US3794593A

US3794593A - Method of electrophoretically covering helically wound heating wires with an insulating layer

Info

Publication number: US3794593A
Application number: US00281345A
Authority: US
Inventors: R Coenen; A Kuiper; M Collaris
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1971-08-18
Filing date: 1972-08-17
Publication date: 1974-02-26
Anticipated expiration: 1991-02-26
Also published as: DE2237318C3; JPS5420827B2; CA977713A; JPS4830363A; FR2149561A1; DE2237318B2; FR2149561B1; DE2237318A1; GB1360169A; NL7111360A

Abstract

A helically wound V-shaped bent reciting wire or bifiller wound heating wire for an indirectly heated cathode is previously dipped in a liquid or connected anaphoretically and then covered in an electrophoresis bath with an insulating layer.

Description

Feb. 26, 1974 R. G. H. COENEN ETAL 3,794,593

- METHOD OF ELECTRCPHORETICALLY COVERING HELICALLY WOUND HEATING WIRES WITH AN INSULATING LAYER Filed Aug. 17, 1972 United States Patent METHOD OF ELECTROPHORETICALLY COVER- ING HELICALLY WOUND HEATING WIRES WITH AN INSULATING LAYER Renrer Gertruda Hubertus Coenen, Adrianus Kuiper, and Maria Antonius Alphonsus Andreas Collaris, Emmasingel, Eindhoven, Netherlands, assignors to US. Philips Corporation, New York, N.Y.

Filed Aug. 17, 1972, Ser. No. 281,345 Claims priority, applicati7olrillggaherlands, Aug. 18, 1971,

Int. Cl. B01k /02; C23b 13/00 US. Cl. 204-181 Claim ABSTRACT OF THE DISCLOSURE A helically wound V-shaped bent heating wire or bifilarwound heating wire for an indirectly heated cathode is previously dipped in a liquid or connected anaphoretically and then covered in an electrophoresis bath with an insulating layer.

The invention relates to a method of electrophoretically covering helically wound heating wires for indirectly heated cathodes with an insulating layer, in particular heating Wires which are bent in the form of a V once or several times or bifilar-wound wires. The invention furthermore relates to a heating wire comprising an insulating layer provided according to said method.

Heating wires for indirectly heated cathodes are usually covered electrophoretically with an insulating layer so that the ends of the helically wound heating wire which is usually bent in the form of a V once or several times or is a bifilar-wound wire, remain free from insulating material. The wire is usually connected to current supply wires by welding with the bare ends.

With the present-day required accuracy it is of importance that the separation between the covered part and the non-covered ends of the heating wire be accurately determined and the separation line be at the same distance from the ends of all the heating wires so as to prevent insulation material from reaching the places of the wire to be welded. It has been found that comparatively large tolerances have to be permitted for the place of said separation since rather large deviations of the desirable place occur.

Experiments have proved that this drawback is caused in that the helically wound wire has capillary spaces so that the suspension destined for electrophoresis is drawnin by capillary action into the ends of the helically wound heating wire not dipped in the suspension.

According to the invention, measures are taken to reduce the influence of the capillary action in the place of separation between covered and non-covered parts. This can be done in a simple manner by filling the spaces of the wound wire with a suitable liquid just prior to dipping the Wire in the electrophoresis bath. Since the capillary spaces are filled by the said liquid, no suspension of insulating material of the electrophoresis bath is drawnin any longer. As a dipping liquid is preferably used the same liquid as that in which the insulating material is suspended.

According to another method, the wound wire may be dipped in the electrophoresis bath and the electrophoresis voltage be applied rapidly as soon as the wire is dipped to the correct depth. The electric field which moves the insulating particles in the suspension to the wire in a direction normal to the wire then prevents the particles from being taken along with the capillary flow parallel to the axis of the wound wire. Moreover, the gaps between the turns of the wire rapidly slit up so that mainly only suspension liquid is drawn above the dipping level by 3,794,593 Patented Feb. 26, 1974 capillary action. If desirable the wire may be kept at an anapheretic potential during dipping the wire in the suspension, after which the voltage is switched in the desired direction for cataphoresis after the wire has been dipped to the correct depth in the electrophoresis bath. During dipping of the wire, the insulating particles are repelled so that substantially only the liquid is drawn in the capillary spaces. Only then is the potential switched.

