Expanded – threaded – driven – headed – tool-deformed – or locked-thr – Headed fastener element with nut – washer – securing means or cap – Headed bolt or screw with threads and complementary nut
Reexamination Certificate
1998-12-18
2001-01-23
Saether, Flemming (Department: 3627)
Expanded, threaded, driven, headed, tool-deformed, or locked-thr
Headed fastener element with nut, washer, securing means or cap
Headed bolt or screw with threads and complementary nut
C411S537000, C411S538000, C411S135000, C411S916000, C285S412000
Reexamination Certificate
active
06176663
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a novel apparatus for reducing stresses due to bending in threaded fasteners, particularly bolts and cap screws, used in connecting flanged connections. Such flanged connections have a wide variety of industrial uses including connecting structural members that will be subjected to critical external loads and in sealing high pressure vessels. The flanges used in such applications must be maintained in face to face contact to ensure proper operation. This face to face contact is maintained by the plurality of threaded fasteners that are tightened to a high percentage of their yield strength to ensure this contact is maintained. It is well known in the art that such flanges bend in an axial or meridional direction during this tightening procedure. This meridional bending is transmitted to the threaded fasteners and is known to contribute significantly to the stress in the threaded fastener and reduces the amount of tightening to which the threaded fastener can be safely subjected. This in turn reduces the face to face contact between the flanges and therefore the load to which the flanged connection can be subjected.
The present invention has particular applicability in the oil and gas industry where large diameter tubular members with flanged end connections, called “risers” or “riser joints”, are used. These risers are used in offshore drilling and production operations and extend from the wellhead at the ocean floor to a surface vessel. These risers have typically been manufactured in fifty and eighty foot lengths. During normal operation the flanged connection between adjacent riser joints must contain internal pressure along with enormous external loads due to environmental conditions. It is critical that these flanged connections, and the threaded fasteners holding them together, function flawlessly. These flanged connections and the threaded fasteners connecting them are the area to which the present invention most closely applies.
2. Description of Related Art
An apparatus for stretching threaded fasteners is shown in U. S. Pat. No. 3,749,362 (O'Connor et al.) The apparatus shown by O° Connor et al. uses an external fastener stretcher threaded onto a bolt to apply an axial load to the bolt wherein the bolt's nut can then be tightened with minimal torque. Removal of the fastener stretcher allows the bolt and nut to retain the load induced in the bolt.
SUMMARY OF THE INVENTION
The current invention uses a uniquely shaped stress reducing washer designed to compensate for the axial or meridional direction bending that the flange undergoes during fastener tightening. The stress reducing washer ensures that as the flanges bend the threaded fastener remains straight and thereby minimizes the bending stress transmitted to the fastener.
According to the present invention, in a first embodiment the stress reducing washer has upper and lower planar surfaces, the lower surface perpendicular to the fastener axis while the upper surface is tapered radially. The stress reducing washer is positioned in a counterbore in the flange backface. A pair of retaining pins ensure the stress reducing washer is properly oriented and is retained in the counterbore. As the fastener is tightened to its predetermined value and the flange deflects, the head of the fastener remains vertical while the washer moves radially. At maximum fastener torque and flange deflection, the tapered upper surface of the stress reducing washer is in substantially face to face contact with the head of the fastener and perpendicular to the axis of the fastener.
In a second embodiment of the invention, the stress reducing washer is of two part construction. The lower washer has a flat lower face with a spherical upper face. The upper washer has a flat upper face with a spherical lower face which engages the spherical upper face of the lower washer. As in the first embodiment, as the fastener is tightened to its predetermined value and the flange deflects, the head of the fastener remains vertical while the spherical faces of the upper and lower washers move radially. Under all conditions, loaded or unloaded, the flat upper face of the upper washer is in substantially face to face contact with the head of the fastener and perpendicular to the axis of the fastener.
In a third embodiment of the invention, the stress reducing washer is of two part construction also. The lower washer has a flat lower face with a cylindrical upper face. The upper washer has a flat upper face with a cylindrical lower face which engages the cylindrical upper face of the lower washer. As in the first embodiment, as the fastener is tightened to its predetermined value and the flange deflects, the head of the fastener remains vertical while the cylindrical faces of the upper and lower washers move radially outwardly. Under all conditions, loaded or unloaded, the flat upperfaceof the upper washer is in substantially face to face contact with the head of the fastener and perpendicular to the axis of the fastener.
In a fourth embodiment of the invention, the backface of the flange is machined with a radially tapered surface adjacent each fastener hole to emulate the stress reducing washer of the first embodiment.
A principal object of the present invention is to provide an apparatus to reduce the bending stress transmitted to threaded fasteners due to deflection of the flange.
Another object of the present invention is to provide an apparatus for increasing the fatigue life of threaded fasteners used in flanged connections.
A final object of the present invention is to provide an apparatus for reducing the bending stress in threaded fasteners without requiring the use of special fasteners.
These with other objects and advantages of the present invention are pointed out with specificness in the claims annexed hereto and form a part of this disclosure. A full and complete understanding of the invention may be had by reference to the accompanying drawings and description of the preferred embodiments.
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Nguyen Lan T.
Walker Stephen J.
Cooper Cameron Corporation
Duke Jackie Lee
Saether Flemming
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