There are certain factors that you have to understand to find a solution for belt weighing.

1) What is the capacity of the belt?

2) The belt load is (belt capacity/3600)/belt speed.

The loadcell capacity will be the belt load + the idler weight + the belt weight per meter.

This ratio of belt load to dead load will determine the accuracy of the system. ■

Hello,

There are two basic variables that are needed. First is a representation of load. Usually described in lbs. per foot or Kg per meter. The other variable is belt travel in feet or meters per second. By combining both signals you can get lbs. per second or alternately kg per second. If you accumulate this combined signal based on distance travelled you can get a total weight. There are alot of influences on get both of these signals correct, and if you have any specific questions, post away! There are a number of experienced people that read this forum. ■

5 tonnes per hour on a 1m belt sounds like the belt will always outweigh the tea. You need some specialist help in calibrating that one because the dynamic disturbances from belt passage over the idlers and uplift from cross wind can cause big percentage errors with that kind of set up. ■

Your Tea Feeder will have light loading. In other words, the change in weight from an empty belt to a loaded belt will need to be at least 10% of the load cell size. Your best bet is to have a conveyor belt speed that is very slow. Here is how the math goes.

5000 kg /hr passing over an idler that is spaced 500 cm apart from the adjacent idlers with a belt speed of 3 meters per minute.

5000 kg/hr = 83.33 kg / min

83.33 kg /min travelling a 3 meter per min = 27.77 kg / meter

27.77 kg / meter applying weight to an idler spaced at 500 cm

would give a weight of 13.89 kg on the idler

a 100 kg load cell would see a change of signal of about 14% at maximum capacity. Slowing the belt down increases the weight proportionally. Increasing the spacing between the idlers also increases the loading seen by the weigh idler.

( I did the math in feet and pounds and then coverted back to metric. As an American farmboy, I can not grasp the metric system unless forced by others.) ■

If this is for tea, the belt has to be synthetic and the idlers have to be of low weight i.e polymer construction.

The weighing arrangement is also crucial as the weight of the idler and belt will have to be nullified. The distance between idlers will have to be as high as possible.

We have supplied a lot of conveyor weighing systems for tea.

You can contact us at sales@ipaindia.com ■

Hello Mr Kurian

I agree with Mr. Dietrich. We have often seen clients using this arrangement to perform weighing. We do recommend using in integrator and speed detector before the PLC for ease of use and ease of calibration and accuracy.

Accurate measurement of speed is essential. If the speed is off by 1%, totalization is off by 1%, assuming you are totalizing.

Compensating for deadload and averaging the deadload of the belt over a complete revolution is also essential. This is called tare or zero.

It is essential that the supported idler is absolutely level with at least two idlers before and after. This is why many clients use a weighframe.

The distance between the idlers should not be so great as to have visible sag in the belt. Absolutely sag should be no more than 4% (of the idler spacing) assuming reasonably correct tension adjustments.

Regards

Mike Tallevi

Siemens ■

Hello,

A friendly suggestion weighfeeder design and construction is a specialised subject. The basics are simple - the implementation not so . One needs to consider appropriate infeed device based on material properties, extraction forces, speed reduction, drive selection , speed pickup and weight pick up arrangements, volumetric capacity needed, belt width selection, belt tensioning and tracking, quality of joint , proper algorithm for speed and belt load derivation, normalising, multiplication, scaling to get the actual feed rate, setpoint - actual comparison , a PI algorithm to generate correction signal to variable speed drive , fault alarms ----- the works! Plus the design details of proper mechanics .

If you are an user - a better bet would be to get an experienced supplier.

If you are a budding manufacturer - rather late in the day . The field is as it is crowded.

Also - tea gardens are very poor in advanced maintenance staffing - and very very cost tight .

Feel free to email me if you want more detailed info . ■

Thayer Scale manufactures weigh belts specifically designed for low loading, low density applications such as yours.

These designs consist of a unique suspension system inter-posed between a mass counter-balanced, multi-idler weigh bridge and a single load cell.

This system permits load cell selection based on net, rather than gross weighing requirements. The suspension system also blocks all extraneous horizontal force vectors (belt tension, friction, torsional forces, etc.) from being misinterpretted as changes in material weight.

These products are used globally to accurately blend tobacco leaf, measure the flow of snack chips and control the flow of natural and synthetic fibers with bulk densities as low as 4 kg/cubic meter.

Visit

href="http://www.thayerscale.com" target="blank">www.thayerscale.com for additional information.

Attachments

low density model m (PDF)

■

Hi Michael,

Speed measurement is an absolute measurement as speed can be measured accurately. The speed can be measured using a pulse tacho/encoder/proximity. To get a better resolution a higher PPR sensor could be used.

The tare weight has to be as low as possible. Also the tare weight can be counter balanced to make the load on the loadcell zero without material.

The type of belt is also critical.

As Mr.Mandal suggests it would be better to go to a vendor who has experience in this area. ■

Dear Sir,

A weigher feeder is a small belt conveyor with the following itemes.

1. Endless belt with flat idlers.

2. A load-cell for load sensing& a Tacho for measuring speed.

3. Electronics to conver the signal into digital figure.

4. It can take the share load.

Belt width & speed you can calculate by very standar relation . Then electronic part you can take help form the supplier.

Before that you must read the book by H. Colinj ( Weighing & Proportioning)

Regards.

A.Banerjee ■

Hello,

Your requirement is 5000kg/hr.

Material to be weighed is tea leaf then it is two posibilities: Wet and Dry.

the density factors of wet and dry is different. e.g. 800Kg/m3 & 500Kg/m3.

Up to this step you can imagine the flow of material.

Now you select the belt speed. Normally, it is 1m/s maximum. Then you can calculate the belt width

The Loadcell selection base on total tare weigh (Idlers+belt) + material

For electronic devices it is better you should refer to some professional supplier as they post ad.

good luck ■

Dear Mr.Arunkish,

This is Sundar from Newton Weighing System Pvt Ltd, Kolkata, India. We have extensive experience in Belt Weighing and to good extent in weigh feeder applications. You may contact us for any of your requirements.

As far as the Belt weighing is concerned it consists of Load cells for measuring the mass and Tacho generator for measuring the speed. An integrator integrates both these parameters and displays the Tph reading. It also calculates the amount of materials passed and displays as total reading.

This Integrator gives Rs232, Rs485, 4-20mA, 0-20mA outputs which can be connected to PLC, Printer, chart recorder etc.

Sure I hv given the basics to you and this will be helpful

Regards

V.Sundar ■