IMPROVED VIBRATORY SCREENING MACHINE

Abstract: A vibratory screening machine such as used for separating solid particulate material from drilling mud to allow the latter to be re-used, is described. The general form of the construction is similar to that of the VSM100 shaker machine as supplied by the applicant company, but significant differences in the design and construction of the new unit allow a higher throughput of mud to be accommodated. An upper filtering screen (88) extends generally horizontally and serves to separate out the larger particles from the rest. The slurry which passes down to the lower screen (90) (of finer mesh than the upper screen) is further filtered and a mud-like residue of particulate material slowly accumulates on the lower screen (90) and migrates up the screen - to be discharged at the upper outlet end thereof, while liquid material filters through the screen to be recovered in a sump below. The drive (14, 16) for the machine comprises two pairs of rotating clump weights, mounted with their individual motors on a platform which spans the basket within which the screens are fitted. The two pairs of clump weights occupy a plane which is generally parallel to the upper (horizontal) screen (88), close to the latter, so keeping the centre of gravity low, which enhances the vibration characteristics of the system.

Description: Title : Improved vibratory screening machine Related Applications This Applications claims the benefit of US Provisional Application No . 60/047 , 124 , filed May 20th 1997. Field of the invention This invention relates to vibratory screening machines such as shale shakers used to separate mixtures of solids and liquids into the separate components such as mixtures which result from drilling of a bore hole or tunnel when a drilling fluid is used. Separation of the components permits re-use of the drilling fluid. Background to the invention There are many examples of such machines which essentially comprise a rigid supporting framework within which is resiliently mounted a box or basket containing one or more mesh screens onto which the mixture of solids and liquids is poured. To assist in the sieving action it is known to vibrate the box or basket so that the solid matter on the mesh screens is itself caused to vibrate . This causes clumps of fine material to break up, and if appropriate , pass through the mesh, together with the liquids , and for larger pieces of material which cannot pass through a mesh to migrate over the surface of the mesh to form part of the solids output of the machine . By using a coarse mesh screen followed by a fine mesh screen so the process of screening is split into two steps in which liquid and fines ( small solid particles ) pass through the coarse mesh leaving the larger solid material thereon, and essentially the liquid alone is able to pass through the very fine mesh screen leaving the fines to be discharged from the second screen. US Patent 4,167,478 describes a modular screening machine having at least one screen for separating particulate material according to size in which the screen box is driven so as to describe an elliptical movement at one end and a substantially rectilinear oscillatory movement at the other end. US Patent 4,340,469 also describes a vibrating screening apparatus containing two screens which are vertically spaced above and below two independently rotatable shafts containing unbalanced weights so that when rotated the screen assembly is caused to move in a generally elliptical path. By choosing the position of the two shaft axes relative to the centre of gravity of the assembly, a rocking movement is introduced at the infeed end of the assembly. US Patent 5,265,730 describes a further arrangement in which rotary eccentric vibrators are mounted outboard of the machine but act on the screen basket in such a way as to introduce an appropriate shaking movement. In such machines the objective is twofold. On the one hand the material has to be vibrated so as to separate liquid from solids as far as possible, and to separate small solids from larger solids particularly where a degree of adhesion can exist as a result of the inherent characteristics of the materials concerned. On the other hand as soon as the liquid and solid separation has occurred, it is desirable to remove the solid material from the screens as quickly as possible to make way for further material which is usually desirably delivered for processing on a continuous basis. Generally movement of solid material off the screen is achieved by incorporating into the motion a backwards and forwards component, as is known in the field of conveyors. This will cause particulate material to migrate along the length of a conveyor in response to an appropriate shaking movement. A drive for such a conveyor is described in US Patent 4,212,731 which also utilises two counter-rotating eccentric weights to generate the forces necessary to introduce conveyance of particulate material along the surface of the conveyor illustrated therein. Whereas the rapid removal of solid material from the screen is obviously advantageous where a high throughput is required, if the rate of removal is too high the migrating solids and fines may carry too much of the liquid phase. Whilst it has been found that improved efficiency, both in terms of screening and throughput, can be achieved using two screens, a first having a relatively coarse mesh and a second having a relatively fine mesh as aforesaid, the actual movement of each of the two screens in order to achieve optimum processing is not necessarily the same for each screen. It has been proposed to drive the two screens separately so as to achieve optimal movement of each screen. Such an arrangement however introduces additional complexities and cost and whilst it does allow for optimisation of a screening process, is not always desirable. The present invention seeks to provide a simple and expedient solution by providing an improved vibratory drive and filter