ON THE APPLICATION OF SAFETY TURBO COUPLINGS IN OVERHEAD CONVEYOR DRIVES

A. V. MURIN, M. M. GOKHBERG, S. I. SHUBOVICH

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

Mechanization of intra-shop and intra-plant transport becomes highly effective when overhead conveyors are used for large freight flows. The well-known advantages of overhead conveyors are enhanced when they can be used on very long routes. However, the widespread use of long conveyors is limited, to a certain extent, by the need in such cases to employ multi-drive systems. Heavily loaded overhead conveyors with a route length on the order of five hundred meters in most cases already require two drives, and there are cases in which the use of a larger number of drives proves necessary.

The use of multi-drive systems entails a number of additional requirements that are not always easy to fulfill. In the case of multi-drive systems it is necessary, above all, to ensure synchronous rotation of the drive-station driving sprockets both during steady-state operation and during conveyor start-up. Of exceptional importance in this respect are the uniform distribution of loads among the drives, as well as reliable protection of each of them and of the conveyor chain against possible emergency overloads.

Synchronous operation of the driving sprockets of a multi-drive overhead conveyor can be achieved by using a combined drive with a common drive shaft, set in motion by means of a common transmission from a single electric motor [1]. This type of drive can be used only when the conveyor branches pass close to one another; otherwise an artificial lengthening of the route is required. Moreover, such a drive has a significant disadvantage, namely that if one of the conveyor driving sprockets, for any reason, sheds its load, the second is overloaded, which causes the nominal stresses in the conveyor chain to be exceeded and may lead to an accident.

When multi-motor drives are used for overhead-conveyor drives, synchronous operation of the driving sprockets of the overhead conveyor can be ensured by the use of special electrical systems, for example, an electrical shaft. This method requires special electrical equipment, motors with increased slip, and a large amount of electrical cable. In this connection the cost of such a drive is high.

Synchronization of the rotation of the drive sprockets is also possible through the conveyor chain, but in that case it is necessary to introduce into the drive devices, au-

automatically changing the slip between its driving and driven elements when the load changes. In this variant, flat-belt transmissions, providing a certain slipping of the belt on the pulleys, and turbo couplings may be used.

At the Altai Tractor Plant, where overhead conveyors have become fairly widespread, all the above-mentioned types of drives were tested.

In the precision-casting shop in 1958 a three-drive overhead conveyor was built, the synchronization of the drives being effected by means of an electric shaft.

A simplified basic electrical control circuit is shown in Fig. 1. This circuit operates as follows. By means of the disconnect switch

Fig. 1. Simplified basic electrical circuit of a three-drive conveyor with an electric shaft

\(R\), voltage is applied to the electrical system. By pressing the \(2\mathrm{KY}\) button, the primary winding of transformer \(T\) is energized, and along with it the time relay \(2\mathrm{RV}\). By closing its contacts, relay \(2\mathrm{RV}\) energizes the coil of intermediate relay \(RP\). The latter closes its contacts \(RP_1\), \(RP_2\), \(RP_3\), \(RP_4\). Thus relay \(RP\) seals itself in through contacts \(RP_1\) and \(RP_2\). The closing of contacts \(RP_3\) and \(RP_2\) ensures energizing of the coil \(1\mathrm{PM}\) and time relay \(1\mathrm{RV}\). Relay \(1\mathrm{PM}\) closes its contacts

in the main network feeding motors (1–4) AD. However, 1PM switches on only two phases, thereby transferring the motors to a synphase position. After a time delay, relay 1RV energizes relay 2PM, thereby applying voltage to the three phases of the motors.

Simultaneously with this, the winding of relay 3PM is switched on and the short-circuited motor 5AD begins to rotate. On the same shaft with this motor is the motor–sensor 4AD, which also begins to rotate. In this case all motors of drive stations (1–3) AD begin synchronous rotation.

After the time delay that ensures the starting process described above, relay 2RV drops out. Its contacts 2RV₁ open (the need for them disappears, since RP is self-powered), and 2RV₂ closes, thereby preparing the overload-protection circuit through blinkers (1–4) B and relay RPM.

