A mining skip is a steel container that runs in guided rails up a vertical or inclined shaft to lift broken ore and waste rock to surface. A wire rope hoist pulls the skip past fixed scrolls or trip arms at the headframe, which tip the body and discharge the load into a bin. Skips replace slower kibble buckets on production shafts because they self-load from a measuring flask and self-dump at speed, moving 5 to 50 tonnes per cycle on shafts running 600 to 1800 m deep.
Mining Skip Hoist Throughput Interactive Calculator
Vary skip payload, shaft depth, hoist speed, and dump time to see cycle time, hourly production, and payload lifting power.
Equation Used
The calculator estimates skip production from the round-trip hoist distance and dump time. Shaft depth D divided by average hoist speed v gives the one-way hoist time; a full cycle is up plus return plus dump time. Hourly production is payload per cycle multiplied by cycles per hour.
- Skip travels up loaded and returns empty at the same average hoist speed.
- Acceleration, braking, loading time, tare mass, rope mass, and hoist efficiency are not included.
- Payload power is the ideal lifting power for ore payload only.
Inside the Mining Buckets and Skip
A skip is the production workhorse of a vertical shaft. The kibble bucket — that round open-top barrel you see on sinking projects — is its slower cousin, used for shaft sinking and small-scale hoisting where round-trip speed doesn't matter much. Once a mine goes into production, you switch to a guided rectangular skip running between fixed timber or steel guides, with a crosshead riding the same guides above the body to keep the rope pull aligned. The hoist drum or Koepe friction wheel at the headframe lifts the skip at 10 to 18 m/s on modern installations.
Loading happens at the shaft bottom in a measuring flask — a chute with a known volume that fills from the loading pocket and dumps a metered charge through a chute door into the skip. You want the flask volume matched to the skip volume within about 2%, because overfilling spills rock into the shaft sump and underfilling kills your tonnes per hour. At the top, the skip's wheels follow the main guides while the body's dump rollers hit a set of curved scrolls bolted to the headframe steel. The scrolls force the body to pivot around its trunnion while the crosshead keeps climbing — the body tips, the bottom door swings open, and the load drops into the surface bin in under 2 seconds.
Get the scroll geometry wrong and you'll know fast. Scrolls set too tight cause the body to slam open and crack the dump door hinge pins. Set too loose, the skip won't fully discharge and you carry rock back down — fines build up in the body, the tare weight climbs, and your payload-to-tare ratio quietly drops from a healthy 1.8:1 down to 1.4:1 over a few months. Worn crosshead liners are the other usual suspect; once liner-to-guide clearance opens past about 6 mm, the skip starts swinging in the shaft and you'll see scrub marks on the guide faces and uneven wear on the rope.
Key Components
- Skip body: The welded steel container that holds the ore. Capacity ranges from 2 tonnes on small shafts up to 50+ tonnes on deep South African gold shafts. Body plate is typically 12 to 25 mm Hardox 450 or similar abrasion-resistant steel to handle the impact of dumped ore from the loading flask.
- Crosshead: Rides the guides above the skip body and carries the rope attachment. Keeps the rope pull centred so the skip doesn't yaw in the shaft. Crosshead-to-guide clearance must stay under 6 mm — beyond that the skip swings and you accelerate guide wear.
- Bottom dump door: Hinged steel door forming the floor of the skip. A latch holds it shut against the load weight; the dump scrolls trip the latch at the headframe. Hinge pins are usually 75 to 100 mm hardened steel and need pin-to-bush clearance kept under 1.5 mm.
- Dump scrolls: Curved cam rails bolted to the headframe steel above the bin level. They force the skip body to rotate around its trunnion while the crosshead continues straight up. Scroll profile must match the skip's roll geometry within 5 mm or the dump cycle goes ragged.
- Loading flask (measuring pocket): A fixed-volume chute at the shaft bottom that pre-measures one skip-load of ore. Sized to match skip volume within about 2%. A pneumatic or hydraulic chute door releases the charge into the skip when the skip is positioned at the loading station.
- Trunnion bearings: The pivot pins around which the body tips during dump. Spherical roller bearings or self-aligning bushings rated for shock load. Failure here strands the skip in the dump position — keep grease intervals tight and inspect for radial play above 0.3 mm.
- Kibble (alternative form): Round open-top bucket used for shaft sinking and small mines. No bottom door — discharged by tipping with a chairing arrangement at the collar. Slower cycle but tolerates oversize muck and works in unguided shafts.
Who Uses the Mining Buckets and Skip
Skips and kibbles cover the full spread of vertical shaft hoisting from a 50 m exploration winze to the deepest production shafts on the planet. Which one you pick comes down to whether the shaft is being sunk or already in production, how much tonnage you need to move per shift, and whether the rock is sized small enough to flow through a measuring flask.
