DEVICE FOR MEASURING DEFORMATIONS IN DISCRETE CONTACT

V. I. Maksak

(Presented by the scientific seminar of the Department of Strength of Materials)

To solve problems associated with friction, wear, stiffness, reliability, and a number of other properties of the contact regions of solid bodies, studies of the deformation of discrete contact are currently being carried out [1, 3]. Experimental work occupies a significant place in these studies. The smallness of the displacements, whose order is determined by microns or even fractions of a micron, requires the development of instruments with high resolving power. Researchers have used various methods and instruments. Thus, the English scientist Rankin made use of electrical and optical methods. Thomson used the change in the amplitude of oscillations of a torsional pendulum. S. E. Khaikin, A. E. Salomonovich, and L. P. Lisovskii applied piezoquartz resonators [1]. Considerable work on the study of the deformation of discrete contact was carried out by I. R. Konyakhin using the instrument of A. V. Verkhovskii, as improved by him [2].

In the works of the above-mentioned researchers, the preliminary displacement was measured only in the direction of a single applied displacing force \(P\).

In the present work, a description is given of an instrument that makes it possible to measure the magnitude of the preliminary displacement in two directions from two mutually perpendicular displacing forces lying in the plane of contact and acting at various ratios. In addition, it is possible simultaneously to measure the magnitude of approach.

Description of the instrument

The upper round specimen 4 (Fig. 1) is pressed at its end face by a force \(N\) against the lower specimen 3, rigidly fastened by screws to the rotary holder 16, which in turn is fixed in the conical socket of cup 17 by means of nut 18 (the design and dimensions of the specimens are shown in Fig. 2). In the plane of contact, the upper specimen is displaced relative to the lower one by horizontal, mutually perpendicular forces \(P_x\) and \(P_y\) through traction ring 2. The force \(N\) can be varied from 0 to 50 kg, and the forces \(P_x\) and \(P_y\) from 0 to 35 kg by stretching the force springs 14 through rotation of screw pairs 24. The magnitude of the force applied to the contact is read from the scales on disks 1, whose rota-

Fig. 1. Diagram of the instrument

Fig. 2. Specimens: a) upper, b) lower

which is connected with the magnitude of the extension of the loading springs by flexible threads 20. Scales 15 have a large division value and serve as control scales. The extension of the loading springs was calibrated by weights and by a specimen made for this purpose—a dynamometer, placed during calibration in the position of the specimens being tested. To reduce friction during transmission of forces, pulleys 13 on ball bearings are provided. Pulleys 21 and the drums of disks 1 are mounted on center pins. Weights 12 are intended for tensioning the flexible threads and balancing the springs. Thus it becomes possible to create zero load on contact 19—a hinge allowing the specimens to rotate relative to the vertical axis. Screws 22 regulate the lengths of rods 23.

To measure displacement magnitudes, microcators 10 with a division value of 0.0002 mm were used. The displacement of the pointer is read with the aid of microscope tubes 11 through mirror prisms 9. The setting of the microcator pointer within the working range of the scale is carried out by a special screw.

The rods of the microcators that measure displacements in the plane of contact directly touch the side surface, polished to grade 12,

machining purity, of the side of the upper specimen. The stem of the microcator, which measures approach, is connected with the upper specimen through lever 7, which has an elastic hinge, needle 6, and disk 5, which has three points of support on the upper specimen.

By means of stand 8, the microcators are rigidly connected with the lower specimen along a collar located at a minimum distance from the contact surface; this makes it possible to reduce the influence of elastic deformation of the lower specimen on the magnitude of the measured deformation in the contact and to exclude completely, during measurement, the contact deformation between the lower specimen and the rotary holder. The point of application of the load to the upper specimen and the point of contact of the specimen by the microcator stem are located at a distance of 1 mm from the plane of contact, which makes it possible to reduce considerably the influence of elastic deformation of the upper specimen, as well as misalignments due to the moment formed when the forces \(N, P_x, P_y\) are applied.

The rotary holder makes it possible to change the position of the specimens relative to the direction of application of the forces by any angle. Thus, when the holder is rotated, it is possible to shift the specimens in the opposite direction.

Checking and Calibration of the Instrument

The use in the instrument of microcators with a scale division value of 0.0002 mm and observation of the pointer displacement through microscopic tubes make it possible to take readings with an accuracy of up to 0.02 micron.

To check the instrument for accuracy and stability, a one-piece specimen is used, made in the same shape as the lower and upper specimens together. As a result of repeated loading of the one-piece specimen by the forces \(N, P_x, P_y\), it was established that deformation in the direction of one of the applied forces has practically no effect on the readings of the microcators in the directions of the other forces, since the greatest magnitude of elastic transverse deformation in this case is about 0.01 \(\mu\).

To eliminate the temperature influence of the experimenter on the readings of the instrument, the control is placed at a distance of 2 m.

Repeated loading of a special elastic specimen showed that the instruments, over ranges of 1 division, likewise have a practically linear scale.

The error of the readings, according to the results of repeated calibration and checking in operation, is about \(\pm 2.5\%\).

The measuring limits of the instrument are 0–12 \(\mu\).

LITERATURE

1. I. V. Kragel’skii, V. S. Shchedrov. The Development of the Science of Friction. Publishing House of the Academy of Sciences of the USSR, 1956.

2. I. R. Konyakhin. Theory of Preliminary Displacements as Applied to Problems of Contacting of Parts. Publishing House of TSU, 1965.

3. N. B. Demkin. The Actual Area of Contact of Solid Surfaces. Publishing House of the Academy of Sciences of the USSR, 1962.