Mr. Butterworth:

There are a number of possible solutions to the impact problem. The one proposed by David Beckly should work for your client, as it provides greater belt impact support than slider beds or garland type impact rollers.

If possible, it would be better to reduce the impact by changing the chute arrangement, by using a grizzly, or by using a sacrificial collector belt or impact slide that would feed the production belt at a reasonable rate.

I cannot advise on the best solution without seeing the operation and discussing what the client will (or will not) accept. If you would like to explore options, please contact me.

Regards,

Dave Miller ■

Rob,

All comments in the replies so far are valid but I don't believe these answer your question as I understand it. Are garland idlers, at the impact area, superior to impact bars?

I believe that they are far superior and will improve the impact absorption.

There is not a concensus on the merrit of impact bars for the purpose of impact absorption. In fact they are more commonly called slider beds. Everyone agrees that they are good for affecting skirtsealing because they elliminate the belt sag that is common between impact idlers.

Energy (or work) is the product of force and displacement. The five roll garland idler is an ideal impact absorber because of its ability to change shape and undergo large local displacement when impacted by large lumps. We have designed garland idler impact areas with special seal skirts that work well with large deflections because they ride the belt and flex to follow its deflections.

The writer has studied the mechanics of impact at large rock belt feeders for run-of-mine (as blasted) material. Impact must be absorbed by components of relatively low mass (low inertia), at the belt and at the rolls. Grouping multiple rigid idlers onto a heavy frame which is then mounted on springs or shock absorbers is largely inaffective because the large inertia will not allow the immediate deflection and a large impact force results.

Rigid impact idlers absorb impact energy in the deflection (compaction) of the rubber discs. Rubber discs are not required at the rolls of the five-roll garland idler. ■

Dear Sir,

Many of the comments are applicable. Obviously reduction of lump size (by grizzly), introduction of rock boxes and modified chute geometry will result in reduced impact loads. As regards the question of impact bars vs garland idlers I believe that Joe Dos Santos has responded most effectively. The 5 roll garland impact idler is perfectly suited to this application. Obviously the bearings need to be carefully considered and the use of spherical roller bearings (high rating and capable of operating under high shaft deflection levels) as suggested elsewhere may be required. The tricky part comes in ensuring that even in the deflected condition an adequate seal between skirts and belt is achieved. The garland string can obviously be made even more flexible by the introduction of special mountings onto the steelwork - springs, rubbers and 'Rosta' type end fixings have been used effectively.

Regards,

ADI FRITTELLA

MELCO CONVEYOR EQUIPMENT

adif@melco.co.za

web-site: www.melco.co.za ■

Dear Mr. Butterworth

Adding to the list of respondents:

1. The damage you experience can be modeled mathematically using discrete element method(DEM). See references below. As such, the impact and shear behavior of large lumps on the belt can be quantified with respect to garland, rigid frame ( rubber covered rolls or not), and all forms of slider beds, grizzlies, rockboxes, and curved chutes. The cut and gouge factor can be ranked from none (below the tensile fracture capacity of the rubber cover) to varying degrees based on the rock impact energy, belt construction, idler system, and idler support system. This provides a NO GUESS method. There are many configurations that reduce the damage. There are also better and less so methods. We design high impact hard rock transfers using this method with excellent success. See the referenced pulblications as a preview. These will lead you to others.

1. "Conveyor Belt Transfer Chute Modeling and Other Applic. ....."

by David Kruse, in SME publication "Bulk Material Handling by Conveyor Belt III

2. "Particle Flow Modeling: Transfer Chutes and Other Applic. .."

By Lawrence Nordell, BELTCON 9 conference RSA 1997 ■

My opinion from experience is that wherever impact occours don't use impact beds. This may sound odd but impact beds in my opinion are only good in loading areas where you don't want spillage. A well selected rubber impact roll far out performs impact beds. As one other person said, impact beds cause the belt to be pinched and cut from impact of material falling.

R.Babin ■

I can share my bad experience on this matter.

Once we replaced impact idlers by impact bars

on a conveyor fed by number of centrifuges.

