## Static Elimination Revisited

PLC Probe

The February column on static elimination inspired a reader to share some experiences he had on this “shocking” subject. He has worked on many web processing machines and encountered static electricity problems often. Static is not necessarily caused by friction, he says…

We all know the cat’s fur rubbing amber experiment from high school science, but when two dissimilar materials are brought into intimate contact with each other (even without friction), there is at least some opportunity for electrons to move from one to the other.

If the first material’s molecular structure tends to have loosely held electrons in its outer bands, while the second tends to have apparent openings in its outer bands, it is likely a few electrons will be stripped from the first material and collected on the second when the two are pulled apart.

Most commonly, we see this problem in the form of charge built up on the surfaces of non-metallic rollers. Silicone rubber rollers running poly films are a nasty example. Static charges accumulate on the surface of the roller that is touched by the passing web. Silicone rubber is an excellent insulator, so the wider the roller, the longer the path to the nearest ground, and therefore, the higher the charge is likely to get before finally finding a discharge path.

Solutions we’ve used in the past included drilling holes in a pattern on the surface of the rubber lagging, thereby reducing the distance from any point on the surface to the nearest ground. In this case, the ground was the surface of the shaft exposed at the bottom of the drilled holes.

A simple answer is to mount the roller in such a position that a grounded section of machine frame comes in close proximity to the roller surface. The tinsel method commonly is used in the same mode, but it is relatively fragile. An equivalent trick I’ve used is to tie a common door spring across the face of the roller with about a 1/4-inch clearance. The charge never accumulates to a level greater than what it takes to reach the spring.

Another tidbit about static electricity that most people don’t appreciate is the severity of injuries incurred by accidental contact with highly charged components. A fellow employee once got too close to a charged silicone rubber roll on a particularly dry winter day. The arc that jumped to his elbow probably jumped about four inches, and he felt as though he’d been hit by a baseball bat.

In fact, this type of injury can be broken into three components. First the entry point of the arc produced a very minor but very real wound. Second, the jolt of electricity causes a muscular spasm that can be painful and last for days. Third, the arc likely caused nerve damage, leaving the victim with at least some pain in that arm for the rest of his life.

I thank the reader for his thoughts and encourage others to weigh in on our monthly offerings when a topic hits home.