Note: Descriptions are shown in the official language in which they were submitted.
S P E C I F I C A T I O N
This invention relates generally to the field of tool
driving or impacting, and more particularly to an impac~ type
wrench having a control system for accurately controlling the
tension in a fastener of a joint.
It is well known in the prior art that tightening a
O fastener 'o its yield point produces optimum joint efficiency.
A fa~ten~d joint havins a grea_er preload value up to the yield
poi~t o f the ma~erial of the joint is more r~eliable and insures
bet~er ~as~ener performance. ~ish fastener preload further
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increas~s fatigue resistance due to the fastener feeling less
added stress from external joint loading, and dynamically loaded
joints have less tendency to slip and loosen.
The prior art reveals various types of impact wrench
S control system3 for controlling the a unt of preload in a fasten-
er. On~ commonly used type employs som~ for~ of torque control,
in which the impact wrench tightens a fastener ~o a maximum pre-
determined v~lue of torqu~ and ther~upon shuts off. Example~ of
impact wranches utilizing torque control can be found in United
State~ P~t~nts to ~ ; Hall, No. 3,833,068;
Schoep~, M~, 3,703,933; Vaughn~ ~o. 3,174,559; Elliott et al, No.
3,018,866 and Maurer, No. Z,543,979. Another means of controlling
impact wrancho~ found in the prior art is commonly known as a
~turn-of-tho-nut" system, in which a fastener is tightened to some
pre8elected initial condition, such as a predetermined torque
value or spin~le speed, and thereupon rotated an additional pre-
determined nu~ber of degrees before shutting off. Examples of
vario~s turn-of-the-nut impact wrench systems are found in United
States Patents to Allen, No. 3,623,557; Ho2a et al, No. 3,318,390
and Spyradakis et al, No. 3,011,479. Another type of control
comprises i~parting a constan~ angular momentum of each lmpulse :
blow, such as found in the United States Patent to Swanson, No.
3,}81,672.
As can be seen from the numerous existing prior art
3ystems, the problem is not a novel one. The ultimate desired
r~sult is to achieve preload of the fastener into the yield region.
The common problem which each of the prior art sys~ems attempts to
solve is determining when the yield point of the fastener has been
reached. In all of the contsol systems described in the above-
no~ed pa~ents, prior knowledge of the fastener and joint character-
i~tic~ ~ust be known or assum~d in order to d~termine either the
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exact predetermined final torque, the exact amount of additional
rotation or the amount of constant angular momentum of each
impact blow. It is well known that tightening to a predetermined
preload condition, such a~ the yield point~ is a function of many
variables, ~m~ng them beir.g joint stiffness9 fastener stiffness,
surface friction between mating threads ancl thread form. Therc-
fore, in aach of ~he prior art systems the yield point cannot
~lways be accurately determined ~ecause the conditions of each
fasten~r and joint vary and may not be known in advance. Thi~
consequence can lead to uneven tightening from joint to joint in
a ~tructure, which can in turn result in loosening of the fastener
~ in the joi~t and premature fatigue failure.
- It is known from the characteristics of fasteners that
a yield phenomena occurs in the applied moment and the preload
simultaneously, so that preload can be controlled by stopping the
tightening process when the applied moment suggests that yield is
occurri~g. Because of the nature of operation of certain types of
wrenches,a contin~ous moment is not applied. For example, in an
impact wrench a series of pulsed impacts of a hammRr onto an anvil
advances the fastener into a workpiece. During each impact, when
the fastener has been tightened unt11 it presents maximum re-
; sistance to further rotation, the anvil which is coupled thereto,
also presents maximum resistance to further rotation and the peak
torque or maximum moment applied by the hammer is r~ached. At
this point, the hammer is subjected to its maximum deceleration
which is proportlonal to its maximum applied moment, and experiences
a recoil, the magnltude of which has been found to be proportional ;
; to the maximum deceleration of the hammer and thus of the maximum
applied moment. In the present preferred embodiment of an impact
wrench in accoxdance with this invention, the deceleration of the
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hammer in the form of its rotary motion i5 sensed by a recoil or
bounce back mechanism. The magnitude of the recoil, either its
duration, force, velocity or total distance of travel, give a
measure of the deceleration of the hammer and, hence, the maximu~
applied moment. However, it has been fow~d to be relatively easy
to mea~ure duration of recoil. Thu~ the recoil timR and the
angle of rotation can be ~onitored simult2lneou~1y, but a graph
showing one as a unction of the o~her is so~ewhat hypothetical
a~ recoil~ only occur at the end of a blow while angular dis-
placement occurs during a blow. ~y convention, therefore, th~graphs are plotted as angular displacement at constant moment
followed by a change of moment at constant angle.
,
, SUMMARY OF THE INVENTION
..:
? Accordingly, it is a general purpose and object of the
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present invention to provide apparatus for tightening a fastener
to the yield point or to some similarly significant point in a
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joint. It is another object of the invention to provide a control
system for tightening a fastener to its yield point and which is
particularly useful with a wrench that applies its tightening
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moment periodically. It is another object of the lnven~ion to
provide an impact wrench having an adaptive control system for
accurately tightening a fastener to a predetermined preload con-
dition and which utilizes measured characteristics of the fastener
and joint being tightened. It is still a further object of the
invention to provide an adaptive control system in an impact
wrench for accurately ti~htening a fastener to a predetermined
preload with minimum prior knowledge of the fastener and joint
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` characteri~tics. It is yet another object to provide an impact
. . .
wrench having an adaptive control system which determines the
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yield point of the fastener by measuring the magnitude of de~
celeration of the hammer after engagement with the anvll, and
issuing a stop control signal when no subsequent deceleration
values exceed a previous peak deceleration value by a predeter-
mined additional amount. It is still a further object to pro-
vide an impact wrench having an adaptive control system which
; measures the magnitude of recoil of the hammer after engagement
with the anvil, measures the angular displacement of -the output
shaft, and issues a shutoff signal to the wrench after a pre-
determined additional number of degrees of rotation subsequent
to measuring a peak recoil value which is not exceeded by sub-
sequent recoil values by more than a fixed or variable additional
amount.
These and other objects are accomplished according to
a preferred embodiment of the present invention by providing a
wrench such as an impact wrench having a control system including
means for developing a signal representative of the deceleration
of the hammer after engagement with the anvil which signal is
also representative of the applied moment, means responsive to
the deceleration signal for determining the yield point or some
similarly significant point of a fastener assembly and means for
producing a control output signal when the fastener assembly is
tightened to the yield point or similarly significant point.
