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  1. #1
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    Default Bolts for couplings

    Hi...can someone tell me if it is written in the VSB1 or other relevant trailer rules, what size and strength bolts are to be used for bolting the coupling to the drawbar? I am referring to trailers less than 3500kg's

    Thanks

    Craig
    Last edited by surfin; 20th May 2010 at 11:47 PM. Reason: Spelling!

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    Dont know of the specs but I would think either 1/2",12mm or 9/16" 14mm or 5/8" 16mm H/T would be fine.
    For a 3.5t weight I would probably go with the 5/8" 16mm.

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    Craig

    I'm not going to guess or "reckon"

    I assume you already have a coupling
    what size bolt is it drilled for?
    is it manufactured for high tensile bolts? or plain black bolts?
    Is your draw bar manufactured for HT or plain bolts?
    There is a major difference in strength for the same size bolt

    Find a vehicle engineer and ask
    regards from Canmore

    ian

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    Its an Alko 3500kg electric coupling.

    I want to make sure I get the right size and strength, rather than guessing. The VSB1 is the bible for small trailers but where is it said what bolts need to be used for various items, including the coupling? Yeah an engineer might say something, but thats just his experience and interpretation of the situation. Is it written somewhere as an ADR or something?

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    Being completely out of my field here can you explain the difference between a Coupling and Coupling plate and Drawbar that is manufactured to suit H/T Bolts and one manufactured to suit MS Bolts.

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    There is no specification in the VSB1 for what strength bolts that are to be used when attaching the coupling. There are two schools of thought. Many people in the trailer industry will tell you to only use High Tensile bolts when attaching any coupling to a trailer. I have personally spoken to Trigg Bros in regard to this very question, and their take on it is that under a high impact load, such as an accident, a high strength bolt can shatter, due to the fact that it is more brittle than a mild steel bolt, which will stretch and hold long before snapping.

    For me, a good comprimise would be Grade 5 rated bolts, as opposed to Grade 8 or 9.

    Quote Originally Posted by pipeclay
    Dont know of the specs but I would think either 1/2",12mm or 9/16" 14mm or 5/8" 16mm H/T would be fine.
    For a 3.5t weight I would probably go with the 5/8" 16mm.
    The holes are only 1/2". And the coupling are Aust. Standard rated, so I wouldn't be modifying it for anything bigger. To comply with this Aust. Standard, they have to be tested to three times their rating, with the 1/2" bolts.
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    Thanks Yonnee.

    I guess another question is would mild steel bolts be up to the task on a 3200-3500kg rated trailer that might carry a fair amount of weight at some stages?

    Its odd that the VSB1 is very particular in saying the couplings need to be ADR compliant etc etc, but you would think that the bolts are almost as important as they are essentially taking most of the load

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    I don't know if I'm missing something but wouldn't you simply contact someone like this

    General

    and ask them which bolts you would purchase for the task at hand or even ring someone like

    Vehicle Components - trailer parts suspension winches brakes axles

    and ask them a quick question???

    When I built mine, I went up to the local mechanic who had to check it for rego and picked his brains for anything I didn't know. I assumed that if he was licenced to check for initial registration he should know the requirements. By memory when I bought the coupling from the trailer spares place (Carasel Trailers in Western Sydney), they gave me the necessary bolts which are 8.8 HT ( it's an un braked box trailer ).

    Bob
    __________________________________________
    A closed mouth gathers no feet. Anon 2009

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    the following is dredging my memeory from over 30 years ago
    Quote Originally Posted by pipeclay View Post
    Being completely out of my field here can you explain the difference between a Coupling and Coupling plate and Drawbar that is manufactured to suit H/T Bolts and one manufactured to suit MS Bolts.
    Quote Originally Posted by surfin View Post
    Yeah an engineer might say something, but thats just his experience and interpretation of the situation. Is it written somewhere as an ADR or something?
    the holding mechanics for a HT bolt are totally different to those for a MS bolt.

    most of the strength and holding power of a MS bolt is in the shank of the bolt.

    with HT bolts, the strength and holding power is in the friction generated between the two pieces of metal being held together. Tightening the bolt to the specified torque generates the required friction. If the pieces being joined are not flat enough, yu can't generate the friction you need.

    Both types of bolted connection can also fail when the metal around the bolt hole tears, but that's a whole other story.

    For a demonstration of the principles, open your left hand and place the index finger of your right hand at right angles between the middle and ring finger. Now push one hand away from you and pull the other towards you.
    repeat the experiment with both hands open and pressed together
    you should find that when you press your hands together with light pressure they slide over each other fairly easily. When the pressure is high sliding the hands apart requires a lot more force. More force than that required to push your hands apart when the connection is only one finger.


