4.2 METHODS OF WORKPARTTRANSPORTThe transfer mechanism of the automated flow line must not only move the partially completed work parts or assemblies between adjacent stations, it must also orient and locate the parts in the correct position for processing at each station. The general methods of transporting work pieces on flow lines can be classified into the following three categories :

Continuous transferIntermittent or synchronous transferAsynchronous or power-and-free transferThese three categories are distinguished by the type of motion that is imparted to the work piece by the transfer mechanism. The most appropriate type of transport system for a given application depends on such factors as :

The types of operation to be performedThe number of stations on the lineThe weight and size of the work partsWhether manual stations are included on the lineProduction rate requirementsBalancing the various process times on the lineBefore discussing the three types of work transport system, we should try to clarify a possible source of confusion. These transfer systems are used for both processing and assembly operations. In the case of automatic assembly machines, we are referring to the mechanisms that transport the partially completed assemblies between stations, not the feed mechanisms that present new components to the assemblies at a particular station. The devices that feed and orient the components are normally an integral part of the workstation. We take a closer look at these devices in Chapter 7 when we discuss automatic assembly in more detail.Continuous transferWith the continuous method of transfer, the work parts are moved continuously at constant speed. This requires the work heads to move during processing in order to maintain continuous registration with the work part. For some types of operations, this movement of the work heads during processing is not feasible. It would be difficult, for example, to use this type of system on a machining transfer line because of inertia problems due to the size and weight of the work heads. In other cases, continuous transfer would be very practical. Examples of its use are in beverage bottling operations, packaging, manual assembly operations where the human operator can move with the moving flow line, and relatively simple automatic assembly tasks. In some bottling operations, for instance, the bottles are transported around a continuously rotating drum. Beverage is discharged into the moving bottles by spouts located at the drums periphery. The advantage of this application is that the liquid beverage is kept moving at a steady speed and hence there are no inertia problems.Continuous transfer systems are relatively easy to design and fabricate and can achieve a high rate of production.Intermittent transferAs the name suggests, in this method the workpieces are transported with an intermittent or discontinuous motion. The workstations are fixed in position and the parts are moved between stations and then registered at the proper locations for processing. All work parts are transported at the same time and, for this reason, the term synchronous transfer system is also used to describe this method of work part transport. Examples of applications of the intermittent transfer of work parts can be found in machining operations, press working operations or progressive dies, and mechanized assembly. Most of the transfer mechanisms reviewed in Section 4.3 provide the intermittent or synchronous type of work part transport.Asynchronous transferThis system of transfer, also referred to as a power-and-free system, allows each work part to move to the next station when processing at the current station has been completed. Each part moves independently of other parts. Hence, some parts are being processed on the line at the same time that others are being transported between stations. Asynchronous transfer systems offer the opportunity for greater flexibility than do the other two systems, and this flexibility can be a great advantage in certain circumstances. In-process storage of work parts can be incorporated into the asynchronous systems with relative ease. Power-and-free systems can also compensate for line balancing problems where there are significant differences in process times between stations. Parallel stations or several series stations can be used for the longer operations, and single stations can be used for the shorter operations. Therefore, the average production rates can be approximately equalized. Asynchronous lines are often used where there are one or more manually operated stations and cycle-time variations would be a problem on either the continuous or synchronous transport systems. Larger work parts can be handled on the asynchronous systems. A disadvantage of the power-and-free systems is that the cycle rates are generally slower than for the other types.Pallet fixturesThe transfer system is sometimes designed to accommodate some sort of pallet fixture. Work parts are attached to the pallet fixtures and the pallets are transferred between stations, carrying the part through its sequence of operations. The pallet fixture is designed so that it can be conveniently moved, located, and clamped in position at successive stations. Since the part is accurately located in the fixture, it is therefore correctly positioned for each operation. In addition to the obvious advantage of convenient transfer and location of work parts, another advantage of pallet fixtures is that they can be designed to be used for a variety of similar parts.The other method of work part location and fixturing does not use pallets. With this method, the workparts themselves are indexed from station to station. When a part arrives at a station, it is automatically clamped in position for the operation.The obvious benefit of this transfer method is that it avoids the cost of pallet fixtures.