What Is The Safety Factor of The Conveyor Belt and How to ...

When we choose or purchase conveyor belts, we will encounter a technical index called &#;safety factor&#; (SF), so what is the safety factor and how should we choose the safety factor?

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&#;Safety factors&#; were first stated in DIN , , it is the figures given by conveyor belt manufacturers for their particular belt styles.  The safety factor is the ratio of a material&#;s breaking strength to the stress applied or maximum load permissible. It is commonly called &#;Tension&#;. This figure is connected to the belt&#;s fabric and therefore the overall strength of the belt. It doesn&#;t matter if the belt is covered with EPDM, Nitrile, or SBR. The safety factor is independent of these. In other words, the safety factor is an important parameter to ensure that the conveyor belt does not break up during operation.

The safety factor is directly related to our market: from the point of view of safety, reliability and long life, it should be larger, and from the economic point of view, it should be smaller.

The safety factor of the previous conveyor belt is relatively conservative&#;for fabric belts, belt safety factor is often a nominal 10:1,and steel cord safety factors were a nominal 6.7:1, for example, a manufacturer wants to build a 220 pounds per inch, 2 ply conveyor belt. He desires to set a 10:1 safety factor standard for his product. This means his finished belt breaks on the &#;snatch and grab&#; machine at a minimum force of pound per inch, the result has led to a substantial increase in user investment.

However, more and more studies have shown that the tensile strength of the conveyor belt has nothing to do with the actual safety factor of the conveyor belt design, but is related to the ability of the conveyor belt to withstand the load and the instantaneous peak load under the stable operation state of the conveyor belt and the splicing fatigue strength, that means it is the multiplier applied to the calculated maximum force to which a conveyor belt splice (as the weakest link in a conveyor belt) will be subjected. A factor of safety accounts for imperfections in materials, flaws in assembly, material degradation, and other uncertainties .

Generally accepted safety factor:

Conventional safety factors usually range from 6,7 to 10 for steady operating conditions, related to the splice strength.

For example, a steel cord conveyor belt from &#;SUNGDA&#; has a tensile strength of N/mm, after testing the splicing fatigue strength of the conveyor belt, the splicing efficiency tested by the fatigue machine reached 38%, therefore, the splicing fatigue strength was N/mm (bearing rate=0.38*N/mm). Another steel cord conveyor belt from &#;SUNGDA&#;, its tensile strength is N/mm, but the splicing technology and strength of the conveyor belt are improved. After the conveyor belt has been tested, its splicing withstand level is 50%, and its net withstand level is N/mm, which is 16% higher than the splicing fatigue strength of the first conveyor belt. Although its tensile strength is 14% lower than the former, the design safety factor of the first conveyor belt is 6.5.

Due to strong splicing strength, the safety factor of the second conveyor belt can be reduced to 4.7, and the corresponding withstand tension is increased by 39%. Conveyor belt and splicing construction materials and modern analysis methods show that it is safe and reliable to use a safety factor lower than 5.0 between the tensile strength of ST-N/mm and ST-N/mm.

Conveyor belts, especially high-strength conveyor belts, account for as much as 30-50% of the investment in belt conveyors. To choose conveyor belts reasonably and reduce the investment of conveyor belts, an important way is to improve the conveyor belt joint technology in order to reduce the safety factor. For example, a conveyor with a length of 20km in Australia reduces the safety factor of the wire rope conveyor belt from the usual 6.7 to 5.0, the weight of the conveyor belt is reduced by 14%, the life of the bearing roller is increased by 7%, and the life of the return roller is increased by 40%. . The reduction of conveyor belt tension reduces the corresponding structural investment, power consumption is reduced by 4%, and investment and operating expenses can be reduced by about 10% within 20 years of service.

The German Industrial Standard DIN- defines the safety factor of the steel cord conveyor belt as the ratio of the tensile strength of the steel cord conveyor belt to the conveyor belt tension when the conveyor is running stably, with a minimum of 6.7. In , Germany revised the original standard, and the new version adopted a new concept and calculation method. The conclusion is that the safety factor of steel cord conveyor belt can be less than 6.7, and the lowest can be 4.5.

In the past ten years, some countries have already broken through the regulations. The belt conveyors in operation in Germany, Britain, New England, Australia and other countries have adopted 4.5-5.5. According to the research results of foreign institutions such as the United States CDI Company, the safety factor of the steel cord conveyor belt can be reduced to below 5.5, which can be 4.5 to 5.0.

