Velocity Diagram of Vessel Movement in a Skip Hoisting Installation

I. K. Khrustalev

Theoretical velocity diagram

The questions involved in choosing velocity diagrams for various types of mine hoisting installations have been well covered in our technical literature; therefore we shall not dwell on them. We shall briefly consider an acceptable velocity diagram for a hoisting installation with skips that unload through the bottom.

It is known that in this case the velocity diagram of the motion of the hoisting vessels must consist of five periods. The time of accelerated motion and the time of retarded motion are each divided into two periods, and the diagram as a whole consists of the following periods: 1—the beginning of accelerated motion up to the exit of the rollers of the empty vessel from the unloading curves; 2—the period of accelerated motion from the moment the rollers of the empty skip leave the unloading curves until the vessels acquire maximum speed; 3—the period of uniform motion at maximum speed (operation of the hoisting motor on its natural characteristic); 4—the period of retardation until the rollers of the loaded skip enter the unloading curves; 5—the end of retarded motion, from the moment the rollers of the loaded skip enter the unloading curves until the vessels come to a complete stop.

Without analyzing the velocity diagram, we note that in our technical literature it is recommended to take the speed of the vessels at the moment they enter the unloading curves as \(v_{\mathrm{in}} = 1.2 \div 1.3\ \mathrm{m/sec}\), the speed at which the empty skips leave the unloading curves as \(v_{\mathrm{out}} = 1.3 \div 1.5\ \mathrm{m/sec}\), and the maximum accelerations and decelerations of the skips as \(j = 1\ \mathrm{m/sec^2}\).

In actual operation of skip hoisting installations, however, as will be seen below, because of fear of one accident or another the indicated limits of speeds and accelerations are sometimes considerably reduced. Thus, for example, \(v_{\mathrm{in}}\) is sometimes lowered to \(0.8\ \mathrm{m/sec}\) and even less, and at many hoisting installations it reaches \(0.3\ \mathrm{m/sec}\). The actual time of the hoisting operation is increased to a considerable extent compared with that for the theoretical velocity diagram. This increase in time occurs mainly at the expense of a reduction in the period of uniform motion and an increase in the time of retarded motion. The latter is caused by the imperfection of the landing devices for skips, which in turn, in order to avoid breakage of the landing and dosing devices, leads to a reduction in the speed with which the skip approaches the dispenser carriage and lands on it. In fact, for the comparatively short distance traveled by the skip immediately before the dispenser carriage and while landing on the latter (\(1 \div 2.5\ \mathrm{m}\)), in individual cases up to \(40\%\) of the time of the hoisting operation is expended.

Actual Velocity Diagram of a Skip Hoisting Installation

We have recorded the actual velocity diagrams of 10 skip hoisting installations in the Kuzbass. To record the actual velocity diagram we used a recording wattmeter and a manual tachodynamo of independent excitation, type Sh-I. The current winding of the wattmeter was connected into a circuit of constant DC voltage (the storage-battery circuit). The voltage winding was connected into the armature circuit of the DC tachodynamo, receiving rotational motion

Fig. 1. Circuit diagram for connecting instruments for taking velocity diagrams.

1—Motor. 2—Armature of the tachodynamo. 3—Storage battery. 4—Stationary current winding of the wattmeter. 5—Wattmeter armature with voltage winding. 6—Wattmeter pointer. 7—Wattmeter drive drum. 8—Warren motor for rotating the wattmeter drum.

1. Excitation current \(=0.45\) A.
2. ” \(=0.85\) A.
3. ” \(=1.1\) A.
4. ” \(=1.5\) A.
5. ” \(=1.9\) A.

Fig. 2. Calibration curves of the tachodynamo.

from the shaft of the hoist electric motor or the shaft of the hoisting machine. For greater sensitivity of the instrument, all the additional resistance of the voltage winding was removed. The ordinate scale was regulated by the magnitude of the tachodynamo excitation current and by the current passing through the current winding of the wattmeter. This was accomplished by connecting a larger or smaller number of cells of the storage battery. The abscissa scale was \(2.1\ \text{mm}=1\ \text{sec}\) (the motion of the tape of the recording wattmeter is carried out at a speed of \(2.1\ \text{mm/sec}\)). The circuit diagram for connecting the instruments for taking velocity diagrams is shown in Fig. 1. The calibration curves of the tachodynamo are shown in Fig. 2.

Speed diagrams were recorded in installations with skips of various types (a skip with bottom discharge by tilting the body during unloading; skips with bottom discharge using sector and lever gates) and with various types of dispenser carriages (rotary and reciprocating movements). It is more convenient to divide the actual speed diagram of the vessels of a skip hoisting installation into four periods: 1—the period of accelerated motion \((t_1)\), 2—the period of uniform motion \((t_2)\), 3—the period of decelerated motion \((t_3)\), and 4—the period of seating the skip on the dispenser carriage \((t_4)\). Figures 3 and 4 show speed diagrams for 2 skip hoisting installations, recorded with a wattmeter.

