How weighing machines work

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Basic Overview

A short description of the way in which items in general can be weighed.

Description

An illustrated description of the different components in a weighing machine and their functions.

Operation

Shows how a typical Easiweigh weighing machine is operated.

Basic Overview

The entire range of Easiweigh weighing machines have the same basic mode of operation and whilst the format, style, size and specification may differ to suit different products, weights and speed requirements basic principles are valid throughout.

The easiest way to understand what the machines do is to compare them with the method of weighing product, by hand, using a set of static scales. Taking a simple example of weighing an ingredient necessary for baking a cake what are the basic requirements?

Some means of weighing - in this case a set of simple kitchen scales will suffice.(We could call this the "weighing system")

Some means of getting the flour from its large packet into the cake mixing bowl - for this purpose a large spoon or scoop will do. (We call this the "product feed")

Someone to carry out the weighing (Lets call this the "control system")

The product you wish to put in the cake - (Lets assume that this is flour)

First you make sure that with no product on the scales (weighpan) the displayed weight is equal to zero - if not adjust it or make a note of the reading and make an allowance for the required weight (target weight).
You then open the large bag of flour (bulk product)
Taking the spoon or scoop you transfer the product from the bulk to the scale pan as quickly as possible (bulk feed) keeping an eye on the scale display.
When the displayed weight nears the target you then slowly trickle flour from the spoon (fine feed) until the desired weight is displayed.
The scale pan is then removed and the contents tipped into the mixing bowl (discharge).

To all intents and purposes this is precisely what the weighing machine does. Product is placed into a holding hopper and is then fed automatically, at a fast speed, into a pan that is attached to a weighing mechanism until the target weight is almost reached at which point the feed rate slows to a trickle. On achieving the required weight feed stops altogether and, after checking that the weight is correct, the weighed product is released into a waiting container.

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Description

Product Feed System

Product Hopper - receives material from bulk supply and acts as a reservoir of product.
Hopper Gate - (Also known as hopper slide) used to regulate depth of product flowing from hopper along the primary (rear) feed tray.
Primary (Rear) Feed Tray - Transports product, under control, from the hopper onto the secondary (front) feed tray.
Vibrator Drive Units - Electro-magnetic vibratory feed units ensure smooth flow of products with accurate control of speed.
Secondary (Front) Feed-Tray - Accepts product from the rear tray and accelerates it away to achieve a smooth constant flow.
Fine Feed Shelf - (Also known as dribble shelf or finger). Fitted in to the front feed tray to divide it into two areas, a large or bulk section and a smaller or fine feed section. The fine feed shelf is normally custom designed to suit the products to be handled.
Level Detector - Normally a photo-electric or other similar device located over the rear section of the front feed tray. The purpose is to control the amount of product on the front tray by switching the rear tray off when product is sensed and on when no product is present. Correct depth of product is ensured by the adjustment of the level detector.
Bulk Gate - A gate that closes off the bulk (large) section of the front feed tray during the period in the feed cycle when only fine feed is required. Can be pneumatic or electrically operated depending on the machine type.
Fine Trap Box - A small catch box located at the outfeed end of the fine feed shelf through which product normally flows. When the required (target) weight is achieved the trap-box lid closes to catch any product that has left the feed system but not reached the weigh-pan thus reducing over-weights.
Outlet Chute - Guides product discharging from the weigh-pan into the desired receptacle (bag, carton, packaging machine etc).
Outlet Chute Extensions - Adapt the outlet chute to suit different sizes of containers being filled.

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Weighing System

Loadcell Weighing Mechanism - Precision device for converting an applied weight into electrical units. All Easiweigh machines are fitted with loadcells that have full overload protection in all direction to minimise possible damage due to mis-use.
Weighpan - Basically an open-top box with lids at the bottom end into which the product is weighed. The lids can only open when correct weight is achieved and on receipt of an external command or signal.

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Control System

All the Easiweigh controls are microprocessor based and are custom designed and manufactured to control all functions of the weighing machine operation. Various systems are available but all are simple to operate, requiring minimal skill or technical know-how. All systems require only a standard, single phase, electrical supply.

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Mode of Operation

*Assuming that the control system has previously been set with relevant details with regard to weights and vibrator speeds. (This only has to be done once as the control systems have a memory function and settings (Product code) can be saved and recalled later.

Relevant product code is selected (see above).
Product to be weighed is placed into the hopper (A).
Hopper gate (B) is opened sufficiently to allow the product to move out freely.
Run button on controls is pressed.
Product moves along rear vibrator tray (C) at its FAST speed.
Level detector (G) controls rear vibrator on/off to maintain correct depth of product in front tray (E).
Product moves along both bulk (large and fine (small)) sections of the front tray (E) at a FAST speed.
Product passes through both the bulk gate (H) and fine trap box (I), which are open, and into the weigh-pan (L). The lids of which are closed.
On reaching about 95% of the required target weight the bulk gate (H) closes and both vibrators (D) change to SLOW speed.
Product now only travels along the fine feed shelf (F) - through the fine trap-box (I) and into the weigh-pan (L) at a SLOW (trickle) rate.
When the target weight is in the weigh-pan (L) the feed stops completely, the fine trap box (I) lid shuts catching any product remaining in-flight.
The weight of product in the pan (L) is checked automatically and providing it has reached the minimum acceptable level can now be discharged.
If manual operation is in progress the user holds the container to be filled under the outlet chute (J) and presses a foot operated switch. The weigh-pan (L) opens releasing the product which travels down the chute (J), through the extension (K) (if fitted) and into the container.

If automatic operation is in progress the same principle applies except the signal to discharge in received from the associated ancillary packing machinery (bagmaker, carton presenter, pouching machine etc).
As soon as the weight is removed the pan lids close and the feed cycle re-starts.

This is only a brief description of events, and many additional operations take place - for example automatic zeroing, automatic tare, automatic in-flight correction, average weight trending etc. but in all cases the same basic rules apply. The real secret of good, fast accurate automatic weighing lies in combining the correct product feed system with a high quality weighing device and in utilising the latest electronic technology to best advantage.

Whatever your requirement whether a single manually operated machine to a complete, fully automatic, weighing and packaging line you can have confidence that through our experience and with our guarantee of quality, reliability, performance and service.

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