Total Manufacturing Assurance; Chapter 5, System Definition (page 2)
MATERIAL HANDLING
          In the previous section, we saw some techniques for laying out the facilities in a manufacturing plant.  These techniques were based on several principles, one of the most important being the minimization of material handling burden.
          Is material handling cost worth worrying about?  Absolutely, especially in a TMA environment.  For one thing, material handling cost can be up to and beyond one-half of total manufacturing cost.  Furthermore, in many applications, material handling is inextricably linked with other manufacturing elements.  Conveyors and transfer mechanisms in assembly lines, part feeders and orienters, and robotic manipulation equipment are examples of devices that handle raw material and work in process at the same time that they are being converted to finished products.
          What materials are handled in a manufacturing system?  Basically, everything from raw materials to work in process to finished products.  Also tools, replacement parts, energy, information, and sometimes even personnel are moved by the material handling systems.
          What are our goals in the handling of these materials?  First of all, we want them to reach the designated destination, and at the proper time, or earlier.  They must arrive safe and undamaged, and in an economical manner.  Further, we should always be aware of each item's location while it is in transit, in case a change in plans makes it necessary to reroute.
          Material handling equipment covers a broad spectrum of sophistication, from trivial to highly automated and intelligent.  In the most basic sense, manpower can be considered a material handling technique: workers can trot over to a storage area and retrieve the materials they need.  This material handling device has the lowest investment cost, but one of the higher unit costs.  As such, it is only advisable for very low production rates.
          Moving up the evolutionary scale, we have human-powered equipment such as dollies and hand carts.  Next come human controlled but auto-powered devices such as fork lifts and motorized carts.  Hoists, gantries, and conveyors require human control only for starting and stopping at desired times.  Finally, at the top of the spectrum, we have intelligent and automated devices such as AGVs (automated guided vehicles) and conveyors with feedback control.
          The choice of the best material handling method for each application depends on a combination of factors.  Quantity of material to be handled is obviously an important consideration, both from a standpoint of how many units per day, and from the view of how many days in the production run.  High material flow rates require large capacity systems, and longer production runs will justify a larger investment in faster and more sophisticated equipment.
          Another critical factor to consider is flexibility.  Will the handling system remain constant for a long time?  Will rerouting become necessary often?  Are changes in flow rate expected?  Naturally, greater flexibility comes at a greater cost, and must be justified by both anticipated changes in material handling requirements, and enhancement of profitability resulting from the flexibility.
          Certain guidelines should be followed when selecting and designing the handling system.  These concepts are basically common sense, but are still worth discussing briefly.  The first guideline is the "bee-line" principle: whenever possible, use straight lines to minimize distance traveled.  Of course, practical considerations may overrule this concept in many situations.
          Next is the "pallet" principle: unless we are dealing with extremely large items (say, locomotives), it is a waste to plop one unit onto a conveyer.  This wastes conveyor capacity, as well as loading and unloading motions.  It is better to palletize parts onto a single holder, or pallet, and manipulate them as a single package.  The pallets should be as large as possible, and should be of consistent size, arrangement, and orientation.
          Along with the pallet principle comes the "buffer" principle: build buffers into the production line so that full pallet loads can accumulate before transportation becomes necessary.  This not only makes palletizing more efficient, but allows different workstations to work at slightly different rates without blocking the flow lines.
          Another guideline is "minimum dead-time" principle: we want to keep the material moving, not sitting around, so we try to speed up the loading and unloading of parts onto and off of the handling equipment.  This means not only well designed pallets and fixtures, but intelligent scheduling of loading and unloading operations during our overall process.
          The "two-way" principle is easily overlooked, but can double handling efficiency:  never send anything back empty if it can be avoided.  Use the same pallet that arrived at station n to take station n's output to station n+1.  Don't send it back to station n-1 for another load!  Arrange flow so that minimal amounts of motion are wasted.  Here is where uniform pallet design pays off.
          Finally, the "information" principle: whenever possible, integrate the information about the material into the flow with the material.  Rather than saving a list of part numbers and their associated pallet number in a computer file for later retrieval by a downstream station, print the part numbers onto the parts or pallet itself.  Bar code printers and readers, or even less sophisticated methods for low volume, make this possible.

Material Handling Equations
          The relationships describing various quantities in material handling systems are fairly simple.  One basic concept is the rate of flow, Rf, usually expressed in parts/minute.  For a one-way conveyor or other handling system, it is given by:
                              Rf = (np)(Vc/Sc) <= np/TL
where:           np is the number of parts per pallet
                     Vc is the flow velocity (feet/minute)
                     Sc is the spacing between pallets (feet)
                     TL is the loading time at each end (minutes)

          A material handling system also has a capacity, or number of parts that can be accumulated in it.  This can be thought of as a temporary storage of work in process.  if nc is number of carriers (pallets) in the system, then:
                              nc = Lf+Lr/Sc
where:           Lf is the length of forward (full) part of route
                     Lr is the length of return (empty) part of route

          Including a return portion (Lr) makes this equation applicable to recirculating types of systems, such as belt conveyors.  Total number of parts in the system, N, is given by:
                              N = (np)(Lf/Sc) = (np)(nc)(Lf/(Lf+Lr))
          One other relationship of importance should be noted here.  That is the relationship between travel time and loading/unloading time.  If TL is time to load a pallet, and TU is time to unload a pallet, then:
                    Vc/Sc must be >= 1/TL and TL must be >= TU
to avoid backups in the system.
          Note that other material handling relations, both specific to individual systems, and applicable in general to all types, are found in the literature.[6]

Automatic Guided Vehicles
          The highest end of the material handling spectrum is occupied by AGVs.  These devices are by far the most expensive, and therefore not applicable to all situations.  But there vast capabilities and inherent flexibility can lead to large payoffs for installations that can justify their cost.
          AGVs are basically independent vehicles, rolling along from station to station under their own power and control.  Some are guided by wires or painted lines on the floor, others by dead reckoning with periodic alignment checks at known landmarks.  They are generally powered by on-board batteries, lasting at least an eight-hour shift before needing recharging.  Often they will have a "safety bumper", a large loop of some flexible material on the front surface which will detect collisions, and stop the vehicle, before the main mass of the AGV does any damage to itself or other objects in its way.  Some AGVs use optical or sonar devices for collision detection.
          Different control schemes can be used with AGVS.  In some systems, the AGV is controlled by typed instructions entered at an on-board control panel.  This is a simple and straightforward method, but requires large amounts of human intervention.  In other systems, a decentralized control scheme is used: a station in need of an AGV will issue a general "service call" via radio frequency.  Any AGV will unused load capacity can detect and answer the call, and service the station.  This is a useful scheme in large factories with "islands of automation", but no centralized control of the entire floor.  If a central computer is used to direct flow in the entire factory, it will generally make the decisions as to which AGV should service which stations, and when.  This method offers greatest overall efficiency, but requires extreme levels of planning and programming.
          Equations to plan an AGV system are similar to those given above for any material handling system, with the addition of a "traffic factor" to account for time wasted in waiting for obstacles to pass, avoiding collisions, adjusting a path, etc.  This factor is usually somewhere between 0.8 and 1.0, and is used to reduce the number of parts/minute that could be handled by an AGV with no need to worry about traffic congestion.  Naturally, the more AGV and other traffic in the system, the lower the traffic factor will become.