basket constructions for a sieve particularly a shale shaker as aforesaid, to obtain a high consistency in the ratio of solids transported therethrough. The invention provides an improved form of such a machine which is more suitable for real time control using closed loop feedback systems for optimising filtering and throughput. The invention also provides an improved shale shaker which can be used as a basic unit or in conjunction with other similar units to provide for greater throughput, and to permit a reliable and controllable screening of mixtures of solids and liquids, with or without real time closed loop feedback control. Summary of the invention According to one aspect of the present invention, a vibratory screening machine comprises a rigid supporting housing within which is resiliently mounted a rigid screening basket having upper and lower screens mounted therein, wherein drive means for vibrating the basket relative to the housing comprises a pair of counter rotating masses mounted for rotation about two spaced apart parallel axes and wherein the centre of gravity of the basket is kept low in that the masses are mounted between the sides of the basket and the axes of the counter rotating masses both occupy a plane which is substantially parallel to the plane of, and is just above, the upper screen. Preferably each mass comprises a pair of similar weights which are mounted on opposite sides of a drive unit therefor and the drive and weights are located on a bridge which spans the basket. Preferably the two weights in each pair are arranged symmetrically relative to their drive unit which is preferably mounted centrally of the bridge, and the low profile produced by mounting the drives and weights just above the basket, and the symmetry of each drive and its weights, produces a balanced assembly and a substantially uniform vibratory motion of the basket relative to the housing, which assists in producing controlled and predictable solids migration over the screens. According to another aspect of the invention, there is provided a method of lowering the centre of gravity of a shaker basket mounted for vibration within a housing and containing upper and lower sieving screens, wherein both of two counter-rotating pairs of weights together with drive means therefor are mounted on bridge means extending between the sides of the basket at the same height above and just clear of the upper screen. According to another aspect of the present invention, there is provided a method of imparting an elliptical motion to a shaker basket having upper and lower screens comprising pairs of eccentric weights driven by drive means, each of the pairs of weights being driven in a counter rotating manner relative to the other, and the assembly of weights and drives is situated beyond the midway point measured along the length of the basket in the direction of movement of particulate solids which occurs therealong when in use. The position of the assembly of weights and drives relative to the basket may be selected to advantage so that the angle of the major axis of the elliptical motion, which tends to act through the centre of gravity of the basket, subtends an acute angle in an upward sense relative to the lower inclined screen. The upper and lower screens both may be of planar form and mounted so that in use the upper screen of coarser mesh is horizontal and the lower screen forms a continuous ramp uniformly sloped upwardly from input end to discharge end, and making an acute angle with the upper screen. According to another aspect of the invention in a vibratory screening machine having upper and lower screens both of planar form, the two screens are mounted so that in use the upper screen of coarser mesh is generally horizontal and the lower screen forms a continuous ramp uniformly sloped upwardly from input end to discharge end and making an acute angle with the horizontal . As employed herein the term "screen" is intended to mean a single framed member having a woven wire mesh filtering panel tensioned thereacross or an assembly of such framed members, arranged in edge abutting relation to form a larger area for filtering more material. A vibratory screening machine as claimed comprising: 1. a stationary rigid housing 2. a rigid screen supporting basket wholly contained within the sides of the housing and resiliently mounted therein 3. two sieving screens mounted one above the other in the basket, the upper screen being of a coarser mesh than the lower screen 4. drive means mounted on the basket and contained wholly between the two sides of the housing for imparting vibratory movement to the basket, wherein 5. both screens are flat and planar and wherein in use the frame and basket are mounted so that the upper screen is substantially horizontal and the plane of the lower screen subtends an acute angle relative to the upper screen and constitutes a rising ramp in the direction in which particulate material will migrate over its surface in response to vibratory forces exerted on the basket by the drive means. Preferably the drive means is such that a resultant force is produced by the drive means acting through the centre of gravity of the basket which at least for some part of each "cycle" of vibration subtends acute angles with the surfaces of the two screens. Particulate material remaining on the upper screen will of course be larger than particulate material remaining on the lower screen. Preferably, the acute angle which the plane of the lower screen makes with the horizontal lies between 2 degrees and 30 degrees, preferably between 2 and 15 degrees. It is important to remove as much liquid as possible from the particulates during passage through the machine. Different sized particles tend to require different treatment to achieve this. The inclined lower screen reduces the velocity of the smaller particles so as to increase their dwell time on the screen and resultant dewatering. By angling the lower screen relative to the upper screen, it has been