GB2034437A

GB2034437A - Regulating the vibration of a in vibrating sieve

Info

Publication number: GB2034437A
Application number: GB7931440A
Authority: GB
Original assignee: Morgardshammar AB
Current assignee: Morgardshammar AB
Priority date: 1978-09-12
Filing date: 1979-09-11
Publication date: 1980-06-04
Also published as: NO792924L; DK379279A; SE445090B; NL7906712A; DE2936541A1; NO150226C; FI792816A7; GB2034437B; FR2435972A1; FR2435972B1; CA1139267A; SE7809573L; NO150226B; AT375282B; ATA598379A

Abstract

A device such as a sieve or feeder (1) is provided with two parallel shafts having respective out-of-balance weight means (5) comprising weights (10) located in casings, each shaft being driven by its own motor (3), preferably of asynchronous type. The out of balance weight means (5) are of different dimensions for each shaft and the centre of gravity of the device is so positioned that the ratio of the distances between the centre of gravity and the respective axes of rotation of the out of balance weight means, is inversely proportional to the products of the weights of the respective out of balance means and their mean distance to the respective axis of rotation so that an elliptical stroke can be obtained. The direction of stroke can be altered by changing the ratio of the values of power supplied to the two motors. <IMAGE>

Description

SPECIFICATION Method for regulating the throw angle of a vibrating sieve or feeder The invention relates to a method which is usable in those driving means for achieving shaking or oscillating movements, in which two out-of-balance weights are rotated in opposite directions around individual shafts and are provided with separate drive motors.

There are two types of this sort of driving unit. In one of them, which has been known for a long time, the two out-of-balance weights are of the same size and weight. A theoretical treatment of this case is to be found in Aufbereitungs-Technik for 1976, pages 108-114, for example. In this first type of drive unit, a linear, reciprocal movement can be obtained along the mid-point normal to a straight line connecting the two shafts.

The other type of drive unit was invented by us and is described in Swedish Patent Appiication 7708140-4 (co-pending U.S. Patent Application Serial No. 922 597), the disclosure of which is hereby included by reference into the present description).

According to this invention, an elliptical stroke is obtained by making the out-of-balance weights of different size, underthe precondition that the center of gravity of the oscillating system lies essentially on an Apollonios' circle to the axes of rotation, so determined that the ratio of the distances from the center of gravity to the axes of rotation is inversely proportional to the products of the weights of the respective oscillation masses and their mean distance to the respective axis of rotation.

The latter invention was developed from an indepth theoretical study of a previously known system with two different sized oscillation masses, compulsorily coupled together with a toothed gearing. The research began with a calculation of the reaction moments on the shafts to the out-ofbalance weights, where, inter alia, the following result was obtained:

where M1 and M2 are the reaction moments around the respective rotational axes; m1 and r1 are the weight of one out-of-balance weight and its distance from its rotational axis, respectively; a and bare the Cartesian coordinates in a Cartesian system of coor dinates with the center of gravity of the oscillating system as the origin; a is the angle between the direction of stroke and a line from the origin to the point (a, b); w is the angular frequency for the rota tion of the out-of-balance masses; and y is the mass moment of inertia of the system.

M1-M2 is then a restoring moment which acts to return the system to the direction a = 0.

The basic idea which is the first step in the present invention is the insight that the equation (1) is valid even when reversed so that if an external moment is applied, the throwing angle is affected to a corresponding degree, that is to say, artificially it is possible to achieve a value for a which is non-zero. A precondition for balance in such a system is that the work of the external moment be equal to the work of the synchronizing moment. The cycle for the sync hronizing moment is 2ir. If we integrate M1-M2 over one revolution, we obtain from equation (1):

One can see from this that a numerical increase or decrease in an external moment results in an increase or decrease in a up to the limit 45Q, whereafter stability can no longer be expected.

In the absence of the external moment, as the description of the invention in Swedish Patent Application 7708140-4 reveals, a purely translational shaking movement is obtained, without rotation (tilting).

