A Double Conic Rope Drum is a winding drum with two tapered (conical) sections joined back-to-back, used on deep-shaft mine hoists and heavy mill winders to keep motor torque roughly constant as the load rises and falls. As one rope unwinds from the small diameter end of one cone, the opposite rope winds onto the large diameter end of the other cone — so the rising load always pulls on a smaller radius than the descending load. Typical industrial units run 200 to 800 tonnes of payload at rope speeds of 8–18 m/s. The Whiting Corporation and South African Robbins-style winders used this geometry to halve peak motor demand on shafts deeper than 1,500 m.
Double Conic Rope Drum Interactive Calculator
Vary shaft depth, rope mass, and payload to see the required conic radius ratio for static torque balancing.
Equation Used
The drum balances torque by putting the heavier rope/load on a smaller radius and the lighter side on a larger radius. For static balance, force times radius is equal on both sides, so the required radius ratio is the heavy-side mass divided by the light-side mass.
- Static torque balance with gravity cancelling from both sides.
- Loaded heavy side includes payload plus the full hanging rope mass.
- Light side is approximated as the cage/skip payload mass from the worked example statement.
- Friction, drum inertia, rope stretch, and acceleration are ignored.
How the Double Conic Rope Drum Works
The whole point of a Double Conic Rope Drum is mechanical honesty about gravity. On a deep shaft, the rope itself weighs as much as the cage and skip combined — a 2,000 m run of 50 mm locked-coil rope weighs roughly 24 tonnes. When the loaded skip sits at pit bottom, the motor has to lift the skip plus the entire descending rope plus the deadweight of the rope already wound on the drum. As the skip rises that imbalance flips. A cylindrical drum gives you a constant lever arm on both ropes, so the motor sees a brutal torque swing from start to mid-shaft. The conical drum cheats this. Wind the heavy rope onto the small end and the light rope onto the large end, and the lever arms shift through the cycle to cancel most of the gravity imbalance.
The drum is built as two mirror-image conical shells joined at the large diameters in the middle, so the two ropes wind in opposite directions and at opposite radii at every instant. The taper angle is not arbitrary — it is calculated from rope mass per metre, payload, shaft depth and drum face length. Get the cone angle wrong by more than about 1° and the torque equalisation falls apart in the middle third of the wind, which is exactly where the motor is doing real work. If you notice the winder pulling heavy current at mid-shaft instead of at start-of-lift, your cone profile is wrong or your rope has stretched and shifted the cross-over point.
The rope must lay in machined helical grooves, one turn per pitch, with the lead matching the rope lay direction. Skip a groove or let the rope crowd and you get rope-on-rope chafe at the cross-over, which kills a 50 mm locked-coil rope in months instead of years. Common failure modes are rope crowding from worn flange liners, fatigue cracking at the weld between the two conical shells (this is the highest-stress joint on the whole drum), and brake-path scoring on the cylindrical centre band where the service brake clamps. Most modern units add a short cylindrical section between the two cones specifically to give the brake a parallel surface to work against — a pure double cone has no good brake seat.
Key Components
- Conical Drum Shells: Two mirror-image welded steel cones, typically 30–50 mm plate, joined at the large diameter. Cone half-angle usually 4° to 12° depending on shaft depth. The tapered profile is what shifts the rope's effective radius through the wind.
- Helical Rope Grooves: Machined into the shell surface at a pitch equal to rope diameter plus 2–3 mm clearance. Groove radius is rope diameter / 2 + 0.5 mm — tight enough to support the rope, loose enough to release it cleanly. Worn grooves let the rope flatten and lose breaking strength.
- Centre Brake Band: A short cylindrical section, typically 600–1200 mm wide, between the two cones. Gives the post and disc brakes a parallel running surface. Without it the brake calipers cannot make even contact across their pad face.
- Main Shaft and Bearings: Forged shaft running in spherical roller bearings, often 800–1400 mm bore on a large winder. Carries the full radial load of drum, ropes, payload and self-weight — typically 400–1500 tonnes static.
- End Flanges: Heavy outer flanges at the small-diameter ends of each cone. Stop the rope walking off at the end of the wind. Flange height should clear the topmost wrap by at least 2.5 × rope diameter.
- Rope Anchorage Sockets: Tapered zinc-spelter sockets bolted into pockets at the small end of each cone. Sockets must be replaced every 5–7 years or after any rope change — re-using an old socket on a new rope is the most common cause of rope-end pull-outs.
Industries That Rely on the Double Conic Rope Drum
You see Double Conic Rope Drums where a single motor has to lift a heavy payload over a long rope run and the engineer cannot afford to oversize the motor for the worst-case start-of-lift torque. The geometry is most common in deep-shaft mining, but it shows up in heavy industrial elevators, paper-mill log haulers, and any winder where rope self-weight is comparable to the payload. If your shaft is shallow and the rope weighs less than 10% of the payload, a cylindrical drum is cheaper and simpler — the conic geometry only earns its keep when rope mass becomes a real fraction of the lift.
- Deep-shaft Gold Mining: Driefontein and Mponeng shafts in South Africa run conical and bi-cylindro-conical winders down to 3,000 m, hoisting 25-tonne skips at 16 m/s.
