Sandwich Belt HACs Broad Horizons - 1992

J.A. DOS Scintos, USA
Courtesy : Trans Tech Publications - Bulk Solids Handling Journal


Developed in 1983. High Angie Conveyors - HACs, are proven versatile systems for elevating materials continuously from a lower to an upper level. Such systems may also be used to lower materials continuously. HACs have found commercial application in widely varying materials (including coal, refuse, coarse copper ore (-250 mm), municipal sludge. gypsum, slag. limestone, phosphate rock, hot clinker, excavated silts, sands. gravels and clays, wood chips, and, various grains), at widely varying throughput rates from 0.27 to 4.000 t/h. The conveying profiles have varied widely and e)evating heights range from 3.66 m to 175 m.

HACs have found application in coal and copper mining, coal preparation, in municKoal waste treatment, rapid transit prolects. pulp and paper mills, cement plants, self unloading grain ships and port facilities.

While noting a wide variety of applications in general, the fatter part of this article cites various applications in continuous ship loading/unloading, storage and transfer yard handling, midstream transfer and blending, elevation from underground hoppers and tunnels, preparation plants etc.. demonstrates the versatility of the concept and the broad market potential.

1. Introduction

Development of the sandwich belt high angle conveyor concept has come a long way since its first introduction in the early 1950s. Over the approximate thirty-year period, until about 1979. significant advances had been few and had only come in spurts. Such advances did not significantly build. on past developments. rather they were independent developments which soon reached their technical limitations. The latest significant development of this technology, beginning in 1979, is the first to take a broad view of the industries to benefit from high angle conveyors. and of all significant developments to date. As a result, these latest developments know few technical imitations, address a broad range of applications and otter a forum for further logical development or evolution.


Fig. 1: idealized model 1 of sandwch bell conveyor

2. The Sandwich Belt Principle

The Continental Conveyor HAC represents logical evolution and optimization of the sandwich belt concept. The sandwich belt approach employs two ordinary rubber belts which sandwich the conveyed material. Additional force on the belt provides hugging pressure to the conveyed material in order to develoo sufficient friction at the and material-to-material interface to prevent sliding back at the design conveying angle.

Fig. 1 is a simplified illustration of the interaction of forces. tithe cover belt is not dnven, then the lineal force N to provide the required hugging pressure at conveyor angle a is given by the following equation:

N > Wm ([sinα / ] - cosα)

Where = m or = n  whichever is the smaller.

A more realistic model is shown in Fig. 2. An ample belt eoge distance assures a sealed material package during operation even when belt misalianmen( occurs. A more comprenensivd treatment of force interaction for a complex model along with the implications of driving both belts is not within the present scope and can be found in [2] and [3].

3. Continental Conveyor HACs

When investigated anew in the late 1970s. It was clear that the sandwich belt concept offered the greatest potential for a cost-effective. operationally-appropriate high angle conveying system to address the broad needs of the mining and materials handling industries.

Following the extensive study of past sandwich belt conveyors. the governing theory and constraints, and development of the governing design criteria, a broad scope effort was undertaken in 1982 at Continental Conveyor & Equipment Company, USA. to develop the, first sandwich belt high angle conveyor to meet these needs.

The resulting HACs, described in Figs. 3 - 14 and Table 1. are truly evolutionary in judiciously selecting and advancing desirable features. write avoiding the pitfalls of the past. They conform entirely to the governing theory and constraint equations, and to the development criteria.

HACs fulfill all established operational requirements. HAC profiles can conform to a wide variety of applications. The HAC is we)) suited to a self-contained modular unit. utilizing nylon fabric belts to achieve short vertical radii of curvature, as it is to a single run approach utilizing steel cord or aramid fiber belts. [3, 4].


Fig. 2: Ideaitzed model 2 of sandwich bell Conveyor


Fig. 3: HAG profiles

4. AdvantageS of Sandwich Belt HACs

HACs can take on various forms (Figs. 3 - 14) and offer many advantages over other systems. including:

Simplicity of Approach

The use of all conventional conveyor hardware. Operating experience thus far has revealed that HACs have very high availability and low maintenance costs.

Virtually Unlimited in Capacity

The use of conventional conveyor components permits high conveying speeds. Available belts and hardware up to 3,000 mm (120 inch) wide make capacities greater than 15,000 f/h possible.

High Lifts and High Conveying Angles

Lifts beyond 300 m (1000 ft) are possible with standard fabric belts, and much higher single-run lifts are possible with steel cord or aramid fiber belts. High angles of up to 90° are possible.

