Ask Joe! Archived Article, Selecting and Installing Conveyor Belt Scales, by Sheldon Shepherd

Fifty years ago belt scales were entirely mechanical devices based on a complex weight balancing design. Today’s systems employ load cells to assess material weight, belt speed sensors, and microprocessor-based integrators to continuously compute the rate of material transferred along a conveyor. 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 belt scale system may seem complicated, but it can be simplified by following a few guidelines.

A typical 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.

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 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.

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 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 ½% 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.

Scale Accuracy	Excess Material Cost		Belt Scale System Cost	Payback Period (days)
Scale Accuracy	Daily	Annually	Belt Scale System Cost	Payback Period (days)
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 ¼% 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 ¼% 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.

After deciding on the desired 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:

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 control system.

The scale measures the forces it senses on the conveyor. A survey of belt scale suppliers on the results of service visits revealed that about eight out of every ten service calls were to resolve a problem with the conveyor and its affect on the scale rather than a component failure or problem with the belt scale system. A properly installed ½% belt scale on a well-maintained conveyor will typically produce an accuracy of ½% or better. Some of the areas of concern are:

Belt scale location – the scale should be located a sufficient distance from the infeed section so the material has time to become properly profiled and settled on the belt. This distance will vary depending on the conveyor design, flow rate, and material; however, about 20 to 30 feet is usually acceptable.

In addition, it needs to be adequately isolated from any vibrations caused by material falling on the belt at the infeed. One must also consider conveyor belt tension because it increases closer to the head pulley. Installing a scale in an area of high tension along the belt can significantly decrease the accuracy. It is possible to configure a scale to operate in an area of high tension; however, special care must be given to the installation, particularly the alignment.

Since many conveyors may curve up or down along some point, it is important to locate the scale an appropriate distance away from the tangent points of the curve. For concave curved conveyors, the recommended minimum distance for mounting the scale is 40 feet from the tangent points of the curve. With convex conveyors, the minimum distance is 20 feet on the approach side and 30 feet on the retreat side.

Conveyor support structure – the conveyor should be rigidly supported so there will be no deflection caused by the weight of the material. The structure and components also need to be free of excess vibration.

Gravity or screw take ups– a gravity take up should move freely and place consistent tension on the conveyor belt. The amount of weight should conform to the conveyor design specifications. Ensure that significant water or debris does not collect in the take up weight, thereby causing excessive tension that can create problems for the conveyor and belt scale. A problem definitely exists if the gravity take up does not move at all and therefore maintenance will be required. The screw take up should be adjusted according to the conveyor specifications.

Carrying idlers – many types of carrying idlers are used on conveyors. Several types of carrying idlers are suitable for use with belt scales: in-line troughed idlers, flat roll idlers, and picking idlers. Offset troughed idlers can be used but special attention needs to be directed toward making sure that all scale area idlers near the scale are properly aligned. Suitable idlers are shown in Figure 6. Idlers not appropriate include impact, adjustable transition, spiral catenary, roll catenary, belt training, and wire rope idlers.

The carrying idler used for the belt scale will often require some minor modification to the supports. Follow the manufacturer’s recommendations for modifying the scale idler. For high accuracy applications, scale quality idlers that are manufactured to close tolerances can be supplied.

It is recommended that all of the idlers in proximity to the scale area idlers be of the same make and model. During installation, replace any worn and damaged idlers to ensure reliable measurement by the scale.

Idler alignment – it is critical that a minimum of two idlers on each side of the scale be aligned with the belt scale, ideally to tolerances of 1/32". In some applications, it may be advisable to go to three idlers on both sides of the belt scale. In high accuracy installations of ¼% accuracy or better it is recommended to strive for even better alignment if possible. Consult the manufacturer’s guidelines for proper idler alignment depending on the belt scale purchased.

Shipping bolts/stops – belt scales are often sent from the factory with bolts or stops installed to protect the load cells and structure during shipping. To offer extra protection during installation, the shipping bolts can be removed after securely mounting the scale to the conveyor stringer. Shipping stops will usually be clearly marked and colored. Be careful not to remove any unknown bolts from the belt scale.

Speed sensor – the speed sensor should be attached to the tail pulley or bend pulley so there is no slippage in the connection. It is important that the speed sensor be securely mounted and free of excessive vibration. Follow the manufacturer’s guidelines for installing the type of speed sensor purchased with the belt scale.

Cabling and wiring – good instrumentation wiring practice should be followed to protect the load cell and speed sensor signals from radio frequency interference and induction. Terminal blocks and grounded metal conduit should be used for all wiring.

Integrator – the integrator should be mounted on a vertical surface that is free of vibration and protected from the weather.

Calibration – there are four commonly used methods to calibrate a belt scale: static weights, test chains, material test, and electronic. Refer to the operating manual for exact details describing the method appropriate for your scale, installation, and desired accuracy.

Electronic calibration basically conducts a system test of the integrator, therefore the scale is not compared to a known weight standard. Electronic calibration should only be used if the scale is installed in a location that cannot be readily accessed. Static weights are simple to use and are fine for the average application, or during a first time calibration before material tests. For other applications, one might consider test chains that are unrolled by a mechanism onto the belt to simulate an actual load.

There is disagreement in the industry as to whether test chains offer higher accuracy in calibration and provide any significant benefit over static weights. Test chains have their place but require consistent maintenance for reliability and can be dangerous if they break during calibration.

Material test is the most accurate method because the scale is calibrated under actual operating conditions to known standards. With material tests, a known weight of material is transferred along the conveyor belt and then compared to the total from the scale integrator. The calibration parameters in the integrator are then adjusted to compensate for the difference. It is important to note that all of the known weight of material should pass across the belt scale. Sometimes the test material can become stuck in the bin or infeed section, or fall from the conveyor.

The known weight of material can be obtained by weighing the test load in a bin supported by load cells or by a truck scale. The material can be weighed before or after the test. It is critical that the weighing device used for the test load be accurate and calibrated to a reliable standard.

A discussion on calibration would be incomplete without addressing repeatability and accuracy. When calibrating a belt scale, the first desired result is repeatability. After obtaining repeatable outcomes during calibration, the scale and integrator can usually be adjusted into accuracy.

Proper and consistent maintenance on the conveyor and belt scale is essential for reliable measurement operation. The scale should be routinely calibrated according to the manufacturer’s recommendations with consideration given to the nature of the application and desired accuracy. Generally, the duration between calibration checks can be extended after the scale has proved reliable for a reasonable period of time.

Routine inspections should be made of the scale area idlers, take-up, belt, speed sensor, and scale to make certain they are in proper working order and that material build up is not hindering operation. Belt scales can be susceptible to problems caused by material build up because debris can collect in structural portions of the scale and prevent the material weight from being sensed by the load cells.

Many companies hire the services of an authorized factory technician to provide on-site assistance and training during installation, startup, and commissioning. This is recommended for first time buyers or for those companies that want to ensure an optimum installation.

A belt scale can be successfully installed and set up by appropriate plant personnel if they follow established procedures. These guidelines along with the instructions in a belt scale manual or installation guide should produce satisfactory results. Contact the factory or an authorized representative if you have concerns about whether a conveyor will work in an application or a scale cannot be installed according to these guidelines.

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