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Tags: conveyor belt safety factor

1. Introduction

Belt conveyors are an indispensable means of transport in a number of industry branches&#;most importantly, in the extraction industry, in smelting and coking plants, and in power plants, but also in the chemical industry, in civil engineering and in agriculture. Belt conveyors are also vital in shipping ports, where they are used to transport, load and unload bulk materials. In the extraction industry, conveyor belt transportation systems are used both in surface and in underground mining. The belt is the most expensive part of the belt conveyor. Its purchase cost represents 50&#;60% of the cost of the entire conveyor. The belt is also the least durable element of the conveyor. The belt is, thus, an element crucial for the effective and reliable operation of the conveyor and significantly influences the transportation costs [ 1 ].

The selection of a conveyor belt capable of performing a particular transportation task when installed on a belt conveyor is primarily informed by the tensile strength of this belt. It is intended to ensure that the forces in the operating belt will not lead to it breaking, i.e., to an event that is dangerous to both the personnel and to the conveyor. Typically, in the belt conveyor design process, the values of safety factors are empirically identified [ 2 3 ].

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The belt is installed on the conveyor in a closed loop. The type of the splices used in the belt depends mainly on the design of the belt core. In most cases, splices have the lowest strength in the entire belt. Nevertheless, occasionally, breaks develop not in the area of the splice, but in the so-called &#;continuous&#; belt section. Reasons for such events are an object of investigation. This article analyzes two aspects, which deserve attention when selecting an appropriate belt to match the conveyor&#;the non-uniformity of belt loads in the transition section of the conveyor, where the belt changes its shape from troughed into flat to enter the pulley, and the influence of the specimen width on the belt strength. Aspects related to belt damage during its operation (punctures, cuts, etc.) are here omitted, although obviously, they are important and monitored by conveyor operators [ 4 7 ]. The analysis here presented is supported by many years of experience gained as a result of testing conveyor belts at the Belt Conveying Laboratory, Wroclaw University of Science and Technology (WUST).

When implementing a particular transportation task, the force levels in the belt and the power of the drive system are identified after calculating conveyor resistances to motion and the minimal tensile force required to ensure frictive engagement between the pulley and the belt. Köken et al. reviewed and compared leading methods used in belt conveyor calculations [ 8 ].

In the transition section of the conveyor, the outermost load-carrying elements of the core accept additional tensile loads, while the loads in the central part of the belt decrease. In the existing calculation methods, the length of the transition section should be defined in such a manner that the non-uniformity of belt loads is reduced and unit forces in the belt are not exceeded. That CEMA method [ 9 ] defines the minimum length of the transition section depending on the geometrical parameters of the section and differentiates between only two groups of belts: with steel cord and with textile core. Importantly, however, textile core belts may significantly differ with respect to their elastic properties and show different reactions in response to geometry changes of the transition section. The Fenner&#;Dunlop method has a similar approach to the identification of the transition section [ 10 ]. In the DIN method [ 11 ], the length of the transition section is calculated on the basis of simplified relationships, but with allowance for the longitudinal elastic modulus of the belt.

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Accurate identification of the stress state in the belt along the transition section requires a model that allows for not only the longitudinal elastic modulus of the belt, but also for the interactions between the adjacent cables or straps in the belt. Oehmen [ 12 ] and Hager and Tappeiner [ 13 ] focused in their research on identifying the strain state in the belt along the transition section. Schmandra [ 14 ] presented a general theoretical model of a steel cord belt, allowing for the interactions between the adjacent cables. The literature also mentions implementations of the finite element method in the modeling of a belt along the transition section of the conveyor [ 15 18 ]. The analysis in [ 17 ] focused on the influence of the elastic modulus in the longitudinal and transverse directions of the belt-on-belt load non-uniformity.

The research here presented involves developing a universal theoretical model of the belt along a transition section of a conveyor in which, in the case of steel-cord belts, the belt is composed of cords and layers of rubber, and in the case of a textile belt, of narrow strips. An analysis was performed into how the non-uniformity of the belt load along the transition section of the conveyor will change if belts with different cores are used. The research also involved tests of the influence of the specimen width on the belt tensile strength. The literature does not mention any results of similar previous studies.

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