Need help from "bolt experts"

[ingrid75]

I have a custom motor mount/alternator mount on my Beta 25 engine. The darned thing keeps breaking the bolts that hold it to the engine. Right now I have 3 bolts broken off below the surface of the casting that I will have to get removed when I get to some civilized place. My question to those who know about these things is this: is there not some super-strong bolt material that will be strong and/or tough enough to withstand the vibration and sheer stress that cause these bolts to break? And, if so, what is it?

[Fiveslide]

If vibration is causing the break, I had that happen on a Cummins diesel. It was many years ago, so I don't remember the grade/material bolt I bought to replace them. If I remember correctly, you're not looking for "super-strong", you need softer and more flexible to withstand the vibration. I'd consult a local fastener supplier like Fastenal.

[JstaRebel]

Look on the head of the bolt and see how many slash marks are on it.. If there are none, it's a grade 2 bolt. If there are three marks, it's a grade 5 bolt which is much stronger. If it has 5 marks, it's a grade 8 bolt. These are what I would suggest using as they are normally used on shaft couplers, and motor mount applications where extra strength is needed. Also, if your alternator belt is aver tightened, this will cause extra stress on the fasteners. Try loosening it just a little bit..

[Pauls]

The advice to use grade 8 bolts is good. In addition you must be sure that they are correctly tightened. Correctly means fully torqued. Even high grade bolts will fail if they are under torqued.

[normdeeley]

It has to be from excess vibration. Fix that and you fix the problem. Possibly a strut to the engine. Pic would help.

[Eigenvector]

Without seeing the layout of the bracket or the fracture surface of the bolt, here’s what I can tell you……….assuming the axis of the bolts are parallel to the alternator shaft.

If the bolts are tight (roughly 80% of yield), the clamping friction during makup of the bolts carries the entire load from the bracket to the block. If a bolt gets loose, now you’ve possibly introduced one or both of the following: shock loading due to a shear clearance / cocking of the bracket which introduces a bending stress.

Vibration will loosen bolts. Short bolts and studs are more problematic than longer ones. If you think of the stretch of the bolt under torque as a spring, the longer the bolt the more stretch at a given torque. Loosening 0.001 inches axially could eliminate 100% of the preload in a short bolt but only 10% in a long one. In high vibration environments, spacers with longer bolts/studs a typically used.

For a proper bolted joint as assumed, the bolt would see a constant stress equal to the initial tightening stress. It will not fluctuate.

If all three broke at the same time you’ve got a displacement driven problem.

Check to make sure the surfaces are parallel. No non-parallel surfaces will cock the bolt during installation, introducing an addition bending stress.

I’d repair, replace, torque, run……..and then check the torque again. Locktite is also an option.

As for stronger grade bolts: The stronger the material, the greater it’s fatigue resistance. The S-N curves for fatigue life are based on percentage of yield of the material.

However, SERIOUS CAUTION here. I see it all the time where people are breaking bolts and replace with Grade 8 only to now start forcing the failure into the Very expensive components they’re bolting together. Best to always find out what the problem is, rather than throwing stronger bolts at it.

My 3 peso’s of advice………

[Dave_S]

Originally Posted by ingrid75

I have a custom motor mount/alternator mount on my Beta 25 engine. The darned thing keeps breaking the bolts that hold it to the engine. Right now I have 3 bolts broken off below the surface of the casting that I will have to get removed when I get to some civilized place. My question to those who know about these things is this: is there not some super-strong bolt material that will be strong and/or tough enough to withstand the vibration and sheer stress that cause these bolts to break? And, if so, what is it?

When you say below the surface of the casting, do you mean just below ?

Just below is the point of greatest streach/stress of the bolt. This could imply,
Overtightend
Poor custom design of the bracket
Incorrect thread
Incorrect thread form
Poor quality bolts
Bolts you may have had plated that haven't been through de-embrittlement

If you are after high tensile bolts, quality Socket Head Cap Screws (SHCS) such as Unbreako are equivalent to a grade 12.9, adding a hardened washer will help.

I'd be looking hard at the custom bracket, even crap bolts should work on a well designed mounting joint.

Make sure of the thread some similar threads can produce serious stress on the bolt before being fully torqued.

[Jd1]

I had a similar issue where one particular bolt on an alternator mount kept breaking even when replaced with a higher grade. It made no sense what was happening. When I eventually took the alternator and mounting hardware off I found that one mounting bracket was an aluminum casting that had cracked in several places allowing all kinds of weird forces to interact with the bolt in question. I replaced the bracket (with a much stronger steel version) and have not had a hint of trouble since.

[Cadence]

Originally Posted by ingrid75

I have a custom motor mount/alternator mount on my Beta 25 engine. The darned thing keeps breaking the bolts that hold it to the engine. Right now I have 3 bolts broken off below the surface of the casting that I will have to get removed when I get to some civilized place. My question to those who know about these things is this: is there not some super-strong bolt material that will be strong and/or tough enough to withstand the vibration and sheer stress that cause these bolts to break? And, if so, what is it?

Grade 8.

[a64pilot]

Know though that high strength steel is usually high carbon steel, and what that means is it will rust just by looking at it, if you go to grade 8, be sure to paint it or corrosion proof it somehow even if its just a coating of grease

[TheThunderbird]

Avoid ss
Use high tensile bolts, in Europe denominated as 8.8 or better 10.8

Use open washer.

Eventually, make the support stiffer!

Mine have 10mm diameter

[Cadence]

Originally Posted by a64pilot

Know though that high strength steel is usually high carbon steel, and what that means is it will rust just by looking at it, if you go to grade 8, be sure to paint it or corrosion proof it somehow even if its just a coating of grease

And a little never seize wouldn't hurt.

[lvictorlucas]

First question is to decide if the problem is yield or corrosion related. Stainless steel cannot be made very strong and must be kept exposed to oxygen in the air or will corrode quickly. Titanium bolts are a good choice for most applications however you will need to verify the different materials and isolate them with it a dielectric. Using a dielectric Greece and an electrical insulator is a good move for almost all typical applications. Another possibility is to consider going to larger diameter bolt if that is feasible in your application. Just following these simple considerations will probably handle 99 percent of typical problems encounter in consumer applications. It may be necessary to call a bolt manufacturer not just a reseller about the specs of the Bolt. More exotic applications will typically require titanium bolts that are both hardened and serfaceis treated.

[zstine]

Grade 8 or Titanium may stop the bolts from breaking, but IMO breaking bolts is not the problem, it is a symptom. Your alt mount is not a high load application, should not vibrate excessively and should definitely not break bolts! If vibration is causing fatigue failure, stronger bolts will just cause the next weakest thing to fail, likely the alternator bearings or something (unassuming the bolts aren't undersized like 1/4-20)... Fix the cause of the vibration. Your custom mount may not be adequately supported allowing vibration, or maybe it has a resonance frequency near the engine vibration at cruising rpm, which again would be solved by stiffening the mount.

[jmschmidt]

Silicon bronze is the strongest and most elastic of all metals generally available and used for marine purposes. Also the most expensive.