Note: Descriptions are shown in the official language in which they were submitted.
iOS~ S
The invention relates to apparatus for and a method of
determining rotational or linear stiffness and is particularly,
but: not exclusively, concerned with the tightening of screw-
threaded or other axially-loaded fasteners. This application
i8 concerned with improvements in the invention described in
our copending Canadian Patent Application No. 170,642 filed
May 8, 1973.
The precise clamping load of a fastener is extremely
important in determining whether or not a joint including the
fastener will fail in service. It is therefore desirable that
fasteners should be tightened to a consistent preload. An
object of the present invention is to provide apparatus for
and a method of tightening a screw-threaded fastener to a
predetermined preload, for example, the maximum preload
attainable without plastic deformation of the fastener or joint.
Another object of the invention is to provide an improved
stiffness meter ox torque gradient meter not only for use in
tightening fasteners but for other applications where stiffness
or torque gradient is required to be determined.
According to one broad aspect, the invention relates to
a stiffness meter for determining the yield point or similar
significant change in the slope of any torque-rotation curve
comprising; means including analog shift register means for
developing a changing signal representative of the torque
gradient of the torque imparted to a member; means responsive
to said gradient signal for storing an information signal
representative of the torq~e gradient developed through the
generally linear portion of the torque-rotation curve; and,
comparator means for comparing said gradient signal and said
information ~ignal and developing a control signal when said
gradient signal has changed to a predetermined percentage of
said information signal.
~ - 2 -
1054825
Features of the invention will be described with
reference to the accompanying drawings, in which:-
Figure 1 is a curve of torque plotted against angle of
turning for a screw-threaded fastener in a typical application,
Figure 2 is an elevation of the first form of the
apparatus;
Figure 3 is a diagram illustrating a logic device to
be used in conjunction with the apparatus shown in Figure 2;
Figure 4 is a particular part of the logic device of
Figure 2 shown in greater detail;
Figure 5 is a derivative curve obtained for Figure 1,
that is the curve of ~ plotted against ~2;
Figure 6 is an alternative form of the apparatus shown
in elevation and part section,
Figure 7 is another alternative form of the apparatus
including the logic device; and
Figure 8 is a section view taken along the line 8-8 of
Figure 7.
The torque required to tighten a screw-threaded
fastener is a function of several variables, namely the joint
stiffness, the fastener stiffness, the surface friction and
the thread form. The general characteristic showing the
relationship between torque and angle of turning of a fastener
is shown by the curve in Figure 1 of the accompanying drawings.
The maximum gradient of the curve at a point A is a function
of the variables set out hereinbefore. The gradient can vary
widely even with a fastener of given diameter and thread
configuration. The clamping load at which the gradient begins
to fall below the maximum value has been found to be xelatively
independent of friction and joint stiffness and to be primarily
dependent upon the yield
- 2A -
1054~Z5
strength of the fastener and/or the joint. The point indicated
at X in Fig. 1 represents a point on the torque/angle of turning
curve to which the fastener is tightened to a consistent tensile
stress close to its yield stress. Further rotation of the fas-
tener would aause the-~rque to~approach a maximum value and the
tensile stress in the fastener to approach its ultimate value.
Depending on the ductility and dimensions of the fastener and/or
the joint, fracture would finally occur at a point representative
at Y.
The general objective in controlling the tightening
of a screw-threaded fastener is to achieve a 60nsistant joint
preload close to the maximum that the fasten~r can apply with-
out yielding commencing. One common method is to use a torque
control by which a specific maximum torque is applied in an
attempt to attain a desired pre~oad for particular thread and
frictional conditions. This methoa has the disadvantage that as
tne exact frictional conditions are not known under practical
fastener assembly conditions, there will be variations in the
torque/tension relationship. This leads to varying tensile loads
2a ''n the fastener for a given applied to~qae. Another known method
which is not dependent upon frictional conditions involves
measuring the elongation of the fastener. In most cases direct
measurement of elongation is impossible and therefore a modifica-
tion of the method employs angle-controlled tightening in which
an estimated elongation is effected by tightening to a precise
angle of tightening. The disadvantage of this method is deter-
mining where to start measuring angles when operating on torque
angle of turning curves which have different initial and maximum
slopes. For preloaas in the elastic range this is extremely
difficult as the deviation from a typical curve may be large.
~054~S
In the plastic range of the fastener, however, an error in angle
does not change the preload appreciably and the deviation is
small; but for very short fasteners this angle error may cause
overstrain and consequent fracture. Angle controlled tightening
is therefore only acceptable in the plastic region for long
fasteners (for example those having more than six free threads)
where some plastic deformation will not cause structural damage
~n the fastener.
It is therefore desirable that the optimum point at
w~ich tightening is to cease is the point where the gradient
of the torque/angle of turning curve has jast started to fall
fro.~ its value in the generally linear region or its maximum
value, that is the point at which the fastener is just beginning
to yield. Taking X in Fig. 1 as a point where the gradient is a
predetermined ratio of the gradient in the generally linear
region or maximum achieved for the fastener, this point determines
a practical position at which tightening should be stopped to
give clamping loads which essentially independent of the fastener
geometry and the conditions of friction. A further object of
the present invention is to provide a method of an apparatus for
enabling the point X to be identified during a tightening opera-
tion as soon as it occurs so that the tightening process can be
immediately stopped at that point and furthermore to enable the
point to be identified independently of the joint or fastener
characteristics, that is without foreknowledge of the joint or
fastener characteristics or calibration.
