Some Results of an Experimental Study of a Two-Motor Overhead Conveyor with Fluid Couplings

A. V. Murin

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

In order to reduce the forces in the traction chain of an overhead conveyor when its route is of considerable length, several drive stations are used, as is known. For the normal operation of a multi-motor conveyor it is necessary to satisfy a very important requirement—to ensure coordinated rotation of the driving sprockets. Another, no less important requirement is the condition of uniform load distribution among the drives. Fulfillment of these conditions presents well-known difficulties. Coordinated rotation of the driving sprockets of a conveyor chain can be ensured by various methods [1], but of special interest is the use, for this purpose, of safety hydrodynamic couplings, since the presence of such couplings in the drives can ensure not only synchronization of the rotation of the sprockets, but also easier starting of a loaded conveyor and automatic protection of the traction chain and drives against inadmissible overloads.

The latter property of these couplings is of particular importance, since in conveyors with long and complex routes, used at mass-production machine-building plants, cases of jamming and inadmissible overloads of the traction chain, causing its rupture, occur comparatively often, which naturally leads to appreciable losses of working time. It should be noted that the use of fluid couplings improves the operating conditions of the drives and of the traction chain also during the period of steady motion. This occurs because of a reduction in the dynamic loads arising as a result of the nonuniform motion of the traction chain. The almost complete disappearance of dynamic loads in this regime is explained by the fact that, when a fluid coupling is present in the drive, there is no rigid connection between the transmission mechanism and the rotor of the electric motor, which, as is known, possesses the greatest kinetic energy during motion and therefore has the principal influence on the formation of dynamic loads in the transmission mechanism and in the chain because of its nonuniform motion.

In view of the absence in the literature of information on the operation of multi-motor overhead conveyors whose drives are equipped with safety hydrodynamic couplings, we carried out, under plant conditions, tests of a two-motor overhead conveyor with such couplings; the principal results of these tests are set forth below.

Fig. 1. Layout plan of the conveyor route (the dash-dot line indicates the walls of the shop building).

Labels in the figure: Unloading area; Direction of motion.

The tests were carried out on a heavily loaded overhead conveyor intended for transporting flash from the forging shop [2]. This conveyor has a complex route (Fig. 1), about 410 meters long, a considerable part of which—about 2/5—passes outside the premises.

Fig. 2. Safety fluid coupling installed in the first drive

As the traction element it uses a standard detachable chain with a pitch of 160 mm, and its drives are equipped with safety fluid couplings developed by us, with asymmetrical wheels and an additional volume on the pump-wheel side. The design of the coupling installed in drive 1 (Fig. 1) is shown in Fig. 2, and its mechanical characteristics are shown in Fig. 3. The dimensions and parameters of the main elements of the coupling of drive 2 differ somewhat from those given here, which naturally led to some difference in their mechanical characteristics.

During the tests of the conveyor under study, the rotational speeds of the pump and turbine wheels of the fluid couplings, the forces in the chain ahead of the driving sprockets, and the instantaneous powers consumed by the drive motors were recorded simultaneously on the oscillograph film. Records of these parameters were made during steady-state motion, during starting periods, and when an insurmountable load was suddenly applied to the conveyor traction chain, causing it to stop.

Fig. 3. Static mechanical characteristics of the fluid coupling of the first drive (an asynchronous squirrel-cage electric motor A 51-4, rated at 4.5 kW at 1440 rpm, was used as the drive);

The speeds of the driving and driven shafts of the drives were recorded by means of contactless induction revolution markers developed by us (one mark on the oscillogram per revolution). The force in the conveyor chain was measured by means of strain-gauge links specially developed by the author for this purpose, with inductive sensors providing an output electric-signal power sufficient for its recording by a loop oscillograph without the use of amplifying devices [2, 3]. The powers consumed by the electric motors of the conveyor drives were oscillographed by means of wattmeter-type vibrators D1-X1 and, simultaneously with this, were measured by wattmeters.

