GB2082940A - Rotary atomizer - Google Patents

Abstract

A coating applicator having a high speed rotary atomizer bell, 12, driven by an air turbine, 23, has a governor 28, which controls the speed of the air turbine to provide more uniform atomization throughout a wide range of loads on the atomizer. Compressed air flows through passages 19, 20, 21, the turbine wheel, 23, and, through openings 24 in the turbine wheel to exhaust ports 25. The passages from chamber 20 to the turbine are blocked when the turbine rotation is too fast, by pressure actuated piston governor, 29, operated by centrifugal vent valve 41, 45. <IMAGE>

Description

SPECIFICATION Rotary atomizing coater Background of the Invention This invention relates to coating apparatus and more particularly to an improved rotary atomizing coater for depositing paint and similar material on a work piece.

One type of commercial coating apparatus for paint, and the like, comprises a rotary disc or bell and an air motor for driving the disc or bell at a high speed. Paint delivered to the surface of the rapidly rotating disc or bell is thrown off in small particles through centrifugal force. The surface of the bell is charged to a high voltage relative to the work piece so as to electrostatically charge the paint particles. The atomized charged paint particles are directed at and coat the work piece through a combination of the charge on the paint particles and, preferably, also a surrounding stream of air discharged from the coater.

In one common commercial application of coaters of this type, the coaters are mounted along the path of a conveyor which carries the work pieces past the coaters. The delivery of paint to the rotary atomizing coater is turned on and off through a suitable valve as work pieces enter and leave the coating area so as to minimize waste.

When the paint flow is shut off, the air motor driven bell tends to accelerate to a speed considerably higher than its normal oeprating speed due to the removal of the paint load. The higher rotational speed of the bell produces a finer particle size in the atomized paint. Consequently, the particle size decreases as the paint flow is terminated and the initial paint particle size is smaller than normal when paint flow is first initiated. Adjustments in the paint flow rate also change the load on the atomizing bell and, consequently, affects the speed of the bell which in turn affects the particle size. Higher bell speeds resulting from a lower than normal paint flow rate may result in such a fine particle size that the paint particles dry before they reach the work piece.The fineness of the atomization affects the color shading, the surface appearance, the transfer efficiency or percentage of paint deposited on the work surface, the wrap-around and the edge coverage of the work piece. Optimizing the particle size can maximize the transfer efficiency, give good edge coverage with acceptable paint buildup on the edge of a panel, and control the wrap-around on the back of a panel. The coverage will be adversely affected by changes in the speed of the atomizer bell as the paint flow is triggered on and off.

The air motor used for driving the high speed atomization bell in a rotary atomizer coater is often an air driven turbine. It is known in the prior art that a governor may be built into an air turbine for limiting the maximum speed of the turbine output shaft. Such a governor is illustrated, for example, in United States patent 3,708,240.

However, this and other prior art does not use a governor in an air turbine which drives a rotary atomizer in order to maintain an optimum paint particle size while the load on the atomization bell changes.

Summary of the Invention According to the present invention, a governor is provided for a rotary atomizer driven by an air turbine so as to optimize paint particle size through varying loads. In one preferred embodiment, the air turbine generally comprises a hollow shaft on which a turbine wheel is mounted.

Compressed air is supplied from a suitable source through a rotating seal and the hollow shaft to the turbine wheel for driving the turbine. A valve located within the turbine shaft modulates the air flow to the turbine wheel in response to the turbine speed to maintain a more uniform turbine speed during changes in loading on the rotary atomizer. An atomization bell or disc is mounted on an end of the turbine shaft. Paint is supplied to a chamber within the bell and such paint is discharged from the bell through centrifugal force as the bell is rapidly rotated. The bell is maintained at a high electrical potential so as to electrostatically charge the paint particles discharged from the bell. In addition, air is discharged in a pattern around the bell and towards the work surface so as to control the pattern of the discharged paint particles.Through the use of the governor for controlling the speed of the atomization bell, the uniformity of the paint particles is greatly increased even though the flow rate of the paint is varied.

In a modified embodiment of the invention, the air modulating valve is located in the air supply hose. The speed of the turbine shaft is sensed electrically, magnetically or optically and is supplied to a microprocessor which generates a control signal for modulating the valve to maintain a constant turbine speed.

Accordingly, it is an object of the invention to provide an improved rotary atomizing coated.

Another object of the invention is to provide a rotary atomizing coater driven by an air turbine in which the uniformity of the coating is maintained even though the flow rate of the coating material to the rotary atomizer is increased or decreased.

Other objects and advantages of the invention will become apparent from the following detailed description, with reference being made to the accompanying drawing.

Brief Description of the Drawing The single figure is a side elevational view in partial section of a rotary atomizing coater constructed in accordance with the present invention.

