Patent Publication Number: US-7721896-B2

Title: Screen separators

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application is a 371 of and claims priority of International patent application Serial No. PCT/GB2004/004813, filed Nov. 12, 2004, and published in English the content of which is hereby incorporated by reference in its entirety. 
   FIELD OF THE INVENTION 
   This invention is concerned with vibrating sieve separators for use in sieving materials using vibratory screens. 
   BACKGROUND OF THE INVENTION 
   Such separators are well-known and generally use one or more taut screens of selected mesh size which are mounted in or on a chassis that is itself supported on a base by elastomeric mounts, springs or other resilient members. The chassis is coupled to a motor which drives offset weights to impart vibration to the chassis and thereby to the screen(s) of the sieve. 
   Separators are widely used in many industries and the use is determined by the materials to be sieved or separated, and these materials themselves determine the mesh size for the screen. 
   One industry in which such separators are widely used is the pharmaceutical industry. In this industry, it is of special importance that, when sieving a product, the probability of any sieved material escaping into the surrounding environment is restricted ideally to zero, but in reality to the lowest possible margin, so that neither workers nor other equipment nor products can become contaminated. 
   There are prior art arrangements to ensure that material fed to such a separator remains within its confines and one such separator is shown in  FIG. 1  of the accompanying drawings. 
     FIG. 1  shows a typical separator  10  comprising a chassis  12  mounted on a rigid base frame  14 . The separator has a motor driven out-of-balance weights vibrator  16  mounted on its side. The chassis  12  contains one or more mesh screens each stretched in a frame which is mounted securely over the hopper. 
   The separator also comprises a cover  18  (or lid) which is mounted on the chassis, and has an inlet  20  through which material to be separated can be loaded into the separator and has an observation port  21 . 
   The cover  18  is fastened to the chassis in sealed engagement therewith by a plurality of toggle or over-centre clamps  22 , the locking action of which secures the cover to the chassis. As good as the seals are that are established with such a structure, in practice, particles of sieved materials do escape from such separators and these particles, being of microfine dimensions can collect on surfaces of the separator and elsewhere, with the attendant possibility of contamination when the separator is used for separating other materials. With the arrangement shown in  FIG. 1 , this is particularly true of the surfaces of the toggle or over-centre clamps  22 , some of which are not readily accessible or visible. Other surfaces which are visible can be cleaned, rinsed and even sterilised where necessary, but the clamps themselves can provide a source of potential contamination because some of their surfaces are not readily visible. 
   In other prior art arrangements, the cover, screen frame, collection hopper and other elements of the sieve can be clamped together using a single band clamp, comprising a V-section hoop which is tightened around the sieve elements to clamp them together. Effective clamping using individual toggle clamps as in  FIG. 1  or the known band clamps requires skill and care on the part of the operator, and the procedure is prone to error. 
   It is therefore an objective of this invention to provide an improved arrangement for securing or clamping the component parts of the separator together. 
   Embodiments of the invention can also improve the sealing of the various components, i.e. chassis, cover, sieve frames, so that finely divided products do not escape from the product space in the sieve. 
   In U.S. Pat. No. 5,226,546, there is disclosed a separator construction in which circular sieve frames can be loaded through slots in a side wall of a cylindrical chassis of the separator and then held in position by inflating an annular inflatable tube with the various components of the separator held in position by one or two strap ties which extend parallel to the axis of the chassis and are located on the outside of the chassis and clamp the component parts together at a maximum of two locations. This, we believe, can lead to distortion of the components of the separator, especially of the screen frames, albeit by a very small amount, but to an extent that does allow leakage of material from the separator. 
   SUMMARY OF THE INVENTION 
   Accordingly, in one aspect, the present invention provides screen separating apparatus comprising a chassis for supporting a screen frame carrying a sieve screen, said chassis providing a first bearing surface; and one or more clamping members which can be located relative to the chassis to provide a second bearing surface or a combination of second bearing surfaces opposed to said first bearing surface such that the screen frame can be clamped between the first and second bearing surfaces; wherein the apparatus further comprises at least one expandable element mountable in the chassis to be between said first and second bearing surfaces and expandable to effect said clamping of the screen frame. 
   The sieve frame is typically circular though it may be of other shape, e.g. elliptical, or even rectangular, and where used herein the terms annular and circumferential or circumference should be understood to include such shapes, unless it is clear that a more specific meaning is intended. Hereinafter, for example, the described and illustrated embodiments use frames which are of circular shape. 
   Thus, where the chassis accommodates one or more annular frames which are circular, the chassis is typically of circular shape also. The annular clamping arrangement which is conveniently shaped to conform to the shape of the chassis is preferably formed as a cylindrical sheath which fits onto the chassis. This sheath may have an in-turned flange portion which is provided, when the separator is assembled for use, to exert a reactive force, directly or indirectly, onto the sieve frame or a stack of such frames where more than one is used, when the expandable element(s) is/are expanded. 
   The sheath itself is preferably formed as a one-piece cylinder, though if more desirable, it may be of multipart form, and where of multipart form, the annular construction of the sheath may be provided by a plurality of sheath segments which are, or can be, linked together with, if appropriate, some spatial separation of the sections from each other. A desirable requirement in the design of the sheath is that when the separator is assembled, and the expandable element(s) is/are expanded, the reactive force exerted by the flange portion of the sheath should be exerted not at localised positions, as in the prior art, but uniformly and continuously along as much of the frame(s) as is required to ensure that the frame(s) and the other components of the separator are subjected to an even distribution of that force. 