found that the vibration will not only cause the appropriate rates of movement of the different particulate materials over each of the two screens, so that neither becomes overloaded in use and a multiphase mixture of larger solids, fines and liquid delivered to the upper screen will be separated in a uniform and reliable manner by the two screens, but in particular the time taken by particulate material to travel a given distance along is substantially consistent. The inclination of the lower screen also provides a hydrostatic head of liquid over the screening surface, which has been found to improve the efficiency of separation and volumetric throughput . The inclined screen also guarantees that its surface remains flooded with liquid at all times during operation, which is important to ensure efficient separation and transport. Whilst it has been proposed in earlier designs of machine to provide upper and lower screens, part of the lower screen being parallel to the upper screen and the remaining part thereof extending upwardly as a ramp so as to converge towards the upper screen and create a weir, it has been found in particular that the abrupt change in direction between the two parts of the lower screen has inhibited regular and uniform migration of particulate material from one end of the lower screen to the other. The material handling characteristics of a machine embodying the invention are found to be better than this earlier design of machine, since the lower screen presents a flat inclined plane to the fines which are to migrate thereover. According to another aspect of the invention the lower screen proposed by the invention is preferably adapted to be self- clearing so as to reduce the risk of fines becoming permanently lodged between the interstices of the smaller mesh used on the lower screen. To this end the lower screen is preferably one which includes two screen cloths one laid above the other over a rigid frame, and which are pretensioned during manufacture so that the tension in the lower cloth is greater than the tension in the upper cloth. Any component of motion perpendicular to the plane of the two cloths forming the lower screen will tend to cause the less tensioned cloth to rise and fall relative to the lower cloth, and the relative movement, and particularly the impact between the cloths during such vibration, will tend to dislodge particulate material from the cloth and prevent particulate material from lodging and blinding the screen. Preferably the cloths are formed from woven wire and the two cloths have a different mesh size and the two cloths are adhesively bonded around their periphery to the rigid frame which itself is secured within the shaker basket. In order to increase the material loading capability of a screen, the rigid frame may include or be formed as part of a rigid plate having a plurality of apertures therein, and the two cloths are not only adhesively bonded to the rigid frame, but also around the apertures in the rigid plate. Typically the apertures are the same size and regularly spaced over the entire area of the plate and in order to reduce weight, the plate may be formed from sheet metal material whose rigidity is increased by bending the edges of the apertures downwardly substantially at right angles to the plane of the plate. The upper screen may be constructed in a similar manner, albeit from a coarser mesh cloth or cloths, so as to provide for differential screening as between the upper and lower screens in the machine. As before, where two cloths are used in the upper screen they may have a different mesh size and be differentially tensioned. Both upper and lower screens may be removable to allow for replacement when wear or damage has occurred. Typically quick release securing means is provided in the form of clamps, which may be hydraulically or pneumatically operated, or more preferably a peripheral inflatable seal is employed. This not only secures the periphery of the rigid frame of the screen in place within the basket, but also prevents the passage of liquid and solid particulate material around the edges thereof thereby avoiding the sieving action of the screen. Preferably the screens are removable rom one end of the basket and typically this is the end over which the solids and fines are discharged in use. Since the screen life may be relatively short (depending on the environment within which they are used and the materials which are being sieved), it is convenient if the individual screens can be handled by one man. Where the area of the screen required to handle the desired throughput is too great to allow a single screen spanning the area to be carried by one man, each of the upper and lower screens may be made up of two or more similar units, each of which is removable separately from the basket so as to be independently replaceable, so that in the event of damage or wear occurring to one of the units, more than another, the damaged/worn unit can be replaced and the undamaged/non worn unit(s) left in place. According to another aspect of the present invention the drive means for vibrating the basket is preferably mounted above the upper screen on a bridge which spans from one side to the other. Preferably the bridge is located nearer to the discharge end of the basket to keep the drive clear of the incoming slurry deluge, and to displace the mass of the drive means away from the centre of gravity of the basket. According to a particularly preferred aspect of the invention, the drive means comprises a pair of counter rotating weights mounted for rotation about two spaced apart parallel axes both of which occupy a plane which is substantially parallel to the plane of the upper screen. Therefore in use the two axes are substantially horizontal. The Centre of Action of the two counter rotating weights will normally lie on a straight line joining the centres of rotation. A straight line resultant will be obtained if the two counter weights are equal in mass and eccentricity and are