A large manufacturer of colored dies used for ink-jet printers was complaining about a machine’s mixing drum speed slowing down after startup of the system. The machine had a digital readout to indicate when the drum’s speed reached that called for in the recipe. Operators would set a potentiometer to give the correct speed when starting a new batch, but 5 to 10 minutes later, the speed dropped 4 to 6 rpm lower than needed. Because speeds usually need to be around 20 rpm, the drop would amount to a speed 20 to 30% slower than needed.
The drum is driven by a hydrostatic drive pump with displacement controlled by a true torque-motor type servovalve. The maintenance department had it rebuilt, which did not improve the situation. They then had both the pump and motor units tested by a reputable rebuild house and reported the two units met the manufacturer’s requirements.
Any idea what the problem was?
Find the Solution
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Solution to Last Month’s Challenge:
Oil Heater Not Getting the Job Done
The system had problems because its tank heater was generating 45 W/in.2, but hydraulic oil should not be subjected to anything greater than 15 W/in.2 The rebuilder purchased a three-phase 460-V heater, which can also run from 230 V three-phase if wired correctly. The rebuilder did not inform the owner that the heater was ordered as a 460 V unit, so technicians wired it as the original 230 V one. Had they known it was a 460 V unit, they would’ve connected the three heating element wires in a Delta configuration, which would’ve provided one fourth the normal 45 W per in.2, or 10.25 W/in.2 The incorrect wiring caused the heater to overheat the oil, turning it into a carbon solid surrounding the heater elements.