Five Questions and Answers on Lubrication

Lubrication occurs when a gas or liquid separates or reduces the contact between two surfaces. Lubrication is used intentionally to reduce wear and increase the life of your car's engine. However, unintended lubrication between your tires and the road can be a bad thing. Water, oil, or snow can separate tires from the road, causing loss of driving, braking, and steering control.

Air lubrication is used intentionally to prevent web contact on air turns. However, unintended air lubrication on rollers and in the winding roll may be a bad thing. Air lubrication needs to be understood and controlled to prevent loss of web tension, guiding, and winding control.

Following is a Q&A primer on air lubrication in web handling.

What problems are caused by air lubrication?
Lubrication reduces the force to drive idler rollers, leading to scratching and debris. Driven roller lubrication breaks down the first assumption of tension control, that the roller speed equals the web speed. When a coater's pacer roller slips, the web speed is uncontrolled, creating coating thickness variation. Lack of friction due to lubrication will increase laterally shifting and position error at web guides and winding rolls. Excessive wound-in air may form “soft roll” defects associated with air's departure over time.

What determines when air lubrication occurs?
Rollers and winding rolls lubricate when the entrained air layer thickness is greater than the combined surface roughness. The entrained air layer thickness increases with larger radius, higher speed, and lower tension. Smooth surfaces lubricate quickly, like films on chromed rollers and winding smooth webs. Porous webs — where air can escape through the web — will see little, if any, lubrication. Web widths of 1 in. or less will see some edge escape effects. Most lubrication problems start above 300 fpm, but smooth surfaces and low tension can air lubricate as slowly as 50 fpm.

How can I tell if I have air lubrication?
The signs of air lubrication are idlers dropping below web speed and driven rollers not maintaining tension set points or deviating from line speed. For undriven rollers, measure the force to hold the roller stopped (please think safety). On driven rollers, run experiments to find the tension differential that causes slip. You have measurable lubrication if this force or differential is lower at higher speeds and lower tensions. At winding, the telltale sign is shifting in the outside layers of the building roll. Air lubrication should be suspected when shifted layers increase with larger diameter, higher speed, lower tension, and smoother products.

How is air lubrication prevented?
For unnipped rollers, surface roughness or texture is the simplest way to prevent lubrication. Don't drive with bald tires, and don't handle webs with overly smooth rollers. Since the air layers are usually less than 0.005 in. thick, a roller roughness feature of 0.010-0.015 in. is a sufficient tread. Nips are effective at eliminating air lubrication but may create additional problems. Vacuum pull rollers eliminate air lubrication without the problems associated with nips. At winding, pack rollers are effective air squeegees, greatly reducing the air entering the roll.

When is air lubrication good?
Traction is required to hold in wrinkles; thus, lubrication reduces wrinkling sensitivity. Air entrained in winding rolls helps to fill the valleys of crossweb caliper variation.

Understanding air lubrication can help you prevent the detrimental effects (scratching, tension, and tracking control problems) and take advantage of the beneficial effects (less wrinkling and less caliper-sensitive winding).

Timothy J. Walker has 20+ years of experience in web handling processes. He specializes in web handling education, process development, and production problem solving. Contact him at 404/373-3771; tjwalker@tjwa.com; tjwa.com


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