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jimo14 (Mechanical) (OP)
30 Jan 02 10:57
What/How is the best way to check a bolt's torque after it has been tightened? Is it to 'break it loose' and read the peak torque value or to tighten the bolt even more and record the value after it first moves?
TheBlacksmith (Mechanical)
30 Jan 02 14:44
I would rather loosen all the bolts and retorque them according to the manufacturer's specs.  The torque to loosen a bolt may not correlate very well to how tight it was, either due to friction or the fact that a lot of fasteners loosen very quickly.  Turning the bolt farther may result in overtightening the bolt, resulting in bolt or substrate failure.  Finally, more manufacturers are specifying "angle torquing", where the fasteners are tightening just enough to draw up the parts, then turned an additional 180-270 degrees.

Blacksmith
ivymike (Mechanical)
30 Jan 02 15:51
I had a discussion of this subject over dinner one time with some engr friends of mine who worked in production at an engine manufacturer.  The only thing I remember is that torque to tighten isn't the same as torque to loosen a bolt, which made it very difficult to verify reliability and repeatability of a bolt tightening machine.  Sorry, I can't remember how they got around the problem.

I'm pretty sure that additional tightening would be the wrong way to go, if the bolt torque is near a limit.
Hush (Mechanical)
30 Jan 02 18:18
To get a true reading you would have to tighten the bolt more until you overcome the break away torque (it starts moving) then quickly take your reading. But as Blacksmith points out this probably wouldn't be a very good idea. ALso if this is an existing install that has been sitting for awhile there are other factors that would throw your reading out to lunch, lubrication breakdown, oxidation creep, to name a few.
brad (Mechanical)
30 Jan 02 20:01
Ivymike's statement is correct.  I remember this discussion in manufacturing processes class.

The real answer (as I recall) to the original is--there is no way.  As ivymike states, the torque to loosen is almost always different than the torque to tighten.

When torque is specified, that is the final tightened torque.  If you want to achieve this torque, you either need to use a torque wrench (at least for the last turn of the installation) or else use a calibrated pneumatic wrench (which gives you the calibrated torque automatically).

That's my recollection (but then, I could be wrong, it's been awhile since I've had that particular class).

Brad
Buhrmann (Mechanical)
4 Feb 02 6:00
Theory predicts that the loosening torque must always be less than the tightening torque.

But what about marking the bolt head angle, then losening it and retightening it to the same position it were before you loosened it.  (That is if the bolts were not plastically stretched during the first installation like what TheBlacksmith mentioned)

If the friction coef between the nut and flange, and on the threads remain the same between the first tighten and the measurement, then it should be reasonable accurate.

Friction is sadly something that is not very reliable, especially with time.

Teo

Helpful Member!  jackboot (Mechanical)
4 Feb 02 10:21
I am offering this- but not as an advertizement to the company who makes these bolts.  However, they work and work well.

http://www.vfbolts.com

The old formula T=K*(Fi)*d (T=torque; K=coef. of friction; Fi= preload on bolt; d= nominal diam of bolt)works well- as long as you know exactly what K is.  

In my experience: torque wrenches are inaccurate and it is hard to determine K accurately.

So don't use torque wrenches. Use preload by either measuring the bolt before and after make-up (strain calculations) or by using one of these bolts on the website.

The Maxbolt at the website has a built-in strain gage.  You can monitor the preload of the bolt anytime-just look at the little indicator.  If your equipment warrents this type of bolt you will find it well worth the cost.
djtim (Mechanical)
4 Feb 02 13:07
The gist of what has been written is correct.  I will add a further complication for you.  With the usual caveat (the design of the specific joint) many joints will go through a relaxation so that the load you created by the original tightening of the bolt will be reduced at some time later. There are many reasons for this and I will leave that as an exercise for you.  At my company when we torque critical fasteners we actually do a 2 stage process.  The first is to torque to a value near the final value.  (This value must correctly chosen.)  Then we wait some period of time (an hour is not unusual) and retorque with the second torque value being the final desired torque.  Can you truly know what the load is with standard hardware?  Nope.. you have to get fancy as mentioned in the one post.  Not practical for most situations.
wktaylor (Aeronautics)
5 Feb 02 9:56
Folks... from an aero's perspective:

In large/critical structures we typically specify the following:

Install temporary fasteners or clamps to pull structure TIGHTLY together.
 
