Convertible centrifugal rock crusher
BACKGROUND OF THE INVENTION 
     The present invention relates to impact-type crushers and, more particularly, to those which utilize centrifugal force to hurl the rocks to be crushed against an impact surface. 
     Impact-type crushers utilizing centrifugal force to hurl rocks to be crushed are generally known. For example, in U.S. Pat. No. 4,126,280, issued Nov. 21, 1978, to Burk, rock is fed into a rotating impeller which hurls the rock against a plurality of anvils disposed in a ring concentric with the axis of rotation of the impeller. As another approach, U.S. Pat. No. 3,970,257, issued July 20, 1976, to McDonald et al., a rotating impeller throws the rock against a bed of crushed rock instead of the anvils. 
     In either case, a primary design consideration is providing for a sufficient useful life span of the apparatus, particularly those portions of the apparatus which come in contact with the rock as it is hurled. For example, portions of the device, such as impeller vanes and upper and lower plates within the impeller, as subject to a great deal of wear while they are accelerating the rock. As a result, any portions of the apparatus subject to wear require periodic replacement, which necessitates substantial down time for the equipment and incurs considerable cost for replacement of worn parts. 
     Of course, in an anvil-type crusher, such as that shown in Burk, the wear problem is very severe on the anvil surfaces, since the rock is specifically directed thereagainst as it is hurled. In general, therefore, it is preferred to use crushers of the rock bed-type, such as shown in McDonald et al., since the high cost and inconvenience of replacing worn anvils can be avoided. 
     Notwithstanding the foregoing disadvantage of anvil-type crushers, there are circumstances in which it is desirable to use a crusher of this type. For example, a specific type of rock can be thrown against an anvil surface at a much lower speed and yet achieve comparably satisfactory results to what is possible when the rock is thrown against a rock bed. As a result, energy requirements to rotate the impeller are much less. This can be an advantage where sufficient horsepower for impeller rotation is not available, or in cases where energy is available only at great expense. As another example, where the objective in using the crusher is to provide broken faces on the rock particles rather than a reduction in their size, an anvil-type crusher will yield superior results. Additionally, certain materials may require the hard anvil surface to obtain adequate crushing. 
     It will readily recognized from the foregoing that an operator of a crusher may have occasions in which use of an anvil-type crusher rather than a rock bed-type crusher is preferable, or vice versa. To obtain the respective advantages of the two types of crushers, it has heretofore been necessary to purchase two different machines, which represents a substantial financial inventment. In addition, the cost to maintain two different crushers is quite substantial. 
     What is needed, therefore, is a crusher which can be converted from an anvil-type to a rock bed-type or vice versa. Such a conversion must be relatively simple to perform, and must involve relatively few parts. In addition, the crusher must be capable of achieving acceptable performance in either configuration. 
     SUMMARY OF THE INVENTION 
     A convertible centrifugal rock crusher includes a housing having upper and lower cylindrical portions, with the upper portion being of a greater diameter than the lower portion. A circular horizontal shelf ring connects the lower end of the upper portion and the upper end of the lower portion concentrically along a vertically disposed axis. A removable cover is attachable to the upper end of the upper portion. A plurality of removable rock crushing anvils are placeable upon the shelf ring radially about the interior of the upper housing portion. Each of the anvils extends radially inward from the shelf ring and has a working face disposed inwardly of the shelf ring to define an impact surface and a working face diameter. Means secures the anvils to the housing when the anvils are placed upon the shelf ring. An insert means includes a circular horizontal shelf extension ring having inner and outer diameters and upper and lower surfaces. The outer diameter of the shelf extension ring is substantially equal to the diameter of the lower housing portion. Means for selectively securing the shelf extension ring within the housing following removal of the anvils is provided. When in place, the shelf extension ring and the shelf ring cooperate to define a supporting surface for rock material that alternatively defines the impact surface. Impeller means is disposed for rotation concentrically within the housing and is adapted to receive rock to be crushed and to throw the rock against the impact surface. 
