1. Introduction
The friction (or Koepe) hoist is a machine where one or more ropes pass over the drum from one conveyance to another, or from a conveyance to a counterweight. In either case, separate tail ropes are looped in the shaft and connected to the bottom of each conveyance or counterweight. The use of tail ropes lessens the out-of-balance load and hence the peak horsepower required of the hoist drive. When compared with a drum hoist for the same service, the tail ropes reduce the required motor HP rating by about 30%, but the power consumption remains virtually the same. Tail ropes have been used for a few double-drum hoist installations to the same effect, but this practice has not gained acceptance by the mining industry.
Because they normally use several hoisting ropes, the largest friction hoists can handle heavier payloads than the largest drum hoists. The drum hoists are normally limited to the capacity of a single rope. Friction hoists require a higher safety factor (SF) on the hoist (head) ropes and are not considered practical for really deep shafts employing high rope speeds.
For mine applications, Koepe hoists compete with drum hoists and the decision concerning which one is best suited for a particular application is considered in the example presented as a side study in Chapter 6 – Feasibility Studies. Drum hoists are discussed separately in Chapter 13. The hoist cycle times developed for drum hoists in that chapter have equal application to friction hoists.
For historical reasons, friction hoists (unlike drum hoists) are usually thought of in terms of metric rather than imperial (British) units. To describe the size of a friction hoist people will say “a 3m wheel diameter” rather than “a 10-foot hoist.” For this reason, the explanations and design calculations that follow are mainly performed in metric units of measure.
2. Rules of Thumb
Hoisting Distance
• A friction hoist with two skips in balance is normally suitable for hoisting from only one loading pocket horizon and for a hoisting distance exceeding 600m (2,000 feet). Otherwise, a counter-balanced friction hoist (conveyance and counterweight) is usually employed (for multilevel, shallow lifts, or cage hoisting). Source: Ingersoll-Rand
• The practical operating depth limit for a friction hoist is 1,700m (5,600 feet) for balanced hoisting and 2,000m (6,600 feet) for counterweight hoisting. Beyond these depths, rope life may be an expensive problem. Source: Jack de la Vergne
• The hoisting ropes (head ropes) for a friction hoist are not required to be non-rotating for depths of hoisting less than 800m (2,600 feet) provided right hand and left hand lays are employed to cancel rope torque effect. Tail ropes must always be non-rotating construction and connected with swivels at each end. Various sources
Static Tension Ratio
• For a tower mounted skip hoist, the calculated static tension ratio (T1/T2) should not exceed 1:1.42, but 1:1.40 is preferable.
For a ground mounted skip hoist, the calculated static tension ratio should not exceed 1:1.44 but 1:1.42 is preferable. For a cage hoist installation, these values may be exceeded for occasional heavy payloads of material or equipment transported at reduced speed. Various Sources
Tread Pressure
• Tread pressure should not exceed 17.5 kg/cm2 (250 psi) for stranded ropes and 28 kg/cm2 (400 psi) for locked coil ropes. Source: A.G. Gent
• For lock coil hoist ropes, the tread pressure calculated for skip hoists should not exceed 2,400 kPa (350 psi), or 2,750 kPa (400 psi) for a cage hoist when considering occasional heavy payloads of material or equipment. Source: Jack de la Vergne
• For stranded hoist ropes, the tread pressure calculated for skip hoists should not exceed 1,700 kPa (250 psi) or 2,000kPa (275 psi) for a cage hoist when considering occasional heavy payloads of material or equipment. Source: Largo Albert
Tail Ropes
• The natural loop diameter of the tail ropes should be equal to or slightly smaller than the compartment centres. Source: George Delorme
Hoist Wheel Rotation
• The total number of friction hoist wheel revolutions for one trip should be less than 100 for skip hoists, but may be as high as 140 for cage hoists. Source: Wire Rope Industries and others
• The hoist wheel rotation at full speed should not exceed 75 RPM for a geared drive, or 100-RPM for a direct drive. Source: Ingersoll-Rand
Position
• The distance between the hoist wheel and the highest position of the conveyance in the headframe should not be less than 1.5% of the distance from the hoist wheel to the conveyance at the lowest point of travel. Source: Largo Albert
• At full speed, a time increment of at least ½ a second should exist as any one section of rope leaves the hoist wheel before experiencing the reverse bend at the deflector sheave. Source: George Delorme
