I worked at Wordsworth holding who at the time manufactured Big d fifth wheels , ramps , sliders etc , the jaws were machined on a mazak cnc Miller , those fifth wheels often came back under warranty claims for various defects .
The fifth wheels were fitted up on a jig ( I did it on accasion ) and there were often issues getting them to work correctly , but as you were on piece work you didn’t want to spend a minute more than you had to getting them to work correctly !!!
I’m not saying this is any thing other than driver error , but the fact they would come back under a warranty claim , you’d think there could be a possibility of it being a defective fifth wheel
The same applies to jaws , after they did away with inspectors you then relied on the machinist to get every component within tolerances on engineering drawing , you’d find a inspector would make sure components were within manufacturing tolerances , but again if your on piece work the odd micron/ thou here and there , it will be ok
Nb I’m going back 20 yrs , think Big d was sold to jost so it maybe of no relevance’s in 2019
dozy:
I worked at Wordsworth holding who at the time manufactured Big d fifth wheels , ramps , sliders etc , the jaws were machined on a mazak cnc Miller , those fifth wheels often came back under warranty claims for various defects .
The fifth wheels were fitted up on a jig ( I did it on accasion ) and there were often issues getting them to work correctly , but as you were on piece work you didn’t want to spend a minute more than you had to getting them to work correctly !!!
I’m not saying this is any thing other than driver error , but the fact they would come back under a warranty claim , you’d think there could be a possibility of it being a defective fifth wheel
The same applies to jaws , after they did away with inspectors you then relied on the machinist to get every component within tolerances on engineering drawing , you’d find a inspector would make sure components were within manufacturing tolerances , but again if your on piece work the odd micron/ thou here and there , it will be ok
Nb I’m going back 20 yrs , think Big d was sold to jost so it maybe of no relevance’s in 2019
Big D’s were never really had to be precision engineered, ish would have been fine and we would use the adjuster after fitting to adjust to work correctly, possibly sent back because they were not adjusted after being fitted.
peirre:
mick.mh2racing:
I can’t see it being driver error. If his pin wasn’t engaged he wouldn’t get out of the yard, it’ll come off the first corner he went round.There has been cases where they’ve managed to get several miles from the yard before loosing it, especially if it’s been partially engaged. ie it’s clicked in, but the arm hasn’t fully snapped into place and they’ve not bothered with the dogclip
New auto safety catch No dog-clip, so drivers have stopped checking correctly.
I have seen 2 trailers come off, one had travelled from Portsmouth to Derby and fell off in the yard, the other came off on the ferry, neither were driver error and both were mechanical failure, the ptfe plastic horse shoe had broken up.
A simple fix to replace with an Allen key.
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peirre:
As part of my routine I duck down and shine a torch to see if the 5th wheel had locked in place
I’m glad I’m not the only one who does this. Never really see anyone do it though
bald bloke:
peirre:
As part of my routine I duck down and shine a torch to see if the 5th wheel had locked in placeAnd me takes approximately 2 seconds at most armed with a torch of course .
And me, every time without fail
In fact I once did a driving assessment, trailer was already coupled and me sat in the driver’s seat and under pressure to be off. I said I wanted to do a walk round to be told to crack on. I wasn’t happy and said I couldn’t drive off without at least checking pin and shining a torch up the jaws.
Driver trainer took it as me being thorough and not always relying on others when I should be doing stings myself. Got the job.
mick.mh2racing:
bald bloke:
peirre:
As part of my routine I duck down and shine a torch to see if the 5th wheel had locked in placeAnd me takes approximately 2 seconds at most armed with a torch of course .
And me, every time without fail
Must’ve banged my head getting back out at least a dozen times, but still check every time
Doesn’t everyone do the “Daylight Test” as part of coupling procedure?
I mean when you look underneath having connected, to see if the trailer bottom is sitting on the plate so you cannot see daylight between the top of the 5th wheel and the bottom of the trailer… Needs a good torch at night of course…
I routinely dip the suspension whilst going under, and then lift it up a bit IF levelling the suspension has not already lifted the legs-down trailer off the ground a bit.
Once off the ground, sliding under the rest of the way - is level, flat, and smooth.
The usual thing that happens if you don’t check the height of your 5th wheel - is under-running the pin, and ending up with it sitting on top of the closed jaws, seemingly passing the tug test, - and then you lose it at the first sharp bend or roundabout leaving the depot! There will be a gaping daylight gap if the pin is resting upon the closed jaws, of course - hence why doing the “Daylight test” is a must, even at night!