     The insert means may include a circular flange having an upper edge and extending upwardly from the inward edge of the shelf extension ring. The means for selectively securing the shelf extension ring secures the ring at a vertical height within the housing such that the upper edge of the flange is at a height generally equal to the lower end of the upper housing portion. 
     The means for selectively securing the shelf extension ring may include a plurality of supporting gussets having upper and outer edges. The gussets are attached in a spaced relationship to the upper surface of the shelf extension ring and extend radially outward therefrom and upward therefrom to a height substantially equal to the height of the upper housing portion. The shelf extension ring is secured by placement of the gussets upon the shelf ring within the housing. 
     Each of the gussets may include a slot defined in its upper edge, with a cylindrical ring being placeable within the slots and being of a height such that placement of the insert means within the housing and placement of the housing cover on the housing causes the cover to contact a rubber seal on top of the cylindrical ring. 
     Means for converting a centrifugal rock crusher of a type utilizing anvils to define an impact surface, from which the anvils have been removed, to a type utilizing a rock bed to define the impact surface, is provided wherein the converting means includes the insert means as described above. The insert means is selectively secured within a crusher such as that also described above. 
     Accordingly, it is an object of the present invention to provide a centrifugal rock crusher that is convertible from a type utilizing anvils to define an impact surface to a type utilizing a rock bed to define the impact surface and vice versa; to provide such a crusher in which the conversion involves the exchange and/or replacement of relatively few parts; and to provide such a crusher which can be operated in either configuration without compromising the performance of the crusher. 
    
    
     Other objects and advantages of the present invention will be apparent from the following description, the accompanying drawings and the appended claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevational view in partial cross-section of a convertible crusher utilizing anvils in accordance with the present invention; 
     FIG. 2 is a portion of a view similar to FIG. 1 showing the convertible crusher utilizing a bed of rock material; 
     FIG. 3 is an exploded perspective view illustrating conversion of the crusher from a type utilizing anvils to a type utilizing a bed of rock material; 
     FIG. 4 is a top plan view of an impeller assembly for use in the crusher with a portion of the upper plate thereof removed; 
     FIG. 5 is a sectional view taken generally along line 5--5 of FIG. 4; 
     FIG. 6 is a sectional view taken generally along line 6--6 of FIG. 4; and 
     FIG. 7 is a perspective view of the impeller assembly with the upper plate removed. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As best illustrated in FIG. 1, the convertible rock crusher of the present invention includes cylindrical housing 10 with concentric upper and lower portions 12 and 14. While the term &#34;rock crusher&#34; of course indicates apparatus for crushing rock, it should be recognized that a rock crusher of the type shown is equally capable of crushing glass, brick, concrete, asphaltic pavement material, and other rock-like materials. 
     Upper portion 12 is of somewhat larger diameter than lower portion 14 and is provided with a top cover plate 16 through which passes a central cylindrical feed tube 18. Feed tube 18 represents one portion of feed means 19 which directs rock to be crushed into the apparatus. The smaller diameter lower portion 14 of housing 10 is open at the bottom 20 in order to permit rock to be discharged from the apparatus once it has been crushed. The cylindrical housing 10 is usually positioned on top of a framework (not shown) which permits the rock to fall from the open bottom 20. 
     Disposed concentrically within housing 10 beneath the feed tube 18 is the impeller assembly 30 which is mounted for rotation in bearing support member 32. The impeller assembly 30 is driven by a central drive shaft 34 having one end extending down through bearing support member 32 into the lower portion of housing 10 where it is connected by pulleys 36 and 38 and V-belt 40 to the drive motor 42. Motor 42 thus rotates impeller assembly 30 at typical peripheral speeds for the assembly 30 in the range of 3500 to 20,000 feet per minute. As a result, considerable centrifugal force is generated within the impeller assembly 30. 
     Disposed concentrically around the impeller assembly 30 within the upper portion 12 of housing 10 are a plurality of adjustably positionable anvils 50. The anvils are disposed so that their end faces 52 form a generally cylindrical impact surface around the impeller assembly 30 against which the rock to be crushed is centrifugally thrown by the impeller assembly. 