   Where the sheath includes an in-turned flange portion, the sheath itself may extend as a skirt around an upper annular portion of the chassis in overlapping relationship and may have a plurality of cut-out locking slots formed therein to co-operate with spaced studs on the exterior of the chassis in a bayonet fitting relationship so that the sheath and the chassis can be locked together, and readily released, when desired, by a single operative to effect maintenance, cleaning and/or replacement. 
   The separator may comprise a lid or cover, as is typical of separators generally. The cover may include a shoulder portion around its lower periphery, the shoulder portion being of external dimensions such that it sits within the confines of the chassis and can be engaged by the in-turned flange portion of the sheath. The shoulder portion may also have an inner skirt depending therefrom which itself has a continuous flange at its lower edge, which flange is arranged to rest upon a sieve frame, or an uppermost sieve frame where more than one is provided. 
   The or each sieve frame is supported within the confines of the chassis. The separator normally comprises a hopper through which sieved materials fall into a collecting receptacle. Such a hopper, in a typical truncated cone shape, may have an upper peripherally-braced shelf portion on which the one or more frames are supported. The shelf portion may have a continuous wall integrally formed with, and upstanding from, the shelf portion, the wall portion being of dimensions such that it forms a snug sliding fit within the chassis. The peripherally-braced shelf portion may itself be supported by the at least one expandable element, which in preferred embodiments of the present invention is a single annular inflatable element which may be provided by an annular inflatable tube or annular bellows mounted in a retaining channel formed by an annular wall extending from an annular ledge welded to the interior of the chassis. 
   The construction and arrangement of this preferred embodiment is such that, when the separator is assembled, the inflatable member is expanded by pneumatic or hydraulic pressure and urges the hopper, the frame(s) and the skirt of the cover upwardly against the in-turned flange portion of the sheath until the shoulder portion of the cover engages the in-turned flange portion and lifts the sheath until its further movement is prevented by the bayonet fittings. Thereafter, increased pneumatic/hydraulic pressure in the expandable element traps the frame(s) between the skirt of the cover and the shelf portion of the hopper within the sheath and the chassis. 
   In an alternative embodiment, the engagement of the sheath and the chassis may be provided by forming the sheath with a lower lip arranged to lie under a lower annular edge of the chassis. If the lower lip extends inwardly of the chassis to a sufficient depth, it can support an annular ring that itself supports the expandable element, thereby simplifying the shape of the chassis or the need to weld channeling to the interior of the chassis. Such an annular ring may itself have an annular wall such that an annular channel is formed between it and the chassis to retain the expandable member therein. 
   The invention also provides screen separating apparatus comprising a chassis for supporting a screen frame carrying a sieve screen, a cover to enclose a space above the sieve screen to contain material to be separated, a hopper to collect fines passing through the sieve screen, at least one inner seal to seal between the cover and the hopper around the circumference of the screen frame when the apparatus is in use, a sheath surrounding said inner seal, at least one outer seal to seal between the sheath and at least one of the cover and the hopper to provide an enclosed volume between the sheath, the cover and the hopper, and an inlet connecting to said enclosed space to enable said enclosed space to be pressurised. This arrangement provides positive leak prevention from the product space in the sieve enclosed by the cover and the collection hopper. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other characteristic features of embodiments of the present invention will now be described by way of example with reference to the accompanying drawings, in which: 
       FIG. 1  is a side view of a separator. 
       FIG. 2  is a cross-sectional view showing the internal construction of a first embodiment of a separator and apparatus according to the present invention; 
       FIG. 2   a  is a close-up cross-sectional view of the right-hand side of the separator shown in  FIG. 2 ; 
       FIG. 3  is a cross-sectional view similar to  FIG. 2   a  but illustrating a second embodiment of the invention; 
       FIG. 4  is a cross-sectional view similar to  FIG. 2   a  but illustrating a third embodiment of the invention; 
       FIG. 5  is a cross-sectional view similar to  FIG. 2   a  but illustrating a fourth embodiment of the invention; 
       FIG. 5   a  is a cross-sectional view of a variant of the fourth embodiment shown in  FIG. 5 ; 
       FIG. 6  is a cross-sectional view similar to  FIG. 2   a  but illustrating a fifth embodiment of the invention; 
       FIG. 6   a  is a cross-sectional view similar to  FIG. 6  of a modification of that embodiment; 
       FIG. 7  is a cross-sectional view similar to  FIG. 2   a  but illustrating a sixth embodiment of the invention; 
       FIG. 8  is a cross-sectional view similar to  FIG. 2   a  but illustrating a seventh embodiment of the invention; 
       FIG. 9  is a cross-sectional view of a detail of  FIG. 8 ; 
       FIG. 10  is a perspective view of a mobile separator according to the present invention illustrating one manner of mounting a sheath of an apparatus according to the invention; 
       FIG. 11  is a perspective view of the separator of  FIG. 10  taken from a diametrically opposite view; 
       FIG. 11   a  is a close-up perspective view of a motor mounting block shown in outline in  FIG. 11 ; 
       FIG. 11   b  is a further close-up perspective view of a motor housing of a separator according to the present invention, the housing being mountable to the mounting block shown in  FIG. 11   a;    
