- February 01, 2003, Timothy J. Walker, TJWalker & Assoc. Inc.
In today's economic climate, belt tightening sounds like a fiscally wise move. Belt tightening, also known as cinching, is good for budgets, trash bags, and keeping your pants up; cinching during roll winding, however, may have less desired results, such as filling trash bags and losing your shirt.
What is cinching? “Cinch” comes from the Latin word for belt. Cinching is used to describe the relative motion of two layers around a curved surface like the motion used to tighten a belt before buckling it. In winding, cinching is any tightening or loosening motion of outer roll layers relative to inner roll layers. This action is sometimes called “clockspringing,” referring to how the roll's layers will tighten like a clock's spiral spring.
When does cinching occur? Cinching occurs when any point in a roll is pushed beyond its torque capacity. Each layer of a roll has a torque capacity defined as the product of the radius, the layer-to-layer traction coefficient, the inter-layer pressure, and the area of contact. Calculating torque capacity is difficult, because inter-layer roll pressures aren't easy to measure or model.
Cinching is more common on center winders than surface winders. Whether rewinding or unwinding, center winders apply torque at the core, transmitting it through the body of a roll to create tension at the roll's outer surface.
Cinching can be a localized event, happening at a distinct radial position in a roll, or it can be a pervasive, full-roll event.
Cinching from unwinding or rewinding tension will always shift layers in a tightening direction. Tightening-direction cinching usually is self-limiting, since the tightening action increases internal roll pressures and torque capacity.
Cinching from inertial torque may shift layers in either a tightening or loosening direction. If a winding roll is decelerated too quickly, its outer layers develop an inertial torque that resists stopping. This loosening cinching action, if great enough, can drive the web tangentially into compressive buckling, forming a crossweb or crepe wrinkle inside the roll. Cinching in the loosening direction is self-promoting, because the loosening action decreases internal roll pressures and torque capacity.
Is cinching a defect? Your product may cinch every roll and show no ill effects, but watch out for these undesirable cinching by-products:
- Scratching, abrasion, and debris generation: When you press two surfaces together and slide them relative to each other, you will have some wear.
- Lateral shifting of roll layers: This is a curious side effect, since cinching is a machine-direction event. When cinching occurs, the applied torsional load has consumed the roll's traction; therefore, there is no traction left over to oppose any internal lateral forces from crossweb nonuniformities such as caliper variations or skew.
- Crepe wrinkles: Inertial cinching that drives internal layers into compressive buckling may create permanent creases or crepe wrinkles in the web.
Can you live with cinching? I have seen a number of operations that simply live with cinching. Cinching is tolerated if your product is insensitive to scratching. Loose-direction cinching is avoided by minimizing acceleration and deceleration rates. Lateral shifting from cinching may be small enough that subsequent web guiding can handle it easily. If the lateral shifting gets severe, using winding flanges or spool walls can contain the shifting layers.
If you are not so lucky and cinching by-product defects fill your cinch sacks and cause you to lose your shirt, join us next month to understand how to eliminate the belt-tightening pain of cinching.
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 or via email at firstname.lastname@example.org; tjwa.com