Flexibility in Planning and in Operation

The Continental Conveyor sandwich belt lends itself to multi-module conveying systems using self-contained units as well as to single-run syStemS usino externally anchored. high angle conveyors. tn either case. t~e conveyor unit may be snortened or lengthened or the conveying angle may be altered according to the requirements Of a new location.

Belts are Easily Cleaned and Quickly Repaired

Smooth surface belts allow continuous cleaning by belt scrapers or plows, This is especially important in handling wet and sticky material. Smooth surface belts present no obstruction to quick repair of a damaged belt by hot or cold vulcanizing.

Spillage-Free Operation

During operation. the material is sealed between the carrying and cover belts. Welt-centered loading and ample belt edge distance result in no spillage along the conveyor length.

5. HAC Installations - General

HACs are well established, with the first commercial unit in operation since June 1984 (Table 1 - Unit 2). Nineteen additional significant HACs have been delivered. and eleven more units are presently in various stages of engineenng and manufactunng. scheduled for operation in 1992 and 1993.

Materials handled vary from various grades of coal (run-of-mine, sized, sized and washed) to coal refuse, to coarse copper ore, excavated earth. dewatered sludge, wood chips. to blast furnace slag, and gypsum, to vancus grains.

Conveying rates vary from a tow of 0.272 f/h (0.3 st/h) to a high of 4,000 f/h (4,409st/h)

Conveying angles vary from 35 to 90. ft is worth noting that thirteen .HACs elevate the conveyed materials at 90.

Elevating heights are as low as 3.66 m (12 ft) and as high as 175 m (574 ft). These same HAG units are respectively the shortest, at 8.6 m (28.2 ft) and the fongest at 454 m (1,490 ft).

6. The Gentleness of HACs

The original HAG development included extensive testing with vanous materials at the HAC demonstration unit. (Table 1, Unit 1). Because of the great potential for elevating and lowering fnaole or damage-prone materials, it was very important to convincingly demonstrate the HAG systern to be gentle.

Damage testing was performed on three USDA Grade 1 grains to demonstrate the gentle distribution of hugging pressure on the sandwiched material. Five one-bushel samples were loaded into oversized burlap sacks from each of a common batch of soybeans. wheat and seed corn. The first bushel of each grain was set aside to serve as the control sample, while the next four bushels were conveyed at 600. the full length of the HAG prototype. two. four, six and eicht times. respectively, for corLesponoing conveying distances of 45. 7 m (150 ft). 91.4 m (300 ft), 137.2 m (450 ft) and 182.9 m (600 ft). Samples (12.555 g) from each bushel sack were then analyzed at a State of Alabama Department of Agriculture laboratory for the various forms of damage and contamination, and at the Alabama State Seed Laooratory Department of Agriculture and industries for germination potential. The results showed no damage to any of the three grains tested, as a result of conveying in the HAG prototype.

The first HAG in wood chips (Table 1 - Unit 7) replaced a positive pressure pneumatic system to elevate screened product continuously to the digester. This system was purchased precisely on this merit. as the pneumatic system resulted in substantial degradation ot the wood chips resulting in expensive fiber toss. Prior to purchase of this unit. Boise Cascade conducted additional independent damage testing at the HAG demonstration unit in Winfietd and once again found the HAG system to result in n& damage to the product.

7. Broad Horizons

This paper so far has rationalized the development of Sandwich Belt High Angle Gonveyors and has described suc -cessful HAG installations in a wide variety of apptications. These have illustrated the versatility of HACs with regard to profile. and the appropriateness for the broad needs of mining and materials handling applications.

Previous writings have pointed out the many possible open pit mining applications [1, 4, 6]. It is the primary purpose of this writing to identify some of the many possible and actual additional HAC applications in continuous ship loading and unloading, storage acid transfer yard handling, midstream transfer and blending. elevation from underground hoppers and tunnels, and prep plant applications.

8. Continuous Ship Loading

Traditional shiploaders are typically rail-mounted traveling machines supported on a long dock structure. These machines are provided with a loading conveyor boom which continuously discharges material through a loading chute into the hold of the ship. The boom can typically alter its luff ing angle during operation and to a non-operating storage position. Depending on specific requirements and/or design preference, the boom may be of the shuttling or of the slewing type. In either case these features permit positioning of the loading chute where desired in the ship's hold. Further possible features includ? articulating and/or telescoping chutes with or without material flingers to trim the ship's hold for complete filling.

