The present invention relates to an axial bolt force measuring instrument and method for measuring an axial force (tightening force) of a bolt, and more particularly, to an axial bolt force measuring instrument for measuring an axial force (tightening force) of a bolt for a hub that couples a wheel and an axle of a vehicle i.e. a hub bolt that fastens the wheel to the hub.
BACKGROUND ART
Generally, a wheel of a vehicle is coupled to an axle through a hub. In this case, the wheel and hub are fastened by a bolt (hereinafter referred to as a hub bolt) and nut. When the nut is fixed and the hub bolt is rotated in the fastening direction, the wheel and hub are compressed, while a tension (axial force or tightening force) is applied to the hub bolt, and the hub bolt extends. In the assembling operation or the like, it is important to adequately control the tightening fore (axial force) of the hub bolt to maintain at a proper value. This is because of a risk that the hub and/or wheel is deformed or broken, when the fastening is not adequate and/or tightening is excessively applied.
Various methods have conventionally been known, as a method for controlling the tightening force (axial force) of a hub bolt. For example, methods have been well known that a tightening torque is set at a predetermined value using a torque wrench, or a skilled mechanic or the like taps each portion of a bolt fastening member using a micro hammer, and based on the tap sound, inspects a tightening state of the hub bolt. Further, an ultrasonic applying measurement tester has also been known which applies ultrasonic pulses to a tightened hub bolt from an end surface in the axial direction, obtains a length (elongation) of the hub bolt from the time required for the reflective wave to return, and using the bolt length, calculates the axial force of the hub bolt.
However, in the method of using the torque wrench, the use is predicated on the muscular strength of a person, and therefore, limited to small-size vehicles or the like. In this torque wrench method, since the tightening torque is set at a predetermined value and the bolt is merely tightened to the set value, it is not possible to examine and/or measure the axial force itself of the hub bolt after the bolt is tightened. Further, in the torque wrench method, an unstable factor of torque coefficients is considered as a defect.
Meanwhile, in the inspection method using the tap sound with a micro hammer, since decision is made by listening based on the skill and intuition without the rating, it is not possible to examine and/or measure a fastening state of the hub bolt with accuracy.
Further, in the method using the ultrasonic applying measurement tester, since the axial force of the hub bolt is obtained by calculating an elongation of the hub bolt targeted for the test using correlation, it is required to perform measurement at least twice, prior to and subsequent to bolt fastening, to determine the elongation of the hub bolt. Accordingly, there are limitations in time required for measurement and reduction in cost. Furthermore, the conventional ultrasonic applying measurement tester has a separate sensor portion and amplifier portion and thus a plurality of components, thereby is not suitable for operation requiring portability, nor does not have a power source, and therefore, can be used only in places with a commercial power source.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide an axial bolt force measuring instrument with excellent portability and general versatility enabling the axial force (fastening state) of a tightened bolt to be measured promptly and accurately.
Therefore, the present invention provides an axial bolt force measuring instrument which measures an axial force of a bolt for a hub that couples a wheel and an axle of a vehicle, and which is integrally provided with a setting portion that sets a predetermined axial force of the bolt, an initiation operation portion that is operated to start measurement, a tapping portion which is brought into contact with the bolt, and is operated by initiation operation from the initiation operation portion to tap the bolt continuously a plurality of times in a predetermined stroke, a sound collecting portion which is brought into contact with the bolt and collects tap sound generated by the tapping portion tapping the bolt, a frequency measuring portion that measures a frequency of the tap sound collected by the sound collecting portion, an axial force converting portion that converts the frequency of the tap sound measured by the frequency measuring portion into an axial force of the bolt, a comparing portion that compares a set axial force set in the setting portion with a converted axial force converted in the axial force converting portion from the frequency of the tap sound, and a display portion that displays a result of comparison in the comparing portion.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a conceptual view of a primary portion enlarged section of an axial bolt force measuring instrument according to the present invention;
FIG. 2 is a schematic block diagram of the axial bolt force measuring instrument according to the present invention;
FIG. 3 is a side view of an axial bolt force measuring instrument according to one embodiment of the present invention;
FIG. 4 is a graph showing the correlation between the axial bolt force and the frequency of tap sound;
FIG. 5 is a schematic internal configuration view of the axial bolt force measuring instrument according to one embodiment of the present invention; and
FIG. 6 is a flowchart showing measuring procedures of the axial bolt force measuring instrument according to one embodiment of the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
A preferred embodiment of the present invention will specifically be described below with reference to accompanying drawings.
FIGS. 1 and 2 conceptually show an axial bolt force measuring instrument according to the present invention. As can be seen from the figures, the axial bolt force measuring instrument according to the present invention taps a hub bolt, and measures an axial force of the bolt from the frequency of the tap sound. First, the principle will be described below.
Generally, when a tension is generated on an object and the object is hit, unique sound is generated. The frequency of the hit sound varies with level of the tension of the hit object (representative examples of application of this principle are musical instruments, and tuning of a piano, guitar or the like is performed by increasing/decreasing the tension of chords). For example, when a rigid body under a tension is hit by an object with a higher degree of hardness than that of the rigid body, the frequency of the hit sound caused by hitting has the correlation with the tension of the rigid body, and increases as the tension is higher, while decreasing as the tension is lower. In practical phenomenon, for example, the sound with a high frequency occurs generally in a normal fastening portion (when normal tension occurs on a rigid body) in a metal component subjected to heat processing, while the dull sound with a low frequency occurs in portions with corrosion, breakage, distortion, crack, etc.
It is a basic concept of the present invention to apply the aforementioned principle to measurement of an axial bolt force. In other words, fastening a bolt and nut is the operation for generating a tension on the bolt, and whether or not the bolt is tightened by a predetermined tightening torque (axial force) can be determined by measuring whether or not a predetermined tension is generated on the bolt. Further, the tension is measured from the frequency of the hit sound by hitting the bolt.
Based on such a principal, as shown in FIGS. 1 and 2, an axial bolt force measuring instrument 1 according to the present invention is provided with an initiation operation portion 2 to be operated to start measurement, a main circuit 4 that receives an initiation signal from the initiation operation portion 2, a tapping device 6 which is brought into contact with a bolt 14 subject to measurement and based on a control signal from the main circuit 4 due to the initiation signal, operates and hits the bolt 14, a sound collecting microphone 8 that collects hit sound (tap sound) from the bolt 14 by the tapping device 6, a setting device 12 to set and input a proper axial force (predetermined value) of the bolt subject to measurement, for example, at a predetermined range, and a display device 10 that displays a result of measurement.
In this configuration, the frequency of the hit sound collected by the sound collecting microphone 8 is measured, and the measured value is compared with a reference value in the main circuit 4. More specifically, the main circuit 4 has a storage portion that stores correlation data (that is beforehand determined by experiments or the like) between the frequency and the axial bolt force (tension), obtains an axial bolt force corresponding to the frequency measured value from the correlation data stored in the storage portion, and determines whether or not the axial bolt force agrees with a proper value (reference value) set and input by the setting device 12 (whether or not the axial bolt force converted from the frequency is in a range of proper values set and input by the setting device 12, when a predetermined range is set and input as the proper axial force) by comparison. Then, the result of determination is displayed on the display device 10. In addition, in FIG. 1, "16" denotes a nut combined with the bolt 14, and "18" denotes a member fastened by the bolt 14 and nut 16.
本文转自：China Industry News