       FIGS. 11   c  to  11   e  are perspective views of details of a closure arrangement for holding the sheath of the apparatus of  FIGS. 10 and 11  in closed position; 
       FIG. 12  is a perspective view of the mobile separator of  FIGS. 10 and 11  with top cover removed and its sheath in partially separated condition; 
       FIG. 13  is a perspective view of a mobile separator illustrating a second manner of mounting a sheath of apparatus according to the present invention; 
       FIG. 14  is a perspective view of the separator of  FIG. 13  showing the sheath in released condition; 
       FIG. 15  is a cross-sectional view of a variant of the first embodiment shown in  FIGS. 2 and 2   a;    
       FIG. 16   a  is a cross-sectional view illustrating a further modification providing positive prevention of leaks and cross-contamination; 
       FIG. 16   b  is a cross-sectional view of a further modification; 
       FIG. 17  is a schematic diagram of a preferred pneumatic control system for embodiments of the invention; 
       FIG. 18  is a perspective view of the embodiments of separator with bayonet-type connections between the sheath and the chassis; and 
       FIG. 19  is a cross-sectional view of a further embodiment with two stacked screen decks. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 2 and 2   a  shows in section a separator embodying the invention comprising a chassis  40  which may be mounted on a mobile carriage  42 , which is shown in  FIGS. 10 to 12 . The chassis  40  is circularly cylindrical having an external diameter normally between 10 cm and 200 cm or more, typically between 40 cm and 120 cm. The chassis  40  is mounted by resilient feet on a platform  44  of the carriage and has a motor  46  attached thereto (not shown in  FIG. 2 ) which is eccentrically weighted to cause the motor to impart vibration to the chassis. 
   Mounted on the chassis and attached thereto by a plurality of bayonet fittings or couplings  48  of an annular clamping arrangement evenly positioned around the chassis is a sheath  50  which carries diametrically opposed handles  52  to enable the sheath to be coupled to or uncoupled from the chassis. Secured by the sheath is a lid or cover which has an inlet  56  at its uppermost part through which material can be poured into the separator. The cover also has an observation port which is not shown in  FIG. 2 . 
   Shown in part section in  FIG. 2  is a hopper  58  which is mounted from within the chassis to depend therefrom to funnel sieved material into an appropriate receptacle. 
   The chassis can be mounted either on a fixed base or a mobile base such as the mobile carriage partly shown in  FIG. 2 . 
   Within the chassis  40 , there is provided an annular ledge  60  which is welded or otherwise fixed to the interior face of the chassis. This ledge  60  supports an annular wall  62  concentric with and spaced from the interior face of the chassis, the ledge, the wall and the interior face of the chassis, thereby defining a channel  64  which houses an expandable element in the form of an annular inflatable tube  66 . The tube is shown in the drawing as being of square cross-section though in practice it may be of any convenient cross-section, or may be of bellows construction. 
   The ledge  60  is provided with one or more apertures  68  through which one or more inlets  69  to the tube  66  can pass, to a source of pressure such as a pneumatic or hydraulic pump. The tube  66  is inflatable to an extent such that it can rise above the height of the annular wall  62 . 
   The hopper  58  rests on the wall  62 , when the tube  66  is not inflated. To this end, the hopper has an annular shelf portion  70  which at its outer periphery extends upwardly as an annular wall  72  which has an external diameter such that it forms a clearance sliding fit within the chassis  40 . The shelf portion is supported by an annular collar  74  which is welded to the hopper and is of elbow cross-section to provide a surface  76  against which the tube  66  can be inflated. 
   The shelf portion  70  of the hopper provides support for a frame  78  of a first or lower sieve screen, the frame  78  being, as previously stated, of circular shape and resting on the upper surface of the shelf portion  70 . The frame in this example is of square-sectioned tubular construction and supports a tautly held screen (not shown) therein. The frame is seated in a U-shaped or L-section gasket  80  formed of a suitable preferably conductive resilient material to provide a sealing and locating function, to prevent product by-passing the mesh or escaping to the outside of the sieve. Mounted above the first, lower, frame  78  is a second, upper, frame  82  of similar construction to that of frame  78 . The two frames  78  and  82  are separated by an annular gasket  86  which provides a similar function to gasket  80 . The gasket  86  is also shaped to brace the two frames away from the wall  72  of the hopper  58 , and extends over the top of the frame  82  to provide a seat for the cover  54 . Frame  78  may carry the primary separator mesh or screen and frame  82  may carry a secondary safety mesh or a magnetic separator. Instead frame  78  may be just a spacer, and frame  82  may carry the single mesh or screen. 
   The cover  54  is of generally circular cylinder shape having a shallow truncated cone top portion  87  which includes the inlet  56  at its apex. Around the lower perimeter of the main cylindrical part  88  of the cover is formed a shoulder portion  90 , depending from which is an annular, inner skirt  92  which lies inside the sheath  50 . At its lower edge, the skirt  92  is formed with an in-turned annular flange  94 , which rests upon the gasket  86  over the spacer  82 . 
   When the sheath  50  is located in position over the cover  54 , an annular in-turned flange portion  96  formed around the upper edge of the sheath  50  overlaps the shoulder portion  90  of the cover  54 . The flange portion  96  is encased in a resilient annular gasket  98  to seal with the shoulder portion  90  of the cover. As with the other gaskets, the gasket  98  may be formed of any suitable resilient material (e.g. silicone, polypropylene). 
