You may have painted yourself into a corner. The yard layout must be according to the required concave curve for a good conveyor design without uplift. A hold down roll does not prevent uplift it just limits how high. Though uplift rolls are ill advised they are regularly used. It reduces the cost of the trailing structure but it is false economics. A proper tripper trailer supports the belt in the case of the locally empty belt subject to maximum tension. A convex curve at the entrance to the tripper structure allows the approaching belt to approach the tripper structure at a tangent to the convex curve regardless of the local belt tension and material load. This too is missed by too many suppliers.

Joe dos Santos ■

Get it right. A hold down will not prevent lift off. I will merely restrict lift off.

Desperate firms recommend them to encourage a punter to believe that the stockpile he wants can actually be achieved. Then it spews material around like a cow with diarrhea.

Hold downs are popular along North Sea type installations where wind can lift nearly any empty belt to the heavens and the hold down catches the belt and prevents it from getting blown into the path of eg. a bucket wheel reclaimer working the adjacent pile. To put it another way, there is an intermediate zone where the correctly designed empty belt can still part company with rollers due to aerodynamics. Wind boards help in such cases.

If you punter is sensible he might consider inserting inclined and skewed conveyors after a short tripper. It's easier and there's plenty of room. We sometimes have kneeling trippers elsewhere and this suggestion is far simpler. Why didn't I come up with this when I was in my pup?

Incidentally, length is not your issue. Height is. Consider how much capacity would you gain by risking a faulty curve against building a higher shorter pile.

Get your punter to evaluate the cost of spillage clean up against the price for retaining walls.

I wish I could insert an emoticon....... ■

I agree with both the replies to this thread, it is not a very good idea at all but there are ways you can shorten the concave curve without creating lift.

> Dave Beckley quite a few years ago wrote a paper wherein he described the methodology of inserting a convex curve in the middle of the concave curve and this significantly reduced the edge buckling and the flattening out of the belt. We have used this principle successfully so we can attest to efficacy of this method.

> You can try to minimise the counterweight tension and at the same time increase the belt mass. Trial and error calculations are required to optimise the options

> You can manipulate the geometry in a way that has the effect of reducing the concave curve length, basically creating two tangent points rather than one. Sometimes works.

A hold down roller can be used to create the third option above but it does require some finessing as hold down rollers crudely installed not only create all sorts of maintenance problems but also limit the volumetric carrying capacity as the belt will flatten out through the curve, i.e. not hold its troughed shape

Cheers

Colin Benjamin

Gulf Conveyor Systems Pty Ltd

www.conveyorsystemstechnology.com ■

Hello,

Your primary requirement is to increase machine operational travel length. This can be achieved by following features, presuming you have unidirectional yard conveyor.

1) The yard conveyor tail end can be made inclined by say 7 to 8 degree for portion tail end to skirt board length + about 3 m. If the required concave radius at this point is say 250 m, it will increase the machine travel length on this side by R.sine(inclination) i.e. 250 x sine7 = 30 m, by deletion of this curvature length. This will amount to about 0 m level for tail pulley top and about -1.2 m deep trench floor. This will introduce some extra cost for civil work at tail end.

2) Thereafter the yard conveyor can be kept straight horizontal till head pulley. No inclination at head end, and outgoing conveyor will be in tunnel of about 3 m depth. Yard conveyor without head end inclination is standard feature in most of the state and national electricity board power station in India since long time, in consensus with prime consultants in India, to get more length of stockpile. This will also introduce cost for civil work for outgoing conveyor but some saving in steel work for gantry and conveyor enclosure on outgoing conveyor.

3) Avoid any transfer tower at head end. Utmost one can have squat removable shed on head end or just weather cover on drive (reference practice for shiftable conveyors). This will enable you to extend machine travel at head end zone, due to absence of boom collision with transfer tower.

Above serial numbers 1, 2 and 3 will result into maximum possible length of stockpile, in available space, but add some cost. Well nothing is free.

Some information is as below about the yard conveyor and S/R machine as a system, concave radius and hold-down pulley. This will be helpful to you to opt for functionally trouble-free system.

The yard conveyor and S/R machine constitute a system, wherein S/R machine and conveyor should be designed as a complementary equipment to each other, to get best result. The issue revolves around the carrying run belt concave radial zone at interface of yard conveyor and S/R machine.

The concave radius formula is same whether for stationary conveyor or at interface of machine and yard conveyor. In case of stationary usual conveyor there is enough space, and hence one can choose radius so that belt will not lift-up in any situation as a general rule and also freedom to design conveyor as per liking. However, in case of yard conveyor, the S/R machine cost put reasonable limitation for the radius to be used. Therefore it becomes imperative for conveyor designer to design conveyor such that the required radius is least. In addition some compromise radius is chosen, allowing certain lift-up of belt some time, but these are technically analysed for acceptability.

