Underwind and overwind protection systems with …

1 411 The Journal of The South African Institute of Mining and MetallurgyAUGUST 2004 IntroductionMine hoist operation carries particular riskof crash-type accidents at the upper and lowerextremities of the hoisting range. Furthermore,a number of factors combine to increase theseverity of the consequences that such anaccident could have. Among these are the largenumber of workers carried by cages and thehigh load of ore carried by skips, large numberof hoists per day, long hoisting distance, highhoisting speed, the fact that a number ofconveyances are operated in a shaft, and thedependency of mining operations on the Underwind incident occurs when theconveyance overruns the design loweroperational limit and is at risk of crashing intothe shaft bottom.

2 An overwind incident occurswhen the conveyance overruns the designupper limit of travel and is at risk of crashinginto the headgear. These events may be causedby the failure of the braking or control systemand may have catastrophic consequences withregard to the loss of human life and/or minecapital equipment and production. In a sample of South African mines,predominantly in the North West region, forthe period January 1988 to July 1995, 46incidents were found1. In three incidents in1973, 1991 and 1997 fatalities numbered 16,7 and 13 respectively1. Because of this riskand of the history of incidents of this nature1,the Safety in Mines Research AdvisoryCommittee (SIMRAC) commissioned a studyon additional end-of-wind protection systems .

3 As workers represent a much more fragileload than material, the design retardation rateswere restricted by human tolerance. To preventinjuries to people transported in conveyances,the maximum deceleration for the underwindsystem is g( m/s2) and for the over-wind system 1 g( m/s2)2. According to theBritish Columbia legislation referred to here,this is deemed a walking away safe Underwind and overwind protectionsystems with enhanced self-sufficiencyby Burger*, von Wielligh*, De Wet*, Steyn*, and Otterman SynopsisUnderwind and overwind protection system concepts for mine hoistshafts were developed in conjunction with and for the Safety inMines Research Advisory Committee (SIMRAC).

4 End-of-windoperation of mine hoists is the most hazardous aspect of minehoisting, carrying the highest risk of loss of life, injuries to workersand loss of operating mechanically and with local actuation weredeveloped. This was done in order to remove the increased riskassociated with dependency on remote or external energy andinformation supply associated with most existing proposed Underwind protection system concept absorbs theenergy of motion of a conveyance overrunning the design lowerlimit of travel by drawing a metal strip through a set of rollers. Thisaction causes dynamic cyclic plastic bending of the strip materialthat converts the kinetic and potential energy of the conveyanceinto strain energy of the metal.

5 In the concept design a pair of steelwire rope slings attached to the strips catch the overrunningconveyance and transfer the retardation force. The proposed overwind protection system concept absorbs theenergy of motion of a conveyance overrunning the design upperlimit of travel by early detaching of the conveyance from the hoistrope. This detaching is carried out via an additional detaching hookactivation mechanism fitted at a sufficient height below thespectacle plate to allow the conveyance to retard to standstill undergravity before it would crash into the spectacle plate.

6 Theconveyance so brought to rest is prevented from falling by means ofjack catches on the conveyance interacting with a rack (toothedprofile) fitted on the guide rails in the retardation models (1:10 scale) of both protection systems weredesigned to conform to established retardation standards. Theretardation standards were limited by the requirement of passengersafety in retarding cages. The models were then built and tested in a1:10 scale shaft model. Retardation performance close to therequired levels was achieved. The retardation distance requiredshould allow such systems to be retrofitted in most existing the simplicity and robustness of the designs, furtherdevelopment was recommended because of their enhanced self-sufficiency and reduced risk of : Underwind , overwind , deceleration, mineshafts, hoisting.

7 *Department of Mechanical and AeronauticalEngineering, University of Pretoria, Pretoria Business Enterprises@University of Pretoria (Pty)Ltd., University of Pretoria, Pretoria The South African Institute of Mining andMetallurgy, 2004. SA ISSN 0038 223 + Paper received Feb. 2004; revised paperreceived May and overwind protection systems with enhanced self-suffiencydeceleration. This legislation on the deceleration value is notincluded in the South African Mines and Safety Act(29/1996). There is a directive C2 to all mining operationsrequiring maximum deceleration values of to m/s2formine winders and to m/s2for rock winders.

8 This is,however, not shafts are already equipped with a number of safetysystems of which the Lilly speed regulator is one, butaccidents still occur. The cause of such accidents can often betraced back to remote- or event-related impairment of signalsand energy supply to the primary safety device. Therefore aprimary requirement for the additional protection systemswas that they should be independent of the existing preferably had to be mechanical devices because of therisk that electrical or hydraulic energy and signals could becompromised by the primary cause of failure.

9 The studyfocused on vertical shafts as these installations present themore stringent requirements. The solutions, however, wererequired to be adaptable to inclined concepts were generated for the protectionsystems. The concept generation process was informed by aliterature and product survey conducted to establish thenature and causes of end-of-wind incidents and to reviewavailable technology for end-of-wind devices. This study wascomplemented by a user needs survey and study of existinginstallations. Specifications were compiled with the aid of afunctional analysis study according to standard designprocedure3.

10 The concepts generated were evaluated againstthe system requirements. Scaled-down models of the selected concepts for bothprotection systems were designed, built and installed fortesting in the 1:10 scale shaft model at the Department ofMechanical and Aeronautical Engineering, University ofPretoria. Underwind The primary design parameters for the Underwind protectiondevice were: The maximum deceleration to prevent injuries topeople transported in the conveyance was taken as ( m/s2) as explained above. The maximum speed of the conveyance is 18 m/s. Thesystem had to provide for Underwind situations butwas not required to provide for free fall.