Patent GB-1226287-A (GB)
IPC: B24C9/00
Filed: 1969-05-27
Abstract: 1,226,287. Blasting. WHEELABRATOR CORP. 27 May, 1969 [14 June, 1968], No. 26774/69. Heading B3D. Apparatus for treating, e.g. cleaning, articles by blasting with abrasive particles of at least in part magnetic materials, comprises a conveying means for feeding articles past a blasting device, the particle flow to which is controlled by an electromagnetic valve. The valve is in turn controlled by electrical pulses emanating from the drive of the conveyer means so that the quantity of blasting material is regulated by the speed of the articles. 10, Fig. 2, is the coil of an electromagnet valve which controls the supply of abrasive particles, comprising at least in part magnetic particles, to a centrifugal wheel for throwing the particles at the surface of a moving workpiece, e.g. for cleaning an elongated strip conveyed by rollers or other means. A number of valves and of centrifugal wheels are used, the volume of abrasive delivered by the valves being controlled in accordance with the speed of the moving workpiece. A cam 12, Fig. 1, coupled to the means for conveying the workpiece, e.g. by being directly or indirectly connected to the shaft of a drive roller, has solid portions 14 with intervening gaps 16. A proximity switch sensing head 18 operates, through power supply 20 and amplifier 22, to deliver pulses to an input " AND " logic device 24. A pulse is transmitted for each of the sequential "ON" and " OFF " signals generated by the head 18, so that six pulses are transmitted for each revolution of the cam 12. For each " ON " signal pulse transmitted to it the device 24 delivers a signal through line 26 to " AND " logic device 28; the same signal is transmitted through line 39 to delay element 32, so that a delayed signal is transmitted through line 34 to the logic device 28. For each " OFF " signal pulse, the inverted output of device 24 delivers (i) a signal through line 36 and (ii) a delayed signal through delay element 42 and line 38, to a logic device 40. The output from devices 28, 40 is fed to an " OR " logic device 44, the output from which is delivered to relay output amplifier unit 50. When the relay in unit 50 is energized, switches 52, 54 are closed, energizing reed relays 56, 58, powered by unit 60. A manual switch 61 permits the delivery of a continuous signal through the device 44, e.g. for testing. A device 63 is provided for setting the devices 32, 42, 28, 40 in the " OFF " position to ensure synchronization at the beginning of an operation. The relays 56, 58 are adapted to be associated with banks of coils 10. For example, relay 56 when energized closes switch 62. Current is fed to silicon controlled rectifier 64 resulting in firing of the rectifier, whereby current is fed to coil 10 through diode 66. A capacitor 67 is connected across the coil 10 to counteract the inductive reactance thereof to ensure that voltage pulses always return to zero thereby giving true pulsing of abrasive flow. A resistor 68 is included in line 70 extending between coil 10 and line 72, so that the coil 10 has a constantly applied voltage; firing of rectifier 64 shorts out the resistor 68 so that full line voltage is delivered to coil 10. Assuming that (i) cam 12 makes one revolution for every six feet of strip of strip travel, so that an " ON " or " OFF " signal will be delivered for every foot of travel; (ii) delay devices 32, 42 are set at 1/5th sec., so that the delayed pulse will shut off the devices 28, 40 <SP>1</SP>/ 5 th sec. after they are turned on by the undelayed pulse, and (iii) resistor 68 permits ¢ full voltage to be constantly applied to coil 10, performance characteristics as shown in Fig. 3 are obtained. If the strip travel is 60 feet/min., there will be one pulse/sec., resulting in full voltage on the valves for 1/5th of each second. If the strip travel is 300 feet/min. full voltage is applied to the valves continuously. Controlling the abrasive flow rates in accordance with the speed of travel (i) avoids or minimizes abrasive built-up on the strip, e.g. where the travel is temporarily halted or slowed, (ii) gives less abrasive break-down and (iii) decreases wear.