Kayem:

I do not claim to be an expert in this area. I believe the tools are available to establish reasonable criteria. I do claim to have a better than average knowledge of polymer applied mechanics and would be able to setforth a procedure for studying the issue. Thus, I give the following comments.

I have not heard of the comment you posted regarding steel cord belt having higher speed capability than fabric belts. Intrinsically it makes some sense if one considers the difference in construction accuracy. Some manufacturers may shed light on the question of construction accuracy that they are able to guarranty for fabric and steel cord belts.

A vibration based model could be constructed, using FEM, that would demonstrate the variation in construction sensitivity with speed and idler spacing.

Criteria for high speed:

1. tensile members pull the same tension across the belt to the accuracy needed.

2. veritcal and horizontal tension member alignment "close" to ideal position again to the accuracy needed.

3. no loose edge tensions - resolve accuracy

4. Belt Modal Natural Frequency Spectrum - increases with higher speed and may place more positions around the belt at critical modes

5. idler geometry rotation tolerances and imbalance control

6. High speed is an issue in cold weather for polymers. Cold weather equates to higher speed through the WLF equation in polymers (rubber and fabrics). Strain sensivity may be an issue.

Fabric vs Steel Cord Construction:

1. Fabric material needs to have a thermal preshrink to stablize the yarn. Many manufacturers do not stablize the yarn which introduces many problems with creep growth duing operation.

2. Fabric yarn prestretch is better than non-prestretch - however, pulling equal tension across full width for each yarn is almost impossible for fabric resulting in comparitively loose edges and/or center regions with respect to steel cord errors or variations in longitudinal or transverse elongation. Watch a fabric belt during construction in factory press - the factory cannot maintain the perfect warp and weft weave yarn orientation and position, during pretensioning. Compare the difference in pretensioning between fabric and steel cord. Thus, with fabric, the belt exhibits more edge flutter (lower tension region) that can be trace to a position in the belt. Center sag can be traced to tight edges and loose center yarn tension. Since these attributes are position repeatable and interspesed, there is a higher propensity to induce critical vibration exitation.

3. Manufacturers usually do not build fabric belts to the same tolerances as steel cords. This may be possible if the demand was there.

4. Although the fabric belt does have more 3-D flexure and axial elongation than the comparable steel cord for the same strength, how important is this issue? Is this flexural fatigue really sensitive in the belt life spectrum?

There are many other design issues that need to be in harmony in order to operate at high speed (> 6 m/s). Certain fabric belt construction errors may be some-what overcome if the belt width, matrial loading, idler spacing, idler roll diameter size, and roll construction tolerances are properly selected and the belt construction tolerances meets the same speed criteria.

Is this statement you note documented in published literature? If so, who and where can it be found.

Success in the New Year

Lawrence Nordell

Conveyor Dynamics, Inc.

www.conveyor-dynamics.com ■

Dear Shri Kayem,

The reply can be some what subjective. It would be better, if you can indicate speed range which you have in mind, and also salient information such as material, lump size and belt width.

Long conveyor can be operated much faster compared to in-plant conveyors, due to technical and economical reasons. The steel cord belts are very widely used for long conveyors or conveyor which have high tension in belt. Therefore, it creates an indirect impression that steel cord belts run faster.

The answer also depends upon the belt ability to accept operational punishment, according to specific case.

Regards

Ishwar G Mulani - Author of book: 'Engineering Science and Application Design for belt Conveyors'

Email ID: parimul@pn2.vsnl.net.in ■

Dear Shri Kayem,

I too have heard the statement, loosely made, but I have never heard anyone take a stand on this issue.

Belt speed is driven by one thing only. The desire to reduce initial cost even at the expense of increased annual operating and maintenance costs. In the case of a long overland conveyor a very long cycle time justifies the former mitigating the latter. In the case of long boom stackers and belt wagons the mass, thus cost, of the machine is directly related to belt width which is inversely related to belt speed. Like Mr. Mulani, I believe the association of higher belt speeds with steel cord belting is coincidence. The high investment systems, having great pressures to reduce cost (increase speed) are more likely to use steel cord belts.

