WO2004055379A1

WO2004055379A1 - A screw conveyor

Info

Publication number: WO2004055379A1
Application number: PCT/GB2003/005461
Authority: WO
Inventors: Robert Maurice Edward Field; Michael Fuller
Original assignee: Aktiebolaget Skf
Priority date: 2002-12-13
Filing date: 2003-12-15
Publication date: 2004-07-01
Also published as: GB0229105D0; AU2003288469A1; GB0329032D0; GB2397352B; GB2397352A

Abstract

A screw conveyor for pumping a fluid or moving a semi-solid, said conveyor comprising a shaft and a lower support bearing assembly, said lower support bearing assembly comprising a housing, a bearing cartridge, a bearing and a seal, wherein: the bearing cartridge is mounted to the housing; the bearing is mounted to the cartridge; the bearing is connected to the shaft; the seal is a self-aligning seal; and the bearing is a self-aligning bearing.

Description

A Screw Conveyor

The present invention relates to a screw conveyor and, in particular, a screw conveyor of the type that has a lower support bearing assembly.

Screw conveyors are used for pumping a fluid or semi solid. Screw conveyors typically comprise a shaft onto which a substantially continuous and essentially uniform helical rib or flange is attached, thus forming a screw. In some applications, the weight of this screw fabrication can be up to 14 tonnes. The screw is typically located in an inclined trough or gutter having a close fit to the outer diameter of the screw. Pumping is achieved by rotating the shaft at a speed of between 30-60 rpm. This type of screw conveyor is known by many names including an Archimedes screw, an Archimedes pump, an Archimedean pump and a screw pump.

The screw shaft is usually connected to a gearbox at the top end and is supported there by a bearing arrangement. A support bearing is also typically provided at the bottom of the shaft.

The Archimedes type screw conveyor is extensively used in water treatment works to lift heavily contaminated sewage water within the water treatment plant. The operational conditions are often hazardous and the bottom of the screw and the screw support bearing will typically be submerged in heavily contaminated sewage water. The^•'•sewage water will often contain coarse solids and abrasives. Many of the screw conveyors currently in use in water treatment works are subject to unpredictable and catastrophic failure, resulting in high replacement costs and a high potential for environmental pollution. Operational lifespan can vary between several months to several years in an apparently random manner.

As a result of their investigations, the present inventors have found that the poor reliability of conventional screw conveyors in water treatment works is often a result of failure of the lower support bearing arising from axial displacements caused by daily and seasonal thermal changes and radial displacement and angular misalignment of the screw shaft to the bearing housing. In addition, the present inventors have found that failure to correctly align the support bearing assembly to the screw/pump shaft on installation and replacement of support bearing units is a also a cause of failure.

Accordingly, the present invention provides a screw conveyor for pumping a fluid or moving a semi-solid, said conveyor comprising a shaft and a lower support bearing assembly, said lower support bearing assembly comprising a housing, a bearing cartridge, a bearing and a seal, wherein: the bearing cartridge is mounted in the housing; the bearing is mounted in the cartridge; the bearing is connected to the shaft; the seal is a self-aligning seal; and the bearing is a self-aligning bearing.

The shaft may be, for example, a flanged stub-shaft, which is connected to the screw conveyor. The flanged stub shaft may comprise two separable parts, i.e. the shaft and the flange.

The term screw conveyor as used herein is intended to encompass also a screw pump, a helical pump, a helical conveyor, a Longbridge pump, an auger, an Archimedes or Archimedes 's screw or pump, and an Archimedean screw or pump.

The present invention may be used to pump a fluid or to move a semi-solid in a vertical or horizontal direction, or in a combination of the vertical and horizontal directions. Examples of semi-solids include sand and grain.

In the present invention, the support bearing assembly comprises a housing, a bearing cartridge, a seal, and a bearing. The bearing cartridge is mounted in the housing. The bearing, the seal and the screw conveyor shaft may be mounted in the bearing cartridge. Alternatively, the shaft may be a part of a flange unit, or the flange unit may be mounted to the shaft.

In use, the lower support bearing assembly is submerged in the medium to be pumped.

The self-aligning bearing and seal preferably accommodate both angular and axial dynamic misalignments.

