Station to Station: Buffers Maximize Uptime in Automated Assembly
February 26, 2008
Contrary to the claims of promotional literature, no automated assembly system can be 100 percent efficient. Part feeders jam. Grippers release parts. Dispense valves clog. Screws miss their mark. Indeed, depending on the production rate, the number of stations, the processes that are automated, and the number of parts to be assembled, an uptime much better than 80 percent is hard to achieve.
“An assembly system that puts two parts together and drives some screws could be 99 percent efficient,” says Bob Rice, director of engineering at ATC Automation. “If you have a 30-station system, and each station is linked together synchronously, you might end up with an overall efficiency of 60 percent to 70 percent-even if each station is itself 99 percent efficient.”
Part feeding jams are the most common reason for downtime in automated assembly systems, so it’s imperative that all parts for the assembly be high quality. At the high volumes of automated assembly, the impact of faulty parts is greatly magnified. Consider a system to assemble a product with five parts. At a relatively modest production rate of 50 assemblies per minute, the system will consume 250 parts per minute. If just five parts out of every 10,000 cause a jam (a defect rate of 99.95 percent), the system will stop an average of every 8 minutes. If it takes 2 minutes to restart, the system will be down 20 percent of the time.
“Feeding parts is the biggest black art,” says Rice. “If the parts are consistent, you won’t have many problems. But, if they’re dirty, oily, damaged or out of spec, they will jam.”
Process failures are another common cause of downtime. Tried-and-true processes, such as screwdriving, are quite reliable in automated assembly. Other processes are less so. Bar codes are great for tracking work in process and directing assembly of product variants. But, the reliability of the reader depends on the quality of the code. Dirty or damaged codes will cause downtime. Vision systems are excellent for inspecting parts before and after assembly. However, if the lighting on the line changes, vision systems can decrease efficiency by stopping assembly for false positive results. Leak testing is a must for many assemblies, but it’s imperative to keep the test station clean to prevent stoppages.
Machine breakdowns are a less common cause of downtime. Nevertheless, pneumatic cylinders do wear out. Bearings do fail. “If the machine has been designed properly, you won’t have a lot of that, as long as the customer does preventive maintenance,” says Rice.
The basic design of the assembly system also influences overall efficiency. Although nonsynchronous systems are typically slower than synchronous systems in terms of assemblies per minute, nonsynchronous systems are generally more efficient than their cam-driven cousins. The reason is that nonsynchronous systems let engineers create buffers between stations, so downtime at one station does not directly affect the output of the subsequent station. With a synchronous system, a fault at one station immediately starves every station down the line.
“As soon as you include a buffer, you improve uptime,” says Rice.
Since line stoppages are inevitable, engineers should endeavor to minimize the time to restart. Such features as quarter-turn screws, snap-in tracks, quick-release levers and hinged doors let operators quickly access feed tracks. A helpful feature is a button on the control panel that signals all the assembly devices to return to a safe “home” position. This saves the operator the trouble of homing each device individually.
Fortunately, the efficiency of an assembly system is not set in stone once it reaches the shop floor. By tweaking a process, adjusting a fixture or altering a part design, engineers can increase uptime significantly over the life a system. Of course, the opposite is true, as well. “If the customer doesn’t do preventive maintenance, the system could run a lot slower over time,” warns Rice.