Ways of improving accuracy

If all of the significant influence factors which bear on the ability of a belt weigher to weigh accurately through a moving conveyor belt are understood, then it becomes possible to engineer a weigh frame which has known and acceptable accuracy.

Belt width, loading, speed and belt tension, and trough angle vary widely from conveyor to conveyor. The place available to situate a belt weigher is often less than ideal, which can have a strong impact on the possibility of accurate weighing. Hence the need to adopt an ‘application engineered’ mentality rather than a merchandising approach to belt weighing.

Responsible belt weigher supply always involves an analysis of each installation. As well, the equipment which is offered will be guaranteed to work in that particular conveyor. Responsible belt weigher supply and application should always include a guarantee of the accuracy which will be achieved ‘in conveyor’.

[The diagram above illustrates the interrelation of belt tension, weigh length and misalignment. Given that some misalignment occurs, we can see that the impact on belt weigher accuracy is devastating. ]

To improve belt weigher performance, the significance of belt tension errors entering the weighing result needs to be reduced. To do this, the weigh frame needs to be longer, idler spacing greater, and the weigh frame deflection less. Chosen locations need to be where belt tension is at a minimum and has the least variability. A proper understanding of the principles alluded to in the [above diagram] becomes a powerful design tool which can be used to confidently predict belt weigher stability and performance in any situation.

With design tools based on the above principles, it is possible to guarantee performance ‘in conveyor’. As a result, instead of using industry ‘conventions’ like “four idler belt weighers are 0.25% accurate” we are able to better understand that “Different weighers are required for different conveyors to achieve the same accuracy”.

Ensuring Suitable Locations

One of the challenges for mining plant designers is to provide suitable places for accurate belt weighers.

Feed Points and Vertical Curves

The best place for a belt weigher is usually near the tail end of a conveyor, just after the feed point. Unfortunately, with the need to keep the plant compact, this is the place where a vertical curve is most likely to be needed. Vertical curves, whether concave or convex are the worst enemy of good belt weighing. The best situation for a belt weigher is on a straight piece of conveyor, with the last of the special weigh quality idler sets on the conveyor being at least 10 idler sets from the start or finish of a curve.

The problem with a concave vertical curve is that the belt often tends to lift off when the belt is empty, making it almost impossible to properly zero the weigher. Also, the belt must pass through the weighing area in a straight line; this is essential to the principle that the belt weigher should be sensitive only to forces perpendicular to the belt.

One way to look at it is to say that the edge tension and the centre tension of the belt should be the same. The belt should be relaxed and should have good contact with the horizontal and the wing rolls of the conveyor. If the belt is not settled on the idlers, experience has shown that large and “seemingly unexplained” systematic errors result.

If it is not possible to avoid a curve on a conveyor which must have an accurate belt weigher, then the belt weigher will have to be located much further up the belt toward the head. Due to the fact that the belt tension is much higher (probably 100% more) than that near the feed point, a much better belt weigher will be required to achieve the same accuracy. It is quite likely that what may have required a dual idler weigh frame may now require a four idler weigh frame to achieve the same accuracy. The fact that the conveyor is more inclined at this point is of no consequence.

Long Conveyors

Long conveyors have never been a good place to locate a belt weigher. Any conveyor over about 300m long is a long conveyor and overland conveyors with lengths of 1000m or more are very long and should be avoided. No doubt it will be possible to find a belt weigher manufacturer who will say that their belt weigher will work on your long conveyor however, the belt weigher supplier will not have to live with the problems.

The two problems of long conveyors are (i)The high belt tensions involved and (ii)The fact that every test result used for calibration will usually require one complete belt revolution, this reduces the quality of maintenance and makes the calibration process very long. To zero the belt properly takes a complete belt revolution and this might be 30 minutes or more.

There are methods which can now be used to reduce the burden of having a long conveyor, these are to (i)use a special weigh frame designed for high belt tension and the other is to (ii)use special features in the belt weigher electronics to reduce the need for all calibration runs to require the circulation of the complete belt. Even when such special equipment is brought into play, it is likely that the unit will still not work as well as one on a much shorter conveyor.

At the Dalrymple Bay Coal Terminal CST have provided belt weighers for loading to ship on L5 conveyor. This belt is 8km in length and each result takes around 25 minutes. On our advice, L6 conveyor, the parallel conveyor installed in the last expansion has a much shorter belt, L6A feeding it. The belt weighers on L6A are greatly superior to those on L5 due to much lower belt tension environment and due to very quick belt revolution time.