1. Introduction
This section is concerned with ropes, sheaves, and conveyances associated with mine hoists (discussed in the two previous chapters). Wire ropes are fundamental to hoisting practice and, therefore, of great importance. What is not so evident is the role that ropes play in designing and selecting mine hoists. Hoist selection depends on having the ropes determined in advance.
For a drum hoist, the ropes dictate the drum diameter, drum width, rope pull, and hoist inertia. For a friction hoist, the ropes dictate the wheel diameter, tread spacing, tread pressure, and wheel inertia. Most non-mining people have a notion that breaking ropes is a constant danger. They feel more comfortable with a friction hoist because it has more than one rope (like an elevator). The fact is that a single rope in mine service very rarely breaks due to static overload. One reason is that when a rope is extended a great distance, it stretches elastically (not unlike a bungee cord). This yo-yo effect is the cause of a number of problems, none of which are normally life threatening.
Unfortunately, non-mining people (politicians and bureaucrats) arbitrarily assign the safety factors (SFs) that govern rope selection. The result is that statutory SFs determining the use of ropes employed in mines are different from one place in the world to another. Because mine hoists are so dependent upon the ropes, a hoist that provides a certain production capacity at one location cannot be counted upon to provide the same service at a second mine elsewhere.
Standardization is impossible; therefore, all new hoists are custom-built for their application and second-hand hoists usually require extensive modifications before being employed at a new location. Lack of standardization applies also to conveyances.
Every mineshaft seems to have a different sized cage compartment and a mine-specific payload requirement; therefore, nearly every new cage is designed from scratch. Some standardization of skip compartment sizes exists based on the space provided in the old 6-foot by 6-foot rectangular timber shaft compartment, but even this convention is often forgotten when a new shaft is designed. The predictable result is that shaft conveyances are very expensive and the delivery time is long.
Sheaves are an exception to the lack of standardization problem. Satisfactory head sheaves for a new application can usually be built to off-the-shelf designs and suitable used sheaves are often obtained from the after-market without requiring modifications. Recently, some important advances have occurred in wire rope design. One example is replacement of the old plastic core with a super plastic (such as Kevlar) applied beyond the core and into the strands resulting in a much stronger rope.
2. Rules of Thumb
Ropes
• The actual rope stretch when a skip is loaded at the pocket is almost exactly double that calculated by statics (PL/AE) due to dynamic effect. Source: L. O. Cooper
• The rope installed on a drum hoist or winch should be pre-tensioned to 50% of the working load. Source: George Delorme
• The tension required for a guide rope is one metric tonne for each 100m of suspended rope. Source: Tréfilunion
• The size of guide rope (steel area of cross section in mm2, S) required for guide ropes is equal to 1½ times the length of suspended rope in metres, H. (i.e. S = 1.5 H). Source: Tréfilunion
• The pitch radius of a wire rope thimble should not be less than 3.5 times the rope diameter. Source: Largo Albert
• The length of a wire rope thimble should not be less than five times the pitch radius. Source: Largo AlbertSheaves
• A change in direction of a rope (around a sheave) of 15° or more is generally accepted as constituting a complete bend. At lesser deflections, a grooved sheave should never be less diameter than one lay length (about seven times rope diameter), nor 1½ times lay length for a flat roller. Source: African Wire Ropes Limited
• For every increase in speed of 1m/s (200 fpm), 5% should be added to the sheave or roller diameter. Source: African Wire Ropes Limited
Conveyances
• Conventional practice at hard rock mines is to employ “Kimberly” skips for a payload capacity of up to 5 tonnes and “bottom dump” skips for a payload between 5 tonnes and 20 tonnes. “Arc-door” skips are usually employed for payloads over 20 tonnes. Source: Jack de la Vergne
• The centre of gravity of a loaded bottom dump skip should coincide with the geo-centre of the skip bridle. Source: Coal Gold and Base Metals of South Africa
• The old rule stating that the bridle of a bottom dump skip should have a length equal to twice the set spacing has been demonstrated to be incorrect. Source: Coal Gold and Base Metals of South Africa
• For a fixed guidance system, the bail (bridle) of a bottom dump skip or the length of an integral skip (between guide shoes) should be of minimum length equal to 1½ times the set spacing. For shaft sinking on fixed guides, the crosshead must be of minimum length equal to 1½ times the face-to-face distance between the guides, otherwise it will chatter. On rope guides, the length of the conveyance is of no concern. Source: Jim Redpath
• A properly designed liner system should allow a skip to hoist 30,000 trips before the conveyance is removed from service for maintenance. Source: Largo Albert
• A properly designed liner system should allow a skip to hoist 500,000 short tons before the conveyance is removed from service for maintenance. Source: Largo Albert
• The regular maintenance refit and repair of an aluminum skip costs approximately 35% of the price of a new skip. Source: Richard McIvor
• A properly designed and maintained aluminum skip should have a total life of 5,000,000 tons (including refits and repairs). Source: Richard McIvor
• The cage capacity will be between 1.6 to 1.8 times the empty cage weight. Source: Wabi Iron Works
3. Tricks of the Trade
• The easy way to design a drum hoist is to first determine the required hoisting speed and payload, then determine the rope that is needed to meet the SF. The hoist parameters can then all be determined only considering the hoist rope and line speed. Source: Tom Harvey
• The mass (unit weight) and strength of a hoist rope may be determined without benefit of a rope catalog (as follows).
