Two of the chief concerns with any new automation design are how many copies of the important processing devices, fixtures, etc., can one afford to build, and what is the cycle time. This will become more apparent as we look at the different configurations and trade-offs.
Linear Indexing
Indexing motion is where a line of products in a similar state is moved into a location to have a process carried out. The spacing of the products is at a constant interval, and if an empty space occurs from a missing product, appropriate measures may need to be taken. For example, if a bottle is missing from an indexing conveyor, automation should not attempt to dispense 20 ounces of beverage, resulting in a mess on the transport and the floor. The two commonly linear indexing configurations are:
Conveyors;
Walking beams.
Conveyors can comprise a continuous belt or be made up of many sections or segments similar to a tank tread, but either design needs to have an attached bar or set of features to assure that the transported product is located at the appropriate place when the process occurs. Figure 4.4 shows both types. These features on a conveyor will classify it as a “flighted” conveyor. Some automation producers will refer to them by the alternate spelling—“flited.” A flighted conveyor is also useful in transferring product up and down inclines without the product’s location and orientation becoming unknown. Some segmented flighted conveyors have customer-specific designed features to facilitate positive locating of a
particular product to be transferred.
There are probably 1000 different types of conveyors and options available. We will look at them in more detail in a different blog post.
As with most continuous conveyors, the belt or series of segments form a contiguous chain that requires a return path to form the total loop. Most often the return path is directly under the working surface. This is usually satisfactory, but does mean that more than twice the length of flighted belt is required to be designed and purchased. For belt conveyors, the rollers on the ends are often tapered so that the belt is likely to stay centered and will not drift to one side or the other. These rollers usually have adjusting screws to keep the centering correct for belt wear.
Conveyors do not work in all applications. Sometimes the expense of the customized flighted belt is too high, or, more likely, the surrounding process or system obstacles do not support a standard belt design. Possible belt conveyor issues include:
The rollers at the ends of the conveyor where the belt reverses direction can be a space issue.
The rollers may be in the way as product is loaded at the start of the conveyor or unloaded at the end.
The segments are large, so the rollers would have to be extremely large and costly to flip the segments around.
Thus there is the need for the Walking Beam. A Walking Beam will transfer a part from one set location to the next location along the index path, and can work right up to the end of the machine. There are no return rollers, so transferring larger parts is not an issue. Often there are some registration features to keep the parts in known positions when the Walking Beam is not in contact over a small part of its cycle. It is usually during this noncontact part of the cycle that a useful process or subsequent transferring occurs.
A side concern with an index machine and its implied coordinated or synchronous motion is the proper timing of all of the operations above the conveyor or Walking Beam. Some machines have avoided the use of sensors and multiple controllers by resorting to a common drive shaft for internal motion devices. This is a somewhat historical approach since this dates back to waterwheels driven by a local river in the 1700s and 1800s. Today, a single
large motor drives a large driveshaft that has gears, chains, and timing belts and that powers all coordinated motion. This is still a viable answer for the right application, but does lend itself to the potential of tremendous frictional and safety shielding problems. As one can image, the use of a common driveshaft can either produce a machine that can last for years, or one that is a perpetual headache. The ability to adjust for a slightly smaller batch of product may not be available with a common drive shaft. Any adjustment in timing and action may be limited. There is little flexibility compared to what is available from good robot software programming.
Linear Continuous
As the name implies, a linear continuous conveyor transfers the product. This is often used for rigid products that can accumulate and are not fragile if they bump into one another, or it can be designed to handle a multitude of products while they are kept separate from each other. It can be one of the lower cost implementations of the many configurations discussed in this text. There are several different modes worth exploring here.
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