D I S T O R T I O N M E T E R FOR R E M O T E M E A S U R E M E N T S

I . M. Z h d a n o v

Translated from Izmer i t e l ' naya Tekhnika, No. 7,

pp. 13-14. July, 1962

OF W E L D I N G D E F O R M A T I O N S

A remote registration and automat ic recording of deformations is often required in studying and controlling

welding deformations during the manufacture of meta l constructions.

The Order of Lenin Kiev Polytechnical Institute has developed for this purpose a lever distortion meter with an annular s t ra in-gauge e lement which transforms welding deformations into e lec t r ica l signals.

Figure 1shows the schemat ic of the distortion meter . Strain gauge 2 is fixed to instrument frame 1. The meas-

ured deformations are transmitted to the strain gauge by means of the equa l -a rm lever 4 with its adjusting screw 3.

The mechanica l design of the instrument and the method of fixing it to the measured surface have been des- cr ibed, in the main in [1].

The measuring circui t is supplied from a 6-V storage battery. The measurements are made by means of pyro- met r ic indicat ing or recording mi l l ivol tmeters and PP ca l ibra ted potent iometers without in termedia te amplifiers.

ht meter R, A

| -2

Fig. i. Fig. 2.

The strain gauge (Fig. 2) consists of ring 1 made of perspex (Plexiglas) with wire transducers Rl, Ra, R 3 and R 4

glued to it. The ring has a rectangular cross section. Its external d iameter is 50 ram, its axia l l ine radius is r = 24 - 24.5 ram, its width amounts to 8 mm, and its thickness is 6 = 1 - 2 mm.

The wire transducer networks are p laced on the ring in such a manner that their middle coincides with the axis perpendicular to the deformation direct ion of the ring. The length of the network equals 20 mm and is chosen so as to p l ace the transducers over sections with the maximum uniformity of strain distribution. For the above d i - mensions of the ring the difference in the stresses over the length of the network does not exceed 25 qo.

The transducers arewound with 0.030-ram constantan wire and have the same resistance of R = 250f~.

The strain gauges are glued with adhesives BF-2 and AK-20, which have been chosen after a series of c o m - para t ive tests. It has been found that by gluing the networks to a paper base with adhesive BF-2 and comple te trans- ducers to the ring with adhesive AK-20 the strongest adhesion of transducers is at tained, and the greatest s tabi l i ty of their operat ion.

Bearings 2 for fixing the ring and sleeve 3 are made of Plexiglas and glued to the ring with a solution of Plexi- glas in dichlorethane. A hardened steel insert 4 is pressed into the sleeve.

549

The s t ra in-gauge transducers are connected to a bridge c i rcui t with four operating arms whose resistances vary byA R. The variations in the resistances of opposite arms of the bridge have the same sign. For such a bridge

the coeff ic ient of sensit ivity is k = 1 and the sensitivity factor c = 1 [2].

The bridge is fed from a storage battery with a vol tage of U = 6V.

The potent ia l difference across the measuring diagonal is equal to [2]

AR a U = ~ : c U - - (1)

R

The re la t ive variat ion in the transducer resistance is

~ - = S a m = s --s (2)

w h e r e s is the coeff ic ient of the transducer 's sensitivity to deformations; e m and o m are , respect ive ly , the re la t ive deformation and the mean stress along the length of the transducer in the direction of its axis; g = 2.8. l0 s k g / c m 2

is the elast ic modulus of Plexiglas.

According to [3] the bending moment at any cross section of the ring is equal to

M=-~- (3)

At cross section A the maximum moment M A is ef fect ive , and at cross section B the minimum moment M B, across the length of the transducer. For a g ivenra t io of ther ing diameter to the length of the transducer, angle ~ B

= 25 ~ If we assume that M m =(M A + MB)/2, then

Mm=O.16OPr. (4)

Force P, which produces deformation A 1 in the ring, can be determined from [3]:

P r 3 h l = 0 . 149 ~-~,

where I is the moment of inert ia of the ring.

