Originally Posted by tshidzumbam

Dear experts,

How does one calculate idler junction stresses?

What parameters are these junctions stresses dependant on?

Regards,

Mpho

One question, are you referring to the stresses the idler set

encounters at the bearings and roller mounting frame? ■

The idler junction stress and junction stress index is a combination of internal and external stress fields in and on the belt that the designers of high speed, high tonnage, and large idler spacing must be aware of.

Externally, the belt is subjected to the force of the belt wing idler pressing down onto the belt center roll near its junction with the wing roll. As idler spacing increases, the junction stress increases. Likewise, the reaction force of the center roll also produces a similar stress pattern on the wing roll near the junction. Excessive surface stresses may result in divots or pitting of the belt cover in contact with the idler roll in these areas just above the steel cord helix high points. We believe this is due to a extrusion effect where the deformed rubber in contact with the idler undergoes a axial and lateral displacement.

Internally, the rubber stress may cause debonding and tunneling of belt's core gum to its steel cord in the overstressed zone that eventually will cause a break in the belt at the debonded section.

We rely on Finite Element Analysis (FEA) to evaluate the idler junction stresses. We have other means to give a hint of the limits based on the study of many installations. The stress analysis and failure method is the same that used for our pipe conveyor technology and the Veyance Technology patent of the CONFINE construction. ■

Shortening the center roller will increase the junction stress. It produces a higher force on the wing which in turn must pass a part of the reaction into the center roller edge zone. ■

[QUOTE=tshidzumbam;70651]

It seems to be a catch 22 between the idler life and belt stresses.

QUOTE]

There is no reasonable relationship between the life of a few idlers and the life of a yard belt.

Safeguard the belt. ■

Originally Posted by tshidzumbam

Thanks for your response Mr. Nordell

We have also noticed the follwoing:

1. The minimum lift (for clearence) from yard belt-line to loading table belt-line is 225mm for narrow centre roller; and 315mm for 3 equal roller configuration;
2. 225mm lift (narrow centre roller) results into a lower centripetal force on the loading table's first and last idler (life and deflection are ok) and
3. while equal rollers idlers (at 315mm) have a higher centripetal force (1st and last idler on loading table) and dont meet the minimum life requirements of 100 000 hours.

It seems to be a catch 22 between the idler life and belt stresses.

Mr. Nordell, you seem to have said the opposite about narrow centre rollers and junction stresses in 2001-December-19 at 15:42 (Subject: 45 degree troughing roll assy use). May you kindly clarify this confusion.

Please quote my 2001 comment you are refering to. I could not retrieve it from the forum archives. ■

Dear Mr. Nordell

Interesting...

Changing side angles or using smooth concave rolls would reduce the contact pressure between rollers and belt?

Also could reduce problems with misalignment.

Could it be simulated using the DEM model?

Best Regards ■

Dear Tshidzumbam,

I did mispeak on the first posting claiming the idler junction pressure would be reduced by shortening the standard idler roller spacing. In most cases, the junction pressure is reduced with wider center roll length. Think of a very large center or no wing = little or no junction stress.

Please see attached images taken from belt failing and our theory of pressure distribution.

"How does one calculate idler junction stresses?

What parameters are these junctions stresses dependant on?"

1. Belt width

2. Idler wing angle (assuming 3 roll)

3. Center roll length

4. Idler spacing

5. Belt cover thickness & separately bottom cover thickness

6. Belt construction such as cord diameter, pitch, and construction

7. Reinforcement fabrics or cross weave of steel cables

8. Idler roll diameter

9. Filling level of ore % ore loading

10. Material bulk density

11. Belt speed

12. Idler roller pressure distribution and belt tracking

13. Rubber viscoelastic properties E', E". G', G", other indices such as carbon/silica loading and crosslink efficiency

14. Operating temperature

I have taken these from memory, others may add to list. Have I posted what an improper idler junction stress will do to the belt?

Attachments

cover failure due to overstress of belt bottom cov (DOC)

■

I have read the comments from the differing sources and would say the following is some of my experiences.

1/ Shorter center rolls do not have the effect of increasing centre roll life but can do a better tracking of the belt system. However, every installation I have worked or done an assessment on, have all been at the expense of the costly belt replacement in a very shortened belt life span and increases centre roll damage and increased wing roll replacement due to shell wear. This is due to extra stresses on the centre rolls and wing roll shell wear.

Take a piece of paper and fold it lineally, at the roll length ratios spoken in this discussion. The flat bottomed 'Vee' is formed and one can see that the lineal forces are much more rigid in the vertical plane but less in the horizontal plane. This action forces the centre roll to have to supply extra lift the and bending the belt dipping between the idler sets and the bowing, from belt mass and loading. The belt wing sections 'Planes' act like stiff planks that don't bend very well on the (edge to wing to centre roll transition) profile to allow the whole belt to rise up and over the centre roll. This causes the belting carcass & covers to 'pucker-up' worse at the idler roll transition points due to the extra loading on the centre roll lifting and rolling action the belting which causes belt cover & carcass failure at this lineal locations between the wing rolls and centre roll and Mr Nordell's document shows one effect of this action.

2/ Reducing the idler spacing helps to reduce these forces and reduces the belt damage dramatically. The belt component of a trough belt system is generally the most expensive and can be around 50% of the total cost. This is the main reason to care for it with designed good structural integrity and maintenance procedures.