     Each anvil 50 is supported by a shelf ring 54, and a cylindrical liner member 56 is positioned within the lower housing portion 14 to protect the wall of housing portion 14 from wear. In addition, rows of fixed anvils 55 and 57 are mounted to the inner wall of lower housing 14, providing further protection for housing 10 and partially supporting anvils 50. 
     A threaded bolt 58 is secured to the rear portion of the body of each anvil 50 and extends through a corresponding hole in the wall of upper portion 12. Inner and outer nuts 60 and 62 threadably engage the bolt 58 on each side of the wall of upper portion 12 in order to position each anvil 50 in a desired radial location and hold the anvil in that position. Passage of bolt 58 through the wall of upper portion 12 enables bolt 58 to be used in adjusting its corresponding anvil 50 inwardly as the end face 52 of the anvil wears. Nuts 60 and 62 are in turn secured by jam nuts 64 and 66, respectively. 
     A removable ring 68 is placed atop the anvils 50 within upper housing portion 12. Ring 68, when secured in place by fastening of cover 16 to upper housing portion 12, secures anvils 50 from any vertical movement during operation of the crusher. In addition, ring 68 serves to prevent rock from rebounding from end faces 52 of anvils 50 to the area behind the anvils. Similarly, ring 70, which is attached to the interior of cover 16, prevents rebounding rock from entering the area above impeller assembly 30. 
     Cover plate 16 is secured to upper housing portion 12 by a plurality of bolts 72 which pass through cover plate 16 and engage with threaded mounting blocks 74 secured to the inside surface of the wall of housing portion 12. 
     An alternative configuration for the rock crusher, and in particular for the impact surface region of the upper housing portion 12, is shown in FIG. 2. Rather than forming the impact surface from the end faces of a plurality of anvils, a bed of crushed rock is used. The exterior of upper housing portion 12, including shelf ring 54, is identical to that shown in FIG. 1. Moreover, as will be described below, the crusher housing 10 may be converted from use for a crusher operating with anvils to use for a crusher operating without anvils. Depending upon the type of rock to be crushed, it may be desirable to use the crusher either with or without anvils, with the conversion to be made when the type of rock being crushed is changed. 
     In particular, an auxiliary liner member 76 is positioned within lower housing portion 14, resting on liner member 56. A circular shelf extension ring 78 is positioned above auxiliary liner member 76, and in turn is supported by a plurality of gussets 80 which rest on shelf 54. Each gusset 80 is provided with a slot 82 into which is placed a cylindrical wear ring 84 that serves to protect bolts 72 from wear. Ring 84 also protects the upper portion of housing portion 12 from wear. An annular flange 86 is mounted to the innermost edge of shelf extension 78. 
     During operation of the rock crusher, rock is retained within upper housing portion 12 along shelf 54 and shelf extension 78 by annular flange 86. The accumulated rock assumes a configuration indicated generally by 88, and rock to be crushed is thrown by the impeller assembly 30 onto the rock bed. The crushed rock then drops through lower housing portion 14. 
     The conversion of the rock crusher from use with anvils to use without anvils can be seen in detail in FIG. 3. Referring briefly back to FIG. 1, the housing 10 is prepared by removal of cover 16 by loosening of bolts 72 from within mounting blocks 74. Ring 68 is removed from upper housing portion 12, and bolts 58 are detached from anvils 50. Anvils 50, bolts 58 and the associated nuts are removed from the housing, along with anvils 55 and 57 which are detached from lower housing portion 14. Openings in upper housing portion 12 for bolts 58 are then blocked by appropriate means (not shown). 
     As shown in FIG. 3, auxiliary liner member 76 is placed within lower housing portion 14 atop liner member 56, blocking the openings (not shown) within lower housing portion 14 for mounting of anvils 55 and 57. Shelf extension 78, flange 86, and gussets 80, which are all interconnected to form assembly 90, are placed into upper housing portion 12 so as to rest on shelf 54. Wear ring 84 is placed into slots 82 formed in gussets 80. Finally, cover plate 16 is replaced, and is secured by bolts 72. 
