Dear B K Bhattacharya

We generally consider 2 percent belt sag between idlers

A R SINGH ■

Singh Shab,

Mr. Bhattacharyay is well aware of the same.As per IS 11592 page 24 the max. limit is 2%.But it various between 0.5 to 2% of idler spacing. Then what is the optimum limit for 1200t/hr?Please think.

Regards.

A.Banerjee ■

Dear Mr Biplab,

For 45 deg. troughing & 1.1 M spacing max. sag you may consider 1.5%.

Regards.

A.Banerjee ■

The more appropriate value should be no more than 1%. The 2% value is an old 1950's standard. It leads to more belt flexure and power loss during normal running. It also leads to greater takeup travel and dynamic difficulty.

For short <300m plant conveyors, it is acceptable but not very wise. For longer conveyors and inclines, the 1% is a more prefered.

The incline is noted due to the compression shock wave at shutdown with the return strand tesnion wave being isolated by a gravity takeup. The compression shock wave will cause much higher sag at the tail. The higher sag leads to belt drooping at the load station that allows rock to get captured under the skirts. ■

Dear B K Bhattacharya

while selecting maximum sag you can select at tail end 1 percent sag considering tail tension and normal carring idler spacing.

At loading point impact idler spacing is generally 300-450 mm accordingly sag is automatically lower . Normaly as go towards head end conveyor tension increases and based on tension rating conveyor sag reduces towards head end

I could not follow your question about optimum value of sag

A R SINGH ■

Dear Bhattacharya,

The answer depends on the conveyor geometry, type of take-up and type of starting method.

I do not wish to speculate on the many alternative solutions to an unknown geometry.

I prefer to place the gravity take-up at the conveyor tail pulley, unless geometry or other factors (acceleration device) prohibit. THe tension at the tail is then well known, this offers the fewest pulleys, controls the belt sag tension at the load point, and has no pulleys contacting the belt's dirty side. Care should be taken to see that drive slip is controlled.

This concept is not acceptable for a geometry where there is a low point closer to the head pulley or you start the motor across the line.

Belt sag is usually worse during a shutdown with a loaded belt and power is cut. The maximum dynamic sag should be controlled to:

a) no more than 2/3 the idler diameter to clear structural members; or

b) 4% sag which ever is smaller

WIthout detail knowledge, general determination of these magnitudes requires you to calculate the sag conditions for all operating states: running state, starting, stopping (drift, braking, aborted stop .......) for (empty, full, winter, summer, worn belt ... )

This should be done with a compute developed to execute the many options, unless you have lots of time or infrequently make these evaluations. ■

Dear Nordell

your reply is very very convincing. This problem will be in long high capacity conveyors . Due to sudden power failure there will be sudden jerk and reduction in conveyor tension. In addition to fly wheel is there any other way to avoid sudden spike

A R SINGH ■

Dear Mr. Singh,

THe sudden jerk you refer to is a set of two shock waves. When power is suddenly removed, such as a motor trip, the drive tension differential (TE = T1-T2) wants to equalize between T1 and T2.

This occurs as the drive pulley decelerates. As you note, applying added inertia (flywheel) retards the rate of deceleration.

THe high tension,T1, will drop causing a compression shock wave to propagate away from the drive pulley. It travels down the belt high tension zone, away from the drive, at the speed of sound in the steel cords or fabric carcass tension member.

The low tension, T2, rises to propagate a tesnile shock wave down the low tension strand. Often, the gravity takeup absorbs the tensile shock wave with an upward jerk.

Yes there are other ways, besides flywheels, to reduce the tensile and compression shock waves. However the answer needs many qualification with respect to the application. THis is the science of conveyor dynamics. Pun intended. THis is the trade we practice to design long overland, high powered inclines and downhill conveyors.

Flywheels and other methods are practiced to not only control shock waves: but many design factors such as:

a) belt sag

b) excessive take-up travel

c) momentary high holdback impulses and higher T1 tension

d) very high belt tension along the return strand

e) loading station belt reversal that drives the ore into the skirts

f) pulling the belt out of concave curves and horizontal curves

g) overstressing the belt in convex curves

h) causing unacceptable stopping coordination between feeding and receiving belts that lead to chute pluggage, skirt damage and spillage,.....

ii) and so on....

I believe you were the VP engineering for Naveen when we designed the Birla Copper overland with Babcock. Is this true? ■

Dear Mr.Bhattacharya:

The sag limit is located at the point of minimum tension assuming equal spacing along the conveyor. It is not an average.

When idler spacing varies, the above may not be true. Idler spacing can be varied to control local sag to the maximum allowed value.

Maximum sag may not be located at the tail.

I do not understand you insistence on 2%. I further do not understand if you are lecturing me of wishing clarification. I have no more to contribute. ■

My congratulations to Larry Nordell for finally speaking out on the quality of forum content. For quite some time there has been a plethora of engineering trivia getting past the moderators. Most posts have already been addressed in long published works. It surprises me that respected experts devote valuable time to involve themselves in futile attempts to convince the nouveau bulk fraternity that conveyors etcetera have been around for quite a few years already.

I stopped taking the forums too seriously after reading a reply propagating pneumatically conveing iron ore in the interests of low wear. That is by no means an isolated example of commercial awareness clouding engineering feasibility. ■

Author/Moderator,

Sirs,

I want to draw you kind attention to the followig words use by Mr. Nordell to me in a reply.

1. Sacastic ( When the FUROM is for serious people.)

2.You do not know (When the FORUM is for exparts.)

3. The FORUM will cease member ship.

I am not in the pay roll of Mr. Nordell of CDI, USA. How can he write all these words.

Really I lost my interest because of two members.

Kindly, reply me what shall I do now.

Regards.

A.Banerjee ■

Thank you gentlemen,

Your discussions gave me a vision on idler spacing. I am sure that we do not have to stick to the tables for spacing conveyor long. As belt tension varies along the conveyor, idler spacing can vary too. The key point seems to decide which would be our allowable belt sag. This may help reducing the costs for longer conveyors. ■