• The clearance between the bottom of the conveyance, at the lowest normal stopping destination in the shaft, and the top of the shaft bottom arrester (first obstruction) is usually 5 feet. This arrangement ensures that the weight of the descending conveyance is removed from the hoist ropes. Source: Largo Albert
• The tail rope loop dividers are generally placed below the arrester. The bottoms of the tail rope loops are then positioned 10 to 15 feet below the dividers. Beneath this, a clearance of about 10 feet will allow for rope stretch, etc. Source: Largo Albert
Hoist Speed
• Where the hoist line speed exceeds 15m/s (3,000 fpm), the static load range of the head ropes should not be more than 11.5% of their combined rope breaking strength. The (ratio of) hoist wheel diameter to rope (stranded or lock coil) diameter should not be less than 100:1, and the deflection sheave diameter to rope diameter should not be less than 120:1. Source: E J Wainright
• The maximum desirable speed for a friction hoist is 18m/s (3,600 fpm). Source: Jack Morris
• The maximum attainable speed for a friction hoist that can be safely obtained with today’s (1999) technology is 19m/s (3,800 fpm). Source: Gus Suchard
• In North America, the desirable speed for cage service is approximately 2/3 of the optimum speed calculated for a skip hoist for the same hoisting distance. Source: Jack de la Vergne
Hoist Wheel Specifications
• The hoist wheel diameter to rope (lock coil) diameter should not be less than 100:1 for ropes up to 1-inch diameter, 110:1 for ropes to 1½ inches diameter, and 120:1 for ropes to 2 inches diameter. Source: Glen McGregor
• A ratio of 100:1 (wheel diameter to lock coil rope diameter) is adequate for ropes of 25-35 mm diameter. This should increase to 125:1 for ropes of 50-60 mm diameter. Source: Jack Morris
• Rope tread liners on the hoist wheel should be grooved to a depth equal to one-third (1/3) of the rope diameter when originally installed or replaced. The replacement (discard) criterion is wear to the point that there is only 10 mm (3/8 inch) of tread material remaining, measured at the root of the rope groove. Source: ASEA (now ABB)
• On most fiction hoist installations, the maximum tolerable groove discrepancy is 0.004 inches, as measured from collar to collar. Source: Largo Albert
Production Availability
• A friction hoist is available for production for 108 hours per week. This assumes the hoist is manned 24 hours per day, seven days per week, and that muck is available for hoisting. Source: Jack Morris
• With proper maintenance planning, a friction hoist should be available 126 hours per week (18 hours per day). Source: Largo Albert
Spacing
• The minimum distance (design clearance) between a rope and bunton or divider is 5 to 6 inches. This is mainly because the hoist rope vibration is normally 2 to 3 inches off centre; 4 inches is considered excessive. Source: Humphrey Dean
• The spacing between head ropes should be 1 inch for each foot diameter of the hoist wheel to get an adequate boss for the deflection sheave. Source: Gerald Tiley
3. Tricks of the Trade
• The easy way to design a friction hoist is to first determine the required hoisting speed and payload then determine the ropes that are needed to meet the required SF. The hoist parameters can then all be determined only considering the hoist ropes and line speed. Source: Tom Harvey
• The distinguishing feature that should be recalled when designing or operating a friction hoist is that “weight is your friend.” In other words, heavier ropes and suspended loads mean higher force of friction and greater facility for braking, etc. Source: Richard McIvor
• The rule of thumb (attributed to Wainright) that indicates a minimum SF of 7 for friction hoist head ropes is not correct. There are a very large number of hoist installations worldwide that have operated satisfactorily for many years at smaller SFs. In this respect, the regulations stipulated for the Province of Ontario in Canada are a good guideline, anywhere. Source: Largo Albert
• To avoid stress concentrations, it is desirable to manufacture a friction hoist wheel in one piece. Wheels up to about 3m (10 feet) in diameter can be shipped complete with shaft to most locations. Source: Gerald Tiley
• When designing a tower-mounted friction hoist, consideration should be given to the possible avoidance of deflection sheaves, as they represent a maintenance headache. Source: Richard McIvor
• At full speed, a time increment of 0.6 second should exist as any one section of rope leaves the hoist wheel before experiencing the reverse bend at the deflector sheave. This adds to the headframe height, but the added clearance is desirable for maintenance and change-out of the sheave wheels. Source: Largo Albert
