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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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Improved understanding of rubber lies at the root of one of the most important advances in conveyor design. Recognizing this improvement, the latest edition of CEMA offers three different horsepower prediction methods.

In April 2014, the Conveyor Equipment Manufacturers Association (CEMA) published the 7th edition of the book Belt Conveyors for Bulk Materials [1] colloquially known as “The Belt Book” or “CEMA-7”. The Belt Book is the de-facto standard of the North American belt conveying industry.

The first edition of this book “CEMA-1” appeared in 1966 [2], and was 300 pages long. To keep pace with advances in conveyor engineering, CEMA expanded the belt book, and today the seventh edition is an 800 page volume.

Improved understanding of rubber lies at the root of one of the most important advances in conveyor design. Recognizing this improvement, the latest edition of CEMA offers three different horsepower prediction methods:

  • The “CEMA Classic Method” which appears virtually unchanged in all editions of The Belt Book.
  • The “Small Sample Method” that first appeared in CEMA-6.
  • A new “Large Sample Method” appearing for the first time in CEMA-7.
     

This paper investigates the history leading to the development of these three methods and provides insight into and justification for adding a new method into the latest edition of The Belt Book.

CEMA’s Classic Horsepower Formula

In the CEMA Classic Method, belt indentation, flexure, and trampling losses are calculated using the following formula:

where:

  • ΔT  =  change of tension in a section of the belt.
  • L  =  length of this section of belt
  • Ky  =  dimensionless constant that is a function of idler pitch and belt tension
  • Kt  = dimensionless constant that is a function of temperature
  • Wb  =  weight per unit length of the belt
  • Wm  = weight per unit length of material carried by the belt
     

This formula differs from the friction factor based formulations in the international standards like DIN-22101 and
ISO-5048 in one important aspect: the CEMA classic method provides designers with charts of friction factor vs belt tension, belt load, temperature, and idler pitch. The other standards simply state that belt friction should be set based on the experience of the designer, but suggest 0.02 be used as a base case [3].

CEMA’s friction factor charts were probably given to CEMA by Hewitt-Robins, Inc (HRI). In the early 1950s, HRI awarded the Department of Mining at Pennsylvania State University (Penn State) a contract to improve the accuracy of their conveyor power prediction methodology.

Between 1954 and 1956 researchers at Penn State tracked power consumption in 14 different conveyors [4] at a wide variety of plants around the East Coast of the U.S.A. The conveyors ranged from small 70 t/h machines to large 1920 t/h systems. In addition to field measurements, the researchers built a 50 ft (approx. 15.2 m) long conveyor in Penn State’s College of Mineral Industries. To determine drag, they suspended the stringers on this conveyor by cables attached to the ceiling and measured the change in cable inclination under different tensions and loads [5].

They also adapted a similar device to measure the friction in conveyors in the field. Ten years before the original publication of CEMA-1, Asman [6] presented a plot of “Carrying Strand Resistance Factor” vs “Carrying Strand Weight” for belt tensions ranging from 1000 to 16 000 lbs (approx. 454 to 7260 kg) that is identical to CEMA’s Ky charts.

The classic CEMA method is a reliable predictor of the power which is consumed by belts constructed from conventional rubbers. It is still widely used in North America today, and its popularity endures because the calculations are easy to understand and implement in spreadsheets.

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