Rock Crushing

Rock Crushing

An objective of the present contract is to “provide a concept for the design of a portable underground hard rock crusher in order to insure that future development will lead to maximum utilization by industry”. The preceding section has concluded that the industry can indeed use such a machine and that, within desired performance and dimensional parameters defined by this study, no standard crushers are suitable for handling hard rock.

As indicated in Section 3, and stated in standard references such as (5),and (6), hard rock of large feed dimensions is best handled by jaw and gyratory crushers. This conclusion is of little value for present purposes unless we can determine fundamentally why these machines, and only these machines, are satisfactory. Using this knowledge, then, we stand a much better chance of devising satisfactory new concepts.

An examination of jaw and gyratory crushers, in comparison to other types used on weaker materials, indicates that the former are distinguished by the following fundamental characteristics:

  1. They are stronger. This characteristic is terribly obvious, and increasing strength (and drive force) is one means to upgrade the capability of a given crusher type in this respect. (For example, compare rotating pick type feeder-breakers used on copper ore with the same type used on coal.) Great strength also forces other fundamental design features that are not so obvious. In particular, strength requires that the machine contact the rock only with broad, extremely blunt surfaces.
  2. Their crushing action utilizes a limited movement relative to the rock surface that is very small in comparison to feed dimensions, though of course capable of great force within that small movement.
  3. They possess a feed means that absolutely prevents advance of any rock fragment to a position that would require a large deformation. This feature is an unavoidable consequence of the limited crushing motion and broad crushing surfaces noted above. However, on some new crusher concepts this feature may not be unavoidable, and unsatisfactory results (jamming, stalling, or machine breakage) may follow.

The following section describes three new crusher concepts, one of which, though earlier thought to be an attractive new concept, can be discarded (for hard rock) because it clearly does not have the third fundamental characteristic mentioned above.

In view of the strong, and perhaps obvious, conclusion that portable crushers will accentuate the need for breaking occasional oversize feed fragments, some thoughts on handling this problem are also presented.

Portable Crushers

Each of the following subsections presents a new crusher concept for hard-rock, portable, underground applications. The first, which will be rejected, is discussed in part to illustrate the importance of the previously noted fundamental characteristics of successful hard rock crusher concepts. The third, on the other hand, indicates that, while valid for reasonably conventional concepts, it would be inappropriate and restrictive to apply such conventional design criteria to unconventional concepts.

“V” Roller Crusher

Based on the successful development of the RAPIDEX conical reamer, a skewed rolling element crusher was conceived using the same principles. The conical reamer is a roller cutter device which s self-advancing by virtue of its wedge-like shape and skewed rollers. A crusher using the principles would be essentially “inside out”, and it would self-feed rock fragments between the rollers.

Figure 13 is a sketch of basic concept, which consists of opposed rollers arranged in a row of “V” shaped pairs. The rollers are powered (i.e., rotated) and skewed (tilted forward) such that a rock fragment placed within the “V” will be simultaneously propelled forward and drawn downward until it is crushed. Product size is determined by the (adjustable) axial space between rollers. Downward and outward flow of product would provide quick clearing of smaller material, thus allowing effective crushing of larger material carried forward between the rollers.


While all of these features would be desirable, it was noted that a large fragment could simply fall downward between two “V” sections rather than feed downward gradually as intended. From this position a fragment would then be driven forward and crushed substantially in a single, large compaction, in violation of the third listed desirable characteristic of a hard rock crusher. Large downward motion between rollers could be avoided, or at least reduced, by placing baffles between rollers, but this would also stop the free discharge of undersize material–one of the major claimed virtues of the concept.

In conclusion, the “V” roller crusher is judged to be unsuitable for hard rock crushing. It would be suitable, and would provide a good, free flowing design, for coarse crushing of softer of friable materials that can now be handled by conventional roll crushers.

Rotary Jaw Crushers

The jaw crusher, either Blake or overhead eccentric as appropriate. is the conventional machine most nearly satisfactory for the subject hard rock portable application. It is entirely satisfactory in terms of crushing performance feed size, hard rock capability, reduction ratio, product characteristics, throughput, and economy. However, it cannot meet the necessary installed dimension requirements, particularly with regard to headroom. Although basic crusher dimensions (i.e., the jaws themselves) are not too bad, the conventional top feed arrangement requires much too much headroom, particular if slabby material (which would have to be vertically oriented) is to be handled.

