Patent GB-305611-A (GB)
Title: Improvements in or relating to methods of enamelling and apparatus therefor
IPC: B05D3/02
Filed: 1927-11-08
Abstract: 305,611. Beatty, W. E., (Bell Telephone Laboratories, Inc.). Nov. 8, 1927. Enamelling ovens.-The temperature in the combustion chamber of an enamelling-apparatus is controlled in accordance with the hardness of the baked enamel. In the apparatus shown, wire is continuously fed from a spool 14 through an enamelling-bath and a heating-chamber 17, to which air is admitted through openings 18 to burn the combustible solvent of the enamel. The wire may make a number of passes through the bath and the heating-chamber, and is finally passed through a mercury bath 19 and wound out to a spool 26. The solvent is vaporized in the lower part of the chamber 17 by an electroheater, and the combustible mixture of gas and air is ignited by a burner 13. The temperature in the heating-chamber is controlled by a valve 12 in the outlet flue, the valve being automatically-operated in accordance with the power factor of the baked enamel, which factor varies with the degree of hardness to which the enamel is baked. The controlling-apparatus comprises equal resistances 21, 22, a continuously variable air dielectric condenser 24, and a condenser formed bv the earthed wire 20, the mercurv in the bath 19, and the enamel dielectric, these elements being arranged as a Wheatstone bridge, to which an alternating current is applied at the points 34, 35. The potential difference between the points 36, 37 is applied between grid and filament of a thermionic valve 29, in the plate circuit of which is a part of relays 31, 32, so adjusted that a current corresponding with a normal power factor for the enamel will energize relay 31 and coil 41 to move the switch arm 44 to a neutral position. A larger plate current, corresponding with the low power factor of enamel baked too hard, will actuate both relays and draw the arm 44 into engagement with the contact 49, while when the enamel is too soft neither relay is actuated and the arm remains in the position shown. A spring-pressed plunger 45 retains the arm in the position to which it has been moved against the action of a spring 47. The position of the arm 44 is utilized to energize a polarized relay 55 at fixed intervals determined by the bridging of the contacts 52 by a rotary arm 51. If the arm 44 is in the position shown when the contacts 52 are bridged, the relay will be so energized by the battery 56 that the arm 57 will engage the contact 58 and will complete a circuit through the battery 61 and coil 64. The plunger 66 will be drawn down, and will first move the pawl 67 into engagement with the ratchet wheel against the spring 69, and will then move the pawl arm 72 against the action of the stronger spring 71. The rotation of the valve in a counter-clockwise direction will restrict the flow of air through the ports 18 and lead to an increase in the baking-temperature. If the arm 44 is in engagement with the contact 49, the coil 65 is energized and the valve is moved in the opposite direction, while no movement is effected if the arm 44 is in neutral position. After leaving the contacts 52, the rotary, arm 51 bridges a second pair of contacts 54 energizing a coil 76, which draws the arm 44 into engagement with the contact 48 where it will either remain or be moved to another position according to the current in the relays 31, 32. The capacity of the condenser 24 is continuously varied to allow for small changes in the capacity of the condenser 19 due to varying thickness of the coating &c., and the grid of the valve is made sufficiently negative to prevent any flow of current in the output circuit until the capacities of the two condensers are nearly equal. In a modification, the valve 12 is in a pipe through which air for combustion is supplied to the heating-chamber.