This invention relates to rotary shaft bearings and, in particular, to bearings which are designed for use in environments where their effectiveness is subject to degradation from airborne particles, for example dust, grit or the like.

Bearings designed to enable a shaft to be rotated when mounted in a dusty environment are often fitted with covers, shields or the like in an attempt to prevent ingress of dust into the interior of the bearing. In particular, if the dust is abrasive, any such penetration leads to wear as the rotary portions of the bearing turn against the fixed portions, and this can lead to a loosening of the shaft in the bearing or even failure of components and the bearing itself. The problem is particularly severe in the case of heavy industrial plant designed to process particulate materials. One example of such plant are so- called mixers for treating flue gas dust arising from combustion processes, for example incineration sites. In order to comply with environmental regulations relating to the emission of particulates, it is customary to treat the flue gas in order to capture potentially harmful particulate materials from the flue gas stream. In one well-known process, powdered activated carbon and hydrated lime are agitated within a mixing vessel through which flue gas dust passes. The potentially harmful particulates in the flue gas are incorporated into the circulating activated carbon-hydrated lime mixture so that the gaseous portions of the flue gas can then be filtered from the gas stream before being exhausted to atmosphere.

It is well-known that the mixers need regular maintenance because the effect of the mixture of powdered-activated carbon, hydrated lime and particulates from the flue gas leads to substantial bearing wear where the mixer shafts are journaled in a wall of the mixing chamber. Typically, the OEM- recommended seal life of six months is rarely met, with failure occurring within a few weeks of start-up.

We have now developed a bearing arrangement which has, if properly designed and manufactured, a substantially extended service life and which does not suffer degradation from the particulate materials.

In accordance with a first feature of the present invention, there is provided a bearing structure comprising of a fixed bearing component having a cylindrical post and a flange enabling the cylindrical post to be fixed in place e.g. to a mixing chamber wall on which the bearing is installed. A double- walled ring member adapted for mounting on a rotatable shaft, where the inner portion of the double wall is spaced from the outer portion of the double wall by a fixed distance corresponding to the wall thickness of the cylindrical post of the fixed bearing component, and wherein, within the cylindrical post on the fixed bearing component, there is an annular chamber into which air may be fed under pressure and wherein the interior and exterior walls of the cylindrical post are provided with a plurality of apertures enabling the air fed to the chamber under pressure to flow into the space between the fixed cylindrical post and the rotating member fitted to the shaft.

Preferably, the inner surface of the outer wall portion of the rotating member has one or more helical grooves formed in it to provide a helical passage through which the air may pass to exit between the end of the rotating double walled member and the flange mounting the post to the equipment. This groove runs contrary to the direction of the shaft rotation, encouraging any dust away from the air chamber.

The air under pressure serves to provide a dynamic cushion of air between the radially inner portion of the double wall attached to the rotating shaft, while the air blown from the outer side of the post acts to ensure that any particulates which do penetrate are blown out as the shaft rotates. The use of a dynamic air cushion in bearing constructions is known, for example from the disclosures of gas- or air-bearings in published patent specifications GB1013351 , GB1 146422, GB2231372A and US4595348. The principles of construction of the bearing arrangement described above may be applied to a wide variety of bearings in a variety of different types of equipment and, for any particular purpose, the bearing materials may be chosen to suit the intended application. Likewise, the dimensions and relative dimensions of the parts of the bearing may be varied as desired, and the number and placement of the passages or apertures through which the air under pressure fed to the static portion of the bearing may likewise be varied as required to maintain the differential pressure between the chamber and atmosphere. By way of example, the accompanying drawings show a bearing for use in a mixer for treating flue gas dusts with a mixture of powdered-activated carbon and hydrated lime.

In the accompanying drawings:

Figure 1 is a diagrammatic view of a mixer unit of known type.

Figure 2 is a sectional view through a bearing in accordance with the invention installed in such a mixer and showing the wall of the mixer chamber and the shaft of the mixer agitation members.

Figure 3 is an exploded view of the components making up the bearing shown in Figure 2.

Referring to the drawings: Figure 1 shows a simplified diagram of a typical mixer unit for mixing dusty or particulate materials. The overall structure is denoted A and it consists of a casing in which there are two horizontally arranged shafts with blades on them, denoted B, which are rotated when the mixer is operated by a motor and gear box assembly C. The bladed shafts B are mounted at their ends in bearings attached to walls D. The known problem in mixers of this type is to stop the often highly abrasive material being mixed getting into and damaging the bearings at the end of the shafts B. Very often, because of this, the service life of such a mixer is only a few months before it has to be stopped, taken apart, the bearings replaced (and possibly other components as well) and the mixer re-built before it can be put back into operation.

