Shackle bolt break

[TucsonJim]

Wileykid - Can you please provide your VIN? You can send it via PM if you wish.

Thanks,

Jim

[TedS]

This looks like classic fatigue failure from multiple cycles of stress. The "beach marks" are the visual clue of progressive crack propagation. These trailer axles generate enormous side loads when we turn on pavement, the maximum being a tight turn of course. 5th wheels can be turned tighter than bumper tow, so they can see the most sideways skidding. Watching from behind when someone is making a tight turn is alarming. The amount of distortion is easily visible between front and back axles. I suspect that proper mechanical engineering stress analysis isn't part of the frame design in the spring hanger area. It should be.

Yes, fatigue failure. A chamfer on the drilled hole would have taken away the stress riser edge.

[Wileykid]

Yeah that's a good set of pictures. It almost looks like brittle fracture starting at the exit of the grease hole spanning across to the raised section where it starts transitioning into a ductile failure.

The interesting thing would be to know the orientation of the bolt in the spring. If the hole was at the top, that would be one of the highest tensile stress areas depending on how much stress the springs put pushing outward during loading.

I am pretty certain that the hole was in fact pointing straight up on the break where the spring detached. I had put a 90* grease nipple on that bolt due to the drain pipe, and it was orientated straight down. Looking at the picture of it (I have removed the nipple), it is 180* to the hole, so the hole would be straight up. On the other one, no idea, it had a straight nipple, and the repair place took it off.

Interesting that it looks like the one that I know was orientated straight up (center picture), looks like it started at the bottom?

[AlexPeterson]

I am pretty certain that the hole was in fact pointing straight up on the break where the spring detached. I had put a 90* grease nipple on that bolt due to the drain pipe, and it was orientated straight down. Looking at the picture of it (I have removed the nipple), it is 180* to the hole, so the hole would be straight up. On the other one, no idea, it had a straight nipple, and the repair place took it off.

Interesting that it looks like the one that I know was orientated straight up (center picture), looks like it started at the bottom?

Yes, the crack initiated on the side opposite the hole in the middle picture. I'd bet the general cyclic loads (of the trailer weight) did not cause the failure, but was cumulative bending loads the springs put on it during sharp turns.

[TedS]

Typical fatigue failure. Origin at the hole.

[Scott'n'Wendy]

Typical fatigue failure. Origin at the hole.

Great....so wet bolts are creating another problem......

At least with heavier trailers.....which means the lighter trailers will go a few years more before seeing the same failures.....

[AlexPeterson]

A .010" chamfer on that hole might 5x or more the life expectancy, particularly if it were then polished. The great photo TedS posted shows this intersection to be virtually sharp. Big no-no.

[TedS]

Another observation is the orientation of the cross-drilled to the top, 12 o'clock., the highest bending stress position. The hole would have less effect if located at 3 or 9 o'clock for least, near 0, bending stress location.

[Jlawles2]

Being a 9/16" diameter bolt, pretty much any material with ductility over a yield strength of 40K (10k required impact factor say 4) should hold up to the shear. Bending and fatigue life I would have to break down and look up (those books were stolen 25 years ago).

[Wileykid]

I brought the trailer home for the weekend, the repair place is waiting on some Nylon bushings. I went under the trailer, and looking at the shackle bracket, even though they tried to straighten them, they are not, and the bolt hole on one side is elongated. Looking from back to front, the springs are not inline like they should be either. When I take it back Monday, the brackets are going to have to be cut off, and replaced with new ones.

[ajg617]

Typical fatigue failure. Origin at the hole.

This is one of my broken wet bolts which doesn't look similar to the the fatigue failure picture above? The other one is somewhere in Montana or North Dakota.

[TedS]

At first blush, that's an ugly looking bolt. The failure is a fatigue failure starting at the side opposite the grease hole. The flat, shiny surface indicates fatigue over time. Final failure was the half diameter of the bolt where the grease hole is. The pebbly texture was the last part to break.

What does the diameter look like? Smooth or is there evidence of machine marks?

[ajg617]

At first blush, that's an ugly looking bolt. The failure is a fatigue failure starting at the side opposite the grease hole. The flat, shiny surface indicates fatigue over time. Final failure was the half diameter of the bolt where the grease hole is. The pebbly texture was the last part to break.

What does the diameter look like? Smooth or is there evidence of machine marks?

I honestly don't know. The GD mobile tech was instructed to bring all the parts back to Elkhart where he was headed for the Rally. This was from the left rear most hanger where the bolt was broken in half but still wedged into the spring and hanger. The right rear bolt simply disappeared and the spring was hanging free. He did show me the left front hanger bolt and all the bolt serrations were worn smooth and the nut end of the bolt had a deep gouge accompanied by an out of round eye and damaged bushing. He changed all of the wet bolts - unfortunately I didn't see them. I've asked for pictures from GD but have not received them.

[Wileykid]

This all makes me wonder why this seems to be predominately with the 8K axles. Not sure one could say it is a bad batch of bolts, as ajg617 and my trailer are about a year apart. I would think the solution is a heavier duty bolt.

My take is, out of all this, if you have a trailer with the 8K axles, and over 10K miles, you should be pulling your wet bolts and checking them, or replacing them.

[ajg617]

This all makes me wonder why this seems to be predominately with the 8K axles. Not sure one could say it is a bad batch of bolts, as ajg617 and my trailer are about a year apart. I would think the solution is a heavier duty bolt.

My take is, out of all this, if you have a trailer with the 8K axles, and over 10K miles, you should be pulling your wet bolts and checking them, or replacing them.

Sadly, might be less than 10K miles. The Momentum owner that joined me at Glacier with the same exact problem only had 10K on his March 2022 build which supports your theory that it isn't just a bad batch of bolts. I also question why the 8K axles seem to suffer.