Install all threaded fasteners "finger tight".

Install rivets [and swagged-collar lock-bolts] undriven, between temp fasteners/clamps. NOTE: tape-over heads to prevent pop-out, allowing rivet to stabilize hole.

Torque To final value [or frangible-collar-break] in (3) stages [50%, 75% and 100%, MIMIMUM] using a typical crossing pattern (perhaps the term "criss-cross pattern" is more "friendly/descriptive").

Note: wait at least 10-miuntes between torquing stages for joint settling and any "sealant squeeze-out].

NOTE: FINAL-TORQUE value is determined by adding the "free-running-torque" of the nut on the male threads to the theoretical required torque value [shear or tension from charts for each thread style]... resulting in a value that will be different for each fastener combination: especially with self-locking hardware!!!

Drive rivets & swagged-collar lock-bolts AFTER securing all threaded fasteners.

Fillet-seal around edges of nuts/washers and structure using sealant-squeeze-out; or apply tamper-evident paint across thread, nut, washer and structure [evidence of movement]. If necessary [IE: vibration environments], we also use cotter-pins and/or lock-wire.

NOTE: for some joints where installed torques are deliberately low but critical [IE: bearings on shafts,  small-headed bolts, etc], we allow a "slight over-torque" to settle parts, wait a short-while, then back-off and final-torque to value. A theme-and-variation to this is "over-torque and back-off slightly concept" is to back-off until a thru-hole in male threads aligns to a "nearest" castelated cut-out in the nut... then install a cotter-pin or lock-wire.

Note: The concept of "checking torque" in a joint is alien... we use indicator mechanisms and inspections to verify that a "job" is finished.... and indicators that joint may have become loosen in-service.

Regards, Wil Taylor

electricuwe (Electrical)
8 Feb 02 11:07
The important issue for a bolted joint is usually not the torque but the force in the bolt. If a reliable bolted joint has to be manufactured one method is to apply a lubricant on the thread and below the head of the bolt so that friction is lower and tightening torque and force correlate more reliable.

For very critical bolted joints (e.g. in the automotive industry) the following method is used:

During tightening of the bolt the angle of rotation of the head and the torque are monitored electronicly. The automatic wrench stops, when the system indicates that the bolt reaches the limit of elastic stretching (The German term for this is "Streckgrenzen-gesteuertes Anziehen" but unfortunalty I don not now the English term)
Hush (Mechanical)
8 Feb 02 11:36
I’ve used the following method on occasion to determine required torques and friction coefficients for different thread lubes:

Drill a stepped hole down the centre of the bolt. Measure the length from the step to the bottom of the hole. Apply a set torque and measure the change in length to determine your bolt load. Of course this doesn’t provide any help with the original question.
gb420917 (Mechanical)
11 Feb 02 6:55
The two ways we tighten large bolts is :-
Angle of turn after all bolts are tightened with an initial preload (torque).

By extension meaturements.

