Steel Cord Belt - Cord Diameter Comment
Could you please clarifiy what you mean by "spot over tensioning".
Your comment on the sensitivity to the cord diameter selection is not clear when you refer to localized cord failure problem. It sounds like you are suggesting the belt or splice are subjected to this "spot over tensioning". A refinement of your rational would be heloful.
Larger steel cord diameters affect many design decisions:
1. Better impact resistance against fretting faliure between wires within the cord from large rock impact kinetic energy. An example is the Canadian Oil Sand conveyors, which are subjected to high impact from dropping frozen oil sand lumps onto the feed point from a wheel reclaimer. Also increase impact capacity due to improved transvers tensile stress capacity.
2. Decreasing the number of splice steps with larger cord sizes - one can argue the reduced splice steps improves splice dynamic fatigue efficiency (splice strength) and reliability.
3. Reduces longitudinal belt flexion over idlers which reducers power consumption
4. Increases transverse belt stiffness which will increase belt pressure on idlers that increases rolling resistance and power
5. Decreases belt transvers flexibility which may require a lower trough angle in order to contact center roll and its belt capacity consequences.
6. Improves idler junction (center to wing roll) stress limiting criteria which then allows wider idler spacing
7. Requires larger pulley diameters and this implication
8. Increases cost of belt due to increase in core rubber volume between cords
9. Increases belt weight which increases power consumption
10. Requires thicker belt bottom covers which increases belt cost and power requirement
11. Can increase number of splices due to increased belt thickness
12. Can resist cord breakage with trapped rock between belt and pulley surface
These are not exhaustive, just what comes to mind. ■
Spot Overtensioning
What is ment by this term is abnormalities such as Carry back material conpacting and forming high points on high tension pulleys - Abnormalities in the belt construction like areas of thicker covers in certain areas, and a general operation of the conveyors close to the maximum design limits. Splice failures have also occured in these cases but of late cord failures along the length have been evident with in some cases belts in service have lasted less than 3 years. The general failure mode of the cords is a simetrical internal "poping" of the cords at regular intervels deteriorating to levels of 11/12 cords, the belt opening up and either tearing along the length or tearing towards the edge. I understand that you have listed pros and cons of the suggested cord dia increase if in theory and calc the belt class is sufficiant it seems that in practice it is not always the case as these type of reliability problems keep surfacing in the case of these type of conveyors. ■
Re: End User Cord Specification In New Belting
I speak of conditions which are void of bad engineering , belt construction, installation errors, bad operation, or housekeeping.
We can generate a long list of possibles. I do not wish to canvas all possibles. There are too many.
Are the damage sites, repeatable, localized to one area of the belt, a consequence of poor chute design, of material build-up, of bad vertical curve designs with extreme buckling which leads to fretting failure of the cords (center with convex; edge with concave).
Dr. Harrison published details of poor press-lap construction that leads to local cord failures from idler hammer.
You speak of regular intervals. What do you mean? Can you diagram the details?
Without more evidence, it is difficult to pin-point the reason(s). Diagram a cause and effect tree. Timing history, as well as location, are important. ■
End user cord specification in new belting
An end user is changing from fabric to steel cord belting in some incline conveyors (± ST630/ST800 class C/C of ± 550m) In recent times very similar conveyors have had inherent cord failure problems in localized areas which was possibly caused by spot over tensioning of the cords. To my way of thinking using thinner cords would give you a smoother tension spread across the belt but when spot over tensioning occurs for what ever reason they do not provide sufficient localized strength to override these types of tension anomalies (Making sure that any localized over tensioning does not occur would alleviate the problem, but hey) Would it be of benefit to suggest to the end user to specify a larger cord diameter to aid in overcoming these problems IF??? they occur as belt manuf are now using standard cord diameters these day ■