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
Patent
1996-05-30
1998-08-11
Wilson, Neill R.
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
411535, 411909, F16B 3300, F16B 4300
Patent
active
057918473
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a screw coupling where a certain amount of prestress is desired in the screw. The pre-tensioning is accomplished by using a pre-tightening washer under the screw head or a nut.
BACKGROUND OF THE INVENTION
Usually this is attempted by placing a washer under a screw or nut to make it easier to tighten the screw by torsional moment, to reduce surface pressure exerted on the support or to ensure the fixing of the screw by shaping the washer suitably. By means of an appropriately shaped washer, a bending effect on the screw by a sloped support can be avoided, or the washer can be used for sealing the coupling or for galvanic insulation of the parts of the coupling. The washer can also be shaped, for example forming it cup-like, so that it will be straightened upon tightening and cause tensile stress in the screw.
In most screw couplings, a sufficient and accurate pre-tensioning of the screw is necessary for the reliable functioning of the coupling. A desired prestress is effected by turning the screw to a tighter position by applying torsional moment or by stretching the screw by a predetermined force corresponding to the desired prestress before the nut or screw is screwed down.
The screw is tightened by means of torsional moment, often with the help of a torsion tool which turns the screw or nut, where often the screw or nut must be held on the opposite side in order to succeed in the tightening. To bring about a certain, predefined tensile stress in the screw, the tool must be provided with a torque restriction or measurement. Because of many factors, such as friction, the prestress brought about in the screw by means of torsional moment is rather inaccurate. Further, the pre-tensioning taking place through the torsional moment causes shear stress in the screw which leads to the dissipation of a part of the pre-tensioning potential of the screw.
The screw can also be warmed up before its assembly, whereby it is stretched by thermal expansion. After the screw has been assembled hot and stretched to the desired extension, it, upon cooling, tightens the coupling, and the desired prestress will remain in the screw. The screw can also be warmed up by means of a heating element placed in a hole bored therein, for example by using an electric resistance. The drawback of the pre-tensioning methods based on thermal expansion is often the high temperature necessary for sufficient elongation, making the assembly difficult and possibly weakening the strength characteristics of the screw metal.
The pre-tensioning can also be performed by stretching the screw by means of pre-tensioning tools, which operate hydraulically or in some other way, as is presented for example in German publication 3733243 or Swedish publication 7701049-4. The required tools are often complicated and are not often suitable for pre-tensioning of standard screws.
U.S. Pat. No. 4,450,616 shows a washer made of shape-memory metal, used for ensuring the tightness of a screw if the screw tends to slacken. According to the publication, an elastic washer punched out of a shape-memory metal is pressed in an austenitic form at an elevated temperature (200.degree. C.) above the A.sub.f point into a cup-like shape. When it is cooled down to the martensitic area below the M.sub.f point (below -150.degree. C.), the screw can be easily screwed down, and the washer deforms to its disc-like shape. As the temperature rises by heat conduction above the A.sub.f point, the washer tends to recover its cup-like shape. Because of its cup-like shape and elasticity, the washer cannot stretch the screw in a manner required for the pre-tensioning.
Japanese publication 62-116292 shows a spring washer intended for tightening of a nuclear reactor container cover. The washer is of shape-memory metal and designed to tighten the bolt in order to maintain the sealing effect as the temperature increases and the bolt is extended by thermal expansion. That invention is not suitable for pre-tensioning of a bolt.
SUMMARY OF THE INVE
REFERENCES:
patent: 4120230 (1978-10-01), Schutzler
patent: 4450616 (1984-05-01), Morita
patent: 4501058 (1985-02-01), Schultzer
patent: 4884934 (1989-12-01), Ikeda et al.
patent: 4897006 (1990-01-01), Blin
patent: 5010949 (1991-04-01), Dehaine
Kvaerner Tamturbine OY
Wilson Neill R.
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