Sandwich Belt High Angle Conveyors – HAC® – Evolution to Date

Dos Santos, J.A. Bulk Solids Handling Vol. 6 (No. 2)  |  April 1986 |  pp. 299-314

ABSTRACT

Conventional belt conveyors offer an economical method for transporting materials at inclines approaching 18°. Sandwich belt conveyors may be used to transport materials at high angles, to 90°, while maintaining the most positive features of conventional conveyors, that is, (1) proven conventional conveyor components, (2) high-speed operation, (3) high single lifts, (4) continuous cleaning of the smooth surfaced belts, and (5) easy, quick repair by hot or cold vulcanizing. The present discussion deals with the evolution and development of sandwich belt high angle conveyors (HAC®s) to satisfy the operational requirements of the open pit mining and materials handling industries.

A look at the limitations of conventional low angle conveyors reveals the conveying angle at which a cover belt is needed. The relationship between conveying angle and hugging pressure by the cover belt is developed mathematically. A study of past developments reveals that the Loop Belt and Beltavator were significant advances. The vertical-radius-of-curvature constraints are discussed in detail, and the constraint equations are developed. Criteria for development are established as basis for the evolutionary development of the latest sandwich belt HAC®s.

Finally, design features of the first large-scale prototype and the latest commercial HAC® units, one in coal, the other in an open pit copper mine, are described.

1. INTRODUCTION

In connection with studies [1], [2] relating to continuous haulage from deep open pit mines, the author had occasion to study, extensively, the use of high angle conveyors as a means of hauling mine products continuously from the pit while maintaining optimum mine slope to minimize total excavation. The many possible high angle conveying methods investigated included skip hoists, bucket ladders, pocket belts, fin belts, cleated belts, sandwich belts, pipe belts, screw conveyors, slurry pipe lines and others. The sandwich belt high angle conveyor appeared to be the most operationally appropriate and economical solution for the mining and materials handling industry. The present paper presents the theory, historical development and state-of-the-art in sandwich belt high angle conveyors.

2. History and Theory of Sandwich Belt Conveyors

2.1 Limitations of Conventional Conveyors

Conventional belt conveyors offer an economical method for transporting bulk materials at recommended inclination angles ranging from a low of 7° for soda ash briquettes to a high of 30° for cinder concrete and ground phosphate fertilizer [3].

Typical recommended inclination angles for open pit mine products such as excavated earth and blasted, primary crushed rock vary from 15° to 22° [3].

The conventional conveyor is often the most economical, reliable and safe means of transporting bulk material [3]. There are, however, many cases which strongly warrant an increase in the conveying angle. In open pit mines with steep mine face angles, or in any case where the surface incline angle in the path of the conveyor substantially exceeds that of the recommended conventional conveyor angle, much excavation or elevated support structure is needed to accommodate the conventional conveyor.

To develop the theory for high angle sandwich belt conveyors logically, one must understand the nature of conveying angle limitations in conventional conveyors.

In a static case, a cohensionless material on a rubber belt will begin to slide back when the incline angle of the belt surface just exceeds the internal friction angle of the material or the material-on-belt friction angle, whichever is smaller. The angle of internal friction is equal to the angle of repose for such materials. Both the internal friction angle and the material-on-rubber friction angle will vary from one material to the next and will be affected by the maximum lump size, the lump size distribution and orientation on the conveyor cross section, and the shape of the particles or lumps. Although there is much published information on angles of repose for various bulk materials [3], very little is available on friction angles for various bulk materials on a rubber surface. H. Colijn[7] lists coefficients of friction of material on rubber for six different fine grain materials. They vary from 0.45 for Ottawa sand with 2% moisture to 0.624 for bituminous coal fines with 15% moisture. An investigation by the U.S. Bureau of Mines [8] revealed much higher friction angles when coarse material was laterally displaced over rubber belting material in a ‘large direct shear tester’. In one set of tests, river run gravel of 0.25 inch by 0.185 inch, 0.375 inch by 0.25 inch and 0.5 inch by 0.375 inch resulted in friction angles at the material-to-belt interface of 38.5°, 42.0° and 38.0°, respectively. The corresponding angles of internal friction are 43.5°, 48.4° and 52.5°. A second set of identical tests on 1-1/2 inch by 0 inch Upper Freeport coal, 1-1/2 inch by Qinch Middle Kittanning coal, 3/4inch by 1/2inch dolomite and 3/4 inch by 1/2 inch limestone yielded respective friction angles of 34.8°, 38.1°, 40.6° and 33.7°.

Recommended conveying angles are, in general, far below either of the friction angles mentioned. This is due to the dynamics in a moving belt conveyor, which result in relative motion between adjacent particles or lumps of the bulk material and between the material and carrying surface of the conveyor belt. The three major sources of dynamic effects are:

1. Agitation of the material on the conveyor belt as it approaches and is carried over each successive carrying idler – this effect is amplified when belt tension is low and idler spacing is high, thus resulting in large belt sag between idlers.
2. Acceleration of the material at loading or transfer points – this results in relative slip and turbulence since velocity and direction cannot change instantaneously.
3. Vertical impact at transfer points – such impact is absorbed by the resilience of the belt and impact idlers and results in bouncing of the material; this adds to the turbulence at the loading area.

These effects are increased when the belt speed is high and even more so when the material handled is loose and contains large-rounded lumps.

In 1934, A. Vierling [9] performed tests on a belt conveyor of V-shaped or wedge-shaped cross section and showed that he could increase the conveying angle simply by increasing the troughing angle above 45°.

Tests conducted with a V-grooved belt conveying overburden having a natural angle of repose of 40° showed that a conveying angle of 38° to 40° could be achieved. It was announced that the grooved belt could convey material with mixed grain size composition at conveying angles 4° to 5° less than the natural angle of repose.