It is appropriate, then, to search for a horizontal feed jaw-crusher concept. One obvious approach, tipping a basically conventional jaw crusher on its edge, (i.e. with the eccentric shaft vertical has been attempted in this country, and several such units are said to be in use in an iron mine hematite) in Europe. All use a horizontal chain conveyor travelling just beneath the lower edge of the jaws to move material through the machine. Although this configuration obviously does work, it must do so at some sacrifice in performance. It seems clear that a feed mechanism working only at one edge of the jaws must be at a disadvantage relative to the uniform (gravity) feed of the standard upright configuration. In fact, gravity acting transverse to the horizontal throughflow causes a downward migration of finer material, thus encouraging early choking in the vicinity of the chain conveyor.

Furthermore, the conveyor is itself a high maintenance item, particularly when simultaneously subjected to some crushing forces (as is also the case in rotating pick feeder breakers ).

The “rotary jaw crusher”, to be described, employe a curved flow path in an attempt to both decrease the vertical dimension of the jaws themselves and provide for horizontal feed without the above problems, it achieves uniform feed distribution across the jaws, with at least a portion of this being gravitational, while avoiding transverse migration of material within the jaws. It uses no conveyor within the crushing zone (although for low headroom applications a conveyor may be used to feed the crusher.)

Figure 14a illustrates a typical jaw crusher profile in simplest schematic form neglecting curved “non-choking” jaw features, all of which can be provided later as necessary. After Me Grew let us assume that the included angle between jaw faces is 200, that is. a small value for hard rock. Then, for vertical jaws having a 30 inch inlet and a 6″ discharge, the bare jaw height must be 69 inches. (Recall the striking uniformity of conventional machine heights noted in Section 3.)

Figure 14b illustrates a schematic of an equivalent jaw crusher in which inlet and discharge dimensions and mean path length (hence convergence angle) are preserved while wrapping the mean path around a 180° curve. For these dimensions, the curved path results in a decrease of 7 inches in height (assuming for the moment a circular mean path). While this is not an enormous saving in itself, the configuration does provide horizontal feed, and this is a substantial improvement. Other advantages will become evident as the concept is further described.

Crushing motion of the curved jaw machine may be provided by several means, the most obvious of which would be oscillation of the “external” jaw (the right-hand element in Figure 14b) against a stationary “internal” member. Jaw motion may be maximum at the discharge, in a Blake-type action, or near the inlet, in an overhead eccentric type action, depending upon the choice of the designer. However, it is believed that neither of these will provide the best design.


Figure 15 illustrates what we shall call a rotary jaw crusher having the preferred inner element crushing motion. A cylindrical inner element is driven in an orbiting motion by a central eccentric shaft, essentially identical to that of an overhead eccentric crusher. It is expected that this orbiting motion will require less force than would oscillation of the outer elements, and less force than is required by conventional jaw designs. The latter must subject their entire rock charge to crushing forces simultaneously as the jaws converge everywhere at the same time. Furthermore, with conventional gravity feed of reasonably graded material, it is virtually certain that rock fragments will in fact be tightly lodged throughout the converging crushing zone as the crushing stroke commences. In contrast, the orbiting cylinder of the rotary jaw crusher produces only a local zone of maximum convergence which travels through the rock charge. Hence, although crushing the enclosed rock charge in approximately 180° of eccentric motion (like conventional designs), it does not crush the entire charge simultaneously. The rotary jaw eccentric bearing should thus see a force that is reasonably uniformly spread through 180°, rather than the conventional force which rises to a peak at the end of 180°.

Orbiting motion of the inner element provides one more major advantage if the motion is in the “forward” direction illustrated in Figure 15. In this case, the crushing action moves through the rock charge in the flow direction providing a peristaltic pumping action to assist throughput.

With regard to throughput, disruption of the simple straight through gravity flow of conventional designs is clearly the major drawback of the rotary design. Refering to the limiting 180° design of Figure 15, gravity feed will be effective only in the middle half of the passage. Feed can no doubt be enhanced in the inlet quarter of the passage by “stuffing” this region with a forcing conveyor feed, but no such assistance is available in the discharge region.