   As can be understood from a study of  FIG. 2  especially, the apparatus is assembled onto the chassis  40  by firstly locating the inflatable tube  66  within its channel  64  and then fitting the hopper  58  into the chassis interior so that the hopper rests on the innermost edge of the wall  62  with the lower surface  76  of the collar  74  overlying the inflatable tube or bellows  66 . 
   Thereafter, gaskets  80  and  86  are fitted to the frames  78  and  82 , the frame  78  is placed on the shelf portion  70  of the hopper  58 , and the second frame  82  is then placed on the first, lower, frame  78  and pressed into position. 
   The cover  54  is placed on the spacer  86 , and the sheath then fitted over the cover so that the protected flange portion  96  seats on the shoulder portion  90  of the cover. The sheath is ‘dropped’ over the cover so that the openings to slots  47  (see  FIG. 18 ) in the sheath are aligned with studs  48  on the chassis  40  to form bayonet fittings. Then, when the sheath is lowered as far as permitted by the slots  47 , the sheath is twisted so that the studs  48  are engaged in the slots to prevent the sheath from being lifted off again. 
   Once the sheath  50  is so engaged, and holds the cover  54  in place, the inflatable tube  66  is inflated via a pump (not shown) filling the tube with compressed air or other suitable fluid causing the tube to inflate. Inflation of the tube causes expansion of the tube, and the expansion exerts an upward force, as indicated by the arrow F, on the collar  74  thereby pressing the shelf portion  70  against the two frames  78 ,  82 , and transmitting this force, via the flange  94 , skirt  92  and shoulder portion  90 , to the flange portion  96  of the sheath  50 . The sheath is thus caused to take up any slack in the bayonet fittings. 
   Thereafter pressure caused by inflation of the tube  66  causes compression of the gaskets  80 ,  86  and  98  until the components of the separator are tightly secured. 
   A perspective view of the separator of  FIGS. 2 and 2   a  is shown in  FIG. 18 , which best illustrates the bayonet couplings with studs  48  and slots  47 . 
     FIG. 2  shows a construction employing two frames  78 ,  82 . More than two frames may be used, in which case a sheath having sufficient depth to accommodate the height of the frames is required. Alternatively, the studs of the bayonet fittings may include a screw-threaded section, and the chassis can then be provided with threaded apertures at different heights so that change in the number of frames can be accommodated by changing the position of the studs. 
   In other embodiments, the separator may have two or more stacked screen decks carrying progressively finer sieve screens. Then the sheath has sufficient depth to clamp together the multiple decks and screen frames. A separator with two sieve decks is shown in  FIG. 19  having an upper deck unit  40  clamped between an upper screen frame  425  and the lower frame  82 . 
   In a second embodiment of the present invention, shown in  FIG. 3 , the sheath  50  is formed integrally with the cover  54 . As can be seen from comparison with  FIG. 2 , the sheath  50  is foreshortened and comprises an extended skirt portion joined to the main cylindrical body of the cover  54  by a shoulder portion  100  providing a peripheral flange that seats against the spacer  86 . This construction removes the necessity for a separate sheath. In all other respects, the second embodiment is substantially identical to the first embodiment. 
   A third embodiment of the present invention is shown in  FIG. 4 , wherein a cross-section of a part of a separator illustrates a chassis  110  which is mounted on a base  112 . The chassis is cylindrical and of annular shape when viewed in plan. The chassis has an annular ledge  114  which is welded or otherwise formed fixed to the interior face of the chassis  110  and this annular ledge supports an annular wall  116  concentric with and spaced from the interior face of the chassis, so that the ledge, the wall and the interior face of the chassis define a channel  118 . As in  FIG. 2 , the channel  118 , like the channel  64 , houses an expandable element in the form of an annular inflatable tube  120  similar to the tube  66  of  FIG. 2 . 
   The ledge  114  is located within the confines of the chassis  110  at a position such that a second expandable element, also in the form of an annular inflatable tube  122  can be located below the ledge  114  and within the confines of the chassis. 
   The separator comprises a hopper  124  which has an annular shelf portion  126  which is protected by a resiliently compressible annular gasket  128 . The shelf portion  126  rests on the inflatable tube  120 . 
   The shelf portion  126  supports first, lower and second, upper sieve frames  130 ,  132  which are mounted in an annular resiliently compressible annular gasket housing  134  on which is supported the cover  136  of the separator. The cover, though of different appearance from that shown in  FIG. 2 , nevertheless has the same function as that of cover  54  therein and includes an inlet through which material can be delivered to the sieve screens and an optional observation port. 
   In this embodiment, a sheath  138  which is of annular form, and of multipart construction, as hereinafter described, envelops the chassis  110  almost completely. The sheath  138  has a skirt portion  140  which embraces the exterior of the chassis  110  and integrally-formed upper and lower in-turned annular flanges  142 ,  144  respectively. The lower flange  144  extends inwardly below the chassis  110  to provide support for the inflatable tube located below the ledge  114 , and the upper flange  142  extends inwardly to overlap a peripheral flange  146  of the cover  136 , thereby trapping the two frames  130 ,  132 , shelf portion  126  and the two inflatable tubes  120   122 . 
