Les Dunn discusses some common problems with Overland Conveyors (OLC) with very large idler spacings.
Over the past few years, I have had the opportunity to inspect and make comments on many overland conveyors. I have become concerned of late that some conveyors are pushing the limits of conventional conveyor designs, from my perspective, to exceed the capacity of normally accepted viable conventions. Whilst I applaud thinking ‘Outside the Square’, I believe that a common sense approach has been exceeded and the viability in ongoing maintenance and failures cost values has been badly compromised.
It is true that Overland Conveyors (OLC) with very large idler spacings are simpler and cheaper to set-up and get running but the conveyor has now become so much more susceptible to minor changes in alignment causing huge running errors of tracking, wear and tear, damage, spillage and numerous other faults to be maintained.
As most of these conveyors are ground mounted over longer distances, the ground strata is such that the conveyor ‘moves’ whenever the ground becomes water logged or dries out due to moisture content changes. The conveyors are also never on a level playing field or straight as most have complex vertical curves combined with horizontal curves which can pose their own complex set of compliance rules.
What I have found through thorough investigation leaves me wondering at the ethics of the sales persons who have advocated such systems to the client/s. I have heard that there is legal repercussions under-way with the supplier/s as the conveyor/s have failed to meet the ‘Fit for Purpose’ as per the contract criteria.
I hope this information will be helpful to those considering this type of conveyor system. The system may be bolstered in future design/s to address the issues and make them ‘Fit for Purpose’ but each client will have to do their own homework on the suitability of them. I have identified many issues which by no means are restricted to those issues I have pointed out but could be much more substantial in volume of sub-standard design/s.
Troubleshooting conveyor problems
Do you have erratic tracking of your conveyor belts?
- Does the belt run (track) true as per installation commissioning but wanders off into structure for no apparent reason?
- Does it track differently during high humidity or wet atmospheric conditions?
- Does the belt track off during sunlight hours and differently during cloudy or night time conditions?
- Does the lack of frictional contact by conventional rolls cause the belting to aquaplane opposite the tracking direction of the trained direction of the idler rolls?
- Does the belting rub and cut the conveyor structure due to continual rubbing damage caused by the belting edges contacting one or both sides of the structure?
- Has the rubbing damage the belting and caused of splice ripping & tearing?
- Do the belting edges have torn strips off the edges?
- Have your idler rolls ‘flat spotted’ or worn ‘elongated wear holes’ in the shell tubing?
(Common to see numerous flat areas around the circumference of the shell)
The reasons might be due to some of the following issues.
- If the shell of the idler rolls too smooth as with aluminium, stainless steel 304/309/316/318L, polypropylene, glass fibre or is painted with non-friction coatings, then the friction required to maintain the idler roll rotating action may be insufficient causing ‘flat-spotting’ or ‘elongated holing’ due to higher than normal ‘rim drag’.
- Subsequent belting covers damage when the worn sharp razor edges peel the covers.
- The friction also is necessary to guide control train (track) the belting within the tolerances laid down in the specifications of +/-2.5% of belt width.
dispersion of buildup clinging to the roll but this has to be weighed up against the ability to maintain rotational actions of the roll and the control training/tracking of the belting.
- Steel shell tubing has good frictional qualities but is subject to corrosion ‘oystering’ or straight out ‘rotting’ and usually in the vicinity of the welded in end (head discs). Some manufacturers have opted for galvanized finished shell tubing but this in reality has exacerbated the ‘rotting’ action. The rolls then part off within weeks due to high sulphur or salt atmospheric conditions as found in coal mines, metalliferous mines and Port Authorities. I believe the galvanizing is burnt to powder form and the ‘bi-metallic galvanic action’ combined with moisture and sulphur (Acid) electrolytically erodes the steel.
- Belting training (tracking) in even new installations does not have every idler set and roll in those idler sets, square to the true ‘centreline’ of the conveyor from loading to discharge ends. During commissioning, the idler sets are bumped forward or back on one side to train (track) the belting to run close to true centre. Roll bearings may be tighter on one side roll which also causes a breaking action pulling the belt to that side. So it is that the rolls need not necessarily be rotating true as square to the true centreline which can and does cause erratic belting tracking.
- Rolls supplied also are delivered in a not so true ‘criteria’ of TIR and Balance causing belt flapping and bounce which reduces continuous frictional contact between the shell surface and with the belting covers which then allows the belting to skip and slide over the roll shell which is theoretically rotating at a slightly slower rpm than the actual belt speed should indicate.
- Bearing size just on the limit have a much greater rate of failure as they are set for as close to 100% the specifications but this does not allow for temporary or prolonged overload situations that is commonplace and difficult to judge.
The roll must be fitted with a concentrically true shell tubing that maintains a corrosion at very much reduced corrosion rates over the whole shell and is not distorted by a weldment to the end heads, thus the roll is not prone to ‘oyster or severe corrosion’ that is caused by welding or other climatic conditions.
As a rule of thumb, the roll shell should not be instrumental in adding further or creating belting flap or bounce. The roll then has a much better chance of continuous frictional contact with the belting covers for control and this also reduces the power requirements as the belting no longer has to cope with varying line tensions.
The roll will also then reduce noise as the tubing is in a balanced state with minimal TIR. This also reduces belting failure rates.
“As a rule of thumb, the roll shell should not be instrumental in adding further or creating belting flap or bounce.”
Rolls in production as ‘weigh scale’ rolls without the need for surface machining and balancing. This is a quality that is lacking in the massed produced rolls and needs very high scrutiny to get a long lasting competent asset for many years of life.
Typically the bearing should in my experience be at least the next size up to give some sort of buffer which would increase roll bearing life by more than 20%, as an estimate, when the B10 life is calculated.
The roll will have less build-up, repellant surface of the shell which causes mis-tracking belting.
The roll shell will also help give a large ROI of cost input and a quality roll that caters for high humidity due to the shell tubing surface texture which helps to maintain good frictional contact with the belting cover. This lack of frictional contact by conventional rolls is the main issue which causes the belting to aquaplane opposite the tracking direction of the trained direction of the idler rolls. This is also one of the main causes of the conveyor structure rubbing damage caused by the belting edges contacting one or both sides. Rubbing damage also damages the belting and is a main cause of splice ripping and tearing strips off the belting edges.