Dear sir

in my openion yhere is no problem in case idler bearings are in good working condition. you can check following

1.0 Replace in case they are not rotating or making noise due to bearing faiure

2.0 Check in case rollers are rubbing with frames

3.0 Check whether material is stuckup between frame and rollers ■

First you must get consensus on the generators and their degree of noise. Many idler manufacturers have experimented with noise abatement. You would do well to talk to them. I am familiar with Melco in South Africa, Prok and Tyson in Australia that have on-going programs.

Noise is produced, amplified, and reduced by the following:

1. Squeal of rubber sqirm in contact with idler roll: rubber has a compression, extension and release or slip contact interface as it passes over each roll. Ideal alignment can minimize the squeal, but not eliminate it. Noise magnitude, can be 75 dba @ 1 meter and up dependent on belt speed, roll diameter, belt bottom cover thickness and compound

2. Idler spacing, larger spacing produces less noise so that measuring at 100 m produces a different noise abatement factor than most criteria which calls for 1 meter measurement. At typical 100 m reading may be 50 dba when it is 75 dba at 1 meter depending again on idler spacing and wind direction and velocity

3. Squeal and bearing rotation noise are transmitted, amplified, and muted by the steel work including:

a) steel roll to steel transom, and transom to steel stringer without vibration isolators produces max echo

b) vibration is deadened by hood cover made of plastic vs steel

c) vibration is deadened by hood cover wrapping idler verses idler frame having open acces from above roll set. This has a typical direction of sound magnitude associated with open side.

d) steel roll noise can be reduced by using aluminum, plastic and/or coated barrels of noise reducing polymers, on exterior or interior depending on pocket-book and final noise level criteria

4. WInd shrouds can amplify noise or reduce it depending on design -- some create a megaphone while other designs block and reduce noise emission of belt-roll squeal and bearing rotation ■

On point one above I failed to include pressure on belt roll interface which is dependent on belt width and mass, mass of material per meter, and idler roll configuration together with speed.

Noise abatement is becoming an issue for overland conveyors. I believe some academics have begun to investigate and catalog the many attributes.

CDI often takes field measurements that over time will create a factorial/latin cube designed data base to provide assistance in documenting the noted conditions. ■

The most cost effective way I have come across is to ensure the idlers are dynamically balanced. This has a payback in lower idler usage. There has also been some excellent work done by looking at various polymer coatings. As Larry has said, noise is a major issue with many overland conveyors. Two particular applications where a lot of work was done was at Alcoa in Western Australia on their overlands and at one the Coal and Allied mines in the Hunter Valley (name escapes me at the moment). In both instances the primary action taken was with idler balance and conveyor system alignment. Belt tension, sag, idler diameter and bearing selection all has an impact as well.

Hope this helps

Col Benjamin ■

Previous replies posted are all valid. Assuming that electrically resistance welded steel tubing is used then achievement of accurate run-out and dynamic balance is critical to the low noise running of the rolls.

Of high significance is the accuracy of scarfing - i.e. the cutting off of the weld bead after tube manufacture. Tube mills often apply a 'broad' cut, effectively producing a 'flat spot' along the length of the roll.



Regards,

Adi Frittella

AFRIPP PROJECTS CC ■

Another set of factors that produce and/or influence noise associated with the idlers are:

1. Roll shell weld seam is usually sufficiently prominent to induce an impact flap

2. Belt modal resonant frequencies coupled to idler spin freqency produces belt edge air beating that should be eliminated for both noise reduction and idler life

3. Idler spacing selection, belt speed, tension range (takeup adjustable and motor locations) must be considered in minimizing belt edge beating or flap and the noise effect ■

Gentlemen,

As a supplier of conveyor rolls we realize that noise issues are one of our client’s biggest concerns. We looked in depth at noise in relation to the interaction between roll and belt and found that the best method to reduce noise was to machine and balance the rolls. Then we looked at the steel selection, quality, thickness of the tube wall as well as its treatment and coating. In this way, we were able to achieve significant noise reduction when measured from a 3 metre distance.

To initially prove the effectiveness of the new low-noise rolls, Alcoa installed 12 complete modules in a major overland conveyor. Performance was closely monitored over a 12-month period. Independent technicians undertook a study to collect sound measurements. They tested both conventional rollers as well as modules with Prok's new low-noise rolls. Tests were taken under full load conditions at 6.5m/sec. and 2,600 tons of ore per hour.

When measuring the sound and vibration of the untreated rollers 1 meter away from the module, the noise level registered 86 dB(A). At a 3-metre distance, the noise level was 82 dB(A). At 7 meters, the level was 78.5 dB(A). A noise level was measured from Prok's new low-noise rollers at the same distances, results showed 80 dB{A) for 1 meter, 67 dB(A) for 3 meters, and 65.5 dB(A) for 7 meters. For all distances, these were significant noise reductions. As you are aware what may appear to be a small reduction in actual noise level numbers represents a huge reduction in noise levels. These reduced noise levels will assist many operators in meeting the new regulatory requirements being introduced in Australia.

I hope this information is of benefit to many.

Paul Attiwell

Group Engineering Manager

Sandvik Materials Handling ■

Dear Paul,

I respect your points on machining. THe weld seam bead can add about 6-10 dba at belt speeds of 6.5m/s with a 152mm roll dia.

I acknowledge the need for balancing to control belt vibration. Can you explain how imbalance generates noise in idler rotation not including the belt? Do you believe that the imbalance produces some impact or slap effect. If so, is this discernable without a belt? If I make a jig and spin the roll at its design speed will there be a measurable noise difference between balanced and unbalanced?