contra-rotated. The dynamic forces acting on the system will cause the rotation of the two weights to synchronise. If the centre of gravity and Centre of Action coincide, the Centre of Action will then describe a generally linear motion substantially perpendicular to the plane containing the two parallel axes of rotation. Where the centre of gravity of the basket and the Centre of Action of the two counter rotating weights do not coincide the resultant force will act along a line joining the Centre of Gravity to the Centre of Action. In accordance with a preferred feature of the invention, the spacing between the two parallel axes and/or the precise position of the pair of axes relative to the mean operational centre of gravity of the basket, is/are selected so that the resultant force is of the desired type and makes a desired angle relative to the screens. Where the counter rotating weights produce a straight line resultant force, this will tend to act along a line between the Centre of Action of the two counter rotating weights and the centre of gravity of the basket. The amplitude of the basket movement from its mean position will be limited inter alia by the stiffness of the basket mounting to the frame. Typically the basket is mounted on rubber or polymer feet or on four helical springs, which support the weight of the basket, which nevertheless can vibrate substantially freely with small amplitudes of up to a few mm's in at least a number of directions relative to the supporting frame, if not freely in all directions. If on the other hand the rotational moment produced by one of the counter rotating weights is different from that produced by the other, (either by using a heavier or lighter mass or a different radius of eccentricity, or a combination of both), two effects are noted. Firstly the position of the Centre of Action will shift from the midpoint between the two centres of rotation nearer to the axis about which the greater rotational moment is produced. The greater disparity of rotational moment, the greater the shift from the mid-position, as described in US 4,212,731. Secondly it has been observed that the locus of the Centre of Action will now tend to describe an ellipse and the greater the disparity, the larger will be the minor axis of the ellipse. Since the forces produced by two contra-rotating weights will combine additively when the weights are moving in the same direction, and will combine subtractively when the weights are moving in opposite directions, the major axis of the ellipse will be generally perpendicular to the plane containing the two axes of rotation and the minor axis of the ellipse will be generally parallel to that plane if the centre of gravity of the basket coincides with the Centre of Action. However if the basket can vibrate and its centre of gravity does not so coincide, then since the resulting force will always act through the centre of gravity of the basket, it will tend to describe an elliptical path with the major axis of the ellipse orientated so as to lie along the line joining the "operational" centre of gravity of the basket to the Centre of Action of the two counter rotating weights. It will be seen therefore that by varying the rotational moment of one of the counter rotating masses of the drive unit so that it is different from the other, so the pure straight line movement which results if the two rotary moments are equal is converted into an elliptical motion in which the minor axis of the ellipse and therefore the extent by which the motion is no longer straight-line is determined by the disparity between the two rotary moments, as is also the angle of the major axis of the ellipse. According to another aspect of the invention, the actual force exerted by the drive on the basket can be adjusted by altering the speed of rotation of the drive to the rotating eccentric weights. According to a further aspect of the invention measuring means may be employed for measuring the throughput of the machine and/or the quality of the screening process, for example by reference to the size of particles in the discharge and/or the volume of material discharged, or both. Feed-back signals may be generated for controlling one or more of the following parameters, namely the speed of rotation of the eccentric weights, the eccentricity of one or both of the weights, or the mass of one or both of the weights, or any combination thereof. Each of the rotatable eccentric masses may be driven by a separate motor. Alternatively a single motor and appropriate drive means may be provided for rotating the various eccentric masses in pairs or other groups. Any drive means for rotating the eccentric masses may be electrically, or pneumatically, or hydraulically powered. In a preferred example, the basket dead-weight unladen is in the range 1000-1750Kg, a typical dynamic loading of slurry is in the range 450-700Kg and a first eccentric weight has a mass of 43Kg and the centre of gravity of the mass is some 40mm from the shaft centre line, whilst the second eccentric weight has a mass of 26Kg and its centre of gravity is some 47mm from its shaft centre line, and the rotational speed is such as to produce vibration in the range 20-40Hz, preferably in the range 25-33HZ. Preferably the opposite sides of the frame of a machine embodying the invention are parallel and upright when the machine is mounted for use. By ensuring that nothing protrudes laterally of the frame, so machines can be placed side by side in close proximity to allow outgoing solids and liquids to be collected by troughs/conveyors adjacent the output ends of the side-by-side machines, and the slurry input to the machines can be discharged into one or two or more of the machines, depending on the volume of slurry to be handled and the capability of the machines to handle the input material. Although reference has so far been made to the rotation of single eccentric weights about each of the two spaced-apart axes of rotation, it is to be understood that each mass or