SIEVING APPARATUS
Field of the Invention
The present invention relates to sieves both for dry particulate solids and for liquids and particularly sieves in which an excitation source provides deblinding excitation of the sieve screen.
Background of the Invention
Most industrial sieving machines include some means of applying a primary vibratory movement to the sieving screen in order to facilitate product movement through the screen and also to create a flow of material over the screen surface. This ensures maximum, utilisation of the active screening area and that oversized product can be transported to an outlet to be removed. The primary vibratory movement is often a combination of horizontal and vertical reciprocating motion which may typically be applied to the frame carrying the sieve mesh or screen in a variety of ways, such as by rotating out-of-balance weights, or a direct drive by a rigid crank or cam system.
A problem with sieving machines is blinding of the screen, particularly when sieving damp or sticky materials. Blinding is a significant problem in the industrial sieving of certain powders and also in the straining of liquids. To overcome the blinding problem secondary vibrations, preferably flexural, have been applied to the screen, for example by impacts from deblinding discs or the application of high and ultrasonic frequencies (see for example EP-A-0369572) .
Typical ultrasonic frequencies are above 20kHz, and typical amplitudes of the ultrasonic vibration supplied to the mesh are a few (1-10) microns. However, ultrasonic energy is quickly dissipated in the screen, making it difficult to excite a large
screen area ultrasonically. Extended resonators to increase the distribution of ultrasonic energy over the screen are disclosed in EP 0652810. However, for large sieve areas, multiple transducers are still normally required.
It is also known to use guide members located above the screen to improve the flow of material to be sieved over the surface of the screen. For example, scroll-shaped guide members are used with circular sieves to ensure material to be sieved moves progressively from the centre of the screen, where it is first delivered, outwards in a generally spiral path, covering nearly all regions of the screen surface before reaching the outlet for oversize particles at or near the screen periphery. This increases the residence time over the screen, to maximise the opportunity for fines to pass through the screen. Other guide member shapes and arrangements are used for different sieve designs, in each case to improve material flow over the screen to increase the time for undersize to separate from oversize.
Summary of the Invention
According to the invention, there is provided a sieve comprising: a base, a rectangular sieve screen frame mounted on the base, a rectangular separator screen mounted in the frame, a vibrator arranged to vibrate the frame relative to the base, a resonator rod secured to or contacting the separator screen, wherein the resonator comprises a rod extending between spaced ends, an ultrasonic transducer at one of said spaced ends to excite the resonator rod at a resonant frequency having a predetermined wavelength along the length of the resonator rod, said resonator rod being s-shaped having a first portion of its length which bends smoothly in a single direction of curvature through at
least 180° and a second portion of its length which bends smoothly in a single direction of curvature of opposite sign to said first portion through at least 180°, and the rod having a minimum radius of curvature at any point between said spaced ends which is greater than said predetermined wavelength.
Brief Description of the Drawings
Embodiments of the present invention will now be described with reference to the accompanying drawings, in which:
Figure 1 is a schematic diagram of a circular sieve which does not embody the present invention;
Figure 2 is a plan view of the sieve of figure 1;
Figure 3 is a plan view of a further arrangement which is also not an embodiment of the invention;
Figure 4 is a scrap cross-sectional view through figure 3 showing an enlarged view of the nodal decoupler;
Figure 5 is a plan view of a still further arrangement which is not an embodiment of the invention;
Figure 6 shows a cross-sectional view along line A-A of figure 5; and
Figure 7 is a cross-sectional view taken along line B-B in figure 5.
Figure 8 is an underneath plan view of a rectangular sieve with an s-shaped resonator rod embodying the present invention;
Figure 9 is an underneath plan view of a variation of the embodiment of Figure 8; and
Figures 10a to 10c are schematic illustrations of additional embodiments of the invention.
Detailed Description of the Preferred Embodiments
Referring now to Figure 1, this shows a circular sieve 2 which is not an embodiment of the present invention but is described below to enable a complete
understanding of embodiments of the present invention which will be described later. The sieve 2 comprises a sieve screen 10 in the form of a mesh, which is held in a sieve screen frame 6, for example by clamping. The frame 6 and sieve screen 10 may be rectangular but a popular circular shape is shown in this example.
