Thursday 16th Jul, 2020

Problems with belts in operation

As an engineer tasked with solving problems, Steve Davis, Senior Bulk Handling Expert at Advisan, follows up on his last article to discuss issues affecting troughing belts.

Many issues with conveyors are the result of cumulative impacts.

As an example: A splice starts to fail. Cleaners are lifted to avoid damaging the splice because there is no time to fix it. Carry back creates significant spillage under the conveyor with build-up on idlers and perhaps take up and some pullies. Build-up and spillage cause mistracking. The belt cuts through structure and damages the edge, dropping more spillage. The belt may run from under the skirts, which become damaged and score the belt. Higher local tension from the take up spillage overstress the carcass and cords pull through covers and splices start to pull apart. The pulley lagging is damaged. The belt eventually must be replaced but other damage and spillage result.

This sequence may take days, or longer. Cost and time to repair is significantly longer than a splice repair, and consequences may be ongoing if all issues are not addressed. There are many chains of events where rectification of the initial problem is far more cost effective.

Damage or upset

When a conveyor has been operating consistently for years and there is a change in belt performance linked to a new belt, many don’t look deeply at the history for a root cause. A new belt does not change loading or temperature, condition and alignment of structure, idlers and pullies, or cleaners and skirts, unless these were changed or damaged too. Large reduction in carcass life, early splice failure and poor tracking compared to the previous belt is often the result of the belt change. Check the history, especially if new to the problem.

Typical issues can include:

  • Vague specifications or low stock results in a belt different to the original. I have seen cover materials change, edge (not centre) split belts, split belts where chemicals are present, lower carcass rating, mismatched carcass and width in repairs. Ensure the correct belt is procured.
  • Splice kits are out of date or inappropriate for new cover rubber.
  • Supplier does not or was not asked to provide an appropriate splice design, so the previous is used.
  • Supplier minimum bend is ignored. Supplier does not see the profile and data so cannot ensure accuracy.

A conveyor, which had been previously upgraded with a new belt specification had a replacement belt of the original type supplied as the spec was not changed in the system.

Results of incorrect supply may be immediately noticeable, such as tracking and splice failure. Other problems may take some time to be obvious, such as wear rate, impact/gouge resistance or chemical attack.

Sudden change to the operating condition of a conveyor are often the result of change in recent history. Conversely, when a conveyor component is changed or repaired, it is likely a subsequent change in belt performance results from change, and not the belt itself.

The quick fix

The mining industry likes a ‘quick fix’. We seek a cheap, short outage solution to every problem. Use the root cause to assess validity, which in some cases may justify a quick fix.

A poorly tracking belt rarely responds to installation of a single tracking frame. I saw one conveyor with six different tracking frames from six analyses, all installed incorrectly and all failed due to being buried in the spillage that caused the tracking issue. The forces that cause mistracking is introduced to the belt, so understand and remove it.

I have seen repeat failures where the solution was to change the failed item in regular maintenance when a small engineering change would cure the problem. Time and money are unavailable for a permanent fix, but unplanned failure or multiple fixes are ok.

The cost of a permanent fix may be higher than a quick fix. Multiple outages and fixes soon exceed the permanent fix and more maintenance increases the risk of injury. Increased cover thickness or ultra-wear-resistant covers, at higher cost, may last longer but addressing the load point issues will reduce wear continuously.

The blind eye

When the cause of damage to a conveyor belt is obvious, and not a result of the belt itself, how does replacing the belt without addressing the cause and expecting a longer life make sense?

  • If the wear on a belt is two parallel lines that match the location of the skirts, it is probably the skirt and load arrangement that is faulty. If lines are elsewhere look for damaged cleaners or jammed rocks and liner plates.
  • If the cover wear is uneven across the belt, then it is likely that the load chute does not present the material at best velocity and direction.
  • If belt edge is torn, cracking or showing cord stress, then most likely causes are overstress from poor transitions, curves and other changes, tension increase, build-up on pulleys and idlers, too small diameter pulleys or contact with structure or frames.
  • If splices fail, determine the cause.

Belt tracking

A belt on a conveyor tracks well when all the lateral forces on the belt are in balance. Experience shows that short belts, feeder belts, and higher-tension belts are less able to accommodate changes in the forces, yet an overland with little elevation might be obviously misaligned, have spillage and the belt tracks acceptably.

Often a belt tracks just off centre, doesn’t move much, has a minor cyclic movement, or tracks a little differently with a different load. If there are no other problems, it is best to monitor rather than trying to correct. Belts can work for years yet never track perfectly.

