Misalinement Detector for Axial Loading Fatigue Machines

16 Pages·1969·8.848 MB·English

by  Donald C. Robinson

\
.f
I
UNITED STATES DEPARTMENT OF COMMERCE
Maurice H. Stans, Secretary
.
NATIONAL BUREAU OF STANDARDS A. V' Astin, Director
TECHNICAL NOTE 4BO
ISSUED APRIL 1969
Misalinement Detector tor
Axial Loading Fatigue Machines
Donald C. Robinson
Engineering Mechanics Section
Mechanics Division
lnstitute for Basic Standards
National Bureau of Standards
Washington, D.C. 20234
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Contents
Page
Abstraet 1
1. Introduclion 1
2, Misalinement Detector Design 2
3. lnstaLLation 3
4. lleasurement Frocadure 3
5. SensiüivitY 4
6. ResulÈs and Dise[ssion 4
7. Conclusions 5
8. References 6
Satigue llvê
large varfat
síred bendin
Ís
necessarY
testing
maeh
resulLs are,
note descr
mênt"
mininiaing
rnachÍning
Personal
Fåctener
s* PennsYl
í
trTrlmbers
of this
n
['ii¡
'l':-
MISALINIX'{E}{I DETECTOR FOR AXIAL LOADING FATIGIIE MACHINES
by Donald C. Robinson
ABSTRACT
Page A strain gage devíce for measuring changes
in alinemenË bet\ìreen the test fixtures of axial
I
loadíng fatigue machines is described. Varia-
tions in fixture orientaËion are determined by
1 measurement of the bending induced in a compliant
beam clamped in parallel mounts attached to the
L test fixtures. Changes Ín bending with applied
tension loads are an indicafion of alËeratíons
3 in the alinement bet\^7eerÌ the load transmitting
members of a machine. An approximately linear
J relationshíp was found to exist between changes
in bending deËermined by the detect.or and the
4 fatígue lífe of high strength fast.eners for vari-
-4 ous levels of misalinement induced in a fatigue
machine.
-5
Key words: Axial loading, fatigue machine,
-6 misalinement, Èest fixtures.
1. Introduction
In order for a fatigue tesLing machine t.o give significant result.s,
iL must produce the desired stress distribution within the specímen"
Fatigue lives of high st.rength metaL fasteners, for example, have shown
large varÍat.ions in tensíon-tension tests due to the presence of unde-
sired bendíng loads:k. For the quality control of such products, it
is necessary to detect undesired stresses due to misalinement of the
testing machine in order to determine when variations in the test
results are due to an inferior lot rat,her than due to the machine. This
note describes the development of a device for detecting such misaline-
menÈ
o
Varíous methods have been used by other investigators for
minimizing misalinemenL in a fat.igue machine. Some reguire preeision
machining of specimens and tthhee mmaacchhiinnee tteesstt ffííxxttuurreessL[l]ll::*kt"> t^ oOtthhe*r.
Personal ConrnunicaÈions with the Engineering Staff of the Precision
FasËener Division, Standard Pressed Steel Company, Jenkintown,
Pennsylvania,
,"r,
Ntlmbers ín brackets indícate the líterature references at the end
of Ëhis paper.
3-
alining procedures have been developed in which a misalínement in-
dícating device ís installed i-n a machine ín pJ.ace of, or in series witt¡ bending of
a specimen , This technique allows Èhe possibility of a change in to be effe
alínement when the device is removed" The device described in ttris
roef paonrt aixsi aal-blolea dtion gd emteaccth imneis faolirn etmhee nrta nogf et hoef llooaadds Ë arapnpslimediË tdinugií nmgem bers test Ifnix tour
tension-tensíon test while a specimen remains in place. changes ina the to a testl
aIínement of the machine are indícated by bending induced in irre ment, it i
detector which is attached Ëo the machine Ëest fixËures. Ëors ionaL
misinterpr,
onLy one p
2. I'lisalinement Detector Design plane with
A schematic of the misalinement detector ís shown in Fígure 1.
