GB2491408A

GB2491408A - Centrifugal sifting apparatus

Info

Publication number: GB2491408A
Application number: GB201109343A
Authority: GB
Inventors: George Tunnicliffe; Stephen Paul Bayley
Original assignee: KEK GARDNER Ltd
Current assignee: KEK GARDNER Ltd
Priority date: 2011-06-03
Filing date: 2011-06-03
Publication date: 2012-12-05
Also published as: GB201109343D0

Abstract

Within a machine housing 10 a paddle assembly 33 is mounted to a first shaft 30 and is rotatable within a stationary cylindrical screen 14. An auger screw 23 provided on a second shaft is rotatable to feed material to be sifted into a first end of the cylindrical screen 14. The second shaft of the auger screw 23 is hollow and the first shaft 30, on which the paddle assembly 33 is mounted, extends co-axially through the hollow second shaft, preferably in cantilever manner. Drive means 28,29,35,36,39 operable to rotate the first and second shafts is provided at a common end of the apparatus. In an alternative version the shafts may be provided the opposite way round such that the first shaft, on which the paddle assembly is mounted, is hollow and the second shaft of the auger screw extends through the hollow first shaft.

Description

CENTRIFUGAL SIFTING APPARATUS

TECHNICAL FIELD

The present invention relates to centrifugal sifting apparatus as used for separation of particles according to size in a wide range of industries, such as food processing, fine chemicals, pharmaceuticals, dyes and pigments, powder coatings and many others. While the invention is described with respect to separation of solid from solid, the principle is equally applicable to separation of solid from liquid.

BACKGROUND ART

A conventional centrifugal sifting machine has a stationary cylindrical screen, often referred to as a sifting basket, arranged inside a machine housing with a rotatable paddle assembly mounted inside the cylindrical screen. Material to be separated is introduced into the screen in an axial direction by an auger screw, also arranged inside the housing, which typically terminates at the inlet of the screen assembly, but may project into one end of the screen enclosure. During operation, material to be separated usually enters the auger screw section of the housing from above by way of a product inlet section. The auger screw is a helically extending blade which conveys the material into the screen where it is picked up by the rotating paddle assembly and thrown centrifugally against the screen. Blades of the paddle assembly are typically, but not always, tilted slightly relative to the axial direction, in what is termed a slow helix disposition, so that they convey the material axially inside the screen as well as around the circumference as the paddle rotates. Particles below the aperture size of the screen are expelled through the screen by the large centrifugal forces exerted as a result of rotation of the paddle assembly and fall as sifted product into an outlet hopper. Larger particles above the screen perforation size gradually progress through the screen to be discharged at the end remote from the auger into a course product outlet.

Centrifugal sifting by means of a rotary paddle assembly within a cylindrical screen is an extremely effective means of sieving particulate material because of the potential high loading of product per unit of screen area, namely the high throughout as well as the possibility of continuous operation.

However, one limitation is that the material to be sifted must usually be fed into the screen at a controlled rate. If not, the operation will be inefficient with too much fine material passing through to the course product outlet if the unit is overloaded, yet less than optimum output if the feed volume is below capacity.

In conventional centrifugal sifting machines both the auger screw and the paddle assembly are mounted on a common shaft which is mounted in the housing by way of respective bearings, one at each end of the housing. However, in some known machines the complete shaft assembly is supported in a cantilever manner via a rigid bearing arrangement, usually at the drive end. This common shaft is rotated by a single drive motor, typically located outside the housing at the end of the housing adjacent the auger screw. Unless controlled by an external feed mechanism, the amount of material fed into the cylindrical screen will be determined by the rate of revolution of the drive motor as well as the configuration of the auger screw. Sometimes where the provision of an external control feed is not practical or desirable, efforts are made, eg. by suitable auger design, to match the optimum rate of revolution for the auger screw to the optimum rate of revolution for transport through the screen by the paddle assembly in the sifting process. For some materials a suitable compromise is possible in the design of the paddle and the auger, but the particular design parameters will then be material specific and it is most unlikely that the same machine can be used satisfactorily for sifting different materials, even different grades or differently sourced types of the same material without having to change the paddle assembly or the auger. That entails removal and replacement of the shaft, a troublesome and time consuming exercise involving considerable down time.