weight along each axis can be divided into two or more (preferably similar) weights, each eccentrically mounted about the same axis . A particularly preferred arrangement involves mounting each motor centrally of the bridge with an output shaft extending axially in both directions from each motor, with eccentrically mounted weights carried by the two output shafts on opposite sides of each motor. Preferably bearings are provided between each motor and each eccentrically rotating weight and a further bearing is provided outboard of the eccentrically mounting weight. Flexible couplings may be provided between the motors and the shafts on which the eccentric weights are mounted so that the eccentric forces created by the rotating eccentrically mounted weights are not transmitted to the motor bearings. Where two weights are associated with each motor not only are the two weights preferably similar, but preferably they are mounted with similar eccentricities. Although an eccentric weight may be distributed (axially) along its shaft, it has been found advantageous to provide a concentrated mass at a relative large radius rather than an axially distributed mass at a smaller radius, since this allows the supporting bearings on either side of the eccentrically rotating mass to be axially closer together. The shorter length of shaft between the two bearings is thus stiffer than if a longer length of shaft is employed to support the eccentrically mounted mass. This reduces the forces acting on the bearings since the bending moment of the shaft between the bearings is now much smaller and the tendency for whip and bend to occur in the shaft is significantly reduced. The invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is a side elevation of the vibratory basket of a shale shaker embodying the invention; Figure 2 is a cross-section through the elevation of Figure 1; Figure 3 is a top view of the basket shown in Figure 1; Figure 4 is an end elevation which also shows the having within which the basket is mounted; Figure 5 is a sectional elevation through a vibratory drive assembly (of which there are two in the embodiments shown in Figures 1 to 4 ) ; Figure 6 is a side view, corresponding to Figure 1, of the basket, showing the centre of gravity of the basket when unloaded and the mean centre of gravity of the basket when under normal operating load (an average 70% of maximum load); Figure 7 corresponds to Figure 6 , and shows the centres of gravity when the basket is loaded, to half capacity, with high or low specific gravity fluids; and Figure 8 is a view corresponding to Figure 7, and showing the centres of gravity when the basket is fully loaded with a high or low specific gravity fluid. Referring to Figures 1 to 3 a vibratory basket designated 10 is spanned by a bridge 12 (see Figure 3) on which are carried two rotational drives 14 and 16. Each of the drives comprises a central electric motor 18, (20) of so-called through-shaft design, having an output shaft protruding axially therefrom on opposite sides of the motor casing, and two equal eccentric "in phase" clump weights, are mounted on each rotating output shaft, so that rotation of the motor and the clump weights produces a rotating force equivalent to the clump weights and their eccentricity. The drives are shown in more detail in Figure 5 and will be described more fully with reference thereto. The basket 10 is supported on helical springs below four brackets 22, 24, 26, 28. Two of the springs can be seen in Figure 4 and are designated by reference numerals 30 and 32. Figure 4 also shows the housing 34 within which the basket is carried on the springs and it will be seen that the two sides of the housing 34 are uncluttered and have no protruding elements, so that two or more such housings can be abutted laterally against one another frame, side by side. This allows for variations in material feed rate to be more accommodated, by conveying slurry to one, or two or more units (if more than two units are mounted side by side), depending on the volume of slurry to be processed, by means of a transversely extending duct over the input ends of the units. Figure 5 shows in more detail one of the drive units 16, based on the motor 18. The motor is a through shaft motor and an output shaft extends therefrom on both sides of the motor housing at 36 and 38 respectively. Each output shaft is connected to a flexible drive coupling 40, 42 respectively, the output shafts of which are carried in journal bearings 44, 46 (in the case of coupling 40) and 48, 50 (in the case of coupling 42). Each journal bearing typically comprises a cylindrical roller bearing. Mounted on the coupling output shaft 52, 54 are eccentric clump weights 56, 58 respectively. Rotation of the motor 18 causes both clump weights to be similarly rotated and their eccentricity generates a rotating out of balance force as they rotate. For safety, and protection against erosion, the clump weights and flexible drives are contained within cylindrical housings 60, 62 respectively. The inboard ends of the housing 60, 62 are coupled to the casing of the motor 18 and are closed at their outboard ends. The housings 60, 62 each have feet (64, 66, 68, 70 in the case of housing 60) by which the housings are secured to the bridge 12. The basket 10 is formed from two side panels 72, 74 between which extend hollow cylindrical tubes 76, 78, 80, 82, 84 and 86. These are welded to the two opposed side panels 72, 74 and create a lightweight, very strong rigid structure. Screen supporting rails (not identified in the drawings) support an upper generally horizontal screen 88 and an inclined lower screen 90, which rises in the direction in which particulate material migrates over its surface during vibration, ie from left to right in Figure 2. A deflector plate or intermediate screen 92 extends over the lower inclined screen below the region onto which slurry (drilling mud) is poured. A collector 94 is located below the discharge end 96 of the lower conveyor. By