SOME WAYS TO INCREASE THE DURABILITY AND OPERATIONAL SAFETY OF CABLES FOR HANDHELD ELECTRIC DRILLS

V. M. VYSOTSKAYA, V. I. ELGAZIN

(Presented by the scientific seminar of the Electromechanical Faculty)

The most frequently encountered cause of failure of handheld electric drills is damage to their cables at the entry point. This type of failure is especially dangerous if the electrified tool is operated in an explosion-hazardous environment. Such tools include, in particular, handheld electric drills used for drilling blastholes in the mining industry. These drills operate under shields, in furnaces, in longwall faces, i.e., in places with the greatest accumulation of methane and coal dust. The cables of these drills are in the most unfavorable conditions. They are constantly moved along the working face, being subjected to systematic bending and accidental mechanical effects.

As our observations have shown, the service life of the drilling cable of type GRShS $3 \times 4 + 1 \times 2.5\ \mathrm{mm}^2$ used for this purpose ranges from several shifts to 2–3 weeks. Then a fracture of the conductors occurs, often accompanied by their short circuit, which can ignite the explosion-hazardous surrounding atmosphere.

An analysis of the nature of the development of failures in the drilling cable showed that, under the action of systematic bending, stresses dangerous to the integrity of the metal arise in the wires of the conductors, and destruction of the conductors begins. The conductors located on the concave part of the cable section are destroyed first. This is explained by the fact that tension is less dangerous for the conductors than compression, since the latter leads to longitudinal bending. Examining the break points, one can note that the break occurs as a result of S-shaped deformation.

As the wires fracture, their bent sharp ends, acting on the conductor insulation at the moment of bending, puncture it. The same phenomenon also occurs with the adjacent conductor. This point is clearly visible in Fig. 1, where the insulation of the lower conductor has already been punctured, while that of the upper one is swollen and will soon be damaged. The molten pieces of copper formed during the short circuit burn through and ignite the hose sheath.

The investigations we carried out showed that the main cause of rapid cable damage is its unsatisfactory design. The choice of cable design must be closely linked

with the conditions of its operation, which was not done when designing the existing drilling cable.

The quality of a cable is determined, first of all, by its correct geometrical construction, by the selection of the required wire diameter and the angle of twisting of the wires in the strands, and of the strands in the conductor. No less important is establishing the angle of lay of the conductors in the cable. The lay angle and, as a consequence of this, the lay pitch adopted for manufacturing the cable have a considerable influence on its individual properties: strength, elasticity, and flexibility.

Fig. 1. Moment of failure development in a GRShS cable \(3 \times 4 \cdot 1 \times 2.5\ \mathrm{mm}^2\).

Cables subjected to systematic bending, which primarily includes cables installed on hand-held drilling machines, must be made with the smallest possible lay pitch of the conductors. Thus, in England, in accordance with the British Standard, the lay pitch of conductors is specified as from 2.5 to 6 outside diameters of the cable, whereas a GRShS \(3 \times 4 - 1 \times 2.5\ \mathrm{mm}^2\) cable has a lay pitch equal to 12.2 cable diameters.

Reducing the lay pitch is necessary so that each conductor in the bending zone is located alternately above and below the axis. This, first, reduces the tensile and compressive loads, which are partly replaced by less dangerous torsional loads on the conductors, and, second—and this is most important—leads to compensation of tensile loads by compressive ones. Indeed, the same turn of a conductor tends in one place to lengthen and in another to compress. The result of this is some displacement of it along its axis without the presence of residual deformation. But such displacement can occur only if there is good mobility of the cable elements. In a GRShS cable this is absent. Here, both the conductor relative to its insulation and the conductor insulation relative to the hose sheath cannot move during bending because of their strong adhesion.

The insufficient mobility of the cable elements is explained by the fact that, when the rubber insulation is applied by the hot method (on injection presses), a large amount of heated rubber flows in between the wires.

In addition to the cable design, the mechanical strength of the conductors is also affected by the design of the cable entry. A drawback of the existing design of the cable entry of hand-held electric drills is the method of fastening the cable and the inconvenience of charging it. The cable is fastened by local compression using a clamping ring. In this case, the mechanical tightening force is transmitted through the cable sheath to the conductors and eliminates their partial mobility.