Extrapolation would lead to the conclusion that a grooved belt is effective in suppressing the dynamic effects due to belt travel over the idlers and is possibly creating an added normal pressure, due to wedging action, at the material-to-belt-surface interface. The conveying angle cannot, however, exceed the angle of internal friction of the material at the free surface.

2.2 Sandwich Belt Conveyors

Conveying angles greater than the angle of internal friction can be achieved by a cover belt which, when pressed against the material, will create a hugging action to prevent sliding of the contact surfaces.

For a cohensionless material, one can idealize the situation, as shown in Fig. 1. The material is idealized as parallel layers spaced infinitely close.

If the cover surface is free to follow the material as it slides back, sliding will occur when the tangential component of the material weight exceeds the frictional forces which resist it:

Where:

where µ = µ_m  or µ = µ_{b ,} whichever is smaller.

W_m,  α, N, µ_m, and µ_b as defined in Fig. 1

To achieve an inclination angle α, a normal lineal hugging load N must be exerted by the top surface such that:

(2)

If the cover surface is fixed and it resits the motion of the material, then slide-back will begin when:

W_m sin α > N (µ” + µ’) + (W_m  cos α) µ’   (3)

Where:

µ = µ_m  or µ = µ_t , whichever is smaller.

µ_t as defined in Fig. 1

To achieve an inclination angle α:

(4)

If  µ’ = µ”, substitution into the above equation shows that the normal hugging load N, needed to prevent backsliding, is only half of that required in the previous case (see Eq. (2)).

When the above principles are related to sandwich belt conveyors, the bottom surface represents the carrying belt, while the top surface represents any cover surface such as a pressed or weighted belt. For a covering surface which depends on self-weight, the normal lineal hugging load N is the normal component of its lineal weight. The second set of equations, (3) and (4), shows that the required hugging load is much less if both surfaces are driven at the same speed.

In his 1958 review of patented high angle conveyor methods [6], P. Rasper breaks down sandwich belt conveyors into three categories:

1. cover belt acting by its own weight
2. cover belt with additional pressure
3. cover belt with carriers.

Fig. 2 illustrates a category a) solution, which was patented in the Federal Republic of Germany in 1953. It consists of a carrying belt conveyor on three-roll, 30° troughing idlers with special spring-suspended impact idlers at the loading area, a chain matt cover surface and a cleated belt to drive the cover surface.

During operation, the material loads onto the bottom horizontal portion of the carrying belt and is carried into the sandwich, where the chain matt cover hugs the material, by virtue of its self-weight, and prevents it from rolling or sliding back. The material is then elevated in the sandwich and is discharged at the top where the two surfaces separate.

The chain matt could conform easily to the irregularities in the material profile, and this was thought to be an attractive feature. It was noted that a cover surface which depends on self-weight becomes uneconomical at higher conveying angles because of the exponential increase in required lineal weight. Some elevating conveyors were built according to these ideas.

Fig. 3 illustrates a category c) solution, which was also patented in the Federal Republic of Germany in 1953 [6]. It consists of a carrying belt conveyor on three-roll troughing idlers and a cover belt with flexible carriers consisting of flexible chain or rope, secured to the belt at prescribed intervals along the length. It requires a loading belt in order to place the material into the sandwich.

During operation, the material is placed into the sandwich by the feed conveyor and it is immediately covered by the carrier. The cover belt and carriers hug the material, by virtue of their self-weight, while it is elevated to the discharge point at the top. As the main selling feature, the cover surface was claimed to be self-cleaning because of the flexibility of the carriers. According to the author, no elevating conveyors were ever built according to this design.

In his book “The Bucket Wheel Excavator” [4], L. Rasper discusses the elevating conveyor illustrated in Fig. 4. It is a category b) solution, patented in the Federal Republic of Germany in 1954. It consists of a carrying belt conveyor on three-roll troughing idlers and a cover belt which is pressed onto the material by rubber tires distributed across the belt and spaced at large intervals along the conveyor length. A feeder belt is used to load the conveyor.

During operation, the material is fed onto a low angle, uncovered portion of the carrying belt at the bottom of the elevating conveyor and is carried into the belt sandwich. The cover belt, which is pressed by the rubber tires, hugs the material as it is elevated to the discharge point at the top. According to L.Rasper [4], this solution was used on several occasions on bucket wheel excavators of the 1950s.

As a result of his survey, P. Rasper [6] lists three important operational requirements of high angle conveyors; the first pertains only to their use in bucket wheel excavators, while the second and third are important in any application:

1. The high angle conveyor must provide for easy removal or lifting of the cover belt when the conveyor is to be operated at shallow angles.
2. The system must lend itself to easy and quick repair of the belts.
3. Any high angle conveyor must lend itself to easy cleaning.

Requirement 1. illustrates a recognition of additional wear and possible damage to the belts and other components when operating with the extra pressure from discrete loads over the cover surface. On a bucket wheel excavator boom, such a conveyor will operate at high angles only when it makes the extreme high and low cuts.

During operation, a bucket wheel excavator can pick up large boulders of different sizes and shapes and tramp iron such as beams, plates etc. from previous underground mines. These can cause tears in the belt. Requirement 2. recognizes that it is impossible to prevent this, so the effort is to minimize the consequential downtime. For a regulated and more predictable material, good design can reduce the frequency of damage. P. Rasper [6] suggests that repairs be made by hot or cold vulcanizing and, on this basis, rules out the use of fins or cleats.

Requirement 3. is based on the realization that belts get dirty and must be continuously cleaned if they are to maintain their alignment. Buildup of material on the belt and other surfaces also leads to additional drag and premature wear of all support and drive components. He states that the most popular way to clean belts on bucket wheel excavators is by screw-type return idlers, and this requires the use of smooth surface belts. This requirement still holds true for the many belt cleaning methods available today.

