Dynamics of Skip Loading

I. K. Khrustalev

Introduction

In our technical literature very little attention is paid to questions of skip loading. Usually all discussions of the choice of a possible speed diagram for skip hoisting installations are conducted without taking this factor into account, which leads to a considerable increase in the time of the hoisting operation, compared with what it would be if the speed diagrams recommended in the technical literature were used. This increase in the time of the hoisting operation is due chiefly to the low actual speed at which the skip is set down on the feeder carriage and to the increase in the time required for loading the skips.

We have made observations of a large number of loading devices in the Kuzbass. On the basis of these observations we would like to share our impressions on this question, which is of no small importance in the operation of skip hoisting installations, especially since the speed at which the skip is set down on the feeder carriage in some cases drops to \(0.1\ \text{m/sec}\), and the time during which the skip moves together with the carriage is sometimes extended to \(16\ \text{sec}\).

Types of Loading Devices Used in the Kuzbass

Feeder Device

In skip hoisting installations of Kuzbass mines, as a rule, self-feeding loading devices with drum gates are used. The drum gate occurs in two varieties. In one case the counterweight, by means of which the feeder gate is closed, is fastened directly to the drum (see Fig. 1). To limit the rotation of the feeder drum gate in this case there is a limiting device (usually a wooden beam fastened to the foundation of the gate). In the other case (see Fig. 2) the counterweight of the drum gate moves in separate guides and is connected with the drum by means of a rope. As experience in the operation of skip hoisting installations in the Kuzbass shows, in the second case the feeder device operates more reliably, permitting a greater impact load and, consequently, a higher speed of the skip when it is set down on the feeder carriage. Conversely, the first variety of drum gate does not permit high landing speeds of the skip. Here, during sharp rotations of the gate as it opens and closes, dangerous mechanical stresses arise in the side plates of the unbalanced part of the drum (at the place where the weights are attached), which causes the weights to tear off and the drum plates to rupture. Such repeated accidents were observed, for example, at the hoisting installation of one of the mines of the “Kaganovichugol” trust, etc. This situation makes it necessary to set the skip down on the feeder carriage at a very low speed, which is extremely

slows the period of skip landing (in this installation it is 15.7 sec). This same circumstance makes it impossible to operate with a large acceleration at the beginning of the starting period, since in this case a large impact force arises when the gate drum strikes the limiting beam. These statements are confirmed by the fact that, as is known, the impact—

Fig. 1. Self-metering loading device

1 — Metering-gate drum.
2 — Counterweight.
3 — Limiting beam.
4 — Metering chute.
5 — Loading-bunker sleeve.
6 — Metering carriage.

Fig. 2.
Self-metering loading device

force is directly proportional to the mass of the colliding bodies and to the velocity of the body before impact, and inversely proportional to the time of impact, i.e.

\[ P_{ud}=\frac{mv}{t_{ud}}. \]

With equal rotational speeds of the gates of both types and equal \(t_{ud}\), the impact force arising in the gate drum shown in Fig. 1 is greater, since in this case its mass \(m\) is greater.

Types of carriages of metering devices for skip landing

In skip hoisting installations of the mines of the Kuzbass, two types of carriages of metering devices are encountered: with translational and with rotary motion. In the first case the carriage frame runs in guides; in the second, it is suspended by means of rods or ropes of the metering device and turns under the action of the descending skip and the counterweight in the shaft sump.

For landing the skip, carriages with translational motion have a part on which, to soften the impact of the skip, a wooden beam 150–220 mm thick is usually placed.

In carriages with rotary displacement, the skip lands either on a landing roller or on a metal beam.

The travel of a carriage with translational displacement is usually \(1.4 \div 1.9\) m, and that of rotary displacement is \(0.6 \div 1.6\) m. Carriages with translational displacement, owing to the presence of a wooden cushioning beam on the landing pedal, operate under more favorable conditions; conversely, in carriages with rotary displacement there have been cases of breakage of the landing roller or of its being torn out of the frame fastening. This circumstance makes it necessary to limit the landing speed and to reinforce the elements of the loading device.