…Then there’s undercutting the pin outright, and slamming your suzie frame up against the fridge…
I must admit, I’d not seen someone jackknife in the middle of the trailer before, either…
Crossroads:
mick.mh2racing:
bald bloke:
peirre:
As part of my routine I duck down and shine a torch to see if the 5th wheel had locked in placeAnd me takes approximately 2 seconds at most armed with a torch of course .
And me, every time without fail
Must’ve banged my head getting back out at least a dozen times, but still check every time
Add me to that list as well, it’s so quick and easy to do I cannot fathom why the majority don’t! (aside from morbid obesity and/or pure laziness of course)
Can’t say as I’ve ever banged my head though, left hand on the chassis beam (this is key I think as I’ve only ever brushed my noggin on my hand or arm) and right hand shining the torch, job done.
Crossroads:
mick.mh2racing:
bald bloke:
peirre:
As part of my routine I duck down and shine a torch to see if the 5th wheel had locked in placeAnd me takes approximately 2 seconds at most armed with a torch of course .
And me, every time without fail
Must’ve banged my head getting back out at least a dozen times, but still check every time
I think dozy has done that more than once 
It don’t have to be a pin failure.
We’ve had instances where the jaws have failed, movement on the road nuts and bolts working loose off the gas pressure is taken off the pin / jaws, they come apart foot back on the accelerator and bang trailer gone.
Gets me everyone thinks driver error
nick2008:
It don’t have to be a pin failure.
We’ve had instances where the jaws have failed, movement on the road nuts and bolts working loose off the gas pressure is taken off the pin / jaws, they come apart foot back on the accelerator and bang trailer gone.Gets me everyone thinks driver error
Nick which nuts and bolts come loose to allow the jaws to fail? also what gas pressure?
nick2008:
It don’t have to be a pin failure.
We’ve had instances where the jaws have failed, movement on the road nuts and bolts working loose off the gas pressure is taken off the pin / jaws, they come apart foot back on the accelerator and bang trailer gone.Gets me everyone thinks driver error
Nick which nuts and bolts come loose to allow the jaws to fail? also what gas pressure?
Trickydick:
nick2008:
It don’t have to be a pin failure.
We’ve had instances where the jaws have failed, movement on the road nuts and bolts working loose off the gas pressure is taken off the pin / jaws, they come apart foot back on the accelerator and bang trailer gone.Gets me everyone thinks driver error
Nick which nuts and bolts come loose to allow the jaws to fail? also what gas pressure?
Ok accelerator pedal and do you know it’s not held together by magic dust it has nuts and bolts in it 
In almost 46 years I have never seen a jaw failure, I have seen fifth wheels ripped from the chassis & I have only ever seen one king pin sheared, has anyone else apart from nick2008 seen a fifth wheel with jaw failure and if so how did it fail?
Christ, American trucks really are made of cardboard aren’t they?!
dave docwra:
In almost 46 years I have never seen a jaw failure, I have seen fifth wheels ripped from the chassis & I have only ever seen one king pin sheared, has anyone else apart from nick2008 seen a fifth wheel with jaw failure and if so how did it fail?
Can’t match your 46 years, but I’ve been around the block a few times.
Same as you: Never seen jaws fail. Seen, felt and heard worn/badly adjusted jaws. A clunk on every gear change. I’ve seen one trailer where you could waggle the pin a couple of inches all around by hand. The faults were so obvious only a numpty would take it anywhere except across the yard to the workshop.
A catastrophic failure, with no warning? Nothing is impossible, but that’s pretty close to it.
dave docwra:
In almost 46 years I have never seen a jaw failure, I have seen fifth wheels ripped from the chassis & I have only ever seen one king pin sheared, has anyone else apart from nick2008 seen a fifth wheel with jaw failure and if so how did it fail?
Nope, me neither, seen overshoots, seen trailers dropped on their knees, known trailers to bounce off having been sat on the jaws, actually had a kin pin sitting on the jaws (sorted immediately never took it on the road), had worn fifth wheels which i’ve defected, but never, and 4 decades too, known of a fifth wheel actually failing.
I watched, amazed, a shunter at Castrol reversing flat out under trailers with a Big J hitting the pin with a deafening bang, even that abuse didn’t break either a pin or fifth wheel.