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Anonymous Poster

Compressive Load From Torque

06/09/2009 11:19 PM

Hiya guys. I've read through few threads here but can't seem to find the answer.

What I need to know is, is it possible to calculate compressive load applied when tightening a bolt to a specific torque?

Eg: tightening a 1/4" bolt to 25 ft.lbs of torque would result in how lbs force applied to the head of the bolt, assuming an effective head diameter of 1/2".

thanks in advance for your time.

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#1

Re: Compressive load from torque.

06/10/2009 12:32 AM

Your question does not make sense.

"Compressive load from torque". yes

"Tightening a 1/4" bolt to 25 ft.lbs of torque would result in how lbs force applied to the head of the bolt, assuming an effective head diameter of 1/2".

Bolt head would be in tension, not compression.

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#2
In reply to #1

Re: Compressive load from torque.

06/10/2009 6:15 AM

If you look at how an assembly is you shall notice that:

-tension is in the bolt shaft

-between bolt head and tightened part there is COMPRESSION which is as force equal to the shaft tension.

The answer is considering above explanation: the compressive force between bolt head and parts is equal to the tension created in the bolt itself.

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#9
In reply to #1

Re: Compressive load from torque.

06/11/2009 5:41 AM

He meant that the item UNDER the head, would be complessed.....!!! Surely that was obvious ?

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#3

Re: Compressive Load From Torque

06/10/2009 10:23 AM

Hello "Guest",

I think that I know what you're looking for. The simple spreadsheets found at this link will give you an idea of what the bolt load would be if the input torque and actual K Factor is known.

* Just remember that "torque" does not indicate how tight a bolt is. The 25 ft-lbs of applied torque could result in any (reasonable) bolt load (or absence thereof). It all depends on the friction encountered when turning the bolt head or nut. Since the friction factors in the torque equations can only be the result of an educated "guess" when torque was calculated, one doesn't know what they will be during actual assembly. Here's another link explaining the pervasive and often dangerous misconception of torque vs bolt load.

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#10
In reply to #3

Re: Compressive Load From Torque

06/11/2009 5:43 AM

GA from me for a really good and accurate post!

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#4

Re: Compressive Load From Torque

06/10/2009 10:32 AM

Tension in the bolt at a particular torque is a function of screw pitch and friction between threads of nut and bolt, so the question can't be answered, at least not easily.

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#5

Re: Compressive Load From Torque

06/11/2009 12:03 AM

Tough question since the terminology can refer to a couple of things within each of our minds as well as within the mechanical world.

Compression is for the ratings of items being forced together, and tension is the rating for the items being forced apart. Threaded parts do both of these things.

The compressive load on the threads alone of both the bolt and the nut or the threaded piece of material in conjunction with the material it is being compressed to would be roughly the torque reading. This is to prevent warpage and such as well as to keep the pieces in place such as with the water-pump you may tightening down. Several bolts spread across an area such as on a water pump are designed to simply keep the pump secure and leakage along with sealant/gaskets for warpage due to heat tension ,etc... yeah, that's a good enough answer for that part.

Since you are torquing this down to 25 ft lbs, the head of the bolt along with the threads and the point of contact below the head of the bolt would be relatively the same (very, very minor differences for temperature and barometric pressure, grease, dirt compression, etc...).

Throw an imaginary plastic washer under the pretend head of the bolt to prove the point... It will compress and the bolt head will eventually grind through it onto the cast iron water-pump thus and forever more, ruining the torque you just applied.

However, if you torque that 1/4" bolt down to 150 ft lbs.. You'll need to chase the easy-out and drill down for you next step. PLUS, if you torque that old stressed out bolt (on the '67 Pontiac Tempest , for example) that had been torqued to the suggested 25 ft lbs throughout the years and hundreds of times... You might need to chase the easy-out and drill down as well, since the ratings are only good for so many turns.

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#6

Re: Compressive Load From Torque

06/11/2009 2:26 AM

yea you can't find this answer in all the normal enginering stuff except with stupid calculations about ramp (pitch) etc, etc. I finally found a simple chart that takes a lot of the stress from the calculations at a company Spaue-Neur. It gave clamp loading based on maximum allowed torque applied to grade 2, 5, 8 and L9 bolts. I do not have the chart or the link but as I recall a 1/4 NC grade 8 bolt at full torque had about 3000 lbs of clamp load. Google the "L9" site they may have it on the web. If not I can supply some comparisons from the chart. Yes it does matter if the threads are lubed, and what kind of lube, so...

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#7
In reply to #6

Re: Compressive Load From Torque

06/11/2009 3:22 AM

The carts you praise were made using the "stupid" calculations but for ONLY 1 friction conditions (usually). Since friction is the MOST disturbing factor in bolting (as Boltintegrity repeats every time which ever the subject is) those so highly appreciated carts have a VERY limited use.