• While it is better to have the rope spacing the same at the hoist wheel and the head sheaves for a ground mounted Koepe hoist, this is not necessary provided that the fleet angle of the outside ropes is 10 or less. This is known because there are single rope friction hoists in Europe with both head sheaves on the same headframe deck that operate satisfactorily, provided the fleet angle is maintained at 60 minutes (10) or less. Source: Tréfileurope
• For a single rope ground mounted Koepe hoist, it is better to have the head sheaves in the same plane as the hoist wheel. However, the head sheaves may be mounted on the same deck of the headframe tower, provided the fleet angle of the outside ropes is not more than 1½ to 2 degrees. Source: Henry Broughton
• While it is better to have the rope spacing the same at the hoist wheel and the skip attachment, this is not necessary provided the fleet angle of the outside ropes is 10 or less when the conveyance is at its upper end of travel. Source: Borje Fredricksson
• The arresters (“last resort”) at the shaft bottom are designed to stop a full-speed conveyance at 2g, while an ascending conveyance must be stopped at less than 1g (i.e. 0.9g), although not necessarily from full speed if it exceeds 15m/s (3,000 fpm). Various sources
• The tail ropes should be oriented to overcome the Coriolis effect. If placed in the East-West direction, the tail ropes will freely open and close. If the compartments are North and South, the ropes will foul the separating spacers (loop dividers) if not widely spaced. Source: Gerald Tiley
• The Coriolis effect can be neglected, as it is much smaller than the movement at acceleration/deceleration and due to rope torque of the tail ropes. Source: Borje Fredricksson
• High-speed friction hoists [over 12m/s (2,400 fpm)] are oriented with the wheel diameter East-West to minimize the effect of Coriolis acceleration on the tail ropes. Source: Jack Morris
• The effect of Coriolis acceleration on the tail ropes is diminished when a fixed guide system is employed, as opposed to using rope guides. Source: Jack de la Vergne
• The tail rope weight is normally designed equal to the head ropes; however, tail ropes slightly heavier than the head ropes will assist acceleration from the loading pocket. Slightly lighter tail ropes will provide a greater SF for the head rope section above the conveyance as it approaches the highest point of travel (the point at which uneven rope tension is most severe). Source: Gerald Tiley
• The distance between head ropes (spacing) varies between 8 inches and 12 inches. At 8 inches, some installations experience rope slap but this is not considered a serious problem, since the ropes are running at the same speed. (Author note: regular slapping is said by others to lead to martensitic alteration, resulting in broken wires.) Narrow rope spacing may require that the rope attachments at the conveyance be staggered. This can be accomplished by including a link at every other attachment. Source: Humphrey Dean
• The guideline for rope spacing is 8 inches up to 11/8-inch rope diameter, 10 inches to 1¼ inches, 12 inches to 1½, 14 inches to 15/8, and 16 inches to 1¾. Drawhead connections can be staggered but this is costly and complicates rope adjustment and maintenance. Source: Largo Albert
4. Friction Hoist Design
Listed below are the steps in designing and selecting a friction hoist.
1. Determine the SF required for the given hoist distance
2. Determine hoisting speed, V
3. Calculate the hoist cycle
4. Define (cage) or calculate (skip) the payload
5. Determine the weights of the conveyances required
6. Select hoist (head) ropes
7. Determine the wheel diameter of the hoist
8. Select balance (tail) ropes
9. Calculate the RMS power requirement
Example
Design and select two friction hoists at the same time. One is required for production hoisting and the other for cage service.
Facts:
1. Both hoists will be tower mounted in the same headframe
2. The skip hoist requires a capacity of 500 tonnes/hour
3. The cage hoist requires a payload of 26 tonnes
4. Each has a hoisting distance, H of 1,000m
5. The statutory SFs of Ontario, Canada are to apply
Solution:
Step 1: Determine the SF required for the given hoist distance.
Following is the SF required by statute for the hoist ropes.
• SF = 8 -.00164D in which D = length of suspended rope, hence D= approximately H + 50m to account for rope suspended above the dump and beneath the loading pocket (in the case of a skip hoist) and similar extra rope length in the case of a cage hoist.
• SF = 8 - (.00164 x 1,050) = 6.3
Step 2: Determine hoisting speed, V.
• The optimum skip hoisting speed, V = 0.44 H ½ = 14m/s (rounded)
• A suitable cage hoisting speed will be about 2/3 V =10m/s (rounded up)
Step 3: Calculate the hoist cycle.