An additional or alternative approach has been to install a separate, independently driven dosing mechanism in or prior to the product inlet section leading (from above) into the auger screw. This allows the amount of material supplied to the screen to be controlled independently of the rate of revolution of the worm conveyor. However, it adds to the complexity and cost of the overall apparatus and requires extra clearance height in the vicinity of the inlet section, as well as additional cleaning and maintenance requirements.

EF-A-0917911 discloses a sifting machine which is also commercial known and has overcome the aforesaid problem by providing the auger screw and the paddle assembly on separate shafts which are mounted in axial alignment with facing ends closely adjacent.

(This is termed a mono axial arrangement as the axes are aligned, but spaced apart, not at the same position). These shafts are independently driven by each having its own separate drive mechanism, the drive mechanism for the auger screw being at its end remote from the paddle assembly and the drive mechanism for the paddle assembly being at its end remote from the auger screw.

Each shaft is mounted in cantilever manner, extending from its respective drive mechanism through a bearing in a closure plate at the respective end of the machine. For purposes of cleaning, maintenance or replacement the auger screw shaft together with its respective bearing plate and drive mechanism can be axially removed, as a unit, from a first end of the machine. Similarly, the paddle assembly on its shaft together with its bearing plate, the cylindrical screen which is mounted thereto, and its drive mechanism can be axially removed, as a unit, from the opposite end of the machine.

1 0 An object of the present invention is to provide an alternative design of centrifugal sifting apparatus which overcomes the same limitations of the conventional machine outlined above, but has advantages over the design disclosed in EP-A-091 7911 in terms of physical size, end clearance requirements and access to the cylindrical screen for cleaning and maintenance purposes.

SUMMARY OF THE INVENTION

The present invention provides centrifugal sifting apparatus comprising a stationary cylindrical screen, a paddle assembly mounted to a first shaft and rotatable within the cylindrical screen, an auger screw provided on a second shaft which is rotatable to feed material to be sifted into a first end of the cylindrical screen, and drive means operable to rotate the first shaft and to rotate the second shaft, characterised in that the drive means operable to rotate the first and second shafts is provided at a common end of the apparatus, in that the second shaft, of the auger screw, is hollow, and in that the first shaft, on which the paddle assembly is mounted, extends through the hollow second shaft.

The first shaft preferably extends co-axially through the hollow second shaft.

The first shaft preferably extends in cantilever manner through the hollow second shaft.

The second shaft is supported in cantilever manner at a common end of the apparatus.

Such cantilevered arrangement is particularly advantageous as it enables a closure plate to be provided at or adjacent a second end of the cylindrical screen, remote from the auger screw, which plate has neither a bearing nor shaft mounting function. Therefore, such plate can easily be opened to provide access to the interior of the screen or to the paddle assembly for cleaning and maintenance purposes. In contrast to the arrangement known from EF-A-091 7911 it is not necessary to displace a drive mechanism and a bearing plate and the entire paddle assembly to achieve such access. The requirement for clearance for provision of a drive mechanism at that end, remote from the auger screw, and the extra clearance for axial displacement of such drive mechanism and its rotor shaft is obviated.

Thus, in accordance with the present invention the operational location of the machine has greater flexibility and both cleaning and maintenance of the screen are facilitated.

As previously, the speed of rotation of the respective shafts are independently controlled.

As previously, this may be by providing each shaft with its own separate drive motor.

However, with the present invention the possibility is provided of using only a single drive motor to power rotation of both shafts, but having separate transmissions to the respective shafts, thus a possibility of different gearing and different shaft rotation speeds. In this way the conditions for feed of material towards and into the cylindrical screen, on the one hand, and for conveying material axially through the screen during the sifting operation, on the other hand, can be optimised.

It is also conceivable that, in accordance with the present invention, the overall design could be reversed compared to that outlined above so that it is the shaft of the paddle assembly which is hollow and the shaft of the auger screw which extends through the hollow shaft. The drive means for rotation of the shafts would then be provided at the end remote from the auger screw. This is less favourable than the above described arrangement as it would be necessary to displace the drive means to obtain access to the cylindrical screen. Also the closure plate at that end of the cylindrical screen, ie. remote from the auger screw, would necessarily be a bearing plate, which would be more complicated to remove and replace than a simple end closure which has no other function.