Figure 4 presents the results of decoding the oscillogram of the forces in the conveyor chain ahead of the driving sprockets, and of the speeds of the driven and one of the driving elements of its drives during steady-state motion. For the decoding, a typical segment of the oscillogram cited by us in [2] was taken. This figure shows the variation of the force in the traction chain 1, of the angular velocities of the pump (4) and turbine (5) wheels, of the slip of the electric motor \(4'\) and of the combined motor-and-fluid-coupling slip \(5'\) of the first drive station, and the variation of the force in the chain 2, of the angular velocity of the turbine wheel 6, and of the combined slip of the electrohydraulic drive \(6'\) of the second drive station. Curve 3 in this figure shows the variation of the relative difference in the forces

\[ \frac{P_1 - P_2}{P_1} \]

in the chain ahead of the driving sprockets of the first and second drives. In view of the smooth character of the variation of the forces in the conveyor chain [2], the plotting of curves 1 and 2 in Fig. 4, and on their basis also curve 3, was carried out from the results of their measurements on the oscillogram at time intervals of 0.04 sec. The curves of angular velocities and slips show the variation of their mean values, calculated from

...from the results of measurements on the oscillogram of time intervals corresponding to 3–4 revolutions of a drive link. The absolute error in measuring slip by the adopted method, due to the inaccuracy of measuring time intervals on the oscillogram and determined according to [4], is, at probability \(P = 0.95\), on average \(0.3\text{–}0.4\%\).

As can be seen from Fig. 4 (curve 3), the difference in the magnitude of the forces in the conveyor chain ahead of the driving sprockets does not exceed 8%, which amounts to 2.2% if the nonuniformity of load distribution is evaluated by the method of V. K. D’yachkov [5]. In this case the deviation from uniform load distribution may be calculated from the following relation:

\[ \Delta = \left( \frac{1}{n} - \frac{P_i}{\Sigma P_i} \right)\cdot 100, \]

where \(\Delta\) is the deviation from uniform distribution, in percent;
\(n\) is the number of drives;
\(P_i\) is the load of the \(i\)-th drive;
\(\Sigma P_i\) is the total load on all conveyor drives.

The measurements carried out show that the greatest deviations from uniform load distribution between the drives, calculated from the relation given above, amount to 6–7%. The change in the speeds of the pump and turbine wheels of the first drive station and of the turbine wheel of the second, caused by a change in the load in the conveyor chain ahead of the driving sprockets, has a smooth aperiodic character. At the same time, an undoubted interrelation is observed between the change in the forces in the traction chain and the change in the rotational speeds of the indicated links. A certain lag in the change of speeds when the forces change is associated with a definite inertia of the rotating links. This is especially clearly manifested in the course of the curve of change in the force in the chain ahead of the first drive station (curve 1) and in the change in the speeds of the turbine and pump wheels of this drive (curves 5 and 4).

Determination of the forces in the conveyor chain ahead of the drive stations, carried out by calculation according to [1], shows that for the first drive the force ahead of the sprocket is almost twice as large as for the second. However, it is seen from Fig. 4 that the angular velocity of the turbine wheel of the first drive (curve 5) in the time interval under consideration is somewhat higher than the speed of the same wheel of the second drive (curve 6). The relative difference in the speeds of these wheels of the first and second drives, \(\frac{n_{T_1}-n_{T_2}}{n_{T_1}}\), amounts in this period to 3–6%. But even with such a noticeable difference in the angular velocities of the conveyor sprockets, as follows from Fig. 4 (curves 1, 2, and 3), a sufficiently uniform distribution of the forces in the chain ahead of them is ensured. The total slip (of the motor and fluid coupling) of the conveyor drives for the analyzed section of the oscillogram is 4–7% for the first station and 9–11% for the second. The slip of the electric motor of the first drive during this time interval was within the range of 0.5–2.0%. The nominal slip of the motors installed in the drives is 4% (according to the catalog). Thus, comparison of the actual and nominal values of motor slip indicates that electric motors with a power several times too high are installed in the drives of this conveyor. Knowing the slip of the electric motor and the total slip of the hydroelectric drive, it is not difficult to find the slip of the fluid coupling as well. Since, in the conveyor under investigation, between the electric motor and the fluid coup-

Fig. 4. Variation of the forces in the conveyor chain ahead of the driving and driven sprockets, of the angular velocities, and of the slips of the driving and driven elements of the drives under steady-state operation.