When any motor of drive station (1–3) AD is overloaded, the maximum-current relay located in two phases of each motor (1–4) MR closes its contacts in the protection circuit, as a result of which the coil of relay RPM is energized. The normally closed contact of RPM opens, the armatures of relays 1PM, 2PM, and 3PM drop out, their contacts in the main circuit open, and the motors of the drive stations are de-energized. After this, they can be restarted only by pressing the start button 2KU.

Emergency stopping is carried out by pressing button 1KU. Operation of this conveyor showed sufficient reliability and serviceability of such an electrical system, but it did not receive further dissemination at the plant because of its shortcomings, which were mentioned above.

For transferring parts from the machine shop to the heat-treatment shop and back, in 1960 a two-drive overhead conveyor about 500 meters long was built at the plant. Synchronization of the operation of the drive stations and overload protection were carried out by means of flat-belt transmissions.

In 1962–1963, a heavily loaded two-drive overhead conveyor about 580 meters long was built for transporting castings; synchronization of the rotation of its driving sprockets was accomplished by means of a common drive shaft.

Operation of these conveyors showed that even when flat-belt transmissions are used in the drives, reliable protection of the drives and chain is not ensured in the event of its sudden (emergency) jamming. The absence of reliable protection for the drives and the conveyor chain led to their comparatively frequent downtimes because of accidents for the reason indicated.

In connection with this, in 1958–1961 the Department of Applied Mechanics of the Tomsk Polytechnic Institute, jointly with the Altai Tractor Plant, carried out work on creating a more advanced drive for overhead conveyors, free of the shortcomings indicated above. Such a drive is a drive with a safety turbo coupling. At the Department of Applied Mechanics of TPI, safety turbo couplings intended for drive stations of overhead conveyors were developed and investigated.

In 1963, in the forge shop of ATZ, a heavily loaded overhead conveyor (for transporting flash) 410 meters long, with a two-motor drive equipped with safety turbo couplings with asymmetrical wheels and an additional chamber on the side of the pump wheel, developed by one of the authors of the article, engineer A. V. Murin, was put into operation. The spatial arrangement of the con-

fan is shown in Fig. 2. As can be seen from the diagram, the conveyor has a fairly complex route, and a considerable part of the route (approximately \(2/5\)) passes outside the building. In order to check operability under real conditions, fluid couplings TM 280 and TM 310 were installed in the drives; these have active diameters of 280 and 310 mm and differ somewhat from one another in the design of the internal cavities. A TM280 fluid coupling was installed at the northern drive station, and a TM310 at the southern one (Fig. 2). The principal parameters of the fluid coupling TM280 are given in [2].

Fig. 2. Route of a two-motor overhead conveyor with safety fluid couplings.

The general view of one of the drive stations of this overhead conveyor is shown in Fig. 3. As drive motors on this conveyor, two squirrel-cage induction motors of type AO51-4 are used, each with a rated power \(N = 4.5\) kW at \(n = 1440\) rpm. The nominal speed of motion of the conveyor chain, equal to \(2.5\) m/min, is provided, with the adopted dimensions of the driving sprocket, by a reducer with a gear ratio \(i = 1556.7\).

The advantages of drives with safety fluid couplings became apparent already from the first days of operation of this conveyor. The fluid couplings provided smooth starting of the conveyor, synchronous operation of the driving sprockets, and reliable protection of both the drive and the chain in the event of its emergency jamming. Experimental investigations of the conveyor operation carried out in March 1964 confirmed these conclusions, which had been drawn on the basis of visual observations. During the experimental investigations, simultaneous oscillographic recording was made of the powers consumed by each electric motor, the forces in the conveyor chain ahead of each driving sprocket, and the rotational speeds of the pump and turbine wheels of the fluid couplings. Records of the indicated parameters were taken during steady-state motion, during starting, and during sudden jamming of the conveyor chain.