Detailed Description Turning now to the single figure, a rotary atomizing coater 10 is shown constructed in accordance with the present invention. The coater 10 generally comprises an air turbine 11 which drives an atomization bell 12. The air turbine 11 drives the bell 12 at high rotational speeds, for example, in the range of 10,000 r.p.m. to 40,000 r.p.m. or more. The air turbine 11 includes an internal governor for maintaining substantially constant the speed at which the bell 12 is rotated, even though the load on the bell 12 may change due to changes in the paint flow rate or changes in the paint density when, for example, the paint color is changed. The substantially constant speed is a function ofthe turbine air supply pressure.

The air turbine 11 includes a main housing section 13 which has a generally circular cross section and mounts a tubular extension 14. A shaft 1 5 is mounted within the housing section 13 and tubular extension 14 and is supported by a plurality of bearings 16. The shaft 15 has an end 1 7 which mounts the atomization bell 12 and has a hollow end 18 which is coupled through a suitable rotary seal (not shown) to a compressed air hose 19. The hollow shaft end 18 defines an interior chamber 20 which communicates through side openings 21 to the interior 22 of a turbine wheel 23.Compressed air normally flows from the air hose 1 9 through the interior shaft chamber 20, the openings 21 and the turbine wheel chamber 22 to radially directed openings 24 on the turbine wheel 23 and thence the air is vented through rear openings 25 in the main housing section 13. The air flowing through the turbine wheel 23 causes the turbine wheel 23 to rotate the shaft 1 5 at a high rate of speed which is dependent upon the load on the shaft 15, the air flow rate and the air pressure.

The speed of the air turbine 11 is controlled by a governor 28 which is located within the shaft chamber 20 for controlling air flow from the air hose 19 to the turbine wheel chamber 22. The governor 28 includes a piston 29 which slides within the chamber 20 into and out of engagement with a valve seat 30. The piston 29 has a valve surface 31 which, when abutting the valve seat 30, blocks air flow from the air hose 1 9 to the turbine wheel 23. A spring 32 urges the piston 29 against the valve seat 30 to block air flow in the turbine 23. At the same time, the compressed air within the chamber 20 acts upon a face 33 on the piston 29 to move the piston 29 against the spring 32, thereby opening the passageway from the chamber 20 to the turbine wheel interior chamber 22.A hole 34 extends through the piston 29 to allow a restricted flow of the compressed air within the chamber 20 through the piston 29 to a chamber 35 located between the piston 29 and an insert 36. As will be shown below, the chamber 35 may be vented to the atmosphere or may be plugged, depending upon the action of the governor required to maintain a constant shaft speed. When the chamber 35 is vented to the atmosphere, the compressed air in the chamber 20 applies a significantly higher force to the piston face 30 than appears at a face 37 on an opposite end of the piston 29. The difference in force is sufficient to compress the spring 32, thereby opening the air flow passages from the chamber 20 into the turbine wheel interior 22.On the other hand, if the chamber 35 is not vented to the atmosphere, the compressed air will flow through the piston hole 34 to pressurize the chamber 35 at the same pressure as the chamber 20. Since the forces applied by the compressed air on the faces 33 and 37 of the piston 29 are identical, the spring 32 acts to move the piston 29 into contact with the valve seat 30 to block the flow of air from the chamber 20 to the turbine wheel interior 22, thereby stopping the turbine. By modulating the position of the piston 29, the flow of air from the chamber 20 to the turbine wheel interior 22 is modulated to control the speed at which the shaft 1 5 is rotated.

A centrifugally actuated ball valve 40 controis, the air pressure within the chamber 35. A valve seat 41 is formed between a passage 42 in the insert 36 which communicates with the chamber 35 and a passage 43 which is vented to atmosphere through an annular region between the shaft 1 5 and the tubular extension 14. A tubular plug 44 retains a ball 45 adjacent the valve seat 41 while leaving clearance for the ball 45 to move into and out of contact with the seat 41. The ball 45 is located so that at all times its center is positioned to the side of the axis of the shaft 15 toward the seat 41. Consequently, as the shaft 1 5 is rotated at an increasing speed, centrifugal force urges the ball 45 against the seat 41 with an increasing force.

In operation, at slow speeds of the turbine wheel 23 the compressed air from the hose 1 9 is allowed to flow through the piston hole 34, the chamber 35, the passage 42 and the passage 43 and thence is vented to the atmosphere.

Consequently, the chamber 35 will have a pressure near or at atmospheric pressure and the pressure on the piston face 33 will move the piston 29 to allow an unrestricted air flow from the chamber 20 to the turbine wheel interior 22.

As the speed of the shaft 1 5 increases, the ball 45 is forced more tightly against the seat 41 and the pressure within the chamber 35 increases. The air pressure within the chamber 35 acts against the piston face 37 to offset the air pressure acting on the piston face 33. At some increasing turbine speed, the increasing pressure within the chamber 35 along with the pressure of the spring 32 moves the piston 29 to progressively close the air flow passages from the chamber 20 to the turbine wheel interior 22. At this point, the speed of the shaft 1 5 is controlled at a constant level. Thus, when load is removed from atomization bell 12 and the shaft 1 5 tends to accelerate, the piston 29 will move to further close the air flow passage and govern the speed of the shaft 1 5. The governor controlled speed is a function of the turbine air supply pressure.