Connection of the carriage with the feeder gate

The connection of the carriage with the feeder gate is made either by means of steel rods or by ropes. Usually two rods or two ropes run from the carriage to the gate drum (one on each side).

It should be noted that a rope connection is more reliable, and in this case the metering device operates under more favorable conditions, since the ropes, possessing cushioning properties, reduce the mechanical stresses in the metering device at the moment when the skip strikes the feeder carriage. Thus, for example, in one of the mines of the Kaganovichugol Trust, even with spring shock absorbers at the point where the rods were attached to the feeder carriage, repeated ruptures of the connecting rods occurred. When the diameter of each rod was increased to 30 mm, the lugs by which the rods are fastened to the gate and carriage were torn out. After the rods were replaced by steel ropes of \(d = 21.5\) mm, no failures in the connections were observed for a long time, although the spring shock absorbers were removed together with the rods. Spring shock absorbers are sometimes used to attach rods or ropes to the feeder carriage. A diagram of such a shock absorber is shown in Fig. 3.

Fig. 3. Spring shock absorber

1 — Shock-absorber body (cylinder), \(d_{\mathrm{c}} = 20 \div 25\) cm.
2 — Spring (conical or cylindrical), \(l = 20 \div 25\) cm.
3 — Rod.
4 — Thrust washer.
5 — Lugs for fastening the shock absorber to the feeder carriage.
6 — Lug of the body with a hole for fastening the shock absorber to the rod (rope).

Such shock absorbers, while not improving the operating conditions of the feeder carriage itself, undoubtedly improve to a considerable extent the operation of the connections and of the gate of the metering device, since they soften the impact shocks when the skip lands on the feeder carriage.

Among the shortcomings of these shock absorbers it must be noted that, in view of the unfavorable conditions of their operation (usually under conditions of heavy contamination and dripping water), springs with a wire diameter of \(16 \div 18\) mm have to be installed, and this reduces the cushioning properties of the shock absorber. In addition, the spring shock absorber causes oscillatory motions of the feeder gate, and consequently oscillatory stresses in it.

Forces Produced in the Elements of the Loading Device and Ways of Reducing Them

The greatest stresses in the feeder carriage, in the connections from the carriage to the metering device, and in the metering device arise, as is known, at the moment when the skip lands (strikes) on the landing device. These stresses are the greater, the shorter the time during which the carriage velocity increases from zero to its maximum when the skip lands on it.

From mechanics it is known that the force causing acceleration of the carriage is equal to \(P = m_k j_k\) \([kg]\), where \(m_k\) is the mass of the carriage in \(kg \cdot m^{-1} \cdot sec^2\); \(j_k\) is the acceleration of the feeder carriage; it is equal to \(j_k = \frac{v}{t}\), where

\(v\) is the maximum speed of the carriage when the skip lands on it,

\(t\) is the time during which the speed of the carriage increases from zero to \(v\).

In view of the fact that the mass of the skip is usually greater than the mass of the carriage, the maximum speed of the carriage \(v\) differs little from \(v_{l.s.}\)—the landing speed of the skip on the feeder carriage. Neglecting this difference, we obtain

\[ j_k = \frac{v_{l.s.}}{t} \ [m/sec^2], \]

and the expression for the force will be:

\[ P = m_k \frac{v_{l.s.}}{t} \ [kg]. \]

From this expression it is evident that, in order to reduce the force \(P\), with the mass of the skip and the mass of the carriage unchanged, it is necessary to strive either to reduce the landing speed \(v_{l.s.}\) or to increase the time \(t\), i.e., to increase the time during which the speed of the feeder carriage increases from zero to its maximum value after it comes into contact with the skip.

