There are two speed “limits”—real and perceived. Most of the real limits are drive related. Maximum motor RPM or horsepower ratings are the most common. Most other limits exist only in the mind.

Has Dr. Roisum gone off his rocker? What about dryer limits? Dryer “limits” are common on many coating, printing and other converting lines. It takes so many BTUs of dryer to evaporate so many pounds of water or solvent at the current coat-weight, width and speed. If you go faster, the web will not be fully dried. Right?

However, even this case is not as black-and-white as it may first appear. What happens is that the defect rate increases, perhaps sharply, as you approach or exceed this “limit.” Thus, speed would appear to be bad. However, speed is also good, because without it there would be no productivity. In short, the unit cost as a function of speed is bathtub-shaped. Very near the bottom will be maximum profitability. It can be shown that the best speed is not a location of zero (speed-related) defects. If you do not bump into the occasional reject due to under-drying, you are not running fast enough to maximize profit. The reader can learn more in an article on optimization that is available for download on my web site.

However, even if we assume that the knee in the defect curve is so sharp that very nearly zero-defect speed is best, we can still improve both speed and profit. What about doing a better job of leveling the coating? You could run slightly faster if you didn't have the occasional wet spots. What about reducing the average coat-weight? With a more level coating, you can reduce the average while still maintaining the minimum required.

Thus, this seemingly obvious speed limit is not a “limit,” any more than automobile speed is limited to 65 mph on the highway. The limit is fuzzy. Other apparent limits are even fuzzier. Coating quality can degrade. Curing and cooling take time. Trim systems plug. Machines can vibrate. The list of items is long—however, none are as sharp-edged as one might believe.

Operators will contribute to limitations with a false dilemma of “Do you want it fast or do you want it good?” This assumes only two choices. I would prefer a third: fast and good. Operators also assume that speed and defects are correlated. However, many defects are speed insensitive. In web handling, only air entrainment and vibration tend to increase with speed. In fact, some defects are inversely related to speed. Wrinkles are one example. Have you ever noticed how wrinkles tend to go away as you speed up?

So we see that most limits are perceptual. However, they can become quite real in our minds if long habit reinforces the “don't push it” psychology. The zero-defects mentality of management further enables the comfort zone. It's not that consistency is not good, it's just that it is also limiting. If you are not happy with where you are, you have to change something. Considering a speed increase is something we should all take a close and honest look at. There is a best speed to drive on the highway, and it is seldom the one posted on the signs.

920/725-7671 drroisum@aol.com, www.roisum.com