   When either or both of the tubes  120 ,  122  is/are inflated, pressure is exerted by the tube  122  on the lower flange  144  and, by inflation of the tube  120 , on the frame housing  134  and thus against the upper flange  142 . Due to the compressible nature of the gasket  128  and of the housing  134 , the structure thus assembled is held fast and can withstand vibration imparted to the separator without leakage of material therefrom. 
   Turning now to  FIG. 5 , there is shown therein a fourth embodiment of the present invention which is similar to that of  FIG. 4  (the same reference numerals are therefore used, where appropriate, as are used in  FIG. 4 ), except in that the lower inflatable tube  122  is removed completely and the annular ledge  114  is supported directly on anti-vibration mounts  210  in spaced relationship to the base  112  of the separator. Without the presence of the inflatable tube, the in-turned lower flange  144  of the sheath  140  only engages the lower edge of the chassis  110 . 
   The resilient mounts  210  are formed of any suitable material, e.g. vulcanised natural or synthetic rubbers, or plastics materials, capable of absorbing vibration transmitted from the motor, so that the base remains substantially vibration free during use, and are located in recesses  212  provided in the base  112 . Mounts in the form of springs may also be used. Further detail of the construction of the fourth embodiment can be gleaned from the variant thereof which is shown in  FIG. 5   a . In this Figure is shown the manner in which the ledge  114  is used to anchor the chassis  110  to the base frame  112 . Each support  214  for the base frame  112  has a bush  216  mounted on its upper extremity, the bush being recessed to receive the head of a bolt  218  which extends upwardly through an aperture in the base frame  112  and partially through a bore  220  in the resilient mount  210  to engage an internally threaded coupling  222  housed in the lower part of the bore  220 . A further bolt  224  countersunk into the ledge  114  engages a further internally threaded coupling housed in the upper part of the bore  220 . 
   As stated above, the construction shown in  FIG. 5   a  is a variant of that shown in  FIG. 5 . In this  FIG. 5   a  embodiment, it can be seen by comparison with  FIG. 5  that the chassis  110  and the sheath  140  are extended upwardly above the upper screen frame  132 , and that the cover  136  has a skirt portion  226  analogous to the skirt portion  92  of the first embodiment shown in  FIG. 2 , the skirt portion  226  extending from a shoulder portion  228  of the cover to an in-turned flange encased in a gasket  232  seated on the upper screen frame  132 . 
   In  FIG. 6 , a further, fifth embodiment of the present invention is illustrated. One factor that has to be considered in the construction of a separator according to the invention is that of earthing the separator to ensure that static charge does not build up on the separator or on the screen frame and so cause potential harm to an operator or charge particles delivered to the sieve screens. The embodiment of  FIG. 6  provides a solution. 
   The construction of the embodiment of  FIG. 6  is very similar to that of  FIG. 5 . The significant difference therefrom is that in the  FIG. 6  embodiment, the sheath  140  is provided with a plurality of earthing probes  240  which extend through apertures provided therefor in the chassis  110 . Where only a single screen frame is employed in the separator, then one or more probes  240  can be provided, aligned with the plane of the frame but, where two or more screens, and therefore screen frames, are provided, probes are provided for both frames. The probes are provided with tapers whereby they can pierce the resilient gaskets protecting the frames. In all other respects, the construction of the embodiment of  FIG. 6  is closely similar to that of  FIG. 5 . It will of course be clearly understood that the provision of earthing probes is equally applicable to all embodiments of the present invention. Instead of the illustrated earthing probes, the parts of the separator may be earthed using electrically conductive material for the resilient gaskets, or interconnecting with earthing braids. 
   A modification of the embodiment shown in  FIG. 6  can be seen in  FIG. 6   a , wherein the chassis is foreshortened and does not include a ledge  114 . Instead, the chassis  110  is formed as a simple cylinder while the sheath  140  has an in-turned flange  145  that extends inwardly further than the flange  144  of  FIG. 6 , and supports an annular ring  147  that in turn abuts against the lower edge of the chassis  110  and supports the inflatable tube  120 . 
   The manner in which a sheath such as is illustrated in  FIGS. 4 to 6   a  can be mounted in a separator according to the invention is shown generically in  FIGS. 10 ,  11  and  12 . In  FIGS. 10 and 11 , the separator is viewed from opposite sides. The motor  46  has a flange  150  secured to a mating flange on chassis  166  (see  FIG. 12 ). Chassis  166  corresponds to chassis  40  of  FIGS. 2 ,  2   a  and  3  and to chassis  110  of  FIGS. 4 to 7 ) 
   As can be seen in each of  FIGS. 10 ,  11  and  12 , the sheath  138 , is formed in two halves  153 ,  154  each of which is a mirror image of the other and extends around almost one half of the chassis  166  save for the motor mount, against side faces of which adjacent ends of the sheath halves abut, as described below. 
   As shown in detail in  FIG. 11   a , which illustrates the manner of mounting of the sheath halves  153 ,  154  on a typical chassis  166  of a separator according to the present invention, the two halves  153 ,  154  of the sheath each have a pair of hinge plates  156  welded thereto, each plate of a pair having an integral bush  158  for receiving a steel hinge pin  160  which passes through both bushes of each pair. 
   Each hinge pin  160  passes through a respective pin hole which is formed in a mounting block  164  which is welded to the chassis  166 , and through a bearing aperture in a retaining plate  168  which is welded in situ between the mounting block  164  and the chassis  166 . 