The sieve screen frame 6 includes an inner support frame 8, which may take the form of an X' frame, although it may take other forms. The sieve screen frame 6 is attached to a lower cylindrical container 7, for example by clamping. An upper cylindrical container 9 is secured, e.g. also by clamping, on top of the screen frame 6 to act as a containment wall for the product to be sieved when it is on the sieve screen surface 10.
The lower container has a domed floor 22. The lower container is secured on a skirt-shaped annular casting 18, e.g. by clamping.
The sieve also has a fixed base 4 which is attached to the floor 3 6, in this embodiment by using sieve stands 38. However, in alternative embodiments the base may simply stand on a suitable surface, may be fixed to a suitable surface or may be arranged on wheeled or other mounts.
The skirt is supported on the fixed base using a suspension support 20. In this particular embodiment the suspension support 20 comprises a rod 19 attached to the skirtcasting 18 and base 4 using elastomeric bushings 21. This arrangement permits both horizontal and vertical movement of the skirtcasting 18 and therefore of the sieve frame 6 and sieve screen 10. Other methods may be used for supporting the sieve screen frame on the fixed base, for example spring mounts.
A motor 23 is mounted on the fixed base 4 and flexibly attached, for example using a rubber coupling 25, to a vibrator 12. The vibrator 12 comprises a bearing housing 2 9 secured in the centre of the skirtcasting 18, a motor shaft 24 which when the motor is at rest is generally vertical, and upper and lower
eccentric weights 26, 28. The upper eccentric weight 26 is attached to the upper end of the motor shaft 24. The lower eccentric weights 28 is are attached to the lower end of the motor shaft 24. In this example the mass of the lower weight is greater than that of the upper weight. However, the effective eccentricity of the mass of one or both of the upper and lower weights may be adjustable and the relative angular positions of the two weights on the motor shaft 24 can also be altered. By altering the effective eccentricity and the positions of the masses the vibration transmitted using vibrator 12 may be varied to give optimum sieve performance for particular applications.
In use, vibrator 12 in combination with the suspension mounting of the skirt 18 will result in vibratory motion being imparted to the sieve screen frame 6 and thereby the sieve screen 10, such a motion having both horizontal and vertical components.
A guide member 14 is located on the sieve screen surface 10 and the guide member is used to control the flow of the material to be sieved over the sieve screen surface. An excitation source 16 is attached to the guide member 14 and excites the guide member, preferably so that it moves in a vertical direction. The guide member 14 thereby preferably drives a vertical vibration of the sieve screen 10. The excitation source 16 of this particular embodiment is additionally attached to the X-frame 8 for support. The various methods of excitation and fixation will be described in more detail subsequently.
For simplicity, how the material to be sieved is supplied to the sieve 12 is not shown. However, this may be at any point on the sieve screen surface, but is typically at or near the centre of a circular sieve or at one end of a rectangular sieve.
An outlet 32 for removal of oversized particles is shown and this will remove particles which remain on the sieve screen surface. Once particles with a size smaller than the apertures in the sieve screen frame have fallen through these apertures they are
directed by the dome 22 towards an outlet 30 for fines. The dome 22 serves an additional purpose of preventing material which has fallen through the sieve screen from fouling the vibrator 12, and in particular the upper eccentric weight 26. Although a dome is depicted in this particular embodiment, this feature may take other forms, for example a cone or a continuous slope across the width of the sieve.
Also shown in this arrangement is a support device 34 which is attached to the guide member and is supported on the X-frame 8. The forms which may be taken by the support device 34 will be discussed subsequently.
Figure 2 shows a plan view of the sieving apparatus 2 of figure 1. The sieve 2 has a circular sieve screen frame 6 in which is secured a circular sieve screen 10 and in addition an X-frame 8. On the surface of sieve screen 10 is located the guide member 14. The guide member 14 is secured to the sieve screen, for example using an adhesive. The guide member 14 takes the form of a spiral-like shape having an inner end approximately at the centre of the sieve screen 10 and extending outwards with a steadily increasing radius of curvature through approximately 540°. The guide member 14 is secured to an excitation source 16 which is located substantially at the centre of the sieve screen 10 and is supported on the X-frame 8. A support device 34 is located at the opposing end of the guide member 14 to support the guide member on the sieve screen 10. There may also be other supports of the same or different type.
