GB2182866A - Sieve shaker - Google Patents

Abstract

A sieve shaker for a stack of precision grading sieves has an overlying clamp plate 18 slidable relative to sleeves 15a attached to clamp piece 15 around vertical hexagonal-section rods 14. Toggle latches 20 on the clamp plate engage catches 21 on the clamp pieces 15. In use, the sieves are stacked on a vibratory location plate 4; clamp plate 18 is placed on top; clamp pieces 15 are slid so that the upper surface of clamp plate 18 lies between indicator marks 32, 33 on sleeves 15a, and clamped by operating knobs 16; and latches 20 are connected to catches 21, at a predetermined pressure. Thus, uniform reproducible pressure on the stack, as necessary for thorough vibration transmission and accordingly reproducible results, is achieved. <IMAGE>

Description

SPECIFICATION Sieve shaker This invention relates to a sieve shaker, of the type by which a stack of precision-made graded sieves of progressivelyfinermesh is shaken to grade a particulate mixture by particle size fractions. Such investigations are used in a wide range of industries, for example ceramics, paints, pharmaceuticals, or metallurgy, with a correspondingly wide range of particulate mixtures.

Conventionally, such sieve shakers posssess a plate called a location casting, supported for, and provided with means inducing, vibration (as described in more detail below) upon which plate a stack of sieves is located and vibrated. Stability ofthe stack is provided by an upper clamp plate, with bores riding upon vertical rods threaded at their upper ends to receive manually operable clamping nutsfor the clamp plate and underlying stack.

Aproblem with such equipment is the lack of reproducibility of results, either between different users, or with the same user at different times. It appears as a result of ourstudiesthatthis is dueto different clamping forces exerted by the clamp plate on the stack, that is to say, different pressures exerted bytheclamping nuts,for different users orat different times. In particular, if stack is clamped too lightly non-reproducibility of results occurs.

ltappearsthatthis has its origin in the transmission of vibration from the location casting to the composite members ofthe stack. Even the slightest discontinuity of stacking, that is to say, play between successive sieves, destroys accurate transmission of vibration and has a detrimental effect on the proportion of material passed by the sieve in question and sieves located thereabove.

We surmise that unduly light pressure ofthis sort is due to the tendency of users, perceiving that the sieves are expensive and precision items, to locate and unite them only by light screw pressure rather than risk spoilage or distortion. Such light pressure may guard against physical dislodgement from the vertical stack condition but does not necessarily ensure uniform positive abutment of each sieve with its neighbours.

Whatever the reason for non-reproducibility of results we have now discovered an expedientwhich gives more uniform stack pressure as between different runs and gives improved reproducibility of grading results.

The invention consists in a sieve shaker for a stack of grading sieves ofthetype in which a clamp plate moveable on vertical rods overlies and exerts pressure on the stack against a vibratory location plate: comprising (a), on each rod, a member slideable to, and immobilisable at, a position indicative of the clamp plate position priorto application of any pressure and (b) attached to the clamp plate in a number, and at locations, corresponding to each such rod latching means each engageable with its relevant immobilised slidable member and exerting a predetermined force when so engaged.

Thus, instead of arbitrary user tightening of pressure screws, there takes place a uniform position check on the unstressed stack and uniform latching pressure thereafter.

Two vertical rods passing through holes in two diametrical extensions of a circular clamp plate and equipped as described above are particularly easy to use if, with suitable design, the adjustment and immobilisation of both slidable members, and operation of both latches, can be carried out simultaneously and from one side.

The rods are preferably polygonal,forexample hexagonal, or otherwise non-circular so as permit easy immobilisation of the slidable member for example byatransverse locking screw. Preferably, each latching means is a toggle latch fastened to the clamp and engageablewith a catch fastened to the immobilised slidable member. If desired the latching means can itself have an adjustment capability, that is to say, be capable of adjustment for different set pressures, either before sale or for example after maintenance, as desired.

More accurately reproducible results have in practice meantthatwe have needed to reconsider the spring structures conventionally supporting the vibratory location plate. Preferably, as described in more detail below, the location plate is supported on a base by three or more elongate flat inclined springs, equispaced around the plate and formed of a laminated synthetic polymer material.

The invention will be further described with reference to the accompanying drawings in which: Figure lisa general view of a sieve shaker without the stack of sieves, Figure 2 is a view from above of the adjustable positioning and clamping slide members and Figure 3 shows a toggle latch mechanism.

The equipment comprises a base plate 1 for rigid support of a motor assembly generally indicated at 2 and for spring support by springs 3 of a location plate 4 upon which is mounted a sieve clamp assembly generally indicated at 5 for a stack of sieve plates, not shown.

Base plate lisa heavy casting with support feet7 on the underside, motorsupport 8firmly bolted to and extending upward from the upper surface, and lifting handles such as handles 9 towards one end.

Motor assembly 2 comprises motor 10 with supply lead 11 (shown partly removed for clarity of illustration),with the motor bolted to support Sand carrying, on a protruding rotary shaft, an eccentric or cam 12.

