Wednesday 20th Nov, 2019

BULKtalk: Choosing the right idler

Rio Tinto’s Steve Davis explains the ins and outs of idler design to help companies reduce labour, downtime and maintenance costs.

An idler roll is relatively inexpensive on its own, but there are many on a conveyor. Saving a few dollars per roll can save many dollars on a compete conveyor, but lost labour and downtime from roll failure can add up to hundreds of thousands of dollars per hour in some operations. Consequential losses from a ruined belt might run to tens of millions of dollars.

Idler design, materials and methods of manufacture have improved rapidly in recent years, and it may be time to consider alternatives to lowcost steel rolls.

There are five key requirements for an idler roll, whether new or replacement:

1.  The conveyor design, installation and operation should be as good as possible, as many interfaces can accelerate idler failure rates.

2.  Rolls must fit into the idler frame. With millions of different sizes, specifications and similar fitments, there are errors. Forced fits have safety and alignment implications for installation and removal. Loose fits allow roll displacement.

3.  Rolls should be manageable. Changeout often requires some difficult physical moves, so weight should be 20 kilograms maximum or less. This can be achieved by use of materials lighter than steel. Alternatively, we can replace three roll troughs with four or five rolls, but this adds cost and can result in other effects.

4.  Rolls should not damage the belt when failure occurs. “Cookie cutters” from weld failure and “potato peelers” from seized bearings can destroy a belt. Corroded and abraded shells result in similar failure modes. Composite rolls can mitigate many of these problems. Common angle steel idler support frames can damage belts, if a roll drops out leaving an exposed fork. Belt-friendly frames do not have this issue.

5.  Rolls should have an acceptable failure rate.

Specifications

I have several idler specifications from across the mining industry. Some have minimal requirements, while others go into a great amount of detail. Most focus on manufacturing specifics and tolerances, but most seem to ignore definition of roll life and how to manage this.

It is common to see requirements for rolls to be capable of carrying a flooded belt load plus several dynamic factors at the highest load point in a conveyor. These can triple the loading over a normal operating situation, or a lower load location. If rolls are standardised across conveyors, what is the correct design?

Rim drag values are included in some specifications, and a maximum is a reasonable expectation.

There is little detail regarding the end cap sealing arrangement, and some are mute on bearing seal arrangements. Many roll suppliers report that failure results mostly from bearing contamination and excellent end cap sealing arrangements have been developed to mitigate the effects. Where open or shielded bearings are specified over sealed bearings, is the minor saving in cost and energy comparable to the cost of earlier bearing failure of multiple idlers?

Some specifications have brief comments on polymer and composite rolls, and other aspects, but detail is lacking. Few mention seal testing for dust and water, as available from TUNRA and others.

What could be considered in selection of idler rolls?

Steel rolls

The most commonly available roll type is a steel shell with a pressed steel end cap that is rim welded to the tube. When this weld fails and separation occurs, the roll often presents a sharp cutting edge to the belt cover. This “cookie cutter” can slice the belt from one end to the other.

There are better designs available that remove end cap weld failure. Rolled tube ends and composite end caps with press fit are readily available. The use of quality steel tube results in good balance and run out values. Steel rolls are typically two or more times heavier than the equivalent composite rolls, even with hollow shafts.

When bearings seize in a steel-shelled roll, regardless of the end cap arrangement, the belt can wear through the shell, leaving a sharp edge that can peel long lengths of rubber from the belt covers.

Corroded steel shells can present many sharp edges to the belt, and cause damage even if the bearings are still turning.

Steel rolls are an option for many applications and are the basis of most impact rolls.

Composite and aluminium rolls

Composite and aluminium rolls of various types have been available for many years. They are better balanced, with less run-out, compared to steel rolls, due to the shell manufacture method. This results in lower weight, lower noise, lower vibration and a potentially longer bearing life. Composite shell and end cap arrangements do not generally damage belts on failure, as there are no sharp edges.

There are several grades of aluminium available. There are many cast, extruded and pultruded polymers and composites available. Some have reinforcement. There appears to be little agreement in the industry as to which offers the best solution and why. New materials and suppliers regularly enter the market. There are some significant differences between the materials in weight, strength and abrasion resistance. Testing offers best guidance.

Composite and aluminium rolls have press fit, adhesive, friction welding or other methods to install end caps, and may have hollow shafts for further weight reduction. These end caps do not fail and cause damage to belts, but separation of the end caps from the shell can occur.

Bearings can still seize, and the belt will wear through composite and aluminium shells.

Wear resistance of the composite and aluminium can be comparable to steel, but chemical and corrosion resistance is higher. Aluminium and caustic materials do not work well together. Composites all have temperature limits lower than steel, though fire resistant composite rolls are available.

Composite and aluminium rolls are suitable for most applications.

Shafts and shells

Hollow shafts save weight if available and are mandatory if weight saving is a goal. One, two- and three-piece hollow shafts are available, and a precision method of assembly and machining is important for achieving long life. Composite shafts are not yet available. There are idlers with stub shafts. However, I am not convinced this is a good solution.

Quality shells give better roll balance, run out and lower noise and vibration. Extruded or pultruded shells are inherently better for this than the welded steel tubes commonly used. However, the best quality steel can be very close in performance.

Shell thickness is a consideration in abrasive environments. Otherwise, strength of the roll should determine thickness. The best quality shells cannot compensate for poor installation. There is more potential variation in steel shell diameter and thickness than other materials. Steel generally conforms to imperial sizes for pipes, whereas aluminium and composites do not have this limitation. Replacement of existing rolls with others could lead to difference in roll height from the stringer, giving local overloading. Manufacturing tolerances should be defined.