   At their opposite ends, i.e. those ends opposite the hinges, the two halves  153 ,  154  of the sheath, when closed onto the chassis are in substantially end-to-end relationship, as shown in  FIGS. 11   c ,  11   d  and  11   e . At these adjacent ends, each half  153 ,  154  of the sheath is formed with a tongue portion  170 ,  172  respectively, which includes an out-turned flange portion  174 ,  176  respectively. These tongue portions  170 ,  172  are positioned flush against the outer surface of the chassis  166  between a magnetic keeper  178  mounted on the chassis  166  and a U-shaped horizontal hinge  180  welded or otherwise attached to the chassis. 
   When the two halves  153 ,  154  are closed onto the chassis  166 , the two flange portions  174 ,  176  are in closely-separated relationship, the separation corresponding to the width of a slot  182  formed in a magnetic locking bar  184  which is used to clamp the two halves  153 ,  154  together on the chassis  166 . The bar  184  is provided with a pair of hooks  186 ,  188  which hook over the horizontal hinge  180  to act as a pivot for the bar  184 . At its opposite end, the bar  184  has a magnet  190  inset into the body of the bar. This magnet  190  co-operates with the magnetic keeper  178  to hold the locking bar  184  in situ when it is moved from the position shown in  FIG. 11   d  to that shown in  FIG. 11   e . The magnetic locking bar/keeper arrangement described above may use permanent magnets or electro-magnets. 
   This construction permits ready locking of the two halves  153 ,  154  of the sheath  152  and quick release when it is required to uncouple them. The two halves can then be parted and swung back from the chassis  166  as shown in  FIG. 12 , and completely removed from the chassis of the separator by removal of the two hinge pins  160  and then lifting away from the chassis  166  with equal ease of re-mounting when the separator is again required for use. Instead of the magnetic arrangement described, an over-centre latch may be used to secure the halves of the sheath together. 
   As mentioned above, the collar  150  of the motor is mounted on the mounting block  164 . 
   The manner in which the separator can be assembled and dis-assembled is shown in  FIG. 12 , which shows a mobile version of the present invention. As illustrated in  FIG. 12 , the cover of the separator has been removed and the two halves  153 ,  154  of the sheath released and are swung away from the chassis  166 , either prior to removal of a screen frame or prior to closure of the two halves after replacement or cleaning of such a frame of frames. 
   In the structure shown in  FIG. 12 , there is also shown a variant of the locking system for locking the two halves  153 ,  154  against the chassis  166 . Instead of the tongue portions  170 ,  172  shown in  FIGS. 11   a  and  11   b , the two halves  153 ,  154  have tongue portions  194 ,  196  ( 194  not shown) which are turned inwards from the surface of the respective half  153 ,  154 , with each tongue portion having a flange portion  198 ,  200  at right angles thereto. 
   Pivotally mounted on the chassis  166  are two toggled hinge pins  202 ,  204  that can pivot between a horizontal position as shown in  FIG. 12  and an upright position parallel with the external face of the chassis  166  in which they engage the flange portions  194 ,  196  to retain the latter in position against the chassis. 
   With such an arrangement, the two halves of the sheath  152  can be quickly released, and subsequently coupled around the chassis  166 . 
   Turning now to  FIG. 7 , there is shown therein a further, sixth embodiment of the present invention. In this embodiment, the ledge  114  does not have an internal annular wall  116  and the shelf portion  126  of the hopper rests on the ledge with the two frames in turn supported on the shelf portion. 
   In this embodiment, a further variation from the preceding embodiments is that the at least one expandable element is provided by a plurality of pneumatic (or hydraulic) piston-and-cylinder arrangements  250  which are mounted on and secured to the base  112  and extend upwardly therefrom to engage the underside of the annular ledge  114 . 
   In this construction, and in that of the seventh embodiment of the present invention, shown in  FIGS. 8 ,  9  and described below, a different arrangement is provided for mounting the sheath  138  relative to the chassis  244  of the separator. 
   The construction shown in  FIG. 7  employs a sheath  246  which is formed by a plurality of sheath segments  252  instead of the two sheath halves shown in  FIGS. 10 ,  11  and  12 . The manner in which the segments are mounted and can be moved is shown generally in  FIGS. 13 and 14 , to which further reference will be made below. 
   In the embodiment of  FIG. 7 , lower in-turned flange portions  253  of the sheath segments  252  are shown pivotally coupled to the piston-and-cylinder arrangements  250  by pivot pins  254  such that each segment can be pivoted from a non-engaging position to the position shown in  FIG. 7 . Each segment  252  has an upper in-turned flange portion  256  which overlaps a peripheral flange  146  of the cover and carries a resilient gasket  258  which is adapted to engage the peripheral flange  146  along the circumferential length of the segment. 
   A similar arrangement is shown in  FIGS. 8 and 9  wherein, instead of being hingedly coupled to the piston-and-cylinder arrangements  250 , the sheath segments  252  are each mounted on a plate  259  which is pivotally coupled to a hinge plate  260  which is itself similarly coupled to a bracket  262  welded to the underside of an in-turned flange  264  of the chassis  244 . The positions of the pivotal couplings of the sheath segment plates to their respective hinge plates, and of the hinge plates to their respective brackets is such as to provide a toggle arrangement as shown in  FIG. 9 . 
   In  FIG. 8  an expandable element  266 , similar to those used in earlier embodiments is located between the flange  264  and the annular shelf portion  126  of the hopper  124 . 
