Method and apparatus for fastener tensioning

A fastener tensioning method and apparatus for obtaining a desired degree of tension in a fastener. The apparatus includes a means for applying rotation to a fastener, a means for measuring the torque applied by the means for rotating the fastener, a means for determining a rate of change of torque applied to the fastener with respect to the rotation applied to the fastener, a means for detecting when the rate of change of torque with respect to rotation becomes a constant, a means for accomplishing a desired amount of rotation of the fastener beyond the rotation required to achieve the constant rate of change of torque with respect to rotation, and a means for shutting off the means for applying rotation to the fastener in response to the total desired rotation being achieved. The embodiment herein described improves the well-known "turn of the nut" method for fastener tensioning by providing that method with a well determined starting point.

BACKGROUND OF THE INVENTION 
The trend toward optimizing equipment design to achieve the maximum 
capability of equipment with relationshp to weight, size, and economy of 
material usage have spurred considerable activities in the area of 
fastener tension and inspection methods. A considerable amount of the 
early development work centered on torque control as a means of fastener 
tension. However, the accuracy of this method is severely limited by its 
sensitivity to such factors as thread condition and other factors 
affecting the coefficient of friction. 
To minimize the effect of friction, a later development, often referred to 
as the so-called "turn of the nut" method, was evolved. The method 
prescribed a combination of torque (to assure the fastener was seated) and 
rotation (using the thread of the bolt as a micrometer to stretch the 
bolt). This method achieves considerable accuracy in tensioning the bolt 
under carefully controlled fastener and joint system conditions. However, 
the torque controlled starting point often leads to difficulties by false 
starts (the fastener or the joint system not properly seated or because of 
thread condition causing high prevailing torque). 
An even more recent development is the method of bringing the bolt to its 
recognizable yield point (a well-defined point of tension) and utilizing 
that point to ultimately arrive at the desired bolt tension either by 
memory of the tightening cycle or an "unturn of the nut" method. While 
these later methods result in reasonably accurate bolt tension, the 
methods have some draw backs in universal application. In many 
applications, it is not desirable to bring the fastener to its yield 
point. The joint may not be capable of sustaining the full tension of a 
yielded fastener without damage such as flange warpage, gasket crushing, 
or thread failure. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to provide a novel method and 
simple apparatus for tensioning a fastener which utilizes a definable 
point in the bolt tensioning sequence below the yield point. In the 
embodiment described herein, the definable point is utilized as a starting 
point for rotation to obtain the accurate tensioning of the fastener 
utilizing its threads as a micrometer to stretch the fastener a 
proportionately determined amount. 
The embodiment is intended as an improvement of the socalled "turn of the 
nut" method wherein the starting point is more accurately determined by 
utilizing the joint characteristics. It is the further purpose of this 
invention to eliminate the variables of the joint and fastener torquing 
sequence occurring prior to the linear portion of the torque rotation 
slope and the unique starting point of the present invention. It is yet 
another purpose of this invention to eliminate the need for driving a 
fastener to its yield point to establish a well-defined point in fastener 
tension from which fastener tension levels may be predicted and achieved. 
It is a further object of this invention to minimize the torque power 
required to achieve a desired level of fastener tension by avoiding the 
overtightening of the fastener prior to achieving the desired level of 
fastener tension. These and other objects are accomplished by an apparatus 
comprising: Means for rotating and applying torque to a threaded fastener; 
means for measuring the rotation of the fastener; means for measuring the 
torque applied to the fastener; means for detecting the rate of change of 
the torque applied to the fastener with respect to the rotation applied to 
the fastener; means for detecting when the rate of change of torque 
applied to the fastener with respect to the rotation becomes a constant; 
means for accomplishing a predetermined amount of rotation of the fastener 
beyond the rotation required to achieve the constant rate; and means for 
shutting off the means for applying rotation to the fastener in response 
to the predetermined amount of rotation being achieved.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Reference is made to FIG. 1 which shows a series of typical joint torquing 
sequences. Curve 1 is typical of a well-prepared hard joint, in this case 
the initial torque build up is relatively rapid and constant once 
established. Of course, the curve could be displaced significantly to the 
left depending on the length of the fastener and the rotation required to 
engage the head of the fastener. Curve 2 is typical of a fastener wherein 
the joint is softer than curve 1 and the threads or the joint itself 
exhibits erratic torquing during initial tightening. This is created in 
typical cases by poor or dirty threads, high spots in the bolt fact or 
local yielding of the joint system. 
