ZhuoHe Intrumentation HomePage > Technical article > >Belt scale principle and how to select one

Belt scale principle and how to select one

2009-09-01 16:32   Article Source:simenz   View Times:

50 years ago belt scales were entirely mechanical devices based on a complex weight balancing design. Today’s electronic weighing systems employ load cells to assess material weight, belt speed sensors, belt scale controller and microprocessor-based integrators to continuously compute the rate of material transferred along a conveyor. Electronic belt scales become an important asset to a plant by helping to maximize the use of raw materials, control inventories, and aid in the manufacture of a consistent product. Choosing and successfully installing a electronic belt scale system may seem complicated, but it can be simplified by following a few guidelines.

A typical electronic belt scale system is composed of a weigh bridge structure supported on load cells, electronic integrator, and belt speed sensor as shown in Figure 1. The rate of the material conveyed is computed using the equation Weight x Speed = Rate. Material weight on the belt is measured by load cells, which produce a voltage signal that is sent to the integrator. The integrator also receives input in the form of electronic pulses per revolution from a belt speed sensor connected to a tail or bend pulley. Using these two points of data, the integrator calculates the rate of material transferred along the belt in pounds or tons per hour.

Work Principle of a typical electronic belt scale system

Experience shows that most training classes on belt scale systems devote about 75% of the time to discussing the conveyor. This is because the reliability of a electronic belt scale is directly proportional to the fitness of the application, quality and maintenance of the conveyor. A basic conveyor configuration is shown in Figure 2 to provide a basic guide for the terminology used to describe conveying systems.

 belt scales components

Selecting a electronic belt scale

Designs range from complete high-performance systems to scales that you can buy and assemble on site. Before selecting a supplier, it is important to consider the desired accuracy, dependability, cost, ease of installation and availability of factory service personnel. Higher accuracy requirements typically increase the cost of a belt scale because of the need for precision components, tighter tolerances and a stronger metal structure. Therefore, one might want to perform an economic analysis of accuracy versus the system cost.

Analyzing the cost of your electronic belt scale system in comparison to the cost of the material you want to weigh is relatively simple. There are many potential costs resulting from errors in estimating or weighing the amount of material conveyed in a process. Some of these include inconsistencies in end product quality, inaccurate inventories and excess use of raw material. For this exercise let’s assume that the cost associated with accuracy is solely the excess raw material fed into a process.

The example in Table 1 presents an economic analysis of a typical conveyor moving approximately 80 t/h while operating 16 hours/day for 300 days out of the year. The cost of the material is about $30/ton resulting in a total cost of $38,400 worth of material conveyed daily. Without a belt scale, the designed rate of the conveyor would be substituted for an actual weighing device, although the accuracy achieved might be around 5% at best. According to the analysis, installing a % electronic belt scale in this application would provide raw material cost savings annually of about $0.5 million and pay for itself in a little over 30 days.

Table 1.

Scale
Accuracy

Excess Material Cost

Belt Scale
System Cost

Payback Period (days)

Daily

Annually

5.0%

$ 1,920

$ 576,000

No Scale

NA

1.0%

$ 384

$ 115,200

$ 3,600

9

0.5%

$ 192

$ 57,600

$ 6,500

34

0.25%

$ 96

$ 28,800

$ 9,000

96

An accuracy of ?% is typically required in instances of high material cost, tight process constraints, or where the scale is being used to weigh materials for sale. For example, an increased material cost of $85/ton would reduce the payback period for a ?% belt scale to about 30 days.

Some belt scales can be certified for custody transfer. The procedure involves calibrating and performing material tests on the scale that are traceable to national standards. While certified scales are needed for a few applications, it is an expensive and timely undertaking. Often, the total cost to obtain certification can be 3 to 5 times the cost of the belt scale. Additionally, there are significant periodic costs to update the certification.

Application Data

After deciding on the desired weighing accuracy, we need to gather application data on the conveyor to provide the belt scale supplier the information required to recommend the appropriate equipment. Most suppliers will have an application data form to complete and return. Contact the supplier if any questions arise as they have personnel and representatives which are available to provide assistance in collecting the information. The data form will at a minimum typically require:

  • Material to be weighed
  • Belt capacity (tons/hour)
  • Belt speed (feet/minute)
  • Belt width (inches)
  • Carrying idler spacing
  • Idler/trough angle and diameter
  • Conveyor incline angle

Any additional information provided to the supplier will help to make certain the most appropriate belt scale system is selected for the application. Other useful information includes minimum, normal, and maximum rate, belt length, environment (outside, inside, corrosive, temperature, etc.), and hazardous area classification.

In addition to accuracy and cost, one should evaluate system reliability, ease of installation, and the availability of service personnel. The cost and quality of a belt scale will vary depending on the supplier. The analysis from Table 1 illustrates that minor differences in the system price are not as critical to the bottom line as consistent accuracy and reliability. The design and manufactured quality of the belt scale should be the most important evaluation criteria after deciding on accuracy.

Next, evaluate the different suppliers in terms of what is needed to install a particular belt scale. Contact the factory and request an installation guide describing the equipment being considered for purchase. Furthermore, ask how many service personnel they have available, the cost, and the typical lead time required before you can have a factory authorized technician at your site.

It is also important to consider the features and functions of the recommended electronic integrator package. Options include rate and total output, alarm relays, and communications capability with your plant’s weigh control system.




Respective topics: Significance of appropriate application o   Belt Scale(Belt Weigher)  
  • Previous:The Evolution Of Weigh Scales
  • Next:A Portable concrete mix batching plant
  •