On this basis, he concludes that only solutions of categories a) and b) lend themselves to the operational requirements and thus warrant further study and development.

1. Rasper also points out that most of the development and use of high angle conveyors on bucket wheel excavators occurred in the 1950s and the bucket wheel excavators of the 1960s had abandoned them in favor of higher tonnage rates and reduced wear of conventional conveyors [4].

Later developments in the category a) solutions include the Retainer Belt by Stephens-Adamson [10] and the Overlay Conveyor by R.A. Hansen Co. [13]. Both claim the capability to convey material at angles of up to 45°.

The Retainer Belt, as illustrated in Fig. 5, consists of a conventional carrying conveyor on three-roll troughing idlers and a heavy cover belt with smooth rubber surface. The cover belt is shot with lead for the extra weight needed in the normal force component.

During operation, the material loads onto the uncovered conventional conveyor, at a nearly horizontal conveying angle, and enters the belt sandwich prior to a gradual increase in angle. The sandwiched material is conveyed at the high angle to the discharge point where the belts separate.

The Retainer Belt conforms well to the second and third operational requirements listed by P. Rasper [6]. The high cost of the special cover belt, however, makes the Retainer Belt expensive at high angles. There is no apparent capacity limitation.

The Overlay Conveyor is illustrated in Fig. 6. It also conforms well to the second and third operational requirements set by P. Rasper, but it fails to address the need for hugging pressure. It uses an ordinary (nonweighted) cover belt which is driven along with the carrying belt by mechanical coupling.

The carrying belt is supported on three-roll troughing idlers. A specially suspended single-roll idler presses the cover belt onto the carrying belt at the entrance to the belt sandwich. A speedup belt is used to feed the high angle conveyor.

During operation, the speedup belt throws the material into the loading area and its momentum carries it into the sandwich entrance. From that point, it is carried in the belt sandwich until it is discharged at the top where the belts separate.

Two test units, the first at 45° and the second at 40°, were able to convey wet sand, but not with rocks exceeding 10% of the material mix [13].

To reflect significant developments in sandwich belt technology, from 1970 to 1976, two new categories d) and e) are added to the possible solution types listed by P. Rasper [6].

The current solution categories are then as follows:

1. a) cover belt acting by its own weight
2. b) cover belt with additional pressure
3. c) cover belt with carriers
4. d) belt sandwich with prying resistance by virtue of the transverse stiffness of the two belts
5. e) belt sandwich with radial pressure by virtue of the belt tension and the conveyor profile geometry.

The Loop Belt by Stephens-Adamson [11] represents a category e) solution. It is illustrated in Fig. 7.

With this concept, a nonweighted inner belt loop is pressed against closely spaced troughing idlers by an outer belt loop. The outer belt is straight at the loading region and supported on troughing impact idlers. Idler support of the carrying strand (outer belt) is discontinued before it joins the inner belt at the sandwich entrance. Throughout the vertical curve, the outer belt tension induces sufficient radial pressure on the curved profile to overcome the normal component of the belt and conveyed material weight and to hug the conveyed material so that it will not slide back.

The material within the sandwich loops around, by approximately 155 °, to the discharge point. The lineal hugging load produced by the outer belt is determined by the following equation:

(5)

where:

T – outer belt tension at the point in question on the conveyor profile

R – radius of curvature corresponding to the belt tension T

P_r – the corresponding lineal load applied by the outer belt.

Typically, the outer belt loop is driven while the inner belt loop follows. However, to exploit the combined tension rating, thus maximizing lift, both belts have occasionally been driven in a load-sharing manner.

The Loop Belt has had great success in self-unloading ship applications and several outdoor installations. It has achieved 10,000 t/h in conveying iron ore pellets and coal and lifts of up to 150 ft (45.7 m). Typical belt speeds are 800 to 1,200 ft/min (4.1 to 6.1 m/s). A special low modulus fabric belt is used to achieve tight vertical curves with a troughed belt.

The Beltavator, by Stephens-Adamson [12], is a hybrid which represents a solution of categories d) and e). It is illustrated in Fig. 8. It is identical to the Loop Belt up to a conveying angle of 90°. At this point, it conveys the material vertically for a desired elevating height and it again becomes like the Loop Belt beyond the 90° conveying angle. It is the straight vertical portion of the conveyor profile that qualifies it as a category d) solution. Along the vertical portion, the belt sandwich is held together by closely spaced, staggered-edge rolls which press the belt edges to keep the sandwiched material sealed between. Hugging pressure is derived from the prying resistance of the two belts as the material is introduced into the sandwich. The required transverse stiffness of the belts imposes capacity limitations on the Beltavator. As belt width requirements become larger, at higher tonnage rates, increasingly stiffer and therefore thicker multi-ply fabric belts are required. The practical belt width is limited to approximately 36inch (914mm), and the maximum capacity is approximately 1,000 t/h when conveying dense material.

The Beltavator, too, has had great success within its capacity range. Its conveying profile can follow a “‘C’’, “Z” or an ‘‘L” belt path, as shown in Fig. 8.

Solutions in categories d) and e) represent a significant advance in the historical development of sandwich-type high angle conveyors. All of categories a) through e) address the need for hugging pressure which increases frictional resistance to material slide-back. The significance is that solution categories d) and e) do not require added weight or a pressing means. Rather, they exploit inherent characteristics of any belt conveyor. ;

Category d) solutions exploit the transverse stiffness of the belts. This stiffness exists whether or not it is exploited.

Category e) solutions exploit the radial force induced on the curved profile by the belt tension. The belt tension is used to an advantage by selection of the conveyor profile geometry.

The belt tension throughout the conveying profile is reasonably predictable and exists whether or not it is exploited to an advantage.

Additional implications in category e) solutions are discussed later when addressing the vertical-radius-of-curvature constraints as specified by CEMA [3].