In use the vibrator 12 produces a substantially gyratory motion of the sieve screen 10. This movement encourages the flow of the material to be sieved outwards from the centre over the sieve screen surface. However, the material may be moved too quickly over the sieve screen surface to the outside of the screen so that fines can be carried with the oversized particles to the outlet 32, reducing efficiency. The guide member 14 controls the flow of
material over the sieve screen surface and thereby increases the residence time of material on the sieve screen surface. This increases the efficiency of the sieve, since there is a greater opportunity for fines to fall through the sieve screen apertures. Although it is known to optimise performance for different materials by adjusting the out-of-balance weights 2 6 and 28 as mentioned above, this is a time consuming adjustment. The guide member 14 can ensure good sieving performance over a wide range of materials. The guide member 14 is a bar-like member, typically having an L-shaped or rectangular section presenting sufficient height above the screen surface to restrict or substantially prevent material from crossing over the guide member during sieving.
As mentioned above, the guide member 14 is excited by excitation source 16 to impart deblinding excitation to the sieve screen 10. As will be described in more detail later, the excitation source 16 is a source of ultrasonic vibration, and is adapted to excite the guide member 14 resonantly. In order to be a good transmitter of ultrasonic energy, the guide member should be preferably of metal; such as aluminium or stainless steel. The guide member 14 ensures the excitation energy from source 16 is distributed over the screen 10, to increase the area of the screen 10 which is sufficiently excited to provide effective deblinding.
Figure 3 shows an arrangement, which is also not an embodiment of this invention, similar to that of Figures 1 and 2 in which the spiral shaped guide member 14 is driven ultrasonically by a centrally mounted excitation source 16. The guide member 14 is supported part way along its length and at its outer end by respective supporting devices 34a and 34b. The device 34a is further illustrated in scrap section in Figure 4 and will be described in detail below with reference to Figure 7.
As has been previously mentioned, the guide
member is ultrasonically excited, commonly at frequencies above 2 0 KHz. Figure 7 provides a detailed illustration of an excitation source 16 configured to provide ultrasonic excitation and a support device 34 which is suitable for use with ultrasonic frequencies.
The excitation source comprises a transducer 42 for converting electrical energy to ultrasonic wave energy, for example by using the piezoelectric effect. The transducer may be a half wave stack-type transducer of a kind which will be familiar to those experienced in ultrasonics. A circular resonator boss 44 is attached to the active end of the transducer 42. The resonator 44 converts the longitudinal vibration of the transducer to a transverse diaphragm mode. The excitation source 16 is supported on the X-frame 8 by the use of a central support 48. The dimensions of the central support 48 are chosen such that it is one half wavelength in length so that a node is formed at a point about half way along the length of the central support 48. A cylindrical sleeve 50 is attached to the support 4 8 at the node point, and the sleeve 50 is •secured to the X-frame 8, for example by welding." Because the connection to the central support is at a node, the mounting arrangement decouples the transducer 42 from the X-frame 8, minimising loss of ultrasonic energy to the frame.
The resonator 44 is attached at its outer periphery to the guide member 14 to transmit ultrasonic energy to the guide member. The dimensions of the guide member 14 are preferably chosen so that the length is approximately a whole number of half wavelengths, so that the guide member 14 can be driven in resonance to maximise the transfer of ultrasonic energy from the transducer 42 into the guide member 14. However, the guide member 14 would normally be a substantial number of half wavelengths long. Therefore, it is not necessary to make the guide member to have a length precisely equal to a whole number of half wavelengths, as it can readily be
brought into resonance by a small change in the drive frequency of the transducer 42, without great loss of efficiency. Also, in some applications, vibration of the guide member 14 may be damped, e.g. by the loading of the sieve screen and material to be sieved, to such a degree that little vibration energy is reflected at the far end of the member. Then, the guide member functions as a non-resonant transmission member rather than as a resonator.
Although resonator boss 44 is illustrated interconnecting the transducer 42 and the guide member 14, in some applications it may be satisfactory to connect the transducer 42 directly to the guide member 14 or through a different coupling system.
