Al-Ko Trailer Tongue failure

I have followed this post with quite some interest. I believe there is some additional information that has been missed however. The bolted joint in question uses M12 x 100 bolts with Nylon lock nuts. The actual joint couples a C channel and a square tube. The bolted joint is subject to transverse vibration forces which translates to shear once the joint has loosened. If the "locknut" nylon has become dry and hardened it may loosen as one is driving down the road and eventually separate. Then the bolt will fall out. Because both elements in the joint are "flexible" as in the square tube being hollow and the C channel can flex or distort some (called fretting) the eventual result is loosening of the fasteners. The design of the joint does include threads in the shear plane which is always a problem. Increasing the length of the fastener and adding washers to eliminate the shear in threads may help. Some of the posts have suggested using a "spacer" which I believe would have to be installed inside the square tube to act as a method to keep the joint from distorting and the prevailing joint torque from loosening as a result of the less flexible / compressible joint, however this would require drilling out the tube and potentially weakening the tube. One would have to probably weld this new spacer and of course it is a galvanized finish which has it's own hazards. And what diameter the spacer? If I have misunderstood the intent of the spacer idea, sorry. OK, so my answer is without significant engineering study, and experiments that could lead to an accident, I suggest checking the existing bolts and nuts for tightness and replacing them with at least 10.9 strength and new "locknuts".. The rated torque value for this bolt is 105 N-m or 77 Ft/lbs. Due to the forces translated to this joint this joint will loosen as the miles pile up. Checking the joint periodically should become part of the pre-towing checklist. Keeping a spare couple of bolts and nuts in the kit is also not a bad idea. I have seen on my own trailer the effect of the bolt necking as it was slipping around in the joint and I replaced it with new bolts and locknuts. Phew! I almost had the same experience as Dave. Just for more information If you look at Bolt Science.com for some interesting ways bolts fail. http://www.boltscience.com/

On Tuesday, August 2, 2016 at 6:26:31 AM UTC-7, Discus 44 wrote:
I have followed this post with quite some interest. I believe there is some additional information that has been missed however. The bolted joint in question uses M12 x 100 bolts with Nylon lock nuts. The actual joint couples a C channel and a square tube. The bolted joint is subject to transverse vibration forces which translates to shear once the joint has loosened.. If the "locknut" nylon has become dry and hardened it may loosen as one is driving down the road and eventually separate. Then the bolt will fall out. Because both elements in the joint are "flexible" as in the square tube being hollow and the C channel can flex or distort some (called fretting) the eventual result is loosening of the fasteners. The design of the joint does include threads in the shear plane which is always a problem. Increasing the length of the fastener and adding washers to eliminate the shear in threads may help. Some of the posts have suggested using a "spacer" which I believe would have to be installed inside the square tube to act as a method to keep the joint from distorting and the prevailing joint torque from loosening as a result of the less flexible / compressible joint, however this would require drilling out the tube and potentially weakening the tube. One would have to probably weld this new spacer and of course it is a galvanized finish which has it's own hazards. And what diameter the spacer? If I have misunderstood the intent of the spacer idea, sorry. OK, so my answer is without significant engineering study, and experiments that could lead to an accident, I suggest checking the existing bolts and nuts for tightness and replacing them with at least 10.9 strength and new "locknuts". The rated torque value for this bolt is 105 N-m or 77 Ft/lbs. Due to the forces translated to this joint this joint will loosen as the miles pile up. Checking the joint periodically should become part of the pre-towing checklist. Keeping a spare couple of bolts and nuts in the kit is also not a bad idea. I have seen on my own trailer the effect of the bolt necking as it was slipping around in the joint and I replaced it with new bolts and locknuts. Phew! I almost had the same experience as Dave. Just for more information If you look at Bolt Science.com for some interesting ways bolts fail. http://www.boltscience.com/

There is one simple solution to bolted joints losing their preload tension (which is what torquing accomplishes): use Belleville spring washers in place or in addition to the flat washers. The washers maintain constant preload tension over a wide range of conditions.

Tom

Thanks for posting this Dave.

Finding bolts with the right unthreaded length wasn't easy. Just replaced the 8-year-old 10.9 grade bolts with AN8-36A and AN365-820A from Aircraft Spruce. $20 with spares and shipping. AN8-37A bolts would work too. The unthreaded part is long enough and the threaded part comes to the top of the Nylock when using locknuts.
Torqued to 45Lb. Does that seem right?