rts principal components are a beam, a flexure, and fíttings for
atËaehing the detecÈor to fatigue test fixËures. The beam ú7as con-
structed from an 11 in. (P79.4 mm) x I in. (25.4 rmn) x 3lL6 j:r." (4.g To mi:
mrn) aluminum bar. The center porÈíon of this mernber was red.ueed. to Èhe beam t.,
approximately 0.625 x 0.016 in. (15.9 x 0.4 mm). A four-arm, temper- ment bar, I
ature compensated bridge circuit constructed of foil-type, resistance clamped to
straín gages was bonded to the reduced section. rË was connected to fittings t,
measure bending and Ëo cancel axial strain components. is removed
The purpose of the flexure \¡ras to transmit the effects of bending One el
due to misalined test fixtures to the gaged sectíon of the beam. rÈ the flexurr
was requíred that the flexure have a low resisËance to axial motion and S ef er¡IS , aS
also provide for a connection to t.he test fixture surfaces having high the flexurr
stiffness ín the horizontal directíon. A leaf flexure, whose basic beam is fii
operating prÍnciple is gíven by Jones and young[2], r*á employed. gaged secË:
A
cross section through rhe flexure is shov¡n ín Figure 1. The flexure tightened.
leaves nere consLructed from 0"005 in. (0.13 mm) brass shims arranged
to províde parallel motion of a rectangular element havíng a 0.2g1
in.z (7.1 tot") surface area which was in contact with the surface of
the beam. Polytetrafluoroethylene tape vras applied Ëo each of these
surfaces and to the end of the beam to minimize the friction due to The br
s 1íding of the benr
specimeq ir
The flexure was used to clamp the upper end of the beam to a steel estimating
fittíng, and the lower end of the beam was bolted to another fitting" range of lt
Both fittings were clamped Èo the circumference of the test fixtures by f ollor¿s:
means of an adjustable metal clamp. In ord.er to mínimize the misaline-
ment betvteen the pair of fittings themselves, they were fabricated from 1. A¡
one píece of material and cut along a plane perpendicular to the mount.- i¡
íng surfaces during the last machiníng operation, Although these
fittings were designed for cyLindrical test fixtures, fittings for 2" B¿
attaching to other fixtures are feasíble.
3.
Re
In order to protect the compliant gaged section of the beam from mi
beíng damaged during assembly, cover plates which attach to the c(
btoy slocoreswens cthoemspel eptlealyte se nwchloesne tdh eth ed erteedcutocrti o!ünassbeeciÈngio nus. eIdË í ¡r-^orarsd neerc beefsoasmrary 4. Re
d
ln-
bending of the beam due to mísalinement between it.s mounting surfaces
ries witt¡
to be ef fecËively t.ransmitted to the st.rain gages.
in
this
In order to ohLain an absolute deLerminatíon of mÍsalinement, the
nembers test fíxtures could be compared to a precision alined surface external
oga
to a testing machine. Due to the beam configuratíon and gage arrange-
s in the
ment, it is dífficult to prevent the beam from transmitting undesired
3 torsional strains to Èhe compliant gaged sectíon which would be
misínterpreted as due ro bending. This device measures bending in
on1-y one plane. In this investigation, care \nlas taken to aline this
plane with the plane of bending.
e 1.
3. Installatíon
r
con-
To minimize aîy misalinement bet.ween the fittings which aËtach
(4 "8 the beam to the machine test fíxtures, a uniform steel fítting aline-
dto
ment bar, having the same nomínal size as the gaged beam, is first
emper- clamped to the fittings. The adjusËable clamps which fasten Ehe
s Ëance fittíngs to Lhe test fixtures are then tightened, the alinement bar
ed to
is removed and the gaged beam is ínstalled"
One end of the beam is fastened to the upper fittíng by means of
bending
Èhe flexure, and its other end is fastened to the lower fítring by
. IË
screüls, as shown in Figure L. To insure that the bearing surface of
t.ion and the flexure is in uniform contact vríth the beam, the upper end of the
ng high
beam is first connected, the scre\4rs attaching the plaÈes coveríng the
asic
gaged section are loosened and the lower end of the member is then
d.A
tightened.
exure
ranged
28L
4. Iuleasurement Procedure
ce of
these
The bending determined by the detector, which ís an indication
" t"o of the bending t.ransmitted between the fatigue test fixtures to the
specimeq is measured using a strain índicator. The procedure for
est.imating Èhe change in alinement betrùeen test, fixtures over Ëhe
a steel
range of loads applied during a tension-tension fatigue test is as
ttÍng
" follows:
tures by
isalíne-
1. Apply a static tension load to a specimen correspond-
ted from
ing to the mean cyclic load.
e mount-
SE 2" Balance the st.rain gage brídge.
for
3. Reduce the load to a value correspondíng to the
minÍmum cyclic 1oad, rebalance the bridge, and re-
tm from cord the indicated strairi.