     Referring back to FIG. 1, feed means 19 includes feed tube 18, which has secured about it clamp halves 92 and 94 (see also FIG. 3). Clamp halves 92 and 94 are each identical, and include a semi-cylindrical flange 96 which is placed in contact with feed tube 18 and a horizontal flange 98 extending radially outwardly from semi-cylindrical flange 96. Vertical radial flanges 100 are mounted at each end of clamp halves 92 and 94, and are disposed perpendicularly with respect to both semi-cylindrical flanges 96 and horizontal flanges 98. Bolt 102 extends through each cooperating pair of vertical flanges 100 to secure clamp halves 92 and 94 about the tube 18. 
     A plurality of holes are formed through each horizontal flange 98. Threaded studs 104, which are fastened in upright fashion to cover plate 16, are placed through the holes formed in radial flanges 98. A pair of nuts 106 are placed on each stud 104, on opposite sides of radial flange 98, to secure feed tube 18 with respect to crusher housing 10. 
     Feed means 19 further includes a hopper 108 which has a pair of legs 110 and 112 extending downwardly from each side of hopper 108. Legs 110 and 112 terminate at mounting pads 114, which are in turn secured to cover plate 16 by bolts 115 or the like. 
     An annular bottom plate 116 is secured to the lower end of hopper 108. Bottom plate 116 includes a central opening 118 which is of slightly greater diameter than that of feed tube 18. Feed tube 18 extends through opening 118 into the interior of hopper 108 to a height substantially above bottom plate 116. Thus, as rock is fed into the crushing apparatus, a portion of the rock will be retained within hopper 108 in the portion thereof around the exterior of feed tube 18, indicated generally at 119. Thus, wear on the inside surface of hopper 108 as rock is placed therein is prevented by the rock retained within hopper 108. 
     Additionally, since feed tube 18 is not attached to either hopper 108 or bottom plate 116, the vertical positioning of feed tube 18 with respect to crusher housing 10 may be adjusted over small distances by movement of nuts 106 along each of studs 104. For vertical movement of feed tube 18 over larger distances, bolts 102 may be loosened, thereby releasing feed tube 18 from within clamp halves 92 and 94. Upon movement to a new position, bolts 102 are retightened. Thus, as the lower end of feed tube 18 is worn, as will be explained in detail below, feed tube 18 may be adjusted downwardly to provide greater time periods between complete replacement of feed tube 18. 
     Further, attachment of feed tube 18 by clamp halves 92 and 94 enables feed tube 18 to be removed for replacement by simply releasing clamp halves 92 and 94 from around the tube. The tube is then pulled upwardly through hopper 108, avoiding removal of the hopper structure as has been required in previous crushers. 
     Referring again to FIG. 3, it can be seen that cover plate 16 is provided with a pair of parallel hinges 120 and 122 extending the full width of cover plate 16. Thus, in the event it is necessary to gain access to the interior housing 10, for example, to observe inward adjustment of anvils, cover plate 16 need not be completely removed. In such a case, those of bolts 72 securing either or both of cover plate portions 124 and 126 are removed, whereupon portions 124 or 126 may be pivoted about hinges 120 or 122, as required. 
     The impeller assembly 30 may be seen in detail in FIGS. 4-7. As shown in FIG. 4, the impeller assembly 30 includes an upper plate 130, a lower member 132, and a generally cylindrical side wall 134 connected therebetween. A landing surface 136 is carried on lower member 132, upon which rock fed into the impeller assembly 30 through feed tube 18 impinges. A plurality of exit openings 138 are formed through side wall 134, spaced equidistantly therearound. Thus, as impeller assembly 30 is rotated in the direction indicated by arrow 140, rock impinging upon landing surface 136 will be thrown from impeller assembly 30 through exit openings 138. A wear ring 139 surrounds side wall 134 and is attached to upper plate 130. Wear ring 139 is provided with a number of openings 141 equal to the number of exit openings 138, with openings 141 being coincident with exit openings 138. 
