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ch-Design of Automatic Machinery

ch-Design of Automatic Machinery - 7 Feeders and Conveyors...

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7 Feeders and Conveyors To the automation novice, it may seem that all of the difficult problems with which one must be concerned have been covered. Yes, one still needs to select the right actuator and add the right sensor to make things function correctly, but are we not done with the challenging stuff yet? Depending on what is being processed, there is one tough nut yet to crack. That nut is the feeding of the products into the work- stations within the machine configuration. Sometimes this is as simple as selecting a feeder from the catalog, other times, your worst nightmare. Feeders, as the name implies, feed parts into the workstation. A conveyor can also function in this capacity, but its role is sometimes a gray area. Does an asynchronous conveyor with a series of pallets constitute a parts feeder or a machine configuration (it can be both)? When is a conveyor a feeder and not part of the machine configuration could be debated by lawyers until the cows come home (perhaps this is a useful way to occupy some lawyers you know). Within this text we will not try to split hairs on these definitions of conveyor func- tions, we will simply look at them for possible solutions and move on. To realize the role of a feeder, imagine the bow tying workstation discussed previously. There needs to be an influx of packages to have bows tied on them. And there needs to be a supply of ribbon. Ribbon traditionally comes on spools, so we will try to use this format as a means of keeping costs low. Requiring ribbon to come in a cartridge, internally jumbled like some older dot matrix printer ribbons cartridges were designed, would cause a supplier to create a 163 Copyright © 2005 by Marcel Dekker
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new packaging machine. This is most likely a higher cost to you, unless you are the one to make the new machine for them too. So we will target ribbon coming on a spool. However, what happens when the spool runs out of ribbon? What happens if the bow tying machine has 90% of the ribbon it needs for the current package, and runs out? . Will sensors stop the process before wasting time trying to use a piece too short? . Will the machine malfunction? . Will it jam like some office copiers, requiring much effort to clear? . Can a huge roll be used? The last point seems to be very practical, making the supply roll huge. This makes the percentage of time one has this issue become smaller, but then the force required to pull off the correct amount of ribbon becomes much higher with the large rotational moment of inertia. And once the inertia is moving, the supply roll might keep going so as to create a giant loop of excess ribbon, per- haps even reaching the floor or becoming a tangled mess. You may find yourself needing to give a motor assist to the supply roll to help pull off the correct amount, and then you might need either a servomotor to reduce the rotation, or a braking system. With the changing diameter of the ribbon as the roll gets smaller, the amount the motor assist operates is not a fixed number of turns. It
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