GB2250698A

GB2250698A - Rotary atomizer

Info

Publication number: GB2250698A
Application number: GB9200795A
Authority: GB
Inventors: Kui-Chiu Kwok; Bernard Joseph Marchand
Original assignee: Graco Inc
Current assignee: Graco Inc
Priority date: 1988-09-23
Filing date: 1992-01-15
Publication date: 1992-06-17
Anticipated expiration: 2012-01-15
Also published as: GB9200795D0; GB2250698B

Abstract

A rotary atomizer for applying coating materials and comprising a shroud (42); a rotary atomizing cup (20) extending from said shroud (42); and means (40) for directing a stream of relatively high volume low velocity air forwardly over said shroud (42), said air directing means (40) being spaced rearwardly from said cup (20) by a distance at least equal to the diameter of said cup (20). <IMAGE>

Description

"ROTARY ATOMIZER" This invention concerns a rotary atomizer.

Rotary atomizers including those with centre feed designs are well known in the painting and coating arts.

Such rotary atomizers, however, have in the past tended to suffer from the problem of an area in the centre of a cup or bell thereof allowing dried paint to accumulate thereon in such a way that the paint thereafter flakes off and lands on the work surface thereby harming the finish on the work surface.

Rotary atomizers have also tended to suffer from the problem of incomplete coating of the atomizing surface in the process of feeding paint from a feed chamber to the surface of the bell. In such devices, the paint is typically fed through a number of apertures but the material between the apertures tends to cause streaks or dry areas on the bell surface during operation. Rotary atomizers also tended to be fairly bulky because of the precision bearings required and the high speeds at which they were run.

Such atomizers have further suffered from the problem of wrapback, that is, paint tends to become deposited on the exterior surfaces of a turbine and drive housing of the atomizer.

According to the present invention, there is provided a rotary atomizer comprising a cup having a forwardly facing surface with a discharge edge; a distributor which is mounted within the cup and is annularly spaced from the inner surface thereof; a diffuser which is mounted in an axial bore in the distributor; and locating means which coaxially locate the distributor and diffuser within the cup.

The diffuser may have an axial passage therein which communicates with at least one radial passage, the or each radial passage communicating with a portion of the axial bore in the distributor.

Preferably, the forward end of the diffuser is spaced rearwardly from the forward end of the distributor.

The axial passage in the diffuser may taper inwardly from its rear end to its front end.

The locating means may comprise a plurality of pins.

The distributor is preferably connected to the cup only by the said pins, the latter extending across the annular space between the distributor and the cup so as to block the said annular space only to a very minor extent.

Preferably, a discharge end of a fixed feed tube is positioned axially of and to the rear of the diffuser.

Preferably, there is a turbine which in operation produces a flow of air over the exterior of the cup.

The turbine may be carried by a rotary shaft which is rotatably mounted on the fixed feed tube by way of bearing means.

There may be releasable retaining means for releasably retaining the fixed feed tube in an housing of the atomizer.

The releasable retaining means may comprise at least one set screw which is threaded in the housing and which engages a groove in the fixed feed tube.

Air directing means may be provided for directing pressurized air onto turbine blades of the said turbine to effect rotation of the latter.

The turbine blades may be rotationally symmetrical, there being two air directing means which direct the pressurized air onto the turbine blades so as respectively to effect rotation of the turbine in opposite angular directions.

The cup may extend from a shroud, the turbine being arranged to direct the said flow of air through a space between the shroud and the cup.

Low velocity air directing means may be provided for directing a flow of low velocity air over the shroud.

The low velocity air directing means may be disposed rearwardly of the cup and spaced from the latter by a distance at least equal to the maximum diameter of the cup.

Preferably, the flow of low velocity air is in operation produced by the turbine.

There may be means for supplying a fluid to the diffuser and thus to the cup, the said means comprising a manifold secured to a housing of the atomizer, the manifold having passages therein which respectively communicate with passages in the housing.

Preferably, intercommunicating passages in the manifold and housing are sealed to each other by an O- ring which is located in a countersunk portion of one of the passages and engages a retaining means mounted in the other passage.

A trigger valve may be provided which is operable to cause a supply of air and/or solvent to be supplied to the fixed feed tube.

A check valve may be mounted between a source of solvent and/or pressurized air and the fixed feed tube downstream of the trigger valve.

In the preferred embodiment, the rotary atomizer can eliminate dried paint build-up on its front rotating surface, and is light, compact and easily manufactured as well as easily serviced. It is further highly resistant to paint build-up on its exterior surfaces.