Cantilever mounting of both shafts may still be provided from the end of the machine where the paddle assembly is located. However, it is also possible in some embodiments that the inner shaft mounting the auger screw may be mounted in conventional manner between bearings at each end of the machine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described further, by way of example, by reference to the accompanying drawings, in which: Figure 1 is a longitudinal cross-section through a first practical embodiment of sifting apparatus in accordance with the present invention; Figure 2 is an end view of the apparatus of figure 1 from the direction of the right of figure 1;and Figure 3 is an end view, comparable to figure 2, of a second embodiment of the apparatus of the invention.

DETAILED DESCRIPTION

1 0 As shown in Figure 1, a machine housing 10 is divided into a feed section 1 0A at the right side and a sifting section lOB at the left side. A product inlet 12 is arranged above the feed section bA. A stationary cylindrical screen or sieve 14 is mounted in the sifting section and below this is a fines outlet 16 for material which has passed through the screen 14 discharging into a sifted product hopper or other means of collection or removal from the outlet (not shown). Beyond the end of the screen 14, remote from the feed section bA, a course product outlet 18 extends from the bottom of the housing. This outlet 18 is for particles, mostly oversize particles, which have not passed through the screen 14.

A bearing housing 11 is mounted to the right of the feed section 1OA and a transmission housing 13 is arranged to the right of the bearing housing 11. Respective bearing assemblies 15, 17 are mounted at a spacing apart to respective end plates 25, 27 of the bearing housing 11.

A hollow shaft 20 is mounted by those bearing assemblies 15, 17 and extends through the plate 27 from a first drive end 21 inside the transmission housing 13, through the bearing housing 11, and through the feed section 1 OA to a second end 22 which extends a short distance into the sifting section lOB of the housing 10. Thus, this shaft 20 is mounted in cantilever manner. In the portion of the shaft 20 which lies in the feed section 1OA it carries a helical blade 23 providing an auger screw for feed of product to be sifted towards the sifting section 1 OB upon rotation of the hollow shaft 20.

Respective bearings 24, 26 are provided in the interior of the hollow shaft 20, adjacent the respective ends 21, 22. A further longer shaft 30 is mounted by these bearings 24, 26 to extend co-axially through the hollow shaft 20 from a first end 31 extending within the transmission housing 13 to a second end 32 which projects into the sifting section lOB.

Thus this shaft 30 is also mounted in cantilever fashion.

A paddle assembly 33 is mounted onto the shaft 30 in the sifting section lOB. This assembly 33 is of any known or yet to be devised configuration and fits within the cylindrical screen 14 with suitable circumference clearance. Upon rotation of the shaft 30, the paddle assembly 33 effects centrifugal sifting of product fed in from the feed section 1 OA in conventional manner by throwing the material tangentially and radially at the screen 14 while also conveying material axially towards the outlet 18.

Drive means for rotation of the respective shafts 20, 30 are provided at the same end of 1 0 the apparatus, namely the right end in Figure 1, the transmission housing 13 being part of said drive means.

The shafts 20, 30 are driven (rotated) completely independently of each other by means of two separate motors 28, 38, shown also in Figure 2. Thus, the motor 28 provides rotary power for the hollow shaft 20 by way of a first transmission belt or similar 29 and suitable gearing 35, while the motor 38 provides rotary power for the inner shaft 30 by way of a second transmission 39 and suitable gearing 36. Thus the speed of rotation of each shaft 20, 30 can be independently controlled. The motors 28, 38 are suitably mounted below the bearing housing 11 to minimise the overall axial length of the assembled apparatus.

In other embodiments one or both motors may be mounted differently, namely, above, to the side or axially beyond the end if there is sufficient clearance and this is preferred.

Indeed, Figure 3 shows a modified embodiment where the motors, designated 128, 138, and their transmission belts 129, 139, are mounted at the sides instead of below the ends 21, 31 of the shafts 20, 30. The shape of the transmission housing 113 is suitably redesigned to allow this. In all other respects the apparatus can be the same. If desired the inner shaft 30 could be directly driven via an in line motor suitably coupled to its end 31 for the normal configuration, or the far end in the alternative configuration.