According to the invention, apparatus for determining
rotational or linear stiffness, i.e., a stiffness meter, com-
prises means for developing a changing signal representative of
the instantaneous torque applied to a rotary member, storage means
for storing a series of signals representative of instantaneous
~054~ZS
torque and gradient register means for sequentially comparing
stored signal wiht an instantaneous torque signal and develop-
ign a signal representative of the borgue gradient.
The invention also provides a stiffness meter for
determining the yield point of similarly significant change in
slope of a torque-rotationicurve, the meter comprising means for
develop~ng a signal representative of the gradient of the torque-
~otation curve; means responsive to said gradient signal for
storing an information signal representative of the gradient
developed in the generally linear portion of the torque-rotation
curve; and comparator means for comparing said gradient signal
and said infor~ation signal and developing a control signal when
5aid grad~ent has changed to a predetermined relationship with
sa~d information signal.
The stiffness meter may also include means for deter-
~nining the largest gradient measured and the information signal
~s then representative of the largest gradient measured up to
any point along the ~orque-rotation~curve. Preferably, the
control signal is developed when the gradient signal is about
50% of the information signal.
The invention also provides apparatus for tightening
a fastener to the yield point or a similarly significant point
including ~rench means for applying torque and rotating the
fastener; means for developing a signal representative of the
instantaneous gradient of the torque-rotation curve through
which the fastener is being tightened; and, means responsive to
the gradient signal for determining the yield point or other
similarl~ significant point on the torque-rotation curve through
~ ch the fastener is being tightened and for developing a
control signal when the fastener is tightened to the determined
point.
10548'~5
The fastener tightening apparatus determines the
yield point or similarly significant point by storing an in-
formation signal representative of the gradient in the generally
linear portion of the torque-rotation curve, preferably the
largest gradient, and comparing the instantaneous gradient
signal with the information signal and by developing the con-
trol signal when the compared signals have a predetermined
relationship, preferably when the instantaneous gradient signal
~s 50% or less of the information signal.
According to certain embodiments of the invention,
apparatus for determining rotational or linear sfiffness, i.e.,
a stiffness meter, comprises output means for imparting move-
me~t to a member; input means for receiving power; deflectable
c~upling means between said output means and said input means
for transmitting power from said input means to said output
mean~; first sensing means responsive to said input means for
develop~ng a first series of signals representative of th~
displacement of said input means; second sensing means res-
ponsive to said output means for developing a second series of
s~gna1s representative of the displacement by said coupling
~eans of saîd output means, and comparator means responsive to
said first and said second series of signals for determining the
difference of the number of signals in said first and said
second series of signals during a datum number of consecutive
signals in said second series of signals as power is trans-
mltted ~y said coupling means from said input means to said
output means, said difference being a function of the stiffness.
The stiffness meter may also include circuit means
responsive to said comparator means for sequentially storing
information representative of the maximum stiffness or torque
gradient developed up to any given point during the driving of
sa1~d member.
-- 6 --
~0~4~'~5
The stiffness meter may be such as to be capable of
being used as a wrench, torque input to said input means being
stopped when said difference reaches a predetermined ratio of
said maximum gradient stored by said circuit means.
Each of said first and second sensing means is con-
veniently an incremental encoder mounted on the respective in-
put or output means and capable of emitting a precise number
of equally spaced signals during a complete rotation of the
respective input or output means.
Certain embodiments of the invention also provide
apparatus, for tightening a fastener, comprising fastener
engaging means for imparting tightening movement to a fastener;
power input means; deflectable coupling means between said
fastener engaging means and said power input means for trans-
mitting power to said astener engaging means; first sensing
means responsive to movement of said power input means for
developing a first series of signals representative of the
displacement of said power input means; second sensing means
responsive to movement of said fastener engaging means for
developing a second series of signals representative of the
displacement by said coupling means of said fastener engaging
means; compara~or means responsive to said first and second
signals for determining the difference of the number of
signals in said first and second series of signals during a
datum number of consecutive signals in said second series of
signals as power is transmitted by said coupling means from
said power input means to said fastener engaging means, said
difference being a function of the instantaneous stiffness;
circuit means responsive to said comparator means for
sequentially storing information representative of the maximum
1054~25
stiffness developed ~uring the tightening of the fastener and
for producing a control signal when said instantaneous stiff-
ness has dropped to a predetermined ratio of the maximum
stiffness developed during the tightening of the fastener,
and control means responsive to said control signal for con-
trolling power input by said power input means.
Apparatus in accordance with certain embodiments
of the invention may particularly, but not exclusively, be
used for tightening a screw-threaded fastener, in which case
the apparatus comprises fastener engaging means for imparting
rotary movement to a threaded fastener; rotary power input
means; torsionally deflectable coupling means between said
fastener engaging means and aaid rotary power input means
for transmitting rotary power to said fastener engaging means;
f~rst sensing meaas responsive to movement of said rotary
power input means for developing a first series of signals
representative of the rotary displacement of said rotary
power input means; second sensing means responsive to said
fastener engaging means for developing a second series of
signals representative of the displacement by said coupling
means of said fastener engaging means; comparator means
responsive to said first and second signals for determining
the difference of the number of signals in said first and
second series of signals as rotary power is transmitted by
said coupling means from said rotary power input means to
said fastener engaging means, said difference being a function
of the instantaneous torque gradient of the fastener; circuit
means responsive to said comparator means for sequentially
storing information representative of the maximum torque
gradient developed during the tightening of the fastener and
1~:)54~;~5
for producing a control signal when said instantaneous torque
gradient has dropped to a predetermined ratio of the maximum
torque gradient developed during the tighteneing of the fastener,
and control means responsive to said control signal for con-
trol.ling power input by said rotary power input means.
