Method for yield tightening of screws

A method for yield tightening of screws by use of a wrench including a device for detecting a tightening torque in the actual tightening process, a device for detecting a tightening angle and an electric motor for applying a torque to each screw, a device for driving the wrench, and a controller including a device for communication with an external device. After the actual tightening torque reaches a certain value, an average torque rate is obtained by an integration using four or more pieces of torque data and is compared with a preset target torque rate, judging a yield point. The integration is performed for each minimum angle for which new torque data can be obtained. The tightening process is stopped when the judgement of the yield point has been given in succession a larger number of times than 1/2 of the number of torque data used for the integration. Further, a value obtained by subtracting a certain value from the actual tightening angle is integrated and the area of a right triangle inscribed in the area of the integrated value is obtained. The tightening process is stopped also when a difference between the integrated value and the area of the right triangle is greater than an area calculated using a preset target angle.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for yield tightening of screws to 
tighten a screw up to the maximum of elastic stress of the screw. 
In the field of screw tightening, attention is now being focused on what is 
called a yield tightening method which tightens a screw up to the maximum 
of elastic stress of the screw itself, and there is also a growing 
tendency toward wide application of the method in the actual production 
line. 
A physical phenomenon commonly called yield refers to a phenomenon that as 
external tensile force is applied to, for example, a rodlike object of 
metal, the external force and the elongation of the object are 
proportional to each other in the elastic area but in the plastic area 
only the elongation of the object increases although the external force 
does not substantially increase. In graphical terms, letting the external 
tensile force be represented on the ordinate and the elongation of the 
rod-like object on the abscissa, the external tensile force shows a linear 
locus at a fixed angle of inclination to the abscissa in the elastic area 
but in the plastic area it shows a locus almost parallel to the abscissa 
at a very small angle thereto. The same phenomenon is observed in screw 
tightening as well. Letting the angle of rotation of the screw be 
represented on the abscissa and the tightening torque on the ordinate, the 
torque locus is very close to the locus of the external tensile force 
mentioned above. This phenomenon has long been known in the art and a 
variety of methods have been proposed for its application to screw 
tightening in the actual production process. 
Since the yield tightening method permits tightening screws with tension 
maximal to their elastic stress, as referred to above, the method is 
advantageous over a conventional method which tightens screws within a 
sufficiently safe range in the elastic area, such as a so-called torque 
tightening method, in that screws of a smaller cross-sectional area could 
be used if they are tightened with the same tension as in the above method 
and that the number of screws used could be reduced if their 
cross-sectional area is the same as in the above method. Since almost all 
industrial products have blocks or parts assembled together through screw 
tightening, it would bring about a considerable advantage in practice if 
the screw size or the number of screws used could be decreased by use of 
the yield tightening method. 
However, many difficulties are encountered in actual applications of this 
yield tightening method. Theoretically, a point of refraction on the 
torque locus is surely a yield point, but it is very difficult to 
correctly find it out on an actual torque curve. 
SUMMARY OF THE INVENTION 
The present invention is intended to fulfil the above most important 
requirement for yield tightening and hance ensure the detection of the 
actual yield point including unknown factors, on the basis of a novel 
concept basically different from the prior art and through use of novel 
logical expressions and methods of analysis. 
An object of the present invention is to provide a method of analysis for 
detecting the actual yield point which is the prime essential to the yield 
tightening. 
To attain the above object of the present invention, a method is proposed 
for yield tightening of screws by use of a wrench including a device for 
detecting a tightening torque in the actual tightening process, a device 
for detecting a tightening angle and an electric motor for applying a 
torque to each screw, a device for driving the wrench, and a controller 
including a device for communication with an external device. After the 
actual tightening torque reaches a certain value, an average torque rate 
is obtained by an integration using four or more pieces of torque data and 
is compared with a preset target torque rate, judging a yield point. The 
integration is performed for each minimum angle for which new torque data 
can be obtained. The tightening process is stopped when the judgement of 
the yield point has been given in succession a larger number of times than 
1/2 of the number of torque data used for the integration. Further, a 
value obtained by substrating a certain value from the actual tightening 
angle is integrated and the area of a right triangle inscribed in the area 
of the integrated value is obtained. The tightening process is stopped 
also when the difference between the integrated value and the area of the 
right triangle is greater than an area calculated using a preset target 
angle.

DETAILED DESCRIPTION 
To readily understand the present invention, the principle of an actual 
screw tightening will first be described. To look for the yield point 
calls for two dimensions of the tightening torque applied to the screw and 
the tightening angle thereof. As is well-known in the art, a torque 
transducer for detecting the tightening torque applied to the screw and an 
angle encoder for detecting the tightening angle of the screw are mounted 
on a wrench which applies the tightening torque, and these devices convert 
detected values into electric signals for application to electronic 
circuits. 
In the actual screw tightening process, the actual torque curve will be 
produced in such a complicated wavy form as shown in FIG. 1 under the 
influence of surface roughnesses of the contact surfaces of screw threads 
and the contact surface of the screw head with a member to be tightened, 
the influence of a dynamic vibration which is produced when the wrench 
applies the tightening torque to the screw, and the influence of electric 
noise which is generated when the above-mentioned devices output the 
detected values after amplifying them. For implementing the yield 
tightening, it is necessary to find out the point of refraction where the 
actual torque curve leaves the elastic area and enters the plastic area. 
