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King pin disconnection from trailer

King pin disconnection from trailer

King pin disconnection from trailer

We have a serious probelm and could not find a solution or initiatives to solve, I hope to find a proper solution or suggest through this forum.

We have a fleet of trailers where the tank is engaged to tractor through two plates and they connected by a KING PIN. This king pin is subjected to repetitive loose/break that caused the tank and tractor disengaged. This happened several times and caused many incidents.

I appreciate if anyone has practical solution or idea to prevent such incidents, thanks.

RE: King pin disconnection from trailer

Hi erdd

I think we need a lot more information ie photo's of the pins that break, drawings or pictures of the joints and materials involved.


RE: King pin disconnection from trailer

erdd...I agree with Desertfox....we need a lot more information in order to offer any comments.  Please explain the conditions a bit more fully and as DF suggested, post a few photos.

RE: King pin disconnection from trailer

This is the sort of question you ought to be asking the manufacturer of your couplings.  They should be very interested indeed, because when this finally kills somebody they are going to have a huge problem.  If they aren't, you have your answer:  Find a different supplier.

In the meantime (and jumping to conclusions about the sort of arrangement you've got), a couple of lines for investigations might be:

Have you got the right size of kingpin?  2 inch is fine for most applications, but there comes a point where you need to step up to 3 1/2".  In among the advertising, there's some useful guidance in

Also worth checking that you've bought pins machined from forgings, not from bar stock.


RE: King pin disconnection from trailer


Also worth checking that you've bought pins machined from forgings, not from bar stock.

Would you mean by this that his equipment breaks because he bought forgings, or that he should buy kingpins machined from bar stock?

RE: King pin disconnection from trailer

Step 1:  Understand what unique failure modes are occurring.  You write loose/break.  This may be one or more failure modes.

Step 2:  Solve the problem.

A problem of this of this criticallity is well beyond what you should be expecting to have solved for free on an internet forum.

While we would love to SPECULATE on the cause of your failures you need to hire a qualified forensic engineer.

You should do this even if the manufacturer is looking into the problem.  Most manufacturers jump directly to step 2.  This rarely produces a satisfactory result.

RE: King pin disconnection from trailer

So we don't give the OP the wrong idea, my preference is:

Forging: More suitable

Bar stock:  Less suitable.

I think that's what my posting implies as well:  Happy to discuss why if you want to raise it in the Engineering Language/Grammar Skills Forum.


RE: King pin disconnection from trailer

not knowing the product these would be questions i would ask.

Typical size king pin for this application.

what is the material normally used.

what are the manufacturing methods & heat treat.
an audit is required of the manufacturing.

what type of test are required for reliability.
Non destructive test to ensure there is no cracks
or other defects.

have an accredited lab analized the failed king pins
spectra analysis of the material type. is it what it's should be. are there any evidence from the micro sections.
is there evidence of embitterment.

take some of the kink pins an do destructive test.
what are the results.
take penetrant & or magnaflux test is there any evidence of

my two cents

RE: King pin disconnection from trailer

If the King Pin is not failing, then as previously suggested, I would look at the size or model with respect to the tractor hitch assembly.

If the King Pin is failing in shear or otherwise, I would look as to where the pin was made and the quality of the steel.   

Mike McCann
MMC Engineering

RE: King pin disconnection from trailer


Your OP is very much lacking in detail.

Also it seems you are not at ease with English language and the wording in the OP does not clearly define your problem.

In order to get a useful response here you need to take the time to formulate a better worded, clearer more detailed question./ As previously suggested, a picture or drawing would help a lot.

The meaning of repetitive loose/break is one statement that is not clear.

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RE: King pin disconnection from trailer

New Zealand Standard NZS5450 (I think that's the right number) has a lot of practical advice on avoiding kingpin retention/engagement problems. It is based partly on an SAE standard - but not sure which one. Some problems which can occur include:
-Lack of proper engagement due to (a)wrong plate thickness for the kingpin model being used or (b) Lack of flatness of the trailer skidplate (often due to how the skidplate is welded to the trailer structure) or (c)  Unauthorised use of plastic 'lube' plate between mating surfaces.
-Lack of strength/rigidity in the skidplate supporting structure
-High torsional loading of the connection due to use of  wrong fifthwheel type (especially important for torsionally rigid trailers like tankers)
This is not an exhaustive list but might be a starting point in your investigation.  

RE: King pin disconnection from trailer

He may have given a hint - though without any reply I can't tell - by saying loose/break.

If the pins were wearing (getting loosened in the connection, or getting looser in the slot and jiont) abormally, the loosened pin is more subject to bending and failure.

So the ultimate failure is the break,
but the symptom is the loosening that leads to the broken pin,
but the root cause is whatever is causing the pin to loosen in the socket and joint in the first place.  

RE: King pin disconnection from trailer

There are so many variables here and the OP has left us hanging without further information since Nov 8.

Is it a welded style king pin?  Failure modes, welds, king pin shaft, fifth wheel jaws.

Is it a bolted king pin?  Failure modes, king pin forging, bolt failure (loosening, shear), fifth wheel jaws.

Is it a drop in king pin?  Failure modes, king pin forging, King pin fit taper damage (king pin or mount), Retaining nut tightness, fifth wheel jaws.

Take your pick.


RE: King pin disconnection from trailer

I wonder if errd is still alive or is just a plain rude person. Note that he has logged in on 2nd of Jan, but didn't even bother to write a line. He also did the same on his other post.
Perhaps they closed the office, so he doesn't have access to the computer anymore...or the person typing on the keyboard is in holiday now.

RE: King pin disconnection from trailer

Well, every time one of us posts, if he/she put their e mail address in correctly when they registered, they get an e mail since they have the thread marked, so he/she is doing a good job of ignoring a lot of people working hard to help them with their "Serious" problem.


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