the prime mover are in line with the front of the trailer.

Apply the park brake, get out of the cabin and check that the fifth wheel and king pin are lined up. (ie the fifth wheel is not turned off-centre on a ballrace turntable).

Check that the height of the trailer skid plate is at least the same height as the fifth wheel.

If the trailer is too low, the prime mover chassis and turntable can hit the trailer front and cause damage to air lines, electrical connections, the body of van and more.

Lower the airbags and select low reverse, and then slowly reverse the prime mover until the fifth wheel is under the skid plate, stopping with the front of the trailer at the edge of the front guards. This prevents all the grease being scraped off the top the fifth wheel.

Raise the air bags to ride height until the legs are clear of the ground.

Visually check the landing legs are clear of the ground and that there is no air gap between the skid plate and the turntable. Resume slow reverse under the trailer until the fifth wheel jaw locks around the king pin. You should hear a distinct 'clunk' as the jaw closes and locks into place.

Step 3: Test
Tug Test. Check the trailer is locked on by selecting first gear then letting up the clutch to the friction point. The prime mover should not move. Apply the park brake.

Get out and visually check that the coupling release lever or pull handle has fully retracted and the secondary locking latch has dropped into its downward position. Some levers require a manual lock tab – this must be used to prevent the lever opening while towing. If the locking tab or the safety chain cannot be fitted, the turntable is not locked.

Using a flashlight, go under the trailer and look into the back of the throat of the fifth wheel. The locking mechanism should be engaged across the entire throat of the fifth wheel and secured around the king pin.

Make sure the head (bottom flange) of the pin is not sitting on top of jaw. Also ensure the trailer is sitting firmly on the fifth wheel by checking there is no gap between the fifth wheel and the trailer skid plate. If there is space, the king pin is not coupled.

In that case, lower the landing gear and slowly ease the prime mover forward until the fifth wheel comes out from under the trailer. Inspect the fifth wheel including the lock for any damage caused by the improper coupling attempt. If damage is found, repair or replace the fifth wheel before attempting to re-couple to the trailer. Repeat the coupling procedure until there is no space between the fifth wheel and the skid plate on the trailer.

Raise the landing gear. Keep winding until fully raised. Leave the landing gear engaged. Make sure the handle is properly stowed, or damage can occur.

If using a trailer for the first time, check the clearance movement between the prime mover (frame and wheel guards) and the trailer. Check also that there is enough clearance between the landing gear and the back of the truck frame to allow for turning.

Connect air hoses and electrical cables. When connecting airlines ensure the connection tabs are locked on the joiners (a quarter turn). If fitted, the shut-off valves are turned on to supply air and charge to the trailer. With prime movers with the air supply valve on the interior dash it is advisable to activate supply while the trailer check is carried out, as this will charge the air systems on the trailer (brakes and air suspension).

Check to ensure that all the lights on the trailer are working.

Remove and stow wheel chocks if required.

Step 4: Check
Reset the air bag level to the normal travel position. Before moving off, do a second tug test by applying the trailer brakes and again taking up the friction point in first gear. This is to confirm your trailer is securely hitched and the trailer brakes are serviceable.

Finally, as you move forward at low speed (5km/h) and in a straight line, do a third tug test by slowly applying pressure to the trailer brake hand piece.

This confirms that the trailer is securely hitched and that the supply and service pressure to the trailer brakes have reached operating levels. The trailer should dip slightly to confirm that the brakes are adjusted correctly.

Step 1: Position
Before uncoupling, ensure your combination is parked on a level surface that is firm enough to support the trailer landing gear and its load. If you have to uncouple on soft ground, place suitably strong timber or other flat supports under the landing gear.

Make sure the prime mover and semi-trailer are in a straight line, then:
• Apply the maxi brakes (parking brake) on prime mover.
• Chock the trailer wheels if required. Modern trailers are fitted with spring brakes, but this is still a requirement at some sites.
• Always use chocks where rear-loading the trailer is performed off a dock and if it is necessary to park on a grade. It is best to chock the semi-trailer's front axle in case the landing legs collapse and the rear axles lift.

Step 2 Disconnect cables and lines
Disconnect the air hoses and electrical cables from the trailer.

Stow them properly on the prime mover, ensuring the connectors are kept free of water and dust and that they cannot get caught on the tail shaft. Ensure air supply taps are turned off (if fitted).

Lower the landing gear until the supports are in contact with the ground. Ensure a visual check is conducted to confirm that both legs have been lowered.

Step 3: Release the jaw
To release the fifth-wheel jaw, raise the secondary safety latch and move the handle towards the front to release the lock, then pull the handle out as far as it will go. With the handle pulled out swing it forward to engage the edge of the plate. Now the fifth wheel is in the open position.

If the release handle cannot be moved, the jaw may be under load. Take the pressure off by rocking the prime mover forward and back and then try to release again.

Step 4: Drive forward
Release the prime mover parking brake and slowly drive forward 10 to 20cm in a straight line. Dump the air bags and wait until there is a gap between the trailer skid plate and the top of the fifth wheel.

This ensures the trailer is sitting firmly on the landing gear – if it isn't it provides an opportunity to rectify the situation without any damage to the equipment. Then drive forward until the fifth wheel is just clear of the trailer skid plate. Ensure the trailer stays put.

For prime movers with air bag suspension, ensure the air bags are down prior to completely clearing the trailer. If they are still fully inflated damage may occur to the mudguards and the trailer, or the king pin could get hooked on the rear cross member of the prime-mover chassis.

If necessary use the dump valve. Note: some prime movers may take longer to dump their air bags than others.

After the prime mover has completely cleared the trailer, reset the airbags to the normal traveling height.

It is advisable when parking the prime mover at the end of a shift or for any length of time and you are disconnecting the trailer, to pull the prime mover clear of the trailer and reset the air ride height to the normal traveling height.

Many companies are now enforcing disciplinary action and seeking compensation from individuals who cause damage through negligence by not following company coupling and uncoupling procedures.

If you take the appropriate care and time to couple or uncouple correctly, you'll avoid embarrassing, costly and potentially very dangerous mishaps.

To see Bob Martin demonstrate the operation of Jost's popular JSK37 fifth wheel, click here.