Certain embodiments of the invention may also pro-
vide a method of determining stiffness using deflectable
coupling means for transmitting power from input means to
output means for imparting movement to a member, the method
consisting of the steps of developing a first series of sig-
nals representative of the displacement of said input means;
developing a second series of signals representative of the
displacement by said coupling means of said output means,
and comparing said first and second series of signals by
counting the difference of the number of signals in said first
and second series of signals during a datum number of con-
secutive signals in said second series of signals as power
is transm~tted by said coupling means from said input means
to said output means, said difference be~ng a function of
the stiffness.
Certain embodiments of the invention may also pro-
vide a method of tightening a fastener ~y power input means
driving fastener engaging means through deflectable coupling
means, the method consisting of the steps of developing a
first series of signals representative of the displacement of
said power input means; a second series of signals represen-
tative of the displacement by said coupling means of said
fastenex engaging means; comparing said first and second series
of signals by counting the difference of the number of signals
in said first and second series of signals during a datum
~OS4~Z5
number of consecutive signal~ in said second series of signals
as power is transmitted by said coupling means from said power
input means to said fastener engaging means, said difference
being a function of the instantaneous stiffness of~the coupling
means; sequentially storing information representative of
the maximum stiffness developed during the tightening of the
fastener and producing a control signal controlling said power
input means when said instantaneous stiffness has dropped to
a predetermined ratio of'the maximum stiffness developed
during the tightening of the fastener.
Particularly, but not exclusively, the method ac-
cording to certian embodiments of the invention may be for
tightening a screw-threaded fastener by rotary power input
means driving fastener engaging means through torsionally
defleetable coupling means, the method consisting of the
steps of developing a first series of signals representative
of the rotary displacement of said rotary power input means;
a second series of signals representative of the displacement
by said coupling means of said fastener engaging means; com-
paring said first and second series of signals by counting thedifference of the number of signals in said first and second
series of signals during a datum number of consecutive signals
in said second series of signals as power is transmitted by
said coupling means from said rotary power input means to
said fastener engaging means, said difference being a function
of the instantaneous torque gradient of the fastener; sequen-
tially storing information representative of the maximum torque
gradient developed during the tightening of the fastener and
producing a control signal controlling said rotary power in-
put means when said instantaneous torque gradient has dropped
-- 10 --
10~4~
to a predetermined ratio of the maximum torque gradientdeveloped during the tightening of the fastener.
Desirably, the control signal may be produced when
saicl instantaneous torque gradient has dropped to substantially
50% of the maximum torque gradient developed during the tight-
ening of the fastener.
The control signal would usually be employ~d to
stop the power input means; but in some applications it may
be used to maintain the power input means to hold the instan-
taneous torque gradient at the aforesaid predetermined ratioof the maximum torque gradient.
The theory involved in the method and apparatus
provided by the certain embodiments of the invention noted
above is as follows:
Assuming that the coupling between the input and
output means is or is equivalent to a torsionally-flexible
shaft with a substantially linear characteristic having an
incremental encoder mounted at each end thereof, if the
torsional stiffness of the shaft is K and ~ and ~ are the
angular displacements of the two ends of the shaft, the tor-
que or turning moment M transmitted by the shaft
( ¢\ 1 q) 2 )
If the end of $he shaft having the angular dis-
placement ~ is connected to the fastener to be tightened and
the other end is attached to a motor or other driving means,
the gradient of the torque/angle of turning characteristic
being followed by the coupling, and therefore by the fastener,
is g~ven by: -
d~2 (~
~0~48'~5
This equation may be evaluated in two alternative
ways; the first of which is to expand the equation, as follows:
~d~
= K
/
= K ~ - ~2
~2
wh~re ~ 2 are the angular veleocities of the two ends of
the shaft.
If the times between successive signals or pulses
from the incremental shaft encoders are tl and t2, then
t ~ 1 t ~ 1
~ 2 ~1~ 2
and thus,
dM = K t2___ t
a~2 tl
The times t~ and t2 can be measured electronically
by means of the signals or pulses produced by the incremental
encoders and after the necessary calculations by the gradient~
determining means, an oupput signal proportional to dM can be
obtained. d~ 2
Alternatively, the equation
d q~ 2 K ( d ~
may be evaluated by employing small incremental values, as
follows:
AM ~ R Q.~ 2
a~,2- ~2
If high resolution incremental encoders giving of the
- 12 -
1054~25
order of four to five thousand pulses per revolution are em-
ployed, a~ and ~ are directly measurable by counting pulses.
For example, if ~ is determined by counting say 100 pulses
by the encoder at the fastener end of the torque-transmitting
coupling, the gradient is directly determined by counting the
number of additional pulses produced by the encoder at the
other end of the coupling during the production of the said
100 pulses by the encoder at the fastener end. If, for ex-
ample, 6 extra pulses are counted at the encoder at the tor-
1~ que input end of the coupling
dMd~2 6
Thus the gradient determining means either has to
t - t
determine 2 ~ 1 by being responsive to the time intervals
between pulses produced by the the two encoders or it has to
a
determine ~1 ~ 2 merely by counting the numbers of
~ 2
pulses produced by the two encoders or by measuring the number
of extra pulses produced by the encoder at the torque input
end while the encoder at the fastener end produces a ~iven
number of pulses, e.g. 100.
