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can you substitute class 12.9 for 8.8 bolts?
Hi all,
Is it safe to use bolts that are stronger than specified by Rotax, or will there be an unintended consequence (e.g., damage to the internal threads, etc.)? I over torqued the metric bolts on my water pump housing and will be pulling the engine to remove & replace the affected bolts. Unfortunately, LEAF is quoting 6 weeks to get replacement M6x90 bolts. I have located a domestic source of M6x90 bolts, but they appear to be considerably stronger as they are Class 12.9, vs Class 8.8 as found on one of the ruined bolts in my possession. A quick internet search shows that Class 8.8 has a yield strength of 660 MPa while Class 12.9's yield strength is 1100 MPa. Is it possible to do damage using bolts that are too strong? Will the torque specifications be the same? If not, is there a known conversion? If not, then I will not use them and will just wait for Christmas. thanks in advance, -dbh |
Stick with 8.8
Quote:
12.9 bolts have a high ultimate tensile strength but a low ductility (they're more brittle). If they're plated they're also much more prone to hydrogen embrittlement. That could be crucial in this instance. They also need to be torqued up to a much higher proof load than 8.8s. In engineering, more is sometimes less. Play it safe and stick with 8.8. |
Bob,
I agree with your observation, but look at the application. I wouldn't lose any sleep over more brittle bolts in the application of water pump pressure boundary retention. Landing gear would be a different story. Rich |
Have you tried contacting Lockwood in FL? They may have what you need in stock.
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Quote:
The fact that the OP is asking this question on this forum suggest to me that he is not an engineer and therefore my recommendation to him is to use the Rotax specified fasteners at the Rotax specified torque. My other concern would be whether when the OP overtorqued the original bolts he damaged the female threads at that time. |
Try McMaster Carr
Dave,
Did you try McMaster Carr. They usually have most in stock Gary |
Don't Change the Torque
There's no change to the assembly torque if you go with a higher-strength bolt, provided -
The thread form hasn't changed, The material remains steel, the head type is the same, the thread engagement is the same, the same washers or nuts are used, and the same lubrication or lack of it is used upon assembly. If all these are true, then the torque to preload relationship is unchanged, and you should use the original assembly torque to obtain the bolt preload, the clamping force, which the designers wanted. A higher-strength bolt DOES NOT need higher torque in this type of a replacement situation. It might well be damaging. You'd use a higher torque in a wholly new design that fully uses the higher strength of the new bolt, but not for a replacement bolt. Dave |
May I request some clarification please? Here's my understanding, and I would appreciate the education if I'm misinformed.
The clamping force applied by a bolt is fundamentally a function of its elastic modulus, and all steels have roughly the same modulus of elasticity, about 3.0^7 psi. The mechanical difference between the 12.9 and 8.8 is yield strength (roughly 90% of 1200 MPa vs 80% of 800 MPa). That means the 12.9 can be stretched further (i.e. torqued to some higher value) before it yields, which of course increases the clamp force. If an 8.8 and a 12.9 have the same modulus, then stretching them equally (same torque, assuming same thread pitch, form, and friction) should result in the same clamp force. There's no reason why a 12.9 mustbe torqued to a higher level. The required clamp force is usually dictated by the anticipated cyclical stress; we want the clamp force to be higher than the cyclical stress which tries to pull the clamped joint apart. If an 8.8 was adequate in this regard, a 12.9 should be fine. Lower ductility for a 12.9 has no bearing on this discussion. It merely indicates a short plastic region on the stress-strain plot. The stress applied here, using the torque specified for the 8.8, would not put a 12.9 anywhere near that plastic region. So what am I missing? POSTSCRIPT....just posted to find Dave has already covered the question. |
My guess is that the original 8.8 bolt will be a mil spec product with cad plating. Cad plating is very lubricious and has a lower coefficient of friction compared to unplated steel. 12.9 fasteners are normally black steel due to the quality control problems associated with plating such a high strength alloy. Of course using a black bolt has it own shortcomings in terms of corrosion. But all other things being equal, the black 12.9 will have to be torqued to a higher level than the cad plated 8.8 to achieve the same clamping force. Now one could go to Milspec-HDBK-60 to get a detailed formula for calculating the torque based on various friction co-efficients. But I don't think the OP is going to do that.