Wet potash penetrated between the belt and

the bars.

As result, after any stoppage, the belt was

practically "glued" to the bars.

Re-start the conveyor was possible only after

profound cleaning the bars, so we had to come

back to the idlers installed very close one

to another.

Regards

M.Rivkin ■

In the last several weeks we have looked critically at the sliding resistance of impact bars (slider beds). It turns out that they don't slide very well either. On a particular project, the troughing idlers of a long slot belt feeder were replaced with slider beds. We were contracted to analize this after the conversion was made and the drives were stalling. Sliding friction at the belt to slider bed interface has a static coefficient of about .5 and a dynamic coeficient of about .33 even for your slickest polyethelene (UHMW) covering. It is higher yet for urethane. This, subject to the material head in the hopper above caused an approximate 30% increase in driving tension and power.

There have been other cases (at high speed belts) where the increased friction caused such heat, under the load, that the belt became fused to the slider bars.

Slider bars are good for sealing and should only be used at the upper half or third of the wing roll where the material load is minor.

Joseph ■

In the last several weeks we have looked critically at the sliding resistance of impact bars (slider beds). It turns out that they don't slide very well either. On a particular project, the troughing idlers of a long slot belt feeder were replaced with slider beds. We were contracted to analize this after the conversion was made and the drives were stalling. Sliding friction at the belt to slider bed interface has a static coefficient of about .5 and a dynamic coeficient of about .33 even for your slickest polyethelene (UHMW) covering. It is higher yet for urethane. This, subject to the material head in the hopper above, caused an approximate 30% increase in driving tension and power.

There have been other cases (at short, higher speed belts) where the increased friction caused such heat, under the load, that the belt became fused to the slider bars.

Slider bars are good for sealing and should only be used at the upper half or third of the wing roll where the material load is minor.

Joseph A. Dos Santos, PE ■

This sounds like a severe application. If the original support bars lasted 2 years this may be the best solution. You may want to simply install new replacement bars.

There are also extra heavy duty impact systems on the market that would increase the life of the impact system. ■

If we can agree that the 5-roll garland idler is the best for impact (because it deflects locally under the local impact load) then it is a matter of effectively sealing the system. We have done this successfully using a thick rubber skirting that lays on and flexes with the belt movement. Additionally, Martin now offers a local slider board that connects between the wing rolls, at the upper area where the pressure is low.

Joe Dos Santos, PE ■

Two years ago CDI designed, installed and successfully commissioned, at Palabora mining company in South Africa, a new primary jaw crusher -300 mm x 2600 t/h hard rock loading station, that has the following beneficial features:

1. Slider bar dust seals fixed to idler frame that allow the belt to articulate in its trough plane. Slider bar principle function is sealing dust outside the impact zone. Slider bars are continuous on the wing idlers above the impact sealing zone, segmented and easily removable. They see no direct impact.

2. Impact idlers that form a fixed trough ( no catenary for the above reasons stated by others) Impact idlers are fixed to a pipe frame that is set on rubber impact bushings which absorb heavy shock while idler impact rubber disks absorb light shock. THe design was modeled by Finite Element Analysis (FEA) to prove the concept.

3. Impact idlers are afixed, in pairs, to a frame the can pivot out of belt contact for easy inspection or replacement. Either or both sides of impact roll assemblies can be dropped for removal of belt contact till maintenance shift can gain access. The underside is free of structures for easy maintenance and cleanup access. The impact roll assemblies have lever arms and cam connections at the pivot for good mechanical advantage in lifting and lowering the assemblies.

4. The noted features are integrated with the skirtboard design and chute geometry.

We offer the service to design this for others.