According to~certain other emhodiments of the invention~,
a tightening apparatus is provided that includes a wrench for
tightening a fastener, torque transducer means for developing
a changing signal representative of the instantaneous torque
being applied to the fastener and sensing means developing
signals representative of fixed increments of rotation of the
fastener. As in certaln embodiments discussed above, there
is al~o provided shift register means receiving signals
reprensentative of instantaneous torque being applied to the
fastener and which is clocked by signals from the sensing means
for sequentially feeding signals to gradient register means,
~054~ZS
in the form of comparator means, subtracting torque signals
outputted from the shift register means from the instantaneous
signals outputted from the transducer means and for developing
a signal representative of the instantaneous gradient of the
torque-rotation curve which could be plotted for the particular
fastener being tightened. Also as in the certain embodiments
discussed above, the instantaneous gradient signal is compared
with a gradient signal determined to be representative of the
gradient of the torque-rotation curve in its generally linear
portion and when the compared signals have a predetermined
relationship, a control signal is developed.
The transducer means may be in the form of a torque
cell associated with the wrench to mea$ure the reaction torque
on the wrench and the sensing means includes a proximity probe
which if operatively mounted adjacent the rotary vanes of the
motor input for the wrench for developing signals corre-
sponding to the passage of the vanes and the rotation of the
fastener.
Turning now, to Fig. 1 o the accompanying drawings,
which as already stated is a typical Torque or Turning Moment
versus Angle of Turning characteristic, the curve followed by
the torque-transmitting coupling and hence by the fastener
may be divided into three regions:-
I. An initial pre-tightening region;
II. A tightening region, and
III. A region of yield and subsequent failure of
the fastener.
In region I the effect of burrs and irregularities in
the threads of the fastener must be ignored and therefore the
gradient-determining means of the torque-transmitting apparatus
- 14 -
~ OS4~S
must first of all determine or be informed that region I has been
left and the fastener is being tightened in region II. Conven-
iently entry into region II is by a torque measurement derived
from the encoder signals. Normally the gradient of the character-
istic will be substantially constant in region II, i.e., the curve
will approximate to a straight-line; but if the characteristic
is curved in region II it will reach a typical maximum value A.
Accordingly, region II can be considered as the generally linear
portion or region of the curve. When the fastener is tightened
beyond region II, region III is reached, the gradient of the
characteristic commences to decrease as the fastener begins to
yield and ultimately the fastener would fail at point Y. The
transition point X between regions II and III may be the point
at which the gradient has been reduced by a certain proportion
of the maximum gradient at A. It is desirable that a fastener
should be tightened to point X and therefore the gradient-
determining means must be capable of determing that region II has
been rea~hed and then of determining the instantaneous gradient
during tightening and continuousiy comparing it with the gradient
in region II, preferably, the maximum value A, to determine when
the point X has been reached. The gradient-determining means may
be any suitable logic circuit or circuits responsive to the
signals received from the two encoders. When the gradient- ¦
determining means has detected that point X has been reached,
it can issue a stop command so that the operator will stop the
tightening of the fastener. Alternatively the stop command may
be used to stop the driving motor or other means automatically.
- 15 -
~054~Z5
Referring l~oW to the drawings, and specifically to Figure
2, a screw-threaded fastener is shown at 1, and this is engaged
by a driver or other tool 2 for turning the fastener and having
a shaft mounted for rotation in a bearing 3. The bearing 3
together with a companion bearing 4 supports a shaft 5 which is
drivingly engaged with the tool 2 and carries a first incremental
encoder 6. The apparatus also includes another bearing 7
supporting a shaft 8 carrying a second incremental encoder 9.
The shaft 8 is arranged to be driven by a torque-applying motor
10. The shafts 8 and 5 are interconnected by a helical spring
11. When the motor 10 is driven, it will rotate the shaft 8 and
this in turn will rotate the helical spring 11 which will drive
the shaft 5. The shaft 5 will turn the tool 2 which will
tighten the fastener 1.
The encoders 6 and 9 may be of an optical, electro-magnetic
or other kind capable of producing signals in association with
fixed light-responsive or other stationary receiver devices (not
shown) which will produce a series of signals effected at
- }6 -
~54~Z5
precise intervals corresponding to the angular turning of the
encoders or the angular velocities thereof and hence of the re-
spective shafts 3 and 8. The frequency of the signals produced
by the respective encoders 6 and 9 determines their respective
angular velocities or a function thereof. The intervals be-
tween the signals produced by the encoders 6 and 9 can be
measured, for example, by electronic means. From the time
intervals between signals produced by the respective encoders
or by counting the number of signals, as aforesaid, the gradient
of the torque/angle of turning curve can be determined as
explained herein and hence when the gradient has fallen to a
value indicating that point X in Fig. 1 has been reached, the
operator would stop the motor 10. The signals produced by the
operation of ~he encoders 6 and 0 are fed into a logic device
which produces a "stop" signal informing the operator that
the motor 10 should be stopped, or the "stop" signal could be
used to stop the motor automatically. Alternatively, instead
of the signal produced by the logic device being a "stop"
signal it may be used to hold the fastener under a substantially
constant load, in which case the signal could be a controlling
signal which prevents additional input power being supplied.
