Checking Bolt Torque 2

[jimo14]

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?

 

[dugal]

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.

 

[fastech]

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:

http://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]

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]

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.