Also shown in Figure 7 is a support device 34 (corresponding to device 34b in Figure 3) designed to support the guide member 14 on the sieve screen 10. At ultrasonic frequencies it is preferable to provide a support device 34 which ultrasonically decouples the guide member 14 from the support frame, to which it is attached.
Accordingly, the support device 34 comprises a cylindrical resonator boss 52, that may be similar to boss 44, which is attached to the guide member 14, so that a diaphragm mode of vibration is excited in boss 52. At least one diaphragm mode node is therefore formed at a predictable position on the resonator boss 52. Decoupling washers 54a, 54b have skirts which are located against the upper and lower surfaces 52a, 52b, of the resonator boss 52, at the diaphragm mode node. These decoupling washers 54a and 54b therefore experience minimal excitation. A support bracket 58 welded to the X-frame 8 engages the lower decoupling washer 54b. A bolt 60 is used to clamp the resonator boss 52 between the washers 54a and 54b and the support flange 58 to secure the boss to the X-frame 8. The bolt 60 extends through an oversize hole in the resonator boss 52, so as not to contact the body of the resonator boss 52. This configuration effectively decouples the guide member 14 from the X-frame 8,
since the only point of contact with the resonator boss 52 is at the diaphragm mode node, i.e. a point of minimum vibration. This nodal decoupling boss is also described in GB-A-2343392. A similar construction is used for the support device 34a of Figures 3 and 4.
The boss 52 may be excited to resonate in other modes, provided the point or points of contact with the boss are made at appropriate nodal points of the resonant mode to ensure decoupling.
Figures 5 and 6 show an alternative supporting arrangement for the guide member 14. Figure 6 shows flange 62 in the form of an inverted J, which is attached to the X-frame 8 and to the guide member 14. Although this construction of support provides less effective ultrasonic decoupling of the guide member 14 from the X-frame 8, this may be sufficient for many purposes, provided the area of contact with the guide member 14 is small compared to a quarter wavelength of the resonant vibration of the member 14.
Although the excitation source or transducer is shown in the previously discussed embodiments as being supported on an X' frame, the excitation source may in fact be wholly supported by the screen,- or may be supported at least partially by a flexible or rigid coupling to the frame or the fixed base.
The "sieve screen" may comprise a number of layers, for example it may comprise a first screen and second screen arranged above and supported by the first. In such multi-screen sieves, one or more of the guide members arranged on the screen may be directly excited by the excitation source.
In all the arrangements described above, a guide member is fastened to the top of the sieve screen in order to control the flow of material to be sieved over the screen surface, as well as to provide for an effective deblinding excitation of the screen itself. Instead a spiral shaped resonator can be fastened beneath the screen. Figure 2 of the drawings is also a schematic representation of this arrangement, except that the; spiral resonator 14 illustrated in the
drawing is secured beneath the sieve screen rather than on top. The spiral shape may have a continuously increasing radius of curvature (as in Figure 2) or the radius may increase in one or more steps. Further the resonator 14 need not have a profile designed to provide a good deflecting action as is necessary when acting as a guide member on top of the screen. Instead, the resonator 14 may be a simple rectangular section tube or solid bar, or else may have a strap shape having a larger dimension secured to the screen. In each case, the resonator 14 should preferably be made of metal or of another material which is an excellent propagator of acoustic energy.
The resonator 14 is excited by an ultrasonic transducer connected to the resonator 14 at the centre of the spiral as shown as 16 in Figure 2. Again the transducer and the spiral may be supported on an X-frame 8 beneath the sieve screen by decoupling arrangements as illustrated in Figure 7, except that the resonant bosses 44 and 52 shown in Figure 7 would be also located beneath the sieve screen.
The spiral resonator 14 is driven to resonance so that deblinding excitation is distributed over the sieve screen to increase the area of the sieve screen which is effectively excited so that deblinding can be minimised. In order to provide effective distribution of the ultrasonic energy over the sieve screen area, the spiral should extend through at least 270° of arc, and preferably more than 360° of arc, as illustrated in Figure 2.
