Abstract:
A system is provided for tightening fasteners. The system has a tightening tool configured to apply torque to a fastener. In addition, the system has a strain sensor configured to sense a parameter of the fastener indicative of an elongation of the fastener. Additionally, the system has a stress sensor configured to sense a parameter of the fastener indicative of magnitude of the applied torque. The system further includes a controller configured to regulate the tightening tool based on a relationship change between the elongation and the magnitude of the applied torque

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure is directed to a fastener tightening system, and more particularly, to a fastener tightening system that utilizes ultrasonic technology. 
       BACKGROUND 
       [0002]    Conventional manufacturing processes typically involve the assembly of individual components into a finished product. Depending on the intended use of the components and type of joints formed during assembly, several methods and devices can be employed to secure the individual components together. Among the devices commonly used to combine components are mechanical fasteners. Mechanical fasteners grip two or more of the components and effectively use compressive forces to minimize movement between the components. 
         [0003]    The strength of joints secured by mechanical fasteners is dependant upon the magnitude of the overall compressive forces applied to the joint, as well as the degree to which the compressive forces acting on the joint are distributed. For example, the joint is strongest when the overall compressive force acting on the joint is evenly distributed over the surfaces of the joined components. 
         [0004]    Typically, the axial load of each fastener and thus the compressive force of the joint is indirectly determined through a measurement of torque and angle of rotation applied to the fastener. In the elastic deformation range of each fastener, the axial load of the fastener has a linear relationship with the applied torque and angle of rotation, and the fastener can be tightened to within 15 percent of a desired axial load using the torque and angle of rotation measurement technique. However, in the plastic deformation range of each fastener, the relationship between axial load, torque, and rotation angle is no longer linear. The axial load on the fastener can vary greatly in relation to applied torque and angle of rotation when in the plastic deformation range, which can make it difficult to predict the axial load acting on the fastener. 
         [0005]    Because this axial load is difficult to determine in the plastic deformation range, conventional assembly systems tighten fasteners as close to the upper limit of the elastic deformation range as possible to achieve the maximum distributed compressive load throughout the joint. This upper limit is often referred to as the yield point. However, conventional systems typically do not have the capability to accurately determine the yield point for each fastener and often insert a safety factor to avoid exceeding the yield point. Unfortunately, inserting a safety factor limits the available compressive force that can be applied to the joint. Moreover, limiting the applied compressive force to avoid the undetermined yield point requires a larger fastener to produce the same amount of force as a smaller fastener used to its full capacity. Utilizing larger fasteners requires more raw materials, which can increase production costs. 
         [0006]    U.S. Pat. No. 6,314,817 issued to Lindback (&#39;817 patent) on Nov. 13, 2001, discloses a system that tightens fasteners to their maximum axial load capacity. To reach the maximum available axial load, the system performs a pre-tightening process on a representative sample of fasteners similar to the ones that are to be used for assembly. The pre-tightening process is performed in a laboratory environment and compares the axial load acting on a fastener to its elongation in both the elastic and plastic deformation ranges. The elongation of each fastener is determined by measuring the length of time an ultrasonic pulse takes to travel up and down the length of the sample fastener. Once the axial loads are determined, the data collected in the pre-tightening process is applied to the tightening of non-tested fasteners in an assembly process. In the assembly process, an axial load along with the related target ultrasonic pulse travel time for each fastener is chosen. The fasteners are tightened until the travel time of the ultrasonic pulse reaches the target time determined in the pre-tightening process. 
         [0007]    Although the system disclosed in the &#39;817 patent may be able to predict the axial load of a sample fastener in both the elastic and plastic deformation ranges, the data may be invalid or inaccurate when used in conjunction with non-tested fasteners utilized during assembly. Because of inherent inconsistencies in the fastener manufacturing process, the mechanical properties may vary from fastener to fastener. In particular, the mechanical properties of the sample fasteners used in the pre-tightening process may not be the same as the mechanical properties of fasteners used to assemble components. For example, the relationship between the yield point and elongation of a fastener may vary 20%-40% from the yield point/elongation relationship of the sample fasteners examined in the lab and can affect the relationship between travel time of the ultrasonic pulse and axial load. Using a travel time of an ultrasonic pulse determined in the pre-tightening process for a particular axial load may actually cause the fastener to be tightened to an incorrect axial load. Without the fasteners being tightened to the desired axial loads, the compressive force may be unevenly distributed, which may weaken the joint. Moreover, without an accurate determination of the yield point of the actual fastener being tightened, the fastener may be tightened dangerously close to or beyond the ultimate tensile strength of the fastener, which may cause the fastener to fail. In addition, using a pre-tightening process to determine the mechanical properties of the fasteners adds an additional step to the tightening process, which can reduce efficiency. 