It may seem surprising at first that Stephens-Adamson typically chooses to drive only the carrying belt in the Retainer Belt, the Loop Belt and the Beltavator when it is clear from Eqs.(2) and (4) that the required hugging pressure is greatly reduced if drag is exerted at both the carrying and cover surfaces. A closer and more realistic look at the belt sandwich model will reveal that both surfaces do indeed exert drag on the material, even though only one belt is driven, provided there is sufficient material-free edge distance.

The belt sandwich model, illustrated in Fig. 1, is instructive, but not accurate. It assumes that the cover surface contacts only the material, but not the edges of the carrying belt. More realistically, a minimum distance from the belt edge to the material is required in order to assure that the material is always covered and does not spill out. To insure ample edge distances and thus a sealed envelope, Stephens-Adamson chose to derate the capacity of the sandwich-type conveyors (as compared to CEMA capacity recommendations for conventional conveyors [3]).

A new, more realistic model (Fig. 9) illustrates the interplay of forces. The minimum normal hugging load (min) N_m which must be exerted on the material to prevent backsliding if both surfaces resist motion is expressed by the following equation:

(min) N_m = W_m      (6)

This follows from Eq. (4). The drag which is exerted on the material by the top surface must be counteracted by the frictional drag between top and bottom surfaces at the edges, as expressed by the following equation:

(min) N_e µ_e = (min) N_m  µ” (7)

The minimum required total normal load (min) N can be expressed by combing Eqs. (6) and (7) to obtain Eqs. (8) and (9):

(min) N = (min) N_e + (min) N_m

= N_m (8)

(min) N = W_m         (9)

If both carrying and cover surfaces are of rubber conveyor belting, then µ’ = µ“.  If µ_e = µ’ = µ”, Eq. (9) reduces to Eq. (2). This requires a material belt edge distance of one-fourth the belt width. Eq. (2) assumes that only the carrying belt is driven.

The required total normal load N is the same when only one belt is driven regardless of which analysis model is used. The new model is more realistic, however, and recognizes that approximately half of the normal load bears directly on the conveyed material while the other half serves as belt-on-belt-edge seal pressure.

What then is the advantage of driving both surfaces? If the type of material to be handled and the environmental constraints do not require a large edge distance, then the total required additional normal load N can be reduced. If there is cause to doubt that the required counteracting edge drag could be developed, that is, µ_e « µ”, then it is advantageous to drive both surfaces. In order to maximize the total elevating height, it is advantageous to drive both surfaces in a load-sharing drive arrangement to exploit the combined tensile strength of both belts.

In light of the more current solutions in sandwich belt conveyors, a new set of operational requirements is established. This set retains requirements 2. and 3. as originally listed by P. Rasper [6] (now numbered 4. and 5.), but omits his requirement 1. since it is applicable only to high angle conveyors when used on bucket wheel excavators.

The combined operational requirements are:

1. The receiving end of the steep angle conveyor must insure that turbulent material at the load point is settled prior to entering the sandwich.
2. The hugging pressure exerted on the conveyed material through the cover surface must be applied by a ‘‘soft” loading system which minimizes load concentrations.
3. The cover must be a floating surface which will not obstruct the flow of lumps larger than anticipated or oriented unfavorably.
4. The system must lend itself to easy and quick repair of the belts.
5. Any high angle conveying system must lend itself to easy cleaning of the belts.

Requirements 4. and 5., which have already been discussed, preclude the use of carriers and make it impossible to cover the material immediately as it is fed into the high angle conveyor. The Overlay Conveyor [13] was loaded at an angle of 40° by using a speedup belt to throw the material into the loading area and allowing the momentum to carry it into the sandwich. This was apparently successful in conveying wet sand, but many difficulties arose in attempting to convey wet sand with 10% rocks. It was found that the orientation of the speedup belt, with respect to the loading area, was very critical and the optimum orientation varied with variations in the mix of the bulk material. Rocks, alone, could not be transferred in this manner since they would bounce and roll back. But even more basically, such a system, which depends on the momentum and angle of incidence of the fed material, could not tolerate an emergency stop of the conveyor. It would be impossible to reaccelerate any material remaining at the loading area after the shutdown. Requirement 1. recognizes the problems associated with loading a sandwich belt conveyor at a high angle. On this basis, it is recommended that the loading angle not exceed 10° to 15° and the material should enter the sandwich at a maximum conveying angle of about 18°.

Compliance with requirement 2. will minimize the wear on the belts and other components due to the additional hugging load. This is very important since L. Rasper [4] cites the reduced belt wear of conventional conveyors as one reason why the bucket wheel excavators of the 1960s had abandoned the use of high angle boom conveyors.

The design must convey material which is discharged from a primary crusher. The crusher setting only assures that one dimension will not exceed the prescribed value. If the feed into the crusher consists of slabby material, then a -8 inch (-203 mm) setting could result in slabs of dimensions of up to Binch by 12inch by 24inch (203mm by 305mm by 610mm). Requirement 3. insures that oversized or unfavorably oriented material will not encounter pinch points and will result only in local lifting of the cover.

2.3 Constraints on the Vertical Radius of Curvature

Operational requirement 1. carries severe implications. After the material is loaded, the conveying angle must be increased from 10° or 15° to a much higher angle. In open pit mining application, the ultimate conveying angle is dictated by the slope of the mine face. Such an angle increase cannot be instantaneous with a troughed belt. The angle change must be sufficiently gradual so that no part of the troughed belt is subject to buckling or overstress. Within these constraints, we must strive to increase the conveying angle along the shortest possible distance. Figs, 10 to 12 illustrate why this is so.

If a represents the highest mine face angle which is attainable because of slope stability or other considerations, then a multi-run high angle conveyor system may be incorporated into the mine, as illustrated in Fig. 10 or Fig. 11.