The introduction of an external moment disturbs this "equilibrium", and a superimposed tilting movement is obtained. However, it has been shown both theoretically and in practice that this is of completely negligible size.

It is now possible to superimpose an "external" moment in many different ways. For example, it is theoretically possible by using springs or weights fastened to the screening box, to apply a couple of forces which tends to turn the suspended mass.

However, we feel that it is most advantageous to act on the two motors which drive the out-of-balance weights.

The invention achieves a method for regulating the throw angle of a vibrating sieve or feeder, through the characteristics disclosed in Claim 1.

Such a regulation can also include the possibility of compensating for uneven load due to uneven distribution of the material to be screened.

The invention will now be described with reference to an embodiment shown in the drawings.

Fig. 1 shows a perspective view of a horizontal sieve.

Fig. 2 shows a schematic sectional view of one of the out-of-balance devices in the sieve according to Fig. 1.

The sieve in Fig. 1 consists of a screen box 1, with a first screening level 7 and a second screening level 8, which is suspended on a bed 9 by means of springs 2.

Two motors 3 each drive one of a pair of out-ofbalance weight means 5, each pair mounted in a dust-proof casing with an intermediate shaft. As can be seen from the detailed drawing in Fig. 2, each motor drives, via a belttransmission ?1, mounted in a protective casing 6 (Fig. 1), and via flexible couplings 12.(in casing 4), a shaft with out-of-balance weights 10 pairwise rotatably journalled in the walls of the screening box 1. The motors are rigidly joined to the bed 9 and thus do not vibrate.

The inventive effect is achieved by virtue of the fact that the motors 3, which are short-circuit asynchronous motors with the same rated speed, can be imparted different propelling forces. If the motors receive current directly from a common alternating current mains source, the oscillation masses 10 (Fig.

2) in the two units, which rotate in opposite directions, will be synchronized and an elliptical thrust will be obtained. Through a transformer unit of a type well-known to the person skilled in the art (either stepwise or continuously variable), the supply voltage to one of the motors can be reduced.

Since the two motors are loaded, they will have a certain lag in relation to the synchronous r.p.m. Due to the difference in voltage, the driving force of one of the motors will drop, and the lag in that motor will increase somewhat. (Other types of motors and other control mechanisms which are well-known to the person skilled in the art are of course conceivable.) It has been shown, quite surprisingly at first, that it is possible to obtain with this simple method a displacement of the direction of the stroke, up to about 45" in either direction, depending on which of the two motors has its supply voltage reduced.

The theoretical explanation above is rather sketchy, but we did not feel it necessary to have a more careful calculation of the dynamic relations.

We were satisfied with finding, by tests with models (regardless of the explanation one wishes to give to the phenomena) that the inventive effect is achieved if one proceeds as specified in the accompanying

Claims

claims. The advantages of the invention are apparent. In both a feeder and a sieve, the transport speed can be varied as needed. It is also possible to compensate for uneven load. By optimizing the throw angle it is also possible to obtain an improved screening effect and control the screening so that the screen meets the material, when falling down, in such a way that the material and the screen have opposite movements, thus achieving the best sorting effect and avoiding plugging. CLAIMS

1. Method for regulating the throw angle of a vibrating device such as a sieve or feeder of the type which has a drive means with out-of-balance means around two shafts which are parallel to each other, characterized in that the two out-of-balance means are of different dimensions, are driven by individual motors and have their shafts arranged in relation to the center of gravity of the device in such a way that the center of gravity lies on an Apollonios' circle to the two shafts, so determined that the ratio of the distances from the center of gravity to the axes of rotation is inversely proportional to the products of the out-of-balance weights of the respective out-ofbalance means and their mean distance to the respective axis of rotation, the throw angle being affected by changing the power of one of the motors in relation to that of the other.

2. Method according to Claim 1, characterized.in that the change in the relation between the powers of the motors is achieved by supplying one of the two motors, which are of the shortwcircuit asynchronictype, with a lower excitation voltage.

3. Method substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

4. A vibrating device operable according to the method claimed in any of the preceding claims.