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Torque To Yield

General engine tech -- Drag Racing to Circle Track

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gruntguru
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Re: Torque To Yield

Post by gruntguru » Thu Sep 12, 2019 1:03 am

A couple of pieces of mis-information above. Corrections below:

1. Torque to yield bolts are designed to be torqued beyond their elastic limit and into their plastic range. This means they will not return to their original length after disassembly.
2. "Torque plus angle" bolts are usually but not necessarily "torque to yield". If the maker says do not reuse, they are torque to yield bolts.
3. The benefit of torque to yield is HIGHER CLAMPING FORCE by operating the bolt closer to its Ultimate Tensile Strength rather than below its Yield Strength.

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Re: Torque To Yield

Post by modok » Thu Sep 12, 2019 3:12 am

That's not so clear cut.
A torque to yield bolt will not give the highest clamping load, but is cheaper and easier to use than comparable re-usable bolt.


While it is always wise to follow the OE instructions, sometimes they just tell you to buy new bolts because they sell more bolts that way, in other cases the new bolts are improved design. equal chance of each IMO.

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Re: Torque To Yield

Post by 4vpc » Thu Sep 12, 2019 4:04 am

Mazda always say use new bolts on their Duratec range, but in the real world mechanics just re-use them and they run fine.
At the other end of the scale the guys at the top say that using ARP on the bottom end distorts the mains meaning they have to be honed true.
There is no S on the end of RPM.

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Re: Torque To Yield

Post by Rick! » Thu Sep 12, 2019 7:27 am

Image
gruntguru wrote:
Thu Sep 12, 2019 1:03 am
A couple of pieces of mis-information above. Corrections below:

1. Torque to yield bolts are designed to be torqued beyond their elastic limit and into their plastic range. This means they will not return to their original length after disassembly.
2. "Torque plus angle" bolts are usually but not necessarily "torque to yield". If the maker says do not reuse, they are torque to yield bolts.
3. The benefit of torque to yield is HIGHER CLAMPING FORCE by operating the bolt closer to its Ultimate Tensile Strength rather than below its Yield Strength.
modok wrote:
Thu Sep 12, 2019 3:12 am
That's not so clear cut.
A torque to yield bolt will not give the highest clamping load, but is cheaper and easier to use than comparable re-usable bolt.


While it is always wise to follow the OE instructions, sometimes they just tell you to buy new bolts because they sell more bolts that way, in other cases the new bolts are improved design. equal chance of each IMO.
The principle of a properly bolted joint is to apply a robust enough clamp force so that the joint doesn't slip during operation.
The way to impart the highest clamping force available in a given bolt is to torque it just into yield.
If you look at the graph, you can see that the highest clamping force is applied just into yield, where the slope of the curve is still positive and fairly steep.
The normal occurrences in a bolted joint during and just after final torque are slight embedment and relaxation.
When this occurs, it is easy to envision the stress, hence clamp in the bolt drops a little but then it falls back into the elastic region of the bolt material.
What this does, in a bolted joint that is properly designed, is to maintain a clamp force that is still higher than just torquing to "85% proof" as seen in the attached graph.

For an engine, head bolts and rod bolts normally see only tension. It makes sense to use TTY fasteners there.

In structural applications, there are vibrations, shear forces, bending forces, tension, and temperature changes to name a few, that all add to the combined stress in the fastener and TTY is not a good bolt clamp strategy as there is little margin for added stress.

I can tell you that the OEMs I have worked for don't look at profits for selling more bolts (though it doesn't stop the parts/accessories vp from making service parts a profit center). Bolt torque strategies are applied that keep the joint in formation for the warranty period, use the minimum expensive torque equipment and the bolt clamp is repeatable within 3 sigma due to friction and bolt variations in production. This saves the OEM money in the long run by not buying warranty parts or assemblies and the associated labor cost from failed bolted joints.

Image

This old englishman has lot of good info on bolted joints. http://www.boltscience.com/

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Re: Torque To Yield

Post by swampbuggy » Thu Sep 12, 2019 8:45 am

Thanks Rick for your post on this topic. Mark H.

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Re: Torque To Yield

Post by strokersix » Thu Sep 12, 2019 8:55 am

boltscience is a good resource.

Bickford's "Introduction to the Design and Behavior of Bolted Joints" Is another if you want more.

Pay special attention to joint diagrams. Then in every bolted joint discussion, design trade-off, parts swap, etc. think about how it affects the joint diagram and you will be a better machinist, engineer, mechanic.

