THERMAL DIFFUSIVITY COEFFICIENTS OF CERTAIN TYPES OF CABLE RUBBERS AND CABLES

A. A. Tatarnikov, A. V. Voloshenko, V. I. Naplekov, V. M. Ostrast

(Presented by the scientific seminar of the Department of ATPPP)

At present, the determination of vulcanization regimes for rubber sheaths of cable products manufactured on continuous vulcanization units (CVU) is carried out by conducting preliminary experiments only at different pressures (temperatures) of heating saturated steam. Carrying out such experiments in production requires a great deal of time and expenditures on expensive materials from which the sheath of the cable product is made (rubber, carbon black, etc.). Therefore, a preliminary analytical calculation of the vulcanization regimes of rubber sheaths of cable products manufactured on CVU, taking into account the principal parameters influencing the vulcanization regime, is of great practical interest for the cable industry.

The calculation of the vulcanization process of the rubber sheath of a cable product is based on the calculation of its heating. To calculate the heating process of a cable product with a rubber sheath of the insulated-core type, it is necessary to have data on the value of the thermal diffusivity coefficient of production cable rubbers \(a_p\). For a cable, which is a body consisting of a rubber compound with copper conductors included in it, when calculating the heating process of the sheath it is necessary to know the equivalent value of the thermal diffusivity coefficient \(a_n^э\) as that of a composite body.

At present, the literature contains practically no data on thermal diffusivity coefficients either for production cable rubbers \(a_p\) or for the equivalent values \(a_k^э\). In the present work, the results of experiments on determining the values of \(a_p\) and \(a_k^э\) for certain types of production rubbers and cable products with a rubber sheath are presented below.

The coefficients \(a_p\) and \(a_k^э\) were determined in an \(\alpha\)-calorimeter by the regular-regime method [1]. The \(\alpha\)-calorimeter consisted of the following main parts: a thermostat, a mirror galvanometer of the GNZ-47 type, and two resistance boxes of the R-33 type. The thermostat was an insulated metal tank measuring \(400 \times 250 \times 250\). To increase the heat-transfer coefficient from the liquid to the test specimen, a stirrer in the form of an impeller was installed in the thermostat. The impeller rotated at a speed of \(\sim 400\) rpm from an electric motor. Water with melting ice was used as the cooling medium. The range of temperature variation of the test specimen during the experiment was \(20\text{—}25^\circ\mathrm{C}\).

Preparation of the rubber specimen for testing was carried out as follows. A vulcanized rubber sheet measuring \(140 \times 210\) and approximately \(2\) mm thick was cut into 8 pieces, which were then glued with rubber cement in the form of a parallelepiped. After processing, the specimens had average dimensions of \(17 \times 43 \times 62\) mm. During preparation, the hot junction of a copper–constantan thermocouple with a wire diameter of \(0.1\) mm was placed in the middle of the specimen. The procedure for conducting the experiment and processing the experimental data was taken from [1]. For each type of rubber, 5 specimens were tested.

The average values of the experimental results for determining \(a_p\) are given in Table 1.

Table 1

Preparation of the cable specimen for testing was carried out as follows. A piece of cable with a vulcanized sheath, 160 mm long, was taken. The ends of the cable were carefully cleaned and leveled; then round plates with a diameter equal to the diameter of the cable and a thickness of approximately \(2\) mm were glued to them.

To avoid warping of the glued-on plates, they were additionally fastened to the cable ends with four metal pins \(\varnothing 0.5\) mm. The specimen was then placed under a press and kept there for 14–17 hours. Next, rubber plugs, each made of 2–3 layers of rubber with a total thickness of 4–6 mm, were glued to the ends. The plug thus obtained was additionally covered along the side surface with rubber, gripping the cable surface by 10–15 mm. Approximately in the middle of the cable, the hot junction of a copper–constantan thermocouple was inserted into the conductor with the aid of a needle. The hole into which the thermocouple was inserted was coated with adhesive. The cable specimen prepared in this way was placed in an \(\alpha\)-calorimeter, and the experiment was carried out in the same way as in testing rubber specimens. The experimental results

Table 2

Note: the value of \(\left(\dfrac{z}{R}\right)_d\) was determined after the experiment was carried out and the sheath was destroyed.

were treated as for an infinite cylinder. In addition, from the cooling curves the calculated values \(\left(\dfrac{r}{R}\right)_{\mathrm{p}}\) were determined for the point \(\left(\dfrac{r}{R}\right)_{\mathrm{b}}\) located on the insulation surface of the largest copper conductor in cross section. For each specimen the experiment was repeated twice. The average values of the results of the experiments for determining \(a_{\mathrm{k}}^{\mathrm{e}}\), \(\left(\dfrac{r}{R}\right)_{\mathrm{p}}\), are given in Table 2.

Conclusions

1. The presence of copper conductors has a substantial effect:

a) on the thermal diffusivity coefficient \(a_{\mathrm{k}}^{\mathrm{e}}\). Thus, for the tested cable types, \(a_{\mathrm{k}}^{\mathrm{e}}\) is greater than \(a_{\mathrm{p}}\) for the sheath by 11–16%;

b) on the position of the point of the cable sheath \(\left(\dfrac{r}{R}\right)_{\mathrm{d}}\), located on the insulation surface of the copper conductor with the largest cross section, as if deepening it by 11–16%.

References

1. G. M. Kondrat’ev. Regular Thermal Regime. GITTL, 1954.