EFFECT OF THE GAP BETWEEN THE PUMP AND TURBINE WHEELS ON THE CHARACTERISTICS OF A SAFETY TURBO COUPLING

A. V. MURIN, S. I. SHUBOVICH

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

Information on the effect of the axial gap between the blades of the pump and turbine wheels on the characteristics of turbo couplings is given in a number of works [1–6]; however, this information is to a considerable extent contradictory and applies only to single-cavity couplings with symmetrical profiles of the working wheels. As for the effect of the axial gap on the characteristics of safety couplings, distinguished by the presence of additional cavities on the side of the pump wheel, this question, as far as we know, has not been investigated and has not been covered in the literature. In order to eliminate this gap, experiments were carried out in the laboratory of the Department of Applied Mechanics of the Tomsk Polytechnic Institute to determine the effect of the axial gap on the characteristics of a safety coupling. The results of this investigation are presented in the present article.

Fig. 1. Diagram of a safety turbo coupling.

The experiments were carried out with a turbo coupling, the diagram and principal parameters of which are presented in Fig. 1. Before recording each series of characteristics of the turbo coupling under investigation on the brake stand, the coupling ...

was disassembled and an axial gap of the desired magnitude was set by moving the turbine wheel on the driven shaft.

In all, five series of characteristics were obtained at gaps of 1, 2.5, 4, and 6 mm. The limits of variation of the gap, expressed in fractions of the active diameter, were from \(0.0035D\) to \(0.021D\).

For each value of the established gap, static characteristics of the torque on the driven shaft of the coupling \(M\) as a function of the slip \(s\) were taken, at a practically constant speed of the driving (pump) shaft and with the coupling filled with spindle oil 2 (GOST 1707-51) to 80, 75, 65, and 60% of the total volume of the internal cavities of the coupling (which in our case was 4.7 l).

The characteristics \(M=f(s)\) were taken with very slow braking of the driven shaft, beginning from idle running to its complete stop, and with slow release of the brake, beginning with starting the electric motor with the turbine wheel completely braked and proceeding to idle running. This method of taking the characteristics is due to the fact that the curves \(M=f(s)\) during braking and during release of the brake have different appearances (Fig. 2), as has already been noted in [7]. In our opinion, the reason for this difference is the unequal

\[ S=\frac{n_1-n_2}{n_1}\cdot 100\%. \]

Fig. 2. Generalized characteristics of a safety turbo coupling during braking (1) and release of braking (2).

degree of filling of the auxiliary chamber of the turbo coupling with working fluid. When the coupling is braked, beginning with regimes in which part of the fluid of the main circulation circuit is discharged into the preliminary and auxiliary chambers, the auxiliary chamber is filled with an air-oil emulsion; therefore a relatively larger amount of fluid participates in the main circulation circuit and, as a result, the coupling develops a larger torque. When the pump wheel stops, the fluid settles; the air emulsifying it is removed from the auxiliary chamber, and the latter is filled to a greater degree with oil, while in the chamber of the main circulation circuit relatively less working fluid remains. Consequently, during the subsequent starting of the coupling under load, the torque on the turbine wheel will have comparatively smaller values.

At small slips, within the range from \(s=0\) to \(s_{\mathrm{H}}\) (Fig. 2), the auxiliary chamber practically does not participate in the operation; therefore the characteristics on these sections are the same both during braking of the coupling and during release of braking.

The characteristics obtained when the coupling is accelerated determine its starting properties, i.e., the properties governing the acceleration process of a machine equipped with a driving motor and a safety coupling; the characteristics obtained during braking characterize the overload properties of the coupling.

Fig. 3. Variation of the principal parameters of the characteristics of the safety turbo coupling during acceleration with an increase in the gap between the wheels: solid lines—coupling filling of 80%; dashed lines—60%.

Fig. 4. Variation of the principal parameters of the characteristics of the safety turbo coupling during braking with an increase in the gap between the wheels: solid lines—coupling filling of 80%; dashed lines—60%.

To judge the changes in the starting properties of the coupling, Fig. 3 gives the curves of the moments \(M'_{\text{start}}\), \(M'_{\max}\), \(M'_{\min}\) (Fig. 2) as functions of the magnitude of the axial gap \(\Delta\) at the greatest (80%) and the smallest (60%) fillings with working fluid; to judge the changes in the overload properties, Fig. 4 gives the curves of the moments \(M_{\text{crit}}\), \(M_{\min}\), \(M_{\max}\), and \(M_{\text{start}}\) (Fig. 2).