Your noise comparison does not appear to obey sound laws - the attenuation seems to be off for still air. Maybe there are other influences.

You have not addressed other very important values:

1. idler diameter and spacing.

2 belt width, mass and tensions

3. where the noise was taken with respect to the conveyor's geometry.

4. wind direction and velocity beyond 1 meter

5. belt vibration beating the air, especially on the return

6. carry & return idlers become homogeneous to the instrument and measurement distance increases - greater than 1 meter

Have you investigated the noise difference between having an idler spacing change with all other factors held constant?

We measured 152mm diameter idlers at 6.5 m/s. THe values at 1 meter were 76-82 dba with only 650 t/h. If this noise eminates from idlers placed at 2 meters verses 4 meters, you will have a significantly different noise reading at 1, 10 and 100 m. ■

Larry,

All our measurements on noise have been taken in conjunction with the belt. We found that using a stand alone jig and measuring noise gives no beneficial results.

I do believe that the imbalance does generate a sound pressure change due to some form of belt flap. We have measurements of many different type of rolls machined, unmachined, balanced and unbalanced and there is definitely a decrease in sound level with balanced rolls.

All the noise measurements made were on site and under the influence of environmental conditions at that time. We could not guarantee a still air scenario.

We did address other important values such as idler diameter, spacing, belt and roll materials etc. These results we are not willing to publish at this stage as this is a commercially sensitive subject for many in the industry and we would like to keep a competitive edge in this area.

Paul Attiwell

Group Engineering Manager

Sandvik Materials Handling ■

Larry, Paul,

The roll has an inherent noise dependant amongst other factors on bearing assembly tolerances and bearing internal clearances. I have not undertaken extensive testing but would presume that the reduction in noise in 'balanced' rolls is due to the reduction in the 'inherent' roll noise.

The major effect remains the interaction between belt, roll face and resonance within the idler base and conveyor structure.

Regards,

Adi Frittella

AFRIPP PROJECTS CC ■

Hi everyone..

If the duty allows, then I reckon the plastic roll is by far the quietest option.

I have three significant overland conveyors using plastic rolls and the difference is amazing.

The dual carry conveyor of ours uses 152 diameter series 30 ones supplied by my good friend Adi, and their seals squeeked like a thousand pigs for the first week, After that though they ran so quiet it was not easy to hear if the conveyor was running.

They seem much more concentric than steel ones too, so you don't get the belt flap.

Cheers

LSL Tekpro ■

Dear Larry,

You knowledge on conveyors is exceptional and your willingness to share your views and experiences is very commendable.

I thank you for all you offer to this forum.

I don't have any idlers in overland conveyors so I could not discuss about the noise generated in that type of environment.

In converting equipment where you can have hundreds of idlers in very small spaces rotating at relatively high speeds the idler generated noise is a big issue.

The surface characteristics of the shell, the connection of the shell to the bearing holders and the unsupported length of the shell are the main generators of noise.

I build an idler that we call raised spiral which is very reliable and easy to maintain. The performance is good but a bit noisy even when well balanced.

I think that an unbalanced idler is much less reliable than a balanced one and at high speeds can create issues with belt tracking, higher energy use etc.

The unbalance on the idler needs to be large to cause flapping.

In some long idlers (62") I found that placing a center bearing in the idler increases stiffness, reduces the noise and since you can use lower diameter bearings, the idler rolls better.

In my opinion, placing the bearing journal in the center of the idler is a neat trick. Can it be done in the conveyor industry?

Antonio Reis

President

Vitrom Mfg Consultants, Inc

Your Process and Manufacturing Solutions

Phone: 209.834.1900

Fax: 209.834.1039

www.vitrom.com ■

Dear Mr. Reis,

I am humbled by your comments and thank you for respect.

Regarding idler noise and belt flap pertaining to roll balancing, I have the following point. The imbalance may induce a centrifigual oscillation that produces a flap effect. Higher speeds (v>6m/s) make impaance more important KE~ (m)(v^2). This is not the main flap inducement for most belts operating at or below v=4m/s.

I believe the main influence of flap has to do with the natural frequency of the belt span support (idler spacing) and its ratio of the roll rotating frequency. We must be concerned with the four first modes. The highest damage is produced at the first mode ( half sine wave over one idler space) which produces the highest oscillating amplitude and vertical acceleration force of the wave (flap).

The belt flap is excited by the change is roll surface speed from roll eccentricity change (irregular diameter, weld bead, and axial runout) and rotating mass change (balance). These effects cause a belt axial acceleration and decelration per each revolution that can cause a system syncopation. System syncopation is like the soldiers on the bridge marching in cadence. You get many rolls coming together to produce the imbalance at the same angualr point in their rotation which results in a long axial response in the belt (longitudinal feeder belt). This can, in extreme cases, cause the product to coalesce into axial waves with their wave profile peaking tranverse to the belt axis. We have pics of this.

THe effect is exacerbated by sympathetic supports (idler transoms, stringers and related supports). that vibrate close to the same mode.

I hope this is stated in a understandable way. We design overlands with much study devoted to this point.

I note Mr. Spriggs comment of plastic rolls. I do respect this point. Vibration theory tells us that when you get close to a fundamental mode, white noise can start the sympathetic vibration. If the system damping is reasonabley strong, you will not get a strong fundamental vibration. This dampening is the area of needed research. CDI practices keeping the working vibration at least 3 db from a fundamantal mode. ■