GB2183002A - Electromagnetic vibrator - Google Patents

Abstract

An electromagnetic vibrator comprises a table 11 having a depending electrical coil 21 in an air gap 16 formed between a ferrous body 12 and a magnetised core 13. The table is supported by means of a circumferential ring of hydrostatic bearings 24 in the core 13 which when pressurised allow the table substantially unrestricted freedom of vibratory movement parallel with the vibrator axis and the bearing axes but which give stiffness perpendicular to those axes. At the lower end of the posts 25 constituting the inner components of the bearings they are interconnected by a ring 34 to give the whole vibrating assembly rigidity. The ring 34 can move in an annular counter bore in the core and can be centred by light opposed pairs of springs 46. <IMAGE>

Description

SPECIFICATION Electromagnetic vibrators This invention relates to electromagnetic vibrators for example of the kind described in British Patent Specifications Nos. 1202526 and 1431757.

That kind of vibrator comprises a pot magnet defining an annular magnetic gap together with a table having a depending annular coil positioned in the gap and spaced from the inner and outer sides of the gap, the table being mounted to be capable of vibration parallel with the axis of the gap due to the interaction between the magnetic field across the gap and the alternating current in the winding or coil. One use of that type of vibrator is to provide a means for applying vibration tests to a sample carried onthe table, and as described in Specification No.1431757 the vibrator can be mounted in trunnions so that the axis of vibration can be vertical or horizontal or at any selectdd angle between them.

Specification No.1202526 disclosed a particular method of mounting the table in relation to the magnet body to be free for axial vibration but to be restrained against movement perpendicular to the axis of vibration and although for many applications the mounting described is perfectly adequate, it is a mechanical mounting which tends to restrict the permissible amplitude of vibration.

It is an object of the present invention to provide a novel form of mounting for the table in relation to the magnet body which allows for a substantial amplitude of vibration.

According to the present invention a vibrator has a body and a table mounted to vibrate in relation to the body on a hydrostatic axial bearing with stiffness perpendicular to the axis of the bearing but allowing substantially unrestricted movement of the table parallel with the bearing axis. Thus the movement parallel with the bearing axis may be limited in extent only by limits of space.

Further according to the invention an electromagnetic vibrator has a magnetic body and core and a table carrying an electrical coil in a magnetic gap defined by the body and core, and an axial bearing for the table, the bearing being a hydrostatic bearing having stiffness perpendicular to its axis but allowing substantially unrestricted movement of the table in relation to the body and core parallel with the bearing axis.

In general when the bearing is pressurised there will be no mechanical contact between the table and the magnetic body and the pressurised hydraulic fluid in the bearing while restricting lateral movement will provide very low restriction against axial movement even for considerable displacements from a reference position. Thus, effectively the length of the vibrator stroke will not be determined by the mechanical characteristics of the bearing as would the prior proposals but merely by the space that is available.

In a preferred form of the invention there are a number of bearings circumferentially spaced around the vibration axis and all having axes parallel with that axis. Two bearings at opposite ends of a diameter could be used but preferably there are at least three or perhaps four hydrostatic bearings arranged around the polar axis of the vibrator in a triangular or square configuration.

Since a hydrostatic bearing in the form of a cylindrical shaft spaced within a cylindrical sleeve and having means for supplying pressurised hydraulic fluid to the annular space between the two components does not define any reference position along the bearing axis, it will be generally convenient to provide resilient or other means defining a reference position of the table in relation to the body and the core. The reference position may be approximately mid-way between extreme positions defined by the available space.

For providing a reference position when the axis of the vibrator is vertical an air cushion is a convenient arrangement. When the coil is not energised the table can rest on the cushion which can be inflated sufficiently to bring the table to a desired reference position and thereafter vibrations towards the cushion will be against the resistance of the inflated cushion but the system can be designed for any distortion due to that resistance to be minimised.

If the vibrator is required to operate with its axis horizontal such an air cushion device would not be needed so that it could be removed or just not inflated. Instead one or more sets of opposed pairs of light springs could be used between the body on the one hand and a component attached to the table on the other hand. Such springs need not act as bearings or provide any guidance or positioning of the table but could merely act to provide a central reference position of the table in relation to the body.

in a preferred arrangement the or each hydrostatic bearing is positioned in a bore formed in the core and or the body of the vibrator with the inner bearing component depending from the table and with the outer bearing component mounted in the bore so that there can be means formed within the body and core for leading pressurised hydraulic fluid to the bearing.

Where there are a number of hydrostatic bearing circumferentially arranged around the vibrator axis there will be a corresponding ring of posts depending from the table one for each bearing and conveniently they are interconnected at their ends remote from the table by a ring which gives the whole structure of table posts and ring some rigidity. The magnetic body and/or core will be machined with an annular groove to accommodate vibration of the ring with the table.

