Ensuring that a hoisting plant is safe and will run when it is needed requires more than the mandatory tests. The true price is constant vigilance aimed at detecting potential problems. Aware of the consequences of unscheduled hoisting interruptions, managers must participate constantly in a program that not only conforms with the regulations but uses them as minimum require ments. The hoisting system starts at the bottom of the shaft at the loading pocket, seldom visited except by maintenance and shaft inspection people. The system ends in the hoistroom, which is usually the showplace of the mine. Normal interruption and delays do not apply to the hoistroom stationary equipment. It is in the balance of the system that hoisting time is lost for various reasons.
Often, important safety and maintenance design features are overlooked or ignored in the rush to get the production shaft going. In their remote inhospitable environment, the measuring and loading pockets somehow perform their functions in the time allotted. And why not? They are simple devices. The only moving parts are gates, air cylinders and latch arms, and there is little movement. Never theless, lubrication is needed and the compressed air should be reasonably dry, two fundamentals that are often only summarily addressed.
Access in the pocket area is always a problem. Loading hang-ups have to be cleared quickly and safely. Liners have to be changed. Below the loading pocket there is continuing need for shaft inspection, rope divider inspection and replacement, alignment of guides and examination of the arrestment gear and crashbeams. This area is vulnerable to damage from spillage. Work is not possible from the tops of conveyances, and special facilities and access are required to enable work to be performed safely and efficiently. Some work can only be done safely from special work platforms suspended from the conveyances.
It is interesting to note that skips in some installations in Europe are loaded from two hoppers on either side of the compartment. This cuts in half the loading time. The normal 30 seconds is reduced to 15 seconds using twin hoppers.
Improvements in the design, fabri cation and installation of mine guides will be important in the future. There is a trend towards the hoisting of greater tonnages from greater depths. Production hoists exist that are capable of carrying 42-ton payloads at 4,000 ft per minute, as well as service hoists handling up to 120 people or loads weighing 20 tons.
Smooth and flexible joints are by far the most important characteristic of a good guide system in hoists. All other characteristics are of relatively secondary importance.
Wood guides have been in use for a long time and are necessary with dogging-type safety devices. Wood guides are less expensive than steel guides and are adequate for hoisting medium payloads at lower speeds. A speed of 2,200 ft per minute (11 m per second) is about the practical limit with a medium-sized skip. Wood guides are subject to changes in dimensions with moisture, and they wear faster than steel guides. This makes it difficult to obtain and maintain a perfect wood guide string. The maximum joint offset should be one- sixteenth of an inch, and the face-to- face measurement should be maintained so that there is always clearance between the conveyance wear shoes and the guides, though this should not exceed a total of one- quarter of an inch.
Wood guides require more maintenance than steel guides. If a safety mechanism is installed on a conveyance, the distance between the surface of the guide and the fastener must be sufficient to ensure that the cutting edge of the safety dog will not strike the bolts when engaged.
When designing a steel guide sys tem, the joint design should permit the changing of guide easily. In prac tice, steel guides rarely have to be changed because of damage. The chief maintenance practice involves checking the bolts for tightness and breakage. In a good system, loose bolts are rare. Breakage may occur where a major shock loading, from misalign ment or high-speed skipwheel break age, is applied at a guide-bunton connection.
Originally, slippers were used on conveyances to provide the riding surface against the guides. Guide rollers are now used in addition to slippers. Their introduction with spring or torsion-type suspension has decreased the shock loading on the guide string. Most guide roller systems do not allow for more than 3/4 inches (20mm) of play before the shoe comes into contact with the guide. Although the diameter of the wheels should increase with the speed and the weight of the conveyance, fairly small wheels (12 inches, or 30.5 cm, in diameter) are a common size.
Rope guides are the smoothest of all guide systems, having no joints. Hoist speeds up to 4,000 ft per minute (20 m per second) are feasible. Rope guides are not satisfactory for multi- level loading operations. They eliminate the need for certain buntons, increasing the ability of the shaft to handle ventilation air. More space is required in a compartment and the deeper the shaft, the easier it is for the rope to be displaced.
Steel is the most common material for conveyance construction. Under corrosive conditions, stainless steel components are sometimes substituted. When rope and hoist capacity are limited, lightweight aluminum alloy conveyances are used. Precautions are needed when steel and aluminum are mixed to keep galvanic corrosion to a minimum. Aluminum alloys are brittle and will fail without noticeable yielding. They are susceptible to fatigue failures under cyclic loading.
Occasionally, large bulky pieces of equipment must be transported under ground by slinging them beneath the cage. An adequate number of slinging points must be available with suitable capacity to handle the hoist load rating. Removable covers on the cage floor can simplify the observation of the load in transit down the shaft.
Wear shoes on cages operating on rigid guides will require replacement several times. If this can be done from inside the cage, the downtime is reduced and conditions are safer. The wear shoe material should be a hard, tough, high-carbon steel for wood guides and a low-carbon mild steel for steel guides.
An examination of reports of hoisting incidents will reveal that a large proportion involve cage doors and cars or material protruding into the shaft.
Early designs of front dump skips with no bails caused excessive guide wear and loose guide connections at speeds over 2,000 ft per min. Decelerometer tests confirmed that misaligned guides resulted in high lateral loading on the guide string because of the rigidity of the skip design. Revisions were made with wear shoes along the length of the conveyance to distribute the load. Most designs now use the box as the load-carrying member and provide a drawhead at each end for suspension attachments. This feature is useful with large payloads because the skip can be made in sections and assembled in the shaft when headroom is restricted.
