Belt Misalignment


The training of a new belt is most important and a study of the following points is well worthwhile.

Unsatisfactory tracking or running off centre of belts is due to the following causes :

  1. Pulleys may not be mounted level and square to centre line of conveyor. the belt will run off the slack side.

  2. Idlers may not be properly lined up

  3. The belt may not trough sufficiently to give proper contact with the horizontal centre idler of trough idlers.

  4. The belt may not be straight.

  5. The belt may not have been joined square and true.

  6. The belt may not be loaded centrally. An off centre load will make the empty side run off. The only remedy is re-design of the loading chute.

  7. The belt may be exposed to wind.

  8. The belt may drag against projecting stationary parts.

  9. The tripper may be askew because of clearance between rails and wheel flange.

When fitting a belt, special care should be taken to make certain that the following requirements are met :

  1. All pulleys must be set up with shafts parallel and square to centre line of conveyor.

  2. All idlers must be lined up exactly and square to centre line of conveyor. The belt must be joined centre and square

In starting up the conveyor, the belt may track more or less off centre, even if pulleys and idlers have been lined reasonably well. Such unsatisfactory tracking of the empty belt is usually due to a combination of causes mentioned under 1, 2, 3, 4 and 5. Assuming that pulleys and idlers have been lined up as well as practicable and that the belt has been joined properly, bad tracking will have to be corrected by slewing of the idlers so far that the belt will be centred.

It should be appreciated that a belt cannot usually be trained by hit and miss methods; methodical procedure and patience are absolutely essential and gradual correction only will have to be done over a period of several days or even a week or two. Over adjustment is also certain to occur if hasty corrections corrections are made, with the result that trouble free running cannot be obtained.

The training or centering of a belt requires that the belt be caused to move towards the centre from the left or right hand side. To cause the belt to do so, the belt should be regarded as if it were a packing case or stick of timber to be moved on rollers. If such a case is to be made to travel towards the right hand, the left end of the roller would be advanced or the right end be shifted back.

In exactly the same manner, if the belt is running to the left of the centre line (looking in the direction of travel) and requires to be shifted towards the right hand, the idlers in question require to be slewed out of centre so that the left side is moved forward and the right side is brought back

to find out where such correction is to be made, it will be found most convenient to look along the edge of the traveling belt in the direction of travel when it is easy to observe the curving along the belt edge due to the belt traveling off centre. Having ascertained where such curve exists, the adjusting at such a place can be made as follows.

Belt Travel

Referring to figure 3 the observer at point A will sight in direction of belt travel along belt edge and observe a definite curving out between idlers marked 2 and 11. Such curve may only extend over 3 or 4 idler spaces, or over a much greater length of conveyor. It is desirable to mark with chalk, idlers 2 and 11, the position of the belt edge, in order to have a reference mark to adjudge the effect of the adjustment which will be made.

Now it is important to note that if the belt starts running off at idler 2, correction of idler setting will always be needed 1 or 2 idlers ahead of this point. In this case, idler 1 or even idler at point B, requires slewing.

To bring the belt towards the centre, i.e. from the left to the right side (as shown by straight arrow), the idler at B should be slewed as shown by the curved arrow, the left end should be moved forward and the right end should be brought back. The amount of movement of the idler is small, and must be determined from the effect obtained. To start with, neither end of the idler is moved forward or back more than about 16mm.

Having adjusted this first idler B, the effect should be noted on idlers 1 and 2.

If the effect is insufficient or cannot be noted, idler 1 is adjusted in the same way, followed by idlers 2, 3, 4 up to 10. If after having done so, the belt has not shifted to the right out of centre at 11, a second adjustment is made starting again at idler B. If the second adjustment proves again insufficient, a third and additional adjustment is made.

It should be noted that the adjusting should never be carried so far that the belt is sent to the opposite side at idler 11, where the belt is already centred.

Furthermore, after each adjustment, it is desirable to observe the effect for a sufficiently long period to permit the belt to travel at least once right around the conveyor before the next adjustment is made. This is necessary as few belts are perfectly straight. Due to this, there is always a certain amount of 'weaving' of the belt at each idler and the position of the belt at any one idler must be judged as the average position of this weaving edge.
To train a belt a start should always be made on the return belt beginning at the head or driving pulley, following the direction of the belt travel. Having centred the return belt, the top belt is trained, starting at the tail pulley and proceeding in direction of belt travel towards head or driving pulley.

If it should happen that the top belt runs so much out of centre that it cannot be run for any length of time, it will be necessary to roughly train the top belt before training the return belt. In such a case the top belt should only be trained so far that it can be run without risk of damage to belt edges after which the return belt is centred, followed by training of the top belt as previously described.

Having trained the empty belt, the belt should also run satisfactorily when it is loaded. If that is not so, it is due to the material not being fed centrally on to the belt. In such a case, the correct thing to do is to modify the feed chute so that the feed on to the belt is well centred. This may be done by changing the shape of the chute or by introducing deflecting plates or bars. If, for any reason, the feed cannot be centred, it may be necessary to train the belt in such a way that the belt will run somewhat off centre while empty and will centre itself when loaded.

A belt may not be straight due to bad joining. If such a belt is run, observation will show that at any given location on the conveyor, the belt will pass centrally for some time and then start to run off centre during the period while the curved portion of the belt passes, after which the belt will be again located centrally.

Looking along the conveyor, it will also be observed that, due to this curved portion of the belt traveling along, it gives the impression as if the belt starts running off centre and returns again periodically, as the curved portion comes along on the next round. Such off centre running due to the belt not being straight cannot be corrected by slewing of idlers. If serious at all, so that correction is essential the belt joint will have to be re-made, if the trouble is due to bad joining.

When starting up a conveyor, it is very important to determine right at the start whether or not any running off centre is due to incorrect alignment of idlers or due to the belt itself not being straight.

In the latter case, re-alignment of the idlers will not assist and it may be necessary to rely upon guide idlers to keep the belt in place until the belt can be straightened by re-joining or splicing.

In practically all cases, the belt should be trained by re-aligning idlers and snub pulleys only. Head or driving pulleys and tail pulleys should never be set out square.

Only as a temporary make-shift, it is sometimes convenient to use the tail pulley for temporary adjustment of the belt if the belt is running off so much when first starting up that running is impracticable. But as soon as the return belt has been sufficiently centred the tail pulley should be again set up square before final training is completed.

If it is found that a belt is running off centre due to the action of the wind, the only correct thing to do is provide suitable wind protection.

If the belt is dragged out of centre by any stationary parts of the structure, such parts must be removed.