Top Web : Slip and Wrinkles

Slip, Alignment and Wrinkles

Do you get scratching, do you have baggy webs, do you have wrinkles that you can't get rid of?

This TopWeb module will let you find out how to fix those problems.

When you wrap a web round a roller, the effect of the roller on the web (e.g. scratching) or the web on the roller (e.g. deflection force) depends on the Wrap angle, the Friction coefficient, the Radius of the roller, the Web roughness and Roller roughness and, of course, on the Tension of the film.

From these inputs, the program calculates the theoretical Air gap based on lubrication theory. If this is greater than the roughness of the web and roller then the web will slip and a Warning: will appear. It also calculates the Tension force and Friction force exerted by the web onto the roller. These are affected by torque on the roller.

If the roller exerts a Torque (either intentionally in a driven roller or unintentionally because of a poor bearing – you can select whether it’s working in Brake or Drive mode) then there will be a tension difference which can cause scratching if the web slips. To avoid scratching you must increase wrap, tension or friction coefficient, or decrease the torque on the roller (e.g. by greasing the bearing). The tension of the web exiting the roller is bigger or smaller (depending on Brake or Drive mode) and is shown as a % of the original tension in the diagram of the wrap.

The stretching of the web under tension causes a shrinkage in the transverse direction. The ratio of the two is called the Poisson ratio, and the Poisson shrinkage is calculate from the Poisson ratio, tension, modulus and web thickness. This shrinkage is normally negligible, but for thin webs and/or low modulus webs, this ‘necking’ can become significant.

General
When you wrap a web round a roller, the effect of the roller on the web (e.g. scratching) or the web on the roller (e.g. deflection force) depends on the Wrap angle, the Friction coefficient, the Diameter of the roller, the Web roughness and Roller roughness and, of course, on the Tension of the film.

From these inputs, the program calculates the theoretical Air gap based on lubrication theory. If this is greater than the roughness of the web and roller then the coefficient of friction will reduce (to zero at an air gap >3xRoughness), the web will slip and a Warning: will appear. It also calculates the Tension force and Friction force exerted by the web onto the roller. These are affected by torque on the roller. The friction force also depends on the coefficient of friction which varies, as explained above, depending on roughness and flying height. The Actual friction coefficient is output for you so you can see when it is decreasing

If the roller exerts a Torque (either intentionally in a driven roller or unintentionally because of a poor bearing - you can select whether it's working in Brake or Drive mode) then there will be a tension difference which can cause scratching if the web slips. To avoid scratching you must increase wrap, tension or friction coefficient, or decrease the torque on the roller (e.g. by greasing the bearing). The tension of the web exiting the roller is bigger or smaller (depending on Brake or Drive mode) and is shown as a % of the original tension in the diagram of the wrap.

Another way to think about it is that there is a maximum difference in tension between the incoming and outcoming nip without slipping. This is calculated as Max Delta T. This is useful if you know you want a tension on one side of, say, 100N/m and a tension the other side of 120N/m, so you want a Delta T of 20N/m. The two things which influence Delta T are the effective friction coefficient and the wrap angle. So by playing with these you can find out the minimum friction or wrap to give you a comfort zone around your 20N/m differential. However, the effective friction coefficient varies with web speed, tension etc. as described above so you should play with these to make sure that what works at one web speed doesn't fail if you go just a bit faster.

The stretching of the web under tension causes a shrinkage in the transverse direction. The ratio of the two is called the Poisson ratio, and the Poisson shrinkage is calculate from the Poisson ratio, tension, modulus and web thickness. This shrinkage is normally negligible, but for thin webs and/or low modulus webs, this ‘necking' can become significant.

Misalignment
If across a Span two rollers have a Misalignment then you get an increase in tension on one edge and a decrease on the other. This can lead to a Slack edge. It also can give rise to unwelcome high tensions (shown as Max tension) which, if it is too high will be a large percentage of the Yield strength, causing a web break. Small misalignments can cause large effects. The misalignment shown in the diagram is highly exaggerated! You enter the misalignment in absolute units of length. The diagram reports the misalignment in radians and degrees for those who like to think in those units

The theory used for calculating Max tension is not applicable once a slack edge appears and the values are shown as n/a.