Calculate the skip hoist cycle time, T. (Since the hoisting distance exceeds 600m, balanced hoisting with two skips is determined.)
• T = H/V + 1.3 V + 25 = 115 seconds
• Trips per hour = 3,600/115 = 31.3
Calculate the cage hoist cycle time (a cage and counterweight is assumed).
• T = 2H/Vc + 2.6 Vc + 70 = 296 seconds
• Trips per hour = 3,600/296 = 12.1
Step 4: Define (cage) and calculate (skip) payload.
The cage payload is given at 26 tonnes and the skip payload is calculated by dividing the capacity per hour by the trips per hour.
• Skip payload, P = 500/31.3 = 16 tonnes (rounded up)
Step 5: Determine the weights of the conveyances required.
The weight of the cage for 26-tonne capacity will be approximately 20 tonnes if it is steel (steel is typical for friction hoists). The weight of the counterweight will be made equal to the empty cage weight plus half the payload = 33 tonnes. (In this case, the counterweight will be designed to readily remove a portion of its weight for regular cage service with lighter payloads.) The weight of the skip, S will be approximately 13 tonnes (refer to Table 15.5a, Chapter 15 of this handbook) for a steel bottom dump skip that would normally be used for this application. However, this weight might not be enough to maintain the required tension ratio (in this case, the skip would be “ballasted” with extra weight).
The empty weight of skip required to maintain a tension ratio of 1.40:1 follows.
• St = P{2.5 - (H x SF/4,500)} = 16 (2.5 - 1.4) = 17.6 tonnes
• The skips will be ballasted to weigh 17.6 tonnes
Step 6: Select hoist (head) ropes.
Cage Hoist
Try 6 lock coil ropes of 32 mm diameter weighing 5.58 kg/m and having a breaking strength (BS) of 890 kN.
• SF obtained = Number ropes x BS/maximum suspended load
= 6 x BS/weight of ropes, payload and cage
= 6 x 890/g (35.5 + 26 + 20) = 6.7 (6.3 required)
• T1/T2 obtained = (35.5 + 26 + 20)/(35.5 + 33) = 1.19
• T2/T3 obtained = (35.5 +33)/(35.5 + 20) = 1.23
Maximum total suspended load = (35.5 + 26 + 20) + (35.5 + 33) = 150 tonnes
Skip Hoist
Try four of the same lock coil ropes – 32 mm diameter, 5.58 kg/m, and BS 863 kN.
• T1/T2 obtained = (23.7 + 16 + 17.6)/(23.7 + 17.6) = 1.39 (1.40, or less, desired) Maximum total suspended load = (23.7 + 16 + 17.6) + (23.7 + 17.6) = 98.6 tonnes
• SF obtained = Number ropes x BS/maximum suspended load
= 4 x BS/weight of ropes, payload and skip
= (4 x 890/g(23.6 + 16 + 17.6) = 6.35 (6.3 required)
Step 7: Determine the wheel diameter of the hoist, D.
The statutory requirement for lock coil ropes is 100 times the diameter of the hoist rope at this location.
Cage and Skip Hoist
(Statutory) D = 100d = 100 x 32 = 3,200 mm = 3.2m
The statutory diameter may not be sufficient. For example, the diameter should be increased if the permitted tread pressure is exceeded, the number of wheel revolutions per trip is too high, or the ropes are greater than 35-mm diameter.
The tread pressure is calculated by dividing the total suspended load by the projected contact area of the ropes on the hoist wheel. Tread pressure should not exceed 2,400 Kpa for a skip hoist or 2,750 kPa for a cage hoist with maximum payload.
• Cage hoist tread pressure = 150g x1,000/ (6 x 3.2 x 32) = 2394 kPa (V)
• Cage hoist revolutions = H/πD = 1,000/3.2π = 99.5 (V)
• Skip hoist tread pressure = 98.6g x1,000/ (4 x 3.2 x 32) = 2360 kPa (V)
• Skip hoist revolutions = H/πD = 1,000/3.2π = 99.5 (V)
A wheel diameter of 3.2m should be satisfactory for both hoists. (On detailed investigation, it may be increased to 3.5m to increase rope life).
Step 8: Select balance (tail) ropes.
Select non-rotating balance (tail) ropes matching the head rope weight and with a natural loop diameter equal to the compartment spacing.