Preferably, the self-aligning bearing is a self- aligning ball bearing, a self-aligning spherical roller bearing or a self-aligning toroidal bearing. Use of a self- aligning ball bearing or a self-aligning spherical roller bearing means that the assembly is able to cope with angular misalignment. Axial misalignments are accommodated by the outer ring of the bearing sliding in the cartridge.

Advantageously, the self-aligning bearing is able to operate with both static and dynamic misalignment present.

A self-aligning toroidal bearing is particularly preferred because it can accommodate misalignment in both axial and angular directions, the outer ring can therefore be fully located in the cartridge. A toroidal bearing also allows for thermal expansion of the screw shaft, or conveyor shaft, and such a bearing can adjust axially to compensate for seal wear. An example is a CARB (TM) bearing from SKF. As well as having a dynamic self-aligning capability, such a bearing is capable of providing the axial displacement requirements of the screw shaft and can also be monitored by vibration analysis techniques. This is in contrast to conventional plain bearings.

More specifically, a CARB (TM) or toroidal bearing is a self-aligning rolling element bearing comprising an inner and outer raceway and a row of rollers arranged between and contacting the raceways, the axes of the raceways being substantially parallel to the axes of the raceways when the latter are coinciding, the rollers and raceways having curved longitudinal section profiles in which the curve radii are substantially greater than the greatest distance between the central axis of the bearing and the surface of the raceways and being obstructed by axial limitations at the raceways for permitting relative inclination and axial displacability being limited only by the rollers being squeezed radially between the raceways.

The self-aligning seal protects the bearing over long periods of operation in hostile environments. It preferably has a dynamic self-aligning capability and is preferably capable of providing the axial displacement requirements of the screw shaft. The seal is preferably shielded from the medium to be pumped by means of a suitable shield. A mechanical self-aligning face-to-face seal prevents ingress of contaminants and leakage of lubricating oil from within the cartridge. The seal is preferably able to accommodate both angular and axial displacement.

The seal is preferably a mechanical seal having both dynamic angular misalignment and axial displacement capabilities. Seals of this type are known and work on the principle that when two parts have mating surfaces that are lapped so that they are flat to a very high tolerance are held together under spring pressure, they form a homogeneous seal. One of these parts is held stationary within the housing, the other rotates with the shaft. The spring is preferably designed so that the pressure between the sealing surfaces changes very little with any axial displacement caused by thermal changes. The radial clearance between the sealing elements and their carriers may be calculated to enable the desired angular misalignment.

The bearing cartridge is designed to support the self- aligning bearing. The bearing cartridge, the bearing and the seal are preferably comprised in a cartridge assembly, which may further optionally comprise one or more of an end cover, flanged stub shaft and transportation screw. The cartridge assembly can be assembled independently from the housing allowing for ease and accuracy of replacement parts. In this manner, when a bearing needs to be replaced, a new bearing cartridge assembly can be fitted relatively simply without resetting the housing. That is, the bearing cartridge is preferably removably located in the housing so as to be replaceable. The cartridge assembly is clamped and supported by the housing. 0 ring seals, for example, - protect the interface between the cartridge outer diameter and the housing bore from corrosion. The cartridge assembly provides accurate location for the bearing and seal.

In a preferred embodiment, the bearing cartridge assembly is a sealed or substantially sealed unit, which preferably comprises a flanged stub shaft carried on a rolling bearing able to dynamically accept both radial and axial misalignment. A face-to-face sealing arrangement is designed to operate reliably under the same misalignment conditions. As stated above, a particularly preferred rolling bearing is a self-aligning toroidal bearing, for example a CARB (TM) bearing (SKF) .

The cartridge may be made of any suitable material, for example steel, preferably stainless steel.

The housing is preferably a split housing so that the bearing cartridge (or bearing cartridge assembly) can be removed or replaced without the need for the housing position to be reset. The housing preferably has a suitably dimensioned base plate to secure an existing mounting plate to make it a permanent fixture within the installation. As entioned above, the housing is preferably a split housing to allow for rapid and easy change of a bearing cartridge unit with no realignment necessary. Thus, in use, the housing is accurately fixed in place, while the bearing cartridge (or cartridge assembly) may be simply replaced when necessary. The housing may be made of any suitable material, for example cast iron.