Figure 4,a presents a typical oscillogram of the instantaneous power consumed by the same phase of each drive motor of the conveyor with safety fluid couplings during steady-state motion. The power records were made using D-1, XI-type power vibrators. The film feed speed when recording this oscillogram was 50 mm/s. The lower trace gives an idea of the power consumed by the motor of the northern drive, and the upper trace by that of the southern drive; moreover, the magnitude of the power consumed at a given instant is proportional to ordinates measured

upward from the zero line for the northern drive and downward from the zero line for the southern drive. Using wattmeters connected simultaneously with the power loops, the records on the oscillograms were calibrated. During the recording of this oscillogram, the ratio between the powers consumed by the northern \(N_{\mathrm{c}}\) and southern \(N_{\mathrm{ю}}\) drives was

\(N_{\mathrm{ю}} = (1.09\text{--}1.12)\,N_{\mathrm{c}}\).

The measurements carried out showed that in other cases as well the difference in the powers consumed by the electric motors most often does not exceed 10–15%. As can be seen from the oscillogram shown,

Fig. 3. General view of one of the drive stations of a two-motor overhead conveyor with safety turbo couplings

the powers consumed by the motors practically do not change. This circumstance is an important property of drives with turbo couplings. It indicates that the turbo coupling smooths out the dynamic loads arising in the chain and in the drive during operation of the conveyor. The slight pulsation of power with a period corresponding to one revolution of the rotor of the southern-drive motor (upper record in Fig. 4a) is explained by us by a certain asymmetry of the motor, which affected the magnitude of the voltage between the phase in which the power measurements were made and the neutral of the stator windings. In measuring the power in the phase of the motor of the northern drive, however, for measuring

of the voltage, an artificial neutral from three identical, specially selected resistances, which provided a more accurate value of the zero voltage simulating the voltage at the center of the star to which the stator windings of the drive electric motors are connected. The use of an artificial neutral for the electric motor of the northern drive was necessitated by its considerable distance (150–200 m) from the southern one, near which the measuring instruments were located; moreover, for safety reasons it was not possible, during the measurements, to connect the natural neutral of the stator windings of the northern drive to the measuring apparatus.

Fig. 4,b also shows a typical oscillogram on which the forces in the branches of the conveyor chain ahead of the driving sprockets are recorded during steady-state motion of the conveyor chain. To record the forces, strain links with inductive sensors were used; these made it possible to avoid the use of amplifying apparatus and to feed the output signal from the measuring circuit directly to the vibrator of the oscillograph. Oscillograph recording of the forces was carried out with type V vibrators. In accordance with the recommendation in [3] and because of the low frequency of variation of the measured quantities, a carrier frequency of 50 cps was used to supply the sensors. The output signal was converted, by means of a bridge assembled from germanium diodes and providing full-wave rectification, into a pulsating current with a frequency of 100 cps, which was recorded on the oscillograph film. Thus, the envelope of the maximum current values on the oscillogram gives an indication of the magnitude and character of the variation of forces in the conveyor chain with safety turbo couplings. The zero line of the force in the conveyor chain ahead of the northern drive is located at the lower edge of the oscillogram, and that of the southern drive at the upper edge of the oscillogram.

Because of the large transverse deformations of the conveyor-chain links, the dynamometers that were made on their basis had nonlinear characteristics. Therefore, when analyzing the oscillogram it is not possible to estimate the magnitude of the variation of force in the chain branches directly from the change in the maximum current values. The variations of force in the branches of the conveyor chain, according to the oscillogram data in Fig. 4,b, are within the range of 4100–3700 kg for the northern drive and 3600–4000 kg for the southern drive (the speed of film advance during recording was 50 mm/sec).

However, it is clear from the oscillogram that the forces in the conveyor chain vary smoothly, i.e., that the dynamic loads are small. Measurements of the forces in the conveyor chain showed that they considerably exceed the nominal values, which for the detachable chain with pitch \(t = 160\) mm used on this conveyor amount to 2500 kg. We explain this by the fact that, during the month preceding the tests, the outdoor air temperature was 20–30°C below zero, which caused additional resistances on the sections of the conveyor route running in the open air. This explanation is confirmed by the fact that tests of this same conveyor carried out a week later, with approximately the same working load, gave force values in the conveyor chain of 1500–200 kg. The outdoor air temperature during that week was considerably higher and ranged from 0°C to −10°C.