This means the necessity of obtaining the angle of inclination of the 
actual torque curve. 
Generally speaking, two points on the torque locus are joined by a straight 
line and then the angle of inclination of the straight line is obtained. 
In FIG. 1, letting the angle of inclination between points P1 and P2 and 
the increment torque and the increment angle between them be represented 
by .alpha.1, dT1 and dA1, respectively, the angle .alpha.1 is given as 
follows: 
EQU .alpha.1=dT1/dA1 
This angle .alpha.1 is commonly referred to as the torque rate. 
In a case where the actual torque curve has such a complicated wavy form as 
depicted in FIG. 1, however, a serious error will occur between the actual 
average torque and the calculated torque rate ".alpha.1" according to the 
sample position where to obtain the angle of inclination. To reduce this 
error, the increment angle "dA1" on the abscissa must be selected large. 
However, where the increment angle is selected extremely large as 
indicated by "dA2", even if the torque rate at terminating end portion of 
the actual torque curve is so small that this portion is almost parallel 
to the abscissa, the calculated torque rate ".alpha.2" will not become so 
small compared with that ".alpha.", as shown in FIG. 1. Therefore, the 
increment angle cannot be chosen so small nor can it be selected so large. 
The angle of inclination is usually obtained by the use of what is called a 
differentiation method, but this method of calculation is not suitable for 
use in connection with such a complicated curve as depicted in FIG. 1. 
Furthermore, the locus of the actual torque curve in the boundary region 
between the elastic and plastic areas may sometimes be gentle over a wide 
range, as indicated by a curve "A" in FIG. 2, and in some cases it may 
have such a complicated refraction area as indicated by a curve "B". It is 
very difficult to find out actual yield points on such curves, with 
accuracy and with certainty. With a view to over-coming the defect of the 
above-mentioned method of calculation, some yield tightening methods have 
been proposed, according to which high and low torque limits are set to 
the actual tightening torque, high and low angle limits are set to the 
actual tightening angle, an acceptable area called a green area is set 
using these limits, and at the end of the actual tightening process check 
is made to determine whether the two tightening values stay within the 
acceptable area, thereby preventing an erroneous detection of the yield 
point. 
However, the actual tightening torque and the actual tightening angle are 
not directly related to the yield point and the limits to these values are 
totally auxiliary and merely expedient measures. The first basic 
requirement of the yield tightening method is to detect the actual yield 
point as accurately as possible. Accordingly, if little importance is 
attached to this, then no reliable yield tightening could not be achieved, 
whatever indirect methods may be used for remedying the above-mentioned 
drawback. 
In view of the above principle of the actual screw tightening process, the 
present invention will now be described. 
As referred to previously, the actual torque curve inevitably takes a 
complex wavy form under the influences of various factors. The measurement 
of the angle of inclination of the torque locus between two point thereon, 
i.e. the calculation of the torque rate, for judging the inclination of 
the locus will not only suffer an error between the calculated torque rate 
and the actual average one but also lead to a misjudgement. 
In the torque rate calculating technique according to the present invention 
it is a first requirement to obtain the average torque rate from four or 
more pieces of torque data. 
In the screw tightening process the torque curve in the elastic area is 
almost straight and its angle of inclination, i.e. its torque rate is 
dependent on the configurations and physical properties of the screw used 
and the member to be tightened. In the plastic area the torque curve is 
nearly parallel to the abscissa, and accordingly the torque rate is close 
to zero. In the boundary region between the elastic and plastic areas may 
vary gently in some cases and may change so abruptly in some cases that 
points of refraction can clearly be discerned. Moreover, a known method 
proposes a technique which detects the yield point by determining how much 
the torque rate and the actual torque rate have changed in the elastic 
area. According to this method, however, if the rate used as the criterion 
for the yield point is selected small, then there will be a fear of 
stopping the tightening process before the final yield point is reached, 
whereas when the rate is selected large, if the torque rate in the elastic 
area is small, the torque rate which is used as the criterion in the 
plastic area will become negative, introducing the possibility of 
tightening the screw to the end of the plastic area. This method is 
incompatible with the concept of quality control regarded as important in 
recent years. 
The ultimate object of the present invention resides in quality control of 
yield tightening of screws. To attain this object, the present invention 
offers a reliable yield tightening method which is free from unstability 
and uncertainty in the detection of the yield point which has limited wide 
application of yield tightening although it superiority has been 
recognized in the art. To this end, the refractive index of the torque 
curve is not used as the criterion for the yield point but a preset yield 
torque rate is employed as a target value of the criterion. This is based 
on a theory that the torque curve in the plastic area becomes nearly 
horizontal regardless of the torque rate in the elastic area. This is a 
second requirement of the present invention. 
In the calculating process according to the present invention, after the 
actual tightening process has proceeded to the rotation angle where the 
next new piece of torque data can be obtained, the new torque data is 
included in the calculation and the oldest torque data is removed from the 
calculation. By this, the calculating process can proceed while exchanging 
torque data one by one no matter how many pieces of data may be included 
in the calculation. This is a third requirement of the present invention. 
