The CEMA Horsepower Equation

Belt Conveyor Design

The CEMA Horsepower Equation

Development of a new Conveyor Power Prediction Methodology
The 7th edition of Belt Conveyors for Bulk Materials (known as “The Belt Book”) includes a new con­veyor power prediction methodology using “Large Sample Indentation Test” (LSIT) data. This article provides background and insight into how LSIT data is used to design conveyors, and describes the relation between this and the older conveyor power prediction methods. It also illustrates the use of LSIT data by using it to predict indentation losses in a recently commissioned conveyor system.
(ed. WoMaMarcel - 07/10/2015)
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Fig. 7 shows a plot of the 20°C LRR data from CEMA-7 fitted with the formula Eq. 2.


Fig. 7: Plot of LSIT data included in CEMA7.

According to CEMA-7, for an LRR belt at 20°C, ‘a’ = 6.59 · 10-3 and ‘b’ = 1.28. Since the CEMA test data was measured on a belt with a 7 mm bottom cover and an idler roll diameter of 219 mm, we must scale the CEMA results to model our conveyor. To estimate how much this friction would change if we retested the sample using a different idler diameter and/or belt thickness we multiply constant ‘a’ by the following equation [13]:

where:

  • Htest  =  belt bottom cover thickness used to produce the LSIT test
  • Dtest  =  diameter of the idler roll used to produce the LSIT data
     

Finally, we substitute Eqs. (4), (5), and (6) into Eq. (2) and integrate the result over the width of the belt as follows:

Thus, using this methodology, the total indentation loss on an idler set is:

Transforming Large Sample Test Data for Use in the Classic CEMA Formulation

For more than half a century conveyor designers used simple friction factor based formulas like Eq. (1) to design conveyors. Experienced conveyor engineers have a “feel” for what friction factors are reasonable on various types of systems, and often the first question auditors ask conveyor engineers is, “what friction factor did you use?”

To get a friction factor, we could simply divide Eq. (3) by the total load on the idler. However, by reconditioning LSIT results in terms of friction factors we can use them in formulas that conveyor engineers are familiar with. This was one of the primary goals of the committee charged with writing the LSIT section in CEMA-7.

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