Figures 2 and 3 show an improved form of bearing for use in mixers or in other analogous situations where bearing life is compromised by abrasive material.

Referring now to Figures 2 and 3, it should be noted that the sectional view shown in Figure 2 is not a diametric section taken in a plane, but rather a section taken in two planes, one angled relative to the other. The diagram on the right-hand side of Figure 2 shows the angled section line as E-E to show the row of holes (50).

Figure 2 shows a bearing in accordance with the invention attached to a wall 1 of a mixer unit, for example of the type shown in Figure 1 . The shaft of, for example, a bladed mixer unit is denoted 2. Its outer end rotates when the mixer is in use in a hole 5 in wall 1 .

The bearing is generally indicated at 3 in Figure 2 and consists of a fixed portion bolted to the wall 1 and a rotary portion which is fixed on to shaft 2. The fixed portion is in the form of a housing 4 made of two semi-cylindrical parts 4A and 4B as shown in the exploded view in Figure 3. The accurate location of the two halves relative to one another is ensured by position pins 7 which fit into holes in an attachment flange 10. Flange 10 has eight counter-bored through-holes 15 which enable the flange 10 to be attached to the inner side of wall 1 by means of a set of bolts 1 1 on Figure 2. Bolts 1 1 are screwed into threaded holes 12 in wall 1 .

Also in flange 10 are eight threaded holes 13 into each of which is threaded a set screw 14. The purpose of set screws 14 is explained below.

Located within the outer cylindrical sleeve 4 is an inner cylindrical sleeve 20 formed of two parts 20A and 20B which are fixed in position relative to one another by positioning pins 22 shown in Figure 3. As can be seen in Figures 2 and 3, the cylindrical sleeve 20 is thicker at its outer ends, each of which carries a groove 24 in which is set an o-ring 23. In the semi-cylindrical portions 20A and 20B located intermediate its ends are six holes 25. These holes connect the space between the outer cylindrical sleeves 4A and 4B and the inner cylindrical sleeves 20A and 20B, forming a chamber denoted 28 in Figure 2. The perforations 25 are open in the concave cylindrical face of the two half sleeves 20A and 20B against which the shaft rotates.

Perforations 25 allow air under pressure to be pumped into the chamber 28 between the fixed and rotary parts of the bearing.

The rotary portion of the bearing consists of two semi-cylindrical sleeves 40 clamped on to the end of shaft 1 by means of bolts 41 . On their inner concave faces, there are two grooves 42 which receive o-rings 43 which are compressed against the exterior surface of shaft 1 when the bolts 41 are tightened up. Bolted to the right-hand end of semi-cylindrical portions 40 as shown in Figures 2 and 3 is an internally-threaded cap 44, which is bolted on by means of four bolts 45. The interior cylindrical surface of cap 44 is threaded, the thread being denoted 46.

As most clearly visible in the detail view at the top left of Figure 3, the semi- cylindrical sleeves 4A and 4B each have two lines of holes 50 running from the inner concave wall to the outer convex wall. The holes do not run radially, but rather at an angle and they serve to direct air under pressure from chamber 28 in the fixed portion of the bearing towards the rotating threaded interior concave surface of cap 44.

The direction of thread 46 is such that when shaft 2 is rotated during normal operation of the mixer fitted with the bearing in accordance with the present invention, it serves to move any particulate materials towards wall 1 . Finally, flange 10 has a port 51 set in it to which a supply of compressed air may be connected which serves to pressurise the interior of the bearing, and, in particular, to provide an air cushion, via holes 25, between the external convex surface of semi-cylindrical sections 40 and the internal concave surfaces of cylindrical sleeve portions 20A and 20B.

During the operation of the bearing, any abrasive material is prevented from entering the bearing by the air under pressure, and even if, for example, because the shaft direction of rotation is changed to free a jam or the like, any abrasive material which might enter between the members 4A and 4B on the one hand and the cap 44 on the other is, as soon as normal operation is commenced, flushed out by the combined effect of the airflow and the screw thread 46.

In order for the bearing to work satisfactorily, it is, of course, vital that the alignment between shaft 2 and the bearing is perfect. This may be ensured by fixing the bearing in place, once it has been located on the end of the shaft 1 , in two stages. In a first stage, nuts 1 1 are loosely fitted into bores 12 and then set screws 14 each adjusted until they are in contact with the interior surface of wall 1 . This compensates for irregularities or deviations from planarity in the contour of the interior surface of wall 1 , which could lead to misalignment. Once all of the set screws 14 are lying against the interior surface of wall 1 , bolts 1 1 are then tightened evenly to hold the bearing in position at a set clearance distance.