WO2000032024A1

WO2000032024A1 - Metering device for particulate material

Info

Publication number: WO2000032024A1
Application number: PCT/NZ1999/000196
Authority: WO
Inventors: Frederick John Larsen; Brian Stewart Taege
Original assignee: Taege Technology Limited
Priority date: 1998-11-27
Filing date: 1999-11-26
Publication date: 2000-06-08
Also published as: AU1418400A

Abstract

A device for metering particulate material (30) which includes a housing (10) supporting a pair of parallel horizontal shafts (21, 22); one shaft (21) carries at least one first roller (13) made of soft resilient material, and the other shaft (22) carries at least one second roller (13a) opposite to the or each first roller (13); the or each first and second rollers (13, 13a) are arranged such that the curved surfaces are in contact; means for driving at least one of the shafts (21) to rotate the corresponding roller (13) or rollers in a predetermined direction; in use, particulate material (30) to be metered is arranged to fall on the upper surfaces of the or each pair of rollers (13, 13a), to pass between the pair of rollers (13, 13a) as the rollers (13, 13a) are rotated.

Description

Title: METERING DEVICE FOR PARTICULATE MATERIAL Technical Field

The present invention relates to a metering device for particulate material and to apparatus incorporating said metering device.

There are a number of applications for which particulate materials need to be accurately dispensed at a preset rate i.e. the quantity of material dispensed in a set time period or in a set distance traveled by a mobile apparatus.

Background Art

The nature of the particulate material itself frequently presents difficulties:- the material may vary in size and/or be fragile and easily damaged by the apparatus and/or may be prone to clogging or bridging.

Ideally, a metering device for particulate material should be capable of accurate dispensing, be able to handle particulate material of a wide range of sizes, regular and irregular (including fragile material), be easy to adjust to vary the dispensing rate and be easy to clean out and to maintain.

Metering devices for particulate material are used in applications as varied as cosmetics and foodstuffs preparation, chemicals processing, preparation of agricultural chemicals such as fertilizer mixes, and mixing sized crushed rocks for applications such as roading.

One of the most demanding applications for dispensing particulate material is in dispensing seeds for agricultural and horticultural applications. In addition to the requirements set out above, a seed drill must be mobile and capable of continued long-term use under sometimes rugged conditions.

For many years the most commonly used type of metering device for seeds has been the "Tullian Drill" and its later modifications. The basic design of such a drill is a hopper containing the seed to be metered into the drilling tines, the base of the hopper opening -into a series of spaced tunnels, one tunnel corresponding to each tine, and being connected to the tine by a dispensing tube. In each tunnel, a hard surfaced multi-grooved dispenser is mounted for rotation about a horizontal shaft. The dispenser is a close fit within the tunnel such that only seed received in a groove of the dispenser can pass from the hopper into the tunnel as the dispenser is rotated. The dispensing rate can be adjusted by moving the dispensers horizontally relative to the opening into the hopper so that a greater or lesser length of the grooved surface of the dispenser is exposed to the hopper. The greater the exposed length of the dispenser, the higher the dispensing rate. In addition, the dispensing rate can be varied by varying the rate of rotation of the common shaft upon which the dispensers are mounted.

This type of drill works well for seed within a size range which is well matched to the size of the grooves but for very small seed (e.g. carrot seed) it is necessary to use a separate small seeds dispenser which is essentially a scaled down version of the dispenser described above, and the dispenser is prone to crush larger seeds. Further, this design of drill is prone to problems with bridging when larger seeds are to be dispensed i.e. the seeds tend to jam together and bridge over the apertures in the base of the hopper which lead to the dispensing devices.

In an attempt to overcome problems of seed damage, metering devices using dispensing sponges have been developed.

The Aitchison drill incorporates a metering device consisting of a sponge-faced disk which extends partially into the base of the hopper and is rotated about a horizontal axis, so that seed to be dispensed falls on the side of the sponge and is carried around with the sponge as it is rotated. The sponge rubs against a stationary scraper surface to dislodge the seed.