Considering that o m = M m 6 / 2I, we obtain

0.536. h l 6 E

From (1), (2) and (6) it follows that

(5)

(6)

~ccOhlbs aU=0.536 - - (7)

r 1

Thus AU, A 1 and ~ are re la ted l inearly to each other. It is possible by calculat ion to obtain the thickness 6 of the ring which is required to produce a potent ia l difference of A U ~ 17 mV for a maximum calcula ted de-

formation of the ring of A1.

The max imum ca lcu la t ed deformation of the ring A1 includes the measuring l imits for welding deformations

for a given measurement length.

Normally the direct ion in which a welding deformation develops is either unknown beforehand, or the deforma- tion changes sign during the process. Hence, the r ing,before measurement , is provided by means of adjusting screw 3 (Fig. 1) with an in i t i a l deformation which is equal to half the maximum ca lcu la ted deformation, and the readings

are thus taken from a conventional zero point.

550

For measurements over a length of 100 and 50 mm the maximum values of welding deformations in the ma- jori ty of prac t ica l cases do not exceed • 1.0 and • 0.5 ram. Deformation A 1 will then amount to at least 1 or 2 ram.

Calculat ions and experience have shown that for measurements over these lengths by means of pyrometric m i l l i -

voltmeters with a PP cal ibra t ion, i t is advisable to use rings with a thickness of 1 and 2 mm, respectively,

L I T E R A T U R E C I T E D

1. K .K . Kreno,~ and !. M. Zhdanov, Svarochnoe proizvodstov, 1959, No. 3. 2. Appl icat ion of Strain Gauges in Engineering[inRussian], Mashgiz, Moscow - Sverdlovsk (I956)o 3. S .P . Timoshenko, Resistance of Materials , v. 2 [in Russian], Gostekhizdat, M0scow-Leningrad (1946).

M E A S U R E M E N T S OF E F F O R T S A N D S M A L L D I S P L A C E M E N T S BY M E A N S

OF F R E Q U E N C Y T R A N S D U C E R S

I . N . V o l k o v

Translated from Izmer i t e l ' naya Tekhnika, No. 7,

pp. 14-16, July, 1962

Strain gauges made of thin wire or foil are extensively used for measuring stresses in various constructional

units, components and separate areas of components. These transducers are only applied once and are unsuitable for further use. This is one of the defects of these transducers. Hence, i t is necessary to produce strain-gauge in-

struments with transducers which can be used repeatedly.

In this work we describe a device for measuring stat ic and dynamic efforts and small deformations. This de- v ice is being used at one of the oi l fields for reproducing by radio te ledynamograms of deep pumping installations.

The device consists of a frequency transducer and a frequency meter connected to a direct reading instrument

or an oscil lograph.

The transducer consists of a germanium transistor LC-osci l la tor whose inductance varies according to thestress appl ied to it (due to the deformation of its base). Variations in the inductance change the frequency of the osc i l l a - tor. Since the disp!acements are reD' smal l , the relat ion between the frequency and the air gap is ! inear.

2.7~ L ~ c

Fig. 1.

output " - - - I t - - -

To a r eac t ance

~60M ~__~0rpF [ _ output

The transducer transforms variations in the effort in- to frequency changes:

af=bAr, (1)

where ZXF is the variat ion in the strain of the component under the effect of any load; AI is the change in the trans- ducer frequency; b is a comtant .

The variations in frequency, which are proportional to the stress, are registered by a frequency meter which consists of a phase-shift ing network from whose input and output the vol tage is fed to a phase meter. The phase m e - ter converts the phase difference between the vol tage at the input and output of the phase-shift ing network into a d c vol tage measured on a vol tmeter whose scale is ca l ibra ted in deformation units A1, or effort units F. The osci l la tor

c i rcui t ( th ree - te rmina l inductance) is shown in Fig. 1.

The generator supply vol tage is 2 0 - 2 4 V; the consumption of the transducer is m 0.5 W. The displacement of the transducer armature changes the inductance of the tuned coi l , which in turn produces Variations in theosc i l la tor

551