On Contamination of the Lubricant in Gearboxes of Mining Handheld Electric Drills

V. T. Gorbenko, V. V. Burkov

(Presented by the scientific seminar of the Department of Applied Mechanics)

In coal-mining, transport, earthmoving, and other machines that operate under conditions of heavy contamination, the principal type of wear of many parts is abrasive wear. Abrasive wear affects not only the moving parts of open mechanisms (transmissions), but also the parts of enclosed mechanisms as a result of abrasive particles entering their housings (for example, the housings of gearboxes, gear-shift boxes, etc.) through the sealing devices of rods and shafts, through joints, and so on.

During the operation of coal-mining machines, their parts are subjected to the action of particles of coal and rock. It is known [1, 2] that coal particles and coal dust are also abrasives and increase the wear of parts by a factor of 3–8 [1, 2], depending on concentration; and the presence of 2% sandstone dust during gearbox tests [1] led to the wear of some parts increasing by more than a factor of 30. Therefore it is necessary to devote the most serious attention to ensuring conditions that exclude the possibility of abrasive particles entering mechanism housings.

The Department of Applied Mechanics of the Tomsk Polytechnic Institute, together with the Tomsk Electromechanical Plant, is carrying out work to study the reliability and durability of mining electric drills produced by the plant. In testing the electric drills and investigating the influence of coal dust on the wear of gearbox parts, we encountered the need to determine the quantitative content of coal dust in the lubricant of electric-drill gearboxes, which could be established on the basis of analyses of lubricant samples taken directly at mines.

For this purpose, the authors examined a large number of handheld electric drills at mines of the trusts “Leninugol,” “Kemerovougol,” “Kuibyshevugol,” “Prokopevskugol,” and “Anzherougol,” and of the combine “Kuzbassugol,” which is the principal consumer of the electric drills produced by the plant.

As a result of the survey it was established that at all mines preventive inspection and repair of handheld mining electric drills is carried out periodically (with the corresponding entry made in a log). The interval between preventive inspections ranges from 7 to 15 days, after which the electric drill, regardless of its serviceability, is brought to the surface and in the workshop undergoes preventive inspection, during which, first of all, the condition of the rubber coating of the handles, back—

the drill, the cable is reterminated, the resistance of the stator windings is checked, and the lubricant in the gearbox is replenished or replaced. However, let us note that if such operations as reterminating the cable, checking the resistance of the stator windings, checking the starter and the condition of the rubberizing of the handles are always carried out, then preventive inspection of the gearbox and replenishment and replacement of the lubricant in it are often omitted; and this leads, as inspection has shown, to the lubricant becoming heavily contaminated with mechanical impurities, while the amount of it in the gearbox proves to be considerably less than is provided for by the instructions.

Contamination of the lubricant may occur owing to the penetration of coal dust into the gearbox housing during operation, owing to careless storage of the lubricant and during preventive maintenance, when particles of coal, rock, and other foreign bodies get into the lubricant, and also owing to wear products of gear wheels, bearings, shafts, and other parts.

The penetration of coal dust into the gearbox housing can, in our opinion, be explained as follows. During operation of the electric drill, because of the heat released in the gearing, in the bearings, and in the electric motor, the pressure in the cavities of the gearbox and electric motor rises, the lubricant is thinned, and it partially leaks (is squeezed) outward through the spindle seal, the joint between the gearbox housing and the electric-motor shield, and also through the front bearing of the rotor shaft into the cavity of the electric motor.

When the electric drill is switched off and cools, a vacuum is created inside the gearbox and electric motor, as a result of which air containing coal dust is drawn in through all the above-mentioned joints and clearances.

To determine the content of mechanical impurities in the lubricant, we analyzed a considerable number (more than 60) of lubricant samples taken from gearboxes, chiefly of handheld mining electric drills that had worked different numbers of shifts in a mine. The operating time of a drill in the mine was established on the basis of entries in the electrician’s registration log. (Here it should be noted, however, that the utilization coefficient of electric drills in the face during one shift is not the same and depends on a whole series of factors, the chief of which is the organization of labor.)

The results of the analyses showed that the contamination of the lubricant by coal dust in the gearboxes of electric drills after 10 days averages: for ER-15 and ER-16 electric drills, 6–8%; for ER-18D and ER-14D electric drills, 1.5–3% (an improved seal design).

In some samples the content of mechanical impurities proved to be very considerable (24–53%), which is explained by an insufficiently high standard in carrying out preventive maintenance of electric drills and by violation of the rules of technical operation. Confirmation of the above is the fact that in some samples (for example, from an ER-15 electric drill, Yaroslavsky Mine of the Leninugol trust) there were particles of coal measuring 2–3 and even 6 mm. Obviously, such coal particles could have entered only during careless disassembly of the electric drill either in the face or on an unprepared and dirty workbench in the shop. (We note that at the Koksovaya-1 mine of the Kirovugol trust, in order to prevent disassembly of electric drills in the face, the nuts on the studs connecting the gearbox to the electric motor are filled with a special mastic.)

Another reason for the high concentration of coal in the lubricant is an insufficient amount of lubricant in the working volume of the gearbox. This is explained mainly by the fact that much less is put into the gearbox

a larger quantity of lubricant than specified in the instructions, as well as partial leakage of lubricant. Thus, in one of the samples, analysis showed 53% coal. Such a high degree of lubricant contamination resulted from the fact that there was practically no solidol in the gearbox, and the housing walls, gears, and other parts were covered with a layer of coal 1–2 mm thick.

The work [2] also gives data on the contamination of oils in cutting machines by mechanical impurities, the content of which ranges from 2 to 10%. In individual cases the amount of coal fines reaches 20–30%.

According to the results of our analyses, the greatest metal content in the lubricant in the form of wear products of parts (mainly gears) was about 3% (ER-16 electric drill); in the remaining samples the metal content was insignificant (0.5% or less). A granulometric analysis of the coal contained in the lubricant gave the following results: particles up to 70 μm in size—11%, from 70 to 800 μm—67%, and 800 μm and larger—22%; i.e., fine particles predominate.

Mechanical impurities contained in gearbox lubricant lead to increased wear of gears and bearings. In the event of bearings becoming clogged with coal fines (the latter is explained by the ability of coal dust to briquette [2]), the bearing rings may rotate, as a result of which the bearing seats are damaged and the gearbox fails.

On the basis of lubricant-analysis data, studies are being carried out on the influence of coal dust on the wear of parts of mining handheld electric drills.

By improving the design of seals in electric drills, together with observance of the rules for technical operation and careful and timely preventive maintenance, it is possible to reduce contamination of the lubricant of mining electric drills by coal dust to such limits that it will no longer have a significant effect on increased wear (according to preliminary data, 1.0–1.5%), since coal is a low-abrasive substance [2]. This will undoubtedly improve their reliability and durability.

REFERENCES

1. A. V. Dokukin, V. N. Istomin, L. I. Tishchenko. Wear, Lubrication, and Repair of Cutting Machines. Publishing House for Mining Literature, Moscow, 1961.

2. M. M. Tenenbaum. Wear Resistance of Parts and Durability of Mining Machines. Gostekhizdat, Moscow, 1960.