The "seed spider" drill disclosed in PCT/NZ95/00059 also uses a sponge. In this drill, seed is dispensed from a hopper onto housing in which a sponge arranged to rotate about a vertical axis. The sides of the housing are formed with a number of grooves which extend vertically but are vertically dis- continuous, so that seed falling into the top of a groove cannot exit from the bottom of the groove until the pressure of the side wall of the rotating sponge has carried the seed into the lower portion of the groove seed leaving the lower portion of the groove falls into one of a number of chutes each connected to a drill tine. This design works well for small seeds but larger seeds tend to bridge over the top of the dispenser. A further drawback with the seed spider type of drill is that a single metering device supplies a large number of chutes. For a standard length of seed drill, this means that some of the chutes are very long and extend at a relatively small angle to the horizontal. Seed traveling down these chutes can be prone to clogging in the chutes as a result.

It will be appreciated that both the Aitchison and Seed Spider designs use a sponge rotating against a hard stationary surface; this tends to wear out the sponges rapidly.

A number of other designs have been proposed over the years, but to date none of these designs has satisfactorily solved the problem of providing a seed drill which can handle a wide range of seed sizes and shapes easily and which is inexpensive to manufacture and rapid and easy to maintain.

It is therefore an object of the present invention to provide a metering device for particulate materials which overcomes the above described disadvantages. The device of the present invention has been designed for a seed drill and will therefore be described with reference to that application. However, it will be appreciated that the device of the present invention may also be used to dispense a wide range of particulate materials, either as a mobile dispenser or as a stationary dispenser.

Disclosure of Invention

The present invention provides a metering device for particulate material, said device including a housing supporting a pair of parallel substantially horizontal shafts, one shaft having at least one first roller mounted thereon and the other shaft having at least one second roller mounted thereon, the or each said first and second rollers being aligned opposite each other on their respective shafts and arranged such that the curved surface of the or each first roller is in surface contact with the curved surface of the or each corresponding second roller; means for driving at least one of said shafts so as to rotate the or each corresponding roller in a predetermined direction; the or each said first roller being a soft resilient roller.

As used herein, the term 'soft' means capable of substantial deformation when in contact with particulate material being dispensed, such that the particulate material is not damaged in any way. In a first embodiment of the invention, the or each said second roller is a soft resilient roller, and said means for driving said shafts are arranged to rotate said first and second rollers inwards and downward towards each other, with said first and second rollers rotating in opposite directions.

In a second embodiment of the invention, the or each said second roller is a hard non- resilient roller having at least one cavity formed in the curved surface thereof, the or each said cavity being dimensioned and configured to receive a predetermined size and shape of particle; and said means for driving said shafts are arranged to rotate said first and second rollers in the same direction, the direction of the or each said second roller being such that in use the or each said second roller rotates towards the corresponding first roller when moving downwards.

The first and second rollers each may extend substantially the full-length of the metering device, or said rollers may be formed as a series of separate spaced first rollers with a corresponding series of separate spaced second rollers, each of which is aligned with the corresponding first roller. In use, particulate material to be dispensed is fed onto the upper surface of the or each pair of rollers, generally by gravity feed from a hopper or similar receptacle. If a single pair of rollers is used, and if the particulate material has to be fed to a number of separate outlets (as in a seed drill) then either the hopper or a housing over the top of the rollers has to provide a corresponding number of separate apertures, so that the particulate material falls onto the rollers at intervals which correspond in position to the separate outlets. However, if the particulate material simply needs to be delivered by the metering device in a continuous line but at a metered rate, then it is unnecessary to provide separate apertures in the hopper and/or housing.

If a series of pairs of rollers is used, then it is necessary to provide separate apertures in the hopper and/or housing, at spaced intervals corresponding in position to the position of the pairs of rollers.

The present invention further provides apparatus for dispensing particulate material, said apparatus incorporating the above described metering device.

