Offset Idlers,

We are not idler manufacturers but we encountered a very serious problem with offset idlers about 10 years ago that led us to do a lot of research and design on how offset idlers contact and interact with a conveyor belt. The benefits of an offset design is mainly maintenance or access to the idlers for changing them out. The problem is most likely to occur with we believe high modulus belts be they fabric and or steel and where there is a liklihood of the idlers surfaces becoming roughened due to corrosion or other causes. What we found was that through slightly different contact angles not all idlers rotated at the same speed as the belt with the consequence the idlers abraded away the bottom cover of the conveyor belt. Since we did this work we have found many very serious instances where very expensive belts have been prematurely changed out because the bottom cover wore out before the top. We have talked to idler manufacturers about this, most don't believe there is an issue, many refer to instances where there is no problems but it is something to keep in mind as we now have quite a catalogue of references as to where it has happened and the design/site conditions that have led to it occuring.

As a consequence we definitely never recommend 5 roll offset designs, we never allow a lead on any idler to assist tracking and we do not recommend 3 roll offsets in high strength applications or if we do we look to the site condition of other idlers plus pay attention to idler spacings.

It will be interesting to get other feedback although I believe this issue has been raised before on the forum.

Col Benjamin

Gulf Conveyor Systems ■

Dear Mr. Felipe Velasco,

Referring to 3-roll troughing idlers, these are manufactured in two categories as below :

- In-line idlers (in this type of arrangement all the three rollers can be said generally as in one vertical plane).

- The offset idlers (In this case, the side rollers are in one vertical plane and middle roller in another vertical plane).

The first type of idlers are somewhat easy to make and they are more in use. The comparative disadvantage is that there is some gap between two consecutive rollers. As per DIN this should be within 10 mm for belt width less than 2000 mm and 15 mm for belt width 2000 mm and above. So, if the belt is thin, it will have pinching at the idler kink.

The second type of idlers provide support to full width of the belt, because rollers are staggered and they are overlapping each other. They are more used for grain conveyors because they can support thin belt without pinching. They are also used in underground mine installations because they have low height. The travelling belt should first enter on middle roller and then it should pass over the side rollers. So, they have direction mark.

There are no other differences in their function.

I hope your query pertains to such 3-roll idlers.

Regards,

Ishwar G Mulani.

Author of Book : Engineering Science and Application Design for Belt Conveyors.

Author of Book : Belt Feeder Design and Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Email : parimul@pn2.vsnl.net.in

Tel.: 0091 (0)20 25882916 ■

There are others benefit to offset idlers rolls when setting the design criteria for overland conveyors.

Some of these benefits are:

1. Belt protection against tearing from roll dislodging that is apparent with conventional in-line design

2. Belt Junction fatigue failure resistance at junction of wing roll to center roll

3. Improved wind profile of structure - better environmental appearance and better ergonometrics.

4. Improved idler failure criterai against catastrophic failure due to idler failure zipper effect ■

Having made the above comment I wish to clarify the fact raised by Colin. The belt rubber cover compound specification must be set to protect against the potential abrasive effect noted. The belt can be specified to allow and control squirm. A rubber compound that is not spec'd properly and there is a higher than normal idler pressure, may result in unacceptable abrasion. ■

The issue of wear on the bottom cover of the belts has nothing to do with rubber "squirm" based on the work we did Larry, nor can it be addressed by changing rubber compounds. The root cause of the issues we investigated was the differential speed between idlers and belt caused by poor contact pressure. Other work has been done that is unrelated to offset idlers on the way rubber and belts interact with idlers and pulleys and some of these issues may well be addressed or minimised by compounding, but these issues are different.

Col Benjamin ■

Dear Colin,

I assume we are only disussing the belt carry strand and the belt wear of the pulley cover.

You may have a point . However, I do not understand the explicit wear mechanism from your discussion. Please clarify your arguement and make it clear to this lay audience.

CDI does often recommend offset idlers for the reasons given. We do wish not to make design mistakes. Therefore, your comments are taken in the most serious way. What is it that we do not understand?

CDI has studied and designed many belts with offset rolls. These designs include belts with high tonnage above 10,000 t/h, high speeds beyond 6 m/s, large idler spacing to 5 m, and idlers with large bearings to 6310 series ball ( indicating high local belt-idler interface pressure).

