• Author / Uploaded
• Joël Mwabi

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ABB Mining Tel: +27 10 202 5000 Fax: +27 11 579 8000 Email: [email protected]

shaft hoisting systems

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1. General



Shaft hoisting system using mine hoist is often the most economical means of transporting men, material and ore from depths ranging from a few hundred meters to more than 3000 m. In many mines, vertical shafts are combined with a ramp for transportation of men and material.

At 180 degrees contact angle and friction coefficient 0.25, rope slip will occur at rope pull ratio 2.19. Since there must be sufficient safety margin to rope slip the static unbalance ratio should not exceed 1.5. This is a general rule of thumb that is valid for tower mounded hoist without deflection sheave. At ground mounting the ratio shall be close to 1.4 due to the inertias of the head sheaves. In order to achieve the unbalance ratio, friction hoists are provided with balance or tail ropes suspended under the conveyances. The tail rope weight per meter should normally be the same as the head rope weight per meter. This also gives a favourable torque characteristics, the driving torque is constant during travel in the shaft with constant speed.

In deep mines and in mines with poor rock stability ramps are not technically or economically viable. In such mines it is common to sink both a production and a service shaft. The production or the service shaft can also be used as ventilation shaft. The optimum hoisting speed depends on shaft conditions and the hoisting distance.

The ratio between the pulley diameter and the head rope diameter (D/d) shall not be smaller than a value that is determined by the relevant mine authority or corresponding document. This is to reduce the bending stresses. The normally accepted minimum D/d ratio is 80 for stranded ropes and 100 for full locked coil types.

The general rule of thumb is that the optimum speed is:

V = k: a:s Where: V k a s

= The friction coefficient between the rope and its friction lining on the pulley, normally 0.25 = The contact angle between the rope and the pulley in radians

Friction hoists are normally provided with several head ropes sharing the suspend load. Multiple head ropes means smaller rope diameter. Thereby the pulley diameter can be reduced as well as the motor torque.

= hoisting speed, m/s = constant in the interval 0.4 - 0.6 = mean value of acceleration and retardation, m/s2, normally ranging from 0.7 - 1.0 m/s2 = hoisting distance, m

The hoisting speed should not exceed 20 m/s. At k>0.5, the motor power increases faster than the production at increased k (=increased speed).

The most common numbers of head ropes are 4 or 6. Earlier 8 and even 10 head ropes were used but due to higher maintenance in keeping the load sharing equal these versions are now abandoned. There are also a few hoists installed with odd number of head ropes - 1, 3 and 5.

2. Types of hoist

2.1.1 Mounting arrangements

There are two dominant types of mine hoists, friction and drum types. Both types can be arranged in several different ways.

Friction hoists can be either tower or ground mounted. Tower mounted hoists often need rope deflection sheaves to get the optimum distance between the head ropes in the shaft i.e. minimize the shaft diameter.

2.1 Friction hoists The friction hoist is the most common type used in northern Europe and many other countries such as Germany, Poland, China and Australia.

Ground mounted hoists have two sets of head sheaves in the head frame above the shaft. This type is normally not used in climates with snow and ice since there is a larger risk of rope slip due to lower friction coefficient and risk of ice falling down from the exposed ropes.

The principle of friction hoist is that its head rope(s) is laid over the pulley making about 180 degrees contact angle between the rope and its friction lining on the pulley. The rope pull of the two sides of the pulley should be within certain limits to obtain the required margin to rope slip. This relation can be expressed as:

2.1.2 Sizes and hoisting depths The hoisting depth range for friction hoists is from a few hundred meters to more than 1500 m. At hoisting distances smaller than about 400 - 500 m it is often economical to use counterweighted friction hoists thereby avoiding large ballast weights on the conveyances to maintain the rope tension ratio required.

T1 na T2 # e Where T1 T2 e

= Rope pull (tension) one side, N = Rope pull on the other side, N = The natural logarithm 2,7182…

With the increased depths and production rates required in today’s mines hoisting systems are getting larger and larger.

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4. Electrical drive system

Pulley diameters up to 7 meters and motor power about 10 000 kW are required to provide production rates up to 10 million tones per year. Skip payload can be more than 50 ton.

Modern hoists are powered by AC motors. Smaller hoists with power requirement about 1500 kW normally use gear box with high speed induction motor. For larger hoists, direct drive with overhung synchronous motor is used. Speed control is made with voltage source inverters, VSI frequency convertors.

2.2 Drum hoists Drum hoists are the most common types of hoists in North America and South Africa. The reasons are both traditional and in particular in South Africa also because of the deep mines that makes friction hoists unsuitable.