This isn't an advertisement for a web site but it has some good articles on bolting and some tutorials.

www.boltscience.com
sprintcar (Mechanical)
11 Feb 02 8:00
We found a torque calculator here

www.futek.com

go to their Bolt Calculator
Not all the info is there, but if you email them with exact specs, they will add it for you
electricuwe (Electrical)
7 Mar 02 16:02
From the site www.boltscience.com I have learned that the method I have described in my post dated Feb 8th is called Yield Controlled Tightening.
alamin (Mechanical)
7 Dec 02 23:33
Check out this website: http://www.rotabolt.com
nashjp (Mechanical)
22 Dec 02 12:14
Some methods to check current torque-

1. Return to mark check - Mark the bolt and abutment with a reference line, then loosen the bolt, retighten the bolt up to the reference line again while monitoring torque. You want the dynamic torque and not the static. (This method has pretty good correlation with the original tightening torque unless corrosion or galling have taken place)

2. Snug check - While recording torque (you need an instrumented torque transducer and recorder) tighten the bolt 3-8 more degrees. Look at the recorded data and pick off the valley after the original static break away torque. (This is easier to do if you also record turn angle and plot turn angle vs bolt torque.) The additional 3-8 degrees will not effect the bolt joint in a negative manner.

3. Ultrasonics - Measures bolt stretch. - Measure the current installed stretch then remove, measure stretch again. (special preparation of the bolt head may be necessary - this method can be very accurate, or frustrating and @#$ depending who does it.)

As the other posts stated, static breakaway torque usually has no correlation with tightening torque. Also if you are seeing what value was used on original assembly, joint setting (which will depend on the joint design) will probably provide a lower torque value unless you have a good solid abutment.

Good Luck,
diamondjim (Mechanical)
23 Dec 02 0:15
Because torque is such a poor indication
of the tension that is in the bolt, you
really should be using more sophisticated
methods that measure tension and not torque.
Turn of the nut method in reverse would give
you some indication of how tight the bolts
were. Ultrasonic method would tell you more
exactly what the tension is.  Do you need
exact information for some type of failure
analysis or is this just a curiosity?  With
the turn of the nut method, you would have to
match mark the bolt and part, loosen it, take
it again to snug position and determine the
angle that it would take to take it to the
original matched mark points.
mechconst (Mechanical)
23 Dec 02 10:43
I'm not sure what the application is, but I run across this in the field all the time in structural bolt ups, using high strenght bolts: A325 and above, 5/8" dia. and larger.  Other methods are: 1) use load indicating washers.  They have a series of raised points on the washer that flaten out when the bolts at the right tension.  2) Use a Skidmore device to check and see what torque is required to achieve the desired tension in the bolt.  This is a bench device that you use to test about 3 or 4 of the bolts from the batch and develop an average torque value, since the torque will vary depending on how a bolt was machined and how much oil is left on the threads among other things.  I think the Skidmore works on compressing a hydraulic cylinder.  3) Use twist off bolts.  They actually have an extended part on the bolt that will twist off at the correct torque.  It requires a special tool.  Hope this helps.
rmp319 (Structural)
23 Dec 02 16:09
Find the Ninth Edition of AISC Manual of Steel Contruction and look in Section 5 for Bolds, Threaded Parts and Rivets. This manual offers information on Torque, washers, tension etc. Mostly geared to Structural Steel but has a wealth of information for general bolting requirements.
boo1 (Mechanical)
23 Dec 02 16:54
Mechconst the Skidmore-Wilhelm Bolt Tension Calibrator is the most commonly used bolt calibration device in the US. The device is a hydraulic load cell. Tightening the bolt creates pressure in the unit. The inter-changeable bushing in the back of the Skidmore sits in a piston. Between the piston and the body of the unit is hydraulic fluid which is compressed by the tension in the bolt. This hydraulic pressure is then read on a dial gage, scaled to read in terms of bolt tension in thousands of pounds (kips), rather than psi of hydraulic pressure.

Because of the slight movement of the piston, the unit underestimates the actual tension that would be put into a bolt for a given rotation. Some of the rotation is taken up by piston movement. In the structure, the steel does not compress and all rotation is used to tighten the bolt.

dugal (Nuclear)
24 Dec 02 17:51
I think everyone has put a lot of good answers in towards this thread.   If I may add my two cents.  When we want to verify torque on an ordinary bolted joint we request "concurrent verification".  That means the the actual torquing process is witnessed by another individual at the actual time of torquing.  To request verification of torque afterwards is useless due to the physics of the joint.
This is simple verification on regular bolts without expensive or complicated equipment.
Helpful Member!  fastech (Specifier/Regulator)
29 Dec 02 23:51
jimo14;

As you have already probably ascertained from each of the responses you have received, there are as many opinions for this as there are writers.