- Deep gold mining: Mponeng Mine in South Africa runs bottom-dump skips on a multi-stage hoisting system from below 3,500 m, with skip payloads around 25 tonnes per trip on the lower stage.
- Hard rock nickel/copper: Vale's Creighton Mine in Sudbury hoists ore from below 2,400 m using guided production skips on a Koepe friction hoist arrangement.
- Shaft sinking contracting: Cementation and Murray & Roberts use kibble buckets of 4 to 10 tonnes during shaft sinking phases — Murray & Roberts deployed kibbles on the Venetia Mine shaft sink in South Africa.
- Potash mining: Nutrien's Cory and Allan potash mines in Saskatchewan run skip hoisting systems lifting ore roughly 1,000 m to surface bins for milling.
- Underground iron ore: LKAB's Kiruna Mine in Sweden uses skip hoists on the Lappmalmen production shafts to bring magnetite ore up from the haulage level to surface crushing.
- Heritage and tourist mines: Restored kibble hoists at the Big Pit National Coal Museum in Wales demonstrate the early sinking-bucket method to visitors.
- Diamond mining: De Beers' Finsch Mine in the Northern Cape transitioned from open pit to underground block cave hoisting using production skips on the main shaft.
The Formula Behind the Mining Buckets and Skip
Skip sizing comes down to one number: hoisting capacity in tonnes per hour. You need to know whether the shaft can move the daily production rate the mine plan calls for. The formula ties skip payload, cycle time, and availability into a single throughput figure. At the low end of cycle time — short shafts, fast hoists, well-tuned loading — you get throughput numbers that look like a conveyor. At the long end, when the shaft is deep and the hoist creeps the last 100 m to spot the skip in the dump position, throughput drops fast. The sweet spot for most production shafts sits at a cycle time roughly equal to 2 × shaft depth / max hoist speed, plus 25 to 35 seconds for loading and dumping combined.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Th | Hoisting throughput | tonnes/hour | short tons/hour |
| Pskip | Skip payload (ore mass per trip) | tonnes | short tons |
| tcycle | Full cycle time (load + hoist + dump + return) | seconds | seconds |
| A | Hoist availability factor (typical 0.85 to 0.92) | dimensionless | dimensionless |
Worked Example: Mining Buckets and Skip in a uranium production shaft in northern Saskatchewan
Cameco's engineering team is checking nominal hoisting capacity on a refurbished production shaft serving a uranium orebody at the Athabasca Basin. The shaft is 750 m deep, the new bottom-dump skips carry 12 tonnes of ore per trip, the maximum hoist speed is 14 m/s, and combined load-plus-dump time is 28 seconds. Hoist availability is budgeted at 0.88. They need to know whether two skips in counterbalance can deliver 5,000 tonnes per day on a single 20-hour production window.
Given
- Pskip = 12 tonnes
- Shaft depth = 750 m
- Max hoist speed = 14 m/s
- Load + dump time = 28 s
- A = 0.88 —
Solution
Step 1 — estimate hoisting time. With acceleration and deceleration ramps, average hoist speed sits around 75% of max, so roughly 10.5 m/s. One-way hoist time:
Step 2 — total nominal cycle time, counting one upward hoist plus the loaded portion's load and dump (the counterbalance skip handles the return on its own clock):
Step 3 — nominal throughput per skip line at 12 tonne payload and 0.88 availability:
With two skips running counterbalanced, effective throughput roughly doubles to about 768 t/h, so a 20-hour window clears around 15,300 tonnes — well above the 5,000 t/day target.
Now check the operating-range bounds. At the low end of typical cycle time — say a shallower 400 m winze with a 60 s cycle — the same 12 t skip delivers Th = (3600 × 12 × 0.88) / 60 ≈ 634 t/h. The hoist barely catches its breath between trips and the loading pocket has to refill at near-continuous duty. At the high end — a 1,500 m hoist with cycle stretched to 180 s — throughput collapses to Th = (3600 × 12 × 0.88) / 180 ≈ 211 t/h, and you'd need a bigger skip or a second shaft to hit the same daily target.
Result
Nominal hoisting throughput is 384 tonnes/hour per skip line, or roughly 768 t/h with both skips counterbalanced. That's the rate at which surface bins fill — at this number you'd see a loaded skip arrive at the headframe roughly every 50 seconds on each side, and the bin operator would barely have a quiet moment between dumps. The range is wide: a 60 s cycle on a shallower shaft pushes per-line throughput to 634 t/h, while stretching cycle to 180 s on a 1,500 m shaft drops it to 211 t/h, so cycle time dominates the answer far more than payload does. If your measured throughput comes in 15 to 25% below this nominal figure, the usual suspects are: (1) loading flask door timing slipping out of sync with skip arrival, costing 3 to 6 seconds per cycle; (2) hoist speed reference detuned to ride below max because of rope-stretch oscillation, which knocks 1 to 2 m/s off the average; or (3) skip tare weight quietly creeping up from rock fines packed behind the body liners, which doesn't show on a throughput readout but lowers the payload-to-tare ratio and raises rope load.