Both for reducing \(v_{l.s.}\) and for increasing \(t\), damping devices may be used. To satisfy the first condition, it is necessary to use a landing device that would make it possible to bring the skip close to the feeder carriage at the greatest possible speed, and then, over a short section of the skip’s path, reduce its speed to the permissible landing speed, i.e., to a speed (not more than \(0.5\ m/sec\)) at which dangerous stresses are not produced in the elements of the loading device.

A damping device can successfully serve as such a landing device. For our conditions, the most suitable are air-oil or oil damping devices, similar to those used as recoil and counter-recoil attachments in powerful artillery systems.

To increase the time \((t)\) of the rise in the carriage speed, in our opinion, it is necessary to use shock absorbers and install them on the feeder carriage. In this case the skip will land not directly on the feeder carriage, but on the indicated shock absorbers. The purpose of these shock absorbers will be: 1) to absorb the impact of the skip descending at the maximum permissible speed; 2) to increase the time \(t\) of the rise in the speed of the feeder carriage from zero to the maximum upon its contact with the descending skip.

From the above formula for \(P\) it is quite clear that by however many times the time \(t\) of the rise in the speed of the feeder carriage increases, by the same number of times the mechanical stresses in the loading device will decrease, which will make it possible to increase significantly the landing speed of the skip. The forces subsequently produced during deceleration of the skip are insignificant and cannot produce dangerous stresses in the metering and landing devices.

Proceeding from the above, the shock-absorbing device must operate under the following conditions:

1) absorb the frequently repeated impacts of the skip during operation of the hoisting installation—hence follows its sufficient strength;

2) increase the time of rise of the speed of the carriage when the skip lands on it, thereby ensuring minimum stresses in the landing and dosing devices—hence follows its sufficient shock-absorbing properties.

Analyzing various kinds of shock-absorbing materials, one may conclude that rubber pads, consisting of a stack of rubber washers, and buffer spring devices most closely meet the operating conditions and the requirements imposed on shock absorbers. However, it should be noted that steel has little capacity to absorb energy; thus, 1 kg of a steel spring can absorb about 45 kgm of energy, whereas 1 kg of rubber absorbs about 450 kgm of energy,

Fig. 4. Rubber laminated shock absorber.

1—Stack of rubber plates. 2—Metal box, which is fastened to the feeder carriage. 3—Box cover. 4—Guide rod. 5—Guide pins. 6—Oblong cutouts, permitting movement of the cover under the impact of the skip.

i.e., 10 times as much; thus, other conditions being equal, steel shock absorption will be 10 times heavier than rubber shock absorption. In addition, steel buffer springs, owing to frequently repeated impact loads, contamination, and the damp mine atmosphere, often fail, and they cannot be recommended for use as shock absorbers on the feeder carriage.

The most reliable device under these conditions will be rubber shock-absorbing pads, which should be installed on the feeder carriage. A diagram of such a shock absorber is shown in Fig. 4.

Dimensions of Shock Absorbers

Many years of experience in the operation of rubber shock absorbers under conditions similar to ours (in aviation, as shock-absorbing devices of landing gear), and the tests we have carried out with various kinds of shock absorbers, make it possible to draw the following conclusions:

1) the best results are given by rubber shock absorbers consisting of separate plates 10–20 mm thick, with metal spacers between the rubber plates, since in this case the entire rubber layer of the shock absorber participates approximately to the same degree in compression and in absorbing the energy of the impacting body;

2) the thickness of the individual plates for uniform compression and energy absorption must be no more than 20 mm;

3) the shock absorber should not be made up of more than 20 plates, since such a shock absorber will have a tendency toward one-sided bulging as a result of curvature and displacement of the plates;

4) for reliable and prolonged operation of the shock absorbers, the safety factor at maximum static load must be not less than \(n = 8\). For soft grades of rubber, on average one may take the specific load on the bearing surface of the shock absorber as \(p \leq 18\ \text{kg}/\text{cm}^2\) under static load.

