GB1592028A

GB1592028A - Centrifuge drum

Info

Publication number: GB1592028A
Application number: GB744378A
Authority: GB
Original assignee: MAN Maschinenfabrik Augsburg Nuernberg AG
Current assignee: MAN AG
Priority date: 1977-03-04
Filing date: 1978-02-24
Publication date: 1981-07-01
Also published as: JPS6315028B2; DE2709410A1; AU3383078A; FR2382271B1; NL180180B; FR2382271A1; JPS53110182A; DE2709410C2; NL7802265A; NL180180C; AU512507B2

Description

(54) A CENTRIFUGE DRUM (71) We, MASCHINENFABRIK AUGsBURG- NURNBERG AKTIENGESELLSCHAFT of Dachauer Strasse 667, 8000 Miinchen 50, Germany (Fed.

Rep.), a German Body Corporate, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a centrifuge rotor drum having a cylindrical drum shell and at least one end cover connected thereto.

The components required for driving and supporting drums of this kind are mounted on the end covers or are integrated therewith.

The cylindrical shell or tube of the drum and some regions of the cover are stressed close to the limit of material failure when in operation.

The static and dynamic design of such rotational systems as well as their components and joints must therefore be optimized while avoiding weak places and near peak loads.

Generally, the end covers are produced by chip forming methods from high tensile, expensive drop forgings, are pressed with an oversized dimension into the centrifuge shell or tube and are joined thereto by means of electron beam welding. In this procedure the electron beam penetrates at the place of the maximum interference dimension from the outside into the tube and produces on the cover rim a weld pool which has a depth of approximately 1 mm and which is produced by rotation of the centrifuge drum about its longitudinal axis. The external diameter of the cover which is greater than the internal diameter of the tube is to compensate for minor expansion at operating speeds.

The use of large quantities of material in covers which are machine-turned from forgings is a disadvantage in such centrifuge drums.

Electron beam welding between shell and covers, performed in the zone of high elastic tube expansion, is also a disadvantage since the existing prestress of the tube at the edges of the weld merges into a stress condition which cannot be readily controlled.

To eliminate these disadvantages it is known to construct the covers of the rotor drum in cup shape of thin metal sheet and to connect them at the free end of the cylindrical part to the drum. The purpose of this is to ensure that no substantial bending stresses occur at the joint if the shell and the "cup edge" of the cover are subjected to approximately the same expansion. However, this advantage is of subordinate significance since these bending stresses do not act in the tangentially oriented principal stress plane. On the other hand, the position of the joint, displaced from the cover base far into the cylindrical shell is a disadvantage because joints are usually weak.

A joint of this kind in which the melting zone is not self-supporting can in this case be supported more readily by the cover base, which is subject to a lower stress level, than by the shell of the centrifuging drum in which supplementary stresses at one place simultaneously reduce the load bearing capacity of the entire drum. This embodiment, as well as drums with drop forged covers, suffer from the disadvantage of a shortening of the drum in operation due to expansion of the drum shell, a feature which has particularly detrimental effects on the bearings of the rotational system.

An object of the invention is to provide a centrifuge drum in which no substantial stress peaks occur under load, and axial shortening of the centrifuge drum is compensated for.

The invention provides a centrifuge drum comprising a cylindrical drum shell and at least one end cover in the form of a disc formed from a metal sheet and having a depending annular peripheral skirt extending into the inside of the shell and being connected thereto, the disc having a dished portion connected to driving or bearing elements of the drum.

The end cover can be inexpensively produced by a forming procedure and is suitable for supporting components of the drive and bearings in a simple and reliable manner. The dished portion of the end cover, which flattens during rotation, results in the required compensating axial motion between the internal and external diameter of the cover. In addition, the maximum tangential stresses which occur on the internal diameter are reduced and are accompanied by a simultaneous increase of the tangential stresses on the external diameter which would be lower without such movement. Furthermore, the inside ot the centrituge drum can be sealed in a vacuum-tight manner without applying harmful stresses to the shell.

Preferably, the annular peripheral skirt is force fitted into the shell.

Preferably, the said dished portion is directed inwardly of the drum symmetrically about the longitudinal axis of the drum. Thus, during use, any flattening of the cover counteracts shortening of the drum shell, so that bearing positions remain unaffected.

Preferably, the outer surface of the annular peripheral skirt of the cover tapers radially inwardly in a direction towards the free end of the skirt. A skirt of this kind facilitates the forcing of the skirt into the drum shell and has a stress-reducing and expansion-reducing action.

Preferably, the largest external diameter of the tapering skirt of the lid is provided with a shoulder against which the endface of the drum shell bears and is connected thereto. The shell is then welded to the cover at the said endface.

Preferably, the shoulder of the cover and hence the connection of the drum shell to the endface is situated substantially level with the base of the dished portion of the end cover.

This kind of weld seam construction corresponds approximately to an I-joint which is situated directly opposite to the arc of the welding operation. Compared with conventional "welding through the tube wall" procedures this offers the following advantages, namely that the melt volume is reduced to approximately one third of that of the conventional weld seam, shrinkage stresses and distortion are reduced, and the amount of molten material which is not self-supporting and represents an additional load for the undisturbed region, is correspondingly lower.