The screw conveyor advantageously further comprises a lubricant reservoir in fluid communication with the support bearing assembly. The lubricant reservoir is preferably in fluid communication with the bearing cartridge. The pressure of the lubricant inside the support bearing assembly is preferably greater than the external pressure. By maintaining a pressure in the lubrication system that is higher than the pressure of the fluid or semi-solid to be pumped, it is possible to prevent or resist ingress of the fluid or the semi-solid into the support bearing assembly. Thus, for example, a suitable lubricant (eg oil) may be fed to the bearing assembly from a static header tank positioned at such a height so as to supply the lubricant at a substantially constant positive pressure over that of the environment surrounding the support bearing assembly. This ensures that contamination of the bearing cannot occur should the sealing start to fail. The lubricant tank is preferably fitted with a sight gauge to allow visual monitoring of lubricant loss and thereby seal condition. In a preferred embodiment, the lubricant tank is associated with either a low lubricant tank level alarm or remote lubricant level monitoring. The lubricant level alarm can be, for example, a light or siren activated by, for example, a float switch in the tank. This offers the advantage of being able to predict seal failure, which makes planned replacement of the bearing cartridge possible. Thus, low oil level switches in the static oil supply tank can be used detect oil loss, thereby give an indication of seal condition. A sight glass, for example, enables a simple check of the oil level to be carried out. Instead of or in combination with a lubricant reservoir positioned at a height above the support bearing assembly, a pump may be used to create the positive pressure difference required.

The screw conveyor advantageously further comprises a condition monitoring sensor, which is preferably remote, and which will typically be linked to a condition monitoring system, either wireless or wired. The sensor may be associated with one or more of the housing, the bearing cartridge (or cartridge assembly) , the bearing and/or end cover. The condition monitoring system monitors the condition of the bearing and offers the advantage of being able to predict eventual bearing failure, which makes planned replacement of the cartridge possible, thus eliminating breakdown due to bearing failure. This is a significant improvement over the known systems where monitoring the condition of plain bearings and seals is not possible under normal operating conditions. Condition monitoring of existing support bearing assemblies incorporating plain bearings is restricted to static displacement measurements indicating wear in the bearing and/or journal requiring physical access. In contrast, in the present invention vibration based condition monitoring can be used and vibration analysis can be relied on to enable the bearing condition to be assessed and the life of the bearing to be predicted remotely. Monitoring of the condition of the bearing assembly during operation is preferably a continuous process. Condition monitoring in the form of vibration analysis of the self-aligning bearing (for example rolling element bearings such as the CARB (TM) bearing) via a suitably arranged transducer and wiring can make the signal available at any appropriate location.

The present invention also provides a support bearing assembly for a screw conveyor for pumping a fluid or moving a semi-solid. The support bearing assembly comprises a housing and a bearing. The bearing is fitted to the housing, and the bearing assembly is designed to receive a screw conveyor. As mentioned above, the bearing is preferably a toroidal rolling bearing. It has been found that a toroidal bearing is advantageous because it caters for misalignment in both axial and angular directions. A toroidal bearing also allows for thermal expansion of the screw shaft, or conveyor shaft, and such a bearing can also adjust axially to compensate for seal wear. Such a bearing arrangement has been found to be particularly suited to screw conveyors. The support bearing assembly preferably further comprises a bearing cartridge, where the bearing cartridge is mounted to the housing, and the bearing is mounted to the bearing cartridge. This offers the advantage that when a bearing needs to be replaced, a new bearing cartridge can be fitted easily without resetting the housing. That is, the bearing cartridge is preferably removably located in the housing so as to be replaceable. The support bearing assembly may further comprise a flange unit, which is mounted to the bearing and is for receiving the screw conveyor. The support bearing assembly may further comprise an end cover fixed to at least one of the housing, the bearing cartridge and the bearing. This offers the advantage of providing a closed space in the support bearing assembly in conjunction with sealing.

Preferably, the support bearing assembly is as herein described and therefore comprises a housing, a bearing cartridge, a seal and a bearing, wherein: the bearing cartridge is mounted in the housing; the bearing is mounted in the bearing cartridge; the bearing is connectable to a screw conveyor; the seal is a self-aligning seal; and the bearing is a self-aligning bearing.