Note that Misalignment is in the plane of the span. Misalignment out of the plane has surprisingly little effect.

The misalignment also steers the web, giving you a TD offset from a straight web path. If the roller is a larger radius at one end, Radius taper, that also steers the web towards the larger radius. The calculated TD offset includes the taper effect and assumes that both are operating in the same direction unless the - checkbox is clicked in which case the direction is opposite. If the TD Force calculated from these misalignments is greater than the Friction Force from the roller then the web will not be offset fully. In this case a "Friction limited" note appears next to the TD Offset and the calculated TD Offset is not affected by further increases in misalignment or taper. Note that the equations used in these Offset calculation are not applicable to all situations (e.g. they become less accurate for large misalignments and tapers) and the results should be regarded as indicative. The span to width ratio is important: outside the range 1 to 10 calculations become less accurate due to shear (<1) and axial tension (>10, depending on the actual tension). Note, too, that these basic calculations don't always match the conclusions from the more sophisticated shear wrinkle calculations below.

Wrinkles
The misalignment (and taper) can also cause shear wrinkles. So-called Type 1 wrinkles will show in the web between rollers but will dissipate on the roller and not cause real problems. Type 2 wrinkles will go over the roller and may be more serious. Type 1 depend on web tension, span, misalignment and modulus of the web. They become Type 2 if the traction on the roller is greater than a critical value. The traction depends on tension, roller radius and coefficient of friction, which in turn can decrease if the web starts to ‘float' over the roller. The critical value depends mainly on the modulus of the web. The approximate angle of the wrinkle is calculated for misalignment wrinkles as this might give you an idea of the level of misalignment present in your own wrinkled web. At present there is no simple formula for the angle of wrinkles from taper. If you select the Anisotropic option then you can enter a ratio of the modulus in the Machine Direction over that in the Transverse Direction, i.e. the web is anisotropic. For materials such as paper this ratio might be 2 (or to put it another way, the TD modulus is half that of the Modulus you have entered for the material), for others it might be a factor of 5. The higher the ratio, the more easily the web wrinkles for a given Modulus.

As a visual guide, if you click the Graph icon you see a plot of Misalignment v Tension. In general you will see a curve split into two colours (Green for Type 1 and Red for Type 2) with a line showing the critical point between the two types. A blue circle shows your current settings. If the circle is below the lines then you are wrinkle free. If it is in the green area then you have Type 1, in the red area then you have Type 2. The vertical position of the line between wrinkle and non-wrinkle and the relative split between Type 1 and Type 2 will change as you alter span, web modulus, roller roughness, air gap (via web speed) etc. The graph is particularly useful for showing if you are near a knife-edge where small changes in your process could tip you into a worrying Type 2 wrinkle. If you want to export the graph to a spreadsheet such as Excel, click the icon and the data will be put onto your Clipboard from which it can be pasted straight into your spreadsheet. For information, the critical misalignment that would cause wrinkles at your current tension is shown in the diagram. When you've finished looking at the Wrinkle Graph, click the T text icon.

Any lateral expansion of the web (thermal or hygroscopic expansion) can cause MD wrinkles of a wavelength and amplitude that can be calculated.

There is a subtle Poisson effect that arises if a driven roller reduces tension in the web. The web will expand in the machine direction and so will also expand in the transverse direction. In many cases this expansion is very small, but occasionally it is significant and can cause wrinkles in the same manner as those from thermal or hygroscopic expansion. Alternatively, if the tension increases, the web shrinks in the transverse direction and any expansion effects are reduced. The modeller takes both effects into account.

The diagram gives a visual feedback on the rollers and web (not to scale!). If you have shear wrinkles a dotted line at the relevant angle appears to remind you. If it's green then the wrinkle only survives up to the roller (Type 1); if it's red the wrinkle passes over the roller (Type 2). If you have MD wrinkles, you have blue lines along the web. The example shows that you have both MD wrinkles and Type 2 shear wrinkles