Ref~rring now to Fig. 3, the logic device is illus-
trated in block diagram form. Commencing at the top of the
diagram, references 6 and 9 indicate the two encoders. The
left-hand encoder, i.e., the one nearer to the fastener, issues
pulses ~ r and the right-hand encoder, i.e., the one nearer
to the motor 10 issues pulses ~ . The two streams of pulses
are supplied to a circuit illustrated by block 20. This counts
the number of pulses ~ and ~ or determines the time intervals
between the pulses of the respective streams of pulses or
computes the gradient from measured angular velocities. A
~o~4~Z5
signal from block 20 passes to blook 21 which is a logic cir-
cuit which decides whether region II has been reached. If the
answer given by block 21 is affirmative, signals pass to block
22 which is a logic circuit which calculates the function of
the gradient by determining ~1 ~2 t2 - tl as aforesaid.
~ 2 tl
Output signals from block 22 then pass to block 23. This is
another logic circuit which compares the gradient function with
a stored maximum function previously determined by block 22
and stored in a circuit indicated by block 24. Block 23 gives
affirmative or negative output signals depending upon whether
or not point X in Fig. 1 has been reached. If the output sig-
nal from block 23 is affirmative, this is the aforesaid stop
command and tightening is immediately stopped, as aforesaid.
If the output signal from block 23 is negative a signal is
fed back to blo~k 22 and the circuit thereof continues to
continuously up-date the gradient function and feed an output
signal to block 23.
Fig. 4 is a schematic block diagram showing the main
components of the electronic circuit indicated by the block 22
in Fig. 3. As already stated block 22 determines the gradient
_ Q~
, or ~ The electronic circuit shown in Fig. 4
calculates the expression QM by measuring the number of
pulses received from the input encoder 9 during a given value
a~ . The value of ~ chosen is referred to as the chord
length because the measurement is equivalent to taking the
di~ference between the readings of torque (M) across a chord
of substantially constant length which is continuously moved
along the M-~ curve as indicated by al bl and a2 b2, in Fig. 1
the pro,iection of the chord length on the ~axis is fixed by
the characteristic or setting of a chord length shift register
- 18 -
lOS4~2S
27 which is operated each time it receives an actuating pulse,
as hereinafter explained.
Pulses (~ ~ received from the input encoder 9 at the
motor end of the spring 11 are gated directly into a residue
register 25 with due regard to the sense of rotation of the
rotor. The function of the residue register 25 is to store
pulses.
Pulses (~ ) from the encoder 6 at the output or
fastener end of the spring 11 are checked by a checking device
28 to determine whether the fastener is rotating in the correct
direction and are then used to decrement the residue register
25. If the encoder 6 at the fastener end were to rotate back-
wards, the amount of backwards th~st is counted and must be
restored by an equal forward twist before any ~ pulses are
gated to the register 25. As soon as a ~ pulse has de-
cremented the residue register 25, the value shown thereby
is examined. If it is greater than ze~o there have been more
~1 pulses then ~ pulses and as a result a signal is passed
through a gate 29 to both a gradient register 26 and the
chord length shift register 27 and also to decrement the
redidue register 25. The chord length shift register 27 is
actuated each time it receives a ~ 2 pulse passed by the
checking device 28, as indicated by arrow 30, and if the
shift register 27 also receives a pulse through the gate 29
i~ registers '1' at its input. If it does not receive a
pulse through the gate 29 it registers '0'. Each time the
output of the shift register 27 registers '1' it decrements
the gradient register by one pulse. When the shift register
output registers '0' it does not affect the gradient register.
Thus each time the shift register is operated on by the pro-
duction of a ~ pulse in the correct sense, the gradient
register either changes by one pulse or does not change de-
-- 19 --
~54~
pending upon whether there are positive "readings" by the
residue register 25 and the output of the shift register 27.
The chord length shift register 27 has a characteristic or a
setting such that the gradient register reading is taken over
the effective chord length. The chord lenght must be suffi-
ciently long to average out the effect of "noise", that is
unwanted signals superimposed on the basic signals. An angle
of twist of 1 corresponding say to one ~ pulse is too small
for this purpose. It has been found that 20 is a suitable
value of angle of twist over which to measure the torque; but
if readings were only taken say every 20, the Torque/Angle
of Twist Curve could not be followed. Therefore the chords,
such as al b1 and a2 b2 in Fig. 1 are overlapped and measure-
ment of ~ over a 20 chord are taken on each ~2 pulse that
is about every 1 of twist. When the gradient register is
operating normally, the indicated reading of the gradient
register is ~M/~ for the fastener being tested. Signals
from the gradient meter 26 are fed to the maximum gradient
store, i.e., to block 24 and to the comparator in block 23
(see Fig. 3).
- 20
lV5~ 5
The signal given by block 23 to stop the motor driv-
ing the fastener is produced when the instantaneous gradient
has fallen to 50% of the maximum gradient attained, that is,
the gradient in the generally linear region of the M-~ 2
curve. The reason for this is that the derivative of the
M-~2 curve, that is the dM ~ ~2 curve shown in Fig. 5, has
a point of inflexion at or near to the 50% of maximum value
as indicated at X in Fig. 5. This point is also the steepest
part of the curve and thus the point where the curve passes
most quickly through a superimposed "noise" curve. Still
referring to Fig. 5, it can be seen that point X lies within
a range of about 25% to about 75% of the maximum value and
a preset relationship utilizing a value within this range
could be utilized to stop the motor driving the fastener.
The point X is therefore the point on the curve which will
give maximum immunity to noise, i.e., spurious signals.
