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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From Fig. 4 it is clear that a 60% splice efficiency increases this reserve tension. That gives us confidence to consider reducing the belt safety factor (SF) from, say, 15% of belt break to 20% of belt break, that is, from a 6.67:1 SF to a 5.0.SF, as there is still adequate reserve belt tension for degradation and accidental damage considerations, Fig. 5.

Fig. 5: 50% and 60% splice efficiency and 5.0:1 SF

This figure shows the 5.0:1 acceleration tension line as an addition 40% of the 5.0:1 running tension. This can be reduced with the use of a soft start fluid coupling or through VFD or DC drive controls.

DIN 22101-2011 offers a convenient approach to determining a suggested belt safety factor for any given application [1]. In this case the degradation factors are embedded in two safety factors called S0 and S1. Suggested values for each of these safety factors are offered in tabular form. Belt dynamic splice efficiency is included in the method and default values are given as 45% for steel cord belts and 30% to 35% for different types of fabric belts splices. The method also considers the maximum belt running tension as the maximum tension at the edge of the belt in the transitions instead of the average belt running tension. Methods to calculate the belt edge tensions and default values are also included. A summary of this SF method is given below. Further details can be obtained from the standard.

As lower safety factors are implemented, typically smaller and/or fewer steel cords are employed in the belt design. This makes the belt more vulnerable to accidental damage from, say, impact or trapped material. In order to reduce the risk of these events having catastrophic consequences for lifeline conveyors it is highly recommended that the condition of the cords be monitored 24/7. Such systems are readily available using well established technology.

Recent field examples justify the cost. For example, a lifeline conveyor in a copper mine in Chile broke in two when the level controller of the stockpile it was feeding failed. The stockpile engulfed the head pulley and the additional material drag on the belt broke the belt in two at a section with 20% of the cords already broken from a previous event. Downtime cost to the mine was stated as US $50 million. Effective cord condition monitoring could have avoided this.

2. Low Rolling Resistance (LRR) Rubber

Energy efficient rubber technology has been offered for 17 years now for conveyor belts. The technology is well established and LRR belt manufacturers are continuing to improve the energy savings offered. The technology has been well reported previously, [2-12], but the following is a brief recap for those new to the industry.

Fig: 6: Idler indentation rolling resistance

Fig. 6 illustrates the source of idler indentation rolling resistance. The pulley cover rubber of a belt deforms on every idler under the load carried by the belt. The deformation creates resistance to belt motion and the action of the rubber being compressed generates heat. This heat is lost energy. On a long horizontal overland conveyor this resistance can easily be 60% of the total belt resistance and belt tension. Rubbers can be designed to minimize this effect and so lose less heat energy. They can reduce the lost energy by up to 40%. The effect is known as indentation rolling resistance (IRR) and several research institutions have developed tests to measure it. A German standard, DIN 22123, describes one test method to measure it. An Australian standard is currently being developed for a similar test method. 

These are both “full scale” belt tests where belt samples are built into an endless loop and driven at a constant speed on a 2-pulley test fixture. The indentation rolling resistance is measured by means of an instrumented idler roll running against the pulley cover of the belt and a simulated normal load is applied through the belt to the idler. The entire fixture is located inside a walk-in environmental chamber. Ambient temperature, normal idler load, belt speed, idler diameter and belt pulley cover thickness are all controlled variables. Indentation rolling resistance is measured for each set of conditions in N/m of belt width.

Smaller scale methods are also used where the rheological properties of the pulley cover rubber are measured. From these tests, the energy absorbed for any given temperature, rubber strain and frequency of strain can be determined. Studies have shown relatively good correlation between the small scale and the large scale test methods.

There are two main benefits from using a low rolling resistance rubber on a conveyor belt.
1. Lower belt tension
2. Lower operational power/energy

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