or in g units

where d is in ram, and G in kg-wt. The ratio

d 2

Jdm=3 14 G '

jdm 3, 14d~G i t = - - - - = 0 , 6 7 .

Jst G4,7d 2

Thus, in the above instances of falling or colliding bodies the actual acceleration is determined as

Jdm = 0"67 Jst"

(34)

(35)

NEW T E N S I L E T E S T I N G M A C H I N E W I T H CAM D Y N A M O M E T E R

B. M. G e r s h k o v i c h

Translated from Izmeritel 'naya Tekhnika, No. 12, pp. 16-19, December, 1962

Existing tensile test machines often limit, owing to the imperfection of their dynamometer mechanisms, the testing of samples with relatively large elongations. Thus, the widely-used and newly-produced tensile machines with pendulum dynamometers have a very low precision. The inertia and insufficient rigidity of the pendulum produces an error up to 10% in measuring force and strains even at medium deformation speeds [1, 2]. Pendulum dynamometers are therefore unsuitable for testing samples at high deformation speeds, which is theoretically es- sential for many types of plastic and visco-plastic samples.

Moreover, tensile machines with pendulum dynamometers require additional correction devices, reading and scale mechanisms for converting readings and automatically recorded strain curves from a sinusoidally rising load on the sample, provided by the pendulum dynamometer, to a linear relationship. This lowers considerably the ac- curacy of measurements [3].

These drawbacks are especially noticeable at higher deformation speeds and in testing samples at various operating temperatures.

Tensile machines with spring dynamometers are in practice unsuitable for testing samples with large elonga- tions or requiring high deformation speeds. The extension of the spring in theory provides a linearly rising load on the sample, but it requires systematic testing and calibration. Moreover, spring-loaded tensile machines must be provided with a lever system for samples with large strains, thus complicating their construction and lowering the accuracy of measurements.

The precision of even the newly-designed and manufactured tensile machines is inadequate, probably owing to these drawbacks. Thus, an instrument for testing plastics PPR-50 and test rak SPP-6 for controlling plastics have a relative measurement error up to J: 3~ [4], whereas GOST (All-Union State Standard) 9550-60 specifies an error not exceeding 1% in evaluating the modulus of elasticity by means of tensile tests.

The tensile machine RM210 for testing plastics, designed by I. V. Provinteev and the author of this article [5], which is free from these deficiencies, has been constructed, tested-out and checked at the All-Union Scientific Research Institute of New Building Materials.*

* The RM21O machine has been manufactured at the S. M. Budennyi Moscow plant. Fitter V. A. Elizarov assembled the machine. I. V. Provinteev and S. G. Ovchiev participated in its testing.

1004

The RM210 machine is intended mainly for testing plast ic and visco-plas t ic samples with large elongations (polythene, rubber, gut tapercha, b i tumen, resin products, asphalt , etc.) for tensile strength and for strain at high or low temperatures and a l inear ly rising load applied to the sample during testing. It is possible by means of this tensi le

machine with various at tachments to carry ou t a set of structural and mechan ica l tests on polymer samples, in- cluding tests for compression, shearing, bending, crushing, adhesion, etc.

Description and pr incip le of operation. The tested sample is p laced in the RM210 machine in the horizontal position, and the machine is provided with a screw drive, and driving and driven carriages. The loading mechan-

ism consists of a motor which transmits its torque by means of a chain drive to a feed screw. The la t ter converts the torque by means of a clasp nut into the re- quired effort for driving the carr iage to which one end of the sample is fixed. The other end of the sample is

fixed to the driven car r iage and through it to the dy- namomete r mechanism consisting of two weights sus- pended from cams. The driving and driven carriages are k inema t i ca l ly coupled to a dif ferent ia l recording

~ . mechanism which au tomat i ca l ly records the "load

versus extension" curve, with the extension plotted to Fig. 1. its natural s ize, i . e . , in the form of a curve which is

equal in length to the deformat ion of the tested sample.

eeed screw 11 (Fig. 1) is driven by motor t through gears 4 (z 2 = 34 teeth) and 5 (z a = 20 teeth) , in terchange- able gears 3 (z 1 = 20 teeth) and 6 (z 4 = 40 teeth), a chain drive and reduction gear 2. The feed screw is k i n e m a t i c a l -

ly coupled to clasp nut 14 whose one side is fixed to driving carr iage 13 which is displaced along guides 10. The

absence of roi l ing bearings in the driving carr iage is due to the fact that its fraction and j a m m i n g do not affect the accuracy of measurements since the carr iage is not connected to the dynamomete r mechanism. The driving carr iage

has a pin a t tached to a thread which is connected only to the drum of the di f ferent ia l recording mechanism.