With the peristaltic action described above, it is quite possible that no throughput problems will be encountered particularly if the discharge region is cut back as discussed below. However, if difficulties are encountered, it is expected that rotation of the cylindrical inner element about its own axis will be very effective in urging material through the crusher. If simple feed enhancement


is all that is desired, rotary drive via a torque source that acts when large crushing forces are absent would suffice. On the other hand, perhaps considerably more rotary torque would benefit crushing action as well, via shearing forces on the rock (like those of an overhead eccentric design). In fact, one might consider a family of designs which distribute orbiting and rotating power differently for different rocks: ranging from pure orbiting on one extreme to pure rotary (i.e., a single sledging roll crusher) on the other.

Rotation of the inner element (either freely or driven) also provides for balanced wear between the two jaw surfaces, since the full circumference of the inner element is about equal to the total length of the outer jaw. Obviously, both jaws would be provided with replaceable wear surfaces. It may also be beneficial to use different surfaces (for example ribbed or smooth), depending on the proportions of crushing and shearing desired.

Although complete rotary jaw crusher design is beyond the scope of this study. Figure 16 illustrates schematically a more complete concept. Refering back to Figure 15, clearly the greatest throughput problems will occur near the discharge, where neither gravity nor force feed are effective, and where choking would be most likely to occur in a “straight” (i.e., continuously converging) design in any case. Proven methods, described by McGrew, can be used to design “non-choking” discharge regions to ease this problem, but it may also be necessary to simply move the crusher discharge point up as shown in Figure 16 to completely eliminate the problem. Furthermore, since the complete crusher must incorporate a discharge conveyor, the higher discharge point (and correspondingly higher inlet) may not result in an overall taller machine if it allows the elevated conveyor placement illustrated in Figure 16.

The rotary jaw crusher concept has been described in schematic form and certain of its important advantages have been cited. Other advantages are also derived from the curved mean path geometry. In summary, the following features are expected to be of special advantage in portable, low head room, hard rock crushing applications:

  1. Use of proven jaw crusher principles for hard rock crushing for the desired feed size range and throughput.
  2. Slightly reduced jaw height.
  3. Horizontal feed at a point below the top of the machine.
  4. Very simple eccentric drive, at less than conventional force levels.
  5. Compact, stiff geometry, probably even more rugged than conventional jaw geometries.
  6. High efficiency.
  7. The possibility of simple performance improvement by varying crushing and shearing actions to match rock properties (not important in hard rock applications, but of interest in extending the range of the device).
  8. Uniform feed across the jaws, with no conveyor inside the crushing zone.
  9. Superior slab-breaking capabilities.
  10. Little or no danger of discharge conveyor damage caused by slabs extending from the discharge to drag against the conveyor.

Some of these advantages, like slab breaking, high efficiency, and compact, rugged design, are obviously of benefit in any situation.

Hammer Crushers

Section 9.2 discusses the use of impact hammers, probably hydraulically actuated, to break occasional abnormally large material feeding the portable crusher. If suitable means are developed for impact breaking occasional large pieces, then it would be a logical extension of that development to attempt automated breakage of all unbeltable material particularly when the latter constitutes a reasonably small fraction of total production. Such development should follow that of the occasional oversize breaking system (particularly its automated actuation) and, although no overall concept is presented here, the idea is suggested as a goal of impact breakage systems. It would seem to promise extreme portability together with the ability to handle widely varying feed dimensions.


This idea is also present to point out the limitations inherent in the preceding selection of hard rock crusher characteristics.

There is great danger in setting up restrictive design criteria and then attempting to apply these criteria too broadly.

Obviously, an impact breaker has to be strong, but meaningful strength parameters for an impacting machine are quite different from those of a conventional machine which uses essentially static forces and brute strength. For example, an impact bit cannot be blunt, at least in the same sense as a crusher jaw, but other design features, like assuring proper orientation, can compensate for this.

In contrast to conventional machines, an impact breaker definitely should not have a limited motion, since the rock to be broken cannot be well restrained at the time of impact. Thus, the second conventional characteristic actually is not correct for this particular unconventional approach. Finally, since an impact breaker would be intended to produce major fracture in a single blow, the third conventional characteristic is also simply not appropriate in this case.

In summary, fundamental characteristics of successful conventional hard rock crushers have been noted. It is believed that these are very useful in judging the suitability of new concepts utilizing the same basic crushing means, but they are not appropriate, and should not be restrictively used, in judging concepts utilizing different crushing or breaking principles.

