IRSST - Institut de recherche Robert-Sauvé en santé et en sécurité du travail

Modernization of Safety Catches for Mine Conveyances: Part 2 – Hoist Rope Failure

Summary

This study was initiated by an October 25, 2013 letter written by a Union member of the sub-committee on hoisting machines of the Commission de la santé et de la sécurité du travail (CSST)1. The letter asked the sub-committee to mandate the Institut de recherche Robert-Sauvé en santé et en sécurité du travail (IRSST) to evaluate emergency arrest systems (safety catches and other systems) for mine conveyances in use worldwide, ultimately with a view to modernizing the safety catches required on mine conveyances in Québec. Part 1, submitted on June 5, 20142, presented a general review of the literature on safety catches and hoist ropes. This second part concerns possible solutions for preventing hoist rope failure and the resulting cage crashes.

Accidents involving crashes of hoisting machine cages are extreme cases if you consider the probability of such an event occurring and the severity of the consequences. In fact, the accident review presented in Part 1 revealed that cases involving cage crashes due to hoist rope failure are rare, but that when they do occur, the number of injured persons or fatalities can be high. This raises the question of the need for safety systems such as safety catches: is it more worthwhile to focus attention and efforts on hoist ropes (as in South Africa), or on safety catches in view of their low cost and the additional safety they offer?

The second chapter of this Part 2 report presents possible solutions for preventing hoist rope severance, as well as the related legislation. Research conducted in South Africa showed that deteriorations in hoist ropes are hard to anticipate and that no general rules can be enacted. However, the overall legislative and standards frameworks in South Africa limit the propagation of the root causes before they result in an accident. Various parameters play a role in the wear and tear on a hoist rope, and a solid knowledge of these mechanisms is needed to anticipate possible weakening. The breaking resistance of a hoist rope can vary greatly over its entire length, and the weakest resistance is rarely at the point of attachment. The criteria for replacing hoist ropes vary from one Canadian province to the other, but none of the provinces suggest considering the cumulative effects, contrary to Standard SABS0293 and Standard ISO 4309. In our view, taking cumulative effects into account is a practice that should be implemented to increase safety, regardless of the safety factor used.

Hoist rope strength can be evaluated through visual examination, electromagnetic examination, or a system for continuously monitoring the state of wear of the rope. Visual examinations should be performed under good lighting and visibility conditions. Two parameters are measured in electromagnetic tests: loss of metallic area (LMA) and localized faults (LFs). Electromagnetic tests provide valuable information on the “state of health” of hoist ropes, particularly on the inside of the rope, which is not visible. However, the apparatus used to perform this type of test has not been perfected and has certain limitations: sensitivity decreases with the depth of the fault, some broken internal wires are not always detectable, and the technician’s experience is a key factor. A training and certification program for hoist rope inspectors appears to be a very worthwhile option. Continuous rope monitoring systems, which are mandatory for lowering the safety factor (SF), allow real-time tracking of the condition of the rope and offer a considerable advantage during periodic electromagnetic inspections as efforts can be concentrated on the zones identified as weakened.

Three strategies should be prioritized to maximize the life span of hoist ropes: regular maintenance, limiting dynamic loads, and using new-generation ropes (mixed-material ropes, synthetic ropes). Automatic lubrication systems are available and allow the lubricant to be sprayed along the entire length of the hoist rope after a certain number of cycles, by connecting them to a programmable logic controller (PLC). Strict control over braking limits the dynamic loads on the rope and can thus prolong its life span. In fact, the majority of root-cause events leading to rope failure due to overloading are related to dynamic loads. Lastly, mixed-material ropes are becoming increasingly common in mine shafts and permit larger allowable loads without compromising worker safety. In the longer term, it will likely be possible to use 100%-synthetic ropes, which will require legislative amendments.

Traditional safety catches in service in Québec today are nearly all of the “Ontario dog” design, i.e. single-tooth safety dogs per side of the shaft guide, equipped with a splitter in order to remove the wood chips from the guide, as well as a heel that prevents complete turnover of the tooth and partially compensates for guide wear.

The deceleration generated by this type of safety system for mine conveyances is relatively high: in the order of 3 g (29.43 m/s2), which is what motivated the request for this expert report. While such decelerations can cause injuries to workers in the cage, a number of cases have been observed in which there were no injuries despite deceleration speeds of this order. The problem arises mainly with cages that are almost empty, as they will decelerate faster.

The braking force F (mean wood resistance) can be calculated by applying the energy conservation principle. The braking force per tooth can be estimated using empirical equations provided by the safety catch manufacturers. It appears that this F force is influenced by the dimensions of the tooth (width and depth of the cut), and by the angle it makes with the horizontal. The smaller the angle, the greater the braking force. While these equations are not perfect, they provide a relatively good – and somewhat conservative – estimate of the braking force per tooth.

