Turn-of-the-Nut w/ DTIs?
Turn-of-the-Nut w/ DTIs?
(OP)
An erector is asking whether it is acceptable to use turn-of-the-nut method to achieve full pre-tensioning of the bolts where there is an uncompressed DTI washer installed at the bolt.
My answer was that turn-of-the-nut is not applicable because the uncompressed DTI washer tells us that full pre-tensioning has not been achieved. Therefore tightening must continue beyond the turn-of-the-nut requirements to compress the tabs on the DTI.
Any thoughts?
Thanks,
ChuckerD
My answer was that turn-of-the-nut is not applicable because the uncompressed DTI washer tells us that full pre-tensioning has not been achieved. Therefore tightening must continue beyond the turn-of-the-nut requirements to compress the tabs on the DTI.
Any thoughts?
Thanks,
ChuckerD






RE: Turn-of-the-Nut w/ DTIs?
There is a range of possible acceptable tensions from the minimum required to achieve the proper friction force to the maximum of getting dangerously close to the bolt capacity. The developers of the turn-of-the nut procedure have chosen a particular rotation to give a desireable tension, enough to keep the connection together but not more than is needed so as to stay away from the limit.
If they follow proper turn-of-the-nut procedure, they will get whatever tension the developers of the turn-of-the-nut procedure have decided counts as "full".
The DTI manufacturers have calibrated their washers to compress at an acceptable tension as well. Different types of DTIs are supposed to be calibrated to match different bolts, so theoretically they should compress at the same tension that matches the turn-of-the-nut proper tension.
So the possibilities are:
1. They're not actually following proper turn-of-the-nut but instead using some torsion-based system.
2. They're using the wrong turn-of-the-nut procedure for the bolts at hand.
3. They're using the wrong DTIs.
4. It's not actually off by that much. If you were so inclined, you could find out what tension in the force unit of your choice corresponds to the proper compression of the DTI and to X turns of that thread bolt, and see if they really are the same.
Hg
RE: Turn-of-the-Nut w/ DTIs?
Hg
RE: Turn-of-the-Nut w/ DTIs?
While properly done turn-of-nut method will give consistent results, they are not quantified as in the semi-quantified DTI method, so there is no real correlation between the two methods.
Either method can be checked in a Skidmore-Wilhelm tensioning device to see if you are getting the desired result.
There are several reasons the DTI might not be working. The contact face of the nut or bolt might be galling, thus indicating you have some bad fasteners (assuming you are using ASTM A325 or A490 fasteners). The DTI could be upside-down. The fasteners are stretching (again, a poor quality fastener).
RE: Turn-of-the-Nut w/ DTIs?
In my earlier post I started with the logic that since there is no One True Tension, the two methods might not necessarily give the exact same tension, but then by the end of the post I wound up sounding like it should be exactly the same. Not what I meant, though they should be in the same ballpark.
I assume spec called for DTIs. Out of curiosity, what justification does the contractor present to justify leaving the connections with uncompressed DTIs?
Hg
RE: Turn-of-the-Nut w/ DTIs?
The erector was able to compress the tabs on the DTIs using a sledge and knocker-wrench (very difficult 35' in the air in a lift). However, compressing the DTIs took nearly a full turn past the snug-tight condition (as opposed to the 1/2 turn required by turn-of-the-nut for these bolts). The end-owner was willing to accept use of turn-of-the nut in place of DTIs to ease installation of the bolts per the erector's request. Several locations already had the DTIs installed prior to this request, but with very little compression of the tabs.
RE: Turn-of-the-Nut w/ DTIs?
Check both in a Skidmore-Wilhelm device...you'll see that a properly applied Turn-of-nut method will favorably compare.
Do the turn-of-nut without the DTI's. You will likely find that the 1/2 turn requirement results in sufficient tension (check it with the Skidmore)
RE: Turn-of-the-Nut w/ DTIs?