         [0008]    The disclosed tightening system is directed to overcoming one or more of the problems set forth above. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect, the present disclosure is directed toward a fastener tightening system. The system includes a tightening tool configured to apply torque to a fastener. In addition, the system includes a strain sensor configured to sense a parameter of the fastener indicative of an elongation of the fastener. Additionally, the system includes a stress sensor configured to sense a parameter of the fastener indicative of magnitude of the applied torque. The system further includes a controller configured to regulate the tightening tool based on a relationship change between the elongation and the magnitude of the applied torque. 
         [0010]    Consistent with a further aspect of the disclosure, a method is provided for tightening a fastener. The method includes applying a torque to the fastener, sensing a first parameter of the fastener indicative of a strain of the fastener, and sensing a second parameter of the fastener indicative of a magnitude of the applied torque. The method further includes adjusting the magnitude of the applied torque in response to a relationship change between the strain and the magnitude of the applied torque. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a diagrammatic illustration of a component assembly system according to an exemplary disclosed embodiment; 
           [0012]      FIG. 2  is a flow diagram of a method according to an exemplary disclosed embodiment; and 
           [0013]      FIG. 3  is a graphical representation of the relationship between elongation and rotation angle of exemplary disclosed fasteners. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]      FIG. 1  provides a diagrammatic perspective of a component assembly station  10  according to an exemplary embodiment. Component assembly station  10  may be used to secure individual components together to create a finished product via mechanical fasteners  12 . Such finished products may include, for example, engine assemblies, engine exhaust assemblies, construction equipment, or any other finished product known in the art requiring threaded fasteners to secure individual components together. Mechanical fasteners  12  may be, for example, screws, bolts, or any other mechanical fastener known in the art. Component assembly station  10  may include a tightening tool  14  for tightening fasteners  12  into mating holes  16  of first and second components  18 ,  20  and a controller  22  for controlling tightening tool  14 . It should be understood that although the exemplary embodiment illustrated in  FIG. 1  discloses two components to be assembled, assembly station  10  can be utilized to simultaneously assemble any number of components. 
         [0015]    While positioned for assembly, first and second components  18 ,  20  may receive fasteners  12  through mating holes  16 . Mating holes  16  may be sized to have approximately the same diameter as a rod portion  24  of fasteners  12 . Although mating holes  16  are disclosed to extend through the entire depth of first and second components  18 ,  20 , mating holes  16  may extend partially rather than completely through first component  18 . In addition, the threading geometry of mating holes  16  may be required to match the threading geometry of rod portions  24 . 
         [0016]    Tightening tool  14  may be an automated torque tool capable of tightening mechanical fasteners. As is shown in  FIG. 1 , tightening tool  14  may include an actuator  26  in communication with a power source  28 , a head portion  30  for engaging fastener  12 , and an angle sensor  32  to determine the angle through which fastener  12  has been rotated. 
         [0017]    Actuator  26  may operationally communicate with power source  28  via power line  34  and may be configured to convert at least a portion of the power output to mechanical energy for applying torque to fastener  12 . It should be understood that power source  28  may be an air compressor, battery assembly, or any other power source capable of driving actuator  26 . Depending on the type of power supplied by power source  28 , actuator  26  may be an AC induction motor, a brushless DC motor, a linear motor, or any other type of motor capable of driving tightening tool  14 . Additionally, power line  34  may be tubing for conducting compressed air, electrical wire for conducting electrical energy, or any other conveyance apparatus that may communicate power generated by power source  28  to actuator  26 . Furthermore, it is contemplated that power source  28  may communicate with controller  22  via communication line  36 . 
