Conveyors: fluid current – With diverse power-driven conveyor – Rotary
Patent
1986-06-09
1988-02-23
Peters, Jr., Joseph F.
Conveyors: fluid current
With diverse power-driven conveyor
Rotary
222413, 222195, 198661, 198662, 1985501, 406 61, B65G 5348
Patent
active
047267151
DESCRIPTION:
BRIEF SUMMARY
This invention concerns the uniform feeding of irregular and regular shaped powder material at low mass feed rates.
There are many occasions on which solid powdered material has to be fed at uniform feed rates. These vary from pharmaceuticals and metal spraying to coal fired boilers and bulk food processing. In consequence numerous powder feeders are available based on a variety of principles, e.g. vibratory feeders, inverted weir and belt feeders, rotary disc feeders and screw feeders.
Nearly all these feeders have problems with "bridging" of the powder, particularly when irregular and/or very fine powders are to be fed. Screw feeders suffer from a further problem of "blinding" of the screw, whereby the powdered material rotates with the screw rather than being fed.
All feeders suffer from some form of slubbing of the mass flow. In certain processes, e.g. laser cladding with blown powder, this slubbing is of critical importance.
Providing a uniform powder flow at ultra-low feed rates is particularly difficult because, whatever the feeding mechanism, there is a conflicting requirement for a small aperture to achieve a low feed rate and a critical aperture to prevent "arching" of the powder, coupled with the additional requirement of high rotary/vibratory speeds to eliminate cyclic effects.
One object of the present invention is to provide a screw powder feeder which avoids, or at least reduces, screw "blinding" and "bridging" near the screw. The screw may then be rotated at higher speeds which in turn reduces the observable slubbing effects.
Another object of the invention is to provide a feeder capable of uniform powder flow at lower feed rates than is possible with existing feeders.
A still further object of the invention is to reduce the cost of screw powder feeders.
According to one aspect of the invention, the screw in a screw powder feeder comprises at least two helical flutes, angularly spaced at 180.degree. to one another.
In one particular embodiment the screw comprises a twist drill bit having a ridge along the leading or `cutting` edge of the helical thread, the ridge normally helping to remove the swarf by providing some clearance against binding when the bit is used in a twist drill. Twist drill bits are inexpensive and widely available, thereby substantially reducing the cost of the screw feeder. Moreover, the long pitch of the flutes on a twist drill bit increase the "pumping" velocity parallel to the screw axis, thus giving extra momentum to the powder particles along the flutes in the required direction of feed. The outwardly projecting ridge which assits the powder feed by cutting through the powder and displacing it into the flutes effectively increases the "aperture" of the feeder for gravity flow.
In one possible alternative, the rotary screw comprises a milling cutter having four angularly spaced helical flutes at 90.degree. to one another.
Ridged threads or finned threads over a short length of the screw may alternatively be used to assist displacement of the powder into the groove or grooves.
The feeder may include a gravity feed hopper and, according to a further aspect of the invention, the feeding of powders which do not flow smoothly under gravity is assisted by discharging a gas into the powder in the vicinity of the screw, the quantity of gas being just sufficient to break down static arching of the powder at the outlet aperture of the hopper. The resulting dynamic arch collapse produces a uniform gravity flow on to the rotary screw which then conveys the powder into a discharge line.
The gas is preferably introduced into a longitudinal bore of the screw and emerges periodically at the surface of the screw through fine holes located in a predetermined sector of the screw and communicating with the bore, the gas being discharged only when the holes are facing upwards.
According to a still further aspect of the invention, the powder is fed under a predetermined pressure to the screw and discharged by the screw into a chamber, the pressure in the chamber being varied against the
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Steen William M.
Weerasinghe Vijitha M.
Kannofsky James M.
Peters Jr. Joseph F.
Quantum Laser Corporation
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