mounting both the two clump weights and their motor drives just above the upper screen, so the centre of gravity (shown in Figure 1) is considerably lower than if the left hand drive 16 is mounted above the position shown in Figure 2, so that the line joining the axes of the two motors is parallel to the inclined lower screen and which is where it has normally been considered that it should be mounted. In use the clump weights of drive 14 are rotated in the opposite sense to the clump weights of drive 16. it has been found in practice that different conveyance characteristics can be produced by different rotations, and in the example shown, if the rotation of the left hand drive 14 in Figure 1 is clockwise, and the rotation of the right hand drive 16 is counter-clockwise, this produces a more consistent and steadier migration of particulates along the screens than if the directions of rotation are reversed. The motors 18 and 20 may be electric motors, but in combinations where flammable gases or liquid are involved, the motors are more preferably hydraulic or pneumatically powered. The springs such as 30, 32 may be replaced by blocks of rubber or plastics, or rubber and plastics composite material. The screens for filtering fluids from solids may be formed from rigid metal, or plastics, or metal reinforced plastic frames having stretched and tensioned and bonded thereto woven wire cloths. Typically each screen has two layers of wirecloth affixed thereto, and the lower cloth has a coarser mesh and higher mesh tension than the upper cloth. Typically the upper cloth has a mesh size in the range 10-80 mesh while the lower cloth has a mesh size in the range 50-350 mesh. Although not shown, each of the screens 88 and 90 (as identified in Figure 2) may be formed from two or four similar screens arranged in edge abutting relation and sealed against the ingress of liquid around or between their frames. An inflatable seal may be provided to secure and seal the frame in place against rails (not shown) on the underside thereof. The centre of gravity of the basket 10 in Figure 1 is shown at 106 in Figure 1, and the instantaneous resultant force produced by the combination of the two rotating pairs of clump weights acts through that point. Where two or more units such as shown in Figures 1 to 4 are mounted side by side, with the side walls of their housings 34 in abutting relation, the feed to the units may comprise a manifold 108 which extends horizontally over the left hand ends (as viewed in Figure 1) of all of the baskets (10), with flow control means 110 selectively allowing or preventing slurry (eg drilling mud) to flow via ducts 112 onto the left hand (input) end of the upper screen 88 (see Figure 2) of some or all of the shakers, as required. A sump (not shown) below the units collects the filtered liquid material containing fines (which cannot be separated) , while solids filtered therefrom and left on the upper and lower screens migrate from left to right to exit off the right hand end of the lower screen, in particular, for collection on a conveyor or in another sump (not shown) mounted along the front of the units. A control unit (not shown) for sensing the volume flow of material to the units and opening and shutting ducts such as 112 and causing the units to operate as required, may be provided, linked to flow sensors and to controls for opening and closing the ducts and turning the shakers on and off. A unit as shown in Figures 1 to 5 of the drawings is of particular use in separating solid particulate material from drilling muds recovered from down-hole oil and gas well drilling operations. Filtering screens for a unit as aforesaid may be of the type produced by United Wire Ltd of Edinburgh, Scotland, UK. With reference to Figure 6, the position 106 is that of the centre of gravity of the basket 10 when no fluids and cuttings are being carried by the screens of the machine. This position lies on a line 108 which, if extrapolated, will pass through the Centre of Action of the weights. The Centre of Action lies on a line (not shown) joining the two axes of rotation of the weights. The machine is so configured that the net force exerted by the weights gives rise to no turning moments being exerted on the basket 10, and this requirement places constraints on the relative sizes of the weights, their effective radii of rotation, and hence the position of the Centre of Action, which can be expressed by the formula m 1 r 1 = b m, where m 1 r 1 is the product of the mass and effective radius of rotation of the clump weights of one drive m 2 r 2 is the corresponding product for the other drive a is the distance of the Centre of Action from the axis of the weights of said drive; and b is the distance of the Centre of Action from the axis of the weights of the other drive. The optimum values for the above parameters are believed to be as follows: m 1 = 33.5 Kg; m 2 = 25.4 Kg; r,, = 0.046m; r 2 = 0.042m and the masses are stated at a speed in the range 1500-2000 rpm. The resultant elliptical path of the basket 10 has a major radius of 3.3 mm and a minor radius of 0.6 mm. Of course, in use, the Centre of Gravity of the basket will be affected by the mass of fluid and cuttings on the screens. Reference numeral 110 denotes the position of the mean operational Centre of Gravity of the basket. In Figure 7, reference numerals 112 and 114 denote the positions of the Centre of Gravity when the basket is loaded, to half its maximum capacity, respectively with examples of high and low Specific Gravity fluids, whilst the positions of the Centre of Gravity when the basket is fully loaded with high and low density fluids are respectively indicated by numerals 116 and 118 in Figure 8. It will be seen that all of the positions 110, 112, 114, 116 and 118 lie on or near the line 108, as a result of the inclination of the lower screen. Thus, the varying of the loads on the basket has a minimal effect on the angular relationship between the Centre of Gravity and the Centre of Action.