Preferably, said apparatus includes a hopper for particulate material to be dispensed, and means for securing the metering device below said hopper such that particulate material from the hopper can contact the upper surface of said rollers. Brief Description of Drawings

By way of example only, a preferred embodiment of the present invention is described in detail, with reference to the accompanying drawings, in which:-

Fig. 1 is an end view showing the overall layout of a seed hopper and metering device of a first embodiment of the present invention;

Fig. 2 is a longitudinal section of part of the apparatus of Fig. 1 , viewed from the front; but with the foremost row of rollers left unsectioned;

Fig. 3 is a view on line Ill-Ill of Fig. 2, on an enlarged scale;

Fig. 4a-c are, respectively, top and bottom plan views and an end view of a roller housing segment;

Fig. 4d is a plan view of the shafts carrying the metering rollers, with all of the housing segments but one removed;

Fig. 5 is a view similar to Fig. 3 but on a larger scale and with some components omitted; and

Fig. 6 is a view similar to Fig. 3, but with the metering device fitted with a choke.

Fig. 7 is an end view showing the overall layout of a seed hopper and metering device of a second embodiment of the present invention;

Fig. 8 is a longitudinal section of part of the apparatus of Fig. 7, viewed from the front, but with the foremost row of rollers left unsectioned;

Fig. 9 is a view on line IV-IV of Fig. 8, on an enlarged scale;

Fig. 10 is a plan view of the shafts carrying the metering rollers, with all of the housing segments but one removed;

Fig.s 11 and 12 are plan views of the inner and outer sides respectively of roller housing segment;

Fig. 13 is a side view of a roller housing segment;

Fig.s 14, 15, 16 are side view of different designs of hard roller; Fig. 17 is a fragmentary view of part of Fig. 15, on an enlarged scale;

Fig. 18 is a side view of a hard roller;

Figs. 19 to 22 show partial end views of the metering device, illustrating variations of the shaft drive; Fig. 23 shows a plan view of the shafts carrying the metering rollers, incorporating a third embodiment of the invention; and

Fig. 24 shows a plan view of the shafts carrying the metering rollers, incorporating a fourth embodiment of the invention.

Best Mode for Carrying out the Invention

Referring to Figs. 1 to 5 of the drawings, a seed drill 2 incorporating the metering device of a first embodiment of the present invention comprises a seed hopper 4 which funnels into a metering device 5 which feeds into a series of spaced outlets 6. In use, each outlet 6 is connected by a chute to a drill tine or other seed - sowing apparatus, (not shown), in known manner.

The hopper 4 has a sealable top opening 7 through which seed to be dispensed is poured into the interior of the hopper. The walls 8 of the hopper funnel inwards at the lower part of the hopper, so that seed in the hopper tends to fall under gravity into the top of the metering device 5.

The metering device 5 comprises a series of paired housing segments 10 (Fig. 2 and 4) secured between end plates 11 to form a housing 10a which provides a series of equidistantly-spaced pairs of cavities 12 to accommodate a series of pairs of rollers 13, 13a.

Each segment 10 is symmetrical about the axis A-A and provides an upper bar 14 with triangular corner portions 15 of the full width 'w' of the segment, a reinforcing portion 16 of reduced width, and a pair of mirror imaged part oval cutouts 17 of further reduced width. The rear walls 18 of the cutouts 17 are completely cut away, to accommodate a pair of rollers, as hereinafter described. The segments 10 are paired with the cutouts 17 of adjacent segments facing each other, so that the opposed cutouts 17, held apart by the widths 2w of the portions 15, form the cavities 12 in which rollers are mounted, as hereinafter described.

The series of paired segments 10 is held together by tie-rods (not shown) threaded through the apertures 19, and secured at the outer faces of the end plates 11.

The rollers 13, 13a are arranged in two sets, the rollers of each set being mounted on a common shaft 21, 22 equidistantly spaced along the shaft.

Each roller 13, 13a consists of a sleeve of resilient material rigidly secured (e.g. by gluing or molding) onto a metal sleeve 23 which is sized to be a sliding fit over the shaft 21 , 22. The resilient sleeve forms a major part of the width of the roller.