Importantly, the spiral design can allow deblinding excitation to be distributed to a screen, particularly a circular screen, of larger sizes by increasing the number of turns of the spiral. In this way almost any practical circular screen size can be excited using a single length of resonator driven by a single transducer. This avoids the problems of tuning the different lengths of a multiple rod resonator to the same driving frequency, and the additional complication of using multiple single rod resonators
with respective separate transducers.
An. embodiment of this invention in the form of a smoothly curved s-shaped rod resonator is illustrated in Figure 8. A rectangular sieve frame 70 is illustrated viewed from beneath. The sieve frame 7 0 carries a rectangular sieve mesh which is omitted in this drawing for clarity. The rectangular sieve frame 7 0 is braced by struts 71 and 72 extending between the long sides of the rectangular frame so as to be spaced beneath the mesh supported by the frame. An S-shaped resonator rod 73 is bonded to the underside of the screen mesh and is supported at each end by decoupling mounts 74 and 75. The de-coupling mounts 74 and 75 may comprise a circular resonator boss bonded to each end of the resonator rod 73 and sized to resonate in diaphragm mode at a preferred resonant frequency of the rod 73. Annular de-coupling extensions are bonded to the bosses at diaphragm mode antinodes, to provide mounting points for attachment to brackets 76 and 77 secured to the frame 70. Accordingly, the de-coupling mounts at the ends of the rod 73 may correspond to mounts 34 illustrated in Figure 7, and also to the de-couplers described in GB-A-2343392.
An ultrasonic transducer is connected to the decoupling boss 74 to excite the rod 73 along its entire length at a resonant frequency. The wavelength of ultrasonic energy along the rod at this resonant frequency is typically about 30 mm.
As illustrated in Figure 8, the rod 73 comprises a first monotonically curving portion 7 8 which bends through about 210°, smoothly connected to a straight portion 79, which is in turn smoothly connected to a further curved portion 8 0 which also bends monotonically (with curvature of opposite sign to the first curved portion 78) back through about 210° to the termination boss at mounting 75. The radius of curvature of each of the curved portions 78 and 80 is about 300 mm.
As can be seen from the Figure, the illustrated
design provides excellent coverage of the rectangular screen 70, so that no part of the screen surface is more than about 400 mm from a source of ultrasonic energy, even though the sieve mesh itself is about 1 metre wide and about 2 metres long.
By using a single gently curved rod resonator secured to the screen, ultrasonic energy is distributed over the area of a screen, thereby reducing or eliminating the regions of the screen which receive insufficient ultrasonic energy to ensure deblinding during sieving operations. Using a rod resonator extending between spaced ends, and excited by an ultrasonic transducer at one of the spaced ends, resonance of the rod over its entire length can usually be ensured. By providing the rod with a gently curved shape, the ultrasonic energy can be delivered efficiently to all parts of a sieve screen. The entire length of the rod should comprise smoothly blended curved or straight portions so that the minimum radius of curvature at any point between the ends of the rod is greater than the wavelength of ultrasonic energy in the rod at the resonant frequency at which the rod is excited. Sharper bends-tend to reduce the efficiency with which ultrasonic energy can travel along the rod around the bend and can give rise to reflections of ultrasonic energy at the bend, so that different parts of the length of the rod may prefer to resonate at different frequencies. By forming the rod with smoothly blended components and gentle curves, the whole length of the rod normally acts as a single resonator with ultrasonic energy distributed along the entire length.
In practice, the ultrasonic transducer may be operated to excite the rod resonator at a resonant frequency between 18 kHz and 40 kHz. A preferred operating frequency is about 35 kHz. The corresponding wavelength of ultrasonic energy along the length of the resonator rod is between 25 mm and 35 mm and typically about 30 mm. In most
applications, the minimum radius of curvature of a resonator rod should be greater than 50 mm, and preferably greater than 100 mm.
Effective coverage of a screen surface can be achieved with a rod which bends with a single direction of curvature monotonically by at least 180°. It should be understood that a monotonically bending portion of a rod that bends with a single direction of curvature may include a straight line portion separating two curved portions bending in the same direction. The rod portion bending in a single direction of curvature can also be described as having a monotonically changing angle with distance along the portion.- This is referred to herein as a monotonically bending or curving portion.