- Coal Mining: British NCB Koepe-conical hybrids at Selby and Daw Mill collieries used double conic profiles for man-riding cages on 1,000 m shafts.
- Heavy Industrial Elevators: Whiting Corporation furnace-charging hoists at integrated steel mills, lifting 40-tonne charge buckets up 60 m blast-furnace stockhouses.
- Paper and Pulp Mills: Log-haul drum winders at Canadian kraft mills lifting bundled softwood from river ponds up 80 m inclines into the debarker line.
- Heavy Construction Hoists: Tower-mounted material hoists on hydroelectric dam projects — the Hoover Dam construction winders used variants of this profile for concrete bucket lifts.
- Salt and Potash Mining: Saskatchewan potash winders at Esterhazy and Lanigan, hoisting potash ore from 1,000 m on twin-rope conical drums.
The Formula Behind the Double Conic Rope Drum
What you actually need to compute is the motor torque demanded at any rope position, because that demand is what forces motor sizing. On a cylindrical drum the torque varies wildly across the wind. On a properly designed Double Conic Rope Drum the torque should stay nearly flat through the middle of the cycle, drop a touch at the cross-over, and only spike at the very ends. At the low end of the typical operating range — short shafts under 500 m with rope mass below 10% of payload — the conic profile barely helps and you are paying for geometry you do not need. At the high end — shafts over 2,000 m with rope mass approaching payload mass — the conic profile is the only thing keeping motor size within reason. The sweet spot is 800–1,800 m shafts where rope self-weight runs 30–80% of payload.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| T(x) | Net motor torque demanded at rope position x | N·m | lbf·ft |
| mload | Payload mass (skip plus ore, or cage plus cargo) | kg | lb |
| mrope(x) | Mass of rope still hanging in shaft at position x | kg | lb |
| g | Gravitational acceleration | m/s² | ft/s² |
| Rup(x) | Effective drum radius where ascending rope is winding on | m | ft |
| Rdown(x) | Effective drum radius where descending rope is paying off | m | ft |
| L | Total shaft depth (full rope wind length) | m | ft |
Worked Example: Double Conic Rope Drum in a copper-mine production hoist
A copper concentrator project in northern Chile is sizing a Double Conic Rope Drum for a 1,500 m production shaft. The skip plus ore weighs 30,000 kg. The locked-coil rope is 48 mm diameter at 12 kg/m. The drum has a small-end radius of 2.0 m and a large-end radius of 3.5 m, with rope laying down each cone over the 1,500 m wind. The team needs to know peak motor torque at start-of-lift, mid-shaft, and end-of-lift to choose between a 4 MW and a 6 MW drive.
Given
- mload = 30,000 kg
- rope linear mass = 12 kg/m
- L = 1,500 m
- Rsmall = 2.0 m
- Rlarge = 3.5 m
- g = 9.81 m/s²
Solution
Step 1 — at start-of-lift (x = 0), all 1,500 m of rope hangs on the ascending side, winding onto the small-diameter end (Rup = 2.0 m). The descending rope is fully wound onto the large end (Rdown = 3.5 m) but carries zero hanging mass.
This is the peak demand. Putting the heavy rope on the small radius is exactly the trick — on a cylindrical drum at R = 3.5 m this same load would demand 1,648 kN·m, nearly 75% more.
Step 2 — at mid-shaft (x = 750 m, nominal operating point), 750 m hangs each side. The ascending rope is now winding onto a radius midway up the cone, roughly 2.75 m. The descending rope is paying off near the large end at roughly 3.0 m.
That is the sweet spot — motor demand at mid-shaft is actually lower than at start-of-lift, which is the whole reason for the conic profile. On a cylindrical drum mid-shaft would peak around 1,030 kN·m.
Step 3 — at end-of-lift (x = 1,500 m), the load reaches the headframe. The ascending rope is now wound entirely onto the large end (Rup = 3.5 m) with zero rope hanging. The descending rope hangs full length on the small end (Rdown = 2.0 m).
So the demand profile across the wind reads 942 → 787 → 677 kN·m. Compare that to a cylindrical drum at the same payload, which would run roughly 1,648 → 1,030 → 1,030 kN·m. The conic drum has chopped peak demand by 43%.
Result
Peak motor torque is 942 kN·m at start-of-lift, dropping to 787 kN·m at mid-shaft and 677 kN·m at end-of-lift. That start-of-lift number is what the drive must deliver, so a 4 MW VSD running at 0.6 rad/s output (about 6 RPM at the drum) gives 6,667 kN·m of stall torque headroom — comfortably enough, you do not need to step up to the 6 MW frame. The flat middle of the curve is the conic drum earning its keep — a cylindrical drum would need a drive sized for nearly 1,650 kN·m and would run at half load for most of the cycle. If you measure peak motor current 15% higher than predicted at start-of-lift, suspect rope crowding at the small-diameter end forcing the wind onto a larger effective radius. If peak demand shows up at mid-shaft instead of start-of-lift, the cone half-angle has drifted from the design value — typically because the drum was rebuilt with replacement shells whose taper was machined to the wrong angle, or the rope has stretched 0.3% or more in service and shifted the cross-over point off the geometric centre.