The discussion of the preferred features of the screw conveyor according to the present invention is equally applicable to the support bearing assembly according ton the present invention.

The support bearing assembly according to the present invention tolerates radial and angular misalignment when static and/or under dynamic operating conditions. This is in contrast to conventional support bearing units, which feature plain or solid support bearings. The stresses induced during operation of such conventional units may result in unpredictable bearing performance. These alignment difficulties present themselves each time a unit is changed. The present invention will now be described further, by^¬ way of example, with reference to the accompanying drawings in which:

Figure 1 is an embodiment of a support bearing assembly for use in the present invention;

Figure 2 is another embodiment of the support bearing for use in the present invention;

Figure 3 is a support bearing and screw pump according to the present invention, together with a condition- monitoring system and a lubrication system;

Figure 4 is a support bearing assembly combined with a transport screw for use in the present invention; and

Figures 5 and 6 show a support bearing assembly with a split housing for use in the present invention.

Figure 1 shows a lower end pump support bearing assembly 1 according to the present invention, which is for use with a screw pump or conveyor 3 for elevating a fluid or semi-solid. In this embodiment, pumping of the fluid is intended to be in a direction comprising both vertical and horizontal components. The assembly 1 comprises a housing 7 and a bearing cartridge assembly 9, which includes a self aligning bearing 11.

The housing 7 is a permanently fixed cast iron housing, which is split along the horizontal axis of the bore. The housing 7 clamps and supports the cartridge assembly 9 in an accurately machined bore. The split along horizontal axis of the bore of the housing 7 allows for easy removal and change of the cartridge assembly 9. The housing 7 is designed to interface with the support plate of the bearing assembly it replaced so that important interface dimensions such as the shaft centre height are left unchanged.

To facilitate fixing of the assembly 1, a base plate 23 is fitted to the housing 7. Although the base plate 23 is shown as a separate base plate, it may be an integral part of housing 7.

The cartridge assembly 9 is accurately located in the housing 7. The cartridge assembly 9 incorporates an end cover 25. This design allows for very simple and speedy replacement of the cartridge assembly 9 should it be necessary. The end cover 25 locates into the cartridge assembly 9 by means of a spigot and facilitates axial location of self-aligning rolling element bearing. The end cover 25 also provides essentially water-tight sealing in conjunction with an 0 ring. In the embodiment shown in Figure 1, the end cover 25 is attached to the cartridge assembly 9. It will be appreciated, however, that it may be attached to the housing 7.

In a preferred embodiment, the cartridge assembly 9 comprises a cartridge, a self-aligning bearing 11, a self- aligning mechanical seal 27, an end cover 25, a flanged stub shaft 15 and transportation screw 39 (see Figures 2 and 4) . The cartridge assembly 9 may be assembled separately from the housing 7 allowing for ease and accuracy of replacement. The cartridge assembly 9 is clamped and supported by the housing 7. 0 ring seals protect the interface between the cartridge 9 outer diameter and the housing 7 bore from corrosion. The cartridge assembly 9 provides accurate location for the self-aligning bearing 11 and self-aligning mechanical face seal 27.

The self-aligning bearing 11 is preferably a toroidal bearing, which is capable of compensating for misalignment in both axial and angular directions. The bearing 11 is located in the cartridge assembly 9 and retained therein by an integral shoulder in the bearing cartridge 9, the end cover 25, and by snap rings 29. The bearing 11 receives a shaft 13, which is attached, in use, to a flanged stub shaft 15, which, in turn, is connected to the screw pump or conveyor 3 (see Figure 3) . The flanged stub shaft 15 provides the interface between the support bearing assembly 1 and the screw pump or conveyor 3. In Figure 1 it is shown as a two-piece unit, but equally could be a one-piece unit. An oil injection port (not shown) allows for easy bearing removal .