Fig. 10 illustrates the adaptation of the conveyor system without additional excavation. The consequences of a long transition radius are twofold. First, the ends of each module divert further from the mine face, thus increasing structural support requirements. Secondly, the ultimate conveying angle α”, corresponding to the long transition radius, is greater than α’, which corresponds to a short transition radius.

If additional local excavation is favored in order to reduce the structural support requirements and to keep the conveyor profile within easy access of the mine benches, then the conveyor system is adapted to the mine, as shown in Fig. 11. A long transition radius of curvature results in increased excavation requirements.

If a single-run conveyor is used to carry the material out of the mine, as in Fig. 12, then a long transition radius of curvature requires that the conveyor loading area extends farther into the mine. In many cases, it is more efficient to locate the loading area several benches above the pit bottom. In such cases, the imposition of the conveyor loading area makes it difficult to recover the material below.

The equations to determine the allowable vertical radius of curvature are listed by CEMA [3] and derived here. The final forms of the equations as derived are equivalent to the CEMA equations, but are presented in a more instructive form.

The vertical belt curve must be designed so that it will not cause buckling nor overstress at any part of the belt cross section. CEMA, in fact, requires that the minimum allowed tension anywhere on the belt is 30 Ib/inch of width.

When any section of linearly elastic material is subject to simultaneous tension T_c and bending M, which is induced by the curvature 1/r, as shown in Fig. 13, the stresses due to the independent forces may be superimposed. The model assumes that the curve is smooth and that plane sections remain plane. When applied to a conveyor belt section, this model predicts the stresses accurately at points sufficiently far from the ends of the curve, if the trough depth and width are much less than the length of the transition curve and the support idlers are very closely spaced when compared to the radius of curvature. With these assumptions, the bending moment can be related to the radius of curvature by the following equation:

(10)

where:

E – modulus of elasticity (in Ib/inch²)

I – moment of inertia (in inch⁴)

M and r as defined in Fig. 13.

Superposition of stresses may then be expressed as follows:

f = fa + fm =  + (11)

Where:

f, fa, fm Tc, A, r and S as defined in Fig. 13.

Recognizing that I|S = y and substituting into Eq. (11)

yields:

F = fa + fm =  +  y (12)

where:

Y  – distance from the neutral axis N.A. to the extreme outer fibers (see Fig. 13).

This equation can be applied to a multi-ply conveyor belt of arbitrary geometry by making the following substitutions:

– replace the cross-sectional area A (in inch²) in Eq. (12) by the product of the belt width and the number of plies b · p (in inch-ply)

– replace the elastic modules E (in Ib/inch²) by the commercially listed belt modulus Bm (in lb/inch-ply).

Eq. (13) then follows from Eq. (12):

F = fa + fm =  +  (in lb/inch-ply) (13)

The units of stress f, fa and fm are now expressed in Ib/inch-ply.

To prevent overstress of the bottom fibers, the combined stresses must not exceed the working tension rating Tr of the belt. This means that the following equation must be satisfied:

≥  + (14)

where:

Tr – working tension rating of the entire belt (in Ib)

Y’ – distance from the neutral axis N.A. to the extreme bottom fibers (in inch).

Solving for r yields the following:

r ≥  (15)

To maintain a minimum tension of 30 Ib/inch on the top fibers, the following equation must be satisfied:

≤  – (16)

where:

y” — distance from the neutral axis N.A. to the extreme top fibers (in inch).

Solving for r yields:

r ≥  (17)

Next, consider a belt which is carried on three-equal-roll idlers of troughing angle Φ. We assume that the belt is divided into three equal parts, as illustrated in Fig. 14.

If the vertical curve under consideration is concave (that is, belt edges are at the inner side of the curve), then y’ = (b/9) sinΦ and y” = (2b/9) sinΦ. Eqs. (18) and (19) follow when these identities are substituted into Eqs.(15) and (17), respectively, and the equations are divided by 12 so that R is the radius of curvature in feet rather than inches:

– to prevent overstress of the middle when the curve is concave:

R ≥   (18)

– to prevent edge buckling when the curve is concave:

R ≥   (19)

If a convex vertical curve is considered (that is, belt edges are at the outer side of the curve), then y’ = (2b/9) sinΦ and y” = (b/9) sinΦ. Eqs. (20) and (21) follow when these identities are substituted into Eqs. (15) and (17), respectively, and the equations are divided by 12 so that R is in feet rather than inches:

– to prevent edge overstress when the curve is convex:

R ≥   (20)

– to prevent buckling of the center when the curve is convex:

R ≥   (21)

Eqs. (18), (19), (20) and (21) are equivalent to the CEMA equations [8] and apply to multi-ply fabric belts. For steel cord belts, these equations are applicable if p = 1 and the elastic modulus Bm is in Ib/inch.

Eqs. (18) through (21) reveal the governing parameters as they relate to the allowable radius of curvature. The belt width b and the troughing angle Φ must be established ahead of time so that they are compatible with the capacity requirements and maximum lump size of the material to be handled. For any solution of category e), the trough depth shall not be less than the maximum lump size. With the belt width and troughing angles established, the remaining variables are the composite elastic modulus of the belt Bm p (in Ib/inch), the rated working tension Tr (in Ib) and the operating belt tension at the point under consideration Tc (in lb). For a concave curve, Eqs. (18) and (19) must be satisfied simultaneously. Tc may be increased by an increase in tail tension in order to counteract the buckling tendency, but not to the point where the working tension rating Tr is exceeded. The same arguments apply to Eqs. (20) and (21) for convex curves. In order to minimize the radius of curvature without violating the governing equations, we must seek a belt which maximizes the ratio of the rated tension to the elastic modulus (maximize Tr/Bm p). However, very low values of Tr cannot be acceptable since this would severely limit the elevating height of any single run. The nylon-reinforced belts offer the best solution of the commercially available multi-ply fabric belts.