There are a small number of long extended idler spacing conveyors in my area with the wide spaced idler sets and all are suffering the same severe damage to:-

a) Belt at the idler roll transition damage lineally

b) Belt breaking completely

c) Idler rolls failures at greater than 100% replacement rate per year due to overloading forces

d) Idler Set breakage failures

e) Structural mounting & stand failures

f) Increased energy required to power the system

g) Drives & Gear Reducers failing due to overloading

h) Large production losses causes quoted in part by conveyor failure

Check the sixth paragraph of the website [ http://www.miningaustralia.com.au/ne...gn=newsletters ].

It does not say what caused the conveyor failure but just that it must have had a significant input to the drop in production to be mentioned.

Other companies have a similar conveyor systems and all appear to be experiencing or having the same issues detailed above.

Anything that is not 'Maxed Out' will last longer.

3/ With regards to the impact table, the system needs to be such that there is a lead-in/out of closer spaced idler sets to absorb the extra belt tensional line loadings within the belting at that point of product transfer. The process is similar to the 'Weigh Scale' Lead in/out stabilizing idler sets.

Of Note:-

i) I have patented idler rolls 'OneFits Rolls' that is shaftless design and have only one bearing combination from 3" to 12" which are massive in radial & axially load capacities. I do not differentiate between Carry, Impact or Garland rolls as the same OneFits Head is the same for all. The only difference is the shell length and thickness with one lubrication seal only and can be fitted with a felt dust seal within the stainless steel weather shield for electrostatic applications. These rolls would stand the overloading applications as per my designed criteria.

ii) In close fitting skirted applications such as the loading table, the patented 'DunnEasy Idler Assembly' allows the rolls to be changed without any dismantling of the componentry and can be close fitted at >300mm idler spacings for increased life of rolls and belt impact protection.

http://www.tamecservices.com.au

Mechanical Doctor ■

Dear Mr. Leslie Dunne, (I assume)

Shortened center roll applications have been applied for decades. The Brown Coal Lignite mines in Germany have used this principle, as have many others around the world, as is described their texts on idler roll designs such as the Precismeca Idler Roller Design Manual emanating from the German coal fields. They do provide benefits in roll life improvement by reducing bearing loads, in contrast to your conclusion. Since you do not offer facts and figures on your criticisms, how can your comments be used by the reader? Why do you make such strong claims against known engineering principles? Maybe your understanding of Cause & Effect is not correct.

Your analogy to a sheet of paper being like a belt conveyor is quite incorrect. I understand your intention in this discussion. The analogy does not hold. A thin sheet of paper is simple isotropic with same X-Y stiffness in the wide plane of the paper, a belt is not. If you wish to criticize on this merit then do more than compare dissimilar materials with dissimilar properties in the X-Y-Z planes of the belt. Do a proper job and apply known principles, otherwise your conclusions cannot be reasoned in a court of law and should not be taken as a serious engineering conclusion.

I read your global comment about Australian overland references in "your area":

I quote you: "There are a small number of long extended idler spacing conveyors in my area with the wide spaced idler sets and ALL are suffering the same severe damage to:" Then you go on to state the large idler spacing is the cause of 8 belt conveyor component failures. These are all serious. You are sure of your facts?

Since you advocate shorter idler spacing, what is your criterion? What is shorter, or shorter than what?

Be more specific: 1. what is "your area", 2. what is "wide idler spacing", and 3. what are the "ALL" in "extended idler spacing, on long conveyors" (reworded for clarity)?

You note 8 serious claims of design flaws by using "Large Idler Spacing". To be clear, that you are not flogging your product, when you make strong claims against operating systems, please give more detail on your claims. I have read your earlier posting with photos. This was not a tool others could use other than as your Dunn Easy product pitch.

You may have valid reasons to make your claims, against "Large Idler Spacing" and they may be accurate. Facts speak louder than words. Are you aware of many installations that have used "Large Idler Spacing" up to 6 m carry side, running at 8.4 m/s, transporting 2500 t/h, using very stiff belts to ST-7100 N/mm (closer to your paper analogy)? They had not such problems you claim to be at the hand of "Large Idler Spacing". They ran for many years. What idler spacing do you say is not appropriate?

Since you are making very strong claims, I suggest you also have "necessary facts", proper reasoning, and engineering backup, to publically make accusations on the efficacy of these designs you claim are flawed.

People may want to buy your product; you can offer this product literature to interested parties.

I also note you do not use your own name. Why not? ■

Hi All,

Larry is right and what is being overlooked is the belt construction. If the belt lacks the thickness or lateral rigidity then failure along the idler junction can be an issue. Therefore with the proviso the belt construction is appropriate the best outcome both for the belt and idler life is designing the idler configuration in a manner that maintains even pressure over the face of all 3 rollers (in a 3 roll system) and making sure this is the case along the entire system. In many instances this is best acheived by increasing the idler spacing particularly in higher tension areas from what is the industry accepted norm, rarely is it achieved by reducing it.

In many instances where we have observed belt failure along the idler junction it has been caused at the loading point. This in turn can be due to changing the idler configuration from say 35 to 45 degrees at the load point to "centralise" the load, loading off centre such there is relatively high impact over an idler junction or poor troughing of the belt at the load area (could also relate to using higher trough angles at the load point).

Colin Benjamin

Gulf Conveyor Systems Pty Ltd

www.conveyorsystemstechnology.com ■