A particular advantage of these particular designs, with both shaft drive means, i.e. motors 28, 38, provided at the upstream end of the apparatus, adjacent the feed section 1 CA, and with the shafts 20, 30 both mounted in cantilever manner so that simple, removable closure plates 40, 42 are all that are needed at the end of the screen 14 and the end of the sifting section 1 OB, respectively. Compared to previously known apparatus these end closures 40, 42 do not incorporate and are not associated with any bearing means for mounting the shaft on which the paddle assembly is mounted, nor any means for mounting a drive mechanism for said shaft. Therefore the opening and closing of these end closure plates 40, 42 is particularly easy to accomplish, facilitating necessary cleaning of the cylindrical screen 14 and the sifting section 1 OB and access for maintenance. Thus with the disclosed design there is no potential for misalignment of bearings, no requirement for clearance to allow drive means to be displaced to allow access, nor any need to move a heavy drive assembly upon opening and closing the end of the sifting section 1 0B.

The foregoing is illustrative not limitative of the scope of the invention and variations in details of design of the apparatus are possible in other embodiments. In particular, in other embodiments a single motor may provide power for rotation of both of the shafts, their respective rotation rates having potential to be set differentially by use of separate transmission and separate gearing fed from the same motor drive shaft.

Also in other embodiments the inner shaft may not be mounted in cantilever fashion and could be mounted in journal bearings at each end. In yet other embodiments the inner shaft may not be coaxial with the hollow shaft, through which it extends, i.e. it may not have a common axis. Instead, the inner shaft may have a different axis, probably substantially parallel to the axis of the hollow shaft through which it extends.

As mentioned at the beginning, while the invention and the specific embodiment have been described with respect to separation of solid from solid, apparatus having the same features could be adapted to separate solids from liquids, namely starting with a slurry from which lumps need to be removed or a liquid from which solid particles need to be removed.

Claims

CLAIMS1. Centrifugal sifting apparatus comprising a stationary cylindrical screen, a paddle assembly mounted to a first shaft and rotatable within the cylindrical screen, an auger screw provided on a second shaft which is rotatable to feed material to be sifted into a first end of the cylindrical screen, and drive means operable to rotate the first shaft and to rotate the second shaft, characterised in that the drive means operable to rotate the first and second shafts is provided at a common end of the apparatus, in that the second shaft, of the auger screw, is hollow, and in that the 1 0 first shaft, on which the paddle assembly is mounted, extends through the hollow second shaft.
2. Apparatus according to claim 1 wherein the first shaft extends co-axially through the hollow second shaft.
3. Apparatus according to claim 1 or 2 wherein the first shaft extends in cantilever manner through the hollow second shaft.
4. Centrifugal sifting apparatus comprising a stationary cylindrical screen, a paddle assembly mounted to a first shaft and rotatable within the cylindrical screen, an auger screw provided on a second shaft which is rotatable to feed material to be sifted into a first end of the cylindrical screen, and drive means operable to rotate the first shaft and to rotate the second shaft, characterised in that the drive means operable to rotate the first and second shafts is provided at a common end of the apparatus, in that the first shaft, on which the paddle assembly is mounted, is hollow, and in that the second shaft, of the auger screw, extends through the hollow first shaft.
5. Apparatus according to claim 4 wherein the second shaft extends co-axially through the hollow first shaft.
6. Apparatus according to claim 4 or 5 wherein the second shaft extends in cantilever manner through the hollow first shaft.
7. Apparatus according to any preceding claim wherein the drive means operable to rotate the first shaft and to rotate the second shaft comprises a single motor as drive actuator, but respective transmission means for each of the first and second shafts.
8. Apparatus according to any of claims 1 to 6 wherein the drive means operable to rotate the first shaft and to rotate the second shaft comprises a separate motor and transmission means for each of the first and second shafts..
9. Apparatus according to claim 1 or 2 wherein a closure plate having neither bearing nor shaft mounting function is provided at a second end of the cylindrical screen, remote from the auger screw, said closure plate being openable for access to the cylindrical screen.
10. Centrifugal sifting apparatus substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.