Conveniently there is also a ring at the other end of the posts for attachment to the table.

In accordance with another aspect of the invention in a method of assembling an electromagnetic vibrator one or more posts are assembled in hydrostatic bearings in the body or core and the bearings are pressurised so as to be centred after which' the table is secured to the posts.

The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawing of which the single Figure is a cross-section through an electromagnetic vibrator. The cross-sections to either side of the vertical centreline are on planes containing the polar axis of the vibrator but inclined to each other by 45".

Since many of the details of the vibrator are already known and indeed described in one or both of the above mentioned British Patent specifications the drawing has been a little simplified and the accompanying description will concentrate on the novel features of the mounting in accordance with the present invention.

The vibrator comprises a stator and a table 11 which vibrates in relation to the stator along the polar axis which is the central vertical axis in the Figure.

The stator is essentially made up of three components, a ferrous metal body 12, a central core 13 mounted on an upstanding central part of the body 12 and an upper ring 14 mounted on the outer periphery of the body 12 and having an inner annular cylindrical surface 15 spaced around and defining a cylindrical annular air gap 16 with the corresponding outer cylindrical surface of the core 13. The body, core, and ring, are all of ferro-magnetic material and they define an annular space accommodating DC exciting coils 17 for magnetizing the stator to produce a radially directed magnetic field extending across the gap 16.

Arrangements for providing DC magnetizing current to the coils, for cooling the stator body by flow of low pressure fluid coolant, for securing the body core and ring together, and for sealing the joints between them against escape of coolant fluid, are conventional and will not be described in detail here.

The table 11 is an magnesium casting having an external annular recess 18 for a depending insulating sleeve 19 carrying closely wound electrical coils 21 which are connected to terminals 22 in turn connected through flexible leads 23 to the stator ring 14 for the application of high frequency alternating current to the coils.

When the AC and DC coils are energized the table vibrates at the AC frequency due to the electromagnetic action in the gap 16 caused by the interaction of the current in the AC coils extending perpendicularly to the radially extending magnetic field. The movement is at right angles to both the magnetic field and the AC current, that is parallel with the axis of the gap as is described in the above mentioned patent specifications.

The Figure shows how the sleeve 19 carrying the AC coils 21 lies centred in the gap 16 and out of contact with the surfaces of the core and the ring defining that gap.

The mounting of the table 11 in relation to the stator is by means of four poster bearings depending from the table into the core and arranged equally spaced from one another in a circumferential ring.

One of the poster bearings 24 is shown at the left hand side of the Figure. It is a hydrostatic bearing comprising an inner cylinder post 25 within an outer bearing sleeve 26 which is seated on a shoulder 27 in a bore 28 drilled downwardly into the top face of the core 13. The bearing sleeve 26 is provided with high pressure hydraulic fluid from a source indicated generally at 29 through passages in the body and core as shown at 31.

The supply of high pressure oil to the bearing nozzles 33 locates the post 25 in the bearing sleeve 26 and holds it with good stiffness against lateral movement but with substantial freedom for axial movement and without the post and sleeve coming into mechanical contact with each other in accordance with the well understood principles of hydrostatic bearings.

The high pressure oil is sent to all four bearing sleeves. The four cylindrical posts 25 are connected at their bottom ends by a lower annular ring 34 housed in an annular counterbore partly in the lower face of the core 13 and partly in the upper face of the body 12 as shown at 35. The posts 25 are interconnected at their upper ends by an upper annular ring 36 which is in turn secured to the table by bolts 37. A bolt 38 extends through the upper ring 36, through each of the posts 25, and into threaded engagement with the lower ring 34 to form a rigid cage suppog the table.

The electrical leads 23 extend between the stator and the table and there is a diaphram 39 ciosing the gap around the table to prevent entry of dirt but those two components are flexible and provide minimum constraint on free axial movement of the table in relation to the stator. The permissible amplitude of vibration is limited by contact of the underside of the table with the core 13 on the one hand and by contact of the lower ring 34 with the hydrostatic bearing sleeves 26 on the other hand. The axial length of the AC coils 21 is made sufficient to ensure that in the extreme positions of the table the coil is in the air gap 16.

It is in most applications necessary to define a mid point position of the cable and coil structure from which vibration takes place and this can be provided simply if the polar axis is vertical with the table at the top by means of a double convoluted air bag 41 with a means 42 for pressurizing them sufficiently to support the weight of the table, its poster bearings, and the upper and lower rings. A plate 42 bolted to the underside of the table 11 rests on the air bag 41 which are inflated sufficiently to raise the table to the desired central or reference position about which vibration is to take place. The characteristics of the bellows are such as not to distort the vibration pattern to an unacceptable degree.

