O. D. Alimov, I. G. Basov, F. F. Zelinger

ON THE WEAR OF A CHAIN WORKING MEMBER WHEN CUTTING FROZEN SOILS

The widespread use of cutting frozen soil with chain working members—bar cutters—is restrained by the insufficient number of bar cutters produced by industry and by the relatively rapid wear of the cutting teeth and chains.

The literature contains a wide variety of data on the wear resistance of cutting chain members. Some [1] assert that bar cutters successfully cut rocky soil, while others [2] state that, when even soils of low strength are broken up, a set of cutting teeth is consumed in one shift, and the chain wears out in six shifts. There is an opinion that, in order to obtain greater wear resistance of the cutting teeth and chains, it is advisable to break up frozen soil at low speeds of their motion, up to 1.5 m/sec, and with the bar sunk under its own weight [2–3].

The authors carried out investigations of the cutting regimes of frozen soils and observations of the wear resistance of the cutting teeth and chain. These observations were conducted with the use of various bar-type earth-cutting machines: BETU, “Moroz,” and BETN. All these machines had devices for combining soil cutting with movement of the machine. Most observations of tooth wear were carried out during operation of the BETU machine. The BETU earth-cutting machine cut slots in homogeneous soil (Table 1), preparing it for subsequent excavation by excavators. The slots were cut with a width of 0.14 m and a depth of 1.3 m. The travel speed of the installation varied from 20.4 m/h to 186 m/h. The speed of move-

Table 1

the speed of the cutting chain (cutting) was constant and was 1.8 m/sec.

As the working member on the BETU, the cutting bar of the KMP-2 cutting machine was used. The chain was assembled in a “herringbone” pattern with nine cutting lines (Fig. 1).

For cutting frozen ground, teeth of four types were used (Table 2): KMZ-1 with a normal holder, KZK, “Shakhter,” and “Moskva” teeth, the heads of which on the side of the front face 4 (Fig. 2) were hard-faced with T-590 electrodes.

Fig. 1. Diagram of the cutting-chain assembly and arrangement of the teeth in the cutting slot of the BETU machine.

The qualitative aspect of the process of cutting frozen soil and of tooth wear may be represented as follows.

Table 2

Penetrating into the frozen soil, the cutter passes through a groove, periodically chipping out a shaving ahead of it. The grooves made by the teeth in the frozen soil (with an average temperature from \(-0.8^\circ\) to \(-2.4^\circ\)C) have almost no lateral breakout, and their profile corresponds to the projection of the front face of the cutter. Ridges of soil remain between the grooves. Therefore, most of the teeth operate under blocked conditions.

The cutting pitch between teeth 1, 2, and 3 along the cutting edges was 24 mm. The ridges of soil formed between the teeth were not destroyed until they were crushed by the holders or by the chain cams. Thus, destruction of the soil ridges between teeth 1, 2, and 3 is carried out by the cutter holders and chain cams, which are not suited for this purpose. Teeth 4–5 operate in a similar manner and also have considerable wear. Teeth 6, 7, 8, and 9, whose cutting pitch is respectively 17 and 11 mm, were subjected to somewhat less wear. Smaller ridges of soil in height and width remain between them. Cutter wear has two characteristic features—

ness. The first feature consists in the specific wear of the cutter heads. The second is the wear of the cutter holder on the side of the front and lateral faces. The heads of teeth of the KMZ-1 type (Fig. 3, I) and KZK (Fig. 3, II), protected by hard-alloy plates, have almost no

Fig. 2. Elements of a tooth and chip separation from the soil during operation of the chain working body.

end wear, but in both of these types of teeth the holders wear very heavily, especially in the KZK, where the hard-alloy insert has the form of a round pin. Such a pin is soldered into the tooth head without covering the front face. Therefore, when the holder wears, the cylindrical reinforcing element 3 (Fig. 2) is exposed during operation and then quickly breaks off and is lost. In view of this, it is inadvisable to use teeth reinforced with cylindrical pins for cutting frozen soil.

“Shakhter” cutters (Fig. 3, III) wear with simultaneous abrasion of the lateral surfaces 1 and 2 and of the front face behind the reinforcing plate 5 (Fig. 2). They also do not exhibit end wear. The abrasion resistance of “Shakhter” cutters is considerably higher than that of KZK and KMZ-1 cutters when operating under identical conditions. Common to the wear of all these teeth is abrasion of the lateral surfaces 1 and 2 with a reduction in the width of the rear face 6. “Moskva” teeth (Fig. 4), deposited with T-590 electrodes, wear on the lateral surfaces and on the rear face. The specific consumption of “Moskva” teeth was the lowest in comparison with cutters reinforced with hard alloys (Table 3).

Table 3

Fig. 3. General view of cutting teeth of the types: I—KMZ-1, II—KZK, III—“Shakhter,” after cutting slots with a chain bar in frozen soil, m²:
a—0, b—1950, c—7800.

Fig. 4. General view of “Moskva” teeth: a—a new tooth; b—a tooth after cutting 7800 m² of soil with a chain bar.

Observations of the operation of other earthmoving machines under various conditions also show that the failure of teeth when cutting frozen soils containing significant inclusions of rock occurs as a result of wear of the holder and of the side surfaces 1, 2 of the cutter head (Fig. 2), and not of end wear of the head, as is observed when cutting coal. This point is also emphasized by other authors [2].