   The general construction using a plurality of sheath segments  252  can be seen in  FIGS. 13 and 14 , and by comparison with the constructions shown in  FIGS. 10 to 12 . In place of the two sheath halves  153 ,  154  of those Figures, the embodiment of  FIGS. 13 and 14  comprises nine equally sized sheath segments  252 . 
   All of the sheath segments  252  are pivotal on mountings such as described with reference to  FIGS. 7 to 9 , and are toggled when in the chassis enclosing position shown in  FIG. 13 , as described with reference to  FIG. 8 , or otherwise can be held in position, as with the embodiment of  FIG. 7 , by expansion of the piston-and-cylinder arrangements thereof. 
   To release the segments from the positions shown in either of  FIGS. 7 and 8 , the pneumatic/hydraulic pressure is first released to allow the segments to be pivoted outwardly from the chassis  244 , as in  FIG. 7 . 
   In either construction, the segments then adopt the position shown in  FIG. 14  to permit the cover to be removed for the purpose of replacing or carrying out maintenance of the screens and/or the interior of the separator. 
     FIG. 15  shows an improvement of the first embodiment described above with reference to  FIGS. 2 and 2   a . Components of the apparatus which are the same or similar as in the first embodiment are identified with the same reference numerals. 
   Instead of the annular wall  62  welded or otherwise fixed to the annular ledge  60  as described and illustrated in the first embodiment and shown in  FIGS. 2 and 2   a , in the modification shown in  FIG. 15 , there is an angle ring  300  providing an upwardly extending flange  301  corresponding to the annular wall  62  of the first embodiment, and a horizontal annular flange  302  which fits inside the outer wall of the chassis  40 . A flat annular gasket  303  supports the angle ring  300  on the annular ledge  60 . 
   The angle ring  300  effectively provides the annular channel  64  of the first embodiment and as illustrated in  FIGS. 2 and 2   a , which contains the expandable element or annular bellows  66 . When the bellows  66  is expanded under pressure as described previously, the annular gasket  303  provides a good seal between the angle ring  300  and the annular ledge  60  of the chassis. 
   Importantly, the annular ring  300  and the gasket  303  can be removed from the chassis structure to facilitate cleaning. 
   An additional modification in the embodiment of  FIG. 15  is the design of the annular shelf portion of the hopper  58 . In the embodiment of  FIG. 15 , the hopper  58  has an outwardly extending flange portion  310  which is now of sufficient radial width that the radially inner part of the flange portion  310  can rest directly on the upper edge of the vertical flange  301  of the angle ring  300 , when the bellows  66  is not inflated. The mesh frame  78  then rests directly on the upper surface of the annular flange  310 , and the bellows  66  acts directly on the under surface of the annular flange  310 . 
   Importantly, this design avoids the enclosed space defined by the annular shelf portion  70  and lower surface  76  of the hopper  58  in the first embodiment as illustrated in  FIGS. 2 and 2   a . Such enclosed spaces can be undesirable, especially when the separator is used for processing pharmaceuticals. 
   Finally, as can be seen in  FIG. 15 , the gasket  80  of the first embodiment in  FIG. 2   a  is now a U-shaped gasket  320  as shown in  FIG. 15 . 
   In other respects, the modified embodiment of  FIG. 15  is the same and operates in a similar fashion to the first embodiment as described previously. The chassis is mounted on the base by resilient mounts  210 . 
     FIG. 16   a  illustrates the modified embodiment of  FIG. 15  with a further modification which allows positive leak prevention from the sieve. An additional circumferential seal  330  is provided between the sheath  50  and an outer surface of the chassis  40 . In the illustrated embodiment, this seal  330  may comprise an annular inflatable tube  331  mounted in a groove  332  formed in the outer face of the chassis wall. The tube  331  is connected by a supply conduit  333  to the pressurised gas inlet  69  for the main inflatable bellows  66 , so that when the bellows  66  is inflated, so also is the tube  331  so as to form a gas-tight seal between the chassis  40  and the sheath  50 . Other sealing arrangements may be used for seal  330 , such as a simple (non-inflatable) O-ring or annular gasket. 
   In addition a gas connection  340  is provided at an upper part of the sheath  50  so that the interior space defined by the sheath unit  50 , the inner skirt  92  of the lid or cover, the gaskets  86  and  320 , the periphery of the hopper  58 , the bellows  66 , the angle ring  300 , the annular gasket  303 , the annular ledge  60 , and the chassis wall  40 , can be pressurised, e.g. to 0.5 bar gauge. This over pressure then prevents any residual leakage from within the sieve product space through the seals between the cover  54  and the hopper  58 . Also, leakage of possibly contaminating ambient air into the product space is prevented. 
   It is not critical to provide a perfect seal between the sheath  50  and the chassis and cover, so long as the desired over pressure can be maintained. However, a pressure sensing leak detection system may be provided if required to monitor the over pressure in the region enclosed by the sheath  50 . 
     FIG. 16   b  illustrates a variation of the arrangement for positive leak prevention using internal bayonet fittings comprising studs  48  engaging in slots provided in an internal flange  340  secured to the inside face of the sheath  50 . This variation provides a smoother exterior surface to the assembled separator. 