If we consider a standard nylon fabric belt, 60inch (1,524 mm) wide, carried on equal-roll -30° troughing idlers and subject to a convex transition curve, we could except an allowed radius of curvature in the range of 50 to 80 ft (15.2 to 24.4m), depending on the tension rating Tr and belt modulus Bm of the commercial belt chosen. One could expect radii of curvature equal to half of this value if a special low modulus belt is used.

With a steel cord belt, the minimum allowed vertical radius of curvature would typically exceed ten times that of the standard nylon fabric belt. In open pit mining applications, the steel cord belt is most practical in a single-run application with the consequential imposition into the pit, as illustrated in Fig. 12. Aramid-fiber-reinforced belts also appear promising in future high lift applications. These offer the strength of steel at lower weight and approximately one-third the belt modulus Bm.

Category e) transition curves (see Fig. 15) are not possible with the large transition curves required for steel cord or aramid fiber belts. If category a) or b) solutions are employed, as shown in Fig. 16, then an additional constraint applies to the transition curve. Since the carrying belt is supported on troughing idlers and is uncovered from the loading area to the sandwich entrance, uplift of the belt in this region must not occur when starting or running empty.

Transition profiles of short radii of curvature lend themselves to a category @) solution, as illustrated in Fig. 15. The belt tension required at the tail end to suspend the material and carrying belt, as dictated by Eq. (5), is low enough to make this solution very attractive.

Utilizing two nylon fabric multi-ply belts, driven by a load sharing system, net elevating heights of up to 350 ft (107 m) could be obtained without oversizing the belts specifically to maximize lift. A single-run conveyor, utilizing two steel cord belts driven by a load-sharing system, could achieve practical elevating heights above 1,000 ft (305 m).

3. The HAC® — Product of Evolution

Following the extensive study of past sandwich belt conveyors and development of the governing design criteria, a broad scope effort was undertaken in 1982 at Continental Conveyor & Equipment Co., Inc., USA, to develop the first sandwich belt high angle conveyor which meets the broad range of needs of the mining and materials handling industries. Towards this end, many mine and terminal operators and planners were consulted at various stages of development.

The resulting HAC®s, illustrated in Figs.17 to 24 and Tables 1 to 3, are truly evolutionary in judiciously selecting and advancing past desirable features while omitting the nondesirable features. They are entirely conforming to the governing theory and constraint equations and to the development criteria previously discussed.

As noted in the previous discussion, conveyors which relied on a self-weighted cover belt did not prove practical for conveying angles approaching 45°. The category b) pressed cover belt conveyor, illustrated in Fig. 4, had limited success on bucket wheel excavators, but was eventually abandoned, for reasons which included the accelerated wear of components. This is not surprising since the total required hugging load was applied as large discrete loads spaced at large intervals along the conveyor. Operational requirement 2. addresses this problem by calling for a soft, floating system which minimizes load concentrations.

The HAC fulfills all of the established operational requirements. It is as well suited to a self-contained modular approach, utilizing nylon fabric belts to achieve short vertical radii of curvature, as illustrated in Figs. 17 to 20 and 22 to 24, as it is to a single-run approach, utilizing steel cord or aramid fiber belts, as illustrated in Fig. 21.

 

Materials conveyed	Lignite, coal, copper ore and waste rock, iron ore pellets, sand, gravel, grain
Conveying angle	variable from 30° to 60°
Design conveying rate at an angle of 60°	1,955, 2,468, 2,684 t(metric)/h (2,155, 2,721, 2,959 short tons/h), respectively, for materials of a density of 0.8, 1.6, 2.4 t(metric)/m3 (50, 100, 150 lb/ft3)
Belt width	1,524 mm (60 inch)
Belt speed	Infinitely variable from 0 to 6.1 ms/ (0 to 1,200 ft/min)
Elevating height	Variable from 7.9 to 19.5 m (26 to 64 ft)
Overall conveyor length	35 m (115 ft)
HAC conveyor drives
Top belt	74.6 kW (100 HP)
Bottom belt	111.8 kW (150 HP)

 

Materials conveyed	coal
Conveying angle	60°
Conveying rate:
Design	1,814 t (metric)/h
	(2,000 short tons/h)
Surge	1,996 t(metric)/h
	(2,200 short ton/h)
Belt width	1,524 mm (60 inch)
Belt speed	4.65 m/s (915 ft/min)
Elevating height, loading point to discharge	32.9 m (108 ft)
Horizontal projection:
Loading point to sandwich entrance at a conveying angle 0°	11.0 m (36 ft)
Sandwich entrance to discharge	25.6 m (84 ft)
Overall conveyor length	56.7 m (186 ft)
HAC conveyor drives
Top belt	112 kW (150 HP)
Bottom belt	149 kW (200 HP)

 

Materials conveyed	copper
Density	2.08 t(metric)/m3 (130 lb/ft3)
Lump size	250 mm (10 inch) max
Conveying angle	35.5°
Conveying rate	4,000 t(metric)/h (4,409 short tons/h)
Belt width	1,524 mm (60 inch)
Belt speed	4.65 m/s (915 ft/min)
Elevating height	93.5 m (307 ft)
HAC conveyor drives:
Top belt	450 kW (600 HP)
Bottom belt	2 x 450 kW = 900 kW (1,200 HP)

3.1 Modular Approach

In a self-contained moderate lift unit (to approximately 350 ft (107 m)), the HAC is a combined category b) and e) solution.

It is the transition curve, from the low angle loading area to the straight high angle incline, that falls into category e). Here all material hugging load is derived from radial pressure due to belt tension and the profile geometry. Nylon fabric belts are used to minimize the transition radii of curvature while conforming to the previously developed constraint equations.

