GB2164722A - Improvement to sieving machines - Google Patents

Abstract

A sieve shaking machine which operates at sub-sonic frequency, about 6 to 10 Hertz, and high amplitude, about 35 mm. In these conditions the particles being segregated lift from the sieve meshes, due to their own inertia, and tumble as the rising mesh changes direction each cycle. This system reduces process time considerably, enables irregular and flaky particles to be segregated efficiently by machine. The low frequency high amplitude action is obtained by applying the principle of a mass freely suspended on a spring and the system operates at its natural resonant frequency; it is therefore highly efficient mechanically. The laboratory type testing machine specified uses not more than 50 watts and may be used for the three standard sieve diameters, 20, 30 and 45 cms. <IMAGE>

Description

SPECIFICATION Improvement to sieving machines This invention relates to mechanical sieve shaking machines, more particularly (although not exclusively) to relatively small machines for laboratory testing purposes.

In order to determine the proportional quantities and sizes of the particles which may comprise an aggregate material, for example; sand and gravel for making concrete for building construction, a small quantity is taken from the bulk and passed through a sieve, or number of sieves in succession. Approved methods of performing this analysis are described in British Standard 812 Methods for Sampling and Testing Mineral Aggregates Sands & Fillers. Although the B.S. recommends shaking the sieves by hand, mechanical sieving is conceded for many routine purposes.

Most sieving machines which are generally available operate at relatively high frequency and are essentially vibratory in action. There is little similarity between vibratory motion and that induced by hand.

The sieve shaking machine, to be described, is designed to simulate the action of hand sieving; that is to say, it operates at sub-sonic frequency and high amplitude, imparting a tossing action to the material being sieved.

Agitation in some form is necessary to disturb the natural repose of the material and cause the particles to spread over the sieving areas, and a vibratory system has been generally accepted. It can be shown however that a low frequency tossing motion may be preferable in terms of, for example: Less damage by attrition to material being sieved.

Less mechanical noise.

Less sieving time for optimum segregation.

Less power consumption, due to operation at resonance.

Angular and even flaky particles may be segregated.

Closer agreement with hand sieving results.

A sieve shaking machine according to this specification operates at sub-sonic frequencies, generates no significant sonic or ultrasonic vibrations, its movements emulate those of hand sieving and the mechanism is simply contrived. The principle of operation upon which the machine works is based upon a spring-mass system, that is; the natural period which occurs when a mass is freely supported by a spring. The resonant frequency of the system depends upon the relationship:

i 1 acceleration due to gravity Frequency Hz=/ 2n static deflexion of spring Essentially the machine comprises two major components: 4 the supporting member which may be fixed to the floor and 6 the active member onto which the sieves may be loaded and clamped.

The active member is attached to the supporting member solely by pairs of springs, each spring acting in opposition to the other and mounted so that they are continually, but variably, stressed at all times.

In the machine which is to be described the active member is suspended between two pairs of helical tension springs; each spring is rated at about 15 kN/m extension. When the machine is at rest each spring is extended by about 40 mm exerting a force of about 60 kgf, total about 240 kgf. The weight of the active member is 36 kg which causes a static deflection of the springs of about 5.7mm. The resonant frequency of the system calculated by the above formula is 6.5 Hz.

Changes in load due, for example, to sieve types and sample quantities will change the resonant frequency; a 2% change in load will cause 1% change in frequency. Compensation for such changes may be made by altering motor speed. The motor is series wound and its speed may be simply controlled by a variable resistance, and this is provided on the control panel of the machine.

Power to start and to maintain oscillation of the active member is provided by eccentric weights mounted on the output shaft of a relatively small variable speed gear reduction electric motor. A 1/30 HP motor, operating at one half its maximum output, provides sufficient power for all laboratory standard sieving.

The motor is mounted on the active member so that its output shaft is radial to the vertical axis of oscillation. The eccentric weights are mounted a suitable distance along the shaft from the vertical axis so as to produce, in addition to the vertical oscillation, rotary impulses which cause the material to be distributed over the mesh areas.

