On the Coefficient of Resistance to the Motion of a Cutting Chain

I. G. Basov, A. N. Shitunov

(Presented by the Department of Mining Machines, Mine Transport, and Mining Mechanics)

Friction of the cutting chain against the surface of the guide groove of the bar always occurs in the presence of particles of the body being broken up. The presence of a medium determines the magnitude of the friction forces opposing the motion of the chain and, moreover, under the confined conditions of the bar groove leads to its “micro-wedging,” which causes nonuniformity over time in the resistance to the motion of the cutting chain, even without taking into account the forces of resistance to cutting any body. This circumstance does not permit the use of ordinary reference data on coefficients of friction when calculating the forces of resistance to the motion of the cutting chain, and requires experimental determination of the coefficient \(\omega_{\mathrm{ц}}\) of resistance to the motion of the cutting chain, by which is also meant the ratio of the pulling force to its weight.

Of greatest interest are data on the values of the coefficient \(\omega_{\mathrm{ц}}\) in the most energy-intensive section of the bar. For modern bars with guide sprockets, this will be the straight-line groove. In this connection, under laboratory conditions we experimentally determined the coefficients of resistance to the movement of cutting chains of the KMP type (single-bar, unbalanced) and “Ural-33” (barless, longitudinally balanced) on straight-line sections of bars with clean sliding surfaces, and also with a filling of coal slack and a wet sandy-clay mixture (the latter corresponds to the case of cutting frozen ground with a bar-type cutting unit).

The experimental setup consisted of a drive mechanism with a series of interchangeable drums for changing the chain-pulling speed; a lifting table on which the working units under study were installed; and a force transducer placed in the cut of a flexible filament connecting the chain with the drive drum. The pulling force of the cutting chain was sensed by a strain-gauge beam made of R-18 steel, with PKB-20 wire resistance gauges glued onto it. The sensor signal was recorded by an MPO-2 loop oscillograph together with a PET-3VM strain-gauge station. For more rapid determination of the chain-pulling force, a Norma microammeter was also used in combination with the strain-gauge transducer.

During the experiments, the chain-pulling speed was varied within the range \(0.22\)—\(3.0\) m/sec.

Processing of the oscillograms was carried out by the method of mathematical statistics set forth in [1]. The relative maximum

the error in determining the coefficient \(\omega_{\mathrm{c}}\) was 13%; the most probable error was 9.5%. The experiments carried out showed that the speed \(v_{\mathrm{p}}\) of motion of the cutting chain has a substantial effect on the magnitude of \(\omega_{\mathrm{c}}\), both under dry (clean) friction and under friction in a “medium” (Fig. 1). Intensive growth of \(\omega_{\mathrm{c}}\) (by a factor of 1.5–2) is observed in all cases when the speed \(v_{\mathrm{p}}\) is increased to 1.5–1.8 m/s. A further increase in speed to 3.0 m/s does not lead to any substantial change in the coefficient \(\omega_{\mathrm{c}}\).

Fig. 1. Dependence of the coefficient \(\omega_{\mathrm{c}}\) of resistance to chain motion in the bar groove on the speed \(U_{\mathrm{p}}\) of motion of the KMP chain (1, 2, 3) and “Ural-33” (4, 5, 6) under dry friction (1, 4), with the addition of coal fines (2, 5), and a wet sandy loam (3, 6)

Under dry friction, the coefficient \(\omega_{\mathrm{c}}\) for the KMP-3 bar-type cutting unit turned out to be even 10–15% lower than for the “Ural-33.” But when coal cuttings or wet sandy loam are present in the KMP bar groove, the coefficient \(\omega_{\mathrm{c}}\) increases by a factor of 1.5–2.5, especially at elevated cutting speeds. Such a significant increase in the coefficient \(\omega_{\mathrm{c}}\) for the KMP-3 bar can be explained by the fact that, when particles of coal or soil are present in its groove, a kind of wedging of the chain cams takes place. As a result, the motion of the chain acquires a clearly expressed dynamic character, owing to which the average force required to pull the chain also increases.

The “Ural-33” chain, owing to its longitudinal balancing on the rectilinear section of the bar groove, acquires a certain rigidity. The resistance to its displacement may be regarded as equivalent to the resistance to displacement of a long beam in guides. This is what provides it, in almost all cases, with a lower coefficient \(\omega_{\mathrm{c}}\) in comparison with the unbalanced planer chain of the KMP-3.

With additional loading of the “Ural-33” chain by 1.25 kg, 2.5 kg, and 4 kg per cam, and with variation of its speed of motion within the range 0.22–0.78 m/s, it was not possible to establish any regularity in the variation of the coefficient \(\omega_{\mathrm{c}}\).

Thus, as a result of the experiments it was established that the coefficient of resistance to chain motion on the rectilinear section of the bar:

1 — increases significantly with increasing chain speed up to 1.8 m/s, and then stabilizes;

2 — depends very strongly on the medium located between the sliding surfaces of the bar elements;

3 — depends substantially on the design of the chain cutting unit.

LITERATURE

1. I. Ya. Al’shits, F. V. Kostyukevich. Equipment and methods for investigating machines for coal extraction. Ugletekhizdat, Moscow, 1957.