     It will be recognized that, although four exit openings 138 are shown, as few as two openings may be provided, with the upper limit being determined by the size of the impeller assembly 30 and the desired production rate of the rock crusher. It should, of course, be clear that regardless of the number of exit openings used, the openings are to be spaced equally about side wall 134. 
     Means for defining a lip is provided adjacent each opening 138 in the form of an elongated lip member 142. A generally right-angled bend is provided near one end of lip body 142, so as to define a lip surface 144 and and end face 146. A curved bend 147 is provided in lip member 142 near its opposite end, providing a means for retaining the lip member 142 within the impeller assembly 30. 
     As seen in FIG. 5, lip member 142 is of a height substantially equal to the height of each opening 138. Openings 138 are formed, however, so that the upper and lower edges 148 and 149, respectively, are remote from the upper plate 130 and lower surface 132. 
     A lower support block 150 and an upper support block 152 are secured to the impeller interior near each opening 138. Lower and upper blocks 150 and 152 are mounted in mutual vertical alignment, and are each of a height such that the space defined therebetween is of a height equal to that of opening 138 and is positioned with respect to side wall 134 at a height identical to opening 138. A cylindrical pin 154 extends between blocks 150 and 152. The curved bend 147 of lip body 142 is engageable with pin 154, as shown in FIG. 4, so that lip body 142 may be secured within impeller assembly 30 by engagement with pin 154. In addition, lip body 142 is placed adjacent the rearward side edge of opening 138 with respect to the direction of rotation of the impeller assembly 30. As impeller assembly 30 is rotated, centrifugal forces will operate to hold the lip bodies 142 firmly in position. Of course, it will be recognized that other means for securing lip body 142 may be used in place of pin 154, such as fixed abutments or pins of other cross-sectional shape (Those skilled in the art may better recognize the rearward side edge of each opening 138 with respect to the direction of rotation of impeller assembly 30 as the leading edge of one of the rock traps, i.e., the leading edge of the portion of side wall 134 defined by two adjacent exit openings 138. In like manner, the forward side edge of each opening 138 may be recognized as the trailing edge of such a side wall portion or rock trap.) 
     To retain lip bodies 142 in position as impeller 140 is stopped, a removable pin 156, which may be formed from a bolt, extends downwardly from upper mounting block 152 along an upper portion of the height of lip body 142. 
     Referring to FIG. 6, lower impeller surface 132 includes a lower plate 158 secured to a central circular block 160 having a central opening for passage therethrough of drive shaft 34. Circular block 160 carries thereon landing surface 136, including a landing cone 164 about which is disposed an annular landing ring 166, having either a flat surface as shown or, alternatively, a conical surface. Landing cone 164 includes a central recess for containing attachment means to the upper portion of drive shaft 34 for rotation of impeller assembly 30. Landing cone 164 and landing ring 166 are positioned directly beneath feed tube 18 through which the rock to be crushed is delivered onto the landing surface 136. 
     Feed tube 18 enters impeller assembly 30 through an annular opening 168 defined in upper plate 130. A downwardly depending annular flange 170 is attached to upper plate 130 within opening 168 so as to encircle feed tube 18. Flange 170 serves to provide a replaceable inner edge for upper plate 130, to restrict somewhat the upper apex of rock trapped within the impeller as will be described, and to restrict airflow through the gap around the feed tube 18 at its entrance to the impeller by trapping fine particles within the gap to the extent that the particles form a seal. 
     As these particles build up, they tend to abrasively wear away the lower portion of feed tube 18. Accordingly, a keyway 172 is formed within the exterior surface of feed tube 18 extending partially upward from its lower end. A key 174, formed of a hard material, such as tungsten carbide, is fitted within keyway 172 acts as a scraper to keep dust accumulated between flange 170 and feed tube 18 from contacting the surface of feed tube 18. Of course, more than one such key 174 may be provided around the exterior of feed tube 18. 