Handling of Oversize

As concluded in Section 8, breaking of oversize feed material will be increasingly important as crusher dimensions are reduced to enhance portability. Indeed, the importance of feed size in crusher design suggests that the handling of oversize should be considered an integral part of the hard rock portable crusher development program. Hence, although impact breaker design and application in general are beyond the scope of this study, it is appropriate to discuss breaker problems and features insofar as they relate to portable crusher development. Although oversize feed may be handled at a variety of locations, that most directly related to crusher development would be immediately upstream of the crusher, and it is primarily this location that will be considered.

Problems of Oversize Breakage in Portable Crusher Feed

As a single example most closely allied to crusher design, consider a hydraulic impactor mounted just ahead of the crusher, positioned to break oversize fragments as feed is conveyed past.

Ideally, the device should run without an operator, breaking all oversize material without interrupting throughput. The following are, very briefly, the major problems that can be expected in the development of such a system.

The first problem will be to identify those fragments which are oversize. Once located, each oversize fragment must be properly positioned relative to the hammer, by moving either the rock, or the hammer, or perhaps both. Preferably, if slabby material is being handled, proper positioning will also include advantageous orientation of the rock. When properly positioned, the hammer should strike the rock with enough energy to fracture the piece in a single blow. If the rock does not fracture, or if fragments are still oversize, this must be quickly determined and another blow struck. Proper support of the oversize rock at impact is important, both to promote effective energy transfer from the impacting device, and to insure that impact does not damage the supporting machinery. In view of the variability of rock size and shape, and the possibility of its motion upon impact, the impacting memeber must be capable of sustaining glancing, or even entirely missed, blows without damage.

Many of these problems are already handled to some degree by present feeder units. For example, the typical feeder that utilizes a chain flite conveyor to pull material from the bottom of a surge bin generally extracts small material first. In slabby material the larger fragments are usually well oriented, with the maximum dimension parallel to the conveyor motion, and the minimum dimension normal to the conveyor surface. Combined with suitable gates, sensing devices, and hammers, it is not unreasonable to expect that such a feeder can be equipped to automatically reduce all feed material to a size which can be handled by a portable crusher. It can also be expected that considerable development effort and operating experience will be required before untended operation of such a system becomes routine.

Handling oversize material is a very important mine problem in general, and worthy of considerable attention. The preceding example, though selected because it relates directly to portable crusher design, illustrates many of the problems that might be expected in the development of any automated impact type breaker system, whether it be applied at a draw point, over a grizzly or on a feeder conveyor, and many of the comments in the following subsection are thus of general interest.

Suggested Impact Breaker Characteristics

In a complete study of handling oversize material it would not be appropriate to assume that hydraulically actuated impact devices represent the best or only breaking means. For the purpose of this crusher study, we shall limit this discussion to such devices simply because they are the most nearly suitable of today’s readily available means. That is not to say, however, that a typical off the shelf hydraulic demolition tool is ideally suited to this task.

For the hard rock, portable crushers contemplated in this study, fragments having minimum dimensions of the order of 30 inches would be considered oversize. It is desireable to break such a fragment in a single blow if possible, both to minimize positioning and holding problems and to avoid throughput interruptions, and because it is more efficient. One manufacturer suggests that this requires 1000 to 3000 foot pounds per blow, obviously depending on rock properties. This same manufacturer has found that repeated blows of too little energy tend to drill holes in large fragments without causing fracture.

In view of the generally poor confinement of target fragments and likely positioning errors at the time of impact, an efficient impactor should be capable of delivering an effective blow throughout a rather long stroke — perhaps as long as 12 inches. In this sense, the typical demolition hammer, although certainly the most suitable off the shelf item, is not ideal.

Depending upon overall system design, rapid automatic blow capability may not be required. Thus the rapid cyclic action of a conventional hammer may be economically omitted in favor of a simpler design that triggers discrete blows after proper hammer position is established.

These two features, very long stroke and discrete blows, suggest that it may be appropriate to reexamine the “hurled bit” or “projectile bit” (after reference 8) concept. As the name indicates, this device uses a one-piece bit-piston which is (hydraulically) hurled directly against the rock without the internal metal-to-metal impact of conventional “struck bit” designs. The major virtue of the hurled bit concept is the substantial reduction of peak stress within the steel for a given rock stress), which in turn, for a given blow energy, permits the use of a lighter machine at higher impact velocities. Many of the admitted design difficulties of the concept have to do with rapid sequencing, a feature that may not be required in this application. Furthermore, with proper actuator design, the hurled bit breaker is compatible with very long effective strokes.