A dock conveyor must feed the material to the boom belt via a tripper. The dock length must therefore be determined for the travel range of the shiploader plus the tnpper length. The tripper length can often add 100 m (328 ft) or more to the dock length depending on the operating belt tension and height of .the boom loading point. This consideration has in many cases ted to elevating the entire dock belt, thus shortening the tripper length. The correct solution is a question of economics, trading extra dock length for support structure at the elevated dock belt.

The linear shiploader design is a notable exception to the above description and was. in part, developed in response to these considerations.

Fig. 4 illustrates yet another solution using a C-profile HAC as an elevating belt from the discharge of a short tripper to the loading point of the ship loading boom. This solution minimizes the required dock length without the need to elevate the entire dock conveyor. Dock length savings in the 30 to 60 m (98 to 196 It) range can reduce dock costs approximately US$1,000,000 to US$2,000,000 depending on the dock design. Savings of US$100,000 to US$200,000 are also estimated at the shiploader with tripper.

Figs. 5 and 6 illustrate a shiploader with loading HAG's of S-profile. Often friable or damage-prone materials such as coke, grains. etc. must be lowered gently into a ships hold to minimize degradation from impact. When allowed to free-fall through a chute, the material grade may be appreciably lowered, thus reducing the commodity's selling price. The lowering HAG's of Figs. 5 and 6 offer solutions to this problem since it is possible to select any desired lowering speed.

  Company & 
Material & Rate 
Conveying angle 
Elevating height 
Belt width
Belt speed
1. Demo Unit
Wintetc. AL. USA 
to 2903
30 to 60 7.9 to 19.5 35 1524 0 to 6.1 75/112 1983
2. Triton Coal Co.
Gillette. WY. USA 
60 32.9 56.7 1524 5.33 149/224 1984
3. Maldanoek Mine
Copper Ore 
35.5 93.5 173.7 2000 2.67 450/900 1992
4. Coal Company
Western USA 
35 29 61.9 1829 4.57 149/224 1987
5. Granite Constr. Co. 
Excavated earth
90 31.7 39.9 914 1.6 22.4/22.4 1988
6. Waste Treatment Co.
90 3.66 8.6 610 0.3 0.0/2.2 1989
7. Boise Cascade
Wallula. WA. USA
Wood chips 
53 32.6 49.3 1219 2.03 22.4/22.4 1989
8. Coal Prep Plant
Eastern USA
Raw coal 
49 21.9 40.2 1372 2.79 56/56 1990
9. BethEnergy Mines
Van. WV. USA
Clean coal 
90 76.2 90.2 1372 2.79 112/112 1991
10. Boise Cascaae
Steilacoom. WA, USA
Wood chips 
90 15.5 31.4 914 2.03 7.5/7.5 1991
11. Valley Camp of Utah
Helper. UT. USA
Raw coal 
65 30.7 44.2 1372 3.56 93.2/93.2 1990
12. Island Creek Corp
Oakwood. VA. USA
Coal refuse 
to 41 174.8 454.2 914 2.34 186/186 1992
13. Steel Cement Ltd
Gypsum, slag 
90 16.2 37.8 600 1.67 7.5/7.5 1991
14. Kimberly Clark
Wood chips 
53 22.9 40.5 1219 2.03 18.6/18.6 1991
15. Cape May County 
90 9.0 17.5 762 1.27 0.0/11.2 1991
16. Cape May County 
90 13.0 31.8 762 1.27 0.0/11.2 1991
17. Shipping Company
90 18.9 27.4 1524 4.06 56/56 1991
18. Shipping Company
90 22.0 181 1829 4.06 112/112 1992
19. Coal Company
Eastern USA
Clean coal 
90 16.1 69.4 1372 2.79 37.3/75 1992
20. Shpping Company
65 30.7 44.2 1372 3.73 75/75 1992
21. Gleason. Peguiven
Phosphate rock
-35.5 Drop 34.0 113.0 914 2.29 0/93.2 1992
22. Cementos Veracruz
Hot clinker 
35 41.3 198.9 1219 1.73 56/112 1992
23. Midwest Conveyor
48 14.2 57.0 1829 3.56 75/112 1992
24. US Gypsum
Gypsum rock
90 36.6 48.5 1067 1.52 37.3/37.3 1992
25. The Conveyor Co. 
90 6.5 15.6 610 1.22 0.0/7.5 1992
26. Mountain Coal Co. 
Raw coal 
51 22.6 44.2 1524 3.56 75/93.2 1992
27. Mountain Coal Co.
Raw coal 
35 15.0 37.5 1219 1.27 11.2/14.9 1992
28. Taulrnan Systems
90 20.0 36.3 762 1.78 11.2/11.2 1992
29. Montague Systems
57 59.4 90.8 1829 3.66 186/298 1993