In this instance, if the whole torque data is exchanged for each 
calculation, no judgement can be made during the tightening process until 
a rotation angle is reached where the next whole torque data to be 
calculated can be obtained; namely, a dead zone is provided. Since the 
accuracy of stoppage of the tightening process is maintained by stopping 
the process upon issuance of a stop command, the presence of the dead zone 
itself impairs the accuracy of stopping the process. The third requirement 
of the present invention is aimed at obviating this showtcoming. 
In general, single screw tightening is extremely rare and multiple screw 
tightening takes place in almost all cases. In case of tightening a 
plurality of screws to fasten one member to another, the torque curve of 
the respective screw often contains spike-shaped undulations superimposed 
on its peculiar wavy locus under the influence of tightening of the other 
screws. The occurrence of this abnormal undulation provides the same 
result of calculation as if the yield point has been reached. When this 
phenomenon occurs relatively early in the tightening process, the final 
tightening angle and tightening torque go out of the aforementioned 
acceptable area referred to as a green area, and consequently this 
phenomenon can be dealt with as an error. However, when this phenomenon 
occurs near the actual yield point, it is impossible, with the above-said 
technique alone, to detect the actual yield point. To avoid this, the 
torque rate is calculated by a predetermined number of times and only when 
the AND operation of the results of calculations is YES, it is judged that 
the actual yield point has been reached. The number of calculations 
necessary for this judgement needs to be larger than 1/2 of the number of 
data to be calculated. This is a fourth requirement of the present 
invention. 
In the basic theory the torque curve in the plastic area is nearly 
horizontal or flat, but this is seen only when the coefficient of friction 
inherent to the screw is always constant. In practice, the torque locus in 
the plastic area may remain extremely flat in some cases and tend to rise 
at a certain angle of inclination in some cases. The latter indicates that 
the coefficient of friction increases little by little as the tightening 
angle increases. In this instance, it is feared that the actual torque 
rate will not become smaller than the preset target torque rate, 
introducing the possibility of misjudging that the calculating process has 
not reached the yield point yet; namely, the tightening process cannot be 
stopped. A reliable device is needed for preventing this over-tightening. 
According to a method which is used in the present invention therefor, 
after the actual tightening process has reached one preset torque limit, a 
value obtained by substracting the preset torque value from the actual 
torque value is integrated as the tightening process proceeds, the area of 
a rightangles triangle inscribed in the integrated area is obtained, and 
when the difference area has exceeded a certain value, the tightening 
process is brought to an emergency stop. 
The emergency stop of the tightening process by this method is 
approximately equivalent to stopping the process when the screw is 
tightened a certain angle after the actual yield point was reached. This 
is a fifth requirement of the present invention. 
The above-described requirements are indispenable for establishing quality 
control of the yield tightening procedure. By fulfilling these five 
requirements in parallel with the progress of the tightening process, 
reliable quality control of the yield tightening can be achieved. 
The ultimate object of the present invention resides in highly reliable 
quality control of the yield tightening, with a view to promoting the 
introduction of the yield tightening technique into the wide field of 
screw tightening. 
A detailed description will be given first of the first requirement of the 
present invention. The defect of the conventional calculating method using 
torque data at two points on an unsmooth torque locus, for obtaining 
accurate average torque data, has been described previously in connection 
with FIG. 1. To clarify the superiority of the calculating technique of 
the present invention over the prior art method, an analysis will be made 
of an effect which would be produced by further inclusion of the 
differentiation of torque data between the above-said two points on the 
torque locus. Since this method includes torque data at many points on the 
torque locus as shown in FIG. 3, it seems that an average value very close 
to the true value can be obtained. 
In FIG. 3, letting the average torque rate be represented by .alpha., it 
can be obtained as follows: 
EQU .alpha.=.SIGMA.(dT/dA)/n (1) 
Further, the above can be modified as follows: 
##EQU1## 
In FIG. 1, however, since it can be defined that .SIGMA.dT=T and 
dA.multidot.n, substitution of these values into equation (2) gives 
EQU .alpha.=T/A. 
Thus, the result of calculation in this case is the same as in the case of 
merely calculating torque data at the start point and the end point on the 
torque locus, and the both methods suffer the same calculation error for 
such a torque locus as depicted in FIG. 1. 
Since the first requirement of the present invention is to use a method of 
obtaining the average torque rate by integrating four or more pieces of 
torque data, a value approximately close to the actual average torque 
value can be obtained. An equation for obtaining the average torque value 
is as follows: 
##EQU2## 
The principle of this equation will be described with reference to FIG. 4. 
The ordinate represents the actual tightening torque, the torque value at 
each point thereon being represented by dT, and the abscissa represents 
the tightening angle. The torque value is processed for each minimum 
resolution of the tightening angle. Let the number of torque value data to 
be preset be represented by n. The number n needs to be an even number not 
less than 4. Dividing a region containing n pieces of torque data into 
two, respective integrated values of their areas are such as indicated in 
FIG. 4. In order to obtain the average torque value, the torque locus in 
this region must be regarded as linear. A difference in area between the 
right-and left-hand regions is equal to the sum total of the areas of the 
hatched portions. Quadrupling the total area and dividing it by the number 
n, an increment t of the torque is obtained. Further dividing it by the 
number n, the average torque rate in this region can be obtained. This is 
the logical contents of equation (3). When the direction of inclination of 
the locus is reverse from the direction in FIG. 4, the answer of equation 
(3) is negative. 
