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HYTORC offers precision bolting process. (Product Development).

Bolting finally has become more precise, according to HYTORC. The company says this new precision is the result of its recently developed and field-proven HYTORC DISC and its widely used HYTORC CLAMP bolting system.

The HYTORC method is said to eliminate the need for longer bolts, reaction arms, backup wrenches, pullers and bridges and two sets of tooling (one for torque and one for tension).

HYTORC says the two most common industrial bolting procedures are torque and tension. Both are applied to obtain even bolt load and thus, consistent circumferential compression to eliminate leaks or joint failure under varying stresses or internal pressures and to avoid inadvertent loosening of fasteners. According to HYTORC, at present the only way to assure such results is through measurement of achieved bolt elongation, an extra procedure that the company says should have been replaced years ago.

Simply put, torque elongates the bolt through the process of turning the nut with force onto the parts to be assembled. Tension elongates the bolt by pulling it hydraulically and by sealing the nut manually onto the parts to be assembled.

Torque is subject to unknown thread and bearing face friction. Tension is subject to unknown bolt relaxation when the stretch-load is transferred onto a hand-tight nut. To achieve and retain an acceptable bolt load with both methods is thus based more on operator's experience than on physical facts.

To achieve at least an acceptable, even, circumferential bolt load while torquing, the spot faces oil which the nuts turn should have an identical finish for a more predictable bearing face friction, nuts should be precision machined and thread friction should be equalized with better lubrication. Torsion should be contained in the tool with a holding pawl to avoid pre-stall and kept at a minimum as later unwinding can add to the load inaccuracies.

To achieve a somewhat constant residual bolt load while tensioning, each nut would have to be manually turned by the same degree. A quarter turn less on an 8TPI nut would already reduce the retained bolt elongation by 31/1000th of an inch. That represents total desired elongation in many cases. In addition, load transfer to the nut should be gradual rather than sudden to avoid embedment on the bearing face surface.

Other methods of tightening industrial fasteners such as Jackscrew nuts or the turn-of-the-nut method have their shortcomings.

Based on these facts, it is clear that something had to be done, according to HYTORC.

In torque, unknown bearing face friction has to become known, thread friction has to become more predictable, torsion and side-load has to be eliminated. In tension, load transfer has to be eliminated and nut turning has to become more precise and repeatable. Both torque and tension have to be made more predictable, safer and simpler to eliminate human error.

The only way to accomplish these goals is to alter the nut. HYTORC invented the DISC, an economical washer placed underneath a regular nut and the CLAMP, a nut of same size and strength as the one it replaces.

The HYTORC-DISC is a flat washer, which threaded segment threads onto the bolt. It is common knowledge that the bolt has a tendency to turn along with the nut when it is turned by a tool. The HYTORC-DISC stops the bolt from turning along with the nut as the tool-driving the nut applies a holding force to the DISC to keep it stationary. Based on the principle that axial pulling force exceeds the pulling force created by torsion, the inner threaded segment of the DISC is engineered to resist the latter but to move axially with the prior to allow torsion-free bolt elongation.

To keep present side clearances identical, the HYTORC-DISC has the same hex size as the nut presently used. This requires a round nut rather than another hex nut. The manufacturing of the round nut permits us to apply dry lubrication to it so as to obtain a more consistent thread friction and as it is machined in conjunction with the DISC the bearing face friction between the two becomes known. The end result is a more even, circumferential bolt load than is now achievable with torque or tension.

How is this accomplished? A threaded segment is machined inside the HYTORC-DISC washer with a common shear section engineered to fail when a given stretch load is applied to it. As the drag friction provided by the nut to the bolt is not sufficient to break the shear section, the bolt is held stationary when a holding force is applied to the DISC by the tool that turns the nut. When bolt elongation breaks the segment loose to rise in the DISC's inner opening, axial grooves eliminate its turning and the break-away sections of both the washer and the segment add the necessary friction to limit all axial movement of the segment to the force applied by the elongating bolt.

For the most critical applications, where only the exact residual bolt load will do, HYTORC invented the CLAMP.

The HYTORC-CLAMP is a replacement nut of equal size and strength. A washer is spline connected and an outer sleeve is thread connected to all inner sleeve connected to the bolt. When the outer sleeve is turned on the inner sleeve and the washer face by a tool that stops the inner sleeve and thus the washer from turning with the outer sleeve, the inner sleeve moves in the axial direction to elongate the bolt. As the outer sleeve turns on precision-machined surfaces, the coefficient of friction is known. The end result is a torsion, side-load and relaxation-free holt elongation with a predictable residual bolt load,

How is it accomplished? By spline connecting the washer to the inner sleeve, the assembly is subject to two turning frictions, one with the bolt and one with the outer sleeve and 2 bearing face frictions, one with the joint and one with the outer sleeve. The outer sleeve, on the other hand, has merely one thread friction with the inner sleeve and one bearing face friction with the washer. By applying a turning force to the outer sleeve with a tool that simultaneously applies an equal and opposite reaction force to the inner sleeve or washer, only the outer sleeve turns stretching the bolt torsion-free. As the outer sleeve turns on known surfaces, the coefficient of friction becomes known and the bolt load, which is established through calibration, becomes predictable with an accuracy of 3 percent. Circle #200
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Publication:Pipeline & Gas Journal
Date:Nov 1, 2002
Previous Article:Advances in materials, methods, help gain new users. (Composite Pipeline Repair).
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