Fig. 3. Speed diagram

\[ \begin{aligned} t_1 &= t_{\text{acc.}} = 12.9\ \text{sec},\\ t_2 &= t_{\text{unif.}} = 10.0\ \text{sec},\\ t_3 &= 7.1\ \text{sec},\\ t_4 &= 15.7\ \text{sec},\\ t_3 + t_4 &= t_{\text{decel.}} = 22.8\ \text{sec}. \end{aligned} \]

Fig. 4. Speed diagram

\[ \begin{aligned} t_1 &= t_{\text{acc.}} = 15\ \text{sec},\\ t_2 &= t_{\text{unif.}} = 19.7\ \text{sec},\\ t_3 &= 13.0\ \text{sec},\\ t_4 &= 14.3\ \text{sec}. \end{aligned} \]

Data on the actual operating speed of various skip installations are given in Table 1.

Table 1

Actual speed of movement of vessels of various skip hoisting installations

Actual Time for Loading and Unloading Skips

An important kinematic element in the operation of a skip hoisting installation is the time for unloading and loading the skip. It makes it possible to determine, for given conditions, the duration of the pause during operation of the installation.

The loading and unloading time of skips is affected by the type of loading device, the capacity of the conveyance, the grade and quality of the coal, and the nature of the change in the speed of the conveyances before they stop.

By means of time-study observations we obtained the loading and unloading times of skips in 9 hoisting installations of the Kuzbass. Data on the loading and unloading times of the skips of these installations are given in Table 2.

Usually the technical literature gives the pause time as:

\[ (\Theta)\quad \begin{array}{ll} \text{for skips with a capacity up to } 6\,t, & \Theta = 8\ \text{sec},\\ \text{for skips with a capacity } 6 \div 8\,t, & \Theta = 10\ \text{sec},\\ \text{for skips with a capacity } > 8\,t, & \Theta = 12\ \text{sec}. \end{array} \]

In actual practice of skip installations, as a rule, a longer pause duration occurs (see Table 2). However, these pauses can be reduced, bringing them down to the time required for loading the skip when it is stopped. In this case the average duration of the pauses, according to Table 2, will be:

\[ \begin{array}{lll} \text{for skips with a capacity up to } 4\,t & — & 7\ \text{sec},\\ \text{for skips with a capacity } 6\,t & — & 8\ \text{sec},\\ \text{for skips with a capacity } 7\,t & — & 10\ \text{sec},\\ \text{for skips with a capacity } 8\,t & — & 11\ \text{sec}. \end{array} \]

Table 2

Loading and unloading time of skips

Here it is necessary to draw attention to two circumstances. First, for all types of skips the unloading time is considerably less than the loading time. This occurs because the outlet opening in the bottom of the skip, through which unloading takes place, is usually larger than the cross-sectional dimensions of the feeder trough. In addition, the chute of the bunker connecting the bunker with the feeder gate has an insufficient angle of inclination ($35^\circ$–$40^\circ$ to the horizontal), which slows the time of coal transfer through it (especially when the coal is fine and wet). In our opinion, the indicated angle of inclination should be $>45^\circ$.

Second, the greater the speed at which the skip is seated on the feeder carriage, the faster the skip is loaded, since the coal located in the feeder in this case more quickly leaves the state of rest. This circumstance is confirmed especially clearly by the operation of the loading device of skip installation 1. There, when loading fine wet coal, it is necessary several times to raise the skip above the feeder carriage and lower it onto it again for loading, as though shaking the coal.

Analysis of the actual velocity diagram

Period of accelerated motion

The time of accelerated motion of the vessels, as is known, depends on the nature of the starting of the hoist motor (automated or non-automated) and on the maximum speed.

With automated starting of the motor, the time of its acceleration depends on the correctness of the setting of the rotor contactor panel; with non-automated starting, it depends entirely on the experience of the hoisting-engine operator.

Among the hoisting installations under consideration, in the skip installations of mines designated by Nos. 6, 7, and 8, liquid starting rheostats are used for the hoist motor. In hoisting installation No. 2, a generator–motor system is used. In these hoisting installations the starting of the hoist motor is not automated. It should be said that, owing to the experience of the hoisting-engine operators, the starting of the hoist motors in these installations proceeds smoothly and quickly. Here the operators have long service records and have studied in detail the operating regime of the hoisting installation. Thus, for example, at mines Nos. 6 and 7 the velocity diagrams were taken when the hoisting-engine operators Krapivin and Glazko were working; each had 23 years’ experience as a hoisting-engine operator.