In a preferred embodiment, the self-aligning toroidal bearing 11 is a roller bearing comprising inner and outer raceways 17, 19. A row of rollers 21 is positioned between and contacting the raceways 17, 19. The axes of the rollers 21 are substantially parallel to the axes of the raceways 17, 19 when the latter are coinciding. The rollers 21 and the raceways 17, 19 have curved longitudinal section profiles in which the curve radii are substantially greater than the greatest distance between the central axis of the bearing 11 and the surfaces of the raceways. Furthermore, the rollers 21 are axially movable between the raceways 17, 19 without being obstructed by axial limitations at the raceways 17, 19 for permitting relative inclination and axial displaceability being limited only by the rollers 21 being squeezed radially between the raceways 17, 19.

A particularly preferred toroidal bearing is a CARB (TM) bearing (available from SKF) , which as well as having a dynamic self-aligning capability, is capable of providing the axial displacement requirements of the screw shaft and can be monitored by vibration analysis techniques (this is in contrast to a plain bearing) . An important feature of the CARB (TM) bearing is that it is able to operate reliably with both static and dynamic misalignment present.

A mechanical self-aligning seal 27 is mounted on at least one of the flange unit 15, the housing 7 and the bearing cartridge 9. In the embodiment shown in Figure 1, the seal 27 is mounted on the shaft 13 in an intermediate position between the flange 15 and the cartridge assembly 9. The bearing 11 is fixed to the shaft 13 by means of, for example, one or more snap rings, a locknut, and/or a bearing retaining plate.

The seal 27 is a mechanical self-aligning face-to-face seal that protects the bearing 11 from ingress of contaminants and leakage of lubricating oil from within the cartridge assembly 9. Like the bearing 11, the seal 27 is self-aligning and is preferably capable of accommodating both axial and radial misalignment. The seal 27 is designed to provide protection to the bearing 11 over long periods of operation in hostile environments. The seal 27 preferably has dynamic self-aligning capability and is capable of providing the axial displacement requirements of the screw shaft. In the Figure, the seal 27 is shown open to allow for natural flushing of clogging media, but it will be appreciated that the seal could be closed to prevent ingress of the clogging media.

With reference to Figure 3, the assembly 1 further comprises a lubrication system comprising a header tank 31 containing a suitable lubricant (eg oil) at a suitable height to ensure that the internal oil pressure is greater (preferably >= 1 bar) than the external fluid pressure (i.e. the pressure in the media to be pumped) , thereby maintaining a pressure limiting entry into the assembly 1. Alternatively (or in combination) , the positive pressure may be achieved using a pump. A conduit 33 fluidly connects the header tank 31 to the bearing 11. An air bleed valve (not shown) enables the bearing cartridge 9 to be fully flooded. The oil inlet is connected when the support bearing assembly 1 has been aligned and fitted and the transport screw 39 removed. A sight glass (not shown) on the header tank has a low oil level point marked. A low level oil alarm (not shown) is fitted to further give warning of low oil level.

With reference to Figure 2, the bearing cartridge 9 has been reversed allowing easy removal away from the flange unit 15. A spacer has been fitted behind the bearing cartridge shoulder. The spacer gives additional adjustment if extra is required, for example if the screw is replaced.

With reference to Figure 3, the assembly 1 further comprises a condition monitoring sensor 35, which is connected to a condition monitoring system 37, and is located in or on one or more of the housing 7, the bearing cartridge 9 and/or the self aligning bearing 11. The condition-monitoring sensor 35 is of a type designed for sensing vibrations. The condition monitoring system 37 is designed to generate a remote alarm signal when the condition-monitoring sensor 35 senses a value exceeding a pre-set value. The condition-monitoring sensor 35 may comprise, for example, an accelerometer.

The condition of the bearing 11 is monitored using vibration based analysis. A transducer is mounted on the housing 7, the resulting signal is transmitted to a remote control box where it can be analysed enabling bearing condition to be assessed and life to be predicted. High and low level alarms are present in the control box, which can be set to warn of any impending bearing failure.

With reference to Figure 4, an embodiment of the support bearing assembly combined with a transport screw 39 is shown. The transport screw 39 is used to locate the bearing 11 at the mid-point of its range of axial movement for transportation purposes. It also prevents excessive and therefore damaging angular misalignment of the bearing and seal during transportation.

In Figures 5 and 6, the split housing 7 of the support bearing assembly and flange unit 15 are more clearly shown.

