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

For a cohesion-less material one can idealize the situation as shown in Figure 1.  The material is idealized as parallel layers spaced infinitely close.

For the case where 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 or

                                                   Wm sin a > (N + Wm cos  α)μ’……….(1)

                                                   where:

                                                   μ = μ_m or μb whichever is smaller.

                                                   Wm, α, N, μ_m, μb, are as defined in Figure 1.

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

                                                   N ≥ Wm ( sin α – cos α) ……….  (2)

If the cover surface is fixed and it resists the motion of the material at the interface, then material will begin to slide back when:

                                                   Wm sin α >N (μ” + μ‘) + (Wm cos α) μ‘………. (3)

                                                   where:

                                                   μ” = μ_m or μ, whichever is smaller. μ, is as defined in Figure 1.

To achieve an inclination angle α;

                                                   N ≥ Wm      μ‘   ( sin α- cos 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 Equation 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 belt, a weighted belt, a link chain matt, etc.  For a covering surface which depends on its self-weight, the normal lineal hugging load N becomes the normal component of the lineal weight of the cover surface.  The second set of equations, 3 and 4, clearly show 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 conveying methods, P. Rasper breaks down the sandwich belt approach into three different categories:

(a)  Cover belt acting by its own weight.

(b)  Cover belt with additional pressure.  

(c)  Cover belt with carriers.

Figure 2 illustrates a category (a) solution which was patented in West Germany in 1953.  It consists of a carrying belt conveyor on three-roll, 30-degree troughing idlers with special spring-suspended impact idlers at the loading area, a cover surface which consists of a link chain matt, and a cleated belt to drive the cover surface.  Special grooved return pulleys are used to carry the link chain cover matt.

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 its self-weight can be uneconomical at higher conveying angles because the required lineal weight of the covering surface increases drastically.  Some elevating conveyors were built according to these ideas.

Figure 3 illustrates a category (c) solution which was also patented in West Germany in 1953.  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.  The main selling feature of this method was the claim that the cover surface was self-cleaning because of the flexibility of the carriers.  No elevating conveyors were ever built according to this design.

In his book The Bucket Wheel Excavator, L. Rasper discusses the elevating conveyor illustrated in Figure 4.  It is a category (b) solution which was patented in West 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, this solution was used on several occasions on bucket wheel excavators of the 1950s.

As a result of his survey, P. Rasper 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
3. Any high-angle conveyor must lend itself to easy cleaning. 

Requirement 1 illustrates a recognition of additional wear on the belt and other components when operating with the extra pressure from 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 guard against this so the effort must be to minimize the consequential downtime.  For a regulated and more predictable material and size distribution, that is, the discharged material after primary crushing, good design can reduce the frequency of damage.  The shape of the lumps cannot, however, be controlled. P. Rasper suggests that the repair be made by hot or cold vulcanizing and, on this basis, rules out the use of fins or cleats.

Requirement 3 is based on a 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.

L. Rasper also points out that most of the development and use of high-angle conveyors on bucket wheel excavators occurred in the 1950s and that the bucket wheel excavators commissioned in the 1960s had abandoned them in favor of higher tonnage rates and reduced belt wear of conventional conveyors.  This reduced the cutting depth of the machine, but operational experience showed that deep-cut cohesive soils and excavated lignite permitted conveying angles of up to 23 and 27 degrees, respectively, with conventional conveyors.

More recent developments in the category (a) solutions include the Retainer Belt by Stephens-Adamson and the Overlay Conveyor by R.A. Hansen Company.  Both claim the capability to convey material at angles up to 45 degrees.

The Retainer Belt, as illustrated in Figure 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 for 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 conveying 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.  Other than the special cover belt, it uses only conventional conveyor components.  The high cost of the special cover belt, however, makes the Retainer Belt expensive at high angles.  There is no apparent capacity limitation other than those which govern conventional conveyors.

The Overlay Conveyor is illustrated in Figure 6.  It also conforms well to the second and third operational requirements set by P. Rasper, but it does not address the need for hugging pressure to keep the material from sliding back.  It uses an ordinary (non weighted) 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 speed-up 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.

It is argued that the ordinary non weighted cover belt is sufficient to keep the large lumps from rolling back and the rest of the material will not slide back.  It is also argued that the speedup belt will throw the material into the loading area at a prescribed speed and angle of incidence so that the momentum of the material will carry it into the belt sandwich with a minimum amount of turbulence.  This justifies loading an uncovered conveyor at the high angle.

Two test units were built to verify the theory.  The first used a 20-in. cleated carrying belt with an 18-inch smooth cover belt and conveyed wet sand with 10 pct rocks at 45 degrees while the second used two 60-in. smooth belts and conveyed wet sand with 10 pct rocks at a conveying angle of 40 degrees.  These test units were successful in conveying wet sand at high conveying rates and wet sand with 10 pct rocks at reduced rates. When the rocks exceeded 10 pct, they would roll and bounce in the loading area without entering the sandwich.  The ability to convey a limited type of material, without additional weight on the cover belt, illustrates the advantage of driving both belts and indicates that the effective friction angles within the sandwich may be quite high.