   The positive leak prevention system may also be applied to the separator with integrally formed cover and sheath of  FIG. 3 . Then an inflatable or O-ring seal such as seal  330  in  FIGS. 16   a  and  16   b  may be provided between the sheath or extended skirt portion  50  of the cover of the  FIG. 3  embodiment and the chassis  40 , conveniently at a location above the studs  48  of the bayonet fittings. Seal  86  for the sieve frame  82  makes a good seal with the shoulder  100  of the cover, when clamping pressure is applied by the bellows  66 . A gas connection (not shown in  FIG. 3 ) can be provided through the skirt portion  50  to pressurise the space bounded by the skirt portion  50 , the shoulder  100 , the frames  82  and  78 , the hopper  58 , the bellows  66  and the chassis  40 . 
     FIG. 17  illustrates a pneumatic control system which may be used with embodiments of this invention. 
   A supply of pressurised air is connected at push connector  400  and is fed to the bellows  66  (together with the annular sealing tube  331  if fitted) by means of an outlet push connector  401 . The pressure supplied to the sieve through the outlet  401  is controlled by a pressure regulator and gauge unit  402 . 
   A T-piece  403  in the pressure line supplies the applied pressure to a pressure sensor  404  which may be connected to disable the main sieve motor on detection of an applied pressure below a predetermined threshold, e.g. 5 bar. For example, the pressure sensor  404  may be connected to one of the starter coils in the starter box of the separator. 
   The pneumatic control system also includes a second T-piece  405  connecting to exhaust via an exhaust valve  406 . In addition an isolator valve  407  enables the inlet pressure supply to be isolated from the sieve. 
   In operation, the bellows  66  of the separator unit is inflated to apply the required clamping pressure by closing the exhaust valve  406  and opening the isolation valve  407 . These valves may be manual. Then, so long as the applied pressure as sensed by the sensor  404  exceeds the threshold (e.g. 5 bar) the sieve can be started in the usual way. If the applied pressure drops below the threshold, the sieve stops automatically and cannot be restarted until the pressure is restored. 
   If the leak detection system of  FIG. 16  is fitted, an additional sensor may detect pressure change in the scaled region behind the sheath  50  and again stop the sieve until corrected. 
   It will be clearly understood from the foregoing description of various embodiments of the invention, that various combinations, other than those described, of the characteristic features of the invention are possible without departing from the scope of the invention so defined by the claims. For example, the sheath could be formed of various combinations as a hybrid of the structures shown in  FIGS. 10 ,  11  and  12  and  FIGS. 13 and 14  whereby the sheath can be formed of segments which are themselves pivotally connected about vertical pivots to fold back about a hinge such as that shown in  FIGS. 10 to 12 . It will also be appreciated that the expandable elements of the various embodiments can be interchanged according to what is optimum for requirements. 
   In embodiments described above, the expandable element is an inflatable continuous tube or annular bellows extending around the circumference of the screen frame. However, in other embodiments, multiple lengths of inflatable tube or bellows elements may be used distributed around the screen frame to provide a substantially even clamping force. 
   In another embodiment the annular bellows may be replaced by an annular ring and at least two pneumatic or hydraulic rams operable to drive the ring to effect said clamping. 
   Other embodiments may use magnetic force to provide the expansion force for said expandable element or elements. Electro-magnets may be employed which can be switched on and off to apply clamping when required. Opposed permanent magnets may also be used and rotated from an opposing position when switching from repulsion (providing clamping pressure) to a released non-repelling position. 
   It should also be clearly understood that, though described in specific arrangements, it is possible to re-arrange the assembly of the frame(s) and expandable element(s) can be adjusted, provided that a clamping arrangement is achieved with the sheath enclosing the chassis. Thus, for example, the arrangement shown in  FIG. 4  could include two inflatable tubes, one above and one below the shelf portion  126  of the hopper  124 . 
   The various embodiments of the invention described above have the following advantages: 
   a) By providing one or more expandable members essentially inside the outer structure of the separator, the outer structure can be substantially “cleaned up” to minimise external crevices and other surface structures which can trap unwanted dirt or contaminants. 
   b) The clamping arrangements described, in particular that using the single clamping sleeve located around the cover of the separator structure, substantially simplifies the operation of clamping the various screening elements together, so that operator error is minimised. 
   c) The provision of pneumatic or hydraulically operated inflatable elements to provide the clamping pressure permits a reliable clamping pressure to be applied and provides the possibility of automatic pressure monitoring for added security. 
   d) Embodiments of the invention ensure that the clamping pressure is applied substantially uniformly around the entire circumference of the sieve frame, thereby improving sealing between the various clamped elements of the separator. 
   e) The additional provision in embodiments of the invention of an over pressure in the enclosed region between the sleeve and the clamped elements of the separator, enables positive leak prevention from the product space within the separator and prevents incursion of contaminants from ambient atmosphere. 
   The embodiment providing positive leak prevention which is described above and illustrated in  FIG. 16 , also incorporates the features of  FIG. 2  relating to securing and clamping together the various elements of the separator. It should be understood, however, that the positive leak prevention arrangement can be used with other designs of separator, for example designs which need not incorporate the one or more expandable elements to clamp the screen frame between a first bearing surface associated with the chassis of the separator and a second bearing surface associated with a clamping element. The positive leak prevention arrangement can be applied to sieves with prior art clamping arrangements sealing together the hopper and cover around the screen frame of the separator, modified to include a sleeve which is sealed to one of the hopper and cover so as to define the required enclosed space encompassing the clamp seals between the separator components. Other possibilities will be apparent to the skilled person.