Finding Maximum Torque for a Bolt

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junquecol

Bruce
User
Finding max torque on a bolt is easy. Keep tightening till head snaps off, then back off 1/4 turn. :rotflm:
 

Hmerkle

Board of Directors, Vice President
Hank
Corporate Member
go ahead and laugh - this is the method - load-up and break 30 pieces - finding the Maximum yield, then calculate appropriate percentage to reduce the clamp load (not torque)

Note: we don't care how much torque you have to apply - we are looking for a number that properly "loads" a clamped joint with a bolt and nut or with a bolt in a tapped hole...

60% of the torque on a bolt goes to under head friction
20% to 30% of the torque goes to thread friction
20% max of the torque goes into clamp load!

so - if you require clamp load in your joint and you use an impact wrench - (Impact wrenches are considered to have a 20% variability in torque scatter) meaning you are likely under torquing or over torquing bolts using this method!

There is you Fastener Bolted Joint 101 class for today!
Tune in tomorrow when we talk about combating vibrational loosening!
 

JimD

Jim
Senior User
I believe what Hank said is correct but I also think his friction percentages look right for a dry fastener. If you lubricate it, those friction loads go down, the clamping loads go up, and the stress in the fastener goes up. So a safe dry torque and a safe lubricated torque are not the same. For some critical applications, the condition of the threads is specified. A washer under the head will also reduce friction under the head.

The time when you determine the amount of torque a bolt can take is when you are designing the connection. You determine the clamping force is appropriate then you find fasteners that can apply that force working well within their capability. In a car engine, you have to worry about dynamic loading of many joints. My point is just that for most of what I do, I am trusting that somebody else determined the safe load on the fastener when they designed the joint and told me the torque to apply.
 

Hmerkle

Board of Directors, Vice President
Hank
Corporate Member
I believe what Hank said is correct but I also think his friction percentages look right for a dry fastener. If you lubricate it, those friction loads go down, the clamping loads go up, and the stress in the fastener goes up. So a safe dry torque and a safe lubricated torque are not the same. For some critical applications, the condition of the threads is specified. A washer under the head will also reduce friction under the head.

The time when you determine the amount of torque a bolt can take is when you are designing the connection. You determine the clamping force is appropriate then you find fasteners that can apply that force working well within their capability. In a car engine, you have to worry about dynamic loading of many joints. My point is just that for most of what I do, I am trusting that somebody else determined the safe load on the fastener when they designed the joint and told me the torque to apply.
VERY good point Jim and all of what you said is true... given more time, I will reply with a recent story where the torque was calculated on a dry joint and a component change caused the current bolts to stretch! I was called in because we had "bad bolts!" While this can happen - it is VERY rare.

Unfortunately too often I am seeing designers have not developed a joint, they are simply choosing a torque from a chart and the variability of the drive method is not taken into account and many are unaware of a "wet" versus a "dry" joint!
 

Dee2

Gene
Corporate Member
Not sure what the OP's intent is but an accurate way to determine clamping force is to measure bolt extension under load, IMHO. Which makes a good case for using studs and knowing material yield strengths.
 

Hmerkle

Board of Directors, Vice President
Hank
Corporate Member
Not sure what the OP's intent is but an accurate way to determine clamping force is to measure bolt extension under load, IMHO. Which makes a good case for using studs and knowing material yield strengths.
I believe the OP's intent was a joke...
I took it a little further to bring it into a more serious realm, and Jim took it even further...

I will take exception that this make a good case for using studs and nuts as this creates a different group of problems in bolted joint technology.

I had a customer years ago that used interference threads on their studs and then asked me why they were breaking studs (5/8-11)

(because your torque is so high to overcome the interference thread that when the nut seats against the spot face of the flange you are taking the joint past yield)
 

Dee2

Gene
Corporate Member
Not even good engineers can fix stupid?

Sounds like that there was not much bolt elongation in that application and therefore not much clamping force, yes?
 
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Hmerkle

Board of Directors, Vice President
Hank
Corporate Member
Not even good engineers can fix stupid?

Sounds like that there was not much bolt elongation in that application and therefore not much clamping force, yes?
Correct - CANNOT fix stupid...
done correctly - a HUGE clamping force in that application,believe it or not... 5/8-18 bolts.

24 of them held a cast flange to the side of a dropped transmission for a mining application...
 

Gotcha6

Dennis
Corporate Member
This all raises a question in my mind about some mechanical fasteners. In years past, I have had applications in both structural steel erection moment connections and engine building where it was specified to tighten the bolt/nut to a specific torque and then turn it an additional xxx degrees. Not sure I understand the purpose of these instructions as it applies to the connection......
 

Hmerkle

Board of Directors, Vice President
Hank
Corporate Member
This all raises a question in my mind about some mechanical fasteners. In years past, I have had applications in both structural steel erection moment connections and engine building where it was specified to tighten the bolt/nut to a specific torque and then turn it an additional xxx degrees. Not sure I understand the purpose of these instructions as it applies to the connection......
what you are using is a "Torque turn strategy"

When the bolt reaches a specific torque it is assumed you have put it into tension, but not yet reached its yield point.
when you turn it an additionall # of degrees that is calculated to pull into a "true" elastic tension (see a good representation of the torque curve here: https://www.extreme-bolt.com/fastener-torque-specs.html)

We know the pitch of a screw and for a 1/4- 20 a full turn advances the bolt 0.05 inches - by telling you to torque the bolt to a given number and then turn it 10 degrees, I get a VERY accurate stretch in the bolt (.005 inches for this example)
 

Gotcha6

Dennis
Corporate Member
Math check, Hank. 10 degrees on a 0.05 pitch wouldn't yield 0.005 bit ~.0014, but 36 degrees would. I assume you meant turn 1/10th of a turn.....
I didn't sleep at a Holiday Inn last night, but I wanted to.....
 