For example, the change in rate may result from a soft sealing gasket which 
bottoms, initial yielding taking place in the joint or thread yielding 
creating a false linear gradient. One thing, however, appears common to 
each of these torquing sequences; at some point each enters a relatively 
linear portion during which the stress in the bolt is considered 
proportional to the strain in the bolt, and the strain is proportional to 
the rotation of the thread. For each of the curves, several points have 
been identified for purposes of further discussion. Point A is the point 
at which the rotation has progressed until the joint is just snug; that 
is, all of the erratic portions or clearances in the joint have been 
eliminated and further rotation of the fastener will result in appreciable 
increase in the torque and tension level experienced in the fastener. 
Point B is the point at which the fastener is entering at its proportional 
range in tension. Point C is an arbitrary intermediate check point, or 
points, for the purpose of this invention. Point D is the point at which 
torque or rotation on the bolt yields the desired bolt tension. Point E is 
the end of the proportional range sometimes referred to as the yield 
point. Point F is a point at which the bolt is experiencing nonelastic 
deformation. 
It will be noted that in each case of the typical joint tightening 
sequence, the curve presented for torque versus rotation exhibits a 
relatively constant slope for at least a portion of the tightening cycle; 
that is (.DELTA..tau./.DELTA..phi.) (increment of torque per increment of 
rotation) beomces a constant K. If a relatively constant speed driver is 
utilized, time may be substituted for the parameter of rotation. Other 
tension associated parameters other than torque may also be utilized. 
However, the preferred embodiment herein described will utilize torque as 
the tension-related parameter because of its relatively common and 
convenient use for fastener tensioning. 
In the past, there have been several attempts as previously described to 
improve the relationship between applied torque and resulting tension. In 
U.S. Pat. No. 3,962,910, Spyridakis, et al, several inspection methods are 
described which improve the reliability of torque as a tension-related 
parameter. In the method of that patent, if certain predetermined levels 
of torque occur within predetermined ranges of rotation for a given 
fastener, after an arbitrarily specified seating torque, then the joint 
tightening system can be assumed to be operating satisfactorily and a 
reasonable tension level achieved in the fastener. The system, however, 
requires predetermination of the acceptable range of torque and/or the 
range of rotation and further assumes a reasonable tension level is 
achieved within these ranges. The method, however, cannot be utilized to 
predict a desired tension level relative to the varying friction and joint 
conditions encountered in typical fastener applications. 
U.S. Pat. No. 3,643,501, Pauley, introduced a method of determining the 
yield point of a fastener as it is rotated. This provided a useful gage of 
fastener tension, in that the yield point of the fastener results from a 
well-defined level of tension in the fastener. This parameter has been 
utilized in several fastener tension systems as both the final point of 
tensioning and the starting point for achieving other levels of fastener 
tension. As previously mentioned, however, this system has the 
disadvantage of requiring that the fastener and its joint first be 
stressed to the yield point of the fastener, which in some cases, is not 
desirable. 
This invention provides an alternative means of determining fastener 
tension levels and may be utilized to achieve any level of fastener 
tension desired with improved accuracy over previous "turn of the nut" 
methods. In this invention, I propose the use of the initial entry to the 
linear portion of the fastener torque (tension-related parameter) and 
rotation curve. Apparatus capable of determining the change of slope of 
the torque-rotation curve, and apparatus for measuring torque and rotation 
are now well-known in the art. 