Ralph Shoberg, RS Technologies, Ltd. has a recent very unique paper on this very thing.  Dr. Shoberg's lab performed most of the torque testing for John Bickford.  Dr. Bickford's Handbook on Fastening (The Bible in the fastening industry), ch 7, page 262, states: "Inspection torque is always applied in a clockwise direction.....the reason for this: the torque required to loosen it is usually less than the torque required to tighten it......the torque required to loosen a UNC thread is usually 70% of that required to tighten it.....for a UNF thread it is only 89%.  That chaptrer makes very interesting reading.

Dr. Shoberg now does what is called an M-alpha plot with his equipment which will actually allow you to determine the actual energy going into making the joint and the actual residual clamp left.

Ingersol-Rand also has a piece of equipment that looks like a common torque wrench but is used only for torque auditng.  I tell all my students that torque auditng, at best, is very complicated and very unpredictable.

Ford Motor Co. spent several hundred thousand dollars to verify that very thing back in the 60's.

Dr. Shoberg's web page is: www.rstechltd.com/  It contains a lot of good info on this topic. I am sure he would be glad to send you a copy of his paper.

Bill P.
CSFT, Inc.

sancat (Mechanical)
30 Dec 02 7:55
Hi,
I have read in the past some information on this device, that seems to read actual load on the bolt without having to loosen or tighten it. Seems that it senses the stress on the bolt by means of an ultrasound wave. I really dont know if it actually works, but if it does, that might help you.

http://www.tentec.co.uk/boltgauge3.htm

sancat
jas001 (Structural)
19 Jan 03 9:43
From this thread it is quite apparent that torque is not a good measure of bolt tightness.  Keep in mind that it is bolt tightness that is important, the tension that is developed in the bolt that holds the two surfaces together.  Torque is merely a measure of the friction developoed between all of the components and is a function of many factors including the tolerances and lubrication of the parts.  There are many good thoughts in this thread but my experience and knowledge have lead me to the conclusion that torque should never be used as a measure of bolt tightness.

Bolt tightness has no impact on the joint capacity if the bolts are only developing shear.  Many tests have demonstrated that the shear capacity of a connection is not increased as a result of bolt tension, even for slip critical connections.  Bolt tightness in shear connections is to make sure that the connection is properly made and won't come loose in normal operations.

Now for tension connections.  It is these that require the bolts to be tightened but not for the bolts but rather for the performance of the connection.  Very simply the total bolt tension in the bolts must exceed the joint load for the connection to perform properly, the faying surfaces do not seperate under load.  The question that needs to be answered is how do you determine if the bolts are adequately tightened?

Well that has been discussed at great length and there are several ways of doing this.  The first is to use indicators that will deform at specified values, this is the simplist and easiest method.  For very large bolts that are critical the bolts may be simultaneously pretensioned using hydralic jacks and then the nuts run down to snug thigh and the force releived.  This si very similar to pretensioning of cables in post or pretensioned structures, very costly but accurate.  Finally there is the turn of the nut method where the nut is made snug tight and then turned a further 1/2 to 3/4 turn, depending on the grip length, to complete the tightening.  This is crude but effective particularly in building structures.

Now for a last word.  If we keep in mind that the tension in the bolt must exceed the joint load, for tension connections, then is it possible to overtighten a bolt?  The rule of thumb is; if the connection is properly designed and the bolts do not break during installation then the connection is okay.  

A simple inspection technique is the visually inspect every bolt and make sure they are tight (not loose).  If you realy need to use a torque wrench then determine the job site torque and use that and do not rely on the troque wrench values unles you have it calibrated every year or any time that it has been dropped.

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