In the preferred embodiment, the rotary atomizer cup comprises a main portion of a cup or bell with a large central bore in which is suspended a distributor assembly. A combination of the cup and distributor assembly forms therebehind a feed chamber which primarily feeds paint through an annular gap between the distributor assembly and the main cup to the front discharge surface of the cup. Moreover, the distributor assembly has located in the middle thereof, a small diffuser which allows a very small proportion of the total paint fed through a centre feed tube to pass to the middle of the front of the distributor.By precisely sizing the passages, a small cup-like area on the front of the distributor/diffuser assembly may be kept full of liquid paint, and that paint then flows off over the front surface of the distributor such that all of the front surface of any portion of the bell can be covered with wet paint. By suspending the distributor assembly on three small pins, total coverage of the front discharge surface of the cup with paint will be very uniform thereby providing superior atomization and finishing. This construction also makes for easy and quick flushing during colour changing.

The preferred embodiment also has a combination centre feed tube and stationary shaft which extends forwardly from a housing base and is held in place by means of three set screws which engage a groove in the stationary shaft. A rotating shaft with a set of bearings is located over the stationary centre shaft and has affixed to the front end thereof the rotary cup or bell atomizing element. Use of this construction allows the turbine assembly to be easily removed and serviced without the need to disconnect any hoses or the like thus minimizing downtime. The discharge end of the centre feed tube extends into a feed chamber in the rear end of the cup. The circumference of the cup extends forwardly and outwardly thereby providing a consistent outward flow of paint from the feed chamber to the forward surface of the bell which in turn leads to the discharge edge.A distributor assembly is suspended over the front end of the feed chamber by means of (preferably) three pins.

In the preferred embodiment, moreover, the distributor has located in the middle thereof a diffuser assembly which has a first axial bore which tapers forwardly and which receives a small proportion of the total paint fed through the feed tube into the feed chamber. The tapered axial bore allows viscous paint to flow through with less restriction. The end of the tapered axial bore connects with a pair of radially extending cross bores which feed paint into a small depression in the front of the distributor. The sizing of these passages and the distributor is such that a small yet constant flow of paint is provided through the diffuser and into the depression and thence outwardly over the front face of the distributor thereby preventing dry spots. The mounting of the distributor with three pins provides a very even distribution of paint over the front surface of the cup and thereby prevents dead spots which do not become evenly covered with paint.

The preferred embodiment also comprises a turbine wheel having rotationally symmetric teeth not unlike gear teeth, and jets are located in the housing which may rotate the assembly in either direction and may also serve to act as a brake for quick stopping. The assembly may be easily and quickly disassembled and repaired by first removing the cup and then the rotating shaft and turbine blade assembly. This allows easy field servicing especially as the centre feed tube may also be easily removed.

The housing may be attached to a manifold through utilization of O-rings which are located in conical counterbores at the junction of passages between the manifold and the housing. This provision allows leak-proof connections while at the same time accommodating some lack of precision in the location of such passage holes.

The invention is illustrated, merely by way of example, in the accompanying drawings, in which: Figure 1 is a cross-sectional view of a rotary atomizer according to the present invention; Figure 2 is a detailed cross-section of a distributor/diffuser assembly forming part of the atomizer of Figure 1; Figure 3 is a broken-away cross-section of a turbine wheel forming part of the atomizer of Figure 1; and Figure 4 shows a detail of an O-ring connection used to connect a housing of the atomizer to a manifold.

A rotary atomizer 10 according to the present invention is shown in general in Figure 1 and is comprised of a housing 12 having a fixed centre feed tube and shaft 14-located therein. The shaft 14 has a passage or central bore 16for passing paint and the like and is provided with a discharge end 18 which extends into a feed chamber 45 (Figure 2). The shaft 14 is releasably held in place in the housing 12 by three set screws 15 which engage a groove 14a at the base of the shaft 14. A rotary atomizing cup 20 is located on the forward end of a rotary shaft 22 which rotates about the fixed shaft 14 on ball or air bearings 24. For assembly, cleaning, changing and the like, the fixed shaft 14, the rotary shaft 22 and the bearings 24 may be removed merely by loosening the three set scress 15. The cup 20 is threaded onto the end of the rotary shaft 22.