To evaluate the results obtained in testing the two-motor conveyor with safety turbo couplings, measurements were made of the power consumption on a two-motor conveyor having drives with flat-belt transmissions, ensuring

transporting parts from the machine shop to the heat-treatment shop and back, as was already mentioned above.

Figure 4c presents an oscillogram on which the powers consumed by the same phase of the drive electric motors are recorded. The power consumed by the motors is proportional to the ordinates located, on the upper record, downward from the zero lines, and on the lower record, upward from the zero lines.

The records on the oscillogram were calibrated with wattmeters connected into the circuit. Thus, if the value of the power consumed by the first motor (upper record) is taken as unity, then the power consumed by the second motor was 1.55–1.64 times greater (according to the wattmeter readings for this section of the oscillogram). The speed of film advance during recording in this section (the second, counting from right to left) was 25 mm/s, and in the first it was 4 mm/s. As can be seen from the oscillogram, the powers consumed by the motors of this overhead conveyor have fairly sharp fluctuations (over 0.02–0.03 s), with the magnitude of the power surges in some places reaching up to 15% of its minimum value. Such fluctuations in the power consumed by the motors indicate still sharper changes in the forces in the conveyor chain; they indicate that the flat-belt transmission does not provide complete smoothing of the dynamic loads in the chain and in the conveyor drives.

Dynamic loads, naturally, lead to a reduction in the service life of the chain and cause fatigue failures of its links. In our opinion, this, as well as the insufficient protective properties of the flat-belt transmission, explains the accidents on this conveyor that became more frequent in the last year because of breaks in its chain.

Thus, the tests carried out showed that flat-belt transmissions, by increasing the slip between the driving and driven links and thereby compensating for the difference in the characteristics of the electric motors of a multi-motor drive, make possible their operation in a multi-drive system; however, they cannot fully satisfy all the requirements imposed on such drives. They do not ensure uniform load distribution between the motors, and the considerable and comparatively sharp fluctuations of the consumed power indicate the presence of large dynamic loads acting on the drive links and the conveyor chain when drives with such transmissions are used.

Comparing the results of studies of the operation of a conveyor having a two-motor drive with safety turbo couplings and a two-motor drive with flat-belt transmissions, we see the indisputable advantages of the first type of drive during operation in steady-state mode. In possible emergency stoppages of the chain, as operating experience with such conveyors shows, belt transmissions do not ensure its protection from destruction, whereas safety turbo couplings successfully cope with this task as well.

The oscillographic recording performed of the forces in the branches of the conveyor chain with turbo couplings during its sudden stoppage shows that the magnitude of the ratio of the maximum force in the chain before the driving sprocket to the nominal value \(P_{\mathrm{n}} = 2500\) kg is 2–2.2 before the northern station, and 1.4 before the southern one; that is, it does not exceed the magnitude of overloads permissible for such a chain. In this experiment the chain was stopped before the northern station, approximately 30–35 m from it.

On the oscillogram obtained one can also see another very important feature of the process of stopping the chain of a conveyor having in the dri-

Fig. 4. Oscillograms of power consumption (a), forces in the conveyor chain ahead of the driving sprockets (b) of a driven overhead conveyor with safety turbo couplings; (c) oscillogram of the power consumption of a two-motor overhead conveyor with flat-belt transmissions.

in drives, safety turbo couplings. Such a feature is the gradual and smooth increase of the load in the chain when it is suddenly stopped.

Thus, comparing the results of studies of the operation of a two-motor conveyor with safety turbo couplings and a two-motor conveyor with V-belt transmissions, and also taking into account operating experience with similar conveyors having other types of drives, it may be said that the use of safety turbo couplings quite simply and reliably solves all the problems that arise when using multi-motor drives. In conclusion, it should be noted that, in our opinion, drives with safety turbo couplings are the most promising types of drives not only for multi-motor overhead conveyors, but also for single-drive overhead conveyors.

REFERENCES

1. V. K. Dyachkov. Overhead Conveyors. Mashgiz, 1961.
2. A. V. Murin. 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, issue 2, 1961.
3. A. M. Turin. Electrical Measurements of Non-Electrical Quantities. Gosenergoizdat, 1959.