So under the circumstances I still think that the conservative approach for the OP to take would be to obtain the engine manufacturer's specified bolts with the same plating and use the engine manufacturer's specified torque. |
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Bob,
As an engineer you know we make our bread and butter answering the questions about what should be done when the OEM specs aren't met. I would not disagree with your statements, but I do think you are being overly conservative in this application. Maybe it's just my experience as a home builder seeing other applications, but I would still be comfortable with his suggested substitution. There are only a few critical applications like landing gear, cylinder head retention and prop bolts that raise the hairs on the back of my neck. Even if one bolt did fail on the water pump (and why would it since preload is most of the stress) you'd just get a leak you could address. I think the probability and consequences in this application result in low risk. Rich |
You guys are way over thinking this. The substitution is fine unless we are talking about cylinder bolts, or case bolts, or rod bolts, but if that were true, they wouldn't be grade 8.8, which is the SAE grade 5 equivalent.
It sounds like they used the low strength bolts to save the case and make it so the bolt would fail if over torqued. I think the only risk with a higher strength bolt is overtorquing and damaging the engine case (or whatever the bolt is threading into). Tim |
Quote:
I have been impressed with the technical calibre of posts made by all the contributors to this thread and I'm confident the OP can now make an informed decision. |
thank you
Quote:
I thank you all for the wealth of information and opinion. Many decades ago I was an engineer, but I think that only makes it dangerous for me to rely on my own analyses; thus the reason I asked the question here. The responses were exactly the kind of engineering discussion that is so informative on what might be perceived as an apparently trivial topic (nothing is trivial in aviation engineering!). Jesse gets the credit for the optimal solution:) : Lockwood's web site shows that not only are they in stock, but they're much cheaper than LEAF's (except for the shipping fees.) Hopefully, I will have the best of both worlds: original specifications and timely availability. In the event that they aren't shipped any time soon, I will wait until I find the original spec 8.8's. thanks again for a lively discussion, -dbh :) |
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Update 11/16/15, notice posted on Van’s Air Force:
Hi all,
Is it safe to use bolts that are stronger than specified by Rotax, or will there be an unintended consequence (e.g., damage to the internal threads, etc.)?
I over torqued the metric bolts on my water pump housing and will be pulling the engine to remove & replace the affected bolts. Unfortunately, LEAF is quoting 6 weeks to get replacement M6x90 bolts. I have located a domestic source of M6x90 bolts, but they appear to be considerably stronger as they are Class 12.9, vs Class 8.8 as found on one of the ruined bolts in my possession.
A quick internet search shows that Class 8.8 has a yield strength of 660 MPa while Class 12.9's yield strength is 1100 MPa. Is it possible to do damage using bolts that are too strong? Will the torque specifications be the same? If not, is there a known conversion? If not, then I will not use them and will just wait for Christmas.
thanks in advance,
-dbh
----------------------------------------------------
I spent a few hours at the local A&P trying to get the carbs balanced, but we never got the engine to run smoothly. That’s the good news. When we were standing around trying to figure out what we were missing, I noted that the front gear strut was wet. Following the moisture trail upwards led to the discovery that one of the bolts holding the water pump to the engine case was ‘loose’. Loose, as in “slides out with no resistance.” I had a sinking feeling and fished it out completely. Yep, it’s fractured right though the threads.
If you go back to http://n76012.blogspot.com/2015/02/46-06-engine-mount.html, you’ll see that I had to remove the water pump housing to get the engine mount on. Thinking back, I remember being amazed at how high the torque settings were to seat those bolts back onto the water pump. In retrospect, I’m guessing I used ft-lbs instead of in-lbs and probably overtorqued these guys by a factor of 12. Damn.
This discovery was last Wednesday. I’m looking for some time to go through the various manuals and catalogs to order the correct replacement bolts from LEAF. I expect it will take two days of labor to remove the engine. A friend has offered to help back out the remnants of the bolt from the engine, and hopefully we won’t find any more broken bolts to deal with. With any luck, I’ll have Thursday & Friday of next week to tackle this setback.
Fractured water pump bolt
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can you substitute class 12.9 for 8.8 bolts?
thank you
: Lockwood's web site shows that not only are they in stock, but they're much cheaper than LEAF's (except for the shipping fees.) Hopefully, I will have the best of both worlds: original specifications and timely availability.
