Mining Rule of Thumb

The primary usage of Rules of Thumb should be in the development of conceptual designs and feasibility studies or, when a quick decision is required in the solution of an operating problem. Although an approximated answer, derived from a Rule of Thumb may solve an immediate problem, Rules of Thumb are not a substitute for the application of sound engineering and design methodologies. Although we firmly believe that the presented Rules of Thumb provide great continuing value to our industry, McIntosh Engineering does not guarantee their validity, nor do we (or the referenced individual sources) accept responsibility for application of the Rules of Thumb by others. Where possible, direct quotes have been provided from individual references; however, it is possible that referenced sources may not have directly stated the Rule of Thumb for which they are assigned credit. Although we have endeavored to accurately quote all individual references contained in the Rules of Thumb compilation, we apologize in advance for any misquotes that may be attributed to individual sources. We will provide updates to the Rules of Thumb compilation, as we become aware of corrections that may be necessary.

Field: Wire Ropes, Sheaves, and Conveyances

Area: 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. Jack de la Vergne
• The centre of gravity of a loaded bottom dump skip should coincide with the geo-centre of the skip bridle. 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. 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. 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. 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. Largo Albert
• The regular maintenance refit and repair of an aluminum skip costs approximately 35% of the price of a new skip. Richard McIvor
• A properly designed and maintained aluminum skip should have a total life of 5,000,000 tons (including refits and repairs). Richard McIvor
• The cage capacity will be between 1.6 to 1.8 times the empty cage weight. Wabi Iron Works

Area: 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. L. O. Cooper
• The rope installed on a drum hoist or winch should be pre-tensioned to 50% of the working load. George Delorme
• The tension required for a guide rope is one metric tonne for each 100m of suspended rope. 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). Tréfilunion
• The pitch radius of a wire rope thimble should not be less than 3.5 times the rope diameter. Largo Albert
• The length of a wire rope thimble should not be less than five times the pitch radius. Largo Albert

Area: Sheaves

• 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. African Wire Ropes Limited
• For every increase in speed of 1m/s (200 fpm), 5% should be added to the sheave or roller diameter. African Wire Ropes Limited