After receiving the end cover skirt the end of the drum shell expands and is stressed on the tapering external surface of the skirt. Accordingly, the stress changes which occur in the joint region during welding do not lead to any undesirable change of relative position of the drum shell and end cover. Welding the endface of the shell would cause the adjoining and less extensively expanded shell region to constrict to a greater extent. However, this is prevented by the support offered by the skirt, so that no movements due to stresses take place.

Notches, cracks or other damage which cannot always be avoided on the endfaces of the highly stressed drum shell are welded over.

The speficied position of the weld can be maintained more accurately since the joints are visible from the outside.

The elimination of "welding through the tube wall" procedures dispenses with the need of obtaining the connection by means of the high-energy and costly electron beam welding method. Furthermore, the joint is situated close to the cover base so that at this place relatively low tangential stresses can take over the additional loading imposed on the welding seam.

The tangential stresses and radial expansions of the end-cover skirt increase with an increasing distance from the cover base until they assume the magnitude of the shell stress. The dished portion of the end-cover becomes more shallow under rotational load and turns the skirt radially inwardly and thus compensates for part of its radial expansion. Accordingly, the tangential stresses, which are normally very high in this zone of the end-cover, are also lowered.

A coaxial bore which can be of cylindrical or cup configuration to establish the maximum stresses on the internal edge can advantageously be provided in the cover base and receives bearing elements or drive elements.

The central drawn-back portion of the endcover reduces the high tangential stresses and the radial expansions so that it is possible for bearing or driving element retaining means to be concentrically positioned by means of small radial pre-stresses in the cover and to connect them thereto by means of simple flanging, clamping or folding.

Due to the stress reduction resulting from the inner edge being drawn through, the thickwalled sockets hitherto necessary for supporting or limiting the radial expansion of the bearing or drive elements can be replaced by sockets produced from ordinary low-cost hardenable steel. The socket is welded to the inner edge.

In a further preferred embodiment the endcover is provided with a hub to support bearing components, as a rule ball and cup systems capable of supporting axial as well as radial bearing forces.

Here too,joining of two components makes use of the possibility of producing the inner, less highly stressed cover part from less expensive materials. A cylindrical or cup shaped configuration on the internal diameter of the metal sheet component serves to reduce stresses at the joint.

For the purpose of applying the drive and depending upon the time thereof, the sockets can be simple flat discs or annular members, the material of which is selected exclusively in terms of optimum electromagnetic properties, by contrast with the cover with the integrated drive element.

Embodiments of the invention are shown diagrammatically in the accompanying drawing, in which: Figure 1 shows in section a centrifuge drum, Figures 2 and 3 each show in section respective ends of the centrifuge drum of Figure 1, Figure 4 is an enlarged sectional view of the joint in Figure 2, and Figure 5 and 6 each show further embodiments of the drum end.

In Figure 1, a centrifuge drum 10 comprises a cylindrical drum shell 11 each end of which is closed by an end-cover 12, 13. One end-cover 12 is connected to a shaft 14 supported in ball bearings, and the other end-cover 13 is provided with a magnet 15, which co-operates magnetically with a magnet 16 fixed to a frame to form a magnetic bearing system. The centrifuge drum 10 is driven by a drive 17 shown diagrammatically.

The end cover 12 associated with the shaft 14 is shown in detail in Figure 2. The cover consists of a thin metal sheet which, after flanging of the edge, comprises an annular peripheral skirt 20 and a central cover disc 21. The cover disc 21 has a dished portion 22 which is directed towards the inside of the drum. The peripheral surface of the skirt 20 tapers radially inwardly towards the free end of the skirt, and the skirt has an annular shoulder 23 which bears on the endface of the rotor shell 11 and is welded thereto to form a joint 25. This joint is shown more clearly in Figure 4.

During manufacture of the centrifuge drum the end cover 12 is inserted with a force fit into the drum shell 11 thus forcing the end of the shell radially outwards as shown in Figure 4.

The stress produced thereby in the drum shell is greatest at its end thus producing good support surface on the cover skirt 20. This constant stress distribution in the axial direction is retained after welding in the region of the joint 25.

Due to the effect of centrifugal forces in operation the shell 11 moves radially away from the support surface on the skirt because of the greater expansion of the shell, so that the cover does not impose any additional load on the shell in this region.

The cover described is also well suited for receiving magnets for the bearing. One advantageous embodiment is illustrated in Figure 3 according to which the cover 12 is provided with a central aperture or bore in the dished portion 22.

To this end the inner periphery 29 of the apertured cover is curved so that a magnet socket 30, which projects into the bore, can be clamped by the cover 12. In one embodiment which is very simple to manufacture, the socket 30 has an annular shoulder 31 on which the inner periphery of the cover 12 bears, and the socket 30 is connected to the cover by a bent over rim 32. This embodiment does not call for strong welding seams, required hitherto, because the high tangential stresses which occur on the internal diameter of the cover base due to rotation are substantially reduced by the dished construction of the cover.

