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Considerations about the Cost of Conveyor Belting – Discussing re-evaluated Belt Safety Factors

Safety Factors for Conveyor Belts

Considerations about the Cost of Conveyor Belting – Discussing re-evaluated Belt Safety Factors

New conveyor belts and belt monitoring technologies reduce conveyor belt capital and operating costs by using lower belt strengths than previously thought possible. Key factors are improvements in splice performance, rubbers and real-time belt condition surveillance systems.
(ed. wgeisler - 01/2/2017)
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Fig. 1: Consequences of a belt break

Lowering a belt’s strength and safety factor reduces belt cost. But, how can you reduce cost without compromising quality or increasing risk? This article explains how recent improvements in steel cord conveyor belt technology in the areas of dynamic splice efficiency (when used with real time belt condition monitoring), and low rolling resistance rubbers make lower belt strengths a safe option.
 

 

Introduction

Fig. 1 shows the consequences of a belt break where over 1 km of belt accumulated at the bottom of a slope on an overland conveyor after a failure. It took two weeks of downtime to sort it out. In order to avoid such situations it is important to consider belt safety factors. Safety factors for steel cord belts were originally developed from many years of experience (first steel cord belt was installed in 1942). They were traditionally designed using conservative values. The most common safety factor being 6.67:1 or 15% of the belt breaking strength. That is, the breaking strength of the belt is 6.67 times the maximum running tension of the conveyor. If we could find a safe way to use a lower safety factor we would be able to use a lower rated, lower cost, belt. Fig. 2 shows the relationship between safety factor and required belt strength.


Fig. 2: Influence of safety factor on belt strength

In Fig. 2, the Y-axis to the right of the chart considers a conveyor with a maximum running tension of 150 kN/m. Using conventional wisdom and a safety factor of 6.67:1, the required belt would be a ST1000, that is, a belt with a breaking strength of 1000 kN/m. On the same Y-axis scale, the chart then shows what would be the required belt strength if we were able to safely reduce the safety factor to a value indicated on the X-axis. The “Safety Factor Index” on the left Y-axis is the multiplier you would use for any belt strength to determine what the required belt strength should be for a different safety factor if you know the ST rating of the belt using the conventional 6.67:1 safety factor. For example, say we have an existing ST5000 belt that was designed using a 6.67:1 safety factor. If you were able to use a 5.0:1 safety factor you could use a 0.75 x ST5000 = ST3750 belt strength.

Not all Safety Factors are created equal!

The generally accepted definition of belt safety factor is the minimum breaking strength of a belt divided by the maximum belt running tension. (Note that DIN 22101-2002 also defines two belt safety factors that when multiplied together, relate to the ratio of the maximum belt edge running tension and the measured dynamic splice strength of the belt). However, some confusion has arisen in the industry due to different definitions of the minimum belt strengths to be used.

Around the world we have a number of international standards for steel cord belts which define the belt construction for standard steel cord belt strengths. They define the number of cords and the minimum cord strengths. In some cases, as in the now obsolete DIN 22131, some additional strength (around 10%) was added to the nominal belt strength to ensure a more robust safety margin. Fig. 3, on the right hand side, shows the actual belt design strengths compared to the stated “ST” (kN/m) rating for Germany’s DIN 22131, Europe’s ISO 15236-2 (nontabular), Australia’s AS1333 and China’s GB/T9770 for two different belt widths, 1000mm and 2000mm. The X-axis is the nominal belt strength or “ST” rating in kN/m. The Y-axis is the actual design belt strength expressed as a percentage of nominal. In practice the belt strengths would be even higher as the actual cord strengths supplied will always be slightly stronger than the specified minimum.


Fig. 3: Belt strength variations according to DIN, ISO, Australian and Chinese standards’ designs

The main point here is that it is important for the end user to understand that there are significant variations between design belt strengths conforming to different standards. If not correctly applied, these differences can lead to incorrect comparisons and conclusions.

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