It can be said that steel cord belts, because of higher elastic modulus, require greater precision to insure good load sharing between the cords and are therefore more precisely manufactured. The properties of steel are also more predictable.

Fabric belts may have hard spots or crimp, due to manufacturing or storage and handling. Slit belts will tend to be bowed laterally. This may be why, generally, fabric belts are harder to train initially. On the other hand, because of the fabrics elongation, these belts will tend to work out the anomalies after running under load.

This writer has worked on and inspected various high speed belts, both of steel cord and of fabric construction. I have not noted general differences in their behavier. Tendencies to excite oscillations are due to the concentricity, roundness and balance of the rotating components, predominantly the idlers. ■

Comment on Mr. Dos Santos statement:

"Tendencies to excite oscillations are due to the concentricity, roundness and balance of the rotating components"

While it is true that irregular shaped idler rolls will tend to excite oscillations, the belt's natural frequency or fundamental vibration modes is the mechanism that will amplify vibration between idler spacings and will vibrate at its fundamental frequency regardless of idler shapes.

Designer do study the coupling of fundamental vibration modes with the spin frequency of the idler roller to keep the two from overamplifying the belt's vibration at or near its natural modes. Designers also must evaluate the idler frame design and support system. Spacing is often staggered to minmize long axial vibration coupling of many idler positions along overland systems.

As I eluded to in my earlier above comments, errors in fabric belt construction may lead to enhanced vibration that could have consequences on the belt's reliablility and/or the idler's life.

As the belt speed increases, eventually, more vibratory modes will be crossed and/or the belt will operate where the idler spin frequency coupling cannot be avoided.

Lawrence Nordell

Conveyor Dynamics, Inc. ■

Larry,

Thanks for your comment. I wasn't going that deeply. Vibrations induced by the belt's natural frequency would produce noticeably different behavior in steel cord belts compared to fabric belts because of their differing modulus, that is, differing pulsing sound and its frequency. I don't doubt your analysis at some level but an inbalanced rotating object does not need additional help to induce vibration at a frequency which is its RPM. Natural frequencies in that range will be excited, be it belt, local frames etc. The like behavior that I mention has a pulsating sound that results after substantial run-in and the idler rolls tend to adjust naturally to bring their inbalances into phase. This frequency can and does excite local structure.

In a somewhat related case in 1984, the first commericial HAC at Triton Coal Company, running at 5.33 m/s (1050 FPM) (with 6" idler rolls) experienced a pronounced and dangerous low frequency lateral vibration at the support bent, about 24.4 m (80') high as I recall. The bent's natural frequency was in phase with the RPM of the tail take-up pulley, which ofcourse floated along on its take-up carriage. Indeed a small inbalance of the pulley was able to propogate through the belt to the front of the structure where the vertical bent supported it. We fixed this by welding tees onto the column flanges, increasing depth, stiffness and reducing its natural frequency decisively out of sinc with the tail pulley RPM and any other possible inducing frequencies that we coud think of. ■

Joe,

Your expertise in structural vibration theory and measurements, as well as your understanding of differing moduli is/was not questioned. I bow to your expertise.

My comments referred to such elementary examples as looking at belt flap (pulsing sound or beating of the wind). After moving the idler assembly spacing a small distance, we observe the belt flap disappears (no more pulsing sound) with no change in idler rolls. Seems to obey vibration theory of string (belt) - close to it.

We calculate the fundamental modes of the belts natural frequencies and make sure to place the idler spacing, spin frequency and belt tension outside of this condition if possible. The idler irregularities exacerbate the problem and we try to keep the belt modes and idler spin from coupling.

Don't question the notion pulleys can excite the belt and structures. White noise can start resonance of structures if they are sympathically inclined with the belt, idlers, et. al. Can cause mountains from mole hills - material on belt.

Larry ■

Dear Shri Kayem,

Further to my earlier answer, I would like to add that for practically used common belt speed, (say up to 5.5 mps), there is no distinction (or preference for one over the other) whether belt is NN or EP or steel cord. When the belt is selected by design, all requirements for impact capability, proper size of pulleys against bending etc. are already fulfilled for each type, and the belts are in a state of equalized technical evaluation on such counts. Subsequent to this, the dominant issue is price, maintenance ability and preference of user.