Figures 5 and 6 each show a cover 12 for receiving bearing and driving elements. The cover 12 according to Figure 5 has a cylindrical extension 35 for a drive with a radial motor 36. An annular extension 38, mounted on the external edge of a bearing socket 39, is provided for a drive with an axial motor 37 according to Figure 6. The socket 39 as well as the socket 40, provided in the embodiment of Figure 5, are connected in accordance with the above-mentioned procedure to an inner flange 41 or 42 of the cover 12.

Reference is made to our copending application No. 8728/78 (Serial No. 1592029).

WHAT WE CLAIM IS: 1. A centrifuge drum comprising a cylindrical drum shell and at least one end cover in the form of a disc formed from a metal sheet and having a depending annular peripheral skirt extending into the inside of the shell and being connected thereto, the disc having a dished portion connected to driving or bearing elements of the drum.

2. A drum as claimed in Claim 1, wherein the annular peripheral skirt is force-fitted into the shell.

3. A drum as claimed in Claim 1 or 2, wherein the said dished portion is directed inwardly of the drum symmetrically about the longitudinal axis of the drum.

4. A drum as claimed in Claim 1, 2 or 3 wherein the peripheral annular skirt of the cover extends inwardly of the shell.

5. A drum as claimed in any one of the preceding claims, wherein the outer surface of the annular peripheral skirt of the cover tapers radially inwardly in a direction towards the free end of the skirt.

6. A drum as claimed in Claim 5, wherein the largest external diameter of the tapering skirt has a shoulder against which an endface of the drum shell bears and is connected thereto.

7. A drum as claimed in Claim 6, wherein the shoulder of the skirt, and hence the connection between the drum shell and the end cover is situated substantially level with the base of the dished portion of the end cover.

8. A drum as claimed in Claim 5, 6 or 7, when appendant to Claim 4, wherein the end of the drum shell is stressed and outwardly flared after, for assembly, the tapering outer surface of the skirt has been force-fitted into the shell so that the shell bears substantially on the said tapering surface.

9. A drum as claimed in any one of the preceding claims, wherein the dished portion of the cover has a coaxial bore receiving bearing or drive elements.

10. A drum as claimed in Claim 9, wherein the coaxial bore is formed with a cylindrical or cup shape wall.

11. A drum as claimed in Claim 9 or 10, wherein the bearing or drive elements are connected to the cover by bent over portions of the said elements.

12. A centrifuge drum substantially as herein described with reference to any one of the embodiments of Figures 3 to 6 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. a magnetic bearing system. The centrifuge drum 10 is driven by a drive 17 shown diagrammatically. The end cover 12 associated with the shaft 14 is shown in detail in Figure 2. The cover consists of a thin metal sheet which, after flanging of the edge, comprises an annular peripheral skirt 20 and a central cover disc 21. The cover disc 21 has a dished portion 22 which is directed towards the inside of the drum. The peripheral surface of the skirt 20 tapers radially inwardly towards the free end of the skirt, and the skirt has an annular shoulder 23 which bears on the endface of the rotor shell 11 and is welded thereto to form a joint 25. This joint is shown more clearly in Figure 4. During manufacture of the centrifuge drum the end cover 12 is inserted with a force fit into the drum shell 11 thus forcing the end of the shell radially outwards as shown in Figure 4. The stress produced thereby in the drum shell is greatest at its end thus producing good support surface on the cover skirt 20. This constant stress distribution in the axial direction is retained after welding in the region of the joint 25. Due to the effect of centrifugal forces in operation the shell 11 moves radially away from the support surface on the skirt because of the greater expansion of the shell, so that the cover does not impose any additional load on the shell in this region. The cover described is also well suited for receiving magnets for the bearing. One advantageous embodiment is illustrated in Figure 3 according to which the cover 12 is provided with a central aperture or bore in the dished portion 22. To this end the inner periphery 29 of the apertured cover is curved so that a magnet socket 30, which projects into the bore, can be clamped by the cover 12. In one embodiment which is very simple to manufacture, the socket 30 has an annular shoulder 31 on which the inner periphery of the cover 12 bears, and the socket 30 is connected to the cover by a bent over rim 32. This embodiment does not call for strong welding seams, required hitherto, because the high tangential stresses which occur on the internal diameter of the cover base due to rotation are substantially reduced by the dished construction of the cover. Figures 5 and 6 each show a cover 12 for receiving bearing and driving elements. The cover 12 according to Figure 5 has a cylindrical extension 35 for a drive with a radial motor 36. An annular extension 38, mounted on the external edge of a bearing socket 39, is provided for a drive with an axial motor 37 according to Figure 6. The socket 39 as well as the socket 40, provided in the embodiment of Figure 5, are connected in accordance with the above-mentioned procedure to an inner flange 41 or 42 of the cover 12. Reference is made to our copending application No. 8728/78 (Serial No. 1592029). WHAT WE CLAIM IS:

1. A centrifuge drum comprising a cylindrical drum shell and at least one end cover in the form of a disc formed from a metal sheet and having a depending annular peripheral skirt extending into the inside of the shell and being connected thereto, the disc having a dished portion connected to driving or bearing elements of the drum.