5. Brake system Spring applied, hydraulically operated disc brakes are used for mine hoists. Two brake discs are required for safety reasons.

Drum hoists are ground mounted and provided with head sheaves. Drum hoists coil the rope on the drum as apposed to friction hoists.

6. Ropes

The most frequent types of drum hoists are:

6.1 Head ropes

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Rope diameters up to about 54 - 56 mm are used. Above this size they become difficult to handle.

Single drum Double drum Blair multi- rope (BMR)

The stranded rope is the most common type used. The triangular strand is the dominant rope for friction hoists in Northern Europe. In deeper shafts stranded ropes with lower torsion coefficient are used. This is achieved by means of two layers of strands with different rotation direction.

Single drum hoists are normally used for smaller payloads although the hosting distance can be up to 2000 m. The double drum type is the most common type of drum hoist. One drum is coiling while the other drum is uncoiling its rope. Thereby balanced hoisting is achieved; one conveyance is moving up while the other moves up.

In North America the full locked coil type rope is commonly used. This rope is more dense than stranded ropes but made of lower grade steel. It has low torsion factor and less stretch during its life time which simplifies maintenance.

Double drum hoists have only one rope per conveyance. This limits the maximum suspended load of the rope as apposed to friction hoists. When this is not sufficient, the BMR type hoist can be used which allows two ropes to carry each conveyance with its payload. Each drum is then provided with one section for each rope the sections separated by a steel flange.

6.2 Balance (tail) ropes Balance ropes need to have low torsion factor and is made as multi layer ropes with clock- and anti-clockwise roping.

Drum hoists are used with diameters and motor power in the same range as for friction hoists.

6.3 Guide ropes

Double drum and BMR hoists are used in shafts deeper than 3000 m.

Half- locked coil are used exclusively, normally with diameter 4048 mm. The profiled outer wires are about 7 mm thick allowing wear caused by the sliding guide shoe of the conveyances.

3. Configurations

7. Skips

Both friction and double drum hoists can be arranged for the following conveyance and counterweight combinations:

There are different types of skips available. The two dominant types in north Europe are the Jetö type with tilting skip box and the Sala type which has a pneumatic cylinder on board the skip that opens the bottom door at dumping. Air supply is provided via air connectors with mating contacts at skip dumping.

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Double skip Skip with counterweight Skip and cage on one side and counterweight on the other Skip and cage Double cage Cage and counterweight

8. Shaft Modern shafts are normally circular. The most common shaft sinking methods are:

In particular in coal mines it is common to use cars on narrow gauge rails for transport of both coal, waste and material. The cars with its payload are loaded into the cage and brought up or down. Automatic car handling systems load and unload the cars to and from the cage.

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Full face raise boring using pilot hole Down the hole full face boring with pilot hole Conventional shaft sinking by drilling, blasting, mucking and hoisting of the rock.

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8.1 Guide System The hoist conveyances can be guided in the shaft by either fixed or rope guides. Fixed guides are today normally made of steel. Earlier, wooden guides were common. The fixed guides are secured to the shaft wall by the buntons located every 5- 7 m in the shaft. Rope guides are supported only at shaft bottom and in the head frame. They are tensioned by either weights at the shaft bottom or by tensioning devices with load cells in the head frame in which case the lower rope end is fixed. 4 rope guides are required for each conveyance and 2 or 4 for the counterweight. Because the ropes do not prevent some side movement of the conveyances, the minimum clearance to the shaft wall and between conveyances needs to be larger for rope guided system. Rope guides also require fixed guides at loading and dumping to keep the conveyances in position. 9. Skip loading Skip loading can be made in two principle ways: 1. Measuring pocket (flask). The measuring pocket is filled from a conveyor that is feed by a ore bunker via a vibrating feeder. 2. Measuring conveyor. The conveyor is suspended in normally 6 load cells that measure the weight of the empty conveyor (tara). The vibrating feeder fills the conveyor when running at slow speed to load the preset payload. When the empty skip arrives at the loading station, the skip runs at high speed dumping the load into the skip. The loading time is somewhat longer than when using measuring pocket but measuring conveyors are becoming increasingly used since the maintenance of measuring pockets are considered higher and more difficult. 10. Ramp hoisting In addition to vertical shaft hoisting, hoisting in ramps is also done using cars on rails. The machinery can be either drum hoist or friction hoist. Rollers between the rails are required for the rope to avoid damages. Applications are found in shallow mines with depths down to about 300 m mainly in coal and industrial minerals mines.

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