Hmerkle

Board of Directors, Vice President
Hank
Corporate Member
Math check, Hank. 10 degrees on a 0.05 pitch wouldn't yield 0.005 bit ~.0014, but 36 degrees would. I assume you meant turn 1/10th of a turn.....
I didn't sleep at a Holiday Inn last night, but I wanted to.....
YEP - note to self - stop posting when you are tired!
 

JimD

Jim
Senior User
My experience on this subject is from work and I wasn't doing the design calculations, just reviewing them. On really big bolted connections to close the head on a nuclear reactor, it is not done by torque, it is done by bolt elongation. That is much more accurate but too much trouble for other situations. If the friction is too high to get the necessary stress in the stud, it is replaced. On a much smaller fastener on nuclear fuel assemblies, we use an automated machine that torques, untorques, then retorques. That apparently helps accuracy. A nice feature for our industry is the tool also records all this so we have records we can review if there is every any issue.

All I personally do is to check the recommended torque, set my torque wrench to that value and torque it. There is a lot of margin in most situations. I have not done anything major on my BMW but many of its fasteners are aluminum and replaced if they are removed. I'm not sure if they are replaced because they are torqued to yield or just to insure the threads are good and the fasteners have not creeped. Steel will absorb a lot of stress as long as you do not go past it's yield strength but aluminum will creep at loads significantly lower than yield. My guess is they use the aluminum fasteners because the block and head are aluminum and they want the same rate of thermal expansion and contraction.
 

Gotcha6

Dennis
Corporate Member
Your observation about thermal expansion differentiation is probably why I will need to replace the exhaust header bolts on my LS aluminum head engine. They eventually snap off and then leak. Not sure I want to use aluminum bolts to replace, though.
 

JimD

Jim
Senior User
Dennis,

I have not had an LS motor yet. Many years ago I had a dune buggy with a VW engine. It had aluminum heads and steel headers secured by bolts. There was no choice in that case because the headers slid over the heads. But on a LS, I would definitely use studs. Steel bolts threading into aluminum threads is almost an invitation to stripped threads. Maybe there is good anti seeze now that will prevent it but I ruined a head in the dune buggy and had to tap new threads in a couple others. You seem like a good mechanic so you probably know this and more. It will not solve the fact that the head expands more than the headers and bolts but I think that shouldn't be a huge issue. It will change the position of the headers a little but the bolts need to change length as the header flange changes thickness so if they are roughly the same material (same thermal coefficient of expansion) it should not cause a leak.

Regular steel is a little better than stainless in this situation too but for a different reason. There is a mild galvanic coupling between stainless bolts and an aluminum block that will increase corrosion of the block. The galvanic potential of regular steel is about the same as the block so that isn't an issue.

Jim
 

Gotcha6

Dennis
Corporate Member
Thanks. I just ordered a quick repair setup from RockAuto made by Dorman that utilizes existing unused bolt holes to clamp the header to the head without having to remove the broken studs. Price was half of what O'Reilly was.
The other option (and probably later on) is to weld a nut onto the broken bolt and remove it, risky at best with electronics possibly getting fried. I had already improvise one such clamp when I bought the van 4 years ago, but now the bolt nearest the temp sensor is snapped and I'm getting false engine data to the computer because of the excessive heat on the sensor. Nonetheless, I also ordered new zinc chromate header bolts for when I can get the broken ones out. That and anti seize compound should last me a few more years......
 

JimD

Jim
Senior User
galvanized steel has the same galvanic couple rating as an aluminum fastener (A) in an aluminum block. An A rating means there should not be any accelerated corrosion of the block.
 

Gotcha6

Dennis
Corporate Member
I wonder if helicoils would help the situation? Also, would it help side post batteries to use a brass insert? I'm always stripping those things. I'm not trying to hijack this thread, but it seems to be drifting that way....
 
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Willemjm

Willem
Corporate Member
Interesting how this thread started and continued.

When I was young and pretty, Mercedes Benz put me through Engineering college and I ended up doing product testing, local country parts QC, vehicle testing and training as well as certifying the technicians who work on cars and trucks.

That was way back in the 80's so things might have changed, but for connection rod to crankshaft bearing caps, we used a bolt with torque developed to move the threads into the yield zone. Meaning the threads were actually permanently stretched or deformed. The idea was that it locks the bolts with more than only friction and they may only be used once and discarded in the case of a rebuild.

For anything else, bolt size (diameter) was designed to give adequate clamping force at a tensile stress of around 80%. There are many variables here, so torque was specified as clean thread surfaces with no lubricant traces. But there were still variables. The 80% was deemed adequate to provide enough friction preventing fasteners to work loose due to vibration, or temperature cycles.

Today I no longer service my cars, but do my motorcycles and personally I only use a torque wrench in a few critical areas, the rest is done by feel and experience. I believe in many applications, the torque suggestions from the manufacturers were a guess, without proper design and aimed more at not stripping threads or damaging a fastener.
 
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JimD

Jim
Senior User
I think whenever you can replace the female threads in a metal like aluminum or lead that creep under load it is a good thing. Helicoils still have threads in the aluminum but they are on a larger diameter so the stress is less and you are not removing and replacing (hopefully).

I don't think I've had a side post battery but if it has female threads in the lead, that would be a significant risk. Lead isn't really good structural material. Wood might be better (but not nearly as good a conductor).
 
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