The invention enables the usual tolerances for the spot of the separation of .covered and uncovered parts to be reduced to A.

The invention will be described in greater detail with reference to the drawing, of which:

FIG. 1 shows diagrammatically an arrangement for carrying out the method according to the invention, while FIGS. 2 and 3 show embodiments of heating wires covered by said method.

Reference numeral 1 in FIG. 1 denotes a shaped heating wire which is wound helically and bent to form a V. The helically wound wire may also be bent back and be a bifilar-wound wire. The wire 1 is clamped in a holder 2. The part of the shaped wire 1 present between the tip of the V and the line AA is to be coated electrophoretically with insulating material. If, as usual, the wire 1 is dipped to the line AA in an electrophoresis bath 4 which is present in a container 3, the suspension 4 will creep up the helically wound wire to slightly above the line AA as a result of the capillary action and reach the ends 9 of the wire, which ends, however, have to remain bare. So during electrophoresis a small quantity of insulating particles will also deposit on the ends 9 which impedes the welding of said ends to current supply conductors.

It has moreover been found that the distance over which the suspension creeps up by capillary action is not constant so that proportionally large tolerances for the place of the separation between the covered and the uncovered parts of the heating wire 9 have to be permitted.

This drawback is considerably reduced by .the various measures which are taken according to the invention in the method of electrophoretically covering such heating wires.

A very suitable measure is that in which the heating wire 1 is dipped in a suitable liquid down to the holder 2, said liquid filling the capillary spaces. Right after this, the wire is dipped in the bath 4. The suspension can now no longer creep up. The dipping liquid is preferably the same liquid as that in which the insulating material of the suspension 4 is suspended.

The suspension 4 preferably consists of aluminium oxide powder which is suspended in methyl alcohol. The liquid being not electrically conductive, covering of the filament is carried out in this case by cataphoresis, the negative terminal of the current source 5 being connectable to the holder 2 by means of a switch 6. The container 3 is connected to a positive potential. It is possible to apply the negative potential to the holder 2 only after the heating wire 2 has been dipped in the suspension 4 down to the line AA. In that case a uniformly thick insulating layer 7 is obtained as is shown in FIG. 2. It is of advantage, however, to give the tip of the V a thickening 8 and to make the thickness of the remaining insulating layer 7 thin. The thickening 8 serves to center the heating member in a cathode support, while the layer 7 does not contact the cathode support. A thin layer 7 has the advantage of being slightly flexible so that the V- shaped parts can be squeezed a little when inserted into a cylindrical support of a cathode without the layer 7 chipping oflf.

In order to obtain such a thickening 8 the cataphoresis potential is already applied before the V-shaped helically the deposition of insulation material on the ends 9 can also be avoided by applying an anaphoretic potential at the wire 1 during the dipping of the wire 1 in the suspension 4. The holder 2 with the Wire 1 is then made positive relative to the suspension 4. The insulating particles from the suspension which are drawn in the wire turn 1 by capillary action are then repelled so that substantially only the suspension medium can reach the capillary spaces. When, after dipping the wire 1 down to theline AA, said liquid has reached its largest height above the line AA, the switch 7 is switched so that the wire 1 becomes negative relative to the suspension 4- and insulating material deposits on the wire 1 down to the line AA. A thickening 8 can be obtained in this case by keeping the tip of the shaped wire in the suspension for some time when the wire 1 is drawn out of the suspension.

Although a single embodiment'of a device for electrophoretically covering articles has been described, it is obvious that other known electrophoresis devices may also be used. For example, the potentials may be applied to separate electrodes placed in the suspension 4.

What is claimed is:

1. A method of electrophoretically covering a helically wound heating wire for indirectly heated cathodes with an insulating layer to a given distance from the ends of the wire, which ends are clamped in a holder during covering, comprising the steps of dipping the heating wire down to the desirable depth into the electrophoresis bath, applying an anaphoretic potential to the wire until the liquid in the capillary spaces has-reached its highest position, and thereafter, applying the electrophoresis potential.

References Cited UNITED STATES PATENTS 3,049,482 8/19 62 Dolan 204181