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Why use TurnaSure DTIs Bolting Services?

Slip Critical Connection – Common connection in steel structures

A non-slip joint is developed when High Strength (Friction Grip) Bolts and Nuts are tightened to a minimum preload force that, in structural steel, exceeds 70% of the minimum tensile strength of the bolt. Insufficiently tensioned bolts can result in major slip of the structure that would produce unacceptable geometric changes. Proper clamping forces on the connections, taking account of the surface conditions (slip factor), produces a structure of high rigidity.

In structural steel applications with a non-slip joint, because it depends on friction between the plies, and not shear or bearing on the bolt shank, bolts can also be assembled in oversize holes or slots. This increases the tolerances that can be applied in fabrication and erection.


Connection subject to Tension Loads - Commonly used in automotive and general engineering, occasionally used in steel structures and can apply to anchor bolts

Insufficiently tensioned bolts in these connections can lead to bolts loosening and falling out or worse failure by fatigue. It is most important that the varying axial tension loads do not exceed the value of the preloaded force in the bolts. (Or forces pulling up on anchor bolts). High preload forces improve bolts fatigue resistance and are the very best guard against loosening. Poor or slack bolt installation can and has lead to catastrophic failure.

Loose Bolts can get you a lot of attention

Connections on gasketed pipe flanges

Connections on gasketed pipe flanges

Example of a pipe flange

On pipe flanges, especially in chemical plant applications, properly tensioned studs are also helpful in preventing the leaking of toxic substances through insufficiently compacted gaskets, or alternatively too much clamping which can damage the gaskets.


Benefits for pipe flanges and valve applications of TurnaSure’s Bolting Services

DTIs are now standardized for many fasteners such as B 7 or B16 studs used in these applications under ASTM standard F2437.

DTIs guard against leakage of toxic substances in flanges with gaskets. Because a precise clamping force can be achieved with a DTI, TurnaSure DTIs for studs, assure that the correct amount of clamping force is applied to a gasketed flange. Too little tension can cause the connection to leak, and too much can damage the gasket with the same consequences of leakage. TurnaSure DTIs for studs can be supplied with a load/gap relationship graph. This gives the gap size to use between the DTI face and the nut face for a desired level of bolt or stud clamping force.


Bearing Connections in Structural Steel Buildings

These connections for certain types of structures or structural applications are often “promoted” by steel construction groups. It is suggested bolts just in bearing, like inserting dowel pins in clearance holes, can contribute to less costly steel structures. It should be noted that when a connection “slips” into bearing on the bolt shanks, few bolts, perhaps one, will be in bearing initially. As the connection is pulled in research studies the connection has the opportunity to “un-zip” as the first bolt shears and the second comes into bearing as the connection is steadily stressed to test its shear strength. More bolts come into bearing as the surrounding material distorts around bolt holes. Consider this picture of a catastrophic failure of a bearing connection, when the adjacent ply “guillotines” the bolt shanks.

bolting services Sheared Bolts

Sheared Bolts

The difference in time to properly tighten bolts is only seconds per bolt. Properly tightened bolts must contribute to structural rigidity and stronger connections.

TurnaSure believes designers should consider these facts when allowing “bearing connections”.

TurnaSure DTIs provide the right clamping forces to prevent slippage, fatigue failure, or other problems related to loose bolts. They contribute to structural rigidity


bolting services direct tension indicators

Find out how TurnaSure DTIs and our bolting technology can bring structural stability and savings to your next project. Contact our experts online or by calling 1-800-525-7193.
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