Yanmar alternator bolts breaking

[D Wayne]

I have twin Yanmar 3YM30s with 900 hours on them. Within a week of each other both alternators broke off the block leaving the bolts in the block and breaking the adjusting arm. I was able to extricate the bolts, re attach the brackets and new adjusting arms. Felt I was on the right track untill after 5 hours running the port alternator fell off again. Obviously I have a problem. The alternators are Balmar 100 amps. Any suggestions? I'm stumped.

[Vasco]

Hi tensile steel bolts.

[sailorchic34]

Originally Posted by Vasco

Hi tensile steel bolts.

Also called grade 8. If I recall right normal bolts are grade 5

[D Wayne]

The bolts are grade 8 and they lasted for hundreds of hours. My concern is that if I use a very hard high tensile blot and the break I may not be able to drill them for extraction with an easy out.

[Dsanduril]

Any chance the bolts are starting to back out due to vibration, then breaking? If so you could go Loctite, although I share your concerns about being able to drill and remove if you Loctite them in.

You might be able to tell if they are backing out by looking at the length of the broken stem compared to the alternator housing and the location of the stub in the engine. If the broken stem is longer than the alternator housing it would indicate that the bolt backed out some before it broke.

Another thought would be that the threads in the engine block have become worn and won't hold a bolt well. I had the same symptoms and had to resort to installing a Heli-coil in the block, but that was after about 18 years of use, so not entirely unexpected. Would be unexpected at the time on your engines.

Lastly, the Balmars do put more load on the bolts than the stock alternator. On my Volvo I ended up fabricating a bracket that attaches to the engine, reaches over the alternator, and supports the front end of the bolt. There were a couple of convenient bolts on the block that I could use to mount the bracket. That reduced the strain on the bolt a lot.

[Nemo55]

Insufficient load bearing points,,Dsandurill has got the right idea,,,fabricate a better bracket/mount...troubles over.

[D Wayne]

Thanks Dsanduril
The weird thing here is that something has changed. It was fine for 4 years and now it isn't. I agree that the Balmar alternators load the bracket more than the stock alternators. That's why we use them right? The stock bracket only has two wimpy bolts and the adjusting arm was a joke. I had stronger adjusting arms built and I'm pretty confident in them. Larger diameter bolts would be nice but reeming the holes with the engine in place would be a stretch. I'll look at the possibility of modifying the bracket to allow for more bolts. But what has changed? I have heard that Yanmars engine mounts are very soft and they do look pretty soft. I'm thinking that the vibration is just tossing the alternators right off the blocks. 900 hours seems pretty soon to be changing mounts.

[Nemo55]

If your drive pulley comes of the crankshaft and it has a vibration damper,,then i would suspect the vibration damper has gone soft and needs replacement,,,because there would have to be a source for all that undue torque which is loading the alternator belt,,,,that's a lot of leverage.

If, on the other hand your drive belt is shared with another device then i would look for a break or weak spot in the system which is allowing such undue torque.

[Dsanduril]

I agree, something has changed.

What I see from a distance is two possibilities; the new bolt just wasn't the same grade as the old one, or over the last 900 hours the bolt hole in the block has worn out and won't hold a bolt properly. In the latter case the bolt could be just as good but would fail quickly as it moved around.

Yannie mounts are soft, but with the whole arrangement moving around I wouldn't expect the movement to cause too much motion in the alternator subsystem. Have seen stranger things though.

Either way, 900 hours seems way short for this kind of issue, without being able to touch/feel its a little hard to know what else to suggest.

I am curious, I have been assuming we are talking about a single bolt that acts as the hinge pin for the alternator. Is that correct, or are we talking about more than one bolt on a bracket, in which case I could be way off base?

[D Wayne]

The alternator is attached to a bracket that is bolted to the block with two bolts. It is the two bolts that attach the bracket to the block that are breaking on both engines!

[Dsanduril]

Wow! That changes things for me a bit, because I was really thinking of the overhung loads. Obviously these are still there, but not in the same way.

Now it does sound like vibration from somewhere. Mounts? Harmonic Balancer? I'm kind of at a loss, sorry.

[Florida Mike]

I would start off comparing the original brackets to the ones you have now and make sure that the surface area is exactly the same. Also check that your bolts are not bottoming out at the same time they seem to be fully tight. You might also check the other engine to make sure those bolts and not back out slightly. This would give you a reference. Checking the damper is a possible cause but would seem unlikely on both engines at the same time.

At this point I would put grade 8 bolts back in and check them daily and maybe remove one at a time after a day’s run and inspect for any possible where. I assume you have put the bolts back in the engine without the brackets and check them for looseness. It is hard to imagine any alternator bracket breaking to grade 8 bolts as that is all that is used on tractor-trailers. Just my two cents, Mike.

[belizesailor]

Yeah, I think you guys are headed down the right path now. Bigger alternators on the standard bracket, vibration, etc. If it was just happening on one engine that wold be more obvious, but what changed that would effect both engines and when?

Also, could be an alignment issue which would make sense given that, as I read it, you've changed alternators and brackets on both engines.

[DeepFrz]

If the bolts are loosening up then try Nord Lock.
Nord-Lock - Your trusted partner for bolt optimization

Also make sure you aren't over torquing the bolts, and as someone above mentioned that the bolts are not bottoming out.

[D Wayne]

The bolts are not backing out and in any case the lock tight would make it neerly impossible to remove them when they do break. The original bolts were grade 8 and I replaced the starboard bracket with aftermarket grade 8 bolts. When the port bracket failed a week later I replaced them with original bolts ordered from Yanmar. They were the ones that broke after a few hours. As it turned out when I extracted them this morning I see that they are grade 7 bolts. I'll replace them with aftermarket grade 8s but I really don't see this as a fix.