Table 1: High Angle Conveyor - HAC Installations - Continental Conveyor & Equipment Company


Fig. 4: HAC elevating to shiploader


Fig. 5: HAC at low soeed. Lowering breakable or damaged material such as coke, grains, etc. into ships in lieu of loading chute


Fig. 6: HAG shiploader at low speed - similar to Fig. 5. but with more versatility at high speed. loading and trimming by flinging material around the entire area of the ships hold

Fig. 6 illustrates the more versatile concept for a more complete trimming of the ship's hold.

With other materials which are not damage-sensitive, the lowenng HAC of Fig. 6 can be operated at high belt speed to fling the material evenly around the ships hold. This permits efficient filling of the hold as the matenal level approaches the combing.

Hubee [12] proposed a yet more novel machine, a " Two in One" Traveling Shiploader Unloader combination with C-shaped elevating HAC to a clam shelf unloader with loading boom belt.

9. Continuous Ship Unloading

Continuous ship unloading equipment may be dock based thus capable of unloading many conventional bulk carriers (Fig. 7) or it may be provided at each ship making it a continuous self-unloader (Fig. 8). The choice depends on many factors including unloading rate, haul distance, port draft etc.

Dock-based continuous ship unloaders (CSUs) were the focus of much attention in the early to mid 1970s. The transshipment of bulk materials was growing and projected to grow at alarming rates along with the size of bulk carriers, beyond the 350,000 DWT class.

Clamshell unloading rates up to approximately 1,200 f/h were thought to be a practical limit for the concept, while continuous unloaders promised much higher unloading rates. This was important since the time spent at port was extremely costly for the large bulk camer.

Early high capacity CSU concepts centered on the chain mounted bucket ladder for the digging and elevating duties. Hampered by the required ruggedness of the digging duty and the limited chain speed (to approximately 400 FPM with free flowing materials) CSU unloading booms and thus the entire machine became massive beyond the original projections. The cost of dock structures to carry these machines also increased.

In the face of these technical problems, along with revised, less ambitious, projections for transshipment of bulk materials, the focus on large capacity CSUs diminished. Instead, the interest shifted to lower capacity CSUs. comparable in untoading rate to the clamshell unloaders.

Various designs emerged, from all bucket elevating systems. to screw-type elevators, to designs which separated the digging and elevating functions. The latter combine bucket wheels with pocket belts or light duty bucket elevators, and augers with air-pressed sandwich belts.


Fig. 7: Continuous ship unloaders - bucket wheel/HAC - CSU and bucket ladder/HAC CSU

HACs. because of the favorable features previously described, make possible CSUs of virtually unlimited capacity and suitable for a wide range of materials. Separation of the digging and elevating functions as indicated in Fig. 7 permits high speed operation of the L-profile HAG, minimizing the cantilevered weight and thus the entire machine wetght.

The concept with short bucket ladder digging means is expected to permit better cleanup of the hold and to be more tolerant of wave action because the sagging chain is in partial suspension as it. drags the ships bottom.

Self-unloading ships have gained prominence in the last decade because of their high unloading rates. exceeding 10.000 f/h. because they need no expensive unloading facilities at the unloading docks and because of their ability to load or top off larger bulk carriers at sea. The last feature 'is especially important for export shipments when the loaded draft of the export carrier exceeds the water depth of the loading ports.

HACs of C-profile with high angle discharge are especia)ly adaptable' to self-unloading ship applications because of unlimited tonnage capacity at high belt speed. suitability for a wide variety of materials, and a vertical elevating profile which minimizes space requirements.

Fig. 8 shows a bulk grain carrier converted to a self-unloader by Continental Conveyor, utilizing a HAC with tong tail plus fourteen other conveyors (Table 1 -Unit 18). Unit 17 of Table 1 is a C-shaped HAG also elevating grain from a self-unloading ship.