Turning now to FIG. 5, it will be described that the method using equation 
(3) is superior to the conventional differentiation method. A value 
obtainable with the differentiation method is as follows: 
EQU .alpha.=t/A=5/7=0.7142857. 
This value has an error of about 30% because the actual torque rate 
.alpha.A is 1. According to equation (3), 
EQU .alpha.=4[(15+14+17+16)-(11+10+13+12)]/8.sup.2 =4(62-46)/64=1 
This is a correct answer. Since equation (3) uses all data in the area of 
calculation, a value close to the true one can be obtained regardless of 
the shape of the actual torque curve. 
Next, the second requirement of the present invention will be described in 
detail. A description will be given first of the relationship between the 
tightening angle and the tightening torque. 
Letting the tightening torque, the tightening angle, the coefficient of 
friction of the screw and the stress of the screw be represented by T, 
.theta., .mu. and .sigma., respectively, the tightening torque T can be 
given by the following approximate expression: 
##EQU3## 
where K1 and K2 are constants. 
If the constant K2 and the coefficient of friction .mu. are constant, then 
the tightening torque T and the stress of the screw .sigma. are in direct 
proportion to each other and the stress of the screw .sigma. is in direct 
proportion to the tightening angle .theta. as well. This means that the 
tightening torque T and the tightening angle .theta. are in direct 
proportion to each other. However, the stress of the screw .sigma. has a 
limit and does not exceed a certain value. This is called to have entered 
the plastic area or yielded area. 
When the stress of the screw .sigma. does not increase after having reached 
its limit, it becomes unrelated to the tightening angle .theta., and 
consequently the tightening torque T will not increase no matter how much 
the tightening angle .theta. increase. However, this is a theoretical 
conclusion, and in practice the torque rate in the plastic area may 
somewhat increase in some cases, as indicated by yA and yB in FIG. 2. 
Judging from the relations between .alpha.A and yA and between .alpha.B 
and yB in FIG. 2, it seems as if the torque rate in the elastic area and 
the torque rate in the plastic area are in proportion to each other. An 
analysis of a large quantity of actually sample data shows 
##EQU4## 
where .alpha. is a standard deviation of the torque rate. 
The value .alpha.(A) is apparently smaller than the value .alpha.(B). This 
indicates that the yielded torque rate in the plastic area is not in 
proportion to the torque rate in the elastic area and has a fixed 
deviation independently of the latter. 
In many known methods, however, the torque rate in the elastic area is used 
as a reference value for judging the yield torque rate. 
This is based on the fact that the yield torque rate is always smaller than 
the torque rate in the elastic area, and is intended to prevent erroneous 
presentting of the target value for judging the yield torque rate. In 
practice, however, the torque locus in the elastic area is not only smooth 
as shown in FIG. 2 but also complicated as shown in FIG. 6. A region "A" 
on the curve in FIG. 6 shows a process in which unparallel contact 
surfaces of members to be fastened together are brought close to each 
other by bending moment, and a region "B" occurs in a case where very 
small protrusions of the contact surfaces or chips of metal still 
remaining thereon after cutting undergo plastic deformation by plastic 
stress applied thereto during the tightening process. It is extremely 
difficult to accurately calculate the torque rate between points T1 and T2 
on the torque curve containing these regions. The torque curve is not rare 
but often seen in the actual production line. Working tolerances are 
always defined for every work-piece and errors in its surface roughness 
and parallelism are never zero; therefore, such a torque curve as shown in 
FIG. 6, though in varying degrees, naturally exists. The concept of 
utilizing the torque rate in the elastic area for judging the yield torque 
rate is based on the assumption that the torque curve in the elastic area 
is straight, but this concept is apparently wrong because a curve from 
which an accurate torque rate cannot be calculated is inevitably involved 
in the judgement of the yield torque rate. 
The portion "B" in FIG. 6 is a false yielding condition, and if the 
judgement of the yield torque rate is started at the point T1, then the 
portion "B" will be judged as the yield point. 
Accordingly, in order to judge the yield torque rate with certainty, it is 
necessary to start the judgement at a point where the torque is as high as 
possible, for example, at the point T2 in FIG. 6, and the target torque 
value must be a fixed preset value in view of the afore-mentioned 
theoretical and statistic conclusions. 
Next, the third requirement of the present invention will be described in 
detail. 
The first requirement of the present invention is to obtain the average 
torque rate by the integration using at least four pieces of torque data, 
as referred to previously. Each torque data are stored for each angle 
corresponding to the minimum resolution of the tightening angle. 
Accordingly, the spacing of the individual pieces of torque data is the 
minimum unit of the tightening angle. Letting the number of pieces of data 
to be integrated be represented by n, the minimum unit of the tightening 
angle is n-1. If the number of pieces of data for each integration is n, 
then the angle range by each calculation is n-1. If each calculation is 
performed using entirely new data different from that used in the previous 
calculation, then the calculation becomes possible after the tightening 
angle proceeds through n-1. That is, the integration is performed only at 
points P1 to P4 at intervals n-1 in FIG. 7. Assuming that the minimum 
resolution of the angle is 1.degree. and the number of pieces of data is 
20, the integration is performed at intervals n- 1=19.degree. alone. 