The end 17 of the shaft 15 extends through and projects from a manifold 48. The atomization bell 12 is mounted on the projecting shaft end 17. A paint hose 49 is connected to deliver paint through a passage 50 in the manifold 48 and thence through an orifice 51 to an annular chamber 52 formed on the back of the atomization bell 21. As the bell 12 rotates at a high speed, the paint within the chamber 52 is thrown radially outwardly and flows along a conical surface 55. The paint continues to flow outwardly along the surface 55 until it is thrown from an outer edge 56 on the bell 12 in the form of small particles. The size of the discharged particles is determined by various factors including the rotational speed of the bell 12, the flow rate of the paint and the properties of the paint.

Air is supplied to the manifold 48 through a fitting (not shown) similar to 49. A passage similar to the passage 50 connects the air supply fitting to an annular chamber 59 located in a manifold face 60. A plurality of openings 61 extend through the manifold face 60 and communicate with the chamber 59. The openings 61 are uniformly spaced around the shaft and cause an air flow with surrounds the bell 12 for directing the paint particles thrown off from the bell 12 towards the work piece.

To prevent paint or solvent from entering the turbine 11 or the bearing 1 6, an annular slinger 62 is positioned on the shaft 1 5. The slinger 62 rotates with the shaft 1 5 and prevents liquid from creeping along the shaft 1 5. The slinger includes a lip 63 which overlaps a fixed annular seal 65. The seal 64 is threaded into the tubular extension 14 and does not rotate with the shaft 1 5. In crosssection, the seal 64 has a cup shaped or hooked lip 65 located radially inward from the slinger lip 63. Flow of any liquid coming into contact with the slinger 63 will be thrown off onto the manifold 48 and will drain through a hole 57. The liquid would have to follow a tortuous path to continue along the shaft 15.A Teflon or polytetrafluoroethylene annular seal 66 has a close running fit to the shaft 1 5. This serves to further restrict any possible fluid flow along the axial length of shaft 1 5. Thus, the slinger 62, the seal 64 and the seal 66 prevent the flow of liquid along the shaft 1 5 into the air turbine 11 and the bearing 16.

In accordance with the present invention, the air turbine which drives the atomization bell for the rotary atomizing coater is governed to maintain a fairly constant speed for the atomization bell throughout varying paint loads on the atomization bell. The governor 28 for regulating the turbine shaft speed has been described as being located within the turbine shaft 1 5. In a modified embodiment of the invention, an air flow control valve may be located external to the coater 10, for example, within the air supply hose 19, rather than within the turbine shaft 1 5. A conventional electrical, magnetic or optical sensor may be connected to generate a signal indicative of the speed of the air turbine shaft. This signal is applied to a microprocessor which in turn generates a signal for modulating the air control valve to regulate the turbine speed.Still another method of achieving an approximately constant rotational speed for the air turbine for uniform atomization with varying paint flow is by measuring the paint flow rate. The paint flow rate then is compared in a look up table with previously measured paint flow rate data versus air flow rate data. The relationship between the paint flow rate, turbine air supply pressure and turbine speed are stored in a microprocessor. The microprocessor selects the proper turbine air supply pressure to produce the desired rotational speed and, accordingly, actuates an air flow control valve.

Various modifications and changes may be made in the above-described preferred embodiments of the invention without departing from the spirit and the scope of the following claims. For example, only an exemplary governor mounted within the air turbine has been described. Other governor designs are recognized in the air turbine art.

Claims (6)

1. A coating applicator comprising'a rotary atomizer mounted on a shaft, an air driven turbine connected to rotate said shaft, means for supplying a controlled flow of coating material to said rotary atomizer, and governor means for maintaining said air driven turbine at a substantially constant speed as the flow of coating material to said rotary atomizer changes.

2. A coating applicator, as set forth in claim 1, wherein said governor means includes valve means for controlling the flow of air through said air driven turbine, and means responsive to the speed of said shaft for positioning said valve means to limit the speed of said shaft.

3. A coating applicator, as set forth in claim 2, wherein said air driven turbine is formed on said shaft and wherein said valve means is located within said shaft.

4. A coating applicator, as set forth in claim 3, wherein said means responsive to the speed of said shaft includes a second valve located within said shaft and having a movable element responsive to an increase in centrifugal force for closing said second valve.

5. A coating applicator comprising a rotary atomizer, means for rotating the atomizer, means for supplying coating material to the atomizer, and means for controlling rotation of the atomizer so as to maintain its speed of rotation substantially constant in spite of variations in the load on the atomizer.

6. A coating applicator substantially as herein particularly described with reference to and as illustrated in the accompanying drawing.