The old bolts didn't look bad. A little corroded, but still tight. The threaded part was not damaged but too long. The unthreaded part was too short for the job.

Do this at home! Chock the trailer. Jack up the tongue aft of the bolted connection. Release the parking brake. 19mm fits metric and STD. Loosen both. Change one at a time, and don't tighten until it's reassembled as the AN8 bolts are a snug fit.
Jim

I was lucky to discover the sheared forward bolt before my 1200 km trip to the French Alps this spring
This bolt was exactly the same position as in your picture. Only the head was gone, with the rest of the bolt still in.

https://dl.dropboxusercontent.com/u/...obra-bolts.jpg

I changed both bolts with original ones and installed them with the correct torque.
My Cobra trailer was build in 2006, holds a Duo Discus xT and weighs 1300 kg.
Average mileage is about 5000 km per year.

I think best practice is to change these bolts at regular intervals ( every 4-5 years?)





Op donderdag 7 juli 2016 23:21:21 UTC+2 schreef Dave Springford:
On my way home from Nephi (towing my trailer with the RV) I stopped for fuel and found one of the bolts that holds the Al-Ko trailer tongue together was gone as can be seen in the picture he

http://www.foxonecorp.com/images/20160703_154501.jpg

In the picture you can see the front hole that has been vacated by the bolt. Fortunately, the second bolt didn't fail before I stopped for fuel and also fortunately, five minutes down the road was a well stocked hardware store. So I was back on the road an hour later.

The original bolts are M12 x 100 with an 8.8 rating giving a tensile strength of 800 MPa (116,000 PSI). The holes in the tongue will accommodate a 1/2 inch bolt so I installed 1/2 x 4 inch grade 8 bolts with a tensile strength of 150,000 psi.

These imperial bolts are about .02 inches larger in diameter than the M12 and also about 30% stronger, not including the added effect of the extra diameter.

I emailed with Alfred Spindelberger and he indicated there is no harm in using stronger bolts.

So... my suggestion to those that tow their trailer with an RV is to check the bolts on the tongue, and maybe just go ahead and replace them before yours fail. It also might be a good idea to replace them every 4-5 years if you tow a lot with your RV (my trailer was new in 2012). Then, keep a spare set of 1/2 x 4 grade 8 bolts in the trailer just in case.

A few dollars in bolts might save you thousands in repairs!

Now, at every fuel stop, as well as visually checking tires and wheel hub temperature on the trailer, I'll also be checking the bolts.

On 08/08/2016 08:26, RuudH wrote:

I was lucky to discover the sheared forward bolt before my 1200 km trip to the French Alps this spring
This bolt was exactly the same position as in your picture. Only the head was gone, with the rest of the bolt still in.

https://dl.dropboxusercontent.com/u/...obra-bolts.jpg

That photo shows a classic fatigue failure. But it has failed through
the shank not the threaded portion. Rather it appears to match the
corrosion marks on the remaining bolt. It also looks like the brittle
fracture area is quite a large portion of the total area, which implies
that it is quite highly stressed.

Solution? Bigger bolts, more bolts or stronger bolts, or all of these.

Ian

I'm no engineer, but seems to me the length of the coupler puts additional stress on bolts. Seems to me that if the bolts were closer to the hitch then there would be less stress on them by a lever type action. I've towed heavier boats with trailer couplers similar to this but rated for 5000lb: https://www.zoro.com/fulton-trailer-...56/i/G3226027/ and you can see the couple is much shorter.

Of course the design with the break away and brake would have to be engineer differently.

I like the idea of Cobra having galvanized tongue but wonder why they did not use galvanized bolts.

Seems like there should be some thought towards using a different type of tongue on trailers that are being towed by RVs and motorhomes.
The tongues used on automobiles that are towed behind RVs allows the tongue to move up and down. Seems like that would eliminate most of the forces that are overstressing the mounting bolts on our glider trailers being towed behind vehicles that have a long distance between the rear axle and the tow ball.

I see your point, but towed autos are sitting on 4 wheels and therefore a hitch that allows vertical movement works. Try that on a 2 wheel trailer and the front of our glider trailers would emediatly drop to the ground.
JJ

Aw. Details.

On Wednesday, August 10, 2016 at 2:41:33 PM UTC-4, wrote:
Aw. Details.

If you're worried about the trailer tongue dragging with a flexible trailer hitch, just leave the front dolly wheel in the "down and swiveling" position while trailering. Might want to upgrade the bearing on it, though.

Chip Bearden
ASW 24 "JB"