For your information many (if not most) bolt failures at 1st. assemby are due to the changing of friction conditions without taking them into consideration and without using for correction the "stupid" equations for torque/pre - load.

You seem not to rightly estimate the effect of friction. Some people who were not confronted to difficult bolting problems do the same and have bigggg surprises.

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#16
In reply to #7

Re: Compressive Load From Torque

06/11/2009 9:25 PM

So what everyone wants to do is to torque the bolt up until it breaks then back it off a half turn.

This for general purposes has been done and the torque charts based on bolt grade gives a good starting point in order to clamp items togeather with suffient load to prevent stress fatigue in the clamping bolts. (ie a loose bolt will break).

Yes I did say lubrication can effect the clamping load. Now what type of lube; moly, oil, grease, lard or whatever and exactly how much and where placed (such as under the bolt head) will effect the clamp load at the same torque value. Is there a chart that tells what co-effient of friction changes depending on lube and of course on temperture and then correlates to the torque to be applied?

The charts are quite generous and include a safety factor to prevent the breakage of bolts. For me torquing bolts to upwards of 1500 ft-lbs works quite well without all the fuss of calculations. I really need to get my job done without spending time with a calculator to decide if my calculated torque is correct or not.

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#18
In reply to #16

Re: Compressive Load From Torque

06/11/2009 10:16 PM

Here's a link to the chart you referred to. This works just fine for many, many, applications, and I suspect it is plenty close enough for the original poster's purposes. (96.4% of all bolts in the world are installed without a torque wrench, after all, and plenty of big bolts are installed with a sledge hammer and a slugging wrench to who knows? what torque.

Of course, the obvious answer is that if you torque a typical 1/4 in bolt to 25 lb ft, it breaks, so there is no clamping force.

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#19
In reply to #18

Re: Compressive Load From Torque

06/11/2009 10:33 PM

There is also limited or no clamping force if you apply 25 ft-lbs to a bolt galled into a blind hole or to a nut galled onto a bolt/stud

Charts should only be used as rough guidelines. In the above case, even if through some shear luck the published friction factors are exactly as the engineer had guessed when calculating torque from his designed bolt loads, you'd still have a failed joint! "Bolter emptor"

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#20
In reply to #18

Re: Compressive Load From Torque

06/11/2009 11:04 PM

See I knew someone was listening. Some times you can engineer things to death instead of just doing the job.

Yes the newest way is to measure the stretch of the bolt but in a lot of cases this is not feasible.

I do know a lot of people will not use a torque wrench but it still is the simplist way to ensure suffient clamp load.

As to bolt thread bottoming out, galling, broken threads etc. this only comes from experiance from the person tightening the bolt. If it doesn't "feel right" trust your instincts it probably isn't. The biggest problem in recent years I have seen is a bolt hole not deep enough or a too long bolt inserted, damaging the first thread. it may bolt up "OK" but when removed will "pull" the rest of the threads on the way out completly damaging the hole.

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#17
In reply to #6

Re: Compressive Load From Torque

06/11/2009 9:38 PM

the lawson bolt distributer, had a great chart for this ,,,,,,,,,,,,and it is actually clamping force that is achieved tightening a bolt.......when you over torque

a bolt it streches once that happens you loose a very signifgent amount of that clamping tension and NO AMOUNT OF TIGHTENING WILL EVER ACHIEVE THAT AGAIN

it will just strech more and more ...signifigntly means 1/3 to 1/2 of the clamping force is lost . THAT EXTRA 1/2 TURN CAN PUSH THE BOLT PAST ITS MAXIMUM LOAD AND START TO STRECH AND YOU WONT FEEL IT AT ALL...

DONT OVER TORQUE BOLTS OR NUTS !!!!!!!!!!

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#21
In reply to #17

Re: Compressive Load From Torque

06/11/2009 11:11 PM

Greetings mrhemightdo,

You've got the right idea and you're quite close... However, a fastener stretches as soon as the associated components are clamped (depending on the respective stiffness factors of everything, of course). The amount of stretch (strain) is proportional to the load (stress) up to the yield point. This is when it stretches elastically; relaxation of load within the elastic region results in the fastener returning to its original length. Beyond this point, the fastener enters the plastic region. It will not return to its original length if the load is released.

Here's an excerpt from a training presention (Powerpoint) that provides a graphic explanation: Stress/Strain (Sorry, it's BIG : 3.6 MB!)

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#22
In reply to #17

Re: Compressive Load From Torque

06/11/2009 11:15 PM

Heres the one I haven't been able to figure out. Some of the newer engines out there (Chrysler, GM) use a "torque to yield" cylinder head bolt. Apparently this is a one time use bolt and must be replaced any time the head is removed and replaced. The problem I see is once you yield a bolt the clamp force decreases because of the yield. How then does this work other than to stuff more money in the parts department cash registers.