In a further aspect of this invention there is provided
in an impact wrench including a hammer impacting with an anvil
to rotate an output shaft operative to tighten a fastener assembly
' to its yield point or some similarly significant point by applying
torque thereto, a control system comprising: means for developing
. a signal representative of the deceleration of the hammer after
engagement thereof with the anvil; calculator means responsive to
said deceleration signal for determining the yield point or some
similarly significant point of the fastener assembly; and control
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means responsive to said calculator means for produclng a control
signal when the fastener assembly is tightened to said point.
In a still further aspect of this invention there is
provided an impact wrench for tightening a fastener assembly
comprising: a motor; a hammer assembly adapted to be driven by
said motor; an anvil adapted to be rotatingly impacted by said
hammer assembly; wrench means operatively attached to said anvil
and adapted to drive the fastener assembly to applying torque
thereto; means for developing a signal representative of the
recoil of said hammer after engagement thereof with said anvil;
calculator means responsive to said recoil signal for determining
,~ the yield point or some similarly significant point of the fast-
:~,
ener assembly; and control means responsive to said calculator
' means for producing a control signal when the fastener assembly is tightened to said point.
In a still further aspect of this invention there is
provided a method of tightening a fastener assembly -to its yield
point or some similarly significant point by applying torque
thereto with an impact wrench of the type including a hammer
impacting with an anvil to rotate an output shaft operatively
coupled to the fastener assembly comprising the steps of:
developing successive signals representative of the recoil of the
hammer after engagement thereof with the anvil; determining
~.
the yield point or some similarly significant point of the
fastener assembly based upon a predetermined relationship of
said recoil signals; and producing a control signal when the
fastener assembly is tightened to said point.
. .
In a still further aspect of this invention there is
provided apparatus for tightening a fastener, said apparatus
comprising: wrench means for periodically applying a tightening
.
:` moment to a fastener in a joint assembly; first means for measur-
ing the moment applied to the fastener during each per:iod and
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for developing a signal representa-tive of the peak moment applied
- during each period; and control means responsive to said peak
!` moment signals for de-termining when a peak moment signal has not
increased by more than a predetermined amount and for developing
; a control signal.
BRIEF DESCRIPTION OF THE DRAWI~GS
Fig. 1 is a side elevational view of an impact wrench
contructed according to the invention partially cut away and in
cross-section, showing an angle encoder and sensing means;
; 10 Fig. 2 is a front elevation view of the angle encoder
; shown in Fig. l;
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Fig. 3 is a transv~rse sectional ~iew taken along the
line 3-3 of Fig. 1 looking in the direction of the arrows,
showing the recoil detection apparatus;
Fig. 3A is a partial transverse sectional view
schematically illustra~ing another embodiment of a recoil
detection apparatus usable with this invention;
Fig. 4 is a block diagram of the control circuit of
the impact wrench of Fig. 1, and
Fig. 5 is a graph showing the various parameters
during the operation of the wrench.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Before proceeding with a description of an apparatus
in accordance with this invention, a brief explanation of a
method in accordance with this invention will be explained.
Referring briefly to Fig. 5 of the drawing there is disclosed
a curve (PRELOAD IN FASTENER) illustrating the relationship
between the preload induced in a fastener tightened by a
periodically or cyclically operated tool such as an impact
wrench and elapsed time during the tightening cycle. From the
noted curve it can be seen that initially the preload increases
rapidly and eventually levels of~ so that only small a~ditional
preload is induced in the fastener. This leveling off occurs
at about the yield point and continues through the remainder of
the tightening cycle. Similar phenomena are observable in the
relationship between applied moment and time as illustrated in
curve L (RECOIL TIME) and curve O (PEAK VALUE). It is merely
noted here that recoil time is representative of the applied
moment.
In accordance with this invention a fastener is
tightened to its yield point by applying a tightening moment to
the fastener and periodically measuring the applied moment.
Preferably the moment is applied periodically and the pea}c moment
applied
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7~75Z
during each period is determined. ~y ~pe~ mvment~ is meant the
largest moment applied during each period. The instantaneous
peak moment is compared with the largest pe~k moment which has
been applied previously during the tightening cycle to determine
if the instantaneous peak moment exceeds the previous la~ges'
peak moment by more tha~ a predetermined am~)u~t. The predetermined
amount may vary slightly for fastener~ of different types but it
ha~ b~en d~term~ned that the predetermlned amount i3 normally about
2~ of the previous large~t peak moment in which case the predeter-
~ned amount i~ variable. It hAs al~o been determined that the 2~
ca~ be approximated and an absolute value can be u-~ed, ior example,
2~ of the peak moment expected to be applied at the yield point.
If the instantaneous peak moment exceeds the previous
largest peak moment by the predetermined amount, the application
of the tightening moment continues and the instantaneous peak
ment is ~tored for comparison with the next instantaneous peak
moment; if the instantaneous peak moment does not exceed the pre-
vious largest peak moment by more than the predetermined amount the
application of the tightening moment can be discontinued since this
indicates that the fas~ener has been tightened to its yield point
as 8hould be understood from the explanation of the relatio~ships
between preload and time and between moment and time.
Referring briefly to curve L(RECOIL T~ME) in Fi~. S of
the drawing it can be ~een that during some periods before the
fastener has been tightened to its yield point the instantan~ous
peak moment is less than the largest previous peak moment. These
occu~rencas are random in the sense that they are not predict-
able and it is possible that the application of the tightening
moment could be discontinued before the yield poin~ is reached.
Accordingly, it is desira~le to not discontinue the application of
the tightening moment until the instantaneouq peak amount has not
3777~'~
exceeded the previous largest peak mom~nt by the predetermined
~mount for a predetermined number of successive period~ during
which the moment is applied. While t~o such detections are
sufficient, three to five is preferable. It h~s been found most
preferable to meaRure a second tightening charasteristic related to
tha period during which the moment is applied, for example, to
mea~ure angul~r xotation o~ the fa3tener during the tightening
cycle, and to ~ot di~continue the application o~ the tightening
moment until the insta~taneou~ peak moment has not exceeded the
previou~ large~t peak ~oment by the predetermlned amount during
a predetermin~d rotation of the fas~tener, for example, during 15
tO 25 degre~. In this way, it can be assured that the applied
moment i9 op~rative to cause rotation of the fastener even though
the torque i9 levelling off. It should be understood that other
characteristic~ could be measured instead of rotation so long a~
these other characteristics are related to the moment in the same
general way as rotation. That is, any characteristic related to
the moment such that the momentlevel~ of~ with respect to that
characteri~tic can be used in place of rotation. Time, for example,
can be used.