End caps and seals

Composite end caps and labyrinth seals are generally lighter than pressed steel, and dimensionally more precise. Sintered steel end caps are available. It is generally easier to mould or press a composite into accurate matching multipath labyrinths, which offer better protection to bearings. Some labyrinth seals add a contact seal for further protection. There are designs with integral stone guards and flingers available.

No two suppliers’ end cap arrangement is the same. The labyrinth must function at all angles from horizontal to 60 degrees for a wing roll and keep out dust and water. Some labyrinths are grease filled, some have heavy-duty rock rings and others have flingers.

End cap sealing keeps out dust, dirt and water, all of which can damage the bearing and lead to early failure. Attention to the design and even testing of the design for dust and water ingress should form part of any idler specification, as there is good evidence that contamination is a significant cause of bearing or idler failure.

Idler and bearing life

Idler roll assemblies have sufficient strength in the design to survive long periods of operation, and not to deflect more than bearings can tolerate. Although cap, shell and shaft failures occur, failure of bearings brings many rolls to a stop. Only one bearing in an idler has to fail. Keeping contamination and water out of bearings will extend life.

Fully sealed bearings offer a final resistance to contamination that might get through the end cap seal. Shields are not as effective. Grease fill recommended by the bearing supplier for the duty offers the best lubrication. Relubrication is not a practical option.

We focus on bearing life to define an expected idler roll life. We use L10 for this, but the meaning seems to be widely misunderstood. The L10 fatigue life calculation, the number of hours in service that 10 per cent of bearings might fail, does not imply similar life for the rest of the roll. Observation and discussion indicates that bearing fatigue failure is rare and that most failures result from other causes, which we do not specify.

An L10 bearing fatigue life of 50,000 hours implies that 90 per cent of bearings should last between six and 60 years in clean operation, under constant design temperature, load and speed, and with suitable lubricant. Ten per cent might fail in the first six years, with high failure rate in the first few months. With no failure for five years, it would still meet the criteria. Would there be a warranty claim if all idlers failed by the 25th year? If small batches of rolls are purchased, any life up to 60 years meets the L10 criteria. I have seen idlers upsized because L10 for the worst case was five per cent less than nominal; the difference is insignificant.

Conveyors rarely operate at constant load, with surging and long periods with no load. Centre rolls take most of the load and wing rolls take less. Loading on idlers varies with the location due to belt tension impact. Standardisation of idler size across several conveyors may increase the variances in loading. Idlers that are not exactly perpendicular to the conveyor belt will induce axial loads on the bearings, tracking idlers being the worst. Many rolls have intermittent contact with the belt, and repeatedly accelerate to full speed and coast to a stop. Temperature is variable. Any carry back on rolls will change the balance and load. Installation adjacent to sources of vibration can reduce bearing life. These are not the “ideal” conditions for an L10 life design. Is there a different approach?

We compensate for non-ideal conditions by increasing L10 fatigue design life and adding factors to the design loading. We then standardise all rolls on a conveyor or conveyors to the highest loaded roll, possibly in the highest load point of a convex curve at the top of an incline, centre roll, with flooded belt, surge factor, and misalignment factor. The ‘empty belt no load’ case for an outer wing idler bearing at the horizontal tail of the conveyor could see one tenth or less load on some bearings.

Is overdesign leading to bearing failure through under loading? Individual rolling elements in a bearing require friction to roll rather than slide. This friction is from load on the bearing. If minimum loads are too low, bearing life reduces. Sliding between rolling elements and raceways leads to smearing damage to the rolling surfaces and causes a temperature increase. Seizure follows.

The bearing cage keeps rolling elements from touching each other under design-load conditions. When minimum loads are not present, and rolling elements are sliding, the cage now drives the rolling elements rather than the friction forces of the races. This can lead to premature cage failure. Under-load does not have to be continuous in operation, even short periods can initiate early failure.

C3 internal clearance bearings may not be correct, as clearance is specific to the internal design of the idler and its operating clearance. Too large or small internal clearance can cause rolling element damage and early failure. Idler suppliers should select clearance to suit.

The above brings into question whether standardisation is a good thing and leads to consideration of different roll sizes for centre and wing and even whether different rolls could be used at different locations on the same conveyor for better idler life. If bearing fatigue failure is only a minor percentage of roll failures, this is less of an issue.

Assembly

Good components require good assembly practice. Bearings and seals are easily damaged during assembly, other parts are more robust.

The assembled unit should be checked for correct process, with some basic checks on components before assembly. Assembled rolls should all be checked for free spinning shafts and randomly for balance, runout, dimensions and rim drag. If a bad roll gets to site, it will likely be installed and fail quickly. Tolerances are critical for press fits.

Selective testing for dust ingress and water ingress through the labyrinth to the bearings will give a comparative understanding of potential bearing life.

Summary

The main cost impact from use of idlers in conveyors is the cost of lost production and labour to replace them after failure. Risking major damage to belting should be a consideration in selection. Savings from lower cost idlers, idlers not designed for the duty, over-standardisation, seeking small energy and drive size savings will soon be lost if failure rate is excessive. Overdesign may not return expected benefits.

Basing idler selection on steady state loaded conditions may be normal engineering practice, but is this sufficient to indicate real life performance?

Good, low weight rolls are available with excellent bearing protection. These should last longer, reduce the amount of collateral belt damage and make manual installation safer. As idler design and quality has changed and improved, the current style specifications have become less relevant, and a collaborative approach to idler selection may be better.

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