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#23
In reply to #22

Re: Compressive Load From Torque

06/11/2009 11:32 PM

The intent is always to apply a known load.

Torquing produces an unknown load unless verified by some other means (ie, measuring resultant elongation). As soon as a fastener has yielded (which happens at a known load), the displacement vs torque changes dramatically. When this is seen by the tightening equipment, torquing stops.

Relatively easy to do on pristine assembly lines with new components but, very tough in real-life applications; It's too easy to overtighten the bolts. And, as you've indicated, new bolts have to be used after disassembly.

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#24
In reply to #22

Re: Compressive Load From Torque

06/12/2009 5:59 AM

I am afraid that you are wrong. If instead of saying "I do not want to learn more since I want to do may job" you would be aware of what happens. In fact learning helps to do a better job. Of course there are many low level tightening cases where knowing more does not help more.

Now to explain you why the trans elastic tightening was developed: When a bolt is stretched over the elastic limit the clamp force DOES NOT decrease it only grows not as steep as in the elastic domain but gros up to a point where it starts to decline and bolt breaks. This not steep growing has a TREMENDOUS advantage with respect to the fatigue life of the bolt since fatigue is related to VARIATION of the force in the bolt and due to the flatter line variation is lower as in the elastic domain and thus fatigue life longer.

Your reaction is characteristic to some body who rejects theory because he has not access to it.

There is a nice story about somebody being not able to reach the grapes, because they were too high, leaves saying "no harm they were not yet ripe"!

Fortunately people so reluctant as you are not as many as you think. In general people want to learn to enlarge their horizont.

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#25
In reply to #22

Re: Compressive Load From Torque

06/12/2009 8:24 PM

They have been around for years, as far as I am aware, since aluminium blocks were first used.....

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#8

Re: Compressive Load From Torque

06/11/2009 5:14 AM

I understand bolts are genrally used in tension and not in compression. Any items that are coming together creating compression may not need to be held together with bolts

When tightening, there is torsional stress developed that can be calculated based on friction, pitch angle, etc. Compression could only be due to resistance to tightening which will be negligible.

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#15
In reply to #8

Re: Compressive Load From Torque

06/11/2009 9:55 AM

Sorry but you're wrong, or never heard of a clutch's plate, just among many compression applications on industry.

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#11

Re: Compressive Load From Torque

06/11/2009 6:06 AM

It should not be too difficult to design a bolt with a strain gauge built in to measure any stretching of the bolt when tensioned and with a little work, to calibrate this fairly accurately (I would use an 8.8 bolt quality at least).

Then using lubricants and washers, to note the differences in torque each time for a particular tension.......even using Axial ball bearings (under say the nut) to reduce the resistive effects of metal to metal contact might be interesting to check out.....

With careful drilling of a tiny hole, the strain gauge could be wired through the middle of the bolt to an external connector for measurement....without reducing the strength of the bolt to much, but it would need to be calibrated against an un-drilled bolt first.....really fairly simple......why has nobody done this up to now????

This could be done for several lengths of the same bolt type to see what difference length brings and then for different diameters.....

Or have they?

This could become a whole new interesting CR4 blog!!!

Many thanks to the OP for starting it in the first place...he should register and join CR4 as quickly as possible please.......yesterday would be just great!!

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#12
In reply to #11

Re: Compressive Load From Torque

06/11/2009 8:19 AM

Hello Andy,

Indeed, bolts have been designed with built-in strain gauges! Some are quite accurate (and very expensive) while others aren't nearly as accurate (and much less expensive). Those in the latter group actually change colour as bolt load changes. They're calibrated to exhibit a certain colour when the target load has been reached, and a different colour if the load has been exceeded. If the bolt loosens, the colour reverts. Since the interpretation is subjective, such bolts should only be considered as "go, no-go" indicators of bolt load (which is much better than simply guessing and hoping when torquing). However, for truly accurate indications of residual load, measurement of bolt load using ultrasonics is the preferred method. Proper calibration results in accuracies of +- 2 % (and better).

Here are a couple of links:

"Smart Bolts"

"Ultrasonic Bolt Elongation Measurement"

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#14
In reply to #12

Re: Compressive Load From Torque

06/11/2009 9:54 AM

Great posts, many thanks......

(Someone stole my ideas already.....)

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#13
In reply to #11

Re: Compressive Load From Torque

06/11/2009 8:33 AM

Bolt integrity reacted faster and gave good answer. Such bolts are used since many many years, I designed and build several myself.

Some gage manufacturers even developed special strain gages for this application.

Such bolts, very carefully calibrated, are used for calibration of friction measuring machines for bolting technology.

The problems are related to the fact that strain gages have a transverse sensitivity and bolts are subject at same time to tensions and torsion so that a special approach to compensate the cross-talk in order to obtain true results is needed.

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