Patent GB-917832-A (GB)
IPC: H01J9/22
Abstract: 917,832. Coating electric discharge tubes internally. GENERAL ELECTRIC CO. Ltd. April 27, 1961 [April 29, 1960], No. 15135/60. Class 39 (2). [Also in Groups III and XL (a)] Apparatus for coating tubes internally with a layer of particles of finely divided solid material, especially for applying a layer of powdered phosphor to the envelopes of low pressure mercury vapour discharge tubes, comprises a conveyer having a plurality of holders for carry - ing the tubes with their axes vertical from a loading to an unloading position, a trough located beneath the conveyer along at least part of the path of the travel of the tubes between the loading and unloading positions and designed to contain a suspension of the material in a carrier liquid, means for introducing the lower end of the envelopes in turn into the coating suspension in the trough, means for connecting the top of each tube in turn to a suction applying means through a control valve while the lower end of the tube is held below the level of the coating suspension in the trough for causing the suspension to be drawn up into the tube, a detecting means associated with each holder to effect the closure of the respective control valve when the suspension reaches a predetermined height in the tube, means for admitting air at a controlled rate into the tube following the closure of the control valve for causing the level of the suspension in the tube to fall leaving a coating on the tube wall and means for withdrawing the tube from the coating suspension in the trough. In the form shown, tubes 4 are fed manually or automatically to a series of holders 2 spaced uniformly around the axis of an intermittently rotating turret 1, each holder consisting of a rubber sleeve connector 7 on the lower end of a vertical metal cylinder 8 slidably mounted in a guide 9 in the turret. A metal tube 11 is hermetically sealed by an annulus 12 of insulating material within each of the cylinders 8 and carries a conducting probe 19, a second probe being carried by the tube 8. An annular trough 20 is mounted below the path of the holders 2 and contains a suspension of powdered phosphor in an aqueous solution of a binder, which is fed into the trough from a reservoir 21 escaping by means of an overflow pipe 22 and thence being pumped back into the reservoir 21. The tubes 8 are supported by members 13 at the upper end of which are rollers 14 travelling on a cam rail 15 so that at one indexing position each tube 4 is lowered so that the bottom end enters the suspension, a microswitch 24 operated by a fixed cam 25 at the next indexing position opening an electromagnetic valve 17 to connect the tube 11 to a vacuum pump 16. The valve 17 is held open by a hold-on relay R1. The suspension therefore rises in the tube 4 until it reaches the probes 19, operating a relay R2 to break the circuit of the relay R1 and of the valve 17 which thereby closes. Air then enters the tube at a predetermined rate through an adjustable permanently open valve 31 in the pipe 11 and the level of the suspension in the tube falls. The cam rail 15 then raises the tube, the excess suspension drains out and the envelope is unloaded from the apparatus manually or automatically, dried by the passage of hot air and finally baked to drive off residual moisture and to remove the binder. The operation of the cam rail 15 may be reversed, the tubes 8 being biased upwards by springs until depressed by the action of a cam. Where the suspension is non-conducting, the probes are arranged in one arm of an alternating current bridge so that the change in capacity between the probes when bridged by the suspension brings about closure of the control valve. Alternatively, a photo-electric device may be used. The turret may be continuously rotated at uniform velocity instead of being indexed.