Referring now to FIG. 2 which shows a block diagram for the circuit logic 
for the embodiment of this invention. The system is comprised of a power 
wrench or nut runner generally identified by reference numeral 1. The 
wrench is provided with a shut-off valve 2. The wrench has its power 
output on a spindle 3 which rotates a socket 4 for driving a typical 
threaded fastener. The output of the power wrench is monitored by an angle 
encoder 5 which converts the rotation of spindle 3 into usable pulse 
signals. In the preferred embodiment, one pulse is produced for each 
degree of rotation. The torque level applied to spindle 3 is monitored by 
torque transducer 6 which creates an analog signal proportional to the 
torque output. 
The angle encoder pulse signals are fed to a sample size counter 7 which 
counts angle encoder pulse signals and produces an output pulse signal for 
every predetermined or set total of input pulses. Typically, one pulse may 
be produced for every 8 input pulses as determined by the joint system to 
be tensioned. 
The output of sample size counter 7 is utilized to produce two repeated 
trigger pulses. This is accomplished in sample trigger circuit 8 which 
produces a signal pulse for approximately 1/2 of the 8 pulse interval. The 
leading edge of the signal pulse is used to produce a short duration "A" 
trigger signal while the collapse or trailing edge of the signal pulse is 
utilized to produce a short duration "B" signal through well-known 
technology. The "A" and "B" signals are alternately and evenly spaced and 
are utilized as timing enable signals in both the slope detection and the 
rate of change of slope logic to be described later. 
The output of torque transducer 6 is utilized to determine the slope of the 
torque rotation curve applied to the fastener as follows: The torque level 
analog signal is first amplified in analog amplifier 9. The "A" trigger 
signal is utilized to enable sample and hold circuit 10 to receive and 
store the output of analog amplifier 9. The sample and hold circuit 10 
will constantly supply a signal proportional to the input signal received 
until it is updated by the next received enable "A" signal. As shown in 
FIG. 2, the output of sample and hold circuit 10 is fed to both 
differential amplifier 11 and sample and hold circuit 12. Sample and hold 
circuit 12 will accept the signal only on an enable command from trigger 
pulse "B". Sample and hold circuit 12 has its output fed to sample and 
hold circuit 13 which accepts the signal only on an enable command from 
trigger pulse "A". The output of sample and hold circuit 13 is fed to a 
differential amplifier 11. 
As can be seen by one skilled in the art, the output of the sample and hold 
circuit 13 is the torque level output at the previous "A" trigger pulse 
while the output of sample and hold circuit 10 is for the present "A" 
trigger pulse. Since the signal output is proportional to the torque rise 
for an "A" pulse interval and the "A" pulse interval is proportional to 
rotation, it can be appreciated that the differential signal applied to 
differential amplifier 11 is the torque differential per interval of 
rotation or proportional to the slope of the torque rotation curve for the 
fastener. 
A similar technique is utilized to determine the rate of change of the 
slope of the torque rotation curve. In this case, the output of the 
differential amplifier 11 (slope) is fed to sample and hold circuit 14 
which accepts the output of differential amplifier 11 on a "B" trigger 
pulse. This is done in order to prevent the signal from being received 
during the updating of the signals to differential amplifier 11 during the 
"A" trigger pulse. The output of sample and hold circuit 14 is fed to 
differential amplifier 15 and also to sample and hold circuit 16 which 
accepts the signal on an "A" trigger pulse. The output of sample and hold 
circuit 16 is fed to sample and hold circuit 17 which accepts the signal 
on a "B" trigger pulse. The output of sample and hold circuit 17 is fed to 
differential amplifier 15. 