The third requirement of the present invention is intended to obviate the 
above defect. According to the present invention, the actual average 
torque rate .alpha. is calculated at the time point where the actual 
tightening process has reached the point P2, and if the calculated actual 
average torque rate is not smaller than the preset target torque rate, 
then the actual average torque rate is calculated again for the region 
"S1" immediately after the actual tightening process has proceeded by the 
minimum resolution dA of the angle. Thereafter the actual average torque 
rate is calculated for regions "S2" to "S4" one after another until the 
afore-mentioned condition is fulfilled. 
The actual calculating process will be described with reference to FIG. 8. 
At first, the actual average torque rate is calculated in a region of the 
angle n-1 from the point "P1" to "P2". If the calculated value is not 
smaller than the preset target torque rate, then torque data T1 is omitted 
from a data area of a microprocessor for the calculation, and when the 
tightening process has reached a point "Pn", new torque data "Tn" is added 
to the data area of the microprocessor and the actual average torque rate 
.alpha. is newly calculated. In this way, the calculating process is 
repeated for each resolution unit d.theta. of angle regardless of the 
number n of pieces of the torque data. 
Next, the fourth requirement of the present invention will be described in 
detail. As referred to previously, screw tightening is multiple screw 
tightening in almost all cases. In these cases, the torque and tension of 
each screw are always subject to the influence of the tightening condition 
of other screws disposed adjacent thereto. 
To clearly explaing this influence, an example of tightening two screws is 
shown in FIG. 9. During the tightening of a screw #1 the other screw #2 is 
subjected to an upward or downward external force "F1" or "F2" due to the 
unparallel contact surfaces of a member "A" to be fastened. The external 
force "F1" applied to the screw produces a positive spike-like torque 
locus "S1" and the external force "F2" produces a negative spike-like 
torque locus "S2" as shown in FIG. 10. During the actual tightening 
process proceeding from the point "P1" to the point "P2" while performing 
the calculation for each minimum unit "dA" of the tightening angle, the 
spike-shaped torque locus "S1" stays in the right-hand half portion "a1" 
of a calculation region "A1". Accordingly, the area of the portion "a1" 
containing the spike-shaped torque locus "S1" is large. Since the torque 
rate is obtained by subtracting the area of the left-hand half portion 
from the area of the right-hand half portion, the calculated torque rate 
".alpha." is larger than in a case where the spike-shaped torque locus is 
not contained. On the other hand, in the actual tightening process from 
the point "P2" to the point "P3" the spike-shaped torque locus "S1" lies 
in the left-hand half portion of a calculation region "A2", so that the 
calculated torque rate ".alpha." is apparently small. Also in the actual 
tightening process from the point "P3" to the point "P4", the negative 
spike-shaped torque locus "S2" lies in the left-hand half portion of a 
calculation region "A3", and accordingly the calculated torque rate 
".alpha." is apparently small. In such a case, if the values of the 
spike-shaped torque loci "S1" and "S2" are large, the calculated torque 
rate ".alpha." will become smaller than the preset target torque rate, 
leading to such a misjudgement as if the yield point has been reached. 
Such a misjudgement can be avoided by making a rule that the judgement on 
the yield point is not regarded as valid unless the judgement has been 
passed in succession a plurality of times more than 1/2 the number of data 
to be calculated. This is the object of the fourth requirement of the 
present invention. 
Finally, the fifth requirement of the present invention will be described 
in detail. FIG. 11 shows tightening torque curves of two screws in the 
same application. Even if the screws are identical in shape and used for 
fastening the same member, a difference between their inherent 
coefficients of friction makes a large difference in the shape of the 
torque curve. When individual yield torque rates "yA" and "yB" of the two 
torque curves "A" and "B" in FIG. 11 are greater than a certain value, it 
is misjudged that the yield points have not been reached. To avoid this, 
it is customary in the art to put the actual tightening process to an 
emergency stop at a point "P1" or "P2" if no yield point can be detected 
even after the actual tightening torque has reached a preset torque level 
"T2" and passed through a preset angle limit "PA". In FIG. 11 a region 
"SA" indicates the state in which the screw and the member to be fastened 
have not yet been completely clamped together. Further, in a region where 
the actual tightening torque is relatively small, the torque curve is 
often complex in locus, as shown in FIG. 6. Accordingly, it is necessary 
that the value of the preset torque "T2", at which the tightening angle 
measurement starts, be as large as possible. This means that the start 
point of the preset angle limit "PA" in each of the torque curves "A" and 
"B" has an error of an angle "dA:; namely, the emergency stop point "P1" 
for the torque curve "A" is set well after the yield point, but the 
emergency stop point "P2" for the torque curve "B" is set before the yield 
point. The fifth requirement of the present invention is intended as a 
solution to this problem. Now, the principle of this requirement will be 
described in connection with FIG. 12. The integration of the area of the 
hatched portion starts at the time point when the actual tightening torque 
has exceeded the preset torque level "T2". At the same time, the area of a 
triangle inscribed in the hatched portion is calculated and a difference 
"Ya" between the hatched portion and the triangle is obtained. 