A turbine wheel 26, which drives the atomizing cup 20, is affixed to the outside of the rotary shaft 22 and is provided with a plurality of teeth or turbine blades 28 which are shown in detail in Figure 3. The turbine wheel 26 is arranged in operation to produce a flow of air over the exterior of the cup 20. It can be seen that teeth 28 are rotationally symmetrical, that is, air blown in (from asource not shown) through an air passage 30 or an air passage 32 will serve to rotate the atomizer in opposite directions respectively. Changing the air passage 30, 32 through which the air is directed may serve to brake the spinning atomizing cup 20. This construction is also easily and inexpensively manufactured.

A portion of the air passes out from the turbine blades 28 toward the front of the atomizer through chambers 31 and 33 and then out through an annular shaping air aperture 34 located adjacent the cup 20. Another portion of the air passes out through chambers 36 and thence through a passage designated by arrow 38 and out through a plurality of apertures 40 located around the circumference of a shroud 42.

The exhaust from the turbine wheel 26 may be augmented by further pressurized air and a sufficient volume of air is provided to form a high volume, low velocity stream of air which will carry all of the paint emitted off a discharge edge 20a of the cup 20 toward a workpiece (not shown) being painted. It is worth noting that the apertures 40 are spaced backwardly from the cup 20 by a distance at least equal to the maximum diameter of the cup 20. While the use of shaping air from passages such as the air aperture 34 as discussed above is well known in the art, such shaping air is ineffective in preventing wrapback.By spacing the jets from the apertures 40 rearwardly of the cup 20 and by providing a relatively high volume of gentle, low velocity air, a much more efficient job is effected than would otherwise be the case of carrying all of the paint or coating material to the workpiece surface thus preventing wrapback or deposition of paint onto the shroud 42 of the rotary atomizer 10. By utilizing turbine exhaust to help pattern flow, total air usage is minimized and a very efficient operation results.

The construction of the central portion 20b of the atomizing cup 20 is shown in more detail in Figure 2.

The central portion 20b is typically machined of a plastics material and is press fitted into the main portion 20c of the rotary atomizing cup 20 as shown in Figure 1. The main portion 20d of the cup 20 is typically machined of a metallic material.

A distributor member 44 is located coaxially in the middle of the central portion 20b and is suspended there by three pins 46. The outer surface of the distributor member 44 is well spaced from the adjacent part of the inner surface 50 of the central portion 20b.

An annular passage 48 between the distributor member 44 and the central portion 20b is substantial enough so that a very large proportion (greater than 90%) of the paint which flows out of the discharge end 18 of the passage 16 and into the feed chamber 45 runs through the annular passage 48. Because of the minimal obstruction to 360 degree passage of paint through the annular passage 48, the pins 46 present a minimal obstruction and in fact the paint closes back together after passing around those pins.

Note that the inner surface or central bore 50 of the central portion 20b is tapered upwardly and outwardly towards the forward end of the cup 20 such that the centrifugal force generated by the spinning of the cup 20 causes the paint to run upwardly and outwardly to a forwardly-facing front surface 20c of the atomizing cup 20 whereupon it runs outwardly to the discharge edge 20a.

A diffuser member 52 is press fitted into the centre or axial bore 55 of the distributor member 44.

The distributor member 44 is provided with an enlarged upper portion 54 of the axial bore 55 which forms a depression or chamber 56. The forward end of the diffuser member 52 is spaced rearwardly from the forward end of the distributor member 44. The diffuser member 52 is provided with a lower tapered axial bore or passage 58 which tapers upwardly and inwardly from its rear end to its front end along the axis of the diffuser member 52 whereupon it terminates in two cross drilled radial passages 60 which lead into the chamber 56. It can be noted that by careful sizing pf the bore 58 and the passages 60, the desired proportion of paint coming out of the discharge end 18 and into the feed chamber 45 will pass into the chamber 56 and will substantially fill the chamber 56 forming a meniscus as shown by a dotted line 62. As best shown in Figure 2, the discharge end 18 is positioned axially of and to the rear of the diffuser member 52. As paint runs out of the chamber 56, it will flow over the top surface 44a of the distributor member 44 and thence outwardly over the front surface 20c of the rotary atomizer cup 20. By utilizing this construction, all parts which are at all weathered by paint on the front of the atomizing assembly are continually wetted such that no paint is allowed to dry and thence flake off.

The two cross drilled radial passages 60, which result in four outlets into the chamber 56,are, in the preferred embodiment, of a diameter of 0.84 mm. With a typical paint, this dimension is sufficient to pass enough paint to keep the front surfaces wetted and to provide the meniscus 62.

