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Tow Force Measurement of Various Mining Equipment

Underground » Mining Technology and Production

Published: May 15Project Number: C23012

Get ReportAuthor: Daniel Carpenter | BMT WBM

Towing of heavy and light equipment is a vital part of coal mining and occurs regularly. The principle risks involved with the failure of towing components include:

· Uncontrolled release of energy when components fail:

o Safety Alert 156 Towing of Longwall chocks almost results in fatalities, 13th February 2007, Queensland department of Mines & Energy,

o MSHA report CAI-2003-24, Fatal powered haulage accident, September 9, 2003,

· Unplanned movements or loss of control when components fail:

o Safety Alert 217 Unplanned movement of towed equipment 9th January 2009, Queensland department of Mines & Energy.

 

In recent years there have been several incidents involving towing equipment resulting in the Department of Industry and Investment releasing Safety Bulletin SB09-03 Broken Pull Chain Results in Fatality on 23 September 2009 with the recommendation that all mines review their towing, pulling and snigging operations. Also in the recommendations is the requirement to consider 'potential impact forces on the towing system resulting from dynamic loading'.

 

The effect of dynamic loading is well understood for lifting applications and detailed in AS1418.1, however there appears to be very little guidance available to design engineers and equipment operators for towing applications. It is tempting to apply lifting equipment guidance to towing applications however some key differences exist between the applications such as: ·

· Dynamic loading occurs once for each normal lifting operation but occurs irregularly throughout a towing operation, thus the frequency of the dynamic loading is different;

· The magnitude of the dynamic effects (the 'dynamic amplification factor') may be quite different to lifting applications. Lifting applications generally involve only one significant moving object, attached to long (soft) couplings with precise speed control. In contrast, Towing applications can involve two heavy pieces of moving equipment connected by relatively short (stiff) couplings and with relatively poor speed control. Thus, the magnitude of dynamic effects is expected to be much larger for towing applications.

 

It would seem that it is possible for towing equipment to be subject to larger dynamic effects than lifting equipment for a potentially large, yet non-apparent, number of load cycles. Dynamic loading consumes the fatigue life of towing equipment so the towing application is expected to incur fatigue damage at a much greater rate than lifting applications. The description of the incident described in Safety Alert 217 indicates that fatigue failure of towing equipment can and has occurred.

 

Despite the differences, towing equipment is generally designed or specified based on commercially rated lifting equipment.

 

There in as increasing trend toward the use of towing 'strops' rather than chains. The strops are made from synthetic fibres and designed to fail without the release of energy. When they are overloaded, they break and fall on the ground without the dangerous recoil that chains exhibit. Strops are less stiff (stretchier) than chains, which, in theory at least, reduces the magnitude of dynamic loading effects. For this reason, strops will be tested alongside and compared with towing chains.

 

Over numerous years, BMT WBM staff has observed a large number of deformed tow hitches, pins and associated equipment. The same staff members have also received numerous requests to design and certify towing equipment.

 

Mining Design Guidelines (MDG-1 and MDG-15) states "The designed minimum factor of safety should be 2.5 times the maximum rated towable capacity of the towing equipment", although it doesn't state whether this should be applied to the gross section yielding (the onset of permanent deformation) or the ultimate strength. An even more conservative interpretation would be to use the value of 2.5 as the 'dynamic amplification factor' and adopt the normal lifting equipment design rules stated in AS 1418.

 

This purpose of this project was to:

· Measure typical towing forces using a strain gauge-based instrumentation and data logging equipment, for a range of machinery and towing equipment;

· Extract key variables from the measured data used for the design and specification of towing equipment including:

o dynamic amplification factor;

o equivalent full-load cycle frequency rate; and

o typical friction coefficients where applicable.

· Determine whether the minimum safety factor described in MDG1 and MDG15 should be applied to the ultimate strength, yield strength or be used as a dynamic amplification factor.

 

The intention is that data from this project will assist engineers design and/or specify suitable towing equipment and could feasibly be used for future development of the Mining Design Guidelines.

 

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