     Referring now to FIGS. 4 and 6, a vertical baffle 180 is attached to the lower impeller plate 158 and to side wall 132 adjacent the forward side edge 182 of each exit opening 138. Each baffle 180 includes an L-shaped base portion 184 and L-shaped plates 186 attached to base portion 184 and extending upwardly and outwardly therefrom. Plates 186 thus define a gap therebetween into which is inserted a baffle segment 188 of a width substantially the same as the gap formed by plates 186. Baffle segment 188 is, therefore, held in place on base portion 184. 
     As impeller assembly 30 is rotated to hurl rock through exit openings 138 to the impact surface, a certain portion of the rock will be retained within impeller assembly 30 through the action of side wall 134 and lip bodies 142. Thus, as shown in FIG. 4, operation of impeller assembly 30 will cause accumulation of trapped rock in the configuration generally indicated as 190. Accumulation 190 extends from the outer edge of lip surface 144 to a relatively well defined apex 192, and then extends in a trailing direction to baffle 180. As rock being delivered to landing surface 136 is accelerated by impeller assembly 30, it will be driven against one of the rock accumulations 190 between apex 192 and lip surface 144. The rock will then be accelerated along the wall of rock defined by rock accumulation 190, after which it will be thrown from impeller assembly 30 through opening 138. 
     This general path for accelerated rock, it should be recognized, holds true regardless of whether the pathway is defined by a rock accumulation or by structural members within the impeller assembly. Thus, by providing lip bodies 142, the abrasive forces of the rock is largely imposed upon the face of rock accumulation 190 rather than impeller assembly 30. Consequently, a significant portion of wear within impeller assembly 30 is eliminated. 
     Notwithstanding the foregoing, a small but significant portion of the rock traverses the wall of rock accumulation 190 at or slightly above or below the upper and lower edges of openings 138. In impeller assemblies wherein the exit openings extend completely to the upper and lower impeller surfaces, this can result in substantial wear upon these surfaces. As a result, many impellers utilize wear plates to protect these surfaces, which must be periodically replaced. 
     In the present invention, however, as can be seen in FIG. 5, the upper and lower edges of each opening 138 are located remotely from upper plate 130 and lower plate 158 of the impeller assembly. Thus, the problem of wear along the upper and lower surfaces is eliminated since the distribution of moving rock along the face of the rock accumulations 190 will not extend sufficiently above or below the vertical positioning of the exit openings 138 to cause the rock to contact and wear either upper plate 130 or lower plate 158. Adjacent side wall 132, however, portions of the rock which approach exit openings 138 slightly above or slightly below exit openings 138 will be collected against side wall 132 above and below openings 138, as indicated by 194. These accumulation extensions 194 will occur even in the case of very dry material, due to the high centrifugal force placed upon the material by the rotation of impeller assembly 30. Thus, rocks which traverse the face of a rock accumulation 190 near the level of the upper or lower edges of an exit opening 138 will not cause wear along these edges of the exit opening since these edges are protected by accumulation extensions 194. 
     It should be recognized from the foregoing discussion that since upper plate 130 and lower plate 158 do not enter into contact with rock moving outwardly from landing surface 136, plates 130 and 158 are only necessary for maintaining the structure of the impeller assembly 30. Thus, plates 130 and 158 could be disposed even further from the upper and lower edges of the openings 138, or could be replaced entirely by, for example, an open framework or the like, so long as the rock accumulations 190 and extensions 194 are sufficiently supported. 