. The described metho~ could be performed by hand, but an
apparatus performing the method will be described. While any
type o~ wrench system applying torque periodically can operate
t perform the method, the preferred emb~diment disclosed herein
is an impact wrench.
Referring to Figs. 1, 2 and 3, an impact wrench lO is
shown, which may be any one of many conventional types that in-
clude an external source of compressed air suitably connected to
the wrench in order to ~uccessively impact a hammer onto an anvil.
An ~nvil 12 i~ rotatably secured within the forward portion of the
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wrench housing 11 by a bearing 13. The forward end 14 of anvil
12 comprises, for example, a ~quare drive for attachment to a
drive socket or some other suitably shaped wrenching member for
driving a fas~ener. A hammer assembly 15 connected to and dxiven
by a conven~ional air motor (not shown~ surrounds and contacts
anvil 12 imparting impact blows thereto to rotate the anvil and
drive a fastener (not shown)~ Wrench 10 a:Lso includes a con-
ventional trigger 22 which, when depressed, allows air from the
external source ~not shown) to enter wrench 10 at an inlet por~
23 connected to an air ~otor ~not shown) driving hammer 15 to
rotate anvil 12.
A bidirectional incremental encoder 16 used in a sys~em
:~ for measuring angular rotation of the fastener is suitably fixed
to anvil 12 for rotation therewith within the forward portion
of wrench housing 11, such as, for example, by key 17 mating with
a corresponding recess 18 in anvil 12. Since the anvil 12 drives
the wrenching mem~er driving the fastener, the encoder 16 rota~es
with the fastener as the fastener is tightened. ~etween i.mpacts
of the hammer 15 against the anvil 12, the anvil and encoder 1~
- recoil, but the fastener does not. Thus the rotation measuring
system in which the encoder 16 i~ used should be ca~able of de-
; tecting and di~regarding the recoil of the encoder. Holes 21 are
each located at a fixed radius on encoder 16. A pair of sensors 19
and 20 are suitably mounted in the forward end of housin~ 11, each
: at a fixed radius from the center line of anvil 12 so that they
line up radially with holes 21. Sensors 19 and 20 are preferably
of a ma~netic type, that is, could include an induction coil whose
~utput varies due to the presence or absence of metal, but any
other suitable proximity type sensor may be used to detect the
passage of ~uccessive ones of hole~ 21 during operation of the
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7752
wrench. As can be seen in Fig. 2, encod~r 16 in the prefexred
embodiment contains eighteen ~18) equally spaced holes, the
center lines of each hole being twenty ~20~ degrees apart at a
fix~d radiu~ from the centar line of the encoder. As will be
explained later the outpu~ signals of the sensors 19 and 20
are ninety ~90) degrees out of phase ~o the sensors are spac~d
a~art to provide that result. Thus, the sensor~ 19 an~ 2n could
be sp~ced apart a distance equal to the sum of five ~53 degreas
plu8 ~ome whole numbe~ multiplied by twenty ~20~) degr,ees, for
example twenty-five (25~, forty-five t45), sixty-five ~65),
etc. degrees. Resolution with this encoder i~ 72 counts per
revolution as will also be explained late~. It should be under
,,
stood that the encoder could contain any reasonable number of
hole~ depending on the degree of accuracy desired, the only
L5 requirement being that the holes are spaced equally apart from
each other. A proximity type sensor 24, which also can include
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an induction coil similar to sensors 19 and 20, is mounted at the
bottom rear portion of the wrench housing for measurinq decelera-
tion in the form of recoil or bounce back o f the hammer. As
noted previously the deceleration of the hammer is proportional
to the pea~ moment applied during each impact.
Referring now to Fig. 3, the bounce back or recoil
indicating mechanism is shown. An output shaft 30 from the air
motor (not shown) is connected t~rough a conventional one-way
clutch 31, to a rotatable cannister 32 having an arm 33 extending
from the surface thereof. The arrows on clutch 31 indicate that
the normal direction of ro~ation of shaft 30 is clockwise when
viewed in a direction opposite the arrows on line 3- 3. Clutch
31 transmit~ rotational force to cannister 32 when hammer 15,
whi~h is suitably connected to rotate shaft 30, rebounds off of
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anvil 12 in the counter-clockwise direction when viewed in a
direction opposite the arrows on line 3 3 after imparting a blow
thereto. Cannister 32 is located inside of cutout 34 at the
rear portion of wrsnch housing 11. A spring 35 i9 at~ached at
one o its ends in some suitable manner at a point 36 adjacent
the di~tal end o~ arm 33, and at its other end at a point 37
adjacent the bottom of w~ench lO. Spring 35 i~ typically an
elongated coil spri~gt but may be any oth~r suitable elastic
te~3ioni~g d~vice for exerting a downward force on arm 33. An
end stop 38 i9 ~ounted at the bottom of wrench lO and extend~
upwardly at an angle with its distal end 39 proxim~te the sensing
end of sensor 24.
; Operation of the bounce back or recoil detection
apparatus will now be described. On each successive impact of
the hammer onto the anvil, as the fastener i9 rotated the energy
stored in the hammer a~d anvil drops to a point where resistance
to furth~r rotation caused by tightening of the fastener in the
workpiece begins to occur. Upon further tightening, a deceleration
oi the hammer at the end of a blow in the form of a recoil occurs,
the duration, total displacement, velocity and force of the re- ?0
coil being proportional to the applied moment. The force of the
recoil is transmitted through shaft 30 and clutch 31 to cannister
32~ which is initially in a position indicated by the dotted lines
in Fig. 3 with arm 33 resting on distal end 39 of end stop 38.