Note
Tail ropes can be custom manufactured to meet precise weight requirements (i.e. kg/m). For the cage hoist (assuming no deflection sheave), select three non-rotating ropes weighing twice the head rope weight. The head ropes weigh 5.58 kg/m; therefore, the tail ropes will weigh 11.16 kg/m with a 53 mm diameter. If the ropes are 34 by 7, the natural loop diameter will be 46 x 53 = 2,438 mm (unsatisfactory). If the ropes are 18 by 7, the natural loop diameter will be 60 x 53 = 3,180 mm (satisfactory).
For the skip hoist (assuming a deflection sheave is required to bring the conveyances closer together in the shaft, say 2m between compartment centres), select three nonrotating ropes of weight = 4/3 x 5.58 = 7.44 kg/m with a 43 mm diameter. If they are 34 by 7, the natural loop diameter will be 46 x 43 = 1,978 mm (satisfactory).
Step 9: Calculate the RMS power requirement.
Assume there is a force-ventilated DC or cyclo-converter drive.
The skip hoist RMS power = a constant(k) x unit weight of the ropes x (speed)5/4
SF obtained
• k=24 for a standard DC (FV) or cyclo-converter drive (FV)
• The skip hoist RMS power = (24/6.3) x 4 x 5.58 x 141.25 = 2,300 kW (3,100HP)
• The cage hoist RMS power = skip hoist factored for speed and out-of-balance loads =
2,300 x 10/16 x (10/14) 1.25 = 944 kW (1250) HP)
5. Production Availability
Confusion and controversy exists in the mining industry as to the meaning of the word “availability” when applied to mine hoists. For hoist maintenance personnel, it may mean the percent of the time the piece of equipment is available to work compared with the total time available. On the other hand, those engaged in selecting and evaluating hoists for mine service must consider the availability of the total hoist system, taking not only maintenance downtime into account, but also downtime due to shaft repairs, power outages, rope dressing, skip change-out, etc. This chapter is concerned with the availability of the total system, and for this purpose, it is described as “production availability.”
To determine the production availability of a friction hoist for the purpose of estimating hoisting capacity per day, a detailed calculation should be made for each case, taking into account the total hoisting system. This will include allowances for empty loading pocket, full bin, hoisting spill, etc. Availability will usually be slightly less than a drum hoist because the friction hoist demands a more sophisticated maintenance routine. It is higher for a five or six days per week operation (because some maintenance work can be performed on the weekend) than a seven days per week operating scenario.
Example
Facts:
1. Estimate is based on a 7-day workweek
2. Automatic hoisting is assumed
3. Two skips are hoisted in balance
4. No cage service is required
5. 12-day annual shutdown is assumed
Solution:
The hoist plant availability is shown in Table 14-1.
Table 14-1 Hoist Plant Availability
(Friction (Koepe) Hoist – Seven Day per Week Operation)
6. Comparisons
Following is a comparison of ground versus tower mount friction hoists.
Ground Mount Friction Hoist
Listed below are ground mount friction hoist advantages.
• Shorter headframe.
• Steel headframe (concrete is preferred in tower mounts for rigidity – reinforced concrete is not subject to residual stresses).
• An elevator is not required in the headframe.
• An overhead bridge crane may not be required.
• Easier access for maintenance.
• A heated headframe is not required.
• A water supply to the top of the headframe is not required.
• Shorter runs of power cables.
• Less susceptible to damage from overwinds, mine explosions, lightning, and earthquakes.
• The longer rope between the hoist and the highest point of conveyance travel makes rope surge and possible subsequent structural upset less likely.
• Most efficient use of available space in the shaft for conveyances.
• Ability to operate without problems at a higher tension ratio (T1/T2). This is likely due to the dampening effect obtained from the wide-angle wrap of hoist rope around the head sheaves and the greater distance between the high point of travel for the conveyance and the hoist wheel.
Tower Mount Friction Hoist
Listed below are tower mount friction hoist advantages.
• Zero or one deflection sheave is required. Two are required for a ground mount – one is subject to reverse bending of the hoist ropes.
• Installing and changing head ropes is less complicated.
• Less real estate is occupied.
• The hoist ropes are not subject to the elements – icing is less of a concern.
• Rope vibration (whip) is less of a concern.
• The headframe tower may be more aesthetically pleasing.
• The headframe shell can be used for shaft sinking simultaneous with Koepe hoist installation above the sinking sheave deck.