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- November 01, 2010, By Timothy J. Walker Contributing Editor
Here are five frequently asked questions about roller deflection with my responses. Next month, we'll talk more about deflection, but instead of rollers, we'll discuss deflection at winding and slitting.
What is roller deflection?
A roller remains cylindrical but no longer straight. This is not the deflection of crushing a pop can, more like bending a garden hose. A roller supported from one side only (cantilevered) will deflect by increasing amounts moving away from the supported end. A roller supported from both ends will have maximum deflection at the center of the roller.
What causes roller deflection?
Gravity, tension, nip loads, and intentional bowing, such as a curved axis spreader roller, all cause roller deflection.
How is roller deflection measured?
Deflection angle caused by gravity is easy to measure with a precision level. For cantilevered rollers, measure the level on the outboard side of the roller with no load and after hanging a weight equal to web tension at the center of the roller. For dual-end supported rollers, measure the level at the left, center, and right positions under no load and with a weight equal to web tension.
Alternatively, use a dial indicator to measure the magnitude of deflection between a reference point and the roller surface. Measure total deflection for low and high loads or tensions.
How much roller deflection is a problem?
For cantilevered rollers, the deflection and alignment specs should be the same. My default is less than 2 mils/ft of width or about 2 mm/m. For lower elastic modulus (stretchier) products, you can relax this to 5 mils/ft. For dual-end supported rollers, the troublesome deflection is more difficult to pin down.
Nipping systems are the most sensitive to deflection for two reasons:
- They have load many times higher than gravity or tension.
- They control many critical processes that are sensitive to gap or engagement variations.
A well-designed nipped roller system will have larger-diameter rollers, rubber-covered rollers, or both to compensate for the pressure variations that would be created by high roller deflections.
It is more difficult to recommend universal deflection specification dual-end supported unnipped rollers. My first instinct is to set a similar specification as roller alignment — keep roller deflection to less than 2 mils/ft of width. However, there is at least one condition in which roller deflection below this specification may cause you troubles, namely wrinkles.
Most of you likely know about bowed rollers (a.k.a. curved-axis or Mount Hope, a brand of Xerium Technologies) and how they are used to spread a web. A bowed roller should be approached in the same manner as a bow and arrow. If you are on the inside of the bow and you shoot an arrow you are safe. If you are on the outside of the bow, receiving the arrow — not so safe.
The same is true with webs. A web approaching a deflecting or bowed roller from the inside of the bow will be encouraged to spread or flatten, but a web approaching a deflecting or bowed roller from the outside of the bow will be encouraged to gather or wrinkle.
Imagine a roller deflection from gravity (let's ignore web tension). If you approach a deflection roller from above, the web will come at the bow from the inside and be encouraged to spread. However, if you approach a gravity-deflection roller from below — look out — this web comes at the outside of the bow and is looking the arrow in the eye or encouraged to gather and wrinkle.
This is the tricky part of deflection — same roller, but depending on the direction from which it enters, above or below, totally different results.
How can roller deflection problems be eliminated?
Here are three ways to solve roller deflection problems:
Eliminate the load that causes deflection | Less weight, less tension and wrap angle, less nip load.
Beef up the roller to resist deflection | Larger diameter, tiffer materials, thicker cylinder walls, narrower widths.
Optimize your process to reduce the effects of deflection | Avoid long rising vertical spans into rollers.
Web handling expert Tim Walker, president of TJWalker+Assoc., has 25 years of experience in web processes, education, development, and production problem solving. Contact him at 651-686-5400; firstname.lastname@example.org; www.webhandling.com.