When Rollers Fight, Webs Lose
- Published: April 01, 2004, By Timothy J. Walker TJWalker & Assoc. Inc.
A tram is a coal bin with wheels. If you want your coal bin to stay on its tracks, you must keep the two axles parallel. If they are not parallel, the front and back wheels will fight each other, leading to a derailment and a coal catastrophe.
Tram is used to describe parallelism in any direction, though most often in the horizontal plane or the plane of the web. Tramming is the action of measuring and setting good parallelism. If you want your web line to stay on track, keep your rollers and equipment sections parallel. If sequential rollers are not parallel, the upstream and downstream rollers will fight each other. When rollers fight, the web loses, leading to misalignment maladies such as wrinkles, tracking error, deformation, breakage, or process variations.
Measuring level — the alignment in the vertical direction — is the first step in tramming. You likely have used a carpenter's level to put up a shelf or hang a picture. For your web process, upgrade to a mechanic's precision level that features a V groove to rest the cylindrical topside of a roller and graduated lines to measure level errors in mils per foot.
Measuring tram — the alignment in the horizontal plane — is more difficult since we don't have gravity to guide us. There are at least five options for measuring tram, including tramming sticks, gauge blocks, pi tapes, optical transits, and laser-based systems. The first three are inexpensive and should be in every web handler's toolbox.
The web-tracking error from roller misalignment will be directly proportional to the error angle times the upstream web span length. A 10-mil/ft misalignment will shift the web about 7 mils for every foot of span length. This “rule of thumb” will overestimate tram-related tracking errors for wide, thick, short span, and high-modulus webs.
The tension variations from tram error are directly proportional to the side-to-side strain change and the web's modulus and thickness. A 10-mil edge-to-edge tram error over a 10-in. span will cause a 0.1% strain change. Multiplying the strain change by modulus (500 kpsi for bond paper or polyester film) produces a 500-psi tension variation or 0.5 PLI for every mil of thickness.
The tram error required to wrinkle a web is a far more complex calculation. Wrinkle analysis starts by assessing whether a roller has enough traction to hold a wrinkle. Next, the web, span, and tension values combine to determine the web's stiffness or resistance to buckling. Most web-tension-span combinations have no problem with an alignment error of 5-10 mil/ft. For wrinkle-sensitive materials (think thin and stiff), I recommend an alignment target of 2 mil/ft of width. I think this is a reasonable and measurable target.
Alignment is more difficult in pivoting assemblies, such as winders, nips, and dancer rollers. Start by ensuring the pivot axis is aligned, design the pivoting arms to translate this alignment, and then ensure the roller is square. Poor alignment at winding leads to crossweb wound-in tension variations and wrinkling. Nip pressure is extremely sensitive to misalignment. A compliant nip roller forgives some degree of misalignment, helping to reduce pressure variations.
How often should my equipment be leveled and trammed? I don't advocate a regular, all-out alignment preventive maintenance plan. Let the process or web tell you when you need to improve your alignment. Wrinkles, coating variations, winding variations, and edge flutter all will beg for improved alignment when they need it. Instead of spending your time and money on periodic alignment, invest in equipment design that holds good alignment. Beef up framework to prevent deflection. Drill and pin your roller or bearing blocks to prevent shifting in bolt hole clearance. Change over to split cap blocks, allowing rollers to be removed and installed without losing alignment.
Good roller alignment is the first step in successful web handling. Watch your web — it will tell you when it is caught in a roller battle of misalignment. Keep your process on track by ensuring roller alignment through design, measurement, and maintenance.
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; This email address is being protected from spambots. You need JavaScript enabled to view it.; tjwa.com.