In the same manner as described before, it should now be obvious to one 
skilled in the art that the slope represented by the output of sample and 
hold circuit 17 is the slope for one preceeding "A" pulse interval. The 
output of differential amplifier 15, therefore, represents the change in 
slope for the interval or the rate of change of slope. The output of 
amplifier 15 is sent to rate of change comparator 18. The signal received 
from amplifier 15 is an analog level signal which increases or decreases 
in relation to the rate of change of slope of the torque rotation curve. 
In the proportional portion of the normal fastener torque rotation curve, 
this value of this signal will approach zero. For practical reasons, a 
rate of change analog reference signal circuit 19 is provided and anytime 
the rate of change of the slope is below the set point value of the 
reference signal, a signal is sent to "and" logic circuit 20. 
The slope output signal of differential amplifier 11 is also fed to slope 
level comparator 21 where it is compared against a preset slope reference 
produced by slope reference generator 22. Whenever the slope signal of 
differential amplifier 11 is greater than the slope reference signal of 
slope reference generator 22, an analog signal will be produced which is 
fed to "and" logic 20. Thus, it can be seen that when the rate of change 
of the slope (output of comparator 18) is below the rate of change 
reference 19 and the slope is greater than the slope level reference 
(output of comparator 4), the "and" logic circuit 20 will produce a signal 
which is fed to counter 23 as an enable function. At this point, counter 
23 will begin to receive and count the angular encoder 5 pulse output 
which is proportional to rotation. When a set point count is exceeded, a 
shut-down signal is sent to the shut-off valve 2. In this manner, a 
predetermined rotation is accomplished after the slope of the torque 
rotation curve is constant and has a preselected minimum value. 
Having described in detail the circuit logic for the preferred embodiment, 
one skilled in the art can appreciate that the nut runner will run the 
fastener down. During this period, there will be an erratic rise in torque 
until the fastener is seated and the joint snugged up. At this point, the 
fastener in the typical case will begin to be elastically deformed at a 
uniform rate for a given uniform increase in applied load. This results in 
the typical (.DELTA..tau./.DELTA..phi.) constant exhibited for the torque 
rotation curve (Point B to Point E of FIG. 1). Utilizing the point (Point 
B of FIG. 1) at which this slope constant K occurs as the starting point 
for rotating the fastener a further predetermined amount of rotation in a 
method similar to the so-called "turn of the nut" method will provide an 
accurate fastener tensioning method having the improvement of a defined 
starting point as opposed to an arbitrarily preselected torque as utilized 
by the "turn of the nut" method. 
As a further inspection method, the slope constant K may be compared 
against a predetermined constant, for example at point C, to assure that 
the fastener system is within a prescribed range of variables including 
thread condition, thread friction, and gasket hardness. 
Utilizing this invention, it is possible then to obtain a desired level of 
tension in the fastener without the necessity of bringing the fastener to 
its yield point. With or without appropriate system checks, the fastener 
may be tightened to any desired level of tension. Utilizing the apparatus 
of this invention, it is necessary for the user to determine the number of 
samples of constant slope required to establish the presence of a constant 
slope and either by theoretical calculation for a given fastener system or 
by experimental result to determine the desired predetermined rotation. 
With normal manufacturing tolerances the resulting tension levels in the 
fastener will be much improved over the tension levels achieved with the 
prior "turn of the nut" method, and the fastener need not be brought to 
its yield point to determine a level of tension. In addition, the system 
apparatus is greatly simplified over that required for yield point 
detection, especially where a tension level other than that yield is 
required. In addition, the system will reduce torquing power required and 
fastener tensioning time, in that the steps of first bringing the fastener 
to its yield point are avoided. 
I have described a unique fastener tensioning system and described in 
detail an embodiment thereof for purposes of assisting one skilled in the 
art in understanding the nature of the invention and its use. It will be 
obvious to one skilled in the art that numerous modifications to the 
circuit will accomplish similar results. I do not wish to be limited in 
the scope of my invention by the decribed embodiment. The invention is to 
be limited only by the scope of the claims.