Metal spray guns of the type to which my invention relates are devices which operate by continuously feeding a metal rod or wire into a zone in which it is melted and from which this melted metal is sub-divided and propelled by a blast of air orother gas. The rod or wire is fed into the melting zone by knurled burs which press against opposite sides of the wire and are driven by a gas turbine operating through reduction gears. The load on the turbine varies from time to time due to changes inthe position of the operator, kinks in the wire, etc., and since it is essential that the rate of feed of the wire be uniformly maintained, it is likewise essential that the turbine have a comparatively stable speed of operation, i.e., that its speed beaffected as little as possible by variations in the load. This may be accomplished by appropriate turbine construction.In the operation of metal spray guns, it is, however, necessary to maintain different rates of feed when spraying different metals; for example, metals, the heats of fusion of which are low, may be fed and sprayed more rapidly than metals with a high heat of fusion and wires of larger diameter are fed and sprayed more slowly than wires of the same material of smaller diameter. In usual practice an effort is made to approximate proper conditions for the particular metal being sprayed by the provision ofreplaceable gearing designed to permit the gun to be operated at the desired rate of wire feed while permitting the turbine to operate within a range of stable operation. Changing the gearing involves a loss of time and the possibility that metalparticles will be picked up by the gears in handling and interfere with the operation of the gun. If an effort is made to reduce the turbine speed by throttling the gas the turbine becomes unstable and a uniform wire feed is no longer maintained.
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