Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Threaded fastener stress
Patent
1995-06-15
1996-11-05
Chilcot, Richard
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Threaded fastener stress
G01L 524
Patent
active
055719716
DESCRIPTION:
BRIEF SUMMARY
CROSS REFERENCE TO RELATED APPLICATIONS
This application is the U.S. national phase of PCT application PCT/FR92/01192 filed 16 Dec. 1992.
FIELD OF THE INVENTION
The invention relates to a method of monitoring and controlling the screws in a threaded element such as a screw, bolt, nut or other threaded mounting element mainly but not exclusively used in the screwed assemblies.
BACKGROUND OF THE INVENTION
The automotive, aeronautical, aerospatial, nuclear and other mechanical manufacturing industries generally use screwed assemblies and this in applications that are extremely varied and complex.
The calculation notes for screwed assemblies are aimed at determining the prestress to apply to the assembly as a function of the useful stress (or service stress) to which the assembly will be subjected when used and of a tightening factor depending on the precision of the means for applying this prestress.
These calculation notes done by engineers studying in these industries are very precise as a function of the defined needs and the conditions of use of the screwed assemblies are more and more severe so that it is getting essential to increase the reliability of these assemblies while respecting the calculation notes in their execution so as to increase the performances and the safety of the systems thus made.
The stress to apply in a threaded mounting element is generally created by conversion of a rotation force into a tractive force by the helicoidal path of the thread of the element. This conversion is imperfect mainly because of the losses from friction of the surfaces in contact; hence it is necessary to apply a rotation force (torque) greater than the rotation force necessary for this conversion.
The efficiency of this conversion is extremely varied because of the variation of the coefficients of friction, the variation of the distance to which the resultant of the friction forces are applied relative to the axis of rotation, and the geometric variation of the parts, mainly the contacting surfaces. Practical observations give a variation of 50% of the induced stress during application of a constant torque on the same lot of mounting elements. It is verified that the improvement of the precision of the applied torque does not bring about a significant improvement in the precision of the tension measured.
A first improvement of the variation of the stress generated by the torque has been associated with the rotation angle starting from a predetermined torque. In effect the lengthening of the mounting element which creates the tension is proportional to the rotation angle. This proportion should be determined in advance because it is dependent on the assembly and not on the mounting element. The dispersion of the stress is improved in particular conditions due to the earlier trials and the geometric specifications of the engaging pieces which can be expensive. In addition the danger of entering into the plastic zone is not excluded because of the variability of the starting point of the measurement of the angle, in theory called prestress torque. It is to be noted that the prestress torque can be an accidental couple, for example from crossthreading; in this case the tension is far from being created.
Starting from the combination of these two measurements of torque and angle a tightening principle has been worked out: the permanent monitoring during the application of the torque of the variation of the gradient of the torque as a function of the angular advance allows one to establish an identifiable point of the function torque/angle, called the point of the elastic limit. This method puts the screw into a state of constraint (or of prestress) which uses all of the prestress possible of the assembly to the detriment of the service load. An advance determination of the tension is done in order to integrate the variations of section and the resistance to traction of the materials forming the mounting element. This method is very sensitive to the reaction forces of the means that apply the torque; for e
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patent: 4016938 (1977-04-01), Rice
patent: 4037282 (1977-07-01), Dahl
patent: 4142266 (1979-03-01), Dahl
patent: 4185504 (1980-01-01), Exner et al.
patent: 4488437 (1984-12-01), Pere
Chastel Jean-Marc
Rey Didier
Chilcot Richard
Ciandar
Dubno Herbert
Noori Max
Wilford Andrew
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