The bottom dump skip consists of a bucket suspended in a bail with a door on the bottom. As the skip enters the dump, the bottom is pulled out of the bail with a scroll, and a 4-bar link mechanism causes the door to open. The jarring is usually sufficient to clear any sticky ore in the bucket. About 8 ft of travel is required during the dumping operation.
Some skips require a positive latch- proving device to ensure that the latches for the doors are closed each time the skip leaves the dump. The devices must be physically tested each working day and, unless proper access is provided, they will not be adequately maintained. The opening of a skip door in the shaft is always disastrous.
Sheave wheels must be inspected each day of hoisting. A tedious and dirty task. Much of the excess rope dressing is flung off at the sheave deck and attempts at housekeeping naturally lack enthusiasm.
One factor that directly affects hoisting rope life is the design of the rope groove in the sheave. The radius at the bottom of the groove where the rope rests should be 10% larger than the nominal radius of the rope on a new or re-machined sheave. When the radius reaches 5%, it should be re-machined. The flange angle should also be opened to the original condition of 36 degrees . Some users prefer to use sheaves with liners to extend the life of the sheave. Usually, when an unlined sheave has reached its maximum wear limit, the sheave itself, through stress, corrosion and general deterioration, has also approached its rated life.
Experience has shown that painting the sheave with a lead-base paint is ineffective against corrosion, but there are several asphalt and coal tar compounds available that are satisfactory. Particular attention should be given to the cavity between the centre of the hub and the sheave shaft. Moisture collects in this cavity without being detected on visual inspections.
The hoist controller is the most important device in the safety system. It monitors the speed of the hoist in relation to the position in the shaft and ensures that the conveyance is operating within the limits of shaft travel. When equipped with a man safety feature, at the operator’s option, hoisting speeds can be adjusted downward, and slowdown and travel limits adjusted to ensure that the workers cannot be hoisted into the skip dump. Controllers for drum hoists, operating in shafts with multi-horizon loading systems, are equipped with level depth indicators that enable the underwind limits to be set for the particular horizon from which ore is being hoisted. This enables deflectors for spillage to be inserted below the underwind position. The deflectors are interlocked to ensure that they are out of the shaft prior to hoisting from a lower level.
It is mandatory that each hoisting compartment be equipped with a switch that is physically actuated by the conveyance when it approaches the highest limit of travel in the headframe. This switch is the track limit switch. When it is actuated, power is removed from the hoist and the brakes are applied at their fastest rate. A backout switch is required in the circuit to allow the conveyance to be moved out of the overwound position safely. On some installations, the backout position is keyed so that it is impossible to back out of the track limit without a key custodian ensuring that no damage has been done to the conveyances, ropes, sheave, or deflectors at the shaft bottom. Ontario government regulations require that minimum clearances be provided in the headframe and in the underwind area, to help ensure that the conveyances will stop before hitting an obstruction.
To meet the requirement of the code limiting deceleration of the conveyance, a number of deep-level hoists installed since 1970 have been equip ped with an electronic brake governor. This compares the rate of change of hoist speed with an adjustable rate of change reference and controls the brake application pressure so that conveyance deceleration is within the required limits.
Braking systems on Canadian- manufactured friction hoists employ disc brakes. Pressure-applied and spring-applied arrangements are used. All designs employ pneumatic systems and pressure-applied brakes are provided with gravity backup. To determine the deceleration rates of the conveyance, a transistorized telemetering decelerometer is used. Brakes can be adjusted without re course to complex circuitry or the use of a brake governor.
Periodically, serious accidents occur when a clutch is withdrawn with the brakes released. This results in a conveyance freefall to the shaft bottom. Defects in the interlock system because of mechanical wear, incorrect adjustments to linkages or electrical devices are the usual cause.
After extended operation, the rope grooves on the hoist drum may wear off-pitch, resulting in reduced rope life with each successive rope change. Machining is required to restore the groove radius and the correct pitch.
Slack rope devices, actuated when the rope sags between the hoist and the sheave, sound an alarm when the hoist is operated manually, or initiate an emergency stop when it is on automatic or semi-automatic control.
Last-resort protection against overwind is provided for all friction hoists by conveyance arresters at the ends of travel in each compartment. The design retardation rate of 90% of the acceleration due to gravity (0.9 g) in the headframe is to prevent men and materials from being thrown upwards and to prevent the balance ropes from overtaking the arrested conveyance. The rate of two times the acceleration due to gravity for the shaft bottom corresponds to the design rate with safety dogs on a drum hoist cage.
Among the many variables that affect selection of a hoisting system are two which reduce hoisting time: the allowance for maintenance in spection, lubrication and minor running repairs and the allowance for interruptions and delays. In his computer program, Fred Edwards of Dynatec Mining uses one hour per working day for maintenance (excluding major routines), as well as a hoist utilization factor to cover interruptions and delays of 85% for a production shaft and 75% for a combined service and production shaft. He finds that these parameters are generally acceptable. REFERENCES Albert, L. and R. A. McIvor, Engineering and Maintenance Considerations in the Selection of a Mine Hoisting Plant, Inco Metals Co., 1981. Redpath, J. S., Towards a Better Understanding of Mine Shaft Guides. * Keith Bowley, a regular contributor to The Northern Miner Magazine, is a Toronto-based maintenance consultant.
Be the first to comment on "Maintenance MAINTAINING THE HOIST"