As shown particularly in Figure 4, the housing 12 is attached to a paint or other fluid manifold 64 for the supply of paint or other fluid to the diffuser 52 and thus to the cup 20. The junction between the housing 12 and the manifold 64 is not shown in Figure 1 but would be located off the bottom of Figure 1 where mounting of the manifold 64 would take place. A number of passages 66 (only one is shown for the sake of clarity) are located in the manifold 64, each passage 66 communicating with a passage 70, having a conically drilled counterbore 68, in the housing 12. An O-ring 72 is located in each counterbore 68 and is retained there by means of a respective retainer 71. Each retainer 71 is press-fitted into the manifold 64 and is provided with a retaining lip 71a which engages the respective 0ring 72.The conical counterbores 68 in combination with the retainers 71 and the O-rings 72 constitute efficient sealing devices and serve to locate the O-rings 72 quickly and easily during assembly without allowing the O-rings 72 to fall out of place. This construction is also quite efficient in accommodating slight inaccuracies in manufacturing concerning the location of the passages 66 and 70 relative to one another.

Figure 1 also shows a trigger valve 74 at the inlet to the passage 16. The trigger valve 74 is a colour valve of conventional construction and may be triggered as desired. A supplementary flushing passage 76 joins the passage 16 just downstream of the trigger valve 74. The supplementary flushing passage 76 allows air and/or solvent from a source thereof (not shown) to be directed through the rotary atomizer for flushing at the same time as a line leading to the trigger valve 74.

This construction allows for quick and efficient colour changing in a production line environment. A check valve can also be installed in the passage 76, before the passage 76 joins the passage 16. This prevents paint from being pushed back to the upstream side of the passage 76.

Claims

1. A rotary atomizer for applying coating materials and comprising a shroud; a rotary atomizing cup extending from said shroud; and means for directing a stream of relatively high volume low velocity air forwardly over said shroud, said air directing means being spaced rearwardly from said cup by a distance at least equal to the diameter of said cup.

2. A rotary atomizer as claimed in claim 1 -in which the cup is secured to a rotary shaft so as to be driven -thereby.

3. A rotary atomizer as claimed in claim 2 in which a distributor is mounted within the cup, and a diffuser is mounted in an axial bore in the distributor.

4. A rotary atomizer as claimed in claim 3 in which the diffuser has an axial passage therein which communicates with at least one radial passage, the or each radial passage communicating with a portion of the axial bore in the distributor.

5. A rotary atomizer as claimed in claim 2 in which the forward end of the diffuser is spaced rearwardly from the forward end of the distributor.

6. A rotary atomizer as claimed in claim 4 or 5 in which the axial passage in the diffuser tapers inwardly from its rear end to its front end.

7. A rotary atomizer as claimed in any of claims 3-6 in which a discharge end of a fixed feed tube is positioned axially of and to the rear of the diffuser.

8. A rotary atomizer as claimed in claim 7 in which there are releasable retaining means for releasably retaining the fixed feed tube in an housing of the atomizer.

9. A rotary atomizer as claimed in claim 8 in which the releasable retaining means comprises at least one set screw which is threaded in the housing and which engages a groove in the fixed feed tube.

10. A rotary atomizer as claimed in any preceding claim in which the flow of low velocity air is in operation produced by a turbine.

11. A rotary atomizer as claimed in claim 3 or in any claim appendant thereto in which there are means for supplying a fluid to the diffuser and thus to the cup, the said means comprising a manifold secured to a housing of the atomizer, the manifold having passages therein which respectively communicate with passages in the housing.

12. A rotary atomizer as claimed in claim 11 in which intercommunicating passages in the manifold and housing are sealed to each other by an O-ring which is located in a countersunk portion of one of the passages and engages a retaining means mounted in the other passage.

13. A rotary atomizer as claimed in claim 7 or in any claim appendant thereto in which a trigger valve is provided which is operable to cause a supply of air and/or solvent to be supplied to the fixed feed tube.

14. A rotary atomizer as claimed in claim 13 in which a check valve is mounted between a source of solvent and/or pressurized air and the fixed feed tube downstream of the trigger valve.

15. A rotary atomizer for applying coating materials and comprising: a shroud; a rotary atomizing cup extending from said shroud; and means for directing a stream of relatively high volume low velocity air forwardly over said shroud, said air directing means being spaced rearwardly from said atomizing member a distance at least equal to the diameter of said atomizing member.