##EQU5## 
In the above, "YA" is an angle starts at the yield point "P1". Assuming 
that the angle "YA" is the preset target angle, the tightening process is 
stopped at a preset target yield angle if it is stopped when the angle 
"YA" has satisfied the condition of the following quation (6). Therefore, 
EQU Ya.gtoreq.YA.multidot.dT/2 (6) 
With this method, the tightening process can surely be stopped at a preset 
angle regardless of the actual tightening torque rate, whatever value it 
may have. 
The five requirements described above are indispensable to perfect yield 
tightening in the actual tightening process, and form the basis of the 
present invention. 
The screw tightening process is a final step in an assembly line, in which 
perfect quality control is required. Therefore, quality control must be 
effected not only for screw tightening but also for all items of the 
clamped member. In other words, quality control of screw tightening alone 
does not mean quality control of the whole screw tightening process. 
As described in detail previously, there is present in the actual 
tightening process such a tightening torque curve as shown in FIG. 6. If 
only the yield tightening is taken into account, the purpose can be 
achieved simply by setting at as high as position as possible the start 
point "T2" for the detection of the yield point. For quality control of 
the whole screw tightening process, however, the torque locus in the range 
between the actual torque curves "T1" and "T2" cannot be ignored. When the 
portion "B" is long, it means that the surface roughnesses of the contact 
surfaces are poor or a foreign substance is held between them. In this 
case, the yield point can be detected by measuring the actual angle "A", 
with the start points "T1" and "T2" used as preset torque limits, and 
determining whether the angle "A" is larger than the preset angle limit. 
When the actual angle "A" is abnormally small, the machining tolerance of 
screw threads or threads of the tap hole is low or the tap hole is filled 
with foreign substances or cross-threaded. With this method, not only the 
actual angle "A" but also the actual torque rate can be employed. 
The method, in which it is determined whether the actual tightening torque 
and the actual tightening angle still remain in the acceptable area or 
green area at the end of the actual tightening process, is not suitable 
for monitoring an erroneous detection of the yield point, as referred to 
previously, but this method is very effective for monitoring the whole 
screw tightening process. 
The acceptable area is, for example, the hatched portion defined by the 
high torque limit "HT", the low torque limit "LT", the high angle limit 
"HA" and the low angle limit "LA" in FIG. 6. When the tightening process 
is satisfactory in all respects, the end point of the actual torque curve 
lies in this acceptable area. 
The actual final torque larger than the limit "HT" indicates that the 
coefficient of friction in tightening exceeds a reference value, and the 
actual final torque lower than the limit "LT" means that the bolt tension 
is below the reference value. When the actual final angle is greater than 
the limit "HA", it means damaged threads or the occurrence of an abnormal 
depression in the contact surfaces, and when the actual angle is smaller 
than the limit "LA", it means a decrease in the stress of the screw due to 
its cracking or the occurrence of excessive galling in the contact 
surfaces. 
Besides, an over time limit is also important as a measure for monitoring 
the whole screw tightening process. In screw tightening there are cases 
where no screws are supplied, where different kinds of screws are 
supplied, where screw threads and threads of the mating tap hole do not 
mesh with each other, and where a wrench and the screw head do not mesh 
with each other. In these cases, the tightening torque will never be 
yielded, and consequently the point "T1" in FIG. 6 at which the tightening 
starts will never be reached. This can be detected by setting the 
above-mentioned over time limit. 
Turning now to FIG. 13, an embodiment of the present invention will be 
described. 
The tightening system comprises, in general, a power wrench 10, a wrench 
controller 20 and a main controller 30. The power wrench 10 and the wrench 
controller 20 form a pair, and in case of the multiple tightening 
application, a required number of such pairs are prepared and are 
connected via a data bus 50 to the main controller 30. For instance, in 
case of two spindles, a second wrench controller is connected at a 
position #2 of the data bus 50, and in case of three spindles, wrench 
controllers are connected at both positions #2 and #3, respectively. This 
embodiment is designed so that a maximum 16 spindles can be connected to 
the main controller 30. The data bus 50 is connected to a parallel signal 
I/F (a parallel signal interface) 21 of each wrench controller and a 
parallel signal I/F (a parallel signal interface) 31 of the main 
controller 30, for transferring therethrough various preset data and 
commands necessary for screw tightening and various tightening data and 
judgement data generated by the wrench controller 20. 
The power wrench 10 comprises a screw socket 11 for engagement with the 
screw head to apply a tightening torque, a driving shaft 12 for driving 
the screw socket 11, a transmission 13 for amplifying the torque of a 
driving motor 14 and transmitting it to the driving shaft 12, a driving 
motor 14 for converting an electric signal from a servo amplifier 22 of 
the wrench controller 20 into the torque, a torque transducer 15 for 
converting the reaction moment of the tightening torque into an electric 
signal and applying it to a torque amplifier 23 of the wrench controller 
20, and an angle encoder 16 for detecting the rotational angle of the 
screw, converting it into a pulse-shaped electric signal and providing it 
to an angle counter 24 of the wrench controller 20. 