Claims: Claims 1. A vibratory screening machine having a rigid housing within which is resiliently mounted a rigid screening basket having upper and lower screens mounted therein, wherein drive means for vibrating the basket relative to the housing frame comprises a pair of counter rotating masses mounted for rotation about two spaced apart parallel axes between the sides of the housing and wherein the axes of the counter rotating masses both occupy a plane which is substantially parallel to the plane of, and is just above, the upper screen. 2. A vibratory screening machine as claimed in claim 1, wherein each mass comprises a pair of similar weights which are mounted on opposite sides of a drive unit therefor and the drive and weights are located on a bridge which spans the basket. 3. A vibratory screening machine as claimed in claim 2, wherein the two weights in each pair are arranged symmetrically relative to their drive unit which is mounted centrally of the bridge, and the low profile produced by mounting the drives and weights just above the basket, and the symmetry of each drive and its weights, produces a balanced assembly and a substantially uniform vibratory motion of the basket relative to the housing, which assists in producing controlled and predictable solids migration over the screen. 4. A method of lowering the centre of gravity of a shaker basket mounted for vibration within a housing and containing upper and lower sieving screens, wherein both of two counter- rotating pairs of weights together with drive means therefor are mounted on bridge means extending between the sides of the basket at the same height above and just clear of the upper screen. 5. A method of imparting an elliptical motion to a shaker basket having upper and lower screens comprising rotating pairs of eccentric weights driven by drive means, each of the pairs of weights being driven in a counter rotating manner relative to the other, and the assembly of weights and drives is situated beyond the midway point measured along the length of the basket in the direction of movement of particulate solids which occurs therealong when in use. 6. A method as claimed in claim 5, wherein the position of the assembly of weights and drives relative to the basket is selected so that the angle of the major axis of the elliptical motion, which tends to act through the centre of gravity of the basket, subtends an acute angle in an upward sense relative to the lower inclined screen. 7. A vibratory screening machine as claimed in any of claims 1 to 6 having upper and lower screens both of planar form and mounted so that in use the upper screen of coarser mesh is horizontal and the lower screen forms a continuous ramp uniformly sloped upwardly from input end to discharge end, and making an acute angle with the upper screen. 8. A vibratory screening machine having upper and lower screens both of planar form and mounted so that in use the upper screen of coarser mesh is generally horizontal and the lower screen forms a continuous ramp uniformly sloped upwardly from input end to discharge end and making an acute angle with the horizontal. 9. A vibratory screening machine as claimed in any of claims 1 to 8, comprising: 1. a stationary rigid housing, 2. a rigid screen supporting basket wholly contained within the sides of the housing and resiliently mounted therein, 3. two sieving screens mounted one above the other in the basket, the upper screen being of a coarser mesh than the lower screen, and 4. drive means mounted on the basket and contained wholly between the two sides of the housing for imparting vibratory movement to the basket, wherein 5. both screens are flat and planar and wherein in use the frame and basket are mounted so that the upper screen is substantially horizontal, and the plane of the lower screen subtends an acute angle relative to the upper screen and constitutes a rising ramp in the direction in which particulate material will migrate over its surface in response to vibratory forces exerted on the basket by the drive means. 10. A machine according to claim 9, wherein the acute angle which the plane of the lower screen makes with the horizontal lies between 2 degrees and 30 degrees. 11. A machine according to claim 9 or claim 10, wherein the drive means produces a resultant force acting through the centre of gravity of the basket. 12. A machine according to any of claims 9 to 11, wherein the lower screen is adapted to be self-cleaning so as to reduce the risk of fines becoming permanently lodged between the interstices of the smaller mesh of the lower screen. 13. A machine according to claim 12, wherein the lower screen includes two screen cloths one laid above the other over a rigid frame, and which are pretensioned during manufacture so that the tension in the lower cloth is greater than the tension in the upper cloth. 14. A machine according to claim 13, wherein the cloths are formed from woven wire, have different mesh sizes and are adhesively bonded at least around their periphery to the rigid frame which itself is secured within the shaker basket. 15. A machine according to claim 13 or claim 14, wherein the rigid frame includes or is formed in part of a rigid plate having a plurality of apertures therein, and the two cloths are not only adhesively bonded to the rigid frame, but also around the apertures in the rigid plate. 16. A machine according to claim 15, wherein the apertures in the rigid plate are the same size and are regularly spaced over the entire area of the plate. 