Figure 9 illustrates a further example of the present invention applied to a shorter rectangular sieve frame of about 1 metre by 1.4 metres. In this illustration, the same reference numerals have been used to indicate corresponding parts as for the embodiment of Figure 8. However, the rod resonator 73 essentially comprises only the first curved portion 78 directly blended into the last curved portion 80, with the intermediate straight portion 79 of the Figure 17 embodiment removed. Each of the curved portions 7 8 and 80 in Figure 9 have a radius of curvature of about 250 mm.
Figures 10a to 10c illustrate further s-shaped rod resonator designs falling within the scope of the invention. In Figure 10a, there is a first portion which bends monotonically through about 270°, smoothly blended with a second portions which bends monotonically in the opposite direction also by about 270°.
Figure 10b illustrates an S-shaped rod comprising a first portion which bends through a 270° monotonically in three 90° curves interconnected by straight portions. The rod then bends 270° monotonically in the opposite direction again by three bends interconnected by straight portions. The bend
radius of curvature is about 100 mm. In Figure 10c an S-shape is illustrated having a first portion bending monotonically through about 210° smoothly connected to a straight diagonal portion, and in turn smoothly connected to a further curved portion bending monotonically in the opposite direction again by about 210°.
For all the above described s-shaped resonators, the transducer may be located at either end of the resonator. Embodiments of the invention may be applied also to sieves with multiple screens, for example multi layer screens with lower screens of increasing fineness for classifying materials into more than two particle sizes. Then one or more of the screens of the sieve may be fitted with the excited guide member, or the spiral resonator, as described above.
In a further example, a so-called cascade sieve has upper and lower screens of the same mesh, with oversize from the upper screen being fed on to the lower screen to retrieve remaining fines which may not have had an opportunity to pass through the upper screen. Fines which do pass through' the upper screen are collected and channelled through an aperture in the centre of the lower screen. In such a cascade sieve design, the lower screen can be fitted with an excited guide member or a spiral resonator having an inner end terminating outside the central aperture of the lower screen.
The excitation induced in the resonator rod in the embodiments of the invention described above has been referred to as one which produces a deblinding excitation in the sieve screen. Generally, secondary excitation of the sieve screen, e.g. at ultrasonic frequencies, is known to speed up the flow of fines through the screen during sieving so that the productivity of the sieve is improved. This enhanced flow through the screen may be the result of other processes than the removal of blind areas on the screen, such as the fluidisation of the material at
the screen interface. It should be understood that the term deblinding used herein to describe the excitation applied to the screen is intended to encompass other processes by which the excitation enhances product flow rate through the screen compared to the rate achieved with only the basic vibratory sieve action.

In the above described examples of the invention, the resonator rod is described as being secured to the sieve screen. In other embodiments, the resonator rod may be only in contact with the screen, e.g. pressing against the screen with sufficient pressure to enable vibrations in the rod to be transmitted to the screen to provide the deblinding excitation.

We Claim:
1. A sieve comprising:
a base;
a rectangular sieve screen frame mounted on the base;
a rectangular separator screen mounted in the frame;
a vibrator arranged to vibrate the frame relative to the base;
a resonator rod secured to or contacting the separator screen, wherein the resonator comprises a rod extending between spaced ends;
an ultrasonic transducer at one of said spaced ends to excite the resonator rod at a resonant frequency having a predetermined wavelength along the length of the resonator rod;
said resonator rod being s-shaped having a first portion of its length which bends smoothly in a single direction of curvature through at least 180° and a second portion of its length which bends smoothly in a single direction of curvature of opposite sign to said first portion through at least 180°, and the rod having a minimum radius of curvature at any point between said spaced ends which is greater than said predetermined wavelength.
2. A sieve as claimed in claim 1, wherein the sieve further comprises a support frame attached to said screen frame and extending beneath the sieve screen.
3. A sieve as claimed in claim 2, having a support device arranged to support the ultrasonic transducer on the support frame and connected to the transducer at a node such as to minimise transmission of ultrasonic energy to said support frame.
4. A sieve as claimed in claim 3, having an additionally support device for the resonator rod connected by a node to the support frame such that transmission of ultrasonic energy to the support frame is minimised.