~- ~he force of the recoil causes shaft 30 and cannister 32, coupled
5
together by clutch 31, to rotate in a counter-clockwise direction
looking forward ~clockwise as seen in Fig. 3), causing arm 33
to move upwardly off o end 39 of stop 38 against the restoring
force of sprinq 35. This restoring force causes arm 33 to return
to it~ initial position resting on distal end 39 of stop 38 after
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some finite duration of time which is proportional to the recoil
energy, and thus the deceleration of hammer lS. Sensor 24
measures the duration of time it takes arm 33 to complete its
~ cycle. The duration of time is, as mentioned hereinabove,
5 dependent upon the amount of recoil energy tran~mitted ro~
hammer 15 to ~haft 30, the maximum amount of recoil snergy
occurring at approximately th~ maximum preload in ~he fastener,
~ at or n~ar the fastener yi~ld point. It sho~ld be understood
~hat either the di~tanc~ t~av~lled or veloclty of arm 33 travel,
or force exerted by spring 36 o~ pin 37 could also be measured,
.. a~ they ~re all proportional to hammer deceleration and thus the
,,.^
applied momsnt. Other parameters proportional to the applied
moment can al80 be measured, for example, the rotation of the
~ fastener.
In another embodiment of the recoil detection apparatus
" shown in Fig. 3, clutch 31 could be replaced ~y a viscous
Newtonian fluid 31A suitably co~tained between shaft 30 and
cannister 32 as shown in Fig. 3A. Viscous drag force of the fluid
would then transmit the recoil force of the hammer which is
coupled to sh~ft 30, to cannister 32 in the same manner as clutch
31 illustrated in Fig. 3. For a more complete descxiption of a
ne-way fluid clutch, reference is made to United States Patent,
No. 2,521,11~, issued to G. B. Du~ois et al. Measurement of the
. total duration of the recoil woul~ be exactly as described above.
: Referring to ~ig. 4, a control system is shown for
controlllng the tightening cycle of wrench l0. The coils of
sensors 19 and ~0 are supplied with a suitable voltage and as the
encoder 16 rotates, the sensorsoutputs vary depending on whether
~ a hole 21 or the l~tal between holes is adjacent their ends. ~or
: example, the sensors 19 and 20 can be arranged ~o pro~ide a high
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output when metal is detected and a low output when it is not.
The output signal from sensor l9 is fed into an amplifier 40, and
the output signal from sensor 20 is similarly fed into an am-
plifier 42, in order to amplify the respective angle signals to a
magnitude at which they are compatible with the rest o~ the control
sy~tem. Signal A from amplifier 40 .is characteristically 90
out of phase (~) with signal B from amplifier 42, the signals
having a ch~racteri3tic ~quare wave shape the pulse width of
which are proportional to th~ radian spacing between hol~s 21.
The square wave shapa of signals A and ~ can be a~sured by using
Schmitt triggers in the amplifier circuits. Output signal A
from amplifier 40 i~ fed concurrently into a first nostable
multivibrator 44 having a positive trigger, a second monostable
multivibrator 46 having a negative trigger, and pulse sorting
logic 48 which separates pulses produced by forward and reverse
recoil rotations of angle encoder 16. Logic 48 will be described
in greater detail hereinbelow. Output signal B from amplifier
42 is fed concurrently into a first monostable multivibrator 50
having a positive trigger, a second monostable multivibr~tor 52
having a negative trig~er and pulse sorting logic 48. Output
signal C from multivibrator 44 is characteristically a sharp
pulse corresponding to the positive going portion of signal A,
and output signal D from multivibrator 46 is a pulse corresponding
to the negative going portlon of signal A. Similarly, o~tpu~
s.ignal E from multivibrator 50 lS a pulse corresponding to the
po~itive going portion of signal B, and output signal F f~om :
multivibrator 52 is a pulse corresponding to the neyative going
portion of signal B. Signals C, D, E and F are each introduced
into pulse sorting logic 48 along with sign~ls A and B. The
pulsas produced by forward a~d reverse rotations of anyle encoder
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16 are separated in logic 4~, which yields output signals G, each
.representing an increment of forward rotation of the encoder 16,
and H, each representing an increment of reverse rotation of the
encoder 16. Signals G and H are fed into a counter/storage unit
S0 which counts the number of forward and reverse rotation pulse~
and stoxes this information. Unit 50 may typically comprise a
synchronous 8-bit up~down binary coun~er w:hich include~ two 4-bit
binary c:oun~era in cascade. Counter/storage unit 51 act~ as an
inhibi'cor of forward rotation pulse~ G through a NAl!ID gate 53
and i9 arranged to count up forward rotation pulses G and count
down reverse rotation pulses H. Counter/storage unit 51 is further
arranged so that it provides a low input signal to NAND gate 5
when it i~ set to zero or is counting up from zero and so that
it provides a high input signal to the NAND gate when it is count-
ing down from zero or counting up to zero. These inputs to NAND
gate S3 are preferably provided by placing a signal inverter
between the output of counter/storage unit 51 and NAND gate 53
and by havin~ the counter/storage unit output a high signal when
it is set at 2ero or counting up from zero and ouput a low signal
: when it is counting down or c~nting up to zeroO The signal
inverter, a~ is conventional, inverts the output of counter/storage
unit 51 before it is fed to N~ND gate 53. Thus, signals G cause
NAND gate 53 to dischar~e only when unit 51 is set at zero or is
counting up from zero.
In addition a second NAND gate can be placed between 25
the output of NAND gdte 100 and the input of signal G to counter/
storage unit Sl so that signals G are fed to unit 51 through this
second NAND gate. For its other input the second NAND gate
receives the output signal from counter/storage unit 51 before
:~ that signal is inYerted.
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Operation of this preferred arrangement will now be
explained. When tightening of the fastener commences, forward
-~ rotation pul~es G are discharged by NAND gate 100 and provide
inputs to the s~cond NAND gate and N~ND gate 53. The output from
counter/~torage u~t 51 is high since the unit i~ set at zero and
thi3 high ~ign~ receiv~d a~ the second input to the ~econd
NAND gate. Thu~ pulses G are not fed to count~r~storage unit
80 it remaln~ set at ~exo. The output from counter/s~orage unit
51 i~ inve~ted by the inverter so that the secor~d input to NAND
gato 53 is low. Thus, pu~ses G cause NAND gate 53 to output a
high ~ignal to mono~table multivibrator 54 causing i~ to outpu~ a
~ignal.
If encoder 16 recoils between an impact of hammer 15
against anvil 12, NAND gate 100 does not output ~ignal G and NAND
gate 98 output~ signals H which are fed to counter~storage unit
51 and counted down. The output of unit 51 i5 now a low signal
which is fed to the second NAND gate and which is inverted and
`~ fsd to NAND gate 53 as a high signal. When forward rota~ion
pul~es G are provided by NAND gate 100 indicating forward rotation,
the second NAND gate discharges to counter/storage unit 51 and
are counted up. At the same time the pulses G canno~ feed past
NAND gate 53 because of the high input signal from the inverter.