Single blow breaking, although fast and efficient, does have one obvious drawback: the required high blow energy may cause damage to the supporting structure. Figure 17 illustrates a novel concept in which the oversize fragment is struck from below, with reaction coming solely from the inertia of the fragment itself, rather than the surrounding machinery. This figure also illustrates a simple gating arrangement which might be used to trigger the impact. Such a design might well use multiple fixed impactors triggered by multiple gates (for example, spread across the width of the feed conveyor) to avoid the complexities of moveable components. The assembly would also require means to contain fragments.

Oversize Breakage in Standard Ore Handling Systems

There is a need, often cited by others, for a better method of controlling oversize, independent of the existance or use of portable crushers. One grizzly-drift block cave mine is experimenting with a low profile crawler mounted impactor, capable of servicing several drifts and many drawpoints, and results to date are promising. The Maysville Operation at Dravo Lime is also using an impactor, mounted on a tractor, to service their “portable” jaw crushers and all the working faces.

Non room and pillar mines using mechanized (non-slusher) face haulage have a common characteristic; namely, the ability to load quite large muck and haul it to a few (relative to production sites) dump points. Grizzlies at the dump pocket represent one method of filtering out problem-causing muck, but the oversize remains, to be handled by costly “secondary” means. These mines may not be able to convert existing rail systems to belts and (if they existed) crusher, but they can consider automatic, untended “devices” at the pockets to break oversize.

A successful “pocket breaker” must be funded (i.e., justified) by savings derived from increased productivity (fewer disruptions), reduced secondary breakage costs, reduced ore pass and chute maintenance, reduced spillage and wear in main haulage, and, perhaps, reduced ore pass costs (size). While these effects are far


reaching, no single item predominates, none are easily estimated, and it is clear that the pocket breaker must be very simple and inexpensive. Impact breakers represent only one potential solution, and since they may not be the most satisfactory or economical, we should consider other means.

Muck at the pocket may have major dimensions exceeding six feet and minor dimensions approaching three feet. Discharge from the pocket breaker should be in the minus 20 to minus 26 inch range in order to eliminate downstream problems (and to enhance eventual conversion to low profile crushers). The tonnage requiring breakage, and the reduction ratio, are therefore quite small, indicating that the pocket breaker need not “run” all the time. A simple jaw, or a vise, perhaps actuated by cylinders, driven by a source of high peak (but low average! power, might be sufficiently simple. Shafts and bearings could be eliminated in favor of less expensive pivots. Servicing should be simple, and the pocket should be useable even if the breaker is not functioning, perhaps by automatically (passively) shunting aside the very large oversize.

Other, and perhaps more exotic, means, may come to mind, but a full treatment is not an objective of this study. Recommendations for this application will follow in Section 10.

Modular Assemblies

The portable crusher performs three basic functions:

  1. It accepts run of mine material from a very unsteady source, such as a load-haul-dump unit carrying perhaps seven tons, or twice that from a telescoping haul unit.
  2. It feeds this material to and through a crusher at an acceptable, essentially steady rate.
  3. It delivers crushed material at an essentially steady-rate to a discharge conveyor or perhaps an ore chute or other haulage element).

Obviously the portable crusher must include some sort of hopper or surge bin to accommodate this highly unsteady delivery, and the hopper design must be compatible with the low head room restrictions and the dumping geometry of the load-haul-dump (or other) haul equipment within those restrictions. Present machines, both the coal feeder-breaker type in use and the horizontal jaw crushers that have been tested, are one-piece machines that feed from the hopper via a chain type conveyor. The feed conveyor also travels through, and is an integral part of, the crushing mechanism.

In soft materials like coal, potash, and trona, feeder-breakers are often self-propelled, offering the ultimate in portability. Applications in harder materials have not enjoyed this degree of portability, although size alone has not been the major problem. Rather, portability has been substantially restricted because of costly and time consuming site preparations deemed necessary in the heavy duty applications. For example, rather extensive foundation structures, requiring subgrade excavation, have been used to avoid damage caused by impact from discharging haul vehicles, and to accomodate the discharge belt. Furthermore, in a wet application any sub-grade excavation must allow additional room to accomodate drainage and clean out functions. Complications such as these make it abundantly clear that the desired hard rock portable crusher should require essentially no site preparation, or at least no site excavation.