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Re: Torque To Yield

Post by digger » Thu Sep 12, 2019 7:05 pm

pretty much all TTY bolts i've used are necked down over a defined length. so you cant always say it generates more clamp simply because the stress is higher as the cross section where it yields is smaller so you need to account for that.

its normally done to have better visibility whether stretch is achieved which gives more resistance to loosening and minimise the amount of external load seen by the fastener.

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Re: Torque To Yield

Post by modok » Thu Sep 12, 2019 9:14 pm

Rick! wrote:
Thu Sep 12, 2019 7:27 am
The way to impart the highest clamping force available in a given bolt is to torque it just into yield.
once again, that depends.
The graph above, note that the even after entering the plastic range, clamping force continues to increase another 10-20% as it stretches.
This would be typical of a grade 5 bolt. A grade 8 bolt does not do that,

heat treated steels, when quenched and tempered for a high hardness, the yield point and break point become closer together, or are the same point.

You could look at it as......with the TTY bolt, stretching it completes the hardening,
while a high grade bolt, it is is already done.

And, if it is tightened to 100% of it's yield strength, not doing anything, then any force put on it will push it over the limit, so what was the point of that?

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Re: Torque To Yield

Post by David Redszus » Fri Sep 13, 2019 3:36 pm

The once simple task of bolting two items together has become an engineering discipline.
A bolt is not just a bolt, but part of a fastener system, consisting of bolt, nut (or threaded hole), washer and clamped material. Each component must be considered as to its contribution to the system. Following are some of the parameters that need to be considered in each section.

Bolt Material
Tensile strength
Yield stress
Bolt major thread dia
Thread pitch
Bolt head dia
Thread pitch angle
Bolt length
Threaded length

Washers are available in several grades and sizes. A washer that is crushed under load does not allow the system to maintain the required clamped force.
Washer Material
Tensile strength
Yield stress
Washer OD
Washer ID
Washer thickness

A nut or threaded hole must be complimentary to the threaded bolt in size, clearance and strength to avoid deformation of the threads and an eventual weakening of the clamped joint.
Nut Material
Tensile strength
Yield stress
Nut width across flats
Nut height

The material to be clamped must not become distorted, crushed or yield as the clamping force is applied.
High grade bolts are often very difficult to clamp aluminum properly due to distortion of the aluminum before the bolt elastic range is reached.
Clamped Material
Tensile strength
Yield stress
Hole dia

A estimation of the required clamping force is made to determine bolt size required, and a safety factor.
A static joint does not require the clamping force safety margin than does a dynamic clamped joint. Vibration and load reversals are evil creatures.
Torque Calculations
Target clamping force
Safety factor
Thread friction
Nut friction

The tensile force applied to a bolt is called stress. The elongation of the bolt is called strain.
A stress/strain curve can be graphed as a straight line as long as the bolt remains in its elastic range.
At some point the elastic linear slope curve approaches a plastic curved slope. The transition point is called the elastic limit. Continued application of stress will stretch the bolt into a non-returnable state. The bolt dimension will have changed; diameter becoming thinner. While the bolt still retains its tensile strength (lbs/sq in), the cross sectional area is reduced and the load carrying capacity has become less.

The Angle Torque procedure simply replaces frictional torque with a more precise method. It uses the thread pitch and number of rotated degrees to determine change in bolt length. The bolt diameter does not matter nor does friction of bolt, nut, washer.

Pitch = 20/in. 1/20*360=in/deg for every degree of rotation the bolt is stretched 0.0001388".
An angle torque of 76 degrees would therefore stretch the bolt 0.0105". (0.0001388in/deg x 76deg = 0.0105")
Now the trick is to know how far the bolt must be stretched to obtain the correct clamping force.

High quality bolt manufacturers have handbooks indicating clamping forces for various types of fastener systems. Get your hands on one if you can.

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Re: Torque To Yield

Post by Rick! » Fri Sep 13, 2019 9:19 pm

digger wrote:
Thu Sep 12, 2019 7:05 pm
pretty much all TTY bolts i've used are necked down over a defined length. so you cant always say it generates more clamp simply because the stress is higher as the cross section where it yields is smaller so you need to account for that.

its normally done to have better visibility whether stretch is achieved which gives more resistance to loosening and minimise the amount of external load seen by the fastener.
For the given bolt geometry, TTY reliably generates the highest clamp force that given bolt will sustain.
If you've ever seen Pankl rod bolts, they are a piece of engineering art with the necked down length perfectly sized to achieve a stretch number that meets requirements.
modok wrote:
Thu Sep 12, 2019 9:14 pm
Rick! wrote:
Thu Sep 12, 2019 7:27 am
The way to impart the highest clamping force available in a given bolt is to torque it just into yield.
once again, that depends.
The graph above, note that the even after entering the plastic range, clamping force continues to increase another 10-20% as it stretches.
This would be typical of a grade 5 bolt. A grade 8 bolt does not do that,

heat treated steels, when quenched and tempered for a high hardness, the yield point and break point become closer together, or are the same point.