Because ~he apparatus, in its fastener-tightening
form, is essentially a torque gradient or torsional stiff-
ness meter and it can compare the instantaneous torque gra-
dient with the torque gradient in the generally linear regionof the M-~ 2 curve and, preferably with the maximum torque
gradient, the limitations of existing systems for controlling
the tightening of fasteners are overcome. These require
relatively accurate pre-knowledge of the torque/angle charac-
teristic, either by knowing the approximate value of themaximum gradient or by knowing the torque to provide a given
clamping load or by knowing the angular rotation to ensure
optimum clamping.
The apparatus described herein does not require pre-
knowledge of a particular torque/angle of turning character-
- 21 -
~054~5
istic because the deviation of the instantaneous gradient
from the gradient in the approximately linear region of the
curve, preferably the maximum gradient, will be determined
automatically and the tightening will be stopped automatically
when the point X at which the instantaneous gradient is any
preset fraction, e.g., 50%, of the maximum gradient has been
reached. It should also be clear that the instantaneous
gradient may never exactly equal the present fraction and,
therefore, the apparatus should stop tightening when the in-
stantaneous gradient is a preset fraction or less than that
fraction of the maximum gradient. The invention, therefore,
provides apparatus and a method preferable to known tighten-
ing systems.
Using a technique analogous to that applied in the
fastener-tightening form of the invention, the aforesaid
torque gradient or torsional stiffness meter may be converted
to a linear stiffness meter either by converting the output
rotation into linear movement, for example with a rotary-to-
linear converter such as a screw-thread or rack-and-pinion
device. Alternatively linear encoders and a linear spring,
or other linearly resilient element, connected between shaft
means carrying the encoders may be employed. In this way,
the point of yield in a linear system can be determined in
a similar way to the determination of the point of yield in
the aforesaid torsional system. Hence, for example, the
yield point of tensile samples in a tension testing machine
or a creep testing machine, can be identified without the
measurement of gauge lengths and without the necessity to
measure separately force and displacement.
Instead of using the helical spring 11 as the coupling
1054~'~5
means between the input and output shafts a spiral spring may
be employed. Alternatively, the torque transmitting coupling
may be a torsion bar or tube connected between the input and
output shaft means or having end portions forming said in-
put; and output shaft means. The torsion bar or tube may be
of metal, rubber, plastics or it may be a composite bar or
tube formed from any of these materials.
Fig. 6 shows an alternative form of the apparatus
which can be used instead of that shown in Fig. 2. In Fig.
2, the spring 11 is rotated about its longitudinal axis in
addition to being twisted as the fastener follows the Torque/
Angle of Turning Curve shown in Fig. 1. In some instances
this may be undesirable and instead the apparatus shown in
Fig. 6 could be employed in which a non-rotatable helical
spxing 31 transmits torque between an input shaft 32 driven
by a motor 33 to an output shaft 34 by which a fastener or
other member to be turned is arranged to be driven. The in-
put shaft 32 is connected through a gear box 35 containing
a gear drive, shown for diagrammatic purposes as a simple
differential gear train 36, to the output shaft 34. When
the input shaft 32 is driving the output shaft 34 at the
same speed, i.e., when the torque M is constant, there will
be no bodily movement of the gear train 36; but when the in-
put shaft starts to move faster than the output shaft, that
is as in region II in Fig. 1, the gear train will swing about
the common aXis of the shafts 32 and 34 and transmit the
swinging mo~ement to the housing of the gear box 35. One
end of the spring 31 is attached to an end wall 37 of the
gear box 35 which is mounted for swinging about the common
axis of the shafts 32 and 34 on a bearing 42. The other end
- 23 -
~354~ZS
of the spring is attached to a fixed mounting plate 38 in
which the output shaft 34 is freely rotatable in a bearing
39. The input shaft 32 carries an incremental encoder 40,
equivalent to the encoder 9 in Fig. 2, and the output
shaft 34 carries an incremental encoder 41, equivalent
to the encoder 6 in Fig. 2. When the point X is ap-
proached and there is a change in the rate of differential
rotation between the input and output shafts, the spring
31 will transmit torque in a similar manner to the spring
ll in Fig. 2; but the spring 31 does not rotate, it only
twists and therefore does not have to be dynamically balanced.
The angular deviation between the shafts will be measured
by the encoders 40 and 41 and their signals will be trans-
mitted through and handled by the logic devices shown in
Figs. 3 ~nd 4 in the way already described. Another advan-
tage of the arrangement shown in Fig. 6 is that the axial
length of the apparatus can be reduced because the gear box
35 can be made short and housed within the spring 31 as
shown. Also the encoders may be positioned within the
spring. Alternatively, the encoder 40 can be positioned at
the input end of the gear box, as shown, and so a low reso-
lution encoder can be used, thereby saving cost.
In either arrangement, (i.e., Fig. 2 or Fig. 6) of
the apparatus, the logic devices may include switch means
to enable the apparatus to be used as a normal tightening
device or wrench in which the torque is transmitted from the
motor to the fastener without indication of deviation from
a linear Torque/Angle of Turning relationship and without
automatic stopping of the driving motor.
Although in Fig. 2, a helical spring ll has been used
as the coupling means between the input and output shafts,
that is the apparatus is employed as a wrench to which input
- 24 -
l(~S~5
torque is applied continuously during tightening, the appa-
ratus could be used as an impact wrench, that is a wrench of
the kind to which input torque is applied intermittently in
steps, by using in place of the spring 11 a coupling means
whlich has a non-resilient, substantially linear M-~ charac-
teristic. The coupling means would in such a case be effec-
tively a non-return type of spring to prevent recoil of the
apparatus between the periods during which input torque is
applied.