Driven car r iage 16, which is connected to the driving carr iage by means of tested sample 15, is displaced on roil ing bearings along guides. The requirement of a min imum friction in displacing the carr iage is due to the kine- ma t i c character is t ic of the tensi le machine. In the absence of a deformat ion (extension) of the sample the vector of the driven car r iage friction force points in the direct ion of the loading force appl icat ion. In this case the friction force does not affect the accuracy of measurements. When the sample is e longated by the requisi te rise in the load

applied to i t , the vector of the friction force st i l l points in the direct ion of the loading force appl ica t ion , whereas the driven carr iage, being connected to the dynamometer weight , is affected by a force pointing in the opposite

direct ion. Thus the friction of the driven carr iage produces a measurement error when the sample is elongated.

Hence, this friction should be reduced to the min imum.

Both the driven and driving carriages are provided with gripping devices which have corrugated jaws for f iat samples or specia l holders for cy l indr ica l samples, or other types of tests, for instance, compression, shear, or other

tests. The driven carr iage also has a pin at tached to two threads which are connected to the drum and pen 8 of the di f ferent ia l recording mechanism, and two pins at tached to wires whose other ends are fixed to pulleys 22 of the dy-

namomete r mechanism.

The dynamometer mechanism consists of two cams 18 mounted on the same axle as pulleys 22 and pul ley 17 which is a t tached to wire 19 connected to damping device 20. The cams are at tached to wires from which weights

21 are suspended. The cams are also provided with counterweights for ba lanc ing the system during their rotation.

The di f ferent ia l recording mechanism consists of a l ight drum ~/ which rotates on roil ing bearings. Thread 9, which is taken over the drum, is connected at one end through tackle 12 to the driving carr iage and at the other

end to the driven carr iage. Pen 8 provided with an inkwell moves along guides by means of a thread which is con-

nected to the driven carr iage alone.

1005

The feed screw, and the driving and driven carriages are placed in a bath which has fixed to its bottom a coiled pipe connected outside the bath to a heating or cooling device. The sample is tested at high or low temperatures when placed into the solution with which the bath is filled.

The principal unit of the dynamometer mechanism consists of the cain which provides a continuous and linear- ly varying loading for the sample. As has already been pointed out this circumstance is of theoretical importance

for attaining greater accuracy of measurement. In addition to reducing the number of kine- ~ ' ~ matic links and mechanisms which serve to convert the nonlinear deflections of the dyna-

mometer into linear readings on the scale, the cam provides greater efficiency of testing. The complex relationship of the variation in the structure of macromolecules in a polymer to the thermoelectrical effect in various ranges and of various values requires, for

.._~~ ~ simplicity of determining the sought-for parameters, a linear rise in the applied force.

i The profile of the cam (Fig. 2, where Pi is the variable force applied to the sample; r 0 is the radius of the pulley; P0 is the load; r i is the variable radius; 1) is the cam; and 2)

8 ~4 is the pulley) is designed on the basis of the following considerations.

Since the rise in force Pi must be linear, the increase in r i for a constant factor K (scale Fig. 2. factor) must also be linear, i .e. , must vary proportionately to the angle of rotation of the cam,

which in turn must vary proportionately to the displacement of the driven carriage.

This condition will be met mathemat ica l ly if the value of r i is made to vary as an Archimedean spiral.

Pi-- Port , since Po=const & ro=const. Pi=l(ri. ( 1 ) ro

The sought-for profile of the cam must be provided with a corrected Archimedean spiral, since it is necessary to account for the vertical direction of the loading force.

The tensile machine operates in the following manner when testing a loaded flat sample for elongation. The tested sample is first fixed in the grip of the driven carriage, and then in that of the driving carriage which is placed by detaching the clasp nut from the feed screw into a position to fit the length of the sample. The pen on the recording drum which carries the mil l imetr ie graph paper is then set to zero (the origin of the elongation of the sample and of the force applied to it). The dynamometer loads are fixed on the basis of an integral relationship between the radius of the pulley and the minimum radius of the cam. The required speed of deformation of the sample is set by the appropriate interchangeable sprockets for the chain drive.

The motor rotates the feed screw and the clasp nut which is now coupled to it and connected to the driving carriage.