If however the vibrator is rotated through 90" in either sense in the plane of the Figure for providing horizontal vibrations the air bag 41, which cannot be used for defining the reference position, are deflated and use is made of a centring device shown in the section at the right hand side of the Figure.

Four bores 43 are formed upwardly into the core from the under side and downwardly into the centre of the body 12 from the upper or mating side so that each forms a cylindrical chamber as shown in the section at the right hand side of the Figure. The lower ring 34 which is locally radially narrower than at the position where it is secured to the poster bearing can move up and down in the annular recess 35 but where that recess joins the four circumferentially spaced bores 43 each angularly position between two of the poster bearingsthe ring has secured to it upper and lower collars 44 and 45. A compression spring 46 is mounted between each endof thebore 43 and the opposed face of the collar 44 or 45 so that the two springs 46 together act to centre the ring 34 and hence define the reference position of the table 11.As with the air bag 41 the springs 46 are not very stiff so that they do not substantially effect the high frequency vibration of the table but merely provide a centring action ensuring that when vibration starts the table will be in the centred position even if the polar axis is horizontal.

In addition to the annular counterbore 35 allowing for the movement of the lower ring 34 the core and to some extent the centre of the body have to be bored to accommodate the four poster bearings 24 and the four spring centering means 44,45 and 46 but it is believed that that does not involve removal of sufficient ferro-magnetic material from the magnetic circuit to prevent satisfactory operation of the vibrator.

In assembling the vibrator the bearing sleeves 26 and inner posts 25, together with the springs 46, are assembled to the lower ring 34 and seated in the counterbore 35, and the core 13 is the fitted over the body 12.

The posts 35 are inserted in the poster bearings, and the upper ring 36 is applied over the posts, and the bolts 38 are screwed loosely into the lower ring 34, before the bearings are pressurised. Then the bearings are pressurised which automaticaliy centres each bearing so that the post is held out of contact with its bearing sleeve and the bolts 38 are tightened and the table is secured to the upper ring 36 by the bolts 37 while the bearings are still held centred. The hydrostatic bearing system can then be relieved of pressure to enable assembly to be completed.

When the vibrator is to be used and the bearings are pressurised they will automatically be re-centred together.

The novel form of hydrostatic bearing and guidance system for the table gives a much lower vertical stiffness than in any of the previously proposed vibrators involving some kind of mechanical linkage syste. It has been found that this substantial vertical compliance which enables a substantial amplitude of vibration to be achieved can be accompanied by the necessary lateral stiffness whether the axis of vibration is vertical or horizontal or at some axis between vertical and horizontal. In one particular design an amplitude of vibration of + and - 30mm has been achieved.

Claims (11)

1. A vibrator having a body and a table mounted to vibrate in relation to the body on a hydrostatic axial bearing with stiffness perpendicular to the axis of the bearing, but allowing substantially unrestricted movement of the table parallel with the bearing axis.

2. An electromagnetic vibrator having a magnetic body and core and a table carrying an electric coil in a magnetic gap defined by the body and core and an axial bearing for the table, the bearing being a hydrostatic bearing having stiffness perpendicular to its axis but allowing substantially unrestricted movement of the table in relation to the body and core parallel with the bearing axis.

3. A vibrator as claimed in either of the preceding claims in which when the bearing is pressurised, there is no mechanical contact between the table and the body.

4. A vibrator as claimed in any of the preceding claims including a number of bearings circumferentially spaced around the vibration axis and all having axes parallel with that axis.

5. A vibrator as claimed in any of the preceding claims including means defining a reference position of the table in relation to the body.

6. A vibrator as claimed in Claim 5 in which the reference means comprise an air cushion.

7. A vibrator as claimed in Claim 5 or Claim 6 in which the reference means comprise one or more sets of opposed pairs of light springs, each spring being connected on the one hand with the body and on the other hand with a component attached to the table.

8. A vibrator as claimed in any of the preceding claims in which the or each hydrostatic bearing is positioned in a bore formed in the body with an inner bearing component depending from the table, and with an outer bearing component mounted in the bore together with means formed within the body for leading pressurised hydraulic fluid to the bearing.

9. A vibrator as claimed in any of Claims 4 to 8 in which there are a ring of posts depending from the table one for each bearing, which posts are interconnected at their ends remote from the table by a rigid ring which can vibrate with the table in an annular groove in the body.

10. A vibrator constructed and arranged substantially as herein specifically described with reference to the accompanying drawing.

11. A method of assembling an electromagnetic vibrator in which one or more posts are assembled in hydrostatic bearings in the body, and the bearings are pressurised so as to be centred and then a table is secured to the posts.