Another embodiment of the apparatus is illustrated
in Figs. 7 and 8 and includes certain means reducing the cost
of the apparatus relative to the embodiment shown in Figs.
2-4. As shown in Fig. 7, the apparatus comprises a wrench
50 including an air motor 52, the operation of which is con-
trolled by a suitable solenoid valve 54, and which drives an
output shaft 56 through a speed-reducing gear box 58 so that
the output shaft does not rotate at the same high speed as
the motor. Output shaft 56 carries an adapter 57 for attach-
ment with a driver bit 59 and is mounted in a suitable ro-
tary bearing assembly 60 facilitating rotation of and ta~ing
up any bending stresses in the output shaft. ~earing assem-
bly 60 may be mounted on a rigid frame 62, but use of the
frame is not necessary for the practice of the invention.
At this point it should be noted that while motor 52 has
been described as an air motor, it may be of any suitable
type, for example electric, hydraulic or any combination of
pneumatic electric or hydraulic. It should also be noted
that the apparatus thus far described is generally conven-
tional and need not be explained in greater detail.
Located between gear box 58 and bearing assembly 60
is transducer means in the form of a torque cell 64 which
develops a signal representative of the instantaneous torque
being applied to the fastener. Torque cell 64 includes a
first mounting base 66 securing the cell to gear box 58 and
a second mounting base 68 securing it to bearing assembly
60. Extending axially of the wrench between mounting bases
66 and 68 are a plurality of strut members 70 which are
somewhat deformable, that is, are relatively rigid members
capable of twisting somewhat about the axis of the wrench.
When wrench S0 is operative to tighten a fasterner, the reac-
tion torque acting thereon causes strut member 70 to twist
about the axis of the wrench, the amount of twist being pro-
portional to the reaction torque which of course, is equal
to and opposite the torque being applied to the fastener.
Each strut member 70 carries a strain gauge 72 which is con-
nected in a wheatstone bridge circuit (not shown) to develop
an electric signal representative of the instantaneous torque
being applied to the fastener. Instead of strain gauges,
contacting or proximity displacement gauges could be used
to develop the electric signal. Thus, with the torque cell
arrangement disclosed in this embodiment, one of the encoders
and the spring arrangement disclosed in the apparatus illus-
trated in Figs. 2-4 have been replaced by a relatively in-
expensive transducer. The exact form of torque cell 64, of
course, may vary somewhat. For example, struts 70 could be
replaced by a somewhat deformable cylindrical member, if
desired.
Since one encoder in the Figs. ~-4 embodiment of
the invention has been replaced by torque cell 64, it should
be clear that only one encoder need be utilized. However, in
- 26 -
~541~Z5
accordance with this embodiment of the invention, it can be
replaced by a relatively inexpensive sensing means further
reducing the cost of the apparatus. Accordingly, further
disclosed in this embodiment of the apparatus is a proximity
probe 74 mounted through the housing of motor 52 adjacent to
and radially spaced from rotary vanes 76 in the motor, as
illustrated most clearly in Fig. 8. Proximity probe 74 can
be in the form of an induction coil which develops an elec-
trical signal when metal passes through its magnetic field.
Thus, as vanes 76 rotate when the fastener is being tight-
ened, signals are provided by proximity probe 74 which re-
present fixed increments of rotation of the fastener. The
size of the increments depends on the number of vanes 76 in
motor 52 and the gear ratio of gear box 58. It should be
understood of course, that proximity probe could cooperate
with one of the gears in gear box 58 in a similar manner.
Still referring to Fig. 7, a control system function-
ally equivalent to that illustrated in Figs. 3 and 4 of the
drawing is illustrated and further reduces the cost of the
system by utilizing certain analog circuitry. The output
signal from torque cell 64 representative of the instantaneous
torque being applied to the fastener is fed through a torque
amplifier 78 which amplifies the torque signal to a magni-
tude wherein it is compatible with the rest of the control
system. From amplifier 78, the torque signal is fed through
shift register means which, since the circuit is analog, com-
prises a series of charge coupled devices in the form of
sample and hold circuits 80, 82, 84 and 86. As in the cir-
cuits illustrated in Figs. 3 and 4, the gradient shift re-
gister means is clocked by signals representative of fixed
~ ~54t~'~5
angular increments of displacement of the fastener. Accord-
ingly, signals fr~m proximity probe 74 which are in the form
of spike shaped pulses are fed through a square wave generator
88 which shapes the signals and feeds the shaped signals and
through a chord length divider 90 to an analog switch driver
92 which sequentially clocks the sample and hold circuits.
Chord length divider 90 is a suitable divider circuit which
electronically divides the pulses from square wave generator
88 by 1, 2, 4, 8, 1~ or 32 so that every pulse, or every
second pulse, or every fourth pulse, etc., is utilized to
clock the shift register. By selecting the appropriate di-
vision to be made in chord length divider 90, it is possible
to adjust the chord length on the Torque-Angle of Turning
curve over which the torque gradient is measured, that is,
chord length a and b in Fig. 1.