When to Use a Mining Buckets and Skip and When Not To
Skip versus kibble versus belt conveyor on an incline shaft is the standard question every shaft engineer faces during pre-feasibility. The right answer depends on whether you're sinking or producing, how deep the shaft is, and whether the rock is sized for flask loading.
| Property | Bottom-dump skip | Kibble bucket | Inclined belt conveyor |
|---|---|---|---|
| Typical payload per cycle | 5 to 50 tonnes | 1 to 10 tonnes | Continuous, no cycle |
| Cycle time at 1000 m depth | 80 to 120 s | 150 to 240 s | N/A — continuous |
| Throughput per line | 300 to 1500 t/h | 50 to 200 t/h | 1000 to 5000 t/h |
| Maximum practical depth | 3000+ m (multi-stage) | 1500 m (rare beyond) | Limited by belt strength, ~500 m vertical lift |
| Capital cost (relative) | High — headframe, scrolls, guides, hoist | Moderate — basic hoist + bucket | Very high in vertical applications, moderate on inclines |
| Rock size tolerance | Limited by flask gate, typically <300 mm | Tolerates oversize, manual loading possible | Limited by belt width and lump size |
| Best application fit | Production hoisting, vertical shafts | Shaft sinking, small or exploration mines | Inclined declines, large continuous orebodies |
| Reliability / typical availability | 0.85 to 0.92 | 0.80 to 0.90 | 0.92 to 0.96 |
Frequently Asked Questions About Mining Buckets and Skip
Rock fines work their way behind the wear liners and pack into voids in the skip body that the design assumed would stay empty. On Hardox-lined skips it's common to gain 200 to 500 kg of trapped fines after 6 to 12 months. The body weighs more, but the rated volume hasn't changed, so payload drops and the rope sees a worse ratio.
The fix is liner removal during a major shutdown — strip the side liners and clean the cavity behind them. If you're seeing the ratio fall faster than 0.05 per year, your liner gasket strips are failing and fines are migrating in faster than they should.
Two smaller counterbalanced skips almost always win on a production shaft past about 800 m. The counterweighted second skip means the hoist motor only fights the payload mass, not the full skip-plus-rope-plus-payload, so installed motor power drops by roughly 40%. You also get redundancy — if one skip is down for liner replacement, you can run the other in single-skip mode at half throughput.
Single skip with a counterweight makes sense only on shallow shafts under about 500 m, or where the orebody is small enough that you'll never need the second line.
Almost always a dump door that isn't swinging through full travel. Check the door stop hardware first — if the stop is set wrong the door opens to maybe 70° instead of 90° and a wedge of rock stays trapped at the back of the body. Second suspect is mud and fines packed into the door hinge, which adds rotational drag and slows the swing during the brief window the scrolls hold the body tipped.
Quick diagnostic: catch the skip at the dump position during a maintenance window and physically check door swing angle with a digital level. Anything under 85° from horizontal and you'll carry rock back down.
Koepe friction hoists win once shaft depth pushes past about 1,000 m because the rope mass on a drum hoist becomes a serious fraction of payload — at 2,000 m a drum hoist can be lifting more rope than ore. Koepe systems use a tail rope to balance head rope mass, so motor power tracks payload only.
Drum hoists stay competitive on shallower shafts and where you need to vary skip levels (multi-level loading pockets) since drum systems give you absolute position control on each rope. Koepe relies on friction and demands strict rope tension matching, so multi-level loading needs careful chairing arrangements.
The formula assumes back-to-back cycling with zero gap. In practice you lose time to creep speeds at the loading and dump positions — the hoist operator dials speed down to 0.5 to 1 m/s for the last few metres of approach to spot the skip accurately. Those creep zones eat 4 to 8 seconds per cycle that don't show up in the headline numbers.
Modern automated hoists with absolute encoder positioning recover most of that loss because the system can hit a known stopping point at full deceleration without the operator's safety margin. Retrofit projects swapping manual hoist control for automated control routinely report 10 to 15% throughput gains with zero hardware change to skips or guides.
Three cases: very small mines where total daily tonnage is under about 200 t and you can't justify the capital for a skip headframe; orebodies producing oversized lump that won't pass through a measuring flask gate; or remote operations where the simplicity of a kibble (no scrolls, no flask, no measuring chamber) outweighs the slower cycle time.
Several Yukon and Nevada small-mine operations run kibbles into production for exactly these reasons. Once daily tonnage targets push past about 500 t/day, the math always swings toward a proper skip installation because cycle time on a kibble is roughly 2× a skip's for the same depth.
References & Further Reading
- Wikipedia contributors. Skip (container). Wikipedia
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