         [0018]    Head portion  30  may engage fastener  12  and be shaped and sized to torsionally grip a receiving portion  38  of fastener  12 . In addition, head portion  30  may communicate with controller  22  via communication line  40 . Furthermore, head portion  30  may interface with a strain sensor  42  located on receiving portion  38  via an interface device  44 . The sensed data from strain sensor  42  may be relayed to controller  22  through communication line  40 . 
         [0019]    Strain sensor  42  may emit a pulse of energy such as, for example, ultrasonic energy along an axial length of rod portion  24  and receive in return, an echo of the pulse. Strain sensor  42  may be an ultrasonic transducer or any other device known in the art capable of emitting such a pulse of energy along rod portion  24  and receiving the reflection of the pulse of energy. It should be understood that an elongation of rod portion  24  measured by strain sensor  42  may be directly related to the strain of fastener  12 . 
         [0020]    Interface device  44  may be located in head portion  30  of tightening tool  14  to contact strain sensor  42  when head portion  30  engages receiving portion  38 . Interface device  44  may receive electrical signals from controller  22  and transmit them to strain sensor  42  through an electrical contact (not shown). Furthermore, interface device  44  may receive electrical signals from strain sensor  42  and transmit them to control device  22  via communication line  40 . 
         [0021]    When tightening tool  14  engages fastener  12 , angle sensor  32  may be actuated to sense a rotational angle of head portion  30  that is equivalent to the rotational angle of fastener  12 . The rotational angle of head portion  30  may be indicative of a torque acting on fastener  12 . It should be understood that angle sensor  32  may be any type of sensor capable of sensing the rotational angle of fastener  12 . For example, angle sensor  32  may embody a magnetic pickup sensor configured to sense a rotational angle of head portion  30  and to produce a signal indicative of the angle. Angle sensor  32  may be disposed proximal a magnetic element (not shown) embedded within a rotational element (not referenced) of head portion  30 , or in any other suitable manner to produce a signal corresponding to the rotational angle of head portion  30 . The rotational angle may be sent to controller  22  by way of communication line  40  as is known in the art. 
         [0022]    Controller  22  may take many forms, including, for example, a computer based system, a microprocessor based system, a microcontroller, or any other suitable control type circuit or system. Controller  22  may also include memory for storage of a control program for operation and control of tightening tool  14 , power source  28 , and/or other components of assembly station  10 . It is contemplated that controller  22  may reference tables, graphs, and/or equations included in its memory and use the sensed information and/or values received from angle sensor  32  and strain sensor  42  to regulate the operation of tightening tool  14  and power source  28 . For example, controller  22  may command tightening tool  14  to disengage from fastener  12  upon a determination that a target axial load has been achieved. The determination may be made by comparing the signals received from strain sensor  42  and angle sensor  32  to tables, graphs, and/or equations included in its memory. Additionally, controller  22  may command tightening tool  14  to disengage from fastener  12  upon a determination that the relationship between elongation and rotational angle sensed by strain sensor  42  and angle sensor  32  is no longer linear signifying that the yield point of fastener  12  has been achieved. 
         [0023]      FIG. 2  and  FIG. 3  illustrate an exemplary method used by controller  22  to tighten fastener  12  and a reference chart, respectively.  FIG. 2  discloses the exemplary method by illustrating the steps utilized by controller  22  and the operator to tighten fastener  12  to its yield point or a target axial load. In addition,  FIG. 3  discloses a graphical representation of a an exemplary relationship between the elongation and rotational angle of fastener  12  referenced by controller  22  when operating tightening tool  14 . 
       INDUSTRIAL APPLICABILITY 
       [0024]    The disclosed assembly system may be able to provide a secure, strong joint bound by mechanical fasteners. In particular, assembly system  10  may be able to determine the yield point and axial load of each fastener  12 , and tighten fasteners  12  up to the determined yield point and/or desired axial load. By tightening fasteners  12  to the determined yield point and/or desired axial load, the joint may be secure and robust, and fasteners  12  may be efficiently used to reduce manufacturing costs. The operation of the assembly system  10  will now be explained. 