A) Conveyor design for least radius

The required radius decreases by

-Reduction in belt tension.

-Increase in belt mass.

So conveyor is to be designed for least tension. This can be often achieved by dual pulley drive at head end. In addition one can opt for scoop coupling or VVVFD drive for very gradual starting of conveyor. The general reduction in tensions also imply tension reduction at concave radius.

The belt mass is to be increased by thicker covers or steel cord belt. Ideally belt mass to be considered for worn-out condition (say 80% of mass).

Presuming you have unidirectional conveyor, the worst situation at radial zone will be for 1) Machine near head end. 2) Empty belt at radial zone. 3) 100% material load from tail end to entry point of curvature. 4) Starting status.

The compromise radius can be calculated for above by considering about 15 to 20% material mass presence on concave zone. The construction radius (as per this or other) is analysed for belt lift-up and take-up behaviour for varied operational situation.

In reality S/R machine manufacturer proposes the machine tripper construction radius, and conveyor designer analyses for its suitability, and also for the complementary design of conveyor, as a 2-way communication, for mutual consideration, support and agreement from both side.

The radius so selected is analysed by the conveyor designer for further check-up about acceptability by finding tension at the applicable points and resulting belt lift-up, take-up behaviour, etc. for varied operational situation (empty, part load, full load, starting, steady running, stoppage) of the conveyor.

Hold-down pulley is not a magical remedy to aforesaid requirement. The hold-down pulley can be actual pulley or some time it has two wheels (with flat rim) fitted on a shaft supported by 2 plummer-blocks. The item has provision to adjust the height from the belt. The belt will come in contact with the hold-down pulley during lift-up beyond this margin. This will not be pleasant for belt, and it will also amount to material spillage. Hold-down pulley as a safety measure after properly selecting the radius is okay, but not as a substitute to aforesaid technical requirements.

Regards,

Ishwar G. Mulani

Author of Book: Engineering Science And Application Design For Belt Conveyors (new print November, 2012)

Author of Book: Belt Feeder Design And Hopper Bin Silo

Advisor / Consultant for Bulk Material Handling System & Issues.

Pune, India.

Tel.: 0091 (0)20 25871916

Email: conveyor.ishwar.mulani@gmail.com

Website: www.conveyor.ishwarmulani.com ■

Dr. Mulani,

Your description and design guidelines are well summarized. I must admit form my experience I find it unacceptable, at major tripper structures to tolerate uplift accept in the extreme case of a hard starting system at max tension with an empty belt. As I said in my previous post, compromise criteria that requires the belt to sit down at a percentage of load is false economics. But much worse than that is the failure to recognize the correct uplift radius and worse, the convex entrance radius. I have had to referee the owner's interest with bad tripper design. A very common and very wrong design utilizes a straight incline at the upper end, a lowering of the incline (in segments) towards the tail with an adjustable tail suspension that attempts to nest the lowest tripper idlers into the yard belt idlers. There is typically no convex entrance curve. This of course results in an abrupt kink of the belt line that buckles the belt's middle on the tail idler and crowds the belt into the idler juncture. The resulting belt juncture failure is typically, erroneously blamed on the belt construction but it is in fact the result of poor understanding of the radius of curvature requirements including continuity of the belt path without abrupt kinks.

Joe Dos Santos ■

Putting a conveyor in a trench in monsoon climates is asking for trouble. Declining the tail end will direct rain run-off that way and increase the water content of the product if left unchecked. This makes the recommendation for a head end drive cover rather superfluous. Why? Because the belt is going to slip like a banana skin and wear away in sympathy thereby inviting frequent replacements. Do you intend putting the GTU in a pit, for Krishna's sake.

At the drive end of a yard belt you can do almost anything because, without a kneeling tripper, the belt doesn't carry anything so talk of spillage is a waste of time and the lift is not so great either so the concave curve radius on a lifted belt along that zone is very easy to accommodate.

Since a lot of coal imported into India is quite wet enough already and the rest has been soaked locally perhaps sinking a trench is not going to affect the coal quality all that much. Elsewhere questions would be asked: regardless of the consensus of prime local consultants....almost anywhere.

Most respondents to this thread have already strongly advised against a hold down. However, if a hold down became incorporated into this system, with its very wet coal, there would be no windblown dust, just a trail of black mud in the trench. So you need a roadway for a Bobcat with entry and exit ramps to recover the mud which you will undoubtedly lash back onto the belt. Unless you slope the trench floor quite drastically: better make it a roadway along each side. Don't forget to increase the boom length while you're at it: always assuming your client has enough width in his poxy little stockpile.

The primary requirement is not to increase travel it is to provide stockpile capacity. Or at least it should be. Play around with retaining walls and see what you can come up with. There is no need for further discussion here. you have been told repeatedly that a hold down is worse than useless and have failed to recognise your fundamental capacity shortage. ■