The driving carriage is displaced along its guides and pulls behind it the driven carriage through the sample together with the wires connected to the carriage and to the pulleys mounted on the same axle as the cams with their suspended weights. As the driving and driven carriages are displaced, the cams rotate about their axis and increase the leverage of the applied weight, i .e. , they increase the torque applied to the axis of the cams and the pulleys. The rise in the torque wilt follow a linear law owing to the shape of the cam and pulley profiles.

In the absence of an elongation in the sample the driving carriage, which is propelled at a given speed, will pull behind it the driven carriage at the same speed. The recording drum which is connected through a tackle to the driving and driven carriages will therefore not be affected, and the pen which is only connected to the driven carriage will draw a straight line on the drum.

With a rising load the sample may be extended. In this case the driving and the driven carriage will have different speeds of propulsion, since the driving carriage will be affected by one move- ment, whereas the driven carriage by two movements. It will travel together with the driving carriage due to the propulsion of the feed screw which affects it through the sample. The second movement of the driven carriage will be provided by the weight of the load, and its direction will be opposite to that of the driven carriage. As a result of this the vector of a driven carriage velocity wilt be smaller than that of the driving carriage when the sample is extending. The recording drum connected to both of them will, therefore, rotate. In conjunction with the movement of the pen which is only coupled to the driven carriage a "load versus

1006

extension" line will be plotted on the drum. Figure 3 shows graphs of the tested polythene samples (the recording was made on mil l imetr ic squared paper placed on the drum of the differential recording mechanism).

The technical characteristics of tensile machine RM210 are as follows: direct maximum load exerted on the sample is 50 kg-wt; operating drive of the extending device (the displacement of the driving carriage) is 700 ram; displacement of the driven carriage is 200 ram; displacement speeds of the driving carriage are 4.0, 4.68, 13.6 and

Elongation, cm

"E J

Fig. 3. 1) Transfer sample o f a length of 35 mm; 2) ditto, 30 ram; 3) ditto, 40 ram; 4) ditto, 33 ram; 5) ditto, 32 ram.

16.0 cm/min; relative error in measuring the force is J: (0.7-0.8)%; the characteristic of the dynamometer mechanism consists of its cams with a special profile and freely suspended weights which increase linearly the loading on the sample; maximum width of the sample is 100 ram, minimum width (without special mountings) is 10 ram; the feed screw has a diameter of 20 mm and a pitch of 4 mm; the gear ratio between the motor and the feed screw is: ima x = 119, and imi n = 39; the motor type is AOA21-4, N = 270 W, n = 1400 rpm; over- all dimensions of the machine are (length x width • height) 1600 x 450 x 1170 ram. Its weight is 150 kg.

Conclusions. A tensile machine has been developed for testing plastic and visco-plastic samples having the property of large elonga- tion under the effect of relatively small forces. The special feature of the machine consists in the use of a quick-response cam dyna- mometer which provides a linearly rising load for the sample by means of weights. The "load versus extension" curve is plotted auto- matical ly on a drum of a differential recording mechanism with the elongation (deformation) of the sample recorded in its natural size.

The tensile machine is also provided with a bath into which the tested sample is immersed in order to subject it to temperature variations. Thus the sample can be tested at high or low temperatures. This machine is capable of providing by means of special mountings and devices a set of structural and mechanical tests for polymerized ma -

terials.

L I T E R A T U R E C I T E D 1. Kh. N. Dement 'ev , Zavodskaya laboratoriya, No. 2 (1958). 2o L . T . Timoshuk and V. S. Zoteev, Zavodskaya laboratoriya, No. 1 (1959). 3. G. Sh. Izraelit , Mechanical Testing of Rubber and Guttapetcha [in Russian], Goskhimizdat, Moscow, 1949. 4. Vestnik tekhnicheskoi i ~konomicheskoi informatsii, NIITEKhlM, No. 1 (1962). 5. I. V, Provinteev and B. M. Gershkovich, Tensile Machine for Testing Plastics. Description of the Invention

Attached to Authors' Certif icate No. 132852.

D I F F E R E N T I A L M E A S U R I N G M E C H A N I S M S

A. I . S o l o v ' e v

Translated from Izmer i te l 'naya Tekhnika, No. 12, pp. 19-22, December, 1962

Examination of the structure of kinematic double-contact measuring mechanisms in modern systems of d imen- sions' production control in engineering has shown that the majority of these systems are a version of a lever dif-

ferential (Fig. la) .

The application of differentials as measuring mechanisms is mainly due m their properties of excluding from

the measurement results the effect of the run-out of the machined component.

1007