         [0025]      FIG. 2  illustrates a flow diagram depicting an exemplary method of operation for assembly system  10 . The method may begin when first and second components  18 ,  20  and fasteners  12  are positioned for assembly (step  100 ). Once first and second components  18 ,  20  and fasteners  12  are positioned for assembly, tightening tool  14  may be brought into contact with fastener  12 , at which time, tightening tool  14  may be activated (step  102 ). The activation of tightening tool  14  may be performed automatically in response to the engagement or, alternately, by manually engaging a switch (not shown). 
         [0026]    Once activated, tightening tool  14  may begin applying an increasing torque to receiving portion  38 , thereby causing fastener  12  to rotate (step  104 ). While fastener  12  is being tightened, controller  22  may send a command signal to strain sensor  42  via interface device  44  to begin emitting an ultrasonic pulse along rod portion  24  of fastener  12 . Upon receiving an echo of the ultrasonic pulse, strain sensor  42  may send an electronic signal indicative of the travel time of the pulse and its echo to controller  22  via interface device  44 . At the same time, controller  22  may receive a sensing signal from angle sensor  32  indicative of the rotational angle of fastener  12 . Controller  22  may use the signals from angle sensor  32  and strain sensor  42  to determine the rotational angle and elongation of fastener  12  (step  106 ), respectively. 
         [0027]    Controller  22  may use the rotational angle and elongation to determine if the relationship between the two parameters is linear (step  108 ).  FIG. 3  illustrates an exemplary relationship between angle of rotation and elongation. In the elastic deformation range (I) of fastener  12 , the axial load may be predictable because the relationship described above is substantially linear. The relationship between the two parameters may, however, become nonlinear in the plastic deformation portion of the graph (II). Because of this non-linearity, the axial load in the deformation range may become unpredictable. The transition point between the linear and non-linear relationship is commonly known as the yield point. Because elongation and rotational angle may both be directly related to an applied torsional force, controller  22  may use the determinations of rotational angle and elongation to predict the axial load in the elastic deformation range of fastener  12 . Therefore, if controller  22  determines that the relationship between rotational angle and elongation has become non-linear ( 108 : No), then fastener  12  has been tightened past the yield point, and controller  22  may send a signal to terminate tightening of fastener  12  (step  110 ). It is contemplated that fastener  12  may be removed if the yield point has been reached before fastener  12  has been tightened to the desired axial load, if desired. In this situation, a new fastener  12  may replace the defective fastener  12  and the entire process may be repeated. 
         [0028]    If controller  22  determines that the relationship between rotational angle and elongation is linear ( 108 : Yes), then controller  22  may determine whether a target axial load has been reached by comparing the determined rotational angle and elongation to graphs, charts, or tables representing elastic deformation axial load values for fastener  12  (step  112 ). If the axial load is less than the target axial load (step  112 : No) then tightening tool  14  may continue applying an increasing torque to fastener  12 . However, if the axial load of fastener  12  is essentially equivalent to the target axial load (step  112 : Yes), then controller  22  may send a signal to power source  28  and tightening tool  14  to terminate the tightening of fastener  12  (step  114 ). Controller  22  may then repeat step  102  through step  112  until all required fasteners  12  are tightened, as desired. 
         [0029]    The ultrasonic tightening method disclosed above can improve axial load tightening accuracy. In particular, ultrasonic tightening can tighten a fastener to within about three percent of a target axial load, while insuring the yield point has not been exceeded. This increased accuracy can improve the distribution of load throughout the resulting joint, thereby increasing its strength and durability. 
         [0030]    Ultrasonic tightening can also be used to determine the yield point of each fastener being used to secure a joint. The largest axial load that can be accurately determined for a given fastener may occur at the yield point of the fastener. By determining each fastener&#39;s yield point, each device can be used to its maximum capacity promoting efficient use of materials and reducing manufacturing costs. In addition, because the process of identifying the yield point of a fastener can be performed on each fastener as it is being tightened, instead of in an extemporaneous step outside of the tightening process, the system&#39;s accuracy and efficiency can be improved. 
         [0031]    It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed system without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.