High tensile bolt modifications.

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coaster
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Joined: Sat Jun 30, 2012 4:10 am

High tensile bolt modifications.

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Hello, I would like to drill a small oil feed hole through the centre of a main bearing bolt.
The thread is M10 fine, 1.25 pitch and the hole might be in the region of 0.8mm.

Given that high tensile bolts are mostly surface hardened, is there a margin for this with the inner core being soft?

Edax
Edax
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Joined: Tue Apr 08, 2014 9:47 pm

Re: High tensile bolt modifications.

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coaster wrote:
Sun Sep 06, 2020 9:31 pm
Hello, I would like to drill a small oil feed hole through the centre of a main bearing bolt.
The thread is M10 fine, 1.25 pitch and the hole might be in the region of 0.8mm.

Given that high tensile bolts are mostly surface hardened, is there a margin for this with the inner core being soft?
For vacuum applications we often drill out bolts to prevent false leaks. Haven’t seen it lead to issues. So intuitively I would say it is OK, There is usually a lot of margin in the core.

But to be sure you might want to calculate it.

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Mudflap
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Joined: Wed Feb 17, 2016 8:36 pm

Re: High tensile bolt modifications.

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M10x1.25 6g external thread minimum minor dia is 8.272mm so the bolt shank dia would be around 8mm. A 0.8mm dia hole will give you a shank stress area of 49.76mm^2.

Assuming a 12.9 grade bolt which is around 1100 MPa 0.2% offset yield strength done up to 70 Nm:

Tension with a nut factor of 0.15 would be 70/(0.15*10*1e-3) ~ 46667 N
Stressin the shank is then 46667/49.76 = 944.5 MPa

Your static margin against yield is 1100/944.5 ~ 1.16 so it should be fine. Soon after tightening tension will relax by 15% or so and the margin will increase accordingly.

To work out the dynamic bolt loads you'd need to calculate the main bearing loads, do the joint diagram and then finally calculate the working bolt load so I can't estimate what the safety factor against fatigue would be, however by drilling the shank the bolt will become more compliant which means it will take a lower proportion of the overall joint load. This may be enough to compensate for the decrease in section area.

Also note that if the tightening is angle controlled (which it should be!), the drilled bolt will give you a different (lower) tension for the same angle.
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coaster
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Joined: Sat Jun 30, 2012 4:10 am

Re: High tensile bolt modifications.

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Thanks edak and mudflap, my grasp of the math held on until the dynamic loads.
I will need to gather more information for presentation, reciprocating weight, cylinder angle and offset, compression, rpm max.

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Mudflap
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Joined: Wed Feb 17, 2016 8:36 pm

Re: High tensile bolt modifications.

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I think the main thing to understand is that total working load on the bolts holding the main bearing cap is smaller than the total load acting on the cap.

So for example say you have a 2 bolt cap with 50 kN acting on the bearing, the actual load per bolt (excluding existing pretension) would be much lower than 25 kN. That is because under external loading the joint behaves as 2 springs in parallel. One spring is the bolt, the other spring is the main bearing panel. The joint force is distributed according to the rates of these 2 springs. A well designed bolted joint will have a bolt which is much more compliant than the section it is clamping so it will take much less force.

If you were to use the same bolt in an aluminium block and in a cast iron block, the bolt in aluminium would see higher working loads since its relative compliance to aluminium (E~70 GPA) is higher. Cast iron E is about 100 GPa so it will be stiffer with respect to the bolt.

Have a look here, from p51 onwards it tells you everything you need to know:
http://www.unbrako.com/images/downloads/engguide.pdf
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Tommy Cookers
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Joined: Fri Feb 17, 2012 3:55 pm

Re: High tensile bolt modifications.

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Mudflap wrote:
Mon Sep 07, 2020 9:55 am
If you were to use the same bolt in an aluminium block and in a cast iron block, the bolt in aluminium would see higher working loads since its relative compliance to aluminium (E~70 GPA) is higher. Cast iron E is about 100 GPa so it will be stiffer with respect to the bolt......
well ....
cast iron E is about 200 GPa

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Mudflap
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Joined: Wed Feb 17, 2016 8:36 pm

Re: High tensile bolt modifications.

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I meant to type 150 GPa
Last time I actually worked on cast iron components was over a decade ago so I'm far from being up to date on this
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