The resilient sleeve is depicted as a regular cylinder, but it is possible that other shapes could be effective. The resilient material could be any of a wide range of non- toxic materials not chemically reactive with seeds or seed coatings. Medium density open cell foam plastics materials have been found to be suitable. Preferably the foam material is a "no memory" foam, i.e. a foam which, after having been depressed, returns to its original shape rapidly, and tends not to become permanently deformed under pressure. Some polyurethane foams have been found suitable in this respect.

Each roller 13, 13a is secured in position on its respective shaft by a set screw (not shown) which passes through an aperture in the metal sleeve 23 into a screw- threaded aperture in the shaft. If the rollers 13, 13a are damaged or worn, they can be removed and fresh rollers secured to the shaft, quickly and easily. Further, if a part emptied hopper needs to be emptied, the shafts complete with rollers can be removed and the hopper quickly swept out.

Each set of rollers on its shaft is mounted between the end plates 11 with the ends of each shaft received in sockets 24, 24a.

The end of each shaft 21 , 22 is formed into a flattened oval shape, and projects as a sliding fit into an aperture 26 of the same shape in the corresponding socket, so as to form a positive, drivable connection with the socket. At one end of each shaft, the socket aperture is longer than the end of the shaft by a length greater than the length of the socket aperture 28 which receives the other end of the shaft. A biasing spring 27 is positioned in the aperture, one end of the spring in contact with the end of the aperture and the other in contact with the end of the shaft, to bias the shaft in the direction of arrow A (Fig. 2). Thus, the shaft and its rollers can be easily removed from the sockets simply by manually pushing the shaft in the opposite direction to arrow A, compressing the spring 27, until the other end of the shaft is clear of the aperture 28.

Drive sprockets 25, 25a (Fig. 1 only) are coupled to the sockets 24a.

The sprockets mesh with each other and one of the sprockets is driven to rotate both sprockets in the directions of arrows B and C (Fig. 1) so that the shafts 21, 22, rotate towards each other. The sprockets 29, 30, are depicted as identical in size and number of teeth, since it is believed preferable for both shafts to be driven at the same speed. However, it is envisaged that the sprockets could be designed to rotate the shafts at different speeds. It will be appreciated that the sprockets drive illustrated in Fig. 1 could be replaced by a chain drive as shown in Fig. 21 :- in this arrangement, the sprockets 25, 25a do not mesh with each other but instead engage a drive chain 50 which is arranged so as to engage the upper surface of the sprocket 25 and the lower surface of the sprocket 25a. A jockey wheel 51 is located adjacent sprockets 25a.

For some applications, the device of the present invention will perform satisfactorily with only one shaft driven and the other shaft simply allowed to idle. The shaft(s) may be driven by any suitable means, e.g. an electric motor, a stepper motor, a drive from a ground wheel (generally through gearing). All such drives are well known and will not be described in detail. Fig. 20 shows a variation in which the shafts 21, 22 are driven by individual electrically powered stepper motors 60, 61 of known type. The stepper motors 60, 61 are individually mounted on the ends of the corresponding shafts 21 , 22 and are electrically powered in known manner through power supplies 62,63.

The sockets 24 may simply rotate freely in the apertures formed in the end-plate 11 or may be driveably coupled to corresponding sockets belonging to an adjacent metering device of the same type. Thus, if a series of metering devices is required to cover the full length of a drill, each device may be driven separately or all the devices may be linked in this way and a single drive used. The lower portions of the segments 10 support a pair of continuous wiper blades 29, 29a each of which extends at a slight angle to the outer surface of each of the corresponding set of rollers, and are in slight contact with said roller surfaces.

The above-described device is used as follows:- the hopper 4 and metering device 5 are mounted on a seed drill and the outlets 6 each connected to a separate drill tine, in known manner.

Seed to be drilled is placed in the hopper 4. When the drill is in position over the area of land to be sown, the shafts 21 , 22 are rotated by any suitable means to rotate the rollers 13, 13a attached to those shafts, in the directions of arrows B and C.