When the forward rotation p~lses G equal the reverse rotation
pulses H counter/storage unit 51 counts zero and its output goes
high. As noted previously, when unit 51 outputs a high siqnal,
signals G are not counted up and are fed thro~gh NAND ga~e 53
to monost~ble multivibrator 54. From the preceding it should
be understood that recoil pulses are made up and slgnal I is
representative of an increment of fastener rotation. Signal I
i~ characteri~tically a single step function. The output from
gate 52 i~ fed into a
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monostable multibibrator 54 whose output signal J is fed into a
selectable ring counter 56, which produces an output signal R after
a predetermined number of forward rotation pulses between 1 and 10
has been received, as will be more fully explained hereinafter.
Counter 56 ~ay also be referred to as a divide-by-lO counter/
divider with ten decoded outputs, and is typically a pair of
5-bit shift registers connected serially. Ou~put signal J from
multivibrator 54 is thus a pulse repre~enting an increment o~ net
orward angular rotation of encoder 16.
~he output signal from sensor 24 is fed into an amplifier
58 which yields an output signal K representative of the magnitude
of the total time for arm 33 (Fig. 3) to move off of, and return
to re~t upon end 39 of stop 38. It should be understood that
force, velocity or distance of recoil could al50 be used with
equally successful results as they are each similarly proportional
to the applied mome~t. Since the rotation of the fastener is
proportio~al to the applied moment, another technique for develop-
ing a ~ignal representative of the applied moment of eaci- lmpact
; would be ~o measure the rotation of the fastener during each im-
pact. The coil of sensor 24 is supplied with a suitable voltage
and its output varies depending on whether arm 33 is seated on the
end 39 of stop 38. For example, sensor 24 and amplifier 58 can be
arranged to provide a high output when no metal is detected and a
output when metal is detected. Signal K, which is a s~uare
wave whose width is proportional to total recoil time, is fed into 25
a ramp generator 60 which produces a characteristic ram~ function
output signal L whose amplitude is proportional to the duration
of signal K. Signal L is then fed into a peak value detector
and storage u~it 62 which stores the maximum or peak value of
recoil tLm~ from sensor 24. Peak value detector and storage unit
- 16
' ~
.
. ~ .
~61777S~
` 62 is generally conventional and includes an amplifier (not shown)
- for detecting whethsr an instantaneous signal L has increased and
a storage unit (not shown) Eor storing the largest signal L plus
a pradetermined increment as will ba explained. The storage unit
can be in the form of a capacitor arrangement. The amplifier
:~ receives input signal L ~rom ramp generator 62 and also the ~ignal
~tored in the storage unit so that it can ~etermine whether the
instantaneous signal L is larger than the stored signal. If it
i~ not the amplifier provide~ no output; if it i5 the amplifier
~ 10 outputs th~ larger signal to the storage unit and provides ano~her
output to a pea~ value increase detector 64, which i5 typically
a monostable multivibrator, producing an output pulse M. Output
~ignal M from detector 64 is characteri5tically a sharp pulse
and is fed simultaneously into an exclusive NOR gate 66 and a
step generator 68 which outputs a signal N which increases the
instantaneous signal L stored in the storage unit of peak value
detector and storage unit 62 by a fixed or variable amount for
each input pulse M received. Output signal N from step generator
68 i5 a square wave of short duration and fixed amplitude. As
will be more fully explained in the description of the operation 20
of the control system, a fixed ~alue of voltage may be added
; (lOOmv, for example), or a fixed percentage of the maximum stored
peak recoil value may be added ~2%, for example). The increased
peak recoil value output signal from the storage unit is fed
back into the amplifier for comparison with incoming signal L.: 25
The storage unit o~ the peak value lncrease and storage unit 62
is also fed as an ~utput signal 0, indicative of the increased
peak value, into a voltage comparator 70, which is typically an
operational ampli~ier, receiving a second input signal from a snug
:' torque setting unit 72. Signal O has a characteristic stepped
- 17
.' .
, ,~ .
:. .
.: ,
~77~i2
r~mp function profile. Unit 72 may be any suitable variable volt-
age producing device, such as a potentiometer, in which a voltage
proportional to so~ne determinable snug torque is generated. By
snug torque i9 meant the torque at which the fastener has pulled
the joint part~ to~ether and wherein prelo,~d ~s being induced.
; The v~ltage l~vels from detector and stora~e unit 62 and ~etting
unit 72 are compared in comparator 70, and when the first i5 at
lea~t equal to the ~econd, an output signal P from comparator 70,
i fed into NOR gate 66 which also receive~ a~ a ~cond inpu~
the signal M from detector 64. Signal P has a characteristic
signle step function shape. As is conventional, NOR gate 66
will provide a high output signal Q only when it has two low
input ~ignaLs or two high input signals. Thus t before the
fastener ha~ been tightened to its snug torque and with no
; 15 increased peak v~lue signal from the storage unit in unit 62,
that i~, with both input~ low, MOR qate 66 outputs signal Q which
reset3 co~nter 56 to zero. When the snug torque is reached, signal
P i9 fed f~om comparator 70 so that the NOR gate does not output
:. signal Q and counter 56 can now count. If, after the snug torque
'~ i9 reached, a signal L exceeds the previous maximum signal L
. 20
by the predetermined amount added by signal N, monostable multi-
. vibrator 64 outputs signal M to NOR gatè 66 so that signal Q is
again ~ed to counter 56 resetting the counter to zero. Thus,
if the instantaneous peak applied moment does not exceed the
prevlous maximum peak applied ~oment by the predetermined amount
. over a~ interval of rotation equal to a predetermined number of
count~ multiplied by the predetermined increm~nt of rotatlon sensed
by the encoder, then counter 56 will output a signal R which is a ~:
single step function amplifed in amplifier 74 and fed to the coil
, ..
; of a conventional solenoid valve 76 for shifting the spindle of
, - 18
:' .
~ .
the valve to its closed position. Solenoid valve 76 is placed
in the air supply line to the impact wrench so that when the
spindle is shifted to its closed position, the air Cupply to
~ort 23 of wrench 10 is closed.
