Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Threaded fastener stress
Reexamination Certificate
2000-08-10
2001-11-13
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Threaded fastener stress
Reexamination Certificate
active
06314817
ABSTRACT:
FIELD OF THE INVENTION
The invention is related to a method for tightening a threaded fastener to and above its yield point, i.e. into the plastic deformation range of the fastener, by using the variation in travel time for ultrasonic waves induced in the fastener.
BACKGROUND OF THE INVENTION
In for instance the motor vehicle industry, it is desirable to keep down the weight of at least some types of threaded joints comprised in various vehicle structures. This is accomplished by utilising the maximum clamping force obtainable by a certain size of the fasteners comprised in the joint, which means that each fastener is tightened to its yield point, i.e. at the upper end of its elastic deformation range. This well known method is described in for instance U.S. Pat. No. 2,600,549.
However, when tightening a fastener to a level above its yield point, i.e. into the plastic deformation range, there is a problem to accurately determine the obtained axial load, because in that range the axial load does not increase linearly in relation to the angle of rotation.
In prior art, there has been no simple and reliable method available for obtaining accurate information about the accomplished axial load when tightening a fastener into the plastic deformation range.
According to one well known method for tightening a fastener into the plastic deformation range, the yield point is determined by detecting the significant change in torque growth obtained in that point, and by tightening the fastener over another predetermined angle from that point on. By this method, a high clamping ability of the fastener is utilised since it is tensioned above the yield point, but there is no possibility to determine the actually obtained axial load on the fastener. Occurring variations in the frictional resistance in the fastener cause a considerable scattering of the obtained axial load, not only in the yield point but also in the final shut-off point.
A well known method for determining the axial load on a fastener and described in for instance U.S. Pat. No 3,969,810, comprises measuring the change in travel time of ultrasonic waves induced axially into the fastener. This method is applicable on tightening a fastener within the elastic deformation range only, because the direct correspondence between the ultrasonic wave travel time and the axial load exists only when there is a linear relationship between the elongation of the fastener and the axial load. However, this is not the case in the plastic deformation range, where the fastener is elongated at a low axial load increase only. This means that this method, as previously described, is not useful to determine the axial load on a fastener at yield tightening.
There is also previously suggested a method for determining the obtained axial load in a fastener by measuring the change in travel time for ultrasonic waves induced into the fastener, also when tightening the fastener into the plastic deformation range. This method is based on a pre-tightening establishment of the relationship between the change in ultrasonic wave travel time and the axial load on the actual type of fastener. Based on this information, the obtained axial load is determined by measuring the change in the ultrasonic travel time and relating it to the previously established relationship between the travel time and the axial load.
This known method, however, does not take into account the variations in the frictional resistance and, thereby, the variations in axial load level obtained at the yield point of the fastener as well as in the plastic deformation range.
As mentioned above, the axial load in and above the yield point does not correspond directly to the elongation of the fastener but depends on the actual friction conditions in the fastener as well. This is due to the fact that a high frictional resistance in the fastener causes increased shearing stresses on the fastener, and a plastic deformation of the fastener takes place at a lower axial load. Accordingly, the axial load in the yield point for a certain type of fastener varies a lot since it is highly dependent on the actual friction conditions in the fastener.
OBJECT OF THE INVENTION
The invention intends to provide a method for accurately determining the axial load in a threaded fastener as the latter is tightened above its yield point by using the change in travel time of ultrasonic waves induced in the fastener for determining the axial load in the yield point as well as in the final pretension level of the fastener.
Further characteristics and advantages of the invention will appear from the following description and claims.
DETAILED DESCRIPTION
The new method according to the invention comprises a pre-tightening investigation process in which the actual type of fastener is subjected to tightening operations including pre-tensioning into the plastic deformation range. This pre-tightening investigation process includes tightening of a number of fasteners to a level above the yield point while measuring by means of a suitable equipment the applied torque, the rotation angle, the axial load, and the travel time for ultrasonic waves induced into the fastener.
Thereby, it is possible to determine the relationship between the axial load, the rotation angle and the ultrasonic wave travel time. It is also possible to investigate the magnitude and the scattering of the torque level at which the fastener starts deforming plastically.
It is important to determine the relationship between the axial load and the travel time for the ultrasonic waves in the plastic deformation range of the fastener, because in that range this relationship is no longer linear. It is of particular importance to investigate how the axial load above the yield point varies with the axial load level in the yield point. The higher the frictional resistance the lower axial load in the yield point. As mentioned above, the explanation to this is that both the axial load and the torsional load contribute to the strain in the fastener and cause together the fastener material to yield at a certain level. This means that a high frictional resistance causes the fastener material to yield at a lower axial load, and, accordingly, it is important to include in the investigation process fasteners having the lowest and highest frictional resistance that can be expected in a normal fastener production, because that would give an indication not only on the scattering range of the frictional resistance but also of the lowest axial load to be expected at the yield point of the fastener. This information is of decisive importance when determining the proper size of fastener to be used.
Since the ultrasonic wave travel time reflects the axial load in the elastic deformation range only, it is not possible to use this travel time to indicate the yield point. Instead, the yield point is suitably detected as a significant change in the torque/angle relationship. It may, however, be detected in any other suitable way. In the yield point, the travel time for the ultrasonic waves is determined, though, so as to make it possible to relate the axial load in the following plastic deformation range to a corresponding axial load/travel time relationship determined during the pre-tightening investigating process.
In practice, a fastener of the actual type is intended to be tightened to a predetermined axial load chosen within the range available for this type of fastener as determined during the previous test runs of the fastener. A torque is applied on the fastener and the instantaneous values of the torque, angle of rotation and the ultrasonic wave travel time are indicated. The ultrasonic wave travel time is indicated from the very start of the process to obtain a value for the travel time through the fastener in unloaded condition.
As the yield point is detected by the change in, for instance, the torque/angle relationship, the tightening is continued to a wave travel time value which according to the pre-tightening investigation corresponds to the desired axial load.
By indicating
Atlas Copco Tools AB
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Noori Max
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