ON THE WEAR OF GEAR WHEELS IN GEARBOXES OF MINING ELECTRIC DRILLS OPERATING WITH LIQUID LUBRICATION

V. T. GORBENKO, S. I. SHUBOVICH

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

In papers [1—2] the results are given of a study of tooth wear in the gear transmissions of ER-5 and ER-16 mining electric drills, the testing of which was carried out with the gearbox filled with consistent grease, solidol M, in accordance with the operating instructions for these electric drills. To determine the effect of liquid lubrication on the process of tooth wear, we carried out tests of the above-mentioned transmissions with liquid lubrication under the conditions and by the method described in paper [1]. The results of these comparative tests are presented in the present article.

The straight-tooth wheels of the second stage of ER-16 gearboxes were tested; for them: $z_1 = 17$, $z_2 = 28$, $m = 2$ mm, $\xi_1 = \xi_2 = 0$. The pinions, made of 20Kh steel, were carburized to a depth of 0.4—0.6 mm and hardened to a hardness of HRC = 56—62. The wheels, made of 40Kh steel, were through-hardened to a hardness of HRC = 40—45. It should be noted here that, in order to reveal the influence of the difference in hardness of the teeth of mating wheels on the process of tooth wear, in some experimental gearboxes wheels with reduced hardness, HRC = 37—39, were used.

The liquid lubricants used were: AK-6 autol (GOST 1862-57) and summer automotive-tractor transmission oil (GOST 542-50). The lubricant level in the gearbox was kept constant during the tests, and the lubricant was changed at the same time intervals as in experiments [1, 2].

The tests were carried out at a peripheral speed of the wheels $v = 1.49$ m/sec and at a constant load, the magnitude of which corresponded to the maximum load in experiments [2]. The stresses, calculated by the Hertz formula (without taking into account the coefficients of concentration and load dynamics), were in this case approximately 10000 kg/cm².

Figures 1 show curves of the increase in wear over time for the pinion teeth, and Fig. 2 for the wheel teeth. The numbers in the figures designate the numbers of the gearboxes tested; moreover, the odd-numbered gearboxes operated on the test stand as step-up transmissions, and the even-numbered ones as step-down transmissions. Numbers 1, 2, 3, and 4 designate the gearboxes tested with consistent grease, the test results for which are taken from paper [2]. The curves (Figs. 1 and 2) show that, with all types of lubrication, tooth wear lies within the same limits; moreover, as a rule, the driving members always have greater wear than the driven ones.

Fig. 1. Graphs of gear wear.
Solid lines—driving gears; dashed lines—driven gears.

Fig. 2. Graphs of wheel wear.
Solid lines—driving wheels; dashed lines—driven wheels.

This phenomenon can be explained by the different nature of friction on the driving and driven teeth of the wheels [3].

It can be seen from Fig. 2 that the teeth of the wheels of reducers 7, 8, 11, and 12 have greater wear with liquid lubrication than with grease lubrication; this, however, is explained, in our opinion, not by the influence of the lubricant, but by the fact that the wheels of these gear pairs had reduced hardness \((\mathrm{HRC} = 37\text{—}39)\) and a poorer initial finish of the tooth surfaces.

The curves (Figs. 1 and 2) show that, with all types of lubrication, there were two periods of wear—the running-in period and the period of monotonic (uniform) wear. A period of intensive wear was not observed in our tests, although in all cases progressive pitting occurred on the wheel teeth; as a rule, it began after the running-in period. This is further evidence that the presence of pitting does not cause intensive wear of the teeth [1].

Figure 3 presents curves of the change in the coefficient of friction in the meshing during tooth wear with different lubricants. It can be seen from this figure that, with all lubricants, the coefficient of friction in the initial test period (during the running-in period) decreases and by the end of this period reaches a certain value for the given type of lubricant, which practically does not change during the subsequent operation of the gear, despite wear and pitting of the working surfaces of the teeth.

Fig. 3. Change in the coefficient of friction in meshing:
1—lubrication with solidol; 2—lubrication with autol; 3—lubrication with transmission automotive tractor oil; 4—lubrication with autol, roughly machined tooth surfaces.

The smallest value of the coefficient of friction occurred when the gear pair operated with grease lubrication, a larger value with lubrication by transmission oil, and the maximum value with lubrication by autol. This regularity can evidently be explained by the fact that, when lubricating with oils of greater viscosity, a thicker oil film is formed on the contact surface of the teeth and, as a result, the friction in the mesh more closely approaches fluid friction in its character. Curves 2 and 4 (Fig. 3) show that the coefficient of friction in the mesh is the higher, the rougher the initial tooth surface.

During tooth wear with liquid lubrication, as well as during wear with grease lubrication [1, 2], three characteristic zones were clearly distinguished on the tooth surfaces, differing in the appearance of the surface and in the outline of the tooth profile. A comparison of the working surfaces of the worn teeth in our tests with worn teeth in the tests [1, 2] showed that the type of lubricant has practically no effect either on the condition of the surface or on the change in the shape of the tooth profile during wear. The exception was the two driven gears of reducers 7 and 13, on the tooth flanks of which

in which, at the end of the tests, the onset of spalling was observed, whereas, as in experiments [1, 2] conducted with grease lubrication, no spalling at all was observed on the tooth flanks of the gears. In the authors’ opinion, spalling on the tooth flanks of gears operating with liquid lubrication can be explained by the fact that, with liquid lubrication, a somewhat higher coefficient of friction occurs in the mesh, as a result of which the friction forces and the accompanying tangential shear stresses in the surface layer increase.

Summarizing the results of the comparative tests carried out on tooth wear in gear wheels with different types of lubrication, it may be concluded that the type of lubricant has no effect on the magnitude of integral wear, or on the character of the surface and the change in the form of the tooth profile during wear; the type of lubricant has some effect on the magnitude of the coefficient of friction in the mesh and, consequently, on the efficiency of the gear pair.

REFERENCES

1. Yu. S. Semenov. Investigation of the wear of gear wheels in gearboxes of mining electric drills. Proceedings of the Conference on Problems in the Calculation, Design, and Study of Gear Transmissions and Flexible-Coupling Transmissions, vol. III, published by the Odessa Polytechnic Institute, 1957.

2. Yu. S. Semenov, V. T. Gorbenko. Wear and cleanliness of the working surfaces of gear-wheel teeth. Izvestiya TPI, vol. 114, published by Tomsk University, 1964.

3. G. K. Trubin. Contact Fatigue of Materials for Gear Wheels. Mashgiz, 1962.