Editor's Note: In Part 1 of this article, we saw that even though a machine is scheduled for production and operators and material are available, it is not always producing. Time is siphoned off by a number of loss factors. Part 2 will take a look at how Faults and Skipped Cycles are impacting the productivity of your high-speed inserter on a daily basis.
In the never-ending search for increased productivity, production mail facilities have made significant investments in high-speed inserting equipment. And unfortunately, too often the expected higher yield rates have not been achieved due to the relationship between accelerated machine speed and increased machine downtime. An understanding of the four primary inserter productivity factors operating in most production mail environments is an important first step towards dramatic improvements in run time ratios and overall cost savings from decreased overtime.
Faults
Ideally, once an operator has loaded a job onto an inserter and pressed the start button, the machine runs continuously until the job is completed. In the real world, this almost never happens. Inserters stop for all sorts of reasons: paper jams, scanning errors, feeders running out of material, breaks in the input web, a meter running out of postage, someone opening an interlocked cover and many others. A fault is defined as any unplanned machine stop requiring operator intervention.
Two factors are critical when measuring the impact of machine faults: How often the machine stops, measured in machine cycles (Mean Cycles Between Faults, or MCBF) as well as how quickly the operator corrects the problem of the machine and restarts the machine, measured in minutes (Mean Time to Clear Fault, or MTCF). How often the machine stops is measured in cycles rather than minutes because a cycle is a better measure of the chance for an error. Every time a machine cycles, there is the probability that an error will occur.
As technology has improved, inserters have become faster. However, operator response time to faults has remained consistent. When a fault occurs, an operator must still recognize that fact, stop what he is doing, move to the location of the error and correct the problem. The fact that the machine is faster doesn't speed up the operator; in fact, it may slow the operator down if the machine is larger or has more complex mechanisms or covers. This means that if the MTCF remains constant, then for a high-speed machine to be cost-effective, to its user, then its MCBF must increase. Otherwise, a point of diminishing returns is quickly reached. ·
Analysis from a number of different production mail sites indicates that the average time for an operator to clear a fault is approximately three minutes. Although this may vary based on the type of machine and application, and the skill level of the operator, the time is remarkably consistent. For an older machine, operating at 5,000 pieces/hour, with an MCBF of 1,000 cycles, the machine will stop on average five times during an hour of production time, adding another 15 minutes, and yielding a net production rate of 4,000 pieces per hour (neglecting the other loss factors). If a new high-speed machine, operating at 20,000 pieces per hour, had the same fault rate, fault time would add an additional 60 minutes onto the production time, which yields a net production rate of 10,000 pieces per hour. The new machine is cycling four times faster than the old but produces at barely more than twice the rate! To achieve the same relative level of efficiency, the high-speed machine would need a MCBF of 4,000.
Understanding the impact of fault time on an inserter is critical to improving productivity. Digging down to the root cause of the faults is essential. There are several contributing factors:
Although machine design or maintenance are often given most of the blame, the other factors can be as important if not more so. The fault-causing factors often intertwine with one another, obscuring clear direction for improvement. It's important not to assume that the solution rests with addressing a single cause. Often, the cure rests in a clever mix of solutions.
The search for a root cause of fault time is best started by building a detailed list of the faults occurring on the machine then categorizing them by type and impact (how much time is required to correct them). Building this list requires capturing a large amount of data and understanding the true causes of each fault (for example, a jam at the insert engine can often actually be caused by a module much further up stream, such as the folder). Careful selection of tools and analysis processes is important to a successful improvement program.
Skipped Cycles
There is an often misunderstood factor that can have a serious effect on productivity. It is the machine's ability to utilize all of its cycles, measured as "Cycle Efficiency." In modern mailing applications, it's rare to find an application that puts a single page, or even a fixed number of pages, into the envelope. Normally, statements contain a variable number of pages. Since there are more statement pages than envelopes, the inserter's statement (or input) feeder is usually designed to operate faster than the envelope insert engine. However, this increased speed may not be sufficient if the average statement page count is high enough. For example, a typical input feed might operate at two or three times the speed of the insert engine, but it is not unusual to find statement mailings with four, five or more pages per mailpiece. When the number of pages in a statement exceeds that which the input can feed in a single machine cycle, the inserter has to skip a cycle, leaving an empty spot to flow through the system and losing that cycle of production.
Inserter manufacturers often include buffering devices to try to eliminate skipped cycles, but these devices are only effective in limited situations, such as when a single high page count statement occurs within a large group of smaller statements. Skipped cycles can occur even if the average page count for a mailing is below the theoretical single cycle capability of the input feeder. All that is required is for there to be clusters of higher page count statements, something that can easily occur in ZIP sorted mailings where customers with similar demographics are grouped together.
Cycle skipping is a big improvement over older technology that required the inserter to stop and wait for high page count statements, because it eliminates the disruption and wear caused by the frequent stops and starts. However, a machine that frequently skips cycles (averages 10% or more skipped cycles) is inefficient. Even though the input feeder might have only needed a fraction of a cycle time to finish feeding the statement, a skipped cycle causes a whole cycle to be lost. Paradoxically, the machine may actually produce more if it is slowed down. The best speed for an inserter is the point at which you have the fewest skipped cycles.
The need to enhance productivity is a given in today's production mail environment. The amount of time invested in examining the effect the four primary productivity factors are having on your production mail site will pay off markedly in increased productivity.
William Hart, Chief Engineer, Lone Oak Technologies, LLC. Loan Oak is a nationally recognized expert in production mail technology. Hart founded Lone Oak Technologies in 2003. For more information, please call 203-270-3367 or visit www.loneoaktech.com.