The bearing cartridge assembly may be assembled and adjusted off site under clean and controlled workshop conditions and may form a standard and easily replaced sealed unit, requiring no complicated and costly installation work on site. Initial installation includes the accurate positioning of the split housing replacing the existing housing. Additionally the installation of the lubrication tank and bearing monitoring is undertaken. Future installation will only involve the replacement of the cartridge assembly, giving very significant savings in time and manpower.

The present invention provides a screw conveyor (and a support bearing assembly therefor) that is reliable, is^" able to compensate for misalignment, and is predictable in its life expectancy. In addition, simple and quick replacement of the bearing is now possible. The use of self-aligning rolling bearings (eg SKF CARB (TM) bearings) with the capability of dynamic misalignment and axial displacement reduce wear and energy consumption. Seal performance and condition can also be constantly monitored. The corollary of which is reduced maintenance costs and reduced possibility of environmental damage.

Claims

Claims :

1. A screw conveyor for pumping a fluid or moving a semi-solid, said conveyor comprising a shaft and a lower support bearing assembly, said lower support bearing assembly comprising a housing, a bearing cartridge, a bearing and a seal, wherein: the bearing cartridge is mounted to the housing; the bearing is mounted to the cartridge; the bearing is connected to the shaft; the seal is a self-aligning seal; and the bearing is a self-aligning bearing.

2. A screw conveyor as claimed in claim 1, wherein the self-aligning bearing is a self-aligning toroidal roller bearing, a self-aligning spherical roller bearing or a self- aligning ball bearing.

3. A screw conveyor as claimed in claim 1 or claim 2, wherein the shaft is a flanged stub shaft which is connected to the screw conveyor.

4. A screw conveyor as claimed in any one of the preceding claims, wherein the bearing cartridge, the bearing and the seal are comprised in a bearing cartridge assembly.

5. A screw conveyor as claimed in any one of the preceding claims, wherein the housing is split.

6. A screw conveyor as claimed in any one of the preceding claims further comprising a lubrication reservoir in fluid communication with the support bearing assembly, wherein, in use, the pressure of lubricant inside the support bearing assembly is greater than the external pressure.

7. A screw conveyor as claimed in claim 6, wherein the lubrication reservoir is in fluid communication with the bearing cartridge.

8. A screw conveyor as claimed in any one of the preceding claims further comprising a condition monitoring sensor associated with a condition monitoring system and mounted to one or more of the housing, the bearing cartridge and/or the bearing.

9. A screw conveyor as claimed in any one of the preceding claims, wherein the bearing cartridge is removably located in the housing so as to be replaceable.

10. A support bearing assembly for a screw conveyor for pumping a fluid or moving a semi-solid, the support bearing assembly comprising a housing, a bearing cartridge, a seal and a bearing, wherein: the bearing cartridge is mounted to the housing; the bearing is mounted to the bearing cartridge; the bearing is connectable to a screw conveyor; the seal is a self-aligning seal; and the bearing is a self-aligning bearing.

11. A support bearing assembly as claimed in claim 10, wherein the self-aligning bearing is a self-aligning toroidal roller bearing, a self-aligning spherical roller bearing or a self-aligning ball bearing.

12. A support bearing assembly as claimed in claim 10 or claim 11, wherein the bearing cartridge, the bearing and the seal are comprised in a bearing cartridge assembly.

13. A support bearing assembly as claimed in any one of claims 10 to 12, wherein the housing is split.

14. A support bearing assembly as claimed in any one of claims 10 to 13, further comprising a lubrication reservoir in fluid communication with the support bearing assembly, wherein, in use, the pressure of lubricant inside the support bearing assembly is greater than the external pressure.

15. A support bearing assembly as claimed in claim 14, wherein the lubrication reservoir is in fluid communication with the bearing cartridge.

16. A support bearing assembly as claimed in any one of claims 10 to 15, further comprising a condition monitoring sensor associated with a condition monitoring system and mounted to one or more of the housing, the bearing cartridge and/or the bearing.

17. A support bearing assembly as claimed in any one of claims 10 to lό", wherein the bearing cartridge is removably located in the housing so as to be replaceable.