Visible figure labels: \(P_k\); \(n\), \(\varphi_{\mathrm{sl}}\), %; \(\dfrac{P_1-P_2}{P_1}\), %; \(L_{\mathrm{sec}}\).

there is no such intermediate transmission, then, as is known, the slip of the hydroelectric drive, of the motor, and of the fluid coupling, respectively, is equal to

\[ s=\frac{n_0-n_2}{n_0}, \qquad s'=\frac{n_0-n_1}{n_0} \quad \text{and} \quad s''=\frac{n_1-n_2}{n_1}, \]

where

\[ \begin{aligned} n_0&\text{—the synchronous rotational speed of the motor rotor,}\\ n_1&\text{—the operating rotational speed of the motor rotor (the speed}\\ &\quad\text{of the pump wheel of the fluid coupling),}\\ n_2&\text{—the speed of the turbine wheel of the fluid coupling.} \end{aligned} \]

Using the last two expressions, the speed of the turbine wheel can be represented as

\[ n_2=n_0(1-s')(1-s''). \]

Substituting the value of \(n_2\) into the expression for determining the total slip of the drive and carrying out simple transformations, we obtain the slip of the fluid coupling

\[ s''=\frac{s-s'}{1-s'}, \]

and, for small values of \(s'\),

\[ s''=s-s'. \]

The absolute error in this case is equal to the product of the slips of the motor and the fluid coupling.

Figure 5, a shows an oscillogram of the forces in the conveyor chain ahead of its driving sprockets, of the speeds of the pump and turbine wheels of the first station and of the turbine wheel of the second drive station during the starting period. In this oscillogram, the numbers 1 and 2 denote the envelopes

Fig. 5. Oscillograms of the forces in the conveyor chain ahead of the driving sprockets, the angular velocities of the driving and driven elements of the drives during starting (a), and of the powers consumed by the motors and the angular velocities of their rotors during starting and steady motion (b)

of the maximum values of the current at the carrier frequency of the output signal delivered by strain-gauge links located in the conveyor chain respectively ahead of the first and second stations. As we noted earlier [2, 3], the recording of these output signals is a recording of a pulsating current of twice the carrier frequency, modulated by the measured parameter. The carrier frequency is 50 Hz. The numbers 3, 4, and 5 denote recordings of signals delivered by revolution counters of the pump and turbine wheels of the first station and of the turbine wheel of the second station. As

as is evident from the oscillogram, during starting of the conveyor a smooth increase in the force in the chain ahead of the driving sprocket is ensured (envelope 1). Unfortunately, at the moment when recording of this oscillogram began, the strain link ahead of the second station was on the turning sprocket, which caused additional deformations of it, and the recording of the electrical signal it produced went beyond the limits of the oscillograph film (envelope 2). A characteristic feature of the starting process being analyzed is that a noticeable increase in the forces in the traction chain begins by the moment at which acceleration of the drive electric motors is completed.

The acceleration time of the electric motors of the drives of this conveyor is 1.5–1.7 sec. A certain difference in the values of the motor acceleration time is explained by the fact that the installed couplings have different active diameters and therefore their pump wheels and the parts connected with them differ somewhat in their inertia, i.e., they have different mass moments of inertia for the elements directly connected to the motor rotors. The turbine wheel of the first drive station begins to rotate 1.0 sec after the electric motors are switched on, and that of the second after 1.5 sec. By the end of acceleration of the motors, the turbine wheel of the first drive makes only 4 revolutions, and that of the second, 2. Thus, the conveyor chain begins to move after the electric motors of the drives have reached speeds corresponding to steady-state motion. The driving sprockets of the conveyor reach the speed of steady-state motion after 1.8–1.9 sec at the first drive station and after 2.8 sec at the second, after the electric motors are switched on. The magnitude of the average acceleration of the chain during this period is 0.03–0.05 m/sec², which is significantly (several times) less than the values of the average accelerations of the traction element during acceleration cited in the literature for belt, belt-chain, and other multi-motor conveyors whose drives are equipped with safety fluid couplings or with wound-rotor electric motors with contactor control.