We do become concerned when the belt has a very high bending stiffness, is thiick and highly reinforced.

CDI has studied/witnessed belt cover wear on ocassion including premature bottom cover wear, which shortened the belt's life. From investigation, we determined some with improper rubber cover compound properties, idler misalignment, unacceptably high localized pressure between belt and idler from a)stiff belts and b) high localized pressure from the idler junction between wing and center roll.

Some idler manufacturers have applied a wear resistant surface treatment which exacerbated belt wear.

I do acknowledge, as said above, that there are case studies that exceed our understanding. So, please assist and enlighten as others do in this forum such as:

1. what causes the differential speed between rollers on the same transom?

2. what is a poor (low or high?) contact pressure?

3. when you state "slightly different contact angles" are you refering to the idler axis not being at right angle to the belt or some other alignment?

4. Is the belt's construction an issue and if so what are your issues and criteria?

5. Do you have a maximum roll-to-belt- pressure criteria that guarantees long life?

Maybe we can compare some installations and their specifications.

I (we) await your wise and cherished comments. ■

Off set idlers generaly carry about 1% more material that in-line idlers due to the changed trough profile but this is not an advantage of any sort. Access for maintenance is the real advantage.

In an ideal world the belt will trough with a slight curve which keeps it out of nip points between the rolls and move smoothly over the rolls.

If we are to be concerned that the belt may get nipped between the rolls on in-line idlers then off set idlers are not the answer. This nip is mostly due to excessive belt sag due to poor design, excessive loading or convex curves all from which off set idlers suffer there own problems.

Offset iders give uneven relief of belt sag which is not such a problem under light loads. The belt is raised either by the centre roll or the wing rolls depending upon the idler set configuration (centre roll leading or trailing). Regardless of the arguments of which convention is better the problem remains.

This uneven lifting results in the profile of the belt changing on heavy load belts or convex curves as it passes over the idlers. The centre flat section widening and narrowing as it encounters the centre roll, followed by the wing rolls or vice versa. This profile change creates a curved path of the corner of the trough, flexing the belt, moving the material and creating additional resistance. The curved route also causes belt wear.

This is shown on the rough (and Exaggerated) sketches below. ■

Hello Felipe..

From my experience the three roll off-set Belt Friendly idler configration is the way to go.

As mentioned above, if you loose a roll or two, the belt simply slides over the pipe forming the idler frame and not on those horrible idler roll brackets as with conventional idler frames.

(On belt friendly idlers, the roll brackets stick out horizontally. On conventional idler frames the roll brackets stick out upwards!)

Also as mentioned above, convex curves can be a problem to the belt on off set idlers...so... calculate the convex curve correctly, and do not try and hide the fault with fancy idlers.

If using five roll idlers, the same applies, but always make sure that the wing rolls are downstream so material can not jam between the idler bracket support pipe and the idler roll, (on the outside vulnerable rolls).

Regards

LSL Tekpro ■

Dear all,

what about the load analysis for offset idler set? As far as I know, inline idler set load analysis has been carried out, but offset idler set loads or forces seem to be unclear, especially from a scientific aspect, instead of industrial engineering simplified attitude.

Xiangwei Liu ■

Offset idle provide superior belt support. They also reduce the squirm between belt cover and roller surface. Contrasting Mr. Benjamins comment, CDI has made extensive study on the squirm effect and can show field damage to an overland belt where the squirm is very evident.

Squirm is a result of the idler deforming the belt bottom cover and the belt accomodating the deformation. Rubber is not compressible unless you consider rubber foam (air pockets). If rubber is not compressible and rubber is attached to the steel cord tension member, where does the displaced rubber go? It squirms is way as slip both axially and transverse. This is especially true over the steel cord twist helix high point. At the high point, the pressure is greatest and therefore can abrade the bottom cover when the rubber properties have been exceeded. Does anyone wish to see photos of the same and FEA analysis of the same? ■

I have stated in another BSH posting the benefit of the offset idlers, per our analysis on many overland conveyors, that confirms our comments. We applied pressure sensitive material to measure the pressure under the belt's steel cords and between cords. We also applied graphite dust and measured the cord footprint for a range of roller diameters (127 -194 mm). Finite Element Analysis was performed. A range of roll pressures was recorded for each roll diameter, mulitple belt constructions, and range belt forces. FEA and measurements were in close agreement.