When a belt does not track centrally on the conveyor, there are several potential causes:

  • Belt or its splicing may be faulty. It is unlikely to be fixed without addressing the belt, Often, no amount of tracking methods or devices will solve the issue. This fault is usually obvious immediately after splicing. Sometimes a ‘bent’ belt can straighten after a period of operation, but not misaligned splices.
  • Off centre loading, variable loading, skirts that are set too wide, differences in skirts along and across the conveyor are common mistracking causes. Rectification may be relatively simple, such as a liner system upgrade, or more complex such as a chute replacement.
  • Poor alignment of the conveyor itself is a major cause. This can be initial installation, change with time, the result of a maintenance event, ground subsidence, or damage to the structure from impact. Poor alignment can be fixed by realigning or repairing so that all pullies and idlers are on the conveyor centreline and according to design inclinations. Tracking frames may correct some of these issues temporarily. 
  • Wind can cause erratic tracking problems, especially if intermittent and the mistracking is short duration. Occasional tracking severe enough to dump all material on the floor yet fully regain normality should consider weather at the time. Wind guards are a solution.
  • Cover wear and carcass damage or cord failure changes belt stress distribution and force balance that produces mistracking.
  • Mechanical issues can cause mistracking and damage to belts. These include seized idler rolls, damaged lagging, jammed tracking frames, change in position of shuttles and trippers, collapsed tripper wheels, take up trolley movement and damage, seized take up sheaves, cleaner and plough damage and many other issues.
  • Spillage causes many issues on conveyors. When it buries or builds up on idler rolls or pulleys, and on structures and walkways this changes the forces acting on the belt and causes mistracking. Build-up on idler rolls and pulleys makes the belt track towards the larger effective diameter. Burying idler rolls in spillage increases drag on rotation requiring more force from the belt to turn them. Eventual seizure occurs, which places more drag on the belt. Build up on pulleys can puncture the belt and eventually the pulley will be damaged.

Spillage on gravity take up weights is commonly seen. With a dense ore, it is easy to double design belt tensions. This changes transition loads, causes belt lift off, can destroy idlers and pulleys, all of which leads to mistracking and often worse.

Carry back falls from the conveyor when the belt passes return idlers and is characterised by piles of spillage under several or many rolls. These piles will continue to build and bury the conveyor.

Belt ploughs are installed on ground level tail ends. There is nowhere for the cleaned material to go, and this can bury the tail end.

Examples

An alumina belt has been replaced every three years. A grade M belt was purchased for a lower price. Lifespan dropped to six months due to splice failure from alumina attack. It took several iterations of the short life before this was evaluated.

A heavy-duty conveyor with a shuttle had erratic tracking issues. Several tracking frames of increasing complexity were installed. A specialist checked and correlated the tracking issues with the shuttle position. Shuttle rails were worn due to poor support structure and caused misalignment. The rails were not inspection items.

Related stories:

A conveyor belt lasted nine years from commissioning to replacement due to cover wear. An ‘identical’ specification belt was sourced at a lower cost. The new belt covers wore out after a third of the tonnage of the previous belt. The cost of belt is less than half the total cost of installation – a 20 per cent saving in purchase price might save 10 per cent on total cost. If the belt does not last as long as before the saving is soon lost. Conversely, if testing is completed on the belt before purchase, and the belt lasts longer at the same price, there is a real saving.

During commissioning of a new plant, a specialist was called to look at conveyor belt damage after running in. Perfect grooves in line with skirts were noted, and the grooves went into the carcass for the full tape length. No-one watched the belt during run in. The belt had to be replaced.

After changing to a thicker belt on an existing conveyor, the conveyor would not pull away. Skirts were adjusted into the top cover and stalled the motor. Skirt adjustment solved the problem.

Chutes and loading

Chute design, wear liners and changes in material flow properties are responsible for many tracking issues.

Chutes should be designed to load materials centrally to the receiving belt as close to the direction and speed of travel as possible. Central loading gives balanced load on the belt. When material is loaded off centre, gravity moves the belt sideways to put the combined centre of gravity on the conveyor centreline. Mistracking results.

To combat this, enough side force must be applied to push the combined weight away from the centreline. A few tracking frames can’t do this, and therefore side rolls are being installed the full length of many conveyors. The side rolls do not move the mistracking belt but stop it from moving too far. In previous times we walked a conveyor tapping idler frames until the tracking stabilised. Safety issues rightly prevent this today, side rolls are an effective option.

  • I still see many chutes that are designed with unchamfered valleys, intersects and ledges that provide good anchor points for sticky material build-up. The chute may not block but build-up and release will change the flow pattern and alter belt loading.
  • Chute performance changes with material flow properties. It is common to see dry hard iron ore load perfectly, but when wetter ores are loaded the chute performance changes. The wetter ore does not flow down the liners the same as the dry ore, changing the point of impact on the belt. This is difficult to predict, and redesign might be the best solution. Some types of chutes, such as Gulf ‘stall flow’ are better able to accommodate ore variation.
  • Chute wear liners are often selected from standard shapes. This is good for standardisation of spares. Selection generally does not consider the impact of gaps between liners, ledges at the top of the lined surfaces, irregular areas where a standard does not fit etc. These gaps, joints, even bolt recesses all provide potential anchor points for build-up of fine material. As the liners wear the effect changes, sometimes better, sometimes worse. Adding in random ‘chocky blocks’ and other stop gap repairs can really impact flow.

Belt problems will always be with us, but by determining the cause we can improve performance.