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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For example, Table 1 shows a number of well-known conveyor belt installations where the reported and the actual safety factors are compared. The reported safety factors are all based on the “nominal ST” belt strength, not the actual belt strength. The conveyor slope is also indicated in the last column.

Table 1: Reported and actual belt safety factors                                                                               

From Table 1, we can make the observations that:
1. There are several belts that have been operating successfully for many years with safety factors between 5.1:1 and 5.5:1
2. Belts with slopes greater than six deg typically have safety factors of 6.0:1 or higher.

Although our sample size is small, there are good reasons to use higher safety factors on high inclined belts. Inclined conveyors have considerably more stored potential energy in the belt than horizontal conveyors, even when the belt is empty.

How Can I Get Lower Cost Belts?

In order to get lower cost belts without compromising belt quality there are two basic approaches:
1. Use lower belt safety factors based on higher dynamic splice efficiencies and 24/7 real-time belt condition monitoring.
2. Use low rolling resistance (“LRR”) rubber to reduce operating belt tension.

1. Lower Safety Factors

Lower safety factors for a given application can be achieved through better dynamic splice efficiency. This is based on the premises that:
1. Maximum belt tension is related to dynamic splice efficiency;
2. Cord fatigue life exceeds dynamic splice life;
3. Dynamic splice life exceeds belt wear life;
4. 24/7 cord monitoring can avert catastrophes.

Dynamic splice efficiencies for steel cord belt splices are most commonly determined by the test defined in DIN 22110 Part 3. In this standard the dynamic splice efficiency (also known as the Relative Reference Strength of the splice) is defined as the maximum test load that can achieve 10 000 load cycles on a 2-pulley splice test. Each load cycle increases the test load from 6.6% of belt break to the test load (e.g. 50% of break) in 42 seconds and returns to 6.6% in 8 seconds to complete a 50 second cycle. During each 50 second cycle, the test belt splice loop also completes 18 revolutions on the fixture. Typically, four tests are conducted at different peak test loads. For each test, the number of cycles achieved before the splice fails are plotted against the peak test load as a percentage of belt break strength. A trend line is drawn through the four points to generate a characteristic fatigue curve, also called a Wöhler curve. The significance of the Wöhlers curve is that it relates the lab test results to anticipated field performance.

The DIN 22110 Part 3 splice test was developed by Hannover University in the 1980’s. For reference, in 1985 the Prosper Haniel ST7500 belt achieved 36.7% dynamic splice efficiency which was thought to be good at the time. In service it had a 6.0:1 safety factor (Table 1). Since then, a new generation of belts with new splice materials and splice patterns has been developed and today, splice efficiencies of 50% and over are commonplace.

Fig. 4 shows Wöhler splice fatigue curves for splices with a 50% and a 60% dynamic splice efficiencies. It also plots:
1. The running tension (= 15% of break) for a conveyor running at a 6.67:1 safety factor.
2. The accelerating tension for the same belt assuming an additional 40% belt tension.

Fig. 4: 50% and 60% splice efficiency fatigue curves
Significantly, the difference between the accelerating tension and the splice fatigue curves at 10 000 load cycles illustrates the reserve tension left in the belt to accommodate belt degradation factors and accidental belt damage. Degradation factors are defined as belt aging, misalignment, pulley bending, splice construction errors, etc.. (For example, in a study conducted by Syncrude in Canada on a movable conveyor, when they deliberately misaligned sets of idler frames, they could increase the power requirement (and belt tension) by 14%.).

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