Checking Bolt Torque
Checking Bolt Torque
(OP)
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?





RE: Checking Bolt Torque
Blacksmith
RE: Checking Bolt Torque
I'm pretty sure that additional tightening would be the wrong way to go, if the bolt torque is near a limit.
RE: Checking Bolt Torque
RE: Checking Bolt Torque
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
RE: Checking Bolt 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
RE: Checking Bolt Torque
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.
RE: Checking Bolt Torque
RE: Checking Bolt Torque
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
RE: Checking Bolt Torque
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)
RE: Checking Bolt Torque
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.
RE: Checking Bolt Torque
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
RE: Checking Bolt Torque
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
RE: Checking Bolt Torque
RE: Checking Bolt Torque
RE: Checking Bolt 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,
RE: Checking Bolt Torque
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.
RE: Checking Bolt Torque
RE: Checking Bolt Torque
RE: Checking Bolt Torque
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.
RE: Checking Bolt Torque
This is simple verification on regular bolts without expensive or complicated equipment.
RE: Checking Bolt Torque
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.
RE: Checking Bolt Torque
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
RE: Checking Bolt Torque
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.