The wrench controller 20 comprises a parallel signal I/F (a parallel signal 
interface) 21 for transmitting and receiving therethrough all data 
necessary for tightening and all data obtained by tightening, a servo 
amplifier 22 for converting data from the data bus 25 into an electric 
signal necessary for rotating the driving motor 14, a torque amplifier 23 
for amplifying the analog signal from the torque transducer 15 and 
converting it into a digital signal, an angle counter 24 for integrating 
and storaging the pulse signal from the angle encoder 16, a ROM (a read 
only memory) 25 for storing all sequences necessary for this unit in the 
tightening process, a RAM (a random access memory) 26 for storing all 
preset data necessary for this unit and all data obtained in this unit 
during the tightening process, and a MPU (a microprocessor) 27 for 
performing all operations necessary for this unit in the tightening 
process and controlling the wrench controller 20. These elements are 
interconnected via an internal bus 25. 
The main controller 30 comprises a parallel signal I/F (a parallel signal 
interface) 31 by which all data and commands necessary for the tightening 
process and all data obtained by the tightening process are transmitted to 
and received from a plurality of wrench controllers 20 via the data bus 
50, a ROM (a read only memory) 32 for storing all sequences necessary for 
the tightening process, a RAM (a random access memory) 33 for storing all 
data necessary for the tightening process and all data obtained during the 
tightening process, a MPU (a microprocessor) 34 for performing all 
operations necessary for the whole tightening process, a CRT display 35 
for displaying all data necessary for the whole tightening process and all 
data obtained during the tightening process, a CRT controller 36 for 
providing display data to the CRT display 35, a keyboard 37 for inputting 
all data necessary for the whole tightening process, a keyboard interface 
38 for receiving the data input from the keyboard 37, a parallel signal 
I/F (a parallel signal interface) 39 through which interlock signals are 
transmitted to and received from an assembly machine in which this 
tightening system is incorporated, a printer I/F (a printer interface) 40 
for transferring to a printer all data necessary for the tightening 
process and all data obtained during the tightening process, a serial 
signal I/F (a serial signal interface) 41 transferring to a data analyzer 
all data necessary for the tightening process and all data obtained during 
the tightening process, and a data bus 42 for interconnecting these 
devices. The storage area of the RAM 33 is divided into two, and the 
storage area for storing all data necessary for the tightening process is 
backed up by a battery for preventing the loss of the stored data in case 
of power failure. 
Next, the operation of this tightening system will be described. 
All data necessary for the tightening process, stored in the RAM 26 of each 
wrench controller 20, are automatically updated when power is connected 
and when the data is rewritten throught the keyboard 37. 
Upon generation of a tightening process start signal from the assembly 
machine, the signal is provided to the MPU 34 of the main controller 30 
via the parallel signal I/F 39 and the data bus 42. The MPU 34 makes a 
self-diagnosis of each device of the main controller 30 and, at the same 
time, sends a diagnosis command to each wrench controller 20 via the data 
bus 42, the parallel I/F 31 and the data bus 50. Each wrench controller 20 
responds to the command to perform the self-diagnosis and, at the same 
time, diagnosis main functions of the power wrench 10 connected thereto. 
The above-said self-diagnosis is essential to quality control of screw 
tightening, because reliable quality control is possible only when all 
functions for tightening are normal. 
After confirming that the results of all the self-diagnosis are good, the 
MPU 34 of the main controller 30 sends a tightening start signal to each 
wrench controller 20. 
Turning next to FIG. 14, the actual tightening process will be described in 
detail. 
At the same time as each wrench controller 20 starts the power wrench 10, 
the MPU 27 of the wrench controller 20 starts to measure the tightening 
time and compares it with a preset over time limit "OT" at all times. If 
the actual tightening process is not completed within the preset over time 
limit, then the MPU 27 sends an abnormality signal via the parallel signal 
I/F 21 to the main controller 30. 
In FIG. 14, references "P1" and "P2" indicate first and second actual 
tightening points and a reference "P3" a final tightening point. A 
reference "a1" indicates an actual tightening angle from the point "P1" to 
"P2", "a2" an actual tightening angle from the point "P2" to the point 
"P3", "a3" the sum of the tightening angles "a1" and "a2", and ".alpha." 
an actual average torque rate calculated using n pieces of data. 
References "A1", "A2", "A3" and "A4" designate first, second, third and 
fourth tightening angle limits, "YA" a yield target angle limit, 
".alpha.T" a target yield rate limit, "n" the total number of torque data 
necessary for calculating the actual average torque rate ".alpha.", and 
"T1", "T2", "T3" and "T4" first, second, third and fourth torque limits, 
which are all preset values input from the keyboard 37 of the main 
controller 30. When input from the keyboard 37, these data are displayed 
on the CRT display 35 and, at the same time, they are once stored in the 
battery-backed-up storage area of the RAM 33 and then transferred to the 
RAM 26. 
These data can be input via the serial signal I/F 41 from the data analyzer 
of an external device as well. 