The HAC transition zone is the most significant advance, to date, in sandwich belt high angle conveyor technology. It follows logically from the previously discussed Loop Belt, but is not limited in geometrical conformance to any desired slope. We recall that the Loop Belt was virtually unlimited in conveying rate, but was constrained to a semi-circular profile geometry. Thus it could not convey along a straight incline. The introduction of an inflection zone at the top of the transition curve, just prior to the straight elevating portion, permits the HAC to conform to any profile geometry while retaining the unlimited capacity feature.

Idler support of the carrying belt sandwich is always on the concave side of the transition curves, with the belt path always entering or exiting along tangents to these curves, regardless of the conveying rate, from empty to severe overload. This insures a smooth material flow path without obstruction to the largest of lumps or to foreign objects that may enter the material flow.

The category b) solution is in the cover belt hugging means along the straight incline. This, too, is a significant advance in sandwich belt conveyor technology. Full equalized pressing rolls, closely spaced along the length and across the belt width, gently hug the material along the straight incline.

Deflection of a pressing spring provides the correct amount of hugging pressure without incurring load concentrations. The spring deflects, thus the pressure increases as material within the sandwich raises each pressing module. The amount of pressure is thus self-regulating. Because they are fully equalized and in independent modules of eight, the pressing rolls conform completely to the cross-sectional and longitudinal configuration of the conveyed material.

The same belt tension which produces the required hugging pressure along the transition also enhances performance of the pressing rolls. The pressure spreads gently over the material because the taut belt resists local point deflections.

3.2 Single-Run Approach

In the single-lift application, illustrated in Fig. 21, (above approximately 350 ft (107 m)), belting must be reinforced with steel cords or aramid fibers. In either case, the radius-of-curvature constraints make a category-e)-type transition curve impractical. High lift HACs are therefore category-b)-type solutions only, with the pressing rolls applied throughout the transition curve. Radii of curvature are large, and belt tensions are typically high, thus also enhancing the performance of the pressing rolls as in the modular solution.

While the modular self-contained unit may be designed for various degrees of mobility or relocation [14], the single-run unit will typically require large foundations, at the terminal ends, to relieve the intermediate structure of the very high belt tensions, thus making such installations more permanently located.

3.3 Advantages of the Continental Conveyor Sandwich Belt HAC

HACs can take on various forms and offer many advantages over other systems, including:

1. Simplicity of Approach The use of all conventional conveyor hardware means interchangeability of components, fast delivery of replacement parts. Operating experience thus far has revealed that HACs have very high availability and low maintenance costs.

2. Virtually Unlimited in Capacity The use of conventional conveyor components permits high conveying speeds. Available belts and hardware up to 120 inch (3,000 mm) wide make capacities greater than 10,000 short tons/h (9,072 t(metric)/h) possible.
3.  High Lifts and High Conveying Angles Lifts of up to 350 ft (107 m) are possible with standard fabric belts, and single-run lifts greater than 1,000 ft (305 m) are possible with steel cord or aramid fiber belts. High angles of up to 90° are possible without excessive wear because of the soft, floating, fully equalized hugging pressure.

4. 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 using externally anchored, high angle conveyors. In either case, the conveyor unit may be shortened or lengthened or the conveying angle may be altered according to the requirements of a new location. High angle conveying modules may be mounted on rails, rubber tires or crawler-type transporters, or they may be equipped with walking feet for optimal mobility.  

6. 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. Quick repair means less costly downtime.
7. Spillage-Free Operation During operation, the material is sealed between the carrying and cover belts. Well centered loading and ample belt edge distance result in no spillage along the conveyor length.

HAC® Applications

The wide variety of possible HAC applications in open pit mining have been previously documented by the author in a Paper entitled “Sandwich Belt High Angle Conveyors – Applications in Open Pit Mining” [14]. Others have also studied their use in specific open pit and underground mining applications [15]—{17]. An upcoming paper will describe the wide variety of possibilities in continuous ship loaders and unloaders, dock elevating conveyors, stackers and stacker/reclaimers, midstreaming, coal preparation and many other yard and dock applications. This is owing to the versatility of the concept, which results from the ambitious broad scope objectives originally set for the development effort.

HACs are now well into the commercial stage, with the first commercial unit in operation since June 1984 (see Figs. 22 and 23 and Table 2) and the second commercial unit (see Fig. 24 and Table 3) in the final stages of engineering, with start-up in a Yugoslavian copper mine scheduled for 1987.

The first large scale HAC, however, began operation in June of 1983. It was at the 60 inch (1,524 mm) belt width prototype (see Figs. 17 to 20 and Table 1) that a year long testing program was conducted to verify the theory and develop specific design criteria. Because of the versatility built into this unit, including various incline angles from 30° to 60° and various belt speeds, it was possible to investigate the limits of the concept.

The program included large scale testing of basic material properties and their relation to conveying characteristics at the HAC at various conveying angles, speeds and degrees of cross-sectional filling.

This unit permits conveying a wide variety of materials at various incline angles from 30° to 60° and at various belt speeds.

Materials tested included Texas lignite, Alabama coal (run-of-mine, lump, sized, sized and washed), Arizona and Tennessee copper ores, river run gravel, soybeans, iron ore pellets and sandstone. Testing proved successful in all cases, with all materials conveyed at various speeds at a conveying angle of 60°. At the 60° angle, the demonstrated conveying rates exceeded 2,000short tons/h (1,814 t(metric)/h) with coal and lignite, 3,000 short tons/h (2,722 t(metric)/h) with copper ore, gravel and sandstone, 51,000 bushels/h (1,795 m3/h) with soybeans and 3,100 short tons/h (2,812 t(metric)/h) with iron ore pellets.