Choosing the Double Conic Rope Drum: Pros and Cons
The Double Conic Rope Drum is not the only way to wind a deep-shaft hoist. Modern South African winders increasingly use Koepe (friction) winders for the deepest shafts, and shallow operations stick with cylindrical drums. The choice comes down to shaft depth, payload, and how much capital you can throw at the headframe.
| Property | Double Conic Rope Drum | Cylindrical Rope Drum | Koepe Friction Winder |
|---|---|---|---|
| Peak motor torque demand | Equalised across wind, ~40-50% lower peak than cylindrical at deep shafts | Highest peak at start-of-lift on deep shafts | Near-constant, lowest peak of the three |
| Practical shaft depth limit | Up to ~2,500 m, limited by drum size | Best below 800 m, impractical above 1,500 m | Up to 3,000 m+ with tail-rope balancing |
| Rope speed capability | 8-18 m/s | 8-15 m/s | 16-22 m/s |
| Payload capacity (skip + ore) | 10-50 tonnes typical | 5-25 tonnes typical | 20-60 tonnes typical |
| Capital cost (winder + drum) | High - large welded shells, complex machining | Lowest of the three | Highest - tower mount, tail rope, friction lining |
| Drum manufacturing complexity | Tapered shell machining, critical weld at centre joint | Simple cylindrical fabrication | Cylindrical with friction lining, simpler than conic |
| Brake mounting | Requires added cylindrical centre band for brake path | Native cylindrical brake surface | Disc brakes on motor shaft, no drum brake path needed |
| Rope life at full duty | 3-5 years on locked-coil at full duty | 3-5 years, similar fatigue profile | 5-8 years - lower bending fatigue, no spooling damage |
Frequently Asked Questions About Double Conic Rope Drum
The cone profile no longer matches the rope mass. Either the replacement shells were machined to the wrong half-angle (check against original drawings — the taper should be within 0.5° of the design value), or the rope has been swapped for a different linear mass without re-checking the profile. Heavier rope shifts the torque-equalisation point lower in the shaft.
Quick check: log motor current at every 100 m of wind for one full cycle. The current curve should peak at start-of-lift, dip through the middle, and end below the start value. If you see a hump in the middle third, the geometry is off and no amount of drive tuning will hide it.
Almost never worth it. The conic drum is wider and heavier than the cylindrical it replaces, the bearing centres rarely line up, and the rope fleet angle into the headframe sheaves changes because the rope departure point moves through the wind. You would need new sheave positions, new headframe geometry, and probably new bearings.
The only retrofit case that pencils out is when you are deepening a shaft and the existing motor is already maxed out — then a conic drum lets you keep the same drive on a deeper run. Otherwise spec a Koepe winder or accept the bigger motor.
The target is to make Rup(x) × (mload + mrope(x)) approximately equal to Rdown(x) × mrope(L−x) plus a constant motor torque, across the whole wind. In practice that means the radius ratio Rlarge/Rsmall should roughly equal 1 + (mrope-total / mload).
For our worked example with 18,000 kg rope and 30,000 kg load, that gives a ratio of 1.6 — which matches the 3.5 / 2.0 = 1.75 in the design, slightly biased to over-correct. Half-angle then falls out of the geometry once you fix drum face length. A 1,500 m wind on a 3 m face length with that radius ratio gives roughly an 8° half-angle.
Almost always fleet angle. The rope leaves the drum at a steeper angle when winding on the small end, and if the headframe sheave is not directly above the small-end groove the rope tries to climb the previous wrap. Industry rule is fleet angle below 1.5° on a grooved drum.
Measure the angle from the small-end first-wrap groove up to the sheave centre. If it exceeds 1.5°, you need to reposition the sheave or accept rope crowding and the rope wear that comes with it. Worn flange liners on the small end make this worse — replace them on a fixed schedule, do not wait for visible damage.
Slightly, but not as much as people assume. Bending fatigue in the rope scales with D/d ratio (drum diameter over rope diameter). On a conic drum the rope sees the small radius at start-of-lift when tension is highest, which is the worst possible combination for fatigue. The large-end wraps see lower tension and a kinder D/d.
Net effect: rope life on a conic drum is roughly the same as on a cylindrical drum sized to the conic's large diameter. The conic does not buy you rope life — it buys you motor size. If rope life is the binding constraint, look at a Koepe winder where bending happens once per cycle on the drive sheave instead of being spooled on and off.
The centre joint is the highest-stress location on the drum because both ropes pull radially inward at that diameter and the bending moments from each cone meet there. A through-crack at the joint will let the two shells flex relative to each other, which immediately throws the rope grooves out of pitch and starts rope crowding within hours.
Inspect the centre weld with MPI or UT every major outage — minimum every 18 months on a production winder. Any indication longer than 25 mm circumferentially is a stop-work. We have seen drums condemned outright when a hairline crack ran 200 mm before being caught.
References & Further Reading
- Wikipedia contributors. Hoist (mining). Wikipedia
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