When breaking frozen soils in which large inclusions of rock, concrete, steel reinforcement, etc. are encountered, teeth fail because of chipping of the hard-alloy plates. This is aggravated by considerable wear of the holder, as a result of which the hard-alloy plates become exposed and chipping of the hard alloy occurs under small impact loads. Under these conditions, cutters whose front faces had been overlaid with T-590 alloy, similar to the “Moscow” cutters, proved to be considerably more wear-resistant.

Since the nature of tooth wear when cutting frozen soil is different from that when cutting coal, the requirements for the design and the criteria of wear of the teeth of earthmoving machines must be different from those for the cutting tools of coal-cutting machines.

The magnitude of the limiting wear of teeth when cutting coal is dictated mainly by the possibilities of their resharpening [4]. When cutting frozen soils without inclusions of hard rock, periodic sharpening of the cutters is not required, since considerable side wear appears before the plate or core of the reinforcing hard alloy changes its geometric shape.

The permissible wear of teeth serially produced by mining-machine-building plants when cutting frozen soils, in our opinion, should be determined by the possibility of restoring the teeth by depositing metal on the worn parts of the holder and on the side surfaces. As experience has shown, such a limit is wear of the side surfaces of the cutter head of 4–5 mm, measured along the cutting edges at the base of the reinforcing element. In such cases the hard-alloy body is still firmly held in the holder. With wear of up to 6–7 mm, the hard alloy is lost in three out of ten teeth; with wear of 7–9 mm, the losses amount to one half.

Fig. 5. Influence of side wear of teeth on their specific consumption.

Investigations showed that, at a soil moisture content of 19%, a cutting speed of 1.8 m/sec, and a feed of 2.5 m/min, with side wear of the teeth up to 4–5 mm their specific consumption increases gradually, and with further wear of the side faces of the head it increases sharply (Fig. 5) because of loss of the reinforcing element. As a result of wear of the holder and of the side surfaces of the tooth head, the width of the cutter is considerably reduced; therefore the width of the chip being removed is also reduced, and consequently so is the area of its cross section. In view of this, as the teeth wear, the energy intensity of the soil-breaking process increases.

Figure 6 presents the pattern of change in the specific resistance to cutting with wear of the side faces of the tooth, as revealed during operation of the BÉTU earth-cutting machine. Comparing the curves plotted at a moisture content of 19% and a soil temperature of −0.3° to −2.4°C, we arrive at the conclusion that they differ only quantitatively; that is, with increasing side wear of the teeth the specific resistance to cutting increases. The “Moskva” cutters provide the best operating conditions. When “Shakhter” teeth are used, the greatest value of the specific resistance is obtained, which is explained by the smaller width of the front face.

Fig. 6. Dependence of the specific resistance to cutting on the magnitude of side wear of the tooth.

Proceeding from this, the authors suppose that the energy intensity of cutting slots in frozen soil can be substantially reduced by decreasing the number of cutting lines and increasing the width of the cutters. When cutting frozen soils, it is advisable to test chains with 5 and 3 cutting lines and cutters whose front-face width is 2–3 times greater than that of cutters used for undercutting coal. The front face of the cutter should be equal to the full length of the part of the tooth protruding from the cam, and its entire surface should be protected by hardfacing or by plates of hard alloy. Such cutters will operate as self-sharpening ones.

It is also advisable to test trenching bars whose cutting chains, in addition to teeth in 2–3 lines, will be equipped with scrapers. In height these scrapers should protrude from the bar by a somewhat smaller height than the teeth, and in width they should be almost equal to the width of the cutting slot. It may be advisable to give the scrapers a triangular shape, which will improve the stability of chain movement in the guide frame of the bar. With such a design of the scrapers, the possibility of using purely scraper chains is not excluded.

The operating conditions of a chain working member when cutting frozen soil differ from the operating conditions in a mine. This is explained by the fact that the bar loosens the permafrost while in a vertical position, as a result of which the conditions for removing the crushed soil from the slot are considerably worsened. For example, in the cutting slot of the BÉTU machine there remained a layer of crushed soil up to 35 cm high. This part of the soil evidently has a substantial effect on the wear of the cutting chain. When earth-cutting installations operate in soils, even with a low compressive strength, wear is observed on the guide strips, connecting hinges (rollers), and cams of the cutting chain. As the guide strips wear, the cams acquire a considerable degree of freedom—they “play” in the groove of the bar. Therefore the teeth deviate from their trajectory of motion toward the side of least resistance, and cases occur in which two teeth located on different cutting lines proceed one after another.

The use of chains with a smaller number of cutting lines, wider cutters, and triangular scrapers will make it possible to eliminate this drawback.

To verify these assumptions and refine the recommendations, special studies must be carried out.

References

1. K. Averyanov. A rock cutter requires recognition. Ekonomicheskaya gazeta, No. 48 (69), 1962.

2. A. P. Bolshakov. Mechanization of frozen-ground excavation. Mekhanizatsiya stroitelstva, No. 2, 1962.

3. A. N. Zelenin. Destruction of frozen soils by cutting, impact, and vibration. TsINTIAM, Series IV, 1962.

4. V. G. Yatskikh, B. L. Rozenberg, A. D. Imas, V. A. Maksimov. Mining machines. Gosgortekhizdat, 1959.