In light of the state-of-the-art and current developments in sandwich belt-type high-angle conveyors and the requirements of the present development, two new categories (d) and (e) are added to the possible solution types as listed by P. Rasper.

The current solution categories are then as follows:

(a)  Cover belt acting by its own weight.

(b)  Cover belt with additional pressure.

(c)  Cover belt with carriers.

(d)  Belt sandwich with prying resistance by virtue of the transverse stiffness of the two belts.

(e)  Belt sandwich with radial pressure by virtue of the belt tension and the conveyor profile geometry.

The Loop Belt by Stephens-Adamson represents a category (e) solution.  It is illustrated in Figure 7.

With this concept, a non weighted 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.  When the conveyed material enters the belt sandwich, the carrying strand (outer belt loop) is no longer supported by idlers.  As the belt sandwich enters the ver­tical curve, the belt tension and the radius of curvature are such that sufficient radial pressure is produced to overcome the weight of the belt and the conveyed material.  The material is thus conveyed in a sandwich made up of the suspended carrying belt and the idler-supported inner belt loop.  This sandwich loops around, approximately 155 degrees, to the discharge point.  The lineal hugging load produced by the outer belt is determined by equation 5.  As the conveying angle increases, the vertical radius of curvature and the outer loop-belt tension must provide sufficient radial pressure to support and hug the suspended material in order to convey it up to a 90-degree angle.  Above the 90-degree angle, the material begins to sit on the supported inner belt loop; and the outer belt loop becomes the cover belt which supplies the hugging force.  The material is conveyed in this manner to the discharge point.  Typically, the outer belt loop is driven while the inner belt loop is forced to follow by the frictional drag of the conveyed material.  There has been occasion to drive both belts by a load-sharing drive arrangement which utilizes the combined tension rating of both belts in order to maximize elevating height.

                                                    Pr = T ……….(5)

                                                    where:

                                                    T = Outer belt tension at the point in question on the conveyor profile.

                                                    R = Radius of curvature corresponding to the belt tension.

                                                    Pr = The corresponding lineal load applied by the outer belt.

The Loop Belt has had great success in self-unloading ship applications and several outdoor installations.  It has achieved 10,000 tons per hour, in conveying iron ore pellets and coal, and lifts up to 150 feet.  Typical belt speeds are 860 to 1,200 feet per minute.  A special low-modulus fabric belt is used to achieve tight vertical curves with a troughed belt.

The Beltavator, by Stephen-Adamson, is a hybrid which represents a solution of categories (d) and (e).  It is illustrated in Figure 8.  The Beltavator is an extension of the Loop Belt.  It is identical to the Loop Belt up to a conveying angle of 90 degrees.  At this point, it conveys the material vertically for a desired elevating height and it again becomes identical to the Loop Belt above the 90-degree conveying angle.  It is the straight vertical portion of the conveyor profile that qualifies as a category (d) solution.  On the straight 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.  The hugging pressure required to convey the material at 90 degrees is provided by the prying resistance of the two belts as the material is introduced into the sandwich.  The prying resistance is by virtue of the transverse stiffness of the two belts.  The required transverse stiffness imposes a capacity limitation on the Beltavator.  As the belt width requirements become large, in order to achieve higher tonnage rates, increasingly thicker multi-ply fabric belts are required to provide adequate transverse stiffness.  Such considerations limit the practical belt width to a maximum of approximately 36 inches, and maximum capacity is approximately 1,000 tons per hour when conveying dense material.  Only the outer or receiving belt is typically driven as was the case in the Loop Belt.

The Beltavator, too, has had great success within its capacity range.  Its conveying profile can follow a “C” Belt Path,” a “Z Belt Path,” or an “L Belt Path,” as shown in Figure 8.

It is important to point out that solutions which fall into categories (d) and (e) represent a significant advance in the historical development of the sandwich-type high-angle conveyors.  All of categories (a) through (e) address the need for hugging pressure which increases the frictional resistance to relative movement between adjacent particles of the conveyed material and between material and belt surfaces.  The significance is that solution categories (d) and (e) do not require added weight or a pressing mechanism.

In the category (d) solution, the hugging pressure results from prying resistance and the transverse stiffness of the two belts,  this transverse stiffness is utilized to an advantage, but it exists whether or not it is utilized.

In the category (e) solutions, the hugging pressure results from radial force by virtue of the tangential belt tension and the curved geometry of the conveyor profile.  Here the belt tension is used to an advantage by the selection of a particular conveyor profile geometry.  The belt tension at any point on the profile is related to the tension at the tail pulley, the lineal material load, and the height above the tail pulley.  As with the belt transverse stiffness, it exists whether or not it is used to an advantage by the selection of the appropriate conveyor profile geometry.