A very important parameter of a fluid coupling is the value of the nominal slip, since it determines the efficiency of the coupling under the nominal load condition. The experiments we carried out showed that, when the axial gap is varied within the limits indicated above, this parameter remains practically unchanged, which agrees with the results of the study published in works [1–3].

From Fig. 3 it can be seen that, as the axial gap increases, the values of the moments \(M'_{\text{start}}\), \(M'_{\max}\), and \(M'_{\min}\) during deceleration decrease; moreover, the first two of these decrease relatively little (by approximately 10–15%). The value of the moment \(M'_{\min}\), which determines the maximum load of the coupling at which acceleration of the drive to nominal speed is still possible, changes substantially as the gap increases at relatively large fillings of the coupling (75–80%). When the gap is increased from 1 to 6 mm, this moment decreases by 25–30%. This property of the characteristic of the safety coupling deserves special attention, since, owing to large axial gaps established during assembly, the coupling may prove unsuitable for operation with respect to the condition of ensuring acceleration of the unit under a static load equal to \(M_n\) ¹.

From Fig. 4 it can be seen that, as the gap increases, the moments of all characteristic points of the curve \(M=f(s)\) decrease. The moments \(M_{\text{crit}}\) and \(M_{\min}\) undergo a particularly strong decrease when the coupling filling is 80%. The value of the first of these determines the degree of overload of the drive at which subsequent stable operation of the coupling will take place only at relatively large slips, amounting to approximately 20–30% for an 80% coupling filling. Therefore, a safety coupling assembled with a large axial gap will not ensure stable operation of the driven machine under relatively small overloads.

The value of the moment \(M_{\min}\) determines the overload of the machine at which stable operation of the coupling with large slip will occur. The smaller \(M_{\min}\) is, the more difficult is the transition from stable operation at large slips to stable operation at small slips of the coupling. Therefore, in the presence of large axial gaps, the coupling may prove to be of little use in operation because it does not ensure a rapid transition to the normal speed operating condition after a brief, small overload of the machine.

The moments \(M_{\max}\) and \(M_{\text{start}}\), with increasing gap, decrease to a lesser extent than \(M_{\text{crit}}\) and \(M_{\min}\); moreover, such a change improves the properties of the coupling, since, with a decrease in \(M_{\text{start}}\) and \(M_{\max}\), the stresses in the machine components during overloads causing stoppage of the driven shaft of the fluid coupling will decrease.

In conclusion it should be noted that changes in the parameters of the characteristic points of the characteristics of the safety coupling are observed to a lesser degree at small fillings and to a greater degree at large fillings of the coupling.

Conclusions

1. Changing the axial gap between the blades of the pump and turbine wheels of the safety coupling within the range from \(0.0035D\) to \(0.021D\) has practically no effect on the values of slip under nominal load.

1. As the axial clearance increases, the minimum torque developed by the coupling during acceleration decreases, as a result of which the coupling’s starting properties deteriorate. With large axial clearances, cases are possible in which the coupling will not ensure acceleration of the machine under the nominal load.

2. An increase in the axial clearance causes a change in the overload properties of the safety coupling. With large clearances, large fluctuations in the speed of the coupling’s driven shaft and unstable operation are possible under conditions of relatively small overloads.

REFERENCES

1. Stroev S. S. On the calculation and design of a fluid coupling. In: Studies in the Field of Automobile Design. Moscow, Mashgiz, 1953.

2. Korotkov A. A. An approximate method for determining the axial forces of vaneless hydrodynamic couplings with radial blades at full filling. In: Collection of Scientific and Technical Information on Hydraulic Machine Building, Issue 4 (10), Moscow, 1959.

3. Study of an Experimental Fluid Coupling. Part 1. Technical Report of NAMI, No. 40, Moscow, Department of Technical Information, 1949.

4. Shtranskii I. A. Study of a fluid coupling with movable wheels. In: Collection of Scientific and Technical Information on Hydraulic Machine Building, Issue 3 (9), Moscow, 1959.

5. Voshchinin A. I. and Savin I. F. Hydrodynamic and Pneumatic Devices on Construction and Road Machines. Moscow, Mashgiz, 1954.

6. Maksimov A. E. Hydroelectric Drive of Mine Hoisting Machines. Moscow–Leningrad, Ugletekhizdat, 1951.

7. Murin A. V. On the influence of the position of the axis of rotation of a fluid coupling on its characteristics. Proceedings of the Tomsk Polytechnic Institute, Vol. 96, Part II, Tomsk University Press, 1961.

1. We encountered this kind of phenomenon in carrying out the experiments of the present study. ↩