You could look at it as......with the TTY bolt, stretching it completes the hardening,
while a high grade bolt, it is is already done.

And, if it is tightened to 100% of it's yield strength, not doing anything, then any force put on it will push it over the limit, so what was the point of that?
A couple things:
Do you mean to say if I tighten a bolt (screw) into yield, I will change its hardness? Though one performs work to tighten a bolt, I have yet to take into account work hardening of a fastener in the joint design. Or maybe I do? On highly loaded structural joints that see regular servicing, I prescribe new bolts every time it's taken apart. On lesser loaded joints, maybe 3 re-uses. 6 re-uses like Mr. Johnson's SVT link, never. It may work for them in their controlled environment but I have to mitigate risk by adding instructions in the service manual to promote success EVERY time.

If you know the fastener loading, the fastener material properties, and at what torque and precise angle of rotation after yield that the fastener breaks, one can confidently apply a torque process that applies the highest clamp for the joint that the fastener will last for the intended lifespan. Do fasteners always need to be "maxed out"? Definitely not. But when you have a fixed number of fasteners, a hard limit on fastener diameter, a high load in a known direction, and reliable fastener properties, one tends to use techniques that allow meeting design goals.

The fastener is initially torqued just into yield, it relaxes just out of yield, and it experiences loads. The loads have been measured and they never push the bolt past the "reserve" of the peak stress (see graph) they can endure before going all gumby. Just because the fastener is living on the elastic/plastic limit doesn't mean imminent doom. Then one does fatigue calcs and fatigue tests to verify the fastener life meets design targets.

Have you ever torqued a fastener and felt that point of "uh oh"? Then rechecked with a torque wrench to convince yourself it's still "run-able"? I bet you have and then felt you got lucky after it lived just long enough for the customer to take it for the weekend before fixing it right. That's one extreme example of a TTY that didn't go kablooey even though by rights it you thought it should have. I know I've done it once or twice in my former career.
David Redszus wrote:
Fri Sep 13, 2019 3:36 pm

A estimation of the required clamping force is made to determine bolt size required, and a safety factor.

The tensile force applied to a bolt is called stress. The elongation of the bolt is called strain.
A stress/strain curve can be graphed as a straight line as long as the bolt remains in its elastic range.
At some point the elastic linear slope curve approaches a plastic curved slope. The transition point is called the elastic limit. Continued application of stress will stretch the bolt into a non-returnable state. The bolt dimension will have changed; diameter becoming thinner. While the bolt still retains its tensile strength (lbs/sq in), the cross sectional area is reduced and the load carrying capacity has become less.

The Angle Torque procedure simply replaces frictional torque with a more precise method. It uses the thread pitch and number of rotated degrees to determine change in bolt length.
The bolt diameter does not matter nor does friction of bolt, nut, washer.
In my world, friction shapes bolt stretch consistency. A guy could math things out but the nut factor of a bolted joint isn't always 0.2 which is why hardened washers, rolled threads, and even some colors of Loctite are an easy way to promote consistent friction and prevent embedment.


Pitch = 20/in. 1/20*360=in/deg for every degree of rotation the bolt is stretched 0.0001388".
An angle torque of 76 degrees would therefore stretch the bolt 0.0105". (0.0001388in/deg x 76deg = 0.0105")
Now the trick is to know how far the bolt must be stretched to obtain the correct clamping force.

High quality bolt manufacturers have handbooks indicating clamping forces for various types of fastener systems. Get your hands on one if you can.
A few clarifications:
"Safety Factor" is a term that is not used in an engineering environment nor in reports or emails or anything that can be called into litigation.
The term has been changed to "Design Factor," at least that's what's I've been taught over the years.

A tensile force divided by the cross section of the subject is stress, F/A. In a fastener like a screw, it is usually called the stress area which is a tick larger than the cross section using the fastener's minor diameter (external thread).
Strain is a unitless value, a scalar, which is the measured stretch divided by a defined length, hence inch/inch or mm/mm, δ/L, etc. The abscissa values are a form of percent of elongation, depending on what the defined length L was.

If I stretch a fastener JUST INTO yield, and through embedment and relaxation, the stress on the fastener lowers, it never goes back down into its elastic region? There will be an offset (hysteresis) in the path of the new stress/strain curve but I bet it can re-enter its elastic region.