Still .ef~rring to F~g. 4, pul~e sorting logic 48 will
b~ doscribed in greater detail. Logic 48 includes a plurality of
N~ND gatos 78, ~0, 82, 84, 86, ~8, 90, 92v 94 and 96, each havi~
: two inputs a~d ~ single output, and 4-input NAND gates 98 and 100,
each havl~ four input8 and a single Outpllt. Gate 78 receivc~ a
~ign~l C ~t ~ first input terminal and sicJnal B at a second input
ter~inal. Gate 80 ~eceives gi~nal B at both input terminal~.
Gate 82 receive~ signal B at a first input terminal and ~ignal D
; at a second input terminal. Gate 84 receives signal E at a fir~t
input terminal and sig~al A at a second input terminal. Gate B6
receives signal A at both input terminal~. Gate 88 receives
lJ signal F at a first input terminal and signal A at ~ second input
~erminal. Gate 90 receives signal C at a fir~t input terminal and
a signal ~A, representing the output signal from gate 80, at a
second input terminalO Gate 92 receives signal D at a first input
~erminal and ~ignal AA from gate 80 at a second input terminal.
Gate 94 receives signal E at a first input terminal and a signal
,~ BB, repre~enting the output from gate 86, at a second input
terminal. Gate 96 receives signal F at a first input terminal
and signal B8 from gate 86 at a 5econd input terminal. Gate 98
receives a signal CC, representing the output from gate 78, at a
fir~t input terminal, a signal DD, representing the output from
gate 92, at a second input terminal, signal EE, representing the
output fro~ gate 94, at a third input terminal, and signal FF,
repxes~nting the output from qate 88, at a fourth input terminal.
Ou~put ~ignal H from g~te g8 is representative ~ the reverse
-- 19
~,
.-
~v a f 0 ~
rota~ion pul~os only of encoder 16. Ga~e 100 receives an input
signal GG~ representing the output from gate 90, at a first
input terminal, a signal HH, representing the output from gate
82, at a ~econd input t~rminal, a signal II represen~ing the
output from gate 8~, at a third input ter~in~l, and a ~ignal U,
repre~enting the output fro~ ~ate 96, at a ourth input terminal.
Output signal G from gate 100 i~ representativa of the forward
rotation pulses only of encoder 16.
A~ should be clear from the preceding description, in
the circuit comprising the pulse sorting logic 48, each transition
from high to low or from low to high of each signal A and B is
oper~tive to cause either of the NA~D gates 98 or 100 to provide
~ s~gnal ~ndicating the encoder 16 has experienced a predetermined
increment of rotation. Since two transitions occur in each of
:
~ two encoders, each hole 21 causes four tr~n~itions per revolution.
:: 15
; Since there are eighteen (18) holes in the encoder 16, the encoder
~- has a resolution of seventy-two counts per t~rn (four multiplied
by eighteen) which in turn means that each signal G and H
`;: repre~ents five (5) degrees of rotation (360 ~ 72)- For each five
; (5) degrees of forward rotation of the encoder, NAND gate 100 out-- 20
puts the pulse G and for each five ~5) degrees of reverse rotation
ox recoil of the encoder, NAND ga~e 98 outputs the pulse H.
; Operation of the pulse sorting logic should be clear :
from the preceding description, but will be explained briefly.
- Assume that encoder 16 is rotating in the ~orward direction, that
25 is, that the fa~tener is being tightened by the impact of hammer
:~ 15 on anvil 12. Assume further that signal ~ is eXperiencing a :;
low to high tr~nsition and signal B, ninety degrees out of phase,
is low. Undex these conditions, pulse C is produced by monostable
multivibrator 44, and monostable multivibrators 46, 50 and 52
. :
,
~:~7~75;~:
have no output. NAND gate 78 receives high input pulse C and
signal B which is at its low level so output signal CC is high;
NAND gate 80 receives the low input signals ~ so output signal
AA is high; NAND gate 82 receives a low input siganl B and low
input signal D so output signal H~ is high; NAND ga~e 84 receives
the low inpu~ aignal E and high input signal A so output signal
II is high; NAND gate ~6 receives ~kae high input signals A so out-
put ~ign~l BB is low; and NAND gate 88 receive~ high input signal
: . A and low input signal F so output signal FF i~ high. NAND gate
90 receiYes high input pulse C and high input signal AA so output
signal GG i8 low; NAND gate 92 receives high input sicJnal AA and
low input signal D so output signal DD is high; NAND gate 94
; receive~ low input signal E and low input signal ~B so output
slgnal EE is high; and NAN~ gate 96 receives low input signal
and low input signal F 90 that output signal U is high. NAND
gate 98 receives high signal CC, high signal DD, high signal EE
and high ~ignal FF so ther~ i5 a low output signal. NA~D gate
". :
100 receives low signal GG, high signal ~H, high signal JI and
,- high signal U so there is provided a pulse G representative of
an increment of forward rotation.
" 20
At the instant signal ~ e.YperienceS a low to high if
~; encoder 16 is rotating forward, signal A is still high so that
monostable multivibrator 50 provides output pulse E while the
output of monostable multivibrators 44, 46 and 52 remain low.
Working the logic through the various NAND gates it can be seen
that NAND gate 98 receives high input signal CC, high input signal
DD, high input signal EE and hiyh input signal FF so there i5 a
low output signal. NAND gate 100 receives high input signal GG,
.~ high input signal HH, low input signal II and high input signal
~ so outpue pulse G is provided.
- 21
"'
. , :
77~S2
At the instant signal A experiences high to low transi-
tions, if encoder 16 is still rotating forward, signal B is still
high so that monostable multivibrator 46 provides output pulse D
while th~ output of mono~ta~le multivibratoss 44, 50 and 52 remaln
low. Working the logic through the various NAND gates it can be
seen that NAND gate 98 receives high input signal CC, high inp~t
signal DD, high input signal EE and high inp~t signal FF so th~re
is a low ou~put ~ignal. NAND gate 100 receiv~ high input qignal
GG, low input ~ignal H~, h~gh input signal II and high input signal
0 U 50 output pulse G is provided.
At the instant signal B experiences a high to low transi-
tion, if encoder 16 is still rotating forward, signal A is low
.qo that mono9table multivibrator 52 provides output pulse F while
the output of monostable multivibrators 44, 46 and S0 remains low.
. 15 Working the logic through the various N~ND gates it can be seen
- that NAND gate 9~ receives high input signal CC, high input signal
DD, high input signal EE and hiqh input signal FF so there is a ~ :
low output signal. NAND gate 100 receives high input signal GG,
.~ high input signal HH, high input signal II and low input signal U
. so output pulse G is provided.
