Standard DTI FAQs
Pattern Tightening and DTI's
The "pattern" referred to here is the sequence of "most rigid" to "most flexible" within the pattern of bolts in a single connection. The RCSC Specification states that all methods of accomplishing fully tensioned bolting (turn-of-nut, calibrated wrench, DTI, and twist-off) MUST be done in two passes: the snug pass and the full tension pass. Both passes must proceed from the most rigid position within a connection outward, so as to iron out the plates and get all the gap closed between the plates. This stipulation by the RCSC is intended to minimize "cross-talk" between bolts as they are tightened sequentially in a connection. In practice, cross-talk is rarely encountered when the plates are as thick or thicker than the bolt diameter, even when the plates are not completely brought together. The RCSC defines the plates in this condition as "... in firm but not necessarily continuous contact ..."
Snug is defined as the point where the plates have been brought completely together. With DTI's, this can usually be accomplished by partial bump compression (with SQUIRTER® DTIs), either no silicone or very little silicone showing).
Bolts must NOT be fully pretensioned during the snugging operation, no matter what bolting method is used, since a fully pretensioned bolt, pretensioned again, will suffer severe distress if it does not fracture. The use of fit-up bolts to bring plates into firm contact is occasionally a good idea, but is seldom seen in recent years.
DTI's must not be fully compressed during the snug pass, of course, since then there will be no further bump compression to indicate correct tension. If this occurs, selectively release the offending bolt, replace the DTI, then crush the DTI immediately.
With SQUIRTER® DTIs, the two pass instruction is simple:
"Snug pass - no (or very little) squirt. Tension pass - let's see squirt."