In the other hoisting installations (see Table 1), metal rheostats are used as starting rheostats for the hoist motors, with the rotor contactor panel adjusted for automatic starting of the motor. It should be noted that in installation 1 the starting of the hoist motor takes place impermissibly rapidly, and the increase in speed occurs in large jumps. Here, in addition to incorrect adjustment of the contactor panel, the total resistance of the starting rheostat is small. In installations 3 and 5, because of incorrect adjustment of the rotor contactor panels, the period of accelerated motion of the hoisting machine is greatly lengthened.

In general, it may be said that, by selecting the starting rheostat and adjusting the rotor contactor panel in accordance with the operating regime of the hoisting installation, the time of accelerated motion can be reduced, on average for all installations, by 5–10%.

Period of Uniform Motion

The speed of motion of the vessels during this period depends on the asynchronous speed of the hoisting motor and on the gear ratio of the hoisting-machine reducer.

Here it is necessary to draw attention to two circumstances.

First, the duration of motion of the vessels at maximum (uniform) speed in all the skip installations under consideration has been reduced by increasing the time of retarded motion. In individual cases this time can be increased by 50% or more, which in turn sharply increases the productivity of the hoisting installations.

Second, in some new hoisting installations the maximum speed is artificially reduced by introducing an additional (ballast) resistance into the rotor circuit of the hoisting motor. In this case the motor operates not on its natural (automatic) characteristic, but on a rheostatic one. This condition, besides reducing the maximum speed, leads to an unnecessary loss of energy in heating the rheostat.

Period of Retarded Motion

The main part of the travel time of the hoisting vessels in all installations is taken up by the period of retarded motion, and in such hoisting installations as those of mines No. 5 and No. 9 it accounts for more than 50% of the skip travel time. A considerable part of the time of retarded motion is spent on the period during which the skip is seated on the feeder carriage. In fact, on installation 1, the distance traveled by the skip together with the feeder carriage amounts to 0.48% of the hoisting height. Yet 15.9% of the travel time is spent on it (see Table 3). On installation 5, correspondingly, the skip travels 0.53% of the path in 17.5% of the travel time. On installation 9, 1.14% of the path is traveled in 34.4% of \(T_{dv}\), etc. This circumstance is caused chiefly by the imperfection of the seating devices for skips, by the presence of rigid seating of the skips. In this case, at a high seating speed, large dynamic stresses arise in the seating and dosing devices, which leads to the failure of the latter. Because of the fear of causing an accident in the loading device, even experienced hoisting-machine operators stretch out this time considerably, and it amounts to 7–11% of the travel time (see Table 3).

Table 3

Analysis of the time of the retardation period \((t_3 + t_4)\) of skip hoisting installations

To reduce the time for seating the skips on the feeder carriage and to reduce the time of the entire period of retarded motion, the need has arisen to use elastic seating of the skips.

Comparison of the Actual Velocity Diagram with the Theoretical One

From the material presented above, it is clear that the actual velocity diagram differs significantly from the theoretically possible one. This is caused mainly by an increase in the time of slowed motion and a reduction in the time of motion of the vessels at maximum speed, for the reasons indicated above.

Comparative data for the theoretical and actual velocity diagrams for various skip installations are given in Table 4.

Table 4

Comparison of the actual velocity diagram with the theoretical one for various skip installations

It follows from this table that the actual cycle time of the skip hoisting installations under consideration is 14–62% greater than the theoretical time, and on average for the 10 skip installations given in the table, it is 36.8% greater than the cycle time under the theoretical velocity diagram. Consequently, by bringing the actual velocity diagram closer to the theoretical one, we will increase the productivity of the skip installations under consideration by an average of 36.8%. In addition, reducing the cycle time will bring about a large reduction in the energy consumption by the drive of the hoisting machine, since under the existing operating mode a considerable amount of energy is expended on “pulling” the skip up to the dispenser carriage and seating it on the latter.

Conclusions

1. The actual velocity diagram of skip hoisting installations differs significantly from the theoretical one. This difference is mainly due to the time of slowed motion: the actual time of slowed motion is greatly increased.

2. The increase in the time of slowed motion is caused by the imperfection of the adopted seating of the skip on the dispenser carriage; instead of the currently adopted rigid seating, elastic seating must be used, i.e., the skip should be seated on cushioning seating devices (damper devices or rubber shock absorbers on the dispenser carriage). This will make it possible to reduce the danger of accidents in the loading device and to reduce the time of slowed motion, which in turn will lead to an increase in the productivity of skip hoisting installations by approximately 30–35%.

1. To reduce the loading time of skips, the sleeve of the loading bunker connecting the latter with the measuring hopper must be installed at an angle of 45–50° to the horizontal. This is especially important for fine and wet coals.

REFERENCES

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Noted Misprints