; Assume now that encoder 16 is rotating in the reverse
; direction, that i5, that the encoder is recoiling between impact~
of hammer lS on anvil 12. Assume further that signal B is e~-
- periencing a low to high transition and signal A, ninety degrees
out of phase is low. Under these conditions, pulse E is produced
by monosta~le multivibrator 50 and monostable multivibrators 44,
~6 and 52 have no output. NAND gate 78 receives low input signal
C and sig~ B which is high so output signal CC is high; NAND
gate 80 receives the high input signals B so output signal AA is
: low; NAND gate 82 seceives high input signal B and low input signal
- 22
. ~
, . . . .
. - . . .
~L~77752
D so output signal HH is high; NAND gate 84 réceives the high ln-
~ put pulse E and low input signal A so output signal II is high;
.; NAND gate 86 receives the low input signals A so output signal B~
is high; and NAND gate 88 receives low input signal A and low in-
put signal F iso output signal FF is high. NAND gate 90 receives
low input signal C and low input signal AA so output signal GG
: i8 high; NAND gate 92 reGeives low input ~ignal AA and low input
~ signal D iso that ou put signal D iis high; NAND gate 94 receives
.: the hlgh input pulse E and high input si~nal BB so output signal
~E is low; and NAND gate 9~ receives high input signal B~ and low
. input signal F so output signal U is high. NAND gate 98 receivQs
.. ~ high input ~ignal CC, high input signal DD, low input signal EE
~ and high input signal DD so there is provided a pulse H representa-
- tive of an increment of reverse rotation. NAND gate lOO receives
high input signal GG, high input signal HH, high input signal II
': 15
~r. and high input siynal U so there is a low output signal.
......
At the instant signal A experiences a low to high transi-
.~:; tion, if enc3der 16 is rotating in the reverse direction, ~ignal
. . .
i5 still high so that monostable multivibrator 44 provides outpu~
; pulse C while the output of monostable multivibrators 46, 50 and 52
. remain low. Working the l~ic through the various NAND ~ates it .
, . . .
. can be seen that NAND gate 98 receives low input signal ~C, high
~ .
;,~, input signal DD, high input signal EE, high input siynal FF so out-
, ~
i`~ put pulse H is provided. NAND gate lOO receives high input signal
ii~.
: GG, high input sicJnal HH, hi~h input siynal II and high input
.~.......... signal~u~so there is a low output signal.
At the instant signal B experiences a high to low transi-
tion, if encoder 16 is still rotating in the reverse clirection,
signal A ii~ still high so that monostable multivibrator 52
provides output pu}se F while the output o~ monosta~le multi-
vibrator~ 4~, g6 and 50 remain low. Working the loyic through th~
. .
: - 23
. ~
~"
~7775Z
various NAND gates it can be seen that N~ND gate 98 receives hlgh
input signal CC, high input signal DD, high input signal EE and
1OW input signal FF so output pulse H is provided. NAND gate l00
recaives high input siganl GG, high input signal HH, h.igh input
3ignal II and high input signal U so there i~ a low output ~ignal.
At the instant ~ignal A experiences a high to low transi-
tion, if ancoder 16 i8 still rota~ing in the reverse direction,
signal ~ i~ still low so that. monostable multiYibrator 46 provide~
output pul~e D while ~he output monostable multibibrators 44, 50
and 52 re~ains low. Working the logic through the variou~ NAND
gates it can be seen that NAND gate 98 receives high input signal
CC, low input signal DD, high input signal EE and high input
~ignal FF ~o output pulse H is provided. NAND gate 100 receives
high input signal GG, hiyh input signal HH, high input signal lI
and high input signal ~ so there is a low output signal.
Operation of the control system will now be described
with ref~rence to all of the figures and particularly with refer-
ence to Figs. 4 and 5. As the impact wrench begins to tighten a
fastener, sensors 19 an~ 20 detect the passage of holes ~1 of
encoder 16 and provide si~na1s A and H which are processed to
provide pulses G representative of anguldr increments of rotation
as explained previously. Pulses G are fed to the NAND gate 53
which also receives the signal from the inverter between the out-
put of up/down counter storage unit 51. Since no reverse rotation
slynals have been produced, the output of unit 51 is high and of
: the inverter is low. Thus, with the low input from the illverter,
each pulse G applied to the NAND gate 53 causes a high ou~put
signal which ires the monostable multivibrator 54 which produces
output ~ignal J similarly representative of the predetermined
incre~ent of rotation. As previously explained ~ignal J is fed
.
- 2~ -
1`'' ~
: ~7~7~;Z
to the ring counter 56. After a preset number of pulses have been
counted in counter 56 it produces output signal R. During the
initial tightenlng impacts, counter 56 is continually reset to
zero by signal Q so that it cannot count the preset number of
pulse~ and, of course, so that signal R cannot be provided. Re-
ferring particularly to Fig. 5, initial tightening produces a.~
steady increa~e in the angle of forward rotatio~ of encoder 16 t
a~ ~hown by curve J at 102, with no corresponding increase in
`;- either fastener preload or recoil time as indicated by curve ~.
Ai should also be clear from curve L, snug torque has not yet
been applied to the fastener nor has the applied moment increased
by more than the predetermined amount so that comparator 70 and
peak value increase detector 64 have low output signals. Thus
, exclu~ive NOR gate 66 outputs signal Q. It should be noted that
; successive pulses 5hown in curve J each denote a 5 increase in
forward rotation of encoder 16 in the paxticular oscillographic
record shown here for illustrative purposes. Actually the amount
of forward rotation between pulses can be set at any deslred value
, . . .
~epending on the degree o~ accuracy desired. When the fastener
has been tightened sufficiently, causing it to contact a mating
workpiece (not shown), a preload beglns to build up in the fastener
as shown by the preload curve at 104 in Fig. 5. The preload was
obtained by well known external instrumentation means (not shown)
for purposes of explaining this invention, but it should be under- -
stood that usually such instrumentation means is not utilized. At
this point in the tightening cylce no measurable recoil of the
hamner against the anvil in the wrench occurs. Upon furthertightening, sufficient resistance to further rotation is encountered
causing the hammer to recoil upon striking ~he anvil, as shown by
~urve L at 106. It should be understood that recoil time is
~- - 25
.....
:.,.
. '! ,
,~. .