l
beam
)cessary 4. Remove the misalinement detector before beginning
rr for a fatigue test" s
5. SensiÈivity
alineme
A calibration of the misarÍnement detector which was used to the fat
estimaËe its sensitÍvity was determined by recording the bending at fixture
itthse agtatgaecdh mseecntti ofintt iningdsu cedr¡. hbiyre v tahreio udse tseiczteo rs híms placed under one of tfeosrat Èan
\,/as clamped to a uniform
cylindrical surface. The calibratíon curve shown in Figure 2 the ave
determined by placing shíms under one fittÍng locaLed approximwaatesry the mea
5.0 ín. (127 nnn) from the gaged secrion whilã Èhe other end of the suggest
bise amca wpaasb lcel aomf pdeedte" rmFrionmin gth ãbseãn ddinagta l,e sits tish aens otÍnmea pteedrc ethnËa t otfh eth ed. eatxeicarolr nheoatd csl or¡
rjfseíigtxcrtÍatudeirindtey s et oxo opaffe rattiehe nmenc saemicodah ncib nh1yien o,ea ai. tdsL. / 4ts rüeinhne.n s í(tthoive ím tydm e) tidesic atamolrse oíis. i Ína,pftrË.u"aei*cn"hcnee dd wtobh yet nht heseu Ëb-est TbaobltlseT ha1,
rhe fix
smalles
6. Results and Discussion relaËÍvr
to 1.1
rn order t.o verÍfy thaË the bending determíned by the detecçor between,
r¡ras representative of the bendíng experienced by a specimen, a strain for vart
bgoafo.g late wbLr/ii4tdh gL tenh "ec ( ibr6ce nun*dit)i n dfgoi adrm .emeteerreamrs iunbreoidnl gi.b rye antha e"ion mgã piws"aarils"ín obenom nodeetn dtr hdtoee tbtehecentdo sírnu,g rf aicne a dlifiree cto. fr
mine Fteirg uteres t3 f,i xrtruasr efosu ninds btayl raetÈda icnh inag 5 1tr0re00 d rebtef c(t2o2r1 0to00 3N Í)n c. a(p7a6c irtmysh,r)o wdnia_
h3y sdhroawusL ítcw, oa rxeigailo nlosa, dríenpgr efasetingluede mbya dcahsinheed. Tanhde dsaotlaid p lrÍontetesd. ín Figure Thr
bdtpdtheayeoes ttras ettt i orcowf¿tilnxoaiod strcuo m0olrfienr .er9staeeh 5slse-au- , rË ecTAeËdu h hrfLvbteieseei nc snrie o dnwtaiedhnrhifeglcfiy c iä.cbfh.aite eesafstnoree rtnte hode atafrh sthec hTtoe habdrsero se inrbeltadea rníttücíedoaadísrn.t, ,egt ifst gohe hxea, pt rrterheyeríeg gesi inootewcnnaeo tdrew e sdhpbe eryTíetrnhsseee otËnhhËfeeed maaiconoen fgmmt eai bnpnlneoldisr aasíairr,rr:
Another investigation r¿as made in r¡hich misalinement Bal
Ín a fatigue machine ¡¿ith beveled washers" The washers, wfhùoasse i nfdruacted Èhe benr
surfaces r¡rere ground non-para1ler., \rrere pt."ãà-rrrder the bolt and benr
of L/4 Ln. (6 trun) diamerei sÈeeI fasreneis. 24,000 1bf (107h,0e0ad0s Linear,
fciaxptuacreiÈsy ,w eelreec t2ro Im/2e cihna ní(c6a41 ,n ranx)i ainl. dloiaamdeinËge r,.n"^ã rrlrr. was used, The teNs)t htni¡gehe ns tttl
fatigue
The values of bendíng índuced by r¿ashers with various angles at
mst(ea8inmn9i0sme Í uoNbmno) larltoesraen ad sgrsieli veo eg fsni v2r eri,en0nn 0 Tis0nar h lbbT loefa bf i(l"Be2- ,6 920ro",no0 . 0.TN0.h" )lreb,rs if1et1", 0fi"a0.sã0to e-lf¿bn eiff;a rigsL(4 idgh, 4uaM5ev0e Nr eNazns*) ätaaslsnl -sodãen im2ä r0b"h0rrey" .r"bf Mr,
knovrn to be particularry sensiti.r" to bending loads when subjected detectol
Lo tension-tension fatigue tests. .. The cuives in Figure 4 give FasÈenel
c(8o9m0p Nar)is aonnd o Èf hteh aev edaraÈgae f rfoamti gTuaeb lrei fIe foorf aan t.e ¡nãsiiao"n road of 200 1bf the sthyelv aJneínal
measured at 200 lbf (gg0 N) to determine wherher t"h+e*T hr,ee sbte nfidx.Ëínugr ewas
il
I
x
;r
li
alinement changed significantLy below the minímum load applied duríng
s used to the faÈigue tests. The data did not indicate any large ãhanges ín
bending at fixture alinement. for the two loads within the range of the fatigue
nder one of tesË and the alinemenË for the load below the test range. The data
o a uniform for a tensíon load of 200 lbf (s90 ñ) rras used for comparison with
ure 2 the average fatigue life of the bolts since Èhe larger scatter ín
proximr¿aatsely the measuremenrs aË 21000 lbf (8,900 N) and 11000 1bf (4,450 N)
rnd of the suggests that the orientation of the larger washers below the bÐlt
the detector heads rdas more crítical aÈ the large4 loads and this oríentat,ion Í¡as
¡f the axial not closely controlled" The questionable data are ldentified in
Table 1.
