Re: Long Belt Conveyors
Dear Kayem,
Although it is true that belts can exhibit a shape retention ?memory? and tend to run on two or three points on the return run (belt edges for relatively narrow belts and belt edges plus middle contact point for wider belts), not all belts react this way. The belt?s weft modulus plays a big part in this. Generally the stiffer the belt is, which is normally associated with higher tension ratings, the greater the chance that it will take a set in the shape of its carry run and cause problems such as erratic training on the return run (at least initially), have rapid edge wear (or delamination, or edge cap break-off), and uneven return idler wear.
With regard to the concept of using a belt turnover to improve return run training of a ?set? belt, this does not guarantee success. You could be trading a three-point contact belt arrangement for a two-point contact one by flipping the belt. The belts that take a set tend to run in a ?W? shape on straight return idlers, with the belt contact points at the trough junction points, as the weight of the sides tends to cause the middle to bow up. The primary reasons for using turnovers are to minimize negative carryover effects, and to prevent extra top cover wear. If return run belt tracking is a major concern, products like Flexco?s Persuader idlers offer a better solution.
However, turnovers should be used wherever space allows ? both lateral and vertical. Typically, a turnover requires at least 10 meters of length. Because of this, a minimum conveyor center-to-center length for using turnovers is approximately 300 meters. Even this is a borderline recommendation due to the cost and belt flexing factors. I believe that a reasonable conveyor length to start using turnovers is 1000 meters.
(NOTE: I have observed this memory problem with short systems, but a turnover is not an option due to the short lengths involved. With overland systems, the belt wander problem resolves itself after the belt relaxes - but the other reasons for using turnovers still exist.)
Regards, ■
Re: Long Belt Conveyors
Kayem,
It has been my experience that the problem of belt set that you refer to often occurs with multi-ply textile reinforced belts that have a relatively stiff carcass. It can also occur with steel cord belts, particularly where the top cover is significantly thicker than the bottom cover. When a belt of this construction is inverted, the steel cords will be above the neutral axis of the belt and the rubber between the cords will be in compression. This compression of the rubber between the cords makes the inverted belt much stiffer.
Personnally, I would avoid the use of belt turnovers at all cost. The turnovers will require a high belt tension on the return strand to prevent the belt from sagging when it is vertical and the helical shape of the belt edge in the turnover will result in high edge tension and low centre tension. If the tension in the centre of the belt falls too low, longitudinal cracking of the belting may also occur.
Finally, there have been many long conveyors built without turnovers that operate reliably and without serious belt tracking problems.
I trust the above will be of some assistance.
Regards.
David Beckley
Conveyor Design Consultants of WA
Perth, Western Australia ■
Long Belt Profile Set
Kayem
This problem can be solved by fitting a pled belt with equal thickness covers, and installing only one turning station onto the conveyor. My company has patented this configuration which has been used on conveyors carrying hot materials.
The principal of turning the belt over each revolution ensures that the belt is flexed in reverse each revolution, and does not suffer a permanent lateral curve set.
We call this arrangement 'Mobi-Ace Turning Station', after Mobius, the German mathematician who first defined the loop with an endless surface, more than 150 years ago.
The engineered solution requires efficient belt cleaning, careful control of alignment through the turning station, and a particular joint design capable of running 'backwards' for half of the time.
All of these reqirements have been successfully achieved in the projects we have completed. If you would like to know more detail, contact me by e-mail.
Rob Drohan
Chief Executive
Ace Conveyor Equipment Ltd, UK ■
Long Belt Conveyors
A very well known technocrat in this field, recently remarked that all conveyor belts have a memory of their own and in case of long conveyors, this memory manifests by way of the belt trying to maintain its troughed profile even in the return run since it was maintaining such a troughed profile for a long span of travel.
As a result, he suggested, such belts would tend to wander and sway on its return run due to inadequate contact with the return idlers.
He suggested that such long conveyors should necessarily have belt turn-over devices at the terminal ends to circumvent this problem.
Is this hypothesis correct ? Beyond what length should one consider turn-overs ? Can I have some inputs on this please ?
Kayem ■