− Mass: If the diameter of the rope is d .. mm, then mass m = k(d/10)^2 .. kg/m.
Where k = 0.40 for 'flattened strand' ropes, and k = 0.55 for 'lock coil' ropes
Example: For a flattened strand rope of 38 mm, mass = 5.8 kg/m.
− Strength: If the diameter of the rope is d .. mm, then strength S = K(d/10)^2 .. kN.
Where K = 70 for 'flattened strand' ropes, and K = 85 for 'lock coil' ropes
The values of K are for steel of breaking strength 1770 MPa.
The value of K for steels of other strengths can be found by proportion.
Examples: For a flattened strand rope of 38 mm & 1770 MPa, strength S = 1011 kN.
For a flattened strand rope of 38 mm & 2000 MPa, strength S = 1142 kN.
Sources: Mike Gray and N. Brook
• When cutting a sample length (usually 8 feet) from a stranded hoist rope for statutory testing, it is often not practical to seize the rope ends with wire. Instead, the rope ends may be secured before cutting with punch-lock clamps placed side by side and in sufficient number to duplicate to length of the original wire seizing. Source: George Delorme
• A friction hoist counterweight conveyance should be designed to facilitate removal of weights with a forklift at the collar elevation. Source: Largo Albert
• New ropes installed on a winch or tugger hoist should be installed with no gap between rope coils on the first layer in order to provide better support for the next layer. Source: George Delorme
• When a stranded hoist rope goes slack, pigtails will form in the rope. A piece of 6-inch by 6- inch wood or piece of shaft timber inserted in a pigtail allows the rope to be drawn taught without fear of a permanent kink. Source: Bob Dengler
• A tail rope on a friction hoist frequently suffers a kink caused by ropes fouling each other or muck spilled from the loading pocket. The kink may be removed simply by drawing the rope through a post-forming tool four or five times. The tool consists of a bracket with a series of offset rollers that may be purchased from a hoist rope manufacturer. Source: Largo Albert
• Excess rope dressing on lock coil ropes is conveniently removed with neat Portland cement (powder). Source: George Delorme
• The outer layer of a lock coil rope consists of a number of “Z” shaped wires. They are interlocked to form a sealed outer cover so that lubricant cannot exude and water cannot enter. Occasionally, a wire will break and pop out of the cover. If this wire snags an obstruction, it can untwist to such a degree that it will damage the rope, requiring a complete replacement. As soon as a broken wire is discovered, it should be repaired by annealing and tapering so that the wires can be braised together and driven back into the rope to take its former position. This repair should last the life of the rope. Source: Largo Albert
• It has been found that when lock coil head ropes are disconnected, healthy ropes will spin out 1½ to two turns after each month’s operation; three or more turns generally indicates the onset of corrosion in the outer cover and means ropes should be dressed at more frequent intervals. Prior to reconnecting the ropes, it was considered good practice to tighten the outer wires by turning up to three turns to restrict entry of moisture and loss of internal lubricant. Because this action tends to loosen the inner wires (opposite lay), this practice is no longer recommended. A rope should be “dead” when connected. Source: Largo Albert
• In deep shafts, a stranded hoist rope will bunch up the rope lay at the bottom end while increasing the lay length at the top end. The torque build-up from this phenomenon can cause problems unless the rope is disconnected once a month or so and allowed to spin out twenty or more turns. Source: George Delorme
• A significant portion of the height of a headframe is accounted for by the overwind distance that is required. This distance may be shortened in a practical manner by installing a “spot pneumatic discharge” instead of regular dump scrolls. Source: Len Deverell
• Ore skips may be used to lower material (such as road dressing or concrete aggregate) in the shaft to a dump pocket above the mining horizon. To clear the travel path required for ore hoisting, dump scrolls must be avoided. This may be accomplished by employing “guillotinedoor” skips or a “spot pneumatic discharge” at the dump pocket. Source: Morris Medd
• In very deep shafts, it is practical (due to the long cycle time) to eliminate the loading pocket and load the skips directly by conveyor. The batch function of the loading flask is replaced by a skip load of ore on the conveyor determined by a belt scale (weightometer). Source: Jack de la Vergne
• Replacement of the steel liners at the back of the skip with 6-inch thick rubber liners has enabled skips to obtain a service life of up to one million tons before being changed-out for maintenance. Source: Largo Albert
• The specifications for a new cage should include provision of a trap door in the bottom deck. This will enable a cage tender to monitor a load slung beneath the cage as it travels through the shaft. Source: Largo Albert
• When faced with the lift of a heavy, long object that is relatively close (25-35 feet) to the "centerpin" of the crane, the crane should be roped with non-rotating cable (non-spin wire rope). The rated lift capacity of the crane should be a minimum of double the weight of the heaviest lift. A typical application is installing a casing (liner) in a newly bored ventilation raise where the liner sections will usually be five to ten feet in diameter and thirty to fifty feet in length. Source: Al Walsh
• More tricks of the trade that relate to wire ropes, sheaves, and attachments are provided on a regular basis by a newsletter on the Internet. A free subscription may be obtained by contacting the publisher (georgedelorme@sympatico.ca).