7775
~`
dependent on the residual strain energy ~tored in the im~act
wrench driving shaft sockets and couplings, and this strain
energy is dependent on the moment being applied, which moment
varles with the instantaneous coefficient of friction as the
fastener ~tops rotating. If signal L is equal to or exceeds some
electrically equivalent predetermined snug torque value, which may
be experimentally d~termined a~d set ~y adjusting ~he output from
unit 72, si~nal P is fed to NOR gate 66 so that output signal Q
which reset~ counter 56 to zero is discontinued and the counter
starts counting forward angle rotation signals J. It has been
determined that the s~lection of a snug torque value from unit 72
is not critical to the operation of the wrench. The cri~eria
used in selecting a snug torque value is that it be set hlgh
enough to assure that preload is beginning to build up in the
fastener, but that it not be set too high in the event that a
maximum recoil value might occur before counter 56 is allowed ~o
count forward rotation pulses J. In the pre~ent preferred embodi-
ment, the snug torque value was set at the level of the first peak :
recoil value in stor~ge unit 62 and in practice is an ap~roxima-
; tion of the torque required to build preload in the fastener.
Signal L representative of the peak recoil value at 106
is stored in the storage unit of peak value detector/storage unit
- 62 and the amplifier unit in unit 62 outputs to peak value increas~
detector unit 64 providing output pulse M which is fed t~ ~tep
generator 68 and NOR gate 66 causing signal Q to res~t counter
. 56 to zero. Step generator 68 in Fig. 4 causes the previously
highe~t recoil pulse L stored in unit 62 to be increased ~y a
preset fixed or variable amount, thus building into the system
; successively higher recoil values than the previously highest
stored value. For example, for the system shown by curve L
: 30
-- 26
:`;
. .
~L~777~
of Fig. S, an incremental fixed amount of abcut 100mv is added
for a peak value store of approximately 6 volts. This incremen~al
i value may be varied depending on the accuracy desired. The
practical cons~ralnts on this incremental value are that it be
small enough ~o that subscquent higher peak recoil values are
detected, but that it be large enough so that subsequent peak
recoil value~ ju~t slightly greater than the previou~ly stored
highe~t peak recoil value do not continue to reset counter 56.
lt should ~lso be u~d~rstood that a fixed percen~age of the
previously ~tored highest peak recoil value could be added,
such as two percent (2%), for example, with equally effective
: result~. It can be seen from Fig. S that the initial peak recoil
value of curve L at 106 causes curve O to increase to a first
3tored peak value at 108. The peak value at 108 of curve O is
; 15 exceeded by the recoil 110, that is the applied moment exceeds
the applied moment at 106 by the previously described predeter-
mined fixed amount. As described signal M ~see 114 curve M~ is
produced causing NOR gate 66 to discharge signal Q resettin~
counter 56 to zero and causing step generator 68 to increase the
value of ~he ignal L at 110 to be increased by the predetermined
amount. This increased peak value is then stored in unit 62~ as
indicated by curve O at 112. Counter 56 then must be~in counting
forward rotation pulses J again. The next peak recoi1 value at
. 116 exceeds the previous peak value at 110 by the predetermined
: fixed amount and in the manner described causes peak value curve
O to increase as shown at 118 and produce reset pulse 120 on curve
:- M. Peak value 118 is stored in unit 62 until the next peak recoil
. . .
value lZ2 of curve L occurs, which value exceeds previously
highest peak recoil value 116 by the prede~ermined amount. A new
peak value ~hown at 12~ of curve N occurs and a reset pulse 126
, 30
;.
~ - 27
~ ,
.:,
,~:
77~
on curve M is produced. Once again counter 56 is reset to ~ero
and ~tarts counting forward rotation pulses J. Subsequent recoil
.~ignals 128, 130, 132 and 134 do not excee~d prevlously highest
recoil value 122 by the predetermined amou,nt, so that no higher
. 5 p~ak value of curve N occurs after 1~4, no.r does a reset pulse
:~ on curv~ M occur after 126. Counter 56 i~ then allowed to count
successive forward ro~ation pulses 136, 138, 140, 142 and 144
of curVe J without interruption. In the present preferred em-
bodiment repre~entated by Fig. 5, the prese~ number of pulses
prGgrammed into counter 56 is five (5), thus causing a st~p signal
146 of curve R to be ~enerated. Stop 5ignal 146 is then fed into
the control coil of solenoid value 76 to shut off the air supply
to port 23 of the impact wrench. The number of angle pulses
before shutoff of the wrench after the ~reviously highest stored
peak recoil value can be varied by adjusting the preset programmed
.: value of counter 56. As shown by the fastener preload curve,
no significant further preload is induced in the Pastener beyond
approximately the third angle pulse 140 after the previously
highest stored peak recoil value 124. Thus the optimum shutoff
point for the present preferrad embodiment occurs between angle
:. 20
.. pulses 140 and 144 (i.e. 15 25 degrees of rotation after the
last re~et pulse 126), but the counter is set at five (S) pulses
to insure that the fastener has reached the yield point.
~ Having thus described the structure and operation of a
-~ preferred embodiment of an impact wrench contxol system, somP of::: 25
the many advan~ages of the present invention should now be readily
,, .
apparent. The control system provides a highly accurate Ind
reliable means for ti~htening a joint ~o the yield point, that is,
Por providing maximum preload in a fastener tightened by an impact-
`~. type wrench, that is, a wrench wherein the tightening momenc is
,~'- 30
- 2~ -
'~
..,,. ~
~ al7~
.
applied periodically. Since the control system is adaptive!
only minimal prior ~nowledge of the joint and fastener character-
i~tics being tightened need be known in orc!er to insure tighteninq
to the maximum attainable preload of the fastener, namely the
yield point. As previously stated, tighteniny to maximum preload
at the yield point of the fastener material insures a joint of
max$~um efficiency with greatest resistance to loosening due to
vibration and fatigue failure. The tightening cycle is very
rapid, making the wrench ideally 5uitable for rapid dsselnbly
line use. In addition to tightening fastenerto the yield point
it should be understood that the method and apparatus according
to this invention can be used to tighten fasteners to a similarly
significant point, for example, preloads other than the yield
point, by bullding into the fastener system a confiquration
causing the fastener to deform at a predetermined preload such
that the applied toxque levels out.
Obviously, many modifications and variations of the
present invention are possible in light of the above teachings.
It i5 therefore understood that within the scope of the appended
claims, the invention may be practiced otherwise than as specific-
ally described.
. .,
,'
.,
i:,
. . .
;,,
;-
..
2 9
'