     While ideally the upper and lower edges of the exit openings 138 are protected from wear by accumulation extensions 194, ultimately some wear of these edges will occur. Rather than have this wear take place on a structural member of the impeller assembly 30, removable inserts 196 and 198 are provided adjacent the upper and lower edges of each exit opening 138 to absorb any wear which may occur. As seen in FIGS. 4 and 5, each insert 196 and 198 comprises a curved bar having notches 200 defined in each end of the bar. Mounting blocks 202 are attached to the inside surface of side wall 134 near the upper and lower edges of each exit opening 138. Inserts 196 and 198 are placed between a pair of mounting blocks 202 so that a block 202 fits within each slot 200. The centrifugal forces generated during rotation of impeller assembly 30 keeps the inserts 196 securely in place against side wall 134. Since wear along upper and lower edges of exit openings 138 will occur only near the rearward side edge due to material exiting along the rock accumulation 190, it will be noted that inserts 196 and 198 preferably extend only partially toward the forward side edge 182 of each exit opening 138. 
     It should be recognized that each baffle 180 serves to prevent the trailing edge of each rock accumulation 190 from interfering with the next adjacent exit opening 138. Thus, it has been found that with only two or three exit openings 138, the distance along side wall 134 between adjacent openings 138 enables baffles 180 to be dispensed with entirely. Similarly, the greater the number of exit openings 138, the further inward each baffle 180 must extend. 
     In addition, it has been found that, depending upon the particular rock being used within the crusher, the distance from which apex 192 of each rock accumulation 190 is spaced from the center of landing surface 136 will vary. Accordingly, it may be desirable to maintain a variety of sizes of baffle portions 188 for use with baffles 180. Portions 188 can then be selected depending upon the type of rock to be crushed. 
     It will be seen from reference to FIG. 4 that end surface 146 of lip body 142 will be subject to wear since the rock will traverse end surface 146 after leaving the side wall of rock accumulation 190. Due to the presence of rock material against lip surface 144, however, wear on end surface 146 will tend toward the outer edge of the surface. Thus, the corner formed by lip surface 144 and end surface 146 will be preserved for a relatively long period, ensuring continued presence of rock accumulations 190. To provide for increased life for end surfaces 146, an insert 204 formed from a hard material, such as tungsten carbide, is imbedded within the end of lip body 142. The insert 204 provides a working surface that is flush with end surface 146, so that there is no effect upon the path of rock being thrown from impeller assembly 30. Also, by having the tungsten carbide working surface flush with end surface 146, the brittle corners of the tungsten carbide are supported by the surrounding material which avoids early breakage of these corners and provides longer life. 
     Notwithstanding insert 204, it will eventually become necessary to replace lip bodies 142. At such times, it will be noted that the ease of removal and installation of lip bodies 142 from the impeller assembly 30 represents a significant advantage of the impeller assembly. 
     To provide further protection against wear within impeller assembly 30, landing surface 136, baffle segments 188, removable inserts 196 and 198, and lip bodies 142 may be formed from a material such as abrasion-resistant alloy white cast iron. 
     It has been found that the angle formed between end surface 146 and an imaginary line 208 passing through the center of impeller assembly 30 and the outer edge of end surface 146 or, alternatively, the angle between lip surface 144 and such a line, has a significant effect upon the performance and wear characteristics of impeller assembly 30. In general, it has been found that an angle 206 between end surface 146 and line 208 within a range of approximately 15° to 45°, and preferably of 30°, is desirable. An angle 206 of less than this range will likely provide a lip surface 144 which is insufficient to maintain a rock accumulation that provides adequate protection for the rearward side edge of the exit openings 138. From FIG. 4, it will be noted that, at the angle 206 shown therein, centrifugal force exerted upon rock particles against lip surface 144 creates a substantial tangential component along surface 144 for retaining rock against lip body 142. As the angle is decreased, however, the tangential component is accordingly reduced so that occasional perturbations in rock particle movement along the side wall of rock accumulation 190 could cause occasional disintegration of the wall along lip surface 144, resulting in premature wear. It will also be seen that reductions in angle 206 could result in the portion of lip body 142 near pin 154 being exposed through the side wall of rock accumulation 190. 
     As the angle is increased beyond this range, it should be recognized that the wearing force on end surface 146 is increased, since the component of force into surface 146 caused by centrifugal force upon rock particles moving therealong is increased. As a result, the useful lifetime of the lip bodies 142 will be reduced. 
     While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus and that changes may be made therein without departing from the scope of the invention.