Hello Nishant,

Some pointers:

1. Off-set idlers supposedly prevent idler junction type belt failure by offering improved belt support. This is particularly significant in case of solid woven PVC belts (thin and light and belt may pull into gap between idlers). Some belt manufacturers dispute its benefits by stating that belt buckles between wing and centre rolls (not proven).

2. Belt line is lower.

3. Easier to replace centre roller.

4. In manufacturing environment easier to introduce forward tilt to wing rolls and ensure that correctly installed to suit this forward tilt.

Regards,

Adi ■

Dear Shri Nishant,

The inline idlers have wide spread use for general application. In case of inline idlers, there is always certain gap between middle roller and side roller. This is to be limited to 10 mm for belt width up to 1800 mm. For belt width 2000 mm and above this gap can be up to 15 mm.

In case of offset idlers, the middle roller and side roller are in different vertical planes and therefore, this nipping gap for belt can be made zero or even the middle roller and side rollers would be somewhat overlapping with respect to belt motion. In view of this the offset idlers are often used for grain conveyors where belt carcass often happens to be thin and belt nipping has to be avoided at the idler kink.

The other important difference is the offset idlers height from its base bottom to the top of middle roller is less compared to the inline idlers. In other words the idler height is less. Therefore, this type of idlers are often used in underground mines where saving of height is beneficial.

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 a number of reasons to select offset roller configurations vs in-line.

Two main reasons:

1. Belt protection against puncture damage if center idler becomes dislodged and support deflection causes belt to sag onto steel support or onto offset roller pipe frame.

2. Reduces junction flexural fatigue of belt (wing to center roll flexure) as it transitions from center to wing to center rollers including elimination of junction gap you noted.

There are many conditions to be considered and solved:

1. length of center roll -- shortened center roll vs equal length

2. diameter of center and wing rollers - influence on power and life with larger center roll

3. angle of wing rolls

4. bearing selection - ditto items 2 and 5

5. idler spacing selection and optimization of power and roll life

6. belt construction selection to control center-to-wing belt junction fatigue stress

7. return roll system

8. influence of selections on horizontal curves, belt speed, spillage

9. offset dimension and its influence on above

10. overall profile of structure is reduced with offset rollers resulting in an ergonometric benefit, and lower wind overturning moment

11. must overcome the ancient idea of seeing belt move with material for its full length. ■

DON"T DO FORWARD TILT!!!!! Big mistake.

This will reduce life of belt bottom covers, cause tracking instability with dry-wet seasons, cause undesirable additional power draw, increase noise squeal, and add to roll shell wear.

You can anticipate a 10-15 db noise increase with forward tilt.

Further acknowledge the ease of changing rollers as noted in prior posting.



We have been practicing offset roller installations for many years. To date no problems with up to 9000 t/h, 7.5 m/s 1800mm wide belts, et al.

Further the rolls can be offset or in-line with belt friendly pipe frames to inhibit belt slitting or tearing from excessive sag when rollers are dislodged. ■

All good points.

I have mentioned this in previous posts. Offset, overlapping rolls, that eliminate the gap at the idler juncture, due offer somewhat better support for thin belts but up to a point. They do not prevent the creasing of very thin belts and such creasing can result in fatigue failure.

Joeseph A. Dos Santos, PE ■

Joe,

The grain industry did migrate to offset rolls when thin fabric belts where used. In-line idlers did fatigue the belt at the junction which led to 3-part belts.

What is your criteria for using and not using offset rolls.

CDI applies a critical junction stress criteria based on FEM modeling of a 3-D model of the belt and idler set.

We use offset rolls exclusively for long overland systems as noted earlier. ■

Larry,

I don't deny that there is a benefit I just question the magnitude of the benefit. This is from my own experience but without any detailed analysis of this issue. Your finite element analysis may quantify this and may be of benefit to the industry if you are willing to share it with the readers of this forum.

I used offset, overlapping roll idlers for the benefit that you mention but the belting still creased conspicuously and I ultimately installed a belt that met the load support criteria.

I am aware of the wide use of the offset idlers in the grain industry. Yet, after more than 30 years of such use I am not aware of a single belt manufacturer acknowledging this quantitatively in there load support tables.

Joseph A. Dos Santos ■

One item not discussed is the difference in carrying capacity. This varies with idler angle and 3-5 roll configuration.

Discussion from the mass meter suppliers would not go amiss since they all insist on inline idlers. There can be lots of problems going from off-set to inline and back again to cater for the mass meters since the conveyor profiles are a different shape.

To look at the effects of belt deformation on a carcass you need to view it in extreme conditions. This ridiculous installation shown in the photo was a luffing boom conveyor. It shows the effects of belt deformation that occur in such extreme conditions, similar deformations occur on straight belts under heavily loaded or high sag conditions and to a lesser extent on normal convex curves when using off-set idlers. A similar photograph with inline idlers would have resulted in a “W” configuration of the belt in the curve. The “W” creates its own problems so best stick to good designs and configurations to get a trouble free installation. ■

Sorry Photo did not attach

Attachments

convex curve w (JPG)

■

In pass designs, I have found the 5-roll configuration was used to carry super capacities or fill-in for in-country lack of bearing size with local manufacturers. 5-roll will cost considerably more all things considered. At the same L10 life rating per roll, you will have a 67% increase in roll failure rate. This is somewhat augmented by standardization or correction for idler larger idler spacing with shortened roll lengths.