Figures 5, 6 present a typical oscillogram of the power consumed by the conveyor electric motors of the first (recording 1) and second (recording 2) drives during the acceleration period (right-hand part of the oscillogram) and during the period of steady-state motion. On the same oscillogram, with the aid of revolution markers, the angular speeds of rotation of the electric motors of the first (3) and second (4) stations are recorded. The envelopes of the maximum values of instantaneous power (upward from the zero line for the first station and downward for the second station) give an idea of the nature of the change in the power consumed by the drive motors and expended on setting the conveyor chain in motion. A characteristic feature, as is evident from the oscillogram, is the practically constant magnitude of the power consumed by the conveyor electric motors (after completion of the acceleration period) and the smooth character of its redistribution. The slight pulsation of the power consumed by the motor of the second station, with a period corresponding to one revolution of the rotor, is explained by a certain asymmetry in the parameters of its stator windings [2]. The absence of sharp fluctuations in the power consumed by the motors during steady-state motion indicates that the fluid couplings smooth out fluctuations of the load in the conveyor chain and do not transmit them to the motor, i.e., they practically eliminate dynamic loads in the drive and chain. This circumstance is of considerable importance for increasing the service life of the chain and of the conveyor drives [6].

The studies carried out on the process of emergency stopping of the conveyor chain caused by a suddenly applied insurmountable load showed that the safety fluid couplings installed in the drives protect them and the chain from impermissible overloads [2]. A characteristic feature of the regime under study was the independent operation of the drive stations during this period. This consisted in the fact that, when an artificially created insurmountable emergency load was applied to the chain not far from the first station, first only its driving sprocket stopped. The second drive continued to operate during this time. It should be noted that, when the driving sprocket of the first drive stopped, the force in the chain ahead of the second drive practically did not increase. The rotation of the driving sprocket of the second drive stopped after the movable carriage with the deflecting sprocket of the load tensioning station of this drive, located immediately after the first drive (Fig. 1), having taken up the entire tensioning travel as a result of the increasing force in the conveyor chain ahead of the second drive station, moved up to the stop. When the driving sprocket of the second drive stopped, its motor continued to operate. At the same time, the force in the conveyor chain ahead of the first drive also practically did not increase, although its motor also continued to operate. The forces in the conveyor chain during the stopping of the sprockets increased rather smoothly. The safety pins present in the conveyor sprockets were not sheared during such emergency locking of the chain. The absence of abrupt changes and the smooth increase of forces in the traction chain at this time is undoubtedly explained by the fact that the kinetic energy of the rotors of the electric motors, during such an emergency stopping of the drive sprockets, does not take part in forming the dynamic load. This circumstance is an important property of drives equipped with safety hydrodynamic couplings.

Conclusions

The studies carried out showed that the safety fluid couplings installed in the conveyor drives, even with some difference in mechanical characteristics, ensure:

1) good equalization of the load on the drives and a smooth character of its redistribution under significant nonuniformity of loading of the branches of the traction element;

2) reliable protection of the drives and the traction chain from impermissible overloads;

3) lead to the almost complete disappearance of dynamic loads in the transmission mechanisms and the traction chain both in steady-state and in unsteady regimes (during starts and emergency locking of the traction element).

References

1. V. K. D’yachok. Overhead Conveyors. Mashgiz, 1961.

2. A. V. Murin, M. M. Gokhberg, S. I. Shubovich. On the application of safety turbocouplings in drives of overhead conveyors. Publ. Tomsk Polytechnic Institute, vol. 147, 1966.

3. A. V. Murin. Use of inductive sensors for measuring forces in the chain of an overhead conveyor. Publ. Tomsk Polytechnic Institute, vol. 147, 1966.

4. L. Z. Rumshiskii. Elements of Probability Theory. Fizmatgiz, 1960.

5. V. K. D’yachkov. Factory tests of overhead conveyors with multi-motor drive. VNIIPTMASH. Coll. “Overhead Ropeways,” No. 22, 1958.

6. I. A. Tsetnarskii. Operating experience with the multi-drive scraper conveyor KSR-1 with turbocouplings. Mining Mechanics. Collection of articles. Gosgortekhizdat, 1961.