Seed 30 from the hopper 4 falls under gravity onto the top of the housing 10a, and through the apertures 31 in the housing to contact the upper parts of the rollers 13, 13a. The rotation of the rollers towards each other (see Fig.s 3 and 5) draws the seeds 30 downwards (arrow D) through the rollers, to fall into the corresponding outlet 6. Any seeds tending to stick to the rollers are removed by the scrapers 29, 29a.

The fact that the rollers are resilient means that a very large range of different sizes of seeds can be metered accurately and without damaging the seeds:- the roller surfaces simply compress to accommodate the seeds, whatever their size or shape. Further, the rollers are not abraded rapidly, since they mainly contact each other, rather than rotating against a hard surface, as in some of the prior art devices.

The rate of metering can be controlled by varying the rate of rotation of the rollers 13, 13a, simply by adjusting the speed at which the sprockets 25, 25a are driven.

However, if very small seeds are to be metered (e.g. carrot seed) and/or the required metering rate is very low, it is desirable to be able to further reduce the dispersing rate below the lowest rate available simply by reducing the drive rate of the sprockets 29, 30. In this case, a choke 32 is used.

The choke 32 is shown in Fig. 6 only and comprises a straight rod or bar which is pushed through the aperture 33 in each segment 10, which is positioned so that the choke 32 pushes the rollers 13, 13a of each pair away from each other just at their highest point of contact. This has the effect of reducing the 'grabbing' action of the rollers on the seed and can reduce the dispersing rate of the seed by up to 80% for smaller seed. The thickness of the choke is not critical - it is its position rather than its size which causes the choking effect.

It has been found that by using a combination of control of the rate of rotation of the sprockets 29, 30 and the choke, seed can be accurately dispersed at rates from fractional kg/Ha to 400 Kg/Ha, depending upon seed type and coulter spacing.

The metering rates of the device are set in known manner i.e. by rotating the drive with the device stationary, measuring how much seed is dispersed per shaft rotation and adjusting the drive/choke as necessary to obtain the required rate.

Fig.s 7 to 18 show a second embodiment of the present invention. In Fig.s 7 to 18, the parts of the apparatus which are identical or substantially identical to those of Fig.s 1 to 6 are identified by the same reference numerals as in Fig.s 1 to 6. It should be noted that the second embodiment differs from the first embodiment only in respect of those matters which are specifically described.

In the second embodiment, the seed drill 2, seed hopper 4 and metering device 5 are arranged as described with reference to the first embodiment. Further, as in the first embodiment, the metering device 5 includes a series of paired housing segments 10 arranged and secured in the same manner as in the first embodiment. However, the shape of the housing segments 10 is slightly different in the second embodiment:- because the wiper blades 29, 29a are not required in the second embodiment, the central portion of the lower surface of each segment 10 is formed as a slightly protruding curved surface 55 (Fig.s 9 and 10) and the fish tail portions shown in Fig.s 4a and 4b, required to support the wiper blades 29, 29a are omitted.

In all other respects the segments 10 are arranged in the same manner as in the first embodiment.

The major difference between the two embodiments is that, in the second embodiment, the rollers 13, 13a are not both made of a resilient material, as in the first embodiment. In the second embodiment, each roller 13 is made of a resilient material selected and arranged in the same manner as described with reference to the first embodiment. However, each of the rollers 13b is a hard roller as hereinafter described with reference to Fig.s 14 to 18. The rollers, 13, 13b are secured to their respective shafts 21 , 22 as described with reference to the first embodiment and the rollers on their shafts are mounted in the housing as described with reference to the first embodiment.

The shafts 21, 22 are rotated by driving the corresponding socket 24a. This may be done by any suitable means e.g., drive sprockets 25, 25a secured to the ends of the shafts 21, 22 and connected to a drive band 40. Alternatively, drive sprocket 25 may be drivingly connected to drive sprocket 25 a by means of an intermediate gear 65 as shown in Fig. 22.