Do threads elastically or plastically deform during the torque process? If they do, does "mathing" out the angle and pitch prove reliable? Make a really, really high resolution FE model of a SAE or ANSI bolted joint using non-linear mat props, apply the intended clamp load and observe what happens - it's eye opening.

Strain gauged fasteners provide data to calculate clamp force. (so do load cells, pancake force gauges, ultrasonic measurements, etc.) DC torque wrenches with really expensive controllers provide angle and applied torque values and graph it out in nearly real time. A competent engineer then uses the compiled data to verify clamp goals are reached to prevent joint slip or separation. When necessary, the DC tool is used to perform torque to failure testing to generate statistics with which to create the proper torque recipe and specs to meet requirements.

Unbrako is pretty much the only bolt OEM that I would trust with a torque table. In general, torque tables are just mathed out values of T=Kfd to a prescribed percent of proof stress, usually 85%. They are a guideline, not an absolute.

I hope the OP has his new bolts and has the bottom end of the engine put back together. If he doesn't, at least he now has enough knowledge to be a real PIA on other forums discussing bolted joints. :)
I'm going to find out how my brother is doing at the Div 5 ET Nats.

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Re: Torque To Yield

Post by strokersix » Sat Sep 14, 2019 12:26 am

Here is another "twist" to bolted joints:

When using torque to tension a fastener there is not only a tensile force which is what we are after and what we all think about but there is also a torsion force in the fastener. Stress in the fastener is the combination of tensile and torsion. Over time the torsion twist can actually unwind and either slip underhead (not good) or actually screw the threads in a little farther (a good thing). Have you ever come back to a fastener and been able to tighten it a bit more with the same torque wrench setting? What may have happened is the unwinding has screwed the threads in a bit more then you came back and wound it up again.

This effect is called "torque ratcheting" and can be done multiple times to achieve greater tensile force from a fastener than if you only twisted it once. Large flanged head fastener to increase underhead frictional torque combined with nice smooth well lubricated threads will encourage torque ratcheting.

Like a lot of things, bolted joints are more complex than at first appear.

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Re: Torque To Yield

Post by Geoff2 » Sat Sep 14, 2019 5:31 am

It is wrong to assume that if a bolt is rotated x amount of degrees, then the bolt will stretch y amount of inches. The amount the bolt stretches will depend on the material it is made of & the thickness/composition of the material it is clamping. The amount of 'stretch' could be shared by the bolt & the material being clamped.

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Re: Torque To Yield

Post by strokersix » Sat Sep 14, 2019 7:22 am

Geoff2 wrote:
Sat Sep 14, 2019 5:31 am
It is wrong to assume that if a bolt is rotated x amount of degrees, then the bolt will stretch y amount of inches. The amount the bolt stretches will depend on the material it is made of & the thickness/composition of the material it is clamping. The amount of 'stretch' could be shared by the bolt & the material being clamped.
Correct. A joint diagram illustrates this.

Tried to paste an image but my skillset does not allow this am. Go to the previously mentioned http://www.boltscience.com/ for more info.

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Re: Torque To Yield

Post by Kevin Johnson » Sat Sep 14, 2019 8:05 am

Rick! wrote:
Fri Sep 13, 2019 9:19 pm
...
A few clarifications:
"Safety Factor" is a term that is not used in an engineering environment nor in reports or emails or anything that can be called into litigation.
The term has been changed to "Design Factor," at least that's what's I've been taught over the years.
https://time.com/103737/gm-words-nhtsa-general-motors/ wrote:always, annihilate, apocalyptic, asphyxiating, bad, Band-Aid, big time, brakes like an “X” car, cataclysmic, catastrophic, Challenger, chaotic, Cobain, condemns, Corvair-like, crippling, critical, dangerous, deathtrap, debilitating, decapitating, defect, defective, detonate, disemboweling, enfeebling, evil, eviscerated, explode, failed, flawed, genocide, ghastly, grenadelike, grisly, gruesome, Hindenburg, Hobbling, Horrific, impaling, inferno, Kevorkianesque, lacerating, life-threatening, maiming, malicious, mangling, maniacal, mutilating, never, potentially-disfiguring, powder keg, problem, rolling sarcophagus (tomb or coffin), safety, safety related, serious, spontaneous combustion, startling, suffocating, suicidal, terrifying, Titanic, unstable, widow-maker, words or phrases with a biblical connotation, you’re toast
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Re: Torque To Yield

Post by Kevin Johnson » Sat Sep 14, 2019 8:07 am


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