In addition, 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 60°, the full length of the HAC prototype, two, four, six and eight times, respectively, for corresponding conveying distances of 150ft (45.7m), 300ft (91.4m), 450 ft (137.2 m) and 600 ft (182.9 m). Samples (2,555 g) from each bushel sack were then analyzed at a State of Alabama Department of Agriculture and Industries laboratory for the various forms of damage and contamination and at the Alabama Department of Agriculture and Industries State Seed Laboratory for germination potential. The results showed no damage to any of the three grains tested as a result of conveying in the HAC prototype.

Conveying at 60° has proved very successful and indicates no limit on the conveying angle up to 90° (vertical). HACs are thus being quoted with conveying angles of up to 90°, and engineering of a model test unit, to demonstrate vertical conveying, is presently underway.

Success in testing and convincing demonstrations led to the sale of the first commercial HAC unit (main features described in Table 2) to a western coal mining company in the latter part of 1983. That unit was commissioned in June of 1984 and has to date operated successfully. (See also Continental HAC advertisement for system description.)

The second commercial HAC unit (main features described in Table 3 and illustrated in Fig. 24) was sold to a Yugoslavian copper company and will be installed within a deep open pit copper mine. This HAC is part of an in-pit crushing and conveying system which incorporates in-pit trucking to portable crushers to an in-pit conveying system which feeds the HAC. The HAC elevates the crushed ore from the pit onto an out-of-pit conveyor system to the coarse ore pile at the plant. The HAC unit incorporates many design features which make it especially suitable for elevating coarse copper ore from the deep open pit mine. It is designed to permit installation of a second future unit to elevate ore from a deeper pit location onto the tail of the first HAC. Only the most rugged conveyor components are used to insure smooth running in a very severe open pit mining environment. The modest 35.5° conveying angle is limited by the stability of the mine slope.

5. Summary and Conclusions

A broad scope development effort has led to the development, at Continental Conveyor, USA, of sandwich belt high angle conveyors – HAC®s – which are virtually unlimited in tonnage rate and geometrical conformance. This development is evolutionary and conforming to the constraints of conventional belting and component technology and to the established criteria for operation in open pit mines and other materials handling industries.

HACs are proven as a result of a year-long testing program and are offered with conveying angles of up to 90°.

The first commercial HAC unit has been in successful operation for nearly two years, with the second commercial unit in its final engineering stages. Start-up is scheduled for 1987.

Because of proven trouble-free performance, mine and terminal operators and planners may now exploit the cost and operational advantages of HAC®s in their present and future systems, with the same degree of confidence that is afforded to conventional belt conveyors.

References

[1] Mevissen, E.A., A.C. Siminerio and J.A. Dos Santos: High Angle Conveyor Study. By Dravo Corp. for the Bureau of Mines, U.S. Department of the Interior, under BuMines Contract No. J0295002, 1981, Vol. 1, 291 pages, and Vol. II, 276 pages.

[2] Dos Santos, J.A., and E.M. Frizzell: Evolution of Sandwich Belt High-Angle Conveyors; CIM Bull. Vol. 576 (1983) No. 855, pp. 51 – 66.

[3] Conveyor Equipment Manufacturers Association (CEMA): Belt Conveyors for Bulk Materials. CBI Publishing Co., Inc., Boston, 2nd Ed., 1979.

[4] Rasper, L.: The Bucket Wheel Excavator – Development, Design, Application. Trans Tech Publications, Clausthal-Zellerfeld, Federal Republic of Germany, 1^st Ed., 1975.

[5] Gartner, E.: Entwicklungstendenzen in der Gerateund Férdertechnik der rheinischen Braunkohlentagebaue (Development Trends of Machines and Material Handling Techniques in the Rheinbraun Open Pit Mines); Braunkohle, Warme und Energie (1955) No. 11/12, pp. 226—241.

[6] Rasper, P.: Bank/Steilférderer im deutschen Braunkohlentagebau (Elevating Belt Conveyors for Lignite Open Mining in Germany); Deutsche Hebeund Férdertechnik im Dienste der Transportrationalisierung (1958) Dec., pp. 25—29.

[7] Colijn, H.: Design Considerations for Belt-Conveyor Systems; Iron and Steel Engineer (1962) Aug.

[8] Wancheck, G.A., and R.S. Fowkes: Materials Handling Research: Shear Properties of Several Granular Materials. BuMines Contract No. RI7731, 1973.

[9] Vierling, A.: Die Keilband-Férderanlage, ein neues Mittel zum steilen Férdern von Massengitern; Braunkohle (1935) No. 11, pp. 161—169.

[10] Stephens-Adamson: The Stephens-Adamson Loop Belt Elevator. Belleville, Ont., Advertisement.

[11] Loop Belt to be Installed in Port Cartier; Engineering and Mining J. (1977) Feb., p. 114.

[12] Stephens-Adamson: Beltavator. Belleville; Ont., Advertisement.

[13] R.A. Hansen, Co., Inc.: Development of Cross Pit Overburden and Waste Material Handling System. BuMines Contract No. H0252084, 1979.

[14] Dos Santos, J.A.: Sandwich Belt High Angle Conveyors — Applications in Open Pit Mining; bulk solids handling Vol. 4 (1984) No. 1, pp. 67—77.

[15] Mitchell, J.J.: High Angle Conveyors Climb to the Top; Coal Mining (1984) Nov., pp. 39—43.

[16] Mitchell, J.J., and D.W. Albertson: High Angle Conveyor Offers Mine Haulage Savings. Materials Handling Conf. BeltCon 3, Sept. 9—11, 1985, Johannesburg, 12 pages.

[17] Huss, C.E., N.G. Reisler and R.M. Almond: Practical and Economic Aspects of In-Pit Crushing Conveyor Systems. SME-AIME Fall Meeting, Oct. 1983, Salt Lake City, UT, USA, pp. 15—31.