The shafts 21, 22 both are rotated in the same direction, as indicated by arrows B and C in Fig. 9. Thus, both sets of rollers 13, 13a, rotate anti-clockwise when viewed end- on, as in Fig. 9.

The sprockets 25, 25a are depicted as identical in size and number of teeth, since it is believed preferable for both shafts to be driven at the same speed. However, it is envisaged that the sprockets could be designed to rotate the shafts at different speeds. The shaft(s) may be driven by any suitable means e.g. an electric motor, a stepper motor, a drive from a ground wheel (generally through gearing). All such drives are well known and will not be described in detail. Fig. 19 shows a variations in which the shafts 21 , 22 are driven by individual electrically powered stepper motors of 66, 67 of known type. The stepper motors 66, 67 are individually mounted on the ends of the corresponding shafts 21, 22 and are electrically powered in known manner through power supplies 68,69.

The sockets 24 may simply rotate freely in the apertures formed in the end-plate 11 or may be driveably coupled to corresponding sockets belonging to an adjacent metering device of the same type. Thus, if a series of metering devices is required to cover the full length of a drill, each device may be driven separately or all the devices may be linked in this way and a single drive used.

As shown in Fig.s 14 to 18, each hard roller 13b comprises a cylinder of suitable rigid material (e.g. stainless steel) with a longitudinal central axis 41 sized to be a sliding fit on the shaft 22. The central portion of the outer curved surface 42 of the roller 13b is formed with a series of equi-distantly spaced cavities 43 indented into the curved surface of the roller. The size, shape and spacing of the cavities 43 vary with the seed to be dispensed. Fig.s 14 and 17 show a roller for dispersing large seed such as maize:- the cavities 43 are sized to receive a single maize seed easily, but to be slightly undersize for two seeds. Also, the shape of the cavity 43 is such that seed can fall into the cavity easily, but does not tend to become stuck.

Fig. 15 shows a roller for dispersing small seed (e.g. grass or clover seed) with many small cavities. •

Fig. 16 shows a roller for dispersing medium size seed (e.g. wheat or barley).

More than one row of cavities 43 may be formed around each roller 13b, and the cavities 43 need not be formed in a straight line around the surface 42. However, it is important that none of the cavities 43 is formed at, or close to, an edge of the roller, since this could lead to seed from the hopper slipping between the roller and the housing.

Seed to be drilled is placed in the hopper 4. When the drill is in position over the area of land to be sown, the sprockets 25, 25a are driven by any suitable means to rotate the shafts 21 , 22, and hence the rollers 13, 13b attached to those shafts, in the directions of arrows B and C.

Seed 30 from the hopper 4 falls under gravity onto the top of the housing 10a, and through the apertures 31 in the housing to contact the upper parts of the rollers 13, 13b.

Each roller 13b rotates so that a cavity 43 adjacent the aperture 31 in the housing next moves downwards towards the corresponding soft resilient roller 13. The roller 13 rotates in the same direction as the roller 13b and thus tends to brush out of the cavity 43 any seed which is not wholly within the cavity:- if more than one seed is in the cavity, the extra seeds are gently pushed away by the roller 13. Then, as the roller 13b reaches the bottom of its rotation, the seed is free to drop from the cavity into the outlet 6. The soft roller 13 serves a dual purpose:- firstly, it removes surplus seed from the corresponding roller 13b as described above. Secondly, because the roller 13 rotates upwards into the hopper, it also keeps the seed in the hopper 4 adjacent the aperture 31 stirred and loose, preventing bridging and packing.

Any seed adhering to the surface of the soft roller 13 is removed when it contacts the edge 31a of the housing 10.

The fact that surplus seed is removed from each cavity 43 by a soft, resilient surface, minimizes damage to the seed and prevents any cracking or grinding of the seed which tends to occur in prior-art equipment. The device of the present invention allows a single seed to be dispensed if single seed is required.

It will be appreciated that the rollers 13, 13b can be transposed - it is simply necessary that the roller 13b rotates downwards and inwards as described above, and that the corresponding roller 13 rotates in the same direction.