When the cable is bent, the conductors, being unable to move along the axis, are deformed and, as a consequence, break.

Work to increase the durability and operational safety of the drill cable of hand-held electric drills was carried out by us jointly with the Tomsk Branch of NIIKPT and the V. V. Vakhrushev Electromechanical Plant.

To eliminate compression of the conductors and reduce deformation at the point where the cable exits the drill, it was decided to use a special fastening of the cable only by the outer protective sheath, without transmitting the mechanical tightening force to the conductors, and also to increase the stiffness of the hose sheath.

The idea of fastening the cable is that an additional cone-shaped rubber covering with a collar is attached to the cable-entry end of the cable (Fig. 2). The collar is intended to prevent the cable from being pulled out and to seal the joint, while the cone is intended to create increased stiffness in the section of the cable most subject to bending. The rubber covering was to be made by rubberizing or by gluing on a previously vulcanized blank.

Fig. 2. Improved design of the cable entry with an additional protective rubber covering: 1 — cable funnel; 2 — cone-shaped rubber covering; 3 — sealing washer.

Experimental samples of the rubber covering were made by rubberizing it onto the cable.

Industrial tests of experimental samples of the cable entry were carried out under the most severe conditions of panel stopes on steeply dipping seams in the city of Prokopyevsk. The tests were comparative and were conducted according to a special procedure. On the basis of the data obtained, it can be said that the modernization of the cable entry made it possible to increase the service life of the cable by approximately 2.5–3 times.

The main attention in developing new designs of drill cable was directed toward increasing the mechanical strength of the conductors.

Three types of cable were developed: KTG \(4 \times 4 \text{ mm}^2\), GI \(4 \times 4 + 1 \times 10 \text{ mm}^2\), and GI \(5 \times 4 \text{ mm}^2\).

The KTG \(4 \times 4 \text{ mm}^2\) cable corresponds, in number of conductors, to the standard GRShS \(3 \times 4 + 1 \times 2.5 \text{ mm}^2\) cable, but differs in the increased cross-section of the grounding conductor, as well as in the system of conductor stranding.

The GI \(4 \times 4 + 1 \times 10 \text{ mm}^2\) cable is intended for installation on electrified tools operating with remote control. It has a new spiral stranding system for the current-carrying conductors, possessing increased resistance to bending and twisting, and also has an increased cross-section of the grounding conductor.

The GI \(5 \times 4 \text{ mm}^2\) cable is made in a lightweight design. Its construction is similar to that of the GI \(4 \times 4 + 1 \times 10 \text{ mm}^2\) cable. The grounding conductor of the cable is more flexible than the power conductors. This provides it with increased mechanical strength.

The characteristics of the KTG and GI cables in comparison with the GRShS cable are given in the table.

Table

To ensure the greatest mobility, the insulation of the current-carrying conductors of the new cable was produced on a cold press; moreover, during stranding and sheathing of the conductors they were liberally talced. The cable conductors were subjected to tinning, which, in addition to increasing the mobility of the conductors relative to the insulation, significantly improved the microstructure of the metal.

Experimental samples of the cable were tested at the Kalinin mine of the Prokopyevskugol trust. The tests showed that the measures taken to increase the mechanical strength of the cable produced the desired result. Whereas the standard GRShS \(3 \times 4 + 1 \times 2.5 \text{ mm}^2\) cable operated during testing until conductor breakage for 3–4 days, the experimental samples of the three cable types, each having operated on average for 20 days, did not fail.

Subsequent opening of the cables showed that only the KTG \(4 \times 4 \text{ mm}^2\) cable had partial damage to the current-carrying conductors (14%); in the GI \(4 \times 4 + 1 \times 10 \text{ mm}^2\) and GI \(5 \times 4 \text{ mm}^2\) cables, all wires in the conductors were intact.

Tests subsequently carried out by us on cable models with continuous spiral stranding of the conductors (like a spring) showed an enormous increase in cable service life compared with the standard cable (by 60–70 times). But even such an enormous increase in cable service life does not completely exclude the possibility of cable damage and the appearance of an open flame source; therefore, along with increasing the mechanical strength of the cable conductors, lightweight designs of shielded cable and reliable protective apparatus should be developed.