Fig. 8. Selt.unioaong ship w,ih HAG system


Fig. 9: Stacker/reclaimer wiih elevating HAG - yard belts may be through-type or reversible


Fig. 10: HAC elevatrtg material from under dump hoppers

10. Materials Handling Yards

HACs offer savings in structure and in real estate in a wide vanety of transfer yard applications. Fig. 9 illustrates savings in yard length and tripper (estimated in the range of US$100,000 to US$200,000) when a C-profile HAG is used to elevate material to the boom of a stacker or stacker/reclaimer. The arrangement shown is equally suitable for stacker/reclaimer operation with reversing or throughtype yard belt. The latter need only incorporate a bifurcated chute with flop gate, at the tripper discharge. able to direct material flow to the HAG or alternately back onto the yard belt.


Fig. 11: HAC elevating from underground excavation protects such as underground mines, tunneling, deep foundations, caissons and underground storage tanks

11. Elevating from Underground

HACs offer potential for large savings in tunneling costs when used to elevate materials from underground. Figs. 10 and 11 illustrate just two of many possible applications. A conventional conveyor at modest incline, to approximately 15, has been the most popular means for elevating large tonnages continuously from underground. A 60 m (197 ft) lift thus required 232 m (761 ft) of 15 slope conveyor and tunneling. Use of the HAG as the elevating means retains all of the positive features of a conventional conveyor while requiring only a 60 m (197 ft) long vertical shaft. Considering the technological breakthroughs in large diameter vertical shalt boring, the savings in tunneling costs can exceed US$1,500,000 for this sample case. Furthermore, vertical shafts tend to be less prone to wall or roof collapse.

Fig. 10 illustrates a case where the slope tunnel is not necessary if the HAG is used to elevate from under dump hoppers. All conveying and elevating equipment remains within the confines of the dump vault.

Table 1 - Units 4, 5. 19. 23. 24 are of this type. elevating materials from underground.

12. Crushing, Blending, Washing and Sampling

In the beneficiation of bulk commodities. materials must he elevated to the top of crushing, sampling, washing towers, blending silos, etc. Gravtfy usually returns the material to the next loaaing point where it is again elevated. Complex beneficiation systems are needed at metal and non-metal mines, coal mines, preparation plants smelters, mills, power plants, grain terminals. etc.

Beneficiation systems are predominantly land based as in Figs. 12 and 13 but can also be mounted on large floating platforms as depicted by the midstream terminal of Fig. 14.

Elevating to the various stages has been traditionally by conventional conveyors and. if space is limited, by bucket elevators and skip hoists.

HACs offer all of the positive features of conventional conveycrs without the profile limitations. High tonnage rates at the least capital, operating and maintenance costs are possible with HACs. and with a minimum space.


Fig. 12: HAC at and based benefication system to elevate between various stages of crushing, blending, sampling. washing. etc.


Fig. 13: HAC to clean coal silo

Actuaf HAG installations of this category include in Table 1, Units 2, 7, 8, 9, 10, 13, 14, 15, 16, 19, 20, 22, 26, 27, 28 and 29.

13. Midstream Terminals

For reasons cited in the earlier discussion of sell-unloaders. midstream terrrtinats have also been the focus of much attention in the past. The grain companies have led the way in this eflort. building several terminals for operation in the lower Mississippi River and Gulf of Mexico. USA and at Europoort, Rotterdarn, The Netherlands. These incorporate continuous unloading of river barges by bucket ladders, blending and sampling systems by combined use of conveyors and large bucket elevators, and ship loading by a stewing conveyor boom.

The coal industry has also developed barge-to-ship midstream terminals. mainly for topping off large export bulk camers in the Gulf of Mexico, USA.


Fig. 14: HACs at barge-to-ship floating transier terminal, to elevate between various stages of blending. sampling. etc. and to the shiploader

These consist of pontoon-mounted grab cranes which transfer material directly from barge to ship with each grab cycle. Because of the previously cited technical limitations. unloading rates are modest. even with several grab cranes.

In either grain or coal or other applications, HACs facilitate midstream operations at the highest throughput rates within the least space.

14. Summary and Conclusions

This and previous papers have described the broad-scope effort at Continental Conveyor, USA. which led to-the developrnent in 1983 of the Continental high angle conveyor-HAC. Previous papers focused on HAG systems for open pit mining. It has been the main purpose of this paper to describe HAC systems for a wide range of materials handling applications, thus illustrating the versatility of the concept.

Review of the commercial installations to date yields the best evidence of the HACs great versatility, varying in profile from S- to L- to C-shape. in elevating angle from 30 to 90. in materials handled from coal, to coarse ore, to sludge. and in throughput rate from 0.27 to 4,000 t/h.

Because of the HACs great versatility and proven performance, its advantages may be exploited with confidence in a wide variety of applications, especially those descnbed in this writing.


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