when sub-
Ëo the tesÈ
I by rhe The values for Èhe faÈigue life of the Ll4 ín" (6 mt) diameter
bolts and the bending determined by the misalinement detect.or when
rhe fixtures úrere aLined (0 degree washer) were most affected by Ëhe
smallesÈ angularity induced by the beveled washers, 0"1g degr.., roitn
relatively smaller changes being observed for larger angularities
up
to 1.1 degrees. The magnitude of the coefficienË of correlaËiot[f]'
e detecçor bet¡¡een the bending derermined at 200 lbf (g90 N) and rhe fatigue life
en, a strain for various washers nas approximetely 0.9; in Figure 5 is sho¡¡n a
he surface direct comparison of the bending at this load and the average fatigue
life of the bolrs.
ending in
a
cÈor,
shown
(76 nm) dia-
apaci ty, 7. Conclusions
¡d in Figure
s. The The device descríbed ln this report utilízes bending induced in a
where the complianË beam to detect changes in the alinement of the Lest fixËures
:ed in the of an axial loading fatigue machine for a range of loads applied during
represent.ed a Èension-tensíon test. The changes in bendíng with applied load are
r sets of an indication of alterations ín Èhe alinement of the load transmitt,íng
:d by members, which should be kept t,o a minimr¡m to prevent. excessive bend-
ing loads from beíng applied Èo a specimen"
v¡as índuced Based on the experimenËa1 measuremenÈs, the relaÈionship between
¡hose flat the bendíng determined by the deËecËor due Lo misalined tesÈ fíxtures
,olt heads and bending experienced by a specimen installed in the fixtures was
(107,000 linear" An approximately linear relationshÍp was found to exist be-
d. The teNs)t t\,)een the bending deÈermined by Ëhe detector and the faÈigue lífe of
high strength bolts for various levels of misalinement. induced in
a
faÈigue machíne.
angles
aË
and 200 lbf
s on the
assembly
Mr" R. Snyder assisted in the consËruction of the misalinement
, and were detecËor. lfr" T. BaumgarÈner, Engineeríng Manager of the precision
ubjected
4 give the Fastener Divisíon, standard Pressed steel company, Jenkintol¡rn, penn-
200 lbf sylvania, arranged for tesËs to be conducted on fatigue equipment at
Èhe Jenkíntoún plant.
rdíng
was
tixture è+i
The project which led to this report was conducted under the
sponsorship and with the financial assistance of the Naval Air Systems
Conrnand, Department of the
Navy"
8.
Referenees
1' I{eibull, w., Fatigue Testing and Analysis of Results, pergamon
Press (1961). b
2. Jones, R. V" and young, I. R., Some Parasitic Deflexions in
Parallel Spring l"Iovements, J. Sci. Instr., f3, p. 11-15 (Jan.
les6).
3. Li, J. C. R., StaËisÈical Inference, Vol. I, p. 301, Edwards
Brothers, Irlc., (L964) .
: the Table 1 - Straín Indicated by the l"lisalinement Detector Due To
-r Systems Beveled l{ashers InstalLed ín an Electromechanical
(a)
Testíng Machine
Applied Èension
Angle of 2000 lbf 1000 lbf 200 lbf
'gamon beveled washer 8900 N 4450 N 890
in./in in./in. in"/in.
deg
"
in
Jan. 58 x 10-6 69 x 10-6 85 x 10-6
0
0.18 L44 t60 L66
,rds
0.4L L76 ]-75 L77
(b)
0.62 1ae 162(b) ].76
0"93 22s(b) 223(b) 180
1"08 238(b) 2!r2(b) 215
(")rn"". values represent changes in bending found aÈ a load
of 2500 Lbf (11,120 N) and the indicated load'
lb)Q,r."tionable daÈa due to orientaËion of beveled washers.