After some thought it becomes clear from the foregoing that the desired portable crusher might better consist of a least two independent pieces: a hopper-feeder unit, and a crusher-discharge unit. The former can be virtually identical to the simple, proven hopper end of present machines. The latter, being independent of the present integral feed conveyor, cannot be identical to the present machines. Several significant advantages may be derived from this multiple piece approach:

  1. The independent pieces can clearly be much smaller and more easily moved than the one-piece present machine.
  2. With proper design, the two pieces need not be precisely located relative to one-another, so that some inadvertant movement of the hopper need not effect either the crusher or the discharge conveyor.
  3. Crushing forces are not imposed on the hopper feeder.
  4. The two separate units, each being simpler and more accessible, are more easily maintained.
  5. Separate pieces, again with proper design, may be used in a variety of arrangements (for example, in line or around a corner, ) thus easing site requirements and perhaps simplifying traffic patterns.

The hopper envisioned in this discussion is a very simple device, similar to the present crushing equipment except that the feed conveyor would be inclined to accept input at the necessary low level while discharging into the top of the crusher. With gravity feed into the crusher, and either a large inlet for the latter or a simple chute arrangement between the two, the hopper – feeder need not be fastened to, or even precisely located, relative to the crusher. This would permit easy set-up and it may provide for much simpler protection against impacts from haul vehicles. For example. Figure 18 illustrates schematically a set-up having the following features:

  1. The feeder-hopper is positioned conveniently at the entrance to a heading, permitting simplified traffic patterns and better visibility.
  2. The feeder-hopper is secured in a position by jacking against ribs (bolting to the floor may be preferred by some operators and this is, of course, always possible).
  3. The crusher is mounted under the feeder discharge at an angle to accomodate minor variations in the distance between the desired hopper position and an established discharge belt.
  4. The crusher discharges onto a normal, floor- or roof- mounted discharge belt.

Actual layout of the equipment is, of course, dependent upon a variety of mining conditions. The sketch is intended to suggest one possiblity, and to illustrate the flexibility inherent in a two-piece design.

Modular assemblies, which offer interesting advantages in this simple two-piece concept, are virtually a necessity if additional crusher features are to be provided. For example, if oversize feed is to be broken on the feed conveyor, as discussed in


a preceding section, it is unlikely that a one piece hopper-feeder-breaker-crusher design will be either portable or maintainable. Furthermore, it has been suggested that feed scalping be employed to avoid additional crushing of already beltable material. Suitable equipment for this feature is well within the present state of the art, and development of a one-piece integrated unit is not only not necessary: it may well be undesirable.

In view of the conclusions reached in this Applications Study, presented in Section 8, and reviews of present equipment together with new concepts presented in Section 9, three recommendations are made for further design, development, and testing of a portable hard rock crusher.

Hard Rock, Low Head Room, Portable Crusher 

It is recommended that a program be initiated to develop a hard rock, low head room, portable crusher of the rotary jaw crusher type. It is believed that this concept is the simplest available based on proven hard rock crushing principles, and therefore, it is the best concept for full development.

Although the machine should ultimately be designed within those parameters cited in Section 8, early experimental work can profitably be done on a smaller prototype of perhaps 20-inch critical inlet dimension. The purpose of this experimental phase of the development would be to establish (above ground) proper jaw shape, eccentric motion, and rotary motion to assure proper feed. Once this is assured, full scale underground prototype development could be undertaken with confidence.

Feed System Design

It has been concluded that feed scalping to avoid unnecessary crushing of beltable material would enhance the performance and capacity of any portable crusher. It is not believed that provision of this feature will require an elaborate development program: therefore, initiation of such a program at this time is not recommended, However, when a full scale prototype crusher design is undertaken, it is recommended that feed requirements be defined in suitable terms to permit procurement of a suitable feed system for use in early field tests.

Handling of Oversize

It is recommended that a program be undertaken, in parallel with crusher development, for the development of suitable means for breaking oversize feed material. This program can be divided into three major subprograms and, in view of the widespread occurance of the problem (it has been cited by others), and the variety of possible applications, it is recommended that all three sub-programs be undertaken simultaneously. They are:

  1. Using existing impact actuators, explore and develop automated means to mount, aim, and control such devices for untended operation. Specific applications within the ore handling system would include use at dump pockets, or in conjunction with limited-inlet crushers.
  2. Develop long stroke impact devices for better performance on loosely restrained chunks of oversize.
  3. Explore the application of, and, if practical, develop other concepts, including non-impacting means, in a search for advance oversize breaking systems capable of functioning economically in realistic applications such as (1) above.