The eccentric weights are mounted at opposite ends of the output shaft on centres of approximately 80 mm. It was found that with the motor assembly fitted so that one of the eccentric weights lay on the vertical axis of the active member, the other weight being then 80 mm from the centre, radial impulses thus generated caused the material being sieved to rotate in the sieves a few degrees at each cycle. To synchronise with the calculated resonant frequency the eccentric weights would be required to rotate at not more than 360 rpm. It is emphasised that this speed of rotation should preferably be the maximum possible speed of the motor output shaft. If shaft speed cannot exceed that required to sustain oscillation at resonance then synchronism will be maintained in all normal operating circumstances.

Due to its very modest power consumption, not more than 50 watts, the machine would be suitable for use in the field. If fitted with a suitable motor it would operate satisfactorily from a car battery.

In normai operating conditions at an amplitude of about 30 mm the material being segregated lifts off the surfaces of the sieving areas each cycle as the active member reaches maximum upward velocity. The combined vertical and rotary impulses cause the particles to tumble, tending to present different faces each time contact is re-made with the mesh as the direction of movement reverses. By this means irregular shaped particles may be segregated with results comparable with those achieved by hand sieving.

Referring now to the accompanying drawing of the machine. The pair of eccentric weights 1 which are located on the motor output shaft are seen immediately beneath the base 2 of the active member 6.

The eccentric weight/motor assembly is rigidly bolted to the underface of the base 2.

Each weight weighs about 0.65 kg their centres of gravity lie on a radius of 50 mm about the motor output shaft, they generate together an eccentric force of about 53N.

Due to the impulses generated by the active member 6 it is necessary to bolt the supporting member 4 to either a heavy plinth, 0.1 m3 of concrete provides an adequate weight, or alternatively bolted direct to a substantial floor.

The sieves, whichmay be either 20, 30 or 45 cms diameter, are placed on the base 2 of the active member 6. The base 2 may be designed to accommodate the alternative sieve diameters. The clamping plate 3 is then placed on the top of the sieves and the adjustable central column 7 brought down to apply sufficient pressure to hold the sieves in position.

When the machine is switched on the active member 4 responds immediately reaching its operational amplitude in about 3 seconds.

The static extensions of the springs 5 of about 40 mm allows a maximum 'safe' amplitude of the active member 6 of about 50 mm.

The amplitude control 8 consists of variable resistances connected in series and arranged so that when the manual control is at maximum (10 on the scale) there remains some resistance which may be set at a value which will limit the voltage that can be applied to the motor. This resistance should be set at the time of installation to suit the voltage of the supply.

Claims (8)

1. A sieving machine the principle of operation of which is a practical application of the principle of Simple Harmonic Motion at an infrasonic frequency.

2. A sieving machine as claimed in Claim 1 in which the effectiveness of operation is independent of any customary mechanical means used to drive it.

3. A sieving machine as claimed in Claims 1 & 2 in which the driving mechanism may be rotary for preference, the speed of which is adjustable but by design cannot exceed that of the natural resonant frequency of the active member 6.

4. A sieving machine in which the vertical oscillation of the active member 6, as it changes direction each stroke, causes the material being sieved to be periodically tossed clear of the sieve meshes.

5. A sieving machine as claimed in preceding claims in which lateral modes of the principal harmonic motion are excited to cause impulses of the active member 6 by which the material being sieved is well-distributed over the sieving areas.

6. A sieving machine as claimed in preceding claims in which laterai impulses of the active member 6 cause the particles of the material being sieved to tumble-turn as they are tossed by the period vertical movement.

7. A sieving machine as claimed in preceding claims in which an unbalance weight or weights excite vertical and lateral modes of a Simple Harmonic Oscillatory sytem.

8. A sieving machine substantially as described herein with reference to the accompanying drawing.