The maximum static load on the shock absorbers for skip landing will consist of the dead weight of the skip \(Q_m\) and the weight of the useful load \(Q\), i.e. \(Q_o = Q_m + Q\).

It is proposed to provide two shock absorbers on the carriage; consequently, the bearing surface for each shock absorber will be

\[ f = \frac{Q_o}{2p} = \frac{Q_o}{36}\ [\text{cm}^2], \]

where \(18\ \text{kg}/\text{cm}^2\) is the permissible specific pressure,

\(Q_o\) is the maximum static force. For example, for a skip with a capacity of 8 t and with a body that tilts during unloading, the dead weight is \(Q_p = 6075\ \text{kg}\).

The bearing surface of the rubber shock absorber will be

\[ f = \frac{14075}{36} = 393\ \text{cm}^2 \]

or rubber plates \(20 \times 20\ \text{cm}^2\) are required.

To improve the operation of the plates, they must be able to expand sideways and, moreover, for more uniform expansion, plates larger than \(15 \times 15\ \text{cm}^2\) must have internal openings.

We have installed such shock absorbers at one of the mines of the Anzherougol Trust. Observations of their operation show that the landing speed of the skip in this case can be brought up to \(1\ \text{m/sec}\) and higher, which substantially reduces the time of landing the skip on the feeder carriage and, consequently, shortens the travel time of the conveyances as a whole. The operating practice of this installation gives grounds to assert that, with the use of rubber shock absorbers 120–150 mm high, the speed of landing the skip on the feeder carriage ceases to be a limiting factor in the operation of skip installations.

Conclusions

1. From the material presented above, it is necessary to conclude that the bottleneck in the operation of skip hoisting installations is the loading device and the adopted rigid methods of landing skips on feeder carriages, which considerably increases the time of retarded motion and the operating cycle of the skip hoisting installation as a whole, and hence reduces productivity. This question of improving the operation of the loading device requires immediate resolution.

2. As gates for dosing devices, in our opinion, the most acceptable is a drum gate in which the counterweight travels along its own guides and is connected to the gate drum by means of a rope. However, when using elastic landing of the skip on the feeder carriage and using rubber shock absorbers (made of rubber washers) as limiters of the drum rotation when closing the gate, a drum gate with a counterweight fastened directly to the gate drum can also be successfully used.

1. As carriages for metering devices, both carriages with translational motion and carriages with rotational motion may equally well be used.

2. In order to improve the mode of skip landing and the operation of the metering and landing devices, in both cases it is necessary to use damping devices that would make it possible to bring the skip up to the feeder carriage at maximum speed without causing breakdowns in the metering and landing devices. The simplest such devices are rubber shock absorbers installed on the feeder carriages.

3. As connections from the carriage to the feeder gate, steel ropes should be recommended; possessing sufficient strength and elasticity in operation (which is especially important under impact loads), they contribute to improving the operation of the feeder.

4. For final fixation of the skip when it is loaded onto the landing timbers or beams used in practice, rubber damping pads must be installed. This will reduce the dynamic stresses in the skip and in the landing timbers when the skip comes to its final stop.

By applying the measures indicated above, it is possible successfully to increase the productivity of existing skip hoisting installations by 35% and more without significant additional costs.

Moreover, these measures will improve the dynamics of skip loading, which, in turn, will improve operating conditions and, consequently, increase the service life of the skips and of the landing and metering devices.

LITERATURE

1. German A. P., Shklyarskii F. N. Mine Hoisting Installations. Ugletekhizdat, 1947.
2. Elanchik G. M. Mine Hoisting Installations. Gostoptekhizdat, 1941.
3. Strugin L. I. et al. Construction and Design of Aircraft, vol. 1. Aircraft Construction, OboronGIZ, 1939.
4. Makarov S. Ya. Hydraulic Shock Absorbers. TVF, No. 7, 1936.
5. Rostovtsev G. G. Structural Mechanics of the Aircraft, part II. ONTI, 1936.