The rate of metering can be controlled by varying the rate of rotation of the rollers 13, 13b, simply by adjusting the speed at which the sprockets 25, 25a are driven. Different rollers 13b, with appropriate sized and shaped cavities 43, are selected for the type of seed to be dispersed.

The metering rates of the device are set in known manner i.e. by rotating the drive with the device stationary, measuring how much seed is dispersed per shaft rotation and adjusting the drive as necessary to obtain the required rate.

It has been found that the above described device is capable of metering a wide size and type range of seed with great precision.

Fig. 23 illustrates a third embodiment of the present invention, in which the housing 10 which supports the pairs of rollers 13,13a/13b does not provide a series of apertures 31 but instead is simply formed with one long continuous aperture for the full-length of the housing. In this embodiment, the spaced apertures providing the correctly spaced access to the rollers 13, 13a/13b are formed in the base of the hopper 4. As shown in Fig. 23 a series of spaced apertures 70 is formed along the base of the hopper 4; if separate pairs of rollers 13, 13a/13b are present, as described with reference to the first and. second embodiments, then in the apertures 70 correspond in position to the spaced pairs of rollers. Alternatively, the rollers 13, 13a/13b instead may be formed as a pair of continuous rollers extending the full-length of the housing, as shown in Fig. 24. In this case, the apertures 70 are spaced to give the desired spaced access between the rollers and the hopper (if necessary). However, it is also possible to use the type of housing described with reference to the first and second embodiments of the invention (i.e. providing a series of apertures 31), with the continuous rollers shown in Fig. 24.

Claims

CLAIMS:

1. A metering device for particulate material, said device including a housing supporting a pair of parallel substantially horizontal shafts, one shaft having at least one first roller mounted thereon and the other shaft having at least one second roller mounted thereon, the or each said first and second rollers being aligned opposite each other on their respective shafts and arranged such that the curved surface of the or each first roller is in surface contact with the curved surface of the or each corresponding second roller; means for driving at least one of said shafts so as to rotate the or each corresponding roller in a predetermined direction; the or each said first roller being a soft resilient roller.

2. The metering device as claimed in claim 1 , wherein the or each said second roller is a soft resilient roller and said means for driving said shafts are arranged to rotate said first and second rollers inwards and downward towards each other, with said first and second rollers rotating in opposite directions.

3. The metering device as claimed in claim 2 further including a choke consisting of a bar removably mountable in said housing, positioned so as to press the contacting curved surfaces of the or each pairs of first and second rollers out of contact with each other at or adjacent their highest point of contact.

4. The metering device as claimed in claim 1 , wherein the or each said second roller is a hard non-resilient roller having at least one cavity formed in the curved surface thereof, the or each said cavity being dimensioned and configured to receive a predetermined size and shape of particle; and said means for driving said shafts are arranged to rotate said first and second rollers in the same direction, the direction of the or each said second roller being such that in use the or each said second roller rotates towards the corresponding first roller when moving downwards.

5. The metering device as claimed in any one of the preceding claims, wherein said housing provides a series of spaced apertures above the upper surface of said first and second rollers.

6. The metering device as claimed in any one of the preceding claims, wherein said first and second rollers each consist of a single roller extending substantially the full-length of the corresponding shaft.

7. The metering device as claimed in any one of claims 1-5 wherein said rollers comprise a series of spaced, separate first rollers individually mounted upon said one shaft and a corresponding series of spaced, separate second rollers individually mounted upon said other shaft.

8. The metering device as claimed in any one of the preceding claims wherein each said shaft is removably mounted in said housing.

9. The metering device as claimed in any one of the preceding claims wherein said drive means is selected from the group consisting of: sprocket drive; chain and sprocket drive; gear drive; stepper motor.

10. The metering device as claimed in any one of the preceding claims wherein said metering device is incorporated in stationary apparatus.

1 1. The metering device as claimed in any one of claims 1-9 wherein said metering device is incorporated in a mobile apparatus.

12. The metering device as claimed in claim 11 wherein said mobile apparatus comprises a seed drill.