    Quote Originally Posted by Yonnee View Post
    There is no specification in the VSB1 for what strength bolts that are to be used when attaching the coupling. There are two schools of thought. Many people in the trailer industry will tell you to only use High Tensile bolts when attaching any coupling to a trailer. I have personally spoken to Trigg Bros in regard to this very question, and their take on it is that under a high impact load, such as an accident, a high strength bolt can shatter, due to the fact that it is more brittle than a mild steel bolt, which will stretch and hold long before snapping.
    Yonnee
    I've never heard of a properly installed HT bolted connection shattering.
    The only instances I know of where a HT bolt has shattered, the bolt had been welded and teh heat of the welding had destroyed the bolt's toughness leading to failure.
    Quote Originally Posted by surfin View Post
    Its an Alko 3500kg electric coupling.

    I want to make sure I get the right size and strength, rather than guessing. The VSB1 is the bible for small trailers but where is it said what bolts need to be used for various items, including the coupling? Yeah an engineer might say something, but thats just his experience and interpretation of the situation. Is it written somewhere as an ADR or something?
    forgive me, but I would have though a 3.5T trailer was anything but "small". Once loaded you'd need something like an 8T truck to tow it.

    If you ask an enginner what you should get is his (or her) experience and interpretation of the applicable standard in respect to the situation you face.
    regards from Canmore

    ian

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    Quote Originally Posted by surfin View Post
    Thanks Yonnee.

    I guess another question is would mild steel bolts be up to the task on a 3200-3500kg rated trailer that might carry a fair amount of weight at some stages?

    Its odd that the VSB1 is very particular in saying the couplings need to be ADR compliant etc etc, but you would think that the bolts are almost as important as they are essentially taking most of the load
    ADR and Australian Standards are two completely different things, and while there is no requirement for what grade of bolts that are to be used for mounting the coupling, there are very strict guidelines for the safety chain.

    Just about all 2000Kg over-ride hydraulic couplings are supplied with mild steel bolts.

    Quote Originally Posted by ian
    Yonnee
    I've never heard of a properly installed HT bolted connection shattering.
    The only instances I know of where a HT bolt has shattered, the bolt had been welded and teh heat of the welding had destroyed the bolt's toughness leading to failure.
    Seen it first hand. All high impact (not necessarily high speed) accidents though. Two similar incidents where the 10.9 grade bolt broke with little stretch, whereas the lower grade bolt showed signs of major stretching, but stayed intact.

    Quote Originally Posted by notenoughtoys
    I don't know if I'm missing something but wouldn't you simply contact someone like this

    General

    and ask them which bolts you would purchase for the task at hand or even ring someone like

    Vehicle Components - trailer parts suspension winches brakes axles

    and ask them a quick question???

    When I built mine, I went up to the local mechanic who had to check it for rego and picked his brains for anything I didn't know. I assumed that if he was licenced to check for initial registration he should know the requirements. By memory when I bought the coupling from the trailer spares place (Carasel Trailers in Western Sydney), they gave me the necessary bolts which are 8.8 HT ( it's an un braked box trailer ).
    Flashy websites don't necessarily mean they're industry experts, and nor does being employed by them. The only 1/2" HT bolts the the Trailer Warehouse show in their list are too long to mount an Electric Coupling.

    Correct me if I'm wrong, Craig's original question was regarding whether the VSB1 states what bolts are to be used, in which case, the simple answer is No. I'm sure Craig has a fair idea of what he'd be comfortable using, and as he pointed out, even talking to an Engineer, you'll get one persons opinion.
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    here is my spin on it(as a boilermaker)On coupleings i was always taught to place a high tensile bolt and a mild steel bolt side by side to cover all bases.another thing to consider is wether you are using metric bolts or imperial,some grades/classes are very differant in terms of strength,shear,etc in equiv diameters.imperial bolts still are superior in some catorgories.just recently i had to inspect and repair a 4.00mx1.2mtr platform manufactured from 150 x75 channel that was hit by a train as it came into the maintenance depot at epping.the bolts were 16 and 20 mm 8.8 structural bolts.out off 40 bolts that were replaced25 stretched visibly but intact,10 sheared thru and 5 no sign of damage but replaced,not one bolt"shattered"but i would imagine that cap" scews" would have had heads poping of everywhere if they were to have been used on this job.cheerrs danny.

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    OK, I'll stand corrected... "shattered" was not the word I was looking for.

    However, the higher the tensile rating, the more 'brittle' the bolt becomes and will break rather than stretch. That's not to say they won't stretch, just not as much. But, we are talking very high impact damage, and probably so far in excess of the kind of stress most trailers would see in their lifetime.
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  13. #13
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    Quote Originally Posted by danielhobby View Post
    here is my spin on it(as a boilermaker) On coupleings i was always taught to place a high tensile bolt and a mild steel bolt side by side to cover all bases.
    snip
    Quote Originally Posted by Yonnee View Post
    OK, I'll stand corrected... "shattered" was not the word I was looking for.

