A small plant makes plastic strips in a process that involves extrusion into sheets and then slicing into strips of exact widths/lengths. These are intermediary components that the plant’s customers use in the manufacturing of their own products. Each length specification has its own slicing line, and all of the slicing lines have a problem with varying length. To compensate, the slicers are set to cut long. The final cut is made manually.
The plant has a single maintenance man, Carl, who used to be an equipment operator. Carl doesn’t have any formal training in any of the trades, but he’s handy with a screwdriver and a socket wrench.
The plant manager asked Carl to figure out why there is variance in the automatic length cutting. Carl determined the problem is the motors that drive the lines vary in speed just a little. Short of replacing them all, he believes there’s probably not a way to fix this problem and suggests bringing in an outside firm to see what they say. Your boss sent you. What is your solution?
Cause of the variance
It’s not the case that the motors are varying a little in speed. Motors don’t gain or lose poles, and neither do the utility’s generators.
Your first quandary here is you can’t start off by telling the plant manager that Carl was completely clueless about motor theory. You want these people to call your firm back for work in the future, and insulting Carl is not the way to make that happen. So privately, explain to Carl what makes motors turn, how that’s a function of the supply frequency, and why the supply frequency doesn’t vary. Tell him he may be on to something, however. It could be that something might be causing the line itself to vary in speed, making it seem as if the drive motor is at fault.
Test one line, and you will likely identify the problem with all of them. It could be due to a lack of maintenance, causing things to shake loose, but don’t say that. Ask Carl to walk you through how the strip is fed into the cutter part of the machine. Ask him to show you exactly how each machine knows when to lower the knife blade and cut. Take notes on what parts might be unstable enough to cause a variation.
Separately and silently, look for and document any Code violations in the area where you are working. Hold on to those for later.
Then look for mechanical issues that can impede or vary the speed of the line. It might seem that all of these will be on the load side of where the motor connects to its gearbox. But a more likely place is the motor mounting system. Issues like a cracked or shaky motor pedestal, weak motor base (metal is too thin to provide the necessary rigidity), or soft-foot mounting will cause wobble in the system — and this problem. A vibration tester can come in handy here, as can a sharp eye and a ruler.
If everything checks out here, maybe the cutting blades are moving. A dial indicator will come in handy. Mount it so the plunger tip touches a vertical surface on the cutting blade arm. You should see almost no play as the arm moves up and down. If that’s the case, then check how the blades themselves are clamped in place. The blade might be moving under the clamp. If you’ve ever used a double-edged safety razor and not securely clamped the blade, you understand the dynamics here.
Next, check any sensors and their cabling. If, for example, the cabling for a sensor that puts out a millivolt signal is routed next to the supply to a motor, induction from the motor supply conductors will alter the signal in the sensor cabling.
Once you’ve determined what is wrong, submit a work proposal and quote to fix the deficiencies noted on this machine. Once that work is completed, do a sample run to see if you solved the length variance problem. Now you’ll know what to check for first on each of the other machines, and you can submit a quote for completing that work also.
The savings realized by this project, both in cost and output capacity, should make the plant manager amenable to hiring your firm to fix the Code violations. Then there’s lighting that can be made better and more efficient with an upgrade