Given we can replicate the field and laboratory measurements of the contact mechanics of both in-line and offset rolls, we claim to be able to classify the degree of difference between these configurations. The offset roll system appears to provide more steel cord support due to the additional belt flexure. About 5-7% additional steel cords are supported. However this depends on belt flexity (construction) and the rubber's elastic modulus temperature properties.

In addition to the above, we design pipe conveyors. Multiple jigs have been built and belts measured that identify the 360 degree pressure field around the 6 roll pipe configuration for both in-line and offset pipe diameter control. Based on these measurements, CDI identified an alternative scheme to reset the belt's internal steel cord placement for pipe conveyors which results in superior pressure gradients that reduce belt line drag, demand power, and provide superior sealing of the pipe seam. A patent has been granted for this concept. Many pipe conveyors have been constructed and measured using this concept. Side-by-side conveyors are witness to the differences between the superior patented construction, superior sealing, reduction in pipe seam rotation, and lower power demand.

All of these facts contribute to our confidence in having the knowledge to predict belt behavior in the idler contact region. ■

If a punter has to specifiy idlers for 25km of conveyor in the present economic, why stop there, climate he is obliged to present the lowest CAPEX to the Client, his management, the BFS or whatever. Suitable roller prices will be the same: lowest. Belt prices will also have to be the lowest. How can the guy justify idler sets which are not the cheaper option?

It will be a rainy day in Hell before finance tries to understand bulk handling sophistication. Here's hoping that offest bracketry is cheaper. ■

Originally Posted by piaozi66

Dear all,

what about the load analysis for offset idler set? As far as I know, inline idler set load analysis has been carried out, but offset idler set loads or forces seem to be unclear, especially from a scientific aspect, instead of industrial engineering simplified attitude.

Xiangwei Liu

Dear Xiangwei Liu,

I think you will gather from the comments above that before any scientific study can be undertaken the ground rules for such an investigation need to be set. Such ground rules will result in highly theoretical results that will only be valid under those conditions and meeting them in the field will be very difficult. Don't forget that the old argument of center roll leading or trailing provides completely different results especially in the case of center roll loading. It all comes down to correct conveyor design to make these idlers or indeed any idlers work efficiently. The idler manufacturers should therefore indicate their preferred direction of travel and design their idlers accordingly. Belt flexing, drag and other factors are variable depending on load, load distribution, curve design etc. ■

Originally Posted by Dave Morgan

Dear Xiangwei Liu,

Don't forget that the old argument of center roll leading or trailing provides completely different results especially in the case of center roll loading. It all comes down to correct conveyor design to make these idlers or indeed any idlers work efficiently. The idler manufacturers should therefore indicate their preferred direction of travel and design their idlers accordingly. Belt flexing, drag and other factors are variable depending on load, load distribution, curve design etc.

Please note, as I indicated earlier, there is a solid case for the central roll always to be up-stream on 3-roll idlers and down-stream on 5-roll idlers...

The wing rolls must be downstream on off-set idlers to stop material jamming between the roll, and its bracket and frame, The rotation of the roll must be self-cleaning.

Cheers

Taggart LSL Tekpro ■

Mr. louispanjang,

I have heard the term "it will be a cold day in hell" or "when hell freezes over". I have never heard "it will be a rainy day in hell". I suppose it is as valid or absurd.

More seriously, originally, in underground offset idlers, the objective was to decrease the backing height thus lower the conveyor. I have heard the argument of offering load support to the flimsy belt but a flimsy belt will crease and fatigue due to the crease even if their is no gap. A belt that needs the offset overlap to save it has not been properly selected. I do agree with the belt friendly idler design but the motivation for it is not a flimsy belt.

Regarding differential speed, this can happen if the roller end is rounded or relieved (thus decreasing diameter) if the belt is in contact with the roll end. This should not happen with a properly designed conveyor.

Joe Dos Santos ■