The Ontario-type dogs provide a compensation mechanism for guide wear. The efficiency of this mechanism has been evaluated. It appears that the smaller the tooth angle, the less effectively the compensation mechanism is able to limit the loss of braking force. The braking force for a shaft guide that is swollen due to moisture will diminish when the tooth angle is small but will increase when the tooth angle is greater than 10°. If the guide width is strictly equal to the nominal width, the braking contribution of the safety-dog heel is approximately 8%.

Numerous results from free-fall tests have been collected and compiled: regardless of both tooth shape and angle and of the moisture content of the guides, the braking force per surface (F/T) is almost always between 3,000 and 5,000 lb/in2. Braking force F can be considered constant for a given tooth geometry and wood species. Hence, based on the results of the fully loaded cage free-fall test, it is possible to estimate the deceleration for an empty cage (or one carrying very few workers). In particular, for tests conducted at the Colorado School of Mines, in which fully loaded cage deceleration was measured at 0.6 g, we can calculate the empty cage deceleration (3.4 g). For this specific case, in most Canadian provinces, the safety catch system would have been rejected (deceleration less than 0.9 g or 1 g). Taking the same cage fully loaded with a deceleration of 1 g, the deceleration can be estimated in the order of 5 g with 1 worker (instead of 72 at maximum capacity).

The fault tree includes 48 root-cause events. Experiential feedback accumulated with traditional safety catches, combined with regulatory requirements and the use of Ontario dog-type teeth, helps eliminate some root-cause events from the fault tree. The mechanism/teeth/guide fit of traditional safety catches generates 11 root-cause events that can result in limited – indeed very limited – braking of the safety catch. It is therefore essential to closely monitor the condition of wooden shaft guides, wear on the guides and dog teeth, and tooth position relative to the guides, and to ensure that the same wood species are used for all pairs of guides.  

Modern safety catches (clamp-type emergency braking system) work on steel shaft guides, and operate on a principle similar to that of disc brakes: the steel guide is gripped by two clamps equipped with braking pads. Two evolved systems available on the market have been studied. Both systems are evolutions of chairing devices.

The mounting of an energy accumulator on one of the chairing devices on the cage roof allows the brakes to be applied in the event of hoist rope failure. A signal is also sent to the hoisting machine as soon as the brakes are applied to prevent, for example, the hoist rope from piling up on the cage roof. However, the reliability of this safety function depends on the communication between the cage and the hoisting machine. This evolved system is found in a few mines outside Québec, but experiential feedback is still very limited. Mining companies fear the unwanted application of the brakes during normal cage operation (untimely braking), which could result in worker injury. It has been equipped with several redundant systems to make it more reliable. Many quick-release tests have been performed, as well as some free-fall tests. The empty-cage test performed in Ontario revealed gentler deceleration (1.4 g) than with conventional safety catches. Fully loaded cage tests are expected to be performed in South Africa. As the pressure applied by the heels on the shaft guides can be adjusted, it is theoretically possible to control the deceleration according to the cage load. The standard chairing devices are installed all over the world, and this type of system has proven itself over the years. Nonetheless, our correspondence with one user of the evolved system brought to light a number of reliability problems that prevent the system from being used on a daily basis. It appears that this system needs to be made more reliable before being brought into general use.

To the best of our knowledge, the second evolved system had not yet been installed on a cage in 2015. Only limited information is found on this system compared to the first system. According to the manufacturer, the main advantage is the use of a closed hydraulic circuit. Unlike the previously evolved system, which requires a compressed air supply at each station, this second system has a battery-driven compressor mounted on the cage. This closed hydraulic circuit prevents pollutants from entering, thus reducing the risk of malfunction. However, it is worth noting that all the documents concerning this second evolved system were removed from the manufacturer’s website during the 2014-2015 study, and that the contact we initiated with them was ultimately unsuccessful.

Modern safety catches help eliminate root-cause events present in the fault trees for conventional safety catches. However, other root-cause events must be added to the FT, thus modifying the probabilities of occurrence of different events. This means that all such modifications must be taken into consideration when evaluating the system’s overall safety. Modern safety catches are still in the very early stages, and their development will be dictated by the use of steel shaft guides. Until then, a few avenues of research could be pursued in order to make them at least as reliable as traditional safety catches, namely, cage/hoisting machine communication, the untimely activation of safety catches on cage ascent, and the use of electronics.

Lastly, the function of safety catches should be enhanced to ensure worker safety during mechanical failure or failure of the hoisting machine control system.

A series of recommendations is proposed at the end of each chapter of this Part 2 report, for a total of 24 recommendations.

 

[1] Now the Commission des normes, de l'équité, de la santé et de la sécurité du travail (CNESST).

[2] Expert Report (QR-1156-fr) published under the reference Giraud and Galy, 2022.

 

Additional Information

Category: Expert Report
Online since: June 20, 2022
Format: Text