Lawrence Nordell

Conveyor Dynamics, Inc.

website: www.conveyor-dynamics.com

email: nordell@conveyor-dynamics.com ■

This may already be stated elsewhere but as important as the ability to absorb impact and to seal the transfer is the ability to control the material transfer so that impact and turbulance are minimized. WEBA high tech transfers, using the cascade and super tube designs, accomplish this by controlling the material's speed and direction through the transfer so that the material doesn't build excessive speed and exits the chute at very nearly the speed and direction of the outgoing belt. Impact is minimized and a sealing system is not required. Indeed WEBA chutes operate successfully and cleanly without skirts or seals. Check out the WEBA high tech transfers at the M&J Engineering website, http://www.mjeng.co.za/

Joe Dos Santos, PE ■

Dear Mr. Butterworth,

As for your impact problem, there could be different solutions, but I am more in line with Mr. Joseph A Dos Santos and Mr. Conrad Kelley.

You have mentioned impact of 5000 ft-pound. I presume your figure signifies energy. When the lump impacts, the system deflects (yields). The value of impact force for typical figures will be as below.

For 6 inch deflection : 5000 = 0.5 x F x 0.5 i.e. F = 20000 lbf.

For 2 inch deflection : 5000 = 0.5 x F x 0.166 i.e. F = 60240 lbf.

The primary rule is that the impact energy cannot be absorbed without deflection (displacement) of reaction surface. Therefore, my suggestion would be to look for :

1) 5 roller garland idler can yield for large deflection

2) Reduce impact coming on belt, by putting heavy impact shelf just over skirt board. The material should fall on the material accumulated on shelf, and then slide / roll on material surface to land on belt.

3) Shelf can have slots (grizzly bar type shelf) so that there is some cushioning layer of smaller size material prior to impacting lump.

Meanwhile, your mentioned figure of 5000 ft-pound looks to be erroneous. 14 inch lump with lump density of 2.0 tonne/m3 (125 lb/ft3) will have a weight (mass) of 198 lb considering cubical shape (which is rare). The 10 feet drop makes impact energy 1980 ft-pound instead of 5000 ft-pound. Please check.

If we have prior impact on shelf with 50% energy dissipation, the lump will be landing with approximately 1000 ft-pound energy only.

Regards,

Ishwar G Mulani.

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

Email : parimul@pn2.vsnl.net.in ■

Further to my earlier response, I would like to add following information.

Regarding 5-roller garland idlers, one generally uses deeper troughing at loading point. The impact reaction force is occurring in central portion. The impact creates large deflection in central portion where it is acting. The idler portion near suspension points have much less deflection. This enables to maintain reasonable sealing between belt & skirt plates, by wide rubber flats, which are pressing by their elasticity. Also, this contact between belt & rubber flat is kept at higher level. Such arrangement is working at Neyveli-India open cast mine where lignite lumps up to 400 / 500 mm are falling from about 10 feet height (discharge by excavator high speed boom conveyor)

This information is neither in favour or against any particular arrangement. It is only for the readers information.

Regards,

I G Mulani.

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

Email : parimul@pn2.vsnl.net.in ■

Mr. I. G. Mulani,

Indeed the skirt arrangement that you describe is similar to what we used at an 84" (2134 mm) wide shiploader boom conveyor (belt speed is 911 FPM= 4.62 m/s). A thick rubber skirt having ample flex resisance bears, broad-side, onto the belt at the upper wing area and is able to follow the belt movement while maintaining the bearing seal. Though we originally proposed and preferred using a deep trough, with 5-roll garland idlers at the impact zone, radius of curvature constraints (the boom luffs at a forward point requiring a convex curve that becomes ever tighter at the lower luffing positions), clearences and transitions precluded this. We used 3-roll garland idlers troughed at 35 degrees. This worked great.

Joe Dos Santos, PE ■

Martin Engineering has developed CAT 5 Impact system for using catenary idlers in high impact applications that over come the disadvantages of these rollers. I agree with the imput that catrenary idlers are much better suited to absorbing impact but they do present other problems. The two primary drawbacks are inablilty to effectively seal the transfer point and increased belt tracking problems. Our system uses HD slider blocks on the edges with catenary rollers in the center. The impact energy is calculated and properly sized elastic restraints abosorb enough of the force to limit the movement of the slider blocks. This sytem is in use in Europe in lignite mines on high speed, high tonnage steel cord belts. ■