Equispaced flat springs 3 constitute a preferred feature ofthe invention. Conventionally, three metal springs are used, ofthe same generally flat configuration as illustrated. We have however found that three or more synthetic polymeric spring materials improvement: the available improved reproducibility ofthe device, as described below, tends to necessitate such improved springs, not subjectto corrosion or fatigue. The polymer spring material is a known flat laminate offibre glass-filled polymer and other polymers, and each such spring is secured by bolts 3a to the base plate and to the location plate.

Location plate 4 is a heavy casting with an adjustable cam-engaging structure 13 protruding from beneath its centre region. As the eccentric or cam 12 rotates it strikes at intervals againstthis structure 13, caUsing the location plate 4to move and vibrate in accordance with the characteristics ofthe springs 3. Structure 13 is not actually a cam-follower but must be generally spaced from the cam or eccentric 12 so as to receive impacts which permit the necessary vibrations. An alternative vibration imparting mechanism, notshown,comprisesan intermittently operable electromagnet acting upon an armature mounted on the underside of the location casting.

Between vibration plate 4 and base 1 there is provided an electrical connection 34, to convey to earth any electrostatic charge built up bythe vibrations.

Apart from the use of flat polymeric springs 3 in place offlat metal springs,the sieve shakeras described so far above is generally conventional.

The sieve clamp assembly 5 is, however, different from such prior art.

Itcomprisestwo upright rods 14, non-circular in cross-section (hexagonal-section roads are illustrated) supporting two slidable clamp pieces 15 immobilisable by operating knobs 16 on threaded shafts in any position on the rods 14 and further possessing marker sleeves 1 5a extending upwards from the clamp pieces slidably around the rods 14.

Markersleevesl5apassslidablythrough bores in respective opposed diametral extensions 17 of clamp plate 18, with an attached circular rubber undersurface 18a and central orifice 19 for sample loading.

Hitherto, vertical rods generally equivalent to rods 14 have extended through the clamp plate, or extensions therefrom, by pressure has always been exerted arbitrarily by nuts on the upper threaded ends of such rods. In the present invention no such nuts are used. To each end of each diametral extension however, is fixed a toggle latch 20, and to each diametrically outer face of each clamp piece 15 is fixed a cooperating catch 21 for the adjacent latch 20.

Atoggle latch 20 is shown in Figure 3. It is per sea well known construction, with a bracket 22, a lever arm 23 pivoting at shaft 24 across the bracket and a latching arm 25 itself pivoted at 26 to the inner end of the leverarm 23 and provided with latching hook 27 at its lower end to engage catch 21 when the lever arm is pushed down (thereby covering the moving part) to liftthe pivot 26. The latching hook 27 itself can be threaded at 28 and meshed with ring 29, located between one end 30 ofthe latching arm and a cross piece 31. This permits adjustmentfrom time to time ofthe effective latching arm length so asto compensate for toggle, or other, wear.

In use, a stack of precision sieves is placed between the clamp plate 18 and the location plate 4.

The position of clamp pieces 15 is adjusted on rods 14 untilthefaceofthe supported clamp plate 18 lies between two closely adjacent lines 32,33 on the sleeve 1 5a in each case. Then the toggle latches are closed so that each latching hook 27 engages its catch 21. The pressure exerted can be preadjusted by the manufacturer, or be adjustable on maintenance, by means of a threaded latch connection 28,29,30, 31.

Thus, instead of a potentially non-uniform and non-reproducible pressure being exerted by nuts on threaded rods, as is conventional, a uniform and reproducible pressure is exerted bythetoggle latches. The clamp pieces 15 do not clamp the plates but immobilise the catches 21 in a known relative locationtothe clamp plate lSunderstandard unstressed stack conditions.

Claims (8)

1. A sieve shaker for a stackofgrading sieves of the type in which a clamp plate moveable on vertical rods overlies and exerts pressure on the stack against a vibratory location plate: comprising (a), on each rod, a member slidable to, and immobilisable at, a position indicative of the clamp position prior to application of any pressure and (b) attached to the clamp plate in a number, and at locations, corresponding to each rod, latching means each engageable with its relevant immobilised slidable member and exerting a predetermined force when so engaged.

2. A sieve shaker as claimed in claim 1 in which two vertical rods, passing respectively through holes in two diametrically opposed extensions of a circular clamp plate, are used.

3. Asieve shaker as claimed in claim 1 or2 in which the rods are polygonal or otherwise non-circular in cross-section.

4. A sieve shaker as clai med in claim 3 in which the rods are hexagonal in cross-section.

5. A sieve shaker as claimed in any one preceding claim in which each latching means is a toggle latch fastened to the clamp plate and engageable with a catch fastened to the immobilised slidable member.

6. A sieve shaker as claimed in any one preceding claim, in which the latching means is itself adjustable to exert different set pressures as desired.

7. A sieve shaker as claimed in any one preceding claim, in which the location plate is supported on a base by elongate flat inclined springs, equispaced around the plate, and formed of a iaminated synthetic polymeric material.

8. A sieve shaker substantially as herein described and as illustrated in Figures 1,2 or 3 ofthe accompanying drawings.