17. A machine according to claim 16, wherein the plate is formed from sheet metal material whose rigidity is locally increased by bending the edges of the apertures downwardly substantially at right angles to the plane of the plate. 18. A machine according to any of claims 9 to 16, wherein both the upper and lower screens are removable from the basket. 19. A machine according to claim 18, wherein quick release securing means for the screens is provided in the form of a peripheral inflatable seal. 20. A machine according to claim 18 or claim 19, wherein the screens are removable from the end of the basket over which the solids and fines are discharged in use. 21. A machine according to any of claims 9 to 20, wherein the drive means for vibrating the basket is mounted above the upper screen on a bridge which spans the basket from one side to the other. 22. A machine according to any of claims 9 to 21, wherein the drive means comprises a pair of counter rotating weights mounted for rotation about two spaced apart parallel axes both of which occupy a plane which is substantially parallel to the horizontal plane of the upper screen. 23. A machine according to claim 22, wherein the spacing between the two parallel axes and/or the precise position of the pair of axes relative to the mean operational centre of gravity of the basket, is/are selected so that the resultant force makes a desired angle relative to the screens. 24. A machine according to claim 22, wherein the rotational moment produced by one of the counter rotating weights is different from that produced by the other, and the centre of action of the weights describes an ellipse. 25. A machine according to any of claims 22 to 24 wherein the actual force exerted by the drive on the basket is adjustable by altering the speed of rotation of the drive to the rotating eccentric weights. 26. A machine according to any of claims 9 to 23, wherein the basket is mounted on rubber or polymer feet or on four helical springs, which support the weight of the basket, and which nevertheless can vibrate substantially freely with small amplitudes of up to a few mms in at least a number of directions relative to the supporting frame. 27. A machine according to any of claims 22 to 26, wherein measuring means is provided for measuring the throughput of the machine and/or the quality of the screening process. 28. A machine according to claim 27, wherein feedback signals are generated for controlling one or more of the following parameters, namely the speed of rotation of the eccentric weights, the eccentricity of one or both of the weights, or the mass of one or both of the weights, or any combination thereof. 29. A machine according to any of claims 21 to 28, wherein each of the rotatable eccentric weights is driven by a separate motor. 30. A machine according to any of claims 22 to 28, wherein a single motor and appropriate drive means is provided for rotating the eccentric weights in groups, such as pairs. 31. A machine according to any of claims 9 to 30, wherein the opposite sides of the other support frame of a machine embodying the invention are parallel and upright when the machine is mounted for use, and nothing protrudes laterally of the support frame . 32. A machine according to claim 22 when appendant to claim 14, wherein the drive means comprises motors each mounted centrally of the bridge with an output shaft extending axially in both directions from each motor, with eccentrically mounted weights carried by the two output shafts on opposite sides of each motor. 33. A machine according to claim 32 wherein bearings are provided between each motor and each eccentrically rotating weight and a further bearing is provided outboard of the eccentrically mounted weight. 34. A machine according to claim 24, wherein flexible couplings are provided between the motors and the shafts on which the eccentric weights are mounted so that the eccentric forces created by the rotating eccentrically mounted weights are not transmitted to the motor bearings. 35. A vibratory screening machine substantially as hereinbefore described with reference to the accompanying drawings. 36. A vibratory screening machine comprising a basket having an inclined screen, and drive means for vibrating the housing to cause solid material deposited on the screen to migrate along the screen, up the incline, to an upper end of the screen from which the solid material is discharged, wherein the drive means exerts on the basket a resultant force along a line which passes through or near the Centre of Gravity of the basket when unloaded, and wherein the inclination of the screen is such that the Centre of Gravity of the basket continues to lie on or near said line for different loadings of the basket. 37. A machine according to claim 36, in which the drive means comprises a pair of weights and means for rotating the each weights eccentrically about a respective one of a pair of spaced apart parallel axes, the weights being counter rotated, and wherein said line joins the Centre of Gravity of the basket, under normal loading conditions, to the Centre of Action of the two weights.

Inventors: BAILEY ARTHUR ROBERT, BURNETT GEORGE ALEXANDER, BAILEY, ARTHUR, ROBERT, BURNETT, GEORGE, ALEXANDER
Applicants: BAILEY ARTHUR ROBERT, BURNETT GEORGE ALEXANDER, RIG TECHNOLOGY LTD, BAILEY, ARTHUR, ROBERT, BURNETT, GEORGE, ALEXANDER, RIG TECHNOLOGY LIMITED
Application year: 1997
Main classification: B07B1
IPC codes: B01D33, B07B1