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February 16, 2007
How does an air pressure gauge set web tension?
Filed under: Tensioning, Web Mechanics --- Tim Walker @ 09:00 AM
Pneumatic systems are popular on low cost converting equipment. Using a brake or clutch and an air pressure gauge avoids sophisticated electronics, wiring, and control systems.
If you set web tension from an air pressure gauge, there are two likely possibilities of what you are doing. If you have a dancer roller, you may be controlling the air pressure to one or two air cylinders that apply an external force to the dancer roller. If you don't have a dancer roller, the air pressure is likely going to a pneumatic brake or clutch, setting the slipping torque transmitted to the unwinding roll, winding roll, or a torque-controlled roller.
In either case, I highly advise spending some time with a force gauge (like a fish weighing scale) and measure the relationship between the air pressure settings and web tension.
[This entry is part of a growing 'book' that I am writing and posting at www.webhandling.com. Convertingblog.com is getting an exclusive first look at this restricted material.]
February 09, 2007
What is elastic? What is viscoelastic?
Filed under: Tensioning, Web Mechanics --- Tim Walker @ 01:18 PM
Definition: Elasticity is the property of returning to an initial form after deformation.
Rubber is the classic example of an elastic material. You push on it, it deforms; you let go, it bounces back. An elastic material responds to load almost immediately (the load travels through the material at the speed of sound). The amount of deformation is proportional to load and independent of time.
Definition: Viscous is the property of having a relatively high resistance to flow.
Molasses is the classic viscous material. When a force is applied to a viscous material, it will flow. The longer the load is on the viscous material, the more it will flow. When the force if removed, they stop flowing, but won't recover.
Definition: Viscoelasticity is the property of having both viscous and elastic properties.
When a viscoelastic (V-E) material is loaded, it will respond with a mixture of viscous and elastic behavior. Upon loading, a V-E material will immediately stretch (elastic behavior) and begin to flow (viscous behavior). When the load is removed from a V-E material, it will recover, some immediately (elastic behavior) and recover more over time (viscous behavior). Vinyl electrical tape is a classic and easily observed V-E material.
Do this test yourself. Pull out a 2-3 foot length of electrical tape. Hang a 1-2 lb weight on it. Note the initial elongation and that the tape will continue to elongate. Take the weight off. Note the initial recover and ongoing recovery.
Congratulations, you've just completed your first creep test.
[This entry is part of a growing 'book' that I am writing and posting at www.webhandling.com. Convertingblog.com is getting an exclusive first look at this restricted material.]