Analog switch driver 92, although not necessary,
assures that each sample and hold circuit has discharged its
stored signal before receiving a new signal. Accordingly,
analog switch driver 92 sequentially clocks the sample and
hold circuits first clocking circuit 86, then circuit 84,
then circuit 82, and finally circuit 80. Accordingly, sample
and hold circuit 86 has discharged its stored signal prior
to receiving a new signal from sample and hold circuit 84,
etc. The output from sample and hold circuit 86 is repre-
sentative of torque a fixed increment of rotation prior tothat particular instant and is fed through a gradient register
or comparator circuit 94 in the form of a differential ampli-
fier which also receives an input signal representative of
the instantaneous torque being applied to the fastener from
torque amplifier 78. As in the circuits illustrated in Figs.
- 28 -
10541~
3 and 4, comparator 94 subtracts its input signals and has
an output signal representative of the instantaneous torque
gradient for the particular fastener being tightened. The
gradient signal from comparator 94 is fed through a suitable
gradient signal amplifier 96 which amplifies it to a magni-
tude compatible with the rest of the control system.
From gradient signal amplifier 96, the instantaneous
gradient signal is fed to means for determining the maximum
gradient and also to means for comparing the maximum and in-
stantaneous gradient signals. Looking first at the meansfor determining the maximum gradient, there is included a
maximum gradient comparator 100 receiving input signals from
gradient signal amplifier 96 and from a sample and hold
circuit 102 which also receives signals from gradient signal
amplifier 96. As will be made clear hereinafter, sample and
hold circuit 102 stores a signal representative of the
maximum gradient encountered up to any point in the tighten-
ing cycle prior to the instantaneous output from the gradient
signal amplifier. Comparator 100 determines whet~er the in-
stantaneous gradient signal from gradient signal amplifier96 or the previously stored signal from sample and hold cir-
cuit 102 is larger. If the instantaneous gradient signal is
larger, comparator 100 feeds an output signal to an AND gate
104 which also receives signals from analog switch driver
92 when the switch driver outputs a clocking signal to sample
and hold circuit 84. When both signals are received by AND
gate 104, it outputs a clocking signal to sample and hold
circuit 102 which allows the sample and hold circuit to re-
ceive a new signal from gradient signal amplifier 96 repre-
sentative of the larger gradient. If the instantaneous gra-
- 29 -
dient is smaller, comparator 100 provides no output, nor
does AND gate 104 so that sample and hold circuit 102 cannot
accept a new gradient signal. By utilizing the clocking sig-
nal from analog switch drive 92 to sample and hold circuit 84,
a time lag is provided which allows the comparison to be
made before a clocking signal can be fed through AND gate
104 and before a new gradient signal can be developed.
Looking now at the means for comparing the maximum and
instantaneous gradient signals, it can be seen that as the
signal representative of the maximum gradient is fed from sam-
ple and hold circuit 102 to comparator 100 it is split and
fed to a division circuit 106 which is operative to divide
the signal by the preset relationship utilized to determine
the point X on the curve illustrated in Fig. 1 or Fig. 2.
If the preset relationship is 50%, as is preferred as noted
above, dividing circuit 106 splits the maximum stored gra-
dient signal in half and feeds the signal to a control com-
parator 98 so that it may be compared with an instantaneous
gradient signal from gradient signal amplifier 96 which is
also fed to the control comparator. When the input signals
to control comparator 98 are equal, or when the gradient
signal is smaller than the divided maximum gradient signal,
the control comparator provides an output signal which is
~ed to another AND gate 108. At this point, it should be
noted that the output signal from comparator 98 could be
fed directly through a valve drive amplifier 110 which would
amplify the signal to a suitable magnitude to close the sole-
noid in valve 54 and stop motor 52. However, to assure that
comparator 98 does not inadvertently provide an output signal
in Region 1 of the curve illustrated in Fig. 1, AND gate 108
- 30 -
10541~S
is utilized and receives an additional input signal from
a snug torque comparator 112. Instantaneous torque signals
are fed from torque amplifier 78 to snug torque comparator
112 which also receives an input signal from a preset snug
torque signal generator 114 which, of course, could be in
the form of a suitable potentiometer for providing a pre-
determined input signal representative of the torque approx-
imately corresponding to that at the point mar~ing the tran-
sition from Region l to Region 2 on the curve illustrated in
Fig. l. The noted point is commonly referred to as the snug
point or snug torque point. The setting in snug torque sig-
nal generator 114 need not be exactly representative of the
snug point and may be an approximation, for example, a signal
representative of about 20% of the torque value expected at
lS the yield point would suffice. When the instantaneous torque
signal from amplifier 78 exceeds that generated by snug
torque signal generator 114, comparator 112 provides an out-
put signal to AND gate 108 which allows the feeding of the
signal from control comparator 98 to valve drive amplifier
110. The output of valve drive amplifier 110 is fed to cont-
trol valve 54, closing same and stopping motor 52. Thus, any
signals inadvertently developed by control comparator 98 in
the pretightening region, that is Region 1, of the curve
illustrated in Fig. 1 would not close control valve 54.
Finally, a reset switch 116 is provided which can be
utilized to clear the circuits and prepare the tool for a new
tightening operation with another fastener. One other point
that should be noted involves the fact that various prede-
termined relationships can be utilized to determine when to
stop the tightening cycle depending on the characteristic of
- 31 -
1~4~S
the torque-rotation curve. For e~ample, if the curve in-
cluded a temporary flattening at a known load less than the
load at the yield point, the tool could be utilized to stop
tightening at that point. Such a temporary flattening of
the~ curve could be caused by a particular fastener configu-
ration.
In the foregoing, there has been disclosed several
embodiments of the invention and it should be obvious to
one skilled in the art that various modifications and changes
can be made without departing from the true spirit and scope
of the invention as recited in the appended claims.
- 32 -