Additional implications in category (e) solutions are discussed later when addressing the vertical radius of curvature requirements as specified by CEMA.

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 equations 2 and 4 that the required hugging pressure is greatly reduced by exerting drag 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.

The Belt Sandwich model, illustrated in Figure 1, is very instructive but it is not totally accurate.  It assumes that the cover surface contacts only the material, but the edges do not touch the carrying belt.  It is known that lateral movement of the cover surface during operation will cause the edges to bear, intermittently, on the carrying belt; thus losing a portion of the hugging load directly to the carrying belt and support idlers, while uncovering the material at the other edge.  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.  On this basis, Stephens-Adamson chooses to derate the capacity of the sandwich type conveyors (as compared to CEMA capacity recommendations for conventional conveyors).  This assures large edge distances and, thus, a sealed envelope.

A new, more realistic model (Fig. 9), illustrates the interplay of forces.  The minimum normal hugging load, Nm; which must be exerted on the material in order to prevent backsliding, if both surfaces resist motion, is expressed by equation 6.

                                                    (min) Nm = Wm sin α cos a) ………. (6)

This follows from equation 4.  The drag which must be exerted on the material by the top surface must be counteracted by the frictional drag between the top and bottom surfaces at the edges, as expressed by equation 7.

                                                    (min) N, µ_r = (min) Nm μ, ………. (7)

The minimum required total normal load, N, can be expressed by combining equations 6 and 7 to obtain equations 8 and 9.

                                                    (min N = (min) Nc + (min) Nm = μ” + 1) (min) Nm ………. (8)

                                                    (Min) N = (μ” + 1) Wm μ” sin α –  cos α)……….(9)

If both carrying and cover surfaces are of rubber conveyor belting, then μ‘ = μ“.  If = μ’ = μ”, equation 9 reduces to equation 2.  It will be shown later that this is, in fact, a reasonable assumption.  Equation 2 assumes that only one surface is driven.

At first impression it would seem that nothing has been gained buy using the new more realistic model, because the required total normal load, N, is the same when only one belt is driven regardless of which model is used.  What is important is the recognition of a more realistic behavior and that only the portion of the normal load which bears directly on the conveyed material actually contributes to an increase in the impact on each successive idler as the material is carried along with the belt, and thus, only this portion contributes to any increase in belt wear.  The required edge distance corresponding to the assumption that μe = μ^‘ = μ” is b/4, where b is the belt width.

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 are circumstances which would cause doubt that the required counteracting edge drag could be developed, that is, μe < μ”, then it would be advantageous to drive both surfaces.  In order to maximize total elevating height, it is advantageous to drive both surfaces by a load-sharing drive arrangement so that the combined tensile strength of both belts is utilized.

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 (now numbered 4 and 5), but omits his requirement 1 because 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 the turbulent material at the load point is settled quickly and experience no backsliding prior to entering the sandwich.
2. The hugging pressure exerted on the conveyed material and, thus, the additional pressure on the cover surface must be applied by a “soft” loading system which minimizes load concentrations.
3. The cover surface must be a floating surface which will not obstruct the flow of lumps larger than anticipated or oriented unfavourably.
4. The system must lend itself to easy and quick repair of the
5. Any high-angle conveying system must lend itself to easy cleaning.

Requirements 4 and 5, which have already been discussed, preclude the use of carriers, and makes it impossible to cover the material immediately as it is fed into the high-angle conveyor.  The Overlay Conveyor by R.A. Hansen Company(¹¹) was loaded at a 40-degree angle 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 when attempting to convey wet sand with 10 pct 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 because 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 re-accelerate any material remaining at the loading area after the shutdown.  Requirement 1 recognized the problems associated with loading a sandwich belt conveyor at a high angle.  On this basis, it is recommended that the conveying angle onto which material is loaded should not exceed 10 degrees, and the material should enter the sandwich at a maximum conveying angle of 18 degrees.

Compliance with requirement 2 will minimize the additional wear on the belts and other components due to the additional hugging load.  This is very important because L. Rasper cites the reduced belt wear of conventional conveyors as one reason why the bucket wheel excavators of the sixties 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 minus 8-in. setting could result in slabs of dimensions of up to 8 by 12 by 24 inches. The loading area should be designed to assure that such slabs are oriented favorably.  Requirement 3 assures that over sized or unfavorably oriented material will not encounter pinch points and will result only in local lifting of the cover and possible spillage of the material.  It should be emphasized that over sized lumps and slabs of the dimensions mentioned are not to be considered part of the normal operation.  If slabby material is to be crushed, it may become necessary to reduce the crusher setting.