Patent Application
20120296578
Bolted joints are used to clamp two parts of a structure and maintain the two parts in clamped relation. Bolts are installed in such joints with an initial tension known as bolt preload, which is proportional to the torque applied to the bolt head. The bolt preload serves to maintain the joined parts in clamped relation and to increase the fatigue resistance of the joint. In structures exposed to high stresses or adverse environmental conditions, such as wind turbines, the preload of a bolt may decrease over time, weakening the joint and increasing the potential for joint fatigue and failure. However, accurately applying the correct initial preload to a bolt can reduce the likelihood of failure. It is thus desirable to achieve high accuracy in applying the initial preload as well as to monitor the preload of the joint over time.
Known torque wrenches allow for measured application of torque to the bolt, and bolt preload may be calculated from the applied torque value and the coefficients of friction between the bolt, the parts to be joined, and the nut on the bolt. As such coefficients may vary depending on the materials used and the presence or absence of lubrication, the preload calculated from the same torque value may vary by as much as 25%.
Bolt preload may also be calculated from the elongation of the bolt. Certain known solutions include ultrasonic sensors disposed on the head of the bolt. However, the use of such sensors requires that the head and the tip of the bolt be machined with high accuracy, so as to return an accurate reading. However, the use of such sensors may effectively double the cost of the bolt. Furthermore, such sensors may not be able to adequately withstand adverse environmental conditions. A low-cost, accurate method of measuring bolt preload in real-time during installation, as well as subsequent to installation, is thus desired.
According to at least one exemplary embodiment, a system for in situ fastener preload measurement is disclosed. The system may include a length measuring tool adapted to grip a fastener by the axially opposed ends of the fastener, and a computing device coupled to the length measuring tool and comprising a database and a display.
A method for in situ fastener preload measurement is also disclosed. The method may include measuring the length of a fastener with a length measuring tool communicatively coupled to a computing device comprising a display and a database and correlating the length of the fastener to a corresponding preload force value stored in the database.
A method of tensioning a fastener or a nut disposed on the fastener is also disclosed. The method may include tensioning the fastener or the nut disposed on the fastener using a torque applying tool communicatively coupled to a computing device comprising a display and a database and simultaneously measuring the length of the fastener with a length measuring tool communicatively coupled to the computing device, correlating the length of the fastener to a corresponding preload force value stored in the database, and stopping tensioning the fastener or the nut disposed on the fastener when a desired preload force is achieved.
In the above-described system and methods, the length measuring tool is provided with tips adapted to seat in indentations disposed in the axially opposed ends of the fastener, with the indentations having a shape complementary to the shape of the tips, and the indentations being disposed such that the deepest points of the indentations are directly axially opposite each other. The tips may have a conical, paraboloid, or hemispherical shape.
Advantages of embodiments of the present invention will be apparent from the following detailed description of the exemplary embodiments. The following detailed description should be considered in conjunction with the accompanying figures in which:
a shows an exemplary fastener for use with the system for in situ fastener preload measurement.
b-2d show exemplary length measuring tool tips and fastener indentations for use with the system for in situ fastener preload measurement.
Aspects of the invention are disclosed in the following description and related drawings directed to specific embodiments of the invention. Alternate embodiments may be devised without departing from the spirit or the scope of the invention. Additionally, well-known elements of exemplary embodiments of the invention will not be described in detail or will be omitted so as not to obscure the relevant details of the invention. Further, to facilitate an understanding of the description discussion of several terms used herein follows.
As used herein, the word “exemplary” means “serving as an example, instance or illustration.” The embodiments described herein are not limiting, but rather are exemplary only. It should be understood that the described embodiment are not necessarily to be construed as preferred or advantageous over other embodiments. Moreover, the terms “embodiments of the invention”, “embodiments” or “invention” do not require that all embodiments of the invention include the discussed feature, advantage or mode of operation.
Further, many of the embodiments described herein are described in terms of sequences of actions to be performed by, for example, elements of a computing device. It should be recognized by those skilled in the art that the various sequence of actions described herein can be performed by specific circuits (e.g., application specific integrated circuits (ASICs)) and/or by program instructions executed by at least one processor. Additionally, the sequence of actions described herein can be embodied entirely within any form of computer-readable storage medium such that execution of the sequence of actions enables the processor to perform the functionality described herein. Thus, the various aspects of the present invention may be embodied in a number of different forms, all of which have been contemplated to be within the scope of the claimed subject matter. In addition, for each of the embodiments described herein, the corresponding form of any such embodiments may be described herein as, for example, “a computer configured to” perform the described action.
According to at least one exemplary embodiment, a system for in situ fastener preload measurement 100 may be provided. System 100 may be used to measure fastener preload in “bolt-through” type joints, or any other joint where both ends of the fastener are accessible. The system may be used to measure fastener preload in situ, for example, so as to verify fastener preload after installing and tightening the fastener, and to measure any changes in preload over time. The system may be calibrated for a variety of fasteners and environmental factors, and may store the calibration data. The system may further be used in conjunction with a torque wrench or other bolt tensioning mechanisms, so as to provide bolt preload information in real time as the bolt is being tightened.
