Question 3 of 7: Hoist Ropes, Overwind, Shaft Mucking and Skip Design

Question 3: Hoist Ropes, Overwind, Shaft Mucking and Skip Design (15 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

3.1 — Regular lay and Lang's lay

A wire rope is built up from individual wires twisted into strands, and the strands twisted around a core; "lay" describes the relationship between the direction the wires are twisted within a strand and the direction the strands are twisted around the rope. In Regular lay the wire lay is opposite to the strand lay, so the visible wires run nearly parallel to the rope's own axis; this gives a rope that resists kinking and untwisting under load, has good structural stability and is easy to handle, at the cost of a smaller effective wire contact area against the sheave/drum (lower resistance to abrasion). In Lang's lay the wire lay is in the same direction as the strand lay, so wires spiral diagonally across the rope surface at a steeper angle; the wires lie against the sheave over a longer contact length, giving better wear/fatigue resistance and flexibility, but the rope has a greater tendency to rotate/untwist under an unrestrained load and requires the free (non-rotating) end to be secured, so it is favoured for friction/Koepe winders and less favoured for a single free-hanging conveyance without a properly designed rope-cap and guide arrangement.

3.2 — Round strand, flattened strand, locked coil and half-locked coil

Round strand rope (round wires wound into round strands, e.g. 6x19, 6x36 construction) is the general-purpose hoisting rope: flexible, straightforward to inspect and terminate, and the default choice for cage and skip hoisting ropes and for haulage/mucking ropes. Flattened strand rope uses trapezoidal (shaped) strands over a triangular core, giving a smoother outer surface with a larger bearing area against the sheave groove; it is used where wear resistance and a smooth running surface matter more than the extra manufacturing cost, such as friction (Koepe) hoisting ropes and heavily used haulage ropes. Locked coil rope has an outer layer of interlocking, shaped (Z- and half-round) wires that lock together to form a smooth, sealed outer surface with no exposed strand valleys; it is the standard for large, non-rotating, high-capacity shaft hoisting ropes (particularly deep single-rope or multi-rope Koepe winders) because the locked surface resists water and dirt ingress and gives excellent abrasion resistance, though it is stiffer and more specialised to terminate. Half-locked coil combines an inner round-strand core with an outer locked layer, giving most of locked coil's smooth wear surface with somewhat more flexibility than a fully locked construction; it is used on medium-to-large hoisting installations where full locked-coil stiffness is not warranted.

3.3 — Overwind and its prevention

Overwind is the condition where the conveyance (skip or cage) travels past its intended stopping point at the top of the shaft and continues into the headframe, potentially striking the sheave wheel, over-running the rope onto the drum, or crashing through the headframe structure — typically caused by hoist operator error, a control or brake malfunction, or an over-speed condition that the normal deceleration/creep-speed control fails to catch in time. It is prevented, in combination, by: (1) an overwind limit switch / depth-indicator interlock (e.g. a Lilly controller or equivalent electromechanical depth-tracking device geared to the drum) that independently tracks conveyance position and cuts power and applies the emergency brake automatically if the conveyance passes a preset limit before reaching bank; and (2) a mechanical overtravel/catch device at the headframe (such as a keps, crash beam, or arrestor system) that physically arrests the conveyance if it travels beyond the electrical limit, providing a redundant, purely mechanical last line of defence independent of the control system.

3.4 — Shaft-sinking mucking methods

All three are mechanical mucking machines used to clear blasted rock from the bottom of a shaft during sinking, where working space is confined and the muck must be lifted into a kibble or bucket for hoisting to surface. The Cryderman mucker is a pneumatically or hydraulically powered clams‌hell-type grab mounted on a rotating, extendable boom suspended from the sinking stage; it swings to pick up muck from anywhere across the shaft bottom and dumps directly into the kibble, and is widely used because one machine can service the full cross-section without repositioning. A clams‌hell (grab bucket) mucker is a two-jawed bucket lowered on its own hoist rope, closed by cable tension to bite into the muck pile and then hoisted clear to dump; it is simple and rugged but less selective and slower per cycle than a boom-mounted grab. A cactus grabber uses a multi-pronged (cactus-like) grab head, typically air-powered, that closes several curved fingers around a bite of muck; it handles blocky, broken rock well and is common on smaller-diameter shaft sinking jobs where a compact grab geometry is needed.

3.5 — Skips and loading pockets; skip design types

A skip is the self-dumping conveyance used to hoist broken rock (ore or waste) up the shaft, as distinct from a cage (which carries personnel/material in a car or cage). A loading pocket is the underground storage and metering structure — typically a set of ore/waste storage bins with a chute and measuring (weightometer or measuring-pocket) gate at the shaft station — that regulates the flow of broken rock into the skip for each hoisting cycle, decoupling the (batch) hoisting cycle from the (continuous) production/haulage system.

Fixed-body bottom dump. The skip body is rigidly fixed to its frame; a hinged door in the bottom is tripped open by a fixed dumping arrangement (rollers/cams) at the headframe dump station, releasing the load downward into the receiving bin. It loads from a conventional bottom-discharge loading pocket and needs only a simple, low-maintenance latch mechanism, but the dump geometry is fixed by the headframe layout and the door mechanism is exposed to impact wear from every dump cycle.

Overturning (Kimberley). The whole skip body pivots (overturns) about trunnions at the dump level, tipping the load out over the top/side as it rotates, guided by fixed dump rails or curves at the headframe. It handles coarser, stickier or wetter muck well (nothing must pass through a bottom gate) and gives a positive, complete dump, but the pivoting mechanism, bearings/trunnions and guide rails see continuous cyclic wear and need more frequent maintenance than a simple bottom-dump door.

Swing-out body. The skip body is hinged near the top and swings outward/sideways to dump, rather than rotating about a full overturn; it is compact vertically (useful where headframe height is constrained) and gives good control of the dump trajectory, but the swing linkage and its guide/latch hardware are a further set of moving parts requiring inspection, and clearance for the outward swing must be built into the headframe layout.