Traditionally, five roll idler sets, tend to have a shallow angled first wing roll set to around 25 degrees and the second set at around 55 degrees. When you compare the carry capacity of a standard 35 and 45 3-roll set, and using the belt edge clearance to contain lumps, the resulting capacities are:

A. Assume:

1. 2000 t/h of 850 kg/cm coal

2. speed is 5 m/s

3. lump size of 150 mm

4. surcharge angle of 20 deg.

5. edge clearance 135 mm on 1200 mm wide belt

B. Capacities of 5 roll 25 x 55 degree vs 3 roll 35 and 45 degree:

6. 5 roll capacity ...................... 2250 t/h = 112 %

7. 3 roll 35 equal length .......... 2010 t/h = basis

8. 3 roll 35 short center .......... 2110 t/h = 105 %

9 3 roll 45 equal roll ................ 2180 t/h = 108 %

10. 3 roll 45 short center ........ 2260 t/h = 112 %

C. Shortened center roll at 29 % of belt with instead of 37 %.

It is interesting to note the 5 roll set in this example does provide some capacity increase. For the same bearing set you can obtain a larger spacing with the 5 roll set thereby reducing the quantities and cost of the 5 roll set.

If we compare spacing and roll length to set bearing capacity equivalence, then the 5 roll length is about 21.8% of belt width and the 3 equal roll length is about 37.1%. Correcting spacing for 5 roll yields a 70% increase in possible spacing when loading rolls in near equal fashion. This means the 5 roll does not cost any more when factored for same bearing capacity. Interesting. I am not sure I believe it given the early moring discourse. Maybe someone can make the check. ■

Mr. Morgan,

I don't see that the photo you posted demonstrates anything regarding the advantages of offset vs, in-line idlers. This is a case of gross violation of radius of curvature constraints. Besides requiring a curve radius ( for the belt modulus and tension) that will not result in belt buckling, the idler spacing must limit the angle break over each idler to not more then 1.5 - 2 degrees.

Joseph A. Dos Santos ■

Larry,

I was referring to differences in the carrying capacity of inline and offset idlers. In your example +-24 ton/hr would be lost on an inline base configuration.

I must re-check my calcs since a 55° 5 roll offset returns much less on my software. It only appears to equal the 35° 3-roll base example. Again with the effects of inline and offset idlers on the 55° 5-roll set, there is almost +-100t/hr involved in the decision. A third assessment would prove useful

Reducing the centre roll to an effective 350mm contact length on the 3-roll sets returns your results. ■

Mr. Dos Santos

The text explains that this is a gross violation. We are not talking about radii. The photo is posted to demonstrate the path of the belt on off-set idlers. This is not a static deformation, its dynamic flexing of the belt as it passes through the set. This type of deformation is visable on the cases referred to in my previous posting. ■

Hi everyone,

Thanks for the questions and explanations.

Could one say that for offset triple idlers the load distribution between side rollers and central roll is better than by the aligned idlers?

Can one count on with a longer life time for the central roll???? ■

Larry,

The idlers are manufactured in vast numbers and typically jig welded (unless tailor made for example for weighing idlers).

Any manufacturing process must have some dimensional tolerance associated with the method of manufacture.

What would you consider to be an acceptable tolerance on roll alignment?

I would suggest that (for offset idlers) it is better to plan to produce some inbuilt forward tilt (1-1,5 degrees) than to have idlers with one wing roll having positive and the other having negative forward tilt. I'm sure that Graham Spriggs will remember that he once resorted to swinging idlers around because they had all been manufactured to the negative side of the tolerance and hence had in-built backward tilt - teh belt could not be tracked.

Larry, Joe, Dave,

This matter was in part discussed at a forum during one of the BELTCON conferences. The discussion related to teh position of the centre roll with respect to the wing rolls - does the belt meet the centre or wing rolls first. (ANOTHER FORUM???).

Testing by Dave Pitcher dis show, in extreme conditions, some lateral buckling of the belt in offset idlers. This buckling occured in teh zone between the centre and wing rolls. ■

Good Grief Adi, don't start the centre roll position debate (just joking).

I must dig out my photos on ply separation caused by this flexing but this also identifies poor quality of belt manufacture.

I've attached a shot of a belt under strain showing the "W" effect that can be generated. This is more common on the inline idlers when the radii and belt selection are in conflict.

To those following this thread; There are merits for both inline and offset idlers providing that you design the system correctly and don't skimp. Minimums and maximums tabled in the various manuals are there for a reason. Deviate and you can expect trouble unless you assess ALL of the effects that your decisions will have on the system.

Attachments

w belt (JPG)

■