Referring to
a shows an exemplary fastener 110 for use with system 100. Fastener 110 may be any known type of fastener. For example, fastener 110 may be a bolt-type threaded fastener having a nominal diameter of 24 millimeters or greater (i.e., an M24 or greater bolt). To adapt fastener 110 for use with system 100, indentations 112 may be made in a first end 114 of fastener 110 and in a second end 116 of fastener 110. The shape of each indentation 112 can be complementary to the shape of a tip 104. Thus, embodiments may have indentations having a conical shape, as shown in
Indentations 112 may be made by any machining process known in the art. As shown in
It should be appreciated that, for system 100 to function as described herein, it is not necessary for the entire surfaces of the first and second ends of the fastener to be precisely machined. Furthermore, a reasonable mismatch in the radial positions of indentations 112 with respect to each other may exist without impeding the functionality of system 100. Thus, the process for machining indentations 112 on fastener 110 may be a simple and low cost process.
Prior to use of system 100 in situ, the system may be calibrated in laboratory or in other controlled conditions. To perform calibration of system 100, a particular fastener 110 having indentations 112 may be selected. The fastener may then be measured utilizing gauge 102 and tips 104 so as to obtain a length for the fastener when no preload forces are being exerted thereon. Measurements performed by gauge 102 may be transmitted to or input in computing device 120 and stored in database 122. Fastener 110 may then be subjected to preload forces in desired increments, and the length of the fastener can be measured subsequent to each increment in preload force. For example, the force may be increased or decreased by an increment of 10% of the initial preload force value. Each length measurement performed by gauge 102 and the value of the corresponding preload force may likewise be transmitted to or input in computing device 120 and stored in database 122. The resultant data set may then be used to generate a two-dimensional plot 126 correlating preload force and fastener length, as shown in
If desired, the calibration procedure described above may be repeated at diverse temperatures so as to account for thermal expansion or contraction of the fastener under varying environmental conditions. Thus, the calibration procedure may be performed at an initial temperature, and the temperature may then be increased or decreased in desired increments. The calibration procedure can then be repeated for each temperature value. Each length measurement performed by gauge 102, the value of the corresponding preload force, and the corresponding temperature value can be transmitted to or input in computing device 120 and stored in database 122. The resultant data set may then be used to generate a three-dimensional plot correlating temperature, preload force and fastener length. The plot may be displayed on display 124.
For some fasteners, the curve fit for the data in the resultant plot may be substantially a straight line, as in a spring equation according to Hooke's law. However, for many fasteners, plastic deformation of the threads and differences in the elongation of different sections of the fastener (i.e. the head, shank, and thread sections of the fastener may each have different spring constants), may be factors that can add complex variation to the curve fit. Some of these factors may be due to unpredictable variation in tolerances and in stress distribution in the fastener. The calibration procedure allows for the consideration of such factors for each type of fastener. Thus, as a result, the curve fit may not be substantially straight and may account for the above-described factors.
Database 122 can be populated with preload force, fastener length, temperature, and any other desired data for a variety of fasteners. For each group of fasteners having identical properties, a representative fastener of that group may be selected and the calibration procedure may be performed on the representative fastener. The resultant data set can then be identified as pertaining to the particular group of fasteners having the properties of the representative bolt. Thus, for in situ measurement of fastener length using gauge 102, database 122 may be populated with data for each group of fasteners that will be measured.
System 100 may be used to perform in situ preload force measurement on fasteners 110 having indentations 112 in first end 114 and second end 116 thereof, and disposed in “bolt-through” joints, or in any other location where both ends of the fastener are accessible. To perform the measurement procedure, gauge 102 can be placed proximate to a fastener 110 such that tips 104 of gauge 102 are seated in indentations 112 of fastener 110 and such that the positions of tips 104 are fixed in three dimensions. Gauge 102 may then be used to obtain a measurement of the length of fastener 110. A measurement of the ambient temperature may also be obtained, if desired. The measured length and temperature values may then be correlated with the data set in database 122 that corresponds to the group of fasteners to which the particular fastener 110 belongs. The preload force value corresponding to the measured length and temperature values can then be obtained.
The correlation of temperature, fastener length, and preload force may be performed automatically by computing device 120. The measured length values may be transmitted from gauge 102 to computing device 120. Likewise, measured temperature values may be transmitted from thermometer 108 to computing device 120. Alternatively, the measured values may be manually input into computing device 120. As shown in
System 100 may be utilized to obtain preload force values in real time while tensioning a threaded fastening member, such as a fastener or a nut disposed on the fastener. To that end, gauge 102 may be utilized to measure fastener length while torque wrench 106 may be used simultaneously to tension the fastener or the nut. Gauge 102 can thus continuously measure the increase in bolt length, which may then be correlated to the preload force exerted on the bolt by computing device 102, as described above. When the desired preload force is achieved, computing device 102 may indicate that no further tensioning is necessary and may transmit a signal to torque wrench 106 to cease tensioning the fastener or the nut. Calculating preload force simultaneously with tensioning the fastener or the nut can thus facilitate obtaining desired preload forces with higher accuracy.
The foregoing description and accompanying figures illustrate the principles, preferred embodiments and modes of operation of the invention. However, the invention should not be construed as being limited to the particular embodiments discussed above. Additional variations of the embodiments discussed above will be appreciated by those skilled in the art.
Therefore, the above-described embodiments should be regarded as illustrative rather than restrictive. Accordingly, it should be appreciated that variations to those embodiments can be made by those skilled in the art without departing from the scope of the invention as defined by the following claims.