When the actual tightening torque has reached the point "P1" where it is 
equal to the first torque limit "T1", the angle counter 24 starts 
measuring the actual tightening angle. When the actual tightening torque 
has reached the point "P2" where it is equal to the second torque limit 
"T2", the MPU 27 determines whether or not the actual tightening angle 
"a1" stays in the range beteen the first and second tightening angles, if 
not, it is judged that an abnormality has occurred, and an abnormality 
signal is sent to the MPU 34. The MPU 34 jumps to a subroutine for 
processing the abnormality and provides to the MPU 27 an execution 
instruction for the abnormality processing. Usually this instruction is 
programmed so that when the actual tightening angle "a1" is smaller than 
the first tightening angle limit "A1", the tightening process is brought 
to an emergency stop in recognition of the emergency condition that screw 
threads or threads of the tap hole have been damaged, cross threading has 
occurred, or the tap hole has been filled with a foreign matter. When the 
actual tightening angle "a1" is larger than the first tightening angle 
limit "A1", the screw is turned back for re-tightening in recognition of 
the condition that contact surfaces of members to be fastened together are 
unparallel or low in surface roughness. It is a matter of course, however, 
that the program of this subroutine can easily be modified as required. 
After the actual tightening process has reached the point "P2", the MPU 27 
begins to calculate the actual average torque rate ".alpha." on the basis 
of the afore-mentioned equation (3) using the stored torque data 
corresponding to the number "n" of preset data, and compares the 
calculated torque rate ".alpha." with the target yield rate limit 
".alpha.T". In this calculating and comparing operation, after the actual 
tightening process has proceeded a very small angle dA from the point "P2" 
in FIG. 14, the calculation of the actual average torque rate ".alpha." is 
performed always using "n" pieces of torque data, omitting the oldest 
torque data each time new torque data is obtained, and the calculated 
torque rate ".alpha." is compared with the target yield rate limit 
".alpha.T", and when the former becomes smaller than the latter, the 
tightening process is stopped. This condition for stopping the tightening 
process will hereinafter be referred to as a condition 1. Similarly, after 
the actual tightening process has proceeded the very small angle dA from 
the point "P2" in FIG. 14, the MPU 27 calculates the difference "dT" 
between the actual tightening torque "TT" and the preset torque "T2", 
integrates the difference upon each increase in the tightening angle, and 
substracts from the integrated value the area of a right triangle to 
obtain the area "Ya". When the area "Ya" has come to satisfy the condition 
of equation (6) which is a conditional equation which is calculated using 
the preset target yield angle "YA", the MPU 27 stops the tightening 
process. This condition for stopping the tightening process will 
hereinafter be referred to as a condition 2. 
Information as to whether the tightening process has been stopped under the 
condition 1 or 2 is applied from the MPU 27 to the MPU 34. According to a 
preset rule, the MPU 34 provides the information via the data bus 42 and 
the CRT controller 36 to the CRT display 35 for display thereon, to the 
printer via the printer I/F 40, to the data analyzer via the serial signal 
I/F 41, and to the assembly machine via the parallel signal I/F 39. 
Usually, the tightening process is judged to be acceptable or unacceptable 
depending on whether the process has been stopped under the condition 1 or 
2, but the program can easily be changed so that the tightening process is 
acceptable in the both cases. 
After the actual tightening process has reached the point "P2", the MPU 27 
always judges whether or not the actual torque curve meets with the 
condition 1 or 2 and whether or not the actual tightening torque "TT" 
passes the preset torque limit "T4". Where the actual tightening torque 
"TT" oversteps the limit "T4", the MPU 24 immediately stops the tightening 
process and provides the information to the MPU 34. 
The MPU 34, when supplied with the information on these abnormalities of a 
spindle, is capable of stopping the abnormal spindle together with other 
normal spindles or allowing the tightening process of the normal spindles 
to proceed. These operations are determined entirely by the contents of 
the program used. 
Immediately when the tightening process is stopped under the condition 1 or 
2, the MPU 27 judges whether or not the final tightening torque "TT" is 
smaller than the preset torque limit "T3" and whether or not the final 
tightening angle "a3" is smaller than the preset angle limit "A3", and 
sends the information to the MPU 34. If these actual tightening values are 
smaller than the preset values, then the MPU 34 will operate on a 
programmed rule. There are many methods for the operation and can also be 
selected by changing the contents of the program. 
All data obtained during the actual tightening process is once stored in 
the RAM 26 and, upon completion of the tightening process, they are 
transferred to the RAM 33. These data are the tightening angle "a1" 
between the preset torque limits "T1" and "T2", the final tightening angle 
"a3", the final tightening torque "TT", the final average torque rate 
".alpha.", and so forth. In case of multiple spindle tightening, 
tightening data of all spindles are stored in the RAM 33 and the MPU 34 
displays the data on the CRT display 35, transfers the data to the printer 
or data analyzer, and sends a judgement signal to the assembly machine. 
Where the data analyzer has a function of analyzing the actual tightening 
condition through utilization of the supplied data, preparing data for 
appropriate tightening and providing the data to the tightening system, 
the MPU 34 stores the data in the battery-backed-up storage area of the 
RAM 33, as is the case with the data input from the keyboard 37. The data 
thus stored in the RAM 33 is transferred to each wrench controller 20 for 
performing an appropriate tightening process. 
The quality control method of the present invention for yield tightening is 
implemented by the system shown in FIG. 13, and various calculating 
processes and various judgement functions necessary for the present 
invention are implemented by making utmost use of the function of the 
imcroprocessor.