This answer is in context of belt speed only. Each type of belt can have its own merit / demerit. For example, steel cord belt needs smaller pulley, but for that other belts are already penalized by using bigger pulleys in design, and thereafter again to give +/- consideration to one or other is duplication.

For unusually high speed, only belt manufacturer data / opinion about carcass flexure fatigue life, as the belt moves on idler, will require consideration, and should be applicable.

Also if the temperature is unusually low / high, the belt manufacturer’s data about carcass flexure fatigue life at that temperature, as the belt moves on idlers, will require consideration, and should be applicable.

The manufacturers publicly committed / guaranteed data do not cover exceptional / unlimited condition. For exceptional situation, one may find reliable information from belt manufacturer, for belt behaviour.

Regards,

Ishwar G Mulani.

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

Email: parimul@pn2.vsnl.net.in ■

Mr. Kayem / Mulani:

I have to add this note to which Kayem suggested in his intial question and to which Mr. Mulani has replied.

Fabric belt undergoes a significant added flexure and hysteresis that steel cord does not. The center-to-wing idler junction deformation strain field is significantly higher with fabric reinforced carcasses. This can be easily tested. The power equations tell us so as well. This higher stain produces more work, heat, and fatigue in rubber and fabric polymers in such belts.

Measurements of fabric and steel cord belt power consumption, applied to the same conveyor, are strong markers. The fabric belt can pull upwards of double the power of a steel cord which we have witnessed. So much more of the belt is under going deformation, in a fabric belt cross-section, especially in the conveyor's lower tension region.

A steel cord becomes axially very stiff as tension is applied. The wire strand helical spin is arrested as its radial compression locks the wire strands by friction. This increases the bending sectional modulus many fold over the simple wrapping of a loose cable. Not so with a fabric belt, at least the magnitude differs to a significant degree.

To calculate the power consumed by a steel cord belt, we need, and do test this stiffness effect, by jigging the cable in question, in an Instron test machine, and pulling it axially to measure the axial and torsional coupled response. This becomes an important part of the steel cord power analysis and splice dynamic fatiue strength calculation. Further to this, a section of constructed belt is placed on a jig where the axial (sag) deflection is measured in four positions - zero and 90 degree orientation for top and bottom cover up, Three jig spacing positions are also recorded. A deflection influence function gives us a equivalent stiffness to check against our theory of the composite construction response.

Sorry for the verbose response. The theorectical procedure is complex. It has taken many years to gain a reasonable understanding on the polymer rheological (viscoelastic) power and strength equations. I believe the above comments suggest that a fabric belt will reach limitations in rolling efficiency and fatigue strength that ultimately will inhibit its use in high speed (>6 m/s) applications.

I do not claim this prognosis will stay true in the future of fabric belt construction, but field measurements do tell us it is true today.

Lawrence Nordell

Conveyor Dynamics, Inc.

website: www.conveyor-dynamics.com

email: nordell@conveyor-dynamics.com ■

To my above latest coments, it may not be clear from the posted description, two flexural regions contribute to the added fabric belt rolling losses. The idler junction is there but its contribution is secondary to the axial plane vertical and transverse deformations. This leads to three power consumption points.

1. Idler Roll Indention Resistance: fabric belt more significantly conforms to the idler shape or roll diameter. This conformance introduces more strain and hence more hysteresis or rolling loss. Review publications on viscoelastic power equations published in Bulk Solids Handling with references to : yours truly, Prof. Hager and his staff at Hanover University, Prof. Spaan of Delft Univ., and others.

2. Belt Flexural Resistance: reverse bending of the belt at and between idler stands. This includes idler junction losses. Again review Spaan's work.

3. Material Trampling Losses: This is the ore agitation loss as a function of Belt Flexural Stiffness in tha axial and tranverse planes.

We have advanced points two and three beyond published works to understand the contribution of the coupled flexural and trampling losses. These have to be simutaneously solved. Here we see another significant difference between fabric and steel. In steel cord this can consume 7-10%. In fabric, it is much higher, depending on the weave and ply construction and idler configuration.

Lawrence Nordell ■