4. Minimum Drum and Sheave Diameters
For many years, it was thought that the drum (or sheave) to rope diameter ratio was a significant factor in determining rope life. Research carried out independently in South Africa and the USA has shown this hypothesis to be false. A report from the
Battelle Institute at Columbus, OH summed it up as follows: “Bending fatigue is not the most dominant factor in rope degradation. Apparently, a corrosion-assisted fatigue process or corrosion itself is more influential on rope failure.” As a result, the statutes relating to drum (and sheave) to rope diameter ratio have been struck from MSHA regulations. Unfortunately, the regulations relating to this ratio remain in effect in the Canadian provinces and elsewhere.
For mine hoists and sheaves, the minimum ratio is best determined by consultation with the selected rope manufacturer. A concern still exists about the minimum sheave to rope diameter ratios for slow hoisting with stage hoists used for shaft sinking. These are not accounted for in mine regulations. Table 15-1 may be used as a guideline for hoisting speeds less than 1m/s (200 fpm).
Table 15-1 Hoisting Speeds less than 1m/s
5. Typical Skip Factors for Mine Hoists
Skip factor is the ratio of the tare (empty) weight of a skip to the payload. An assumption of the skip factor is needed to determine the hoist rope(s) required. The values in Table 15-2 are for typical conditions and should be slightly modified for unusual situations (see notes that follow the table).
Table 15-2 Skip Factors
Notes
• These factors are for standard skip compartments: 6 feet by 6 feet.
• These factors are for standard Lakeshore bottom dump skips.
• These factors are for skips with steel liners.
• These factors are for an ore with an SG =2.67 (bulk density of broken ore approximately 100 Lbs./cubic foot = 20 cubic feet per short ton = 1.67 tonnes/m3).
• Adjust these figures slightly downwards for sulfide ore (heavy), rubber liners or “throw away” skips.
• Adjust these figures slightly upwards for skips equipped with safety dogs.
• These factors are not to be used for friction hoists that hoist two skips in balance from depths less than 4,000 feet (1,200m) without investigating the requirement for the skips to be ballasted with extra weight to maintain a safe tension ratio.
6. Clearances and Rub Rope Requirements for Rope Guided Hoisting Shafts
The following minimum spacing should be maintained in installations where rope guides are used.
• 300 mm between a conveyance or counterweight and the shaft wall or shaft installations, except in areas of fixed guides at the ends of the travel range.
• 500 mm between the conveyances or between conveyances and counterweights. This spacing can be reduced to 300 mm if rubbing ropes are employed. Source: George Delorme (For shaft sinking clearances, refer to Chapter 10 - Shaft Sinking.)
7. Rope Stretch for Skip Hoist Ropes
The hoist rope will stretch at a station when the cage is loaded and even more at the loading pocket due to the dynamics of the ore falling and impacting on the suspended skip. To overcome this problem, chairs may be employed to hold the cage or skip in place while being loaded. Chairs are not normally required for friction hoists; however, they may be necessary for drum hoist installations in deep shafts.
Example
Calculate the rope stretch for the following case.
Facts:
1. Skip payload = 18 short tons
2. Collar to LP = 4,684 feet
3. Collar to dump = 60 feet
4. Sheave to dump = 33 feet
5. Rope diameter = 2.25 in.
6. Rope Construction = Flattened (triangular) strand
P = Payload = 18 short tons = 36,000 Lbs.
L = Suspended Rope = 4,684 + 60 + 33 = 4,777 feet
A = Rope cross sectional area = 3.98 in.2
E = Modulus of elasticity of the rope = 10,000,000 Lbs./in.2 (selected from the following tabulation)
E = Modulus of elasticity = 8,000,000 Lbs./in.2 (round strand rope)
E = Modulus of elasticity = 9,000,000 Lbs./in.2 (shaft sinking rope)
E= Modulus of elasticity = 10,000,000 Lbs./in.2 (flattened/triangular strand rope)
E= Modulus of elasticity = 14,00,000 Lbs./in.2 (lock coil rope)
E= Modulus of elasticity = 16,00,000 Lbs./in.2 (½ lock coil rope)
ΔS = 36,000*4,777/3.98*10,000,000 = 4.3 feet
The total (dynamic) rope stretch, Δd = 2 × ΔS = 8.6 feet