    However, the higher the tensile rating, the more 'brittle' the bolt becomes and will break rather than stretch. That's not to say they won't stretch, just not as much. But, we are talking very high impact damage, and probably so far in excess of the kind of stress most trailers would see in their lifetime.
    can I correct a few misconceptions and outright falsehoods here

    MS and HT bolts are two completely different beasties. as you have noted Yonnee, HT bolts are not as ductile (stretchable) as MS ones.

    when you use MS bolts to join two bits of metal together, the strength of the joint is in the diameter of the bolt used. The bits of metal being held together must both bear against the bolt before the joint has any strength at all and the main function of the nut is to stop the bolt falling out of the hole. The joint fails when the shear force on the bolt is enough to either deform the material being joined or the bolt stretches, or abit of both.

    With HT bolts, a joint's strength comes from the clamping force generated when the bolt is torqued to specification. Torquing the bolt stretches the bolt which clamps the bits being joined together, meaning much more force is required to overcome the friction between the things being joined together. The higher the clamping force generated (ie the higher the strength rating) the more force the joint can resist. Leave the nut off and the joint efectively has zero strength.

    looking at failed joints and saying the bolts in this one stretched, the bolts in that one broke is a a very poor comparison of the two types.
    a joint which uses 12mm MS bolts installed in 15mm holes will "fail" as soon as the load comes on when one piece moves relative to the other
    the same joint using 10mm HT bolts installed in 15mm holes (yes I know in practice the heads will pull through the holes, but let me exagerate to make the point) will not fail when the load comes on because even thogh the holes are way oversize, the clamping force exerted by the bolts will still generate sufficient clamping force to hold the plates together and provide enough friction for the joint to carry the load.

    Daniel, because of the dfferent way the two types of bolt work, there is no point in putting HT bolts next to MS ones "to cover all the bases" because it doesn't work.

    please excuse the loud voice, but it's important that readers remember this point:
    because of the fundamentally different way MS and HT bolts hold a joint together, a joint designed for six 12mm MS bolts will be WEAKER if some of the bolts are replaced with HT ones.
    A joint designed for six 12mm HT bolts will also be WEAKER if some of the HT bolts are replaced with MS ones
    regards from Canmore

    ian

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    To reiterate what Ian was saying, the joint design determines the type of bolt.
    If you don't know which to use there are some clues but they are not rules. (and simplified somewhat)

    Typically the more bolts required, the better chance they will be a friction type joint and thus require HT bolts. These bolts can be tensioned up much tighter than they can withstand in shear. The more bolts means the more force clamping the joint together thus using friction as the predominant "glue".

    If there are fewer bolts required than maybe expected, then the load would be one of shear. Since HT bolts are prestretched as part of the tightening process and have higher resistance to breaking in tension than in shear, they must be tight to perform. Whereas MS bolts cannot be subjected to such high tensions without breaking and are used as shear pins in joints and as such the bolt shank is the "glue".

    If you leave a HT bolt loose to act in shear, it may still perform better than a MS bolt. Higher graded bolts have higher shear rates when tightened correctly. I am currently looking at a suppliers engineering data and that is a resultant fact.

    In most cases where HT bolts fail, there is usually a reason and often that may be post manufacturing treatment (ie: galv, cad plating etc). Either that or the joint deisgn was wrong.

    Look also for the surface of the jointed faces for clues. If they are rough you can be fairly sure they will be relying on the shear of the bolt to keep them together. A friction joint relies on clean and consistent faces that can grip under the tensions specified.

    The joint you are speaking of can also bring dynamic load into the design as the constant forward/aft motion of the forces can induce fatigue into low strength bolts.

    If it were me, I would be using HT bolts torqued up to the bolt manufacturers spec (look in a bolt handbook for advice). I would reducethe paint from between the faces and I would sand/file the faces flat (not pedantically so). I would also not reuse the bolts once undoing them. It is easy to overtorque MS bolts and not know it and as such they are not worth the risk.

    By the same token, if the hitch is chinese crap, you can disregard all that and just take your chances. In the event of a high loaded crash, you have more things to worry about than whether the hitch will stay together.

    Best of luck (my advice above is general bolting info rather than coupling specific advice).

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    Hmmm... well, they say if you don't learn something each day, you've wasted a day. So today was a good day. The two prevoius post have been quite informative.

    But I will add this. The bottom of a cast coupling is not particularly flat, and the pre-punched coupling plates you buy off the shelf are not exactly made to very tight tolerances, both with regard to hole size and placement, so based on information from the previous two posts, the "joint" between coupling and plate is not specifically made for High Tensile bolts.

    However, there are murmurings throughout the trailer industry saying that you must use HT bolts to attach couplings, and yet you don't need any qualifications to be a trailer industry "expert". As I said earlier, I have personally spoken to Trigg Brothers who informed me they test all their couplings to 3 times their rated capacity for Australian Standards compliance, with MS bolts.

    And lastly, there is no specification in the Vehicle Standards Bulletin #1 for building